Adding upstream version 86.0.1.upstream/86.0.1 upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
author: Daniel Baumann <daniel.baumann@progress-linux.org> 2024-04-28 14:29:10 +0000
committer: Daniel Baumann <daniel.baumann@progress-linux.org> 2024-04-28 14:29:10 +0000
commit: 2aa4a82499d4becd2284cdb482213d541b8804dd (patch)
tree: b80bf8bf13c3766139fbacc530efd0dd9d54394c /third_party/rust/tracing
parent: Initial commit. (diff)
download: firefox-2aa4a82499d4becd2284cdb482213d541b8804dd.tar.xz
firefox-2aa4a82499d4becd2284cdb482213d541b8804dd.zip
33 files changed, 10147 insertions, 0 deletions
diff --git a/third_party/rust/tracing/.cargo-checksum.json b/third_party/rust/tracing/.cargo-checksum.json
new file mode 100644
index 0000000000..51b9e3ffd2
--- /dev/null
+++ b/third_party/rust/tracing/.cargo-checksum.json
@@ -0,0 +1 @@
+{"files":{"CHANGELOG.md":"31b10e6cb5c73a58c5181b37d6595832ccc98318a66f2368561b34d9de276b4a","Cargo.toml":"dfa3ce66aa1c75c611d7e35f9e6e7cdfcb2600a5975e561bcdde2437e355baa1","LICENSE":"898b1ae9821e98daf8964c8d6c7f61641f5f5aa78ad500020771c0939ee0dea1","README.md":"00f72d1c89ccf8036512d17a761ddfea2b7481b93f569c5c542d5ee93680945a","benches/no_subscriber.rs":"799c293113c78255e1cb45eec3656ee85c0f8910458bc3412f3175c44c2bff46","benches/subscriber.rs":"066a69119ffc8dc5ccf2801b0d75fb73cbd563dbaadb824121b9a68301a68a4e","src/dispatcher.rs":"8279c019ecceae81e47407323cebe8f7a017abbf4fd6b5314fb88b7b1c9ec26e","src/field.rs":"f5537ea28cc98850a27f86dc6b937e8baff40869b8999b17751ae3f559500ab0","src/instrument.rs":"fe0a173f404b970f0ce78785f151f56a8f53ded97a9ac56214864f7670527a9d","src/level_filters.rs":"8b81db76629b00a802b40a91f4a617b97a6f78b3d9fda233b6d9ec99ecb4bb8a","src/lib.rs":"46d4f3e75506c793921ef026fa7a4d2a7604351790ba9d264f973fe5cd863fe6","src/macros.rs":"aba201cd59e0ff119794b1af6774db678d556e18ddedb4a39415680e8fc20fdb","src/span.rs":"24304f752eb893aa912bcf73ac748f6df679327d5a75ba4a052bc91349e1c3b4","src/stdlib.rs":"248514a9bae6106e436358aee44c92abf8e7f79022895c4a25136ddef211d198","src/subscriber.rs":"4d489dfa55b29db439b447bb42eedd5fa030579634b9f403603afb0055fb9371","tests/event.rs":"7c043893e2ec6202906043ac0a2b9433b53ba6156b59afdd264bf4b9e38fc712","tests/filter_caching_is_lexically_scoped.rs":"0845255a0cf0b3288f8d641fd7d72277051b759fdbce6ff24e266523219ee68c","tests/filters_are_not_reevaluated_for_the_same_span.rs":"d3287dabe73e321870128e87b4084e20548ec18964004cd73cc3ca6383b73c26","tests/filters_are_reevaluated_for_different_call_sites.rs":"98c891a0ebd8c2297c9f4a38a7a0406b0a55757f99cc0deea968929d2a998092","tests/filters_dont_leak.rs":"f302ea2ba976f32400206b81a8de6110aa53a4d4555cd4c284242d0417220dab","tests/macro_imports.rs":"d5de857162185d4a2384f3cb644bfcf76c7f5c1a3b5f72bfa0d2620ac6e3873c","tests/macros.rs":"846a3ce9fbd3f781000b8557485b5dc99f8ff89915e602d4e0c17e3775a50627","tests/max_level_hint.rs":"973124e340f6250ecb2ef8a8e5602623702ddfc75885187b15107accfb378e08","tests/multiple_max_level_hints.rs":"c8339fe81dafdc89ab4a86658be2a44d0ddd70d639522a76cd273f03f563d09c","tests/span.rs":"399615f8a2d8e2f4cce9c63adb0c1f2bbf7eaa70347adb3dc1e4c5ba3522f449","tests/subscriber.rs":"51035f71f9642dea4c5b9348691d47611b1d77fe78dd217931187f7b7549e5db","tests/support/event.rs":"ffcfa68f305782b393bb30ac4514308751757d2d81001ef5e774588141b595db","tests/support/field.rs":"bc221de91feb5f6705fedf1440fb6fccc5d7e1e0906c8a8d01a3029b1af77054","tests/support/metadata.rs":"408f9cb169e7a9330f1db0db25bdff7eaa8580143c99b3aca0d3653d8e155ea8","tests/support/mod.rs":"c67ea2e17f9cf8642069972ee6b02acd23e6d782a4382b57828810d3130de24d","tests/support/span.rs":"769690c93c2766a7a3ba683c59c41ccbd24e183d14e8151f028960785918e636","tests/support/subscriber.rs":"f51424720f46a73fc6b65a540b9a4653473ffeca1f58367e6f98744268c49584"},"package":"b0987850db3733619253fe60e17cb59b82d37c7e6c0236bb81e4d6b87c879f27"}
+\ No newline at end of file
diff --git a/third_party/rust/tracing/CHANGELOG.md b/third_party/rust/tracing/CHANGELOG.md
new file mode 100644
index 0000000000..0a6c648e5c
--- /dev/null
+++ b/third_party/rust/tracing/CHANGELOG.md
@@ -0,0 +1,368 @@
+# 0.1.21 (September 28, 2020)
+
+### Fixed
+
+- Incorrect inlining of `Span::new`, `Span::new_root`, and `Span::new_child_of`,
+  which could result in  `dispatcher::get_default` being inlined at the callsite
+  ([#994])
+- Regression where using a struct field as a span or event field when other
+  fields on that struct are borrowed mutably would fail to compile ([#987])
+
+### Changed
+
+- Updated `tracing-core` to 0.1.17 ([#992])
+
+### Added
+
+- `Instrument` trait and `Instrumented` type for attaching a `Span` to a
+  `Future` ([#808])
+- `Copy` implementations for `Level` and `LevelFilter` ([#992])
+- Multiple documentation fixes and improvements ([#964], [#980], [#981])
+
+Thanks to @nagisa, and new contributors @SecurityInsanity, @froydnj, @jyn514 and
+@TaKO8Ki for contributing to this release!
+
+[#994]: https://github.com/tokio-rs/tracing/pull/994
+[#992]: https://github.com/tokio-rs/tracing/pull/992
+[#987]: https://github.com/tokio-rs/tracing/pull/987 
+[#980]: https://github.com/tokio-rs/tracing/pull/980
+[#981]: https://github.com/tokio-rs/tracing/pull/981
+[#964]: https://github.com/tokio-rs/tracing/pull/964
+[#808]: https://github.com/tokio-rs/tracing/pull/808
+
+# 0.1.20 (August 24, 2020)
+
+### Changed
+
+- Significantly reduced assembly generated by macro invocations (#943)
+- Updated `tracing-core` to 0.1.15 (#943)
+
+### Added 
+
+- Documented minimum supported Rust version policy (#941)
+
+# 0.1.19 (August 10, 2020)
+
+### Fixed
+
+- Updated `tracing-core` to fix incorrect calculation of the global max level
+  filter (#908)
+
+### Added
+
+- **attributes**: Support for using `self` in field expressions when
+  instrumenting `async-trait` functions (#875)
+- Several documentation improvements (#832, #881, #896, #897, #911, #913)
+
+Thanks to @anton-dutov, @nightmared, @mystor, and @toshokan for contributing to
+this release!
+  
+# 0.1.18 (July 31, 2020)
+
+### Fixed
+
+- Fixed a bug where `LevelFilter::OFF` (and thus also the `static_max_level_off`
+  feature flag) would enable *all* traces, rather than *none* (#853)
+- **log**: Fixed `tracing` macros and `Span`s not checking `log::max_level`
+  before emitting `log` records (#870)
+  
+### Changed
+
+- **macros**: Macros now check the global max level (`LevelFilter::current`)
+  before the per-callsite cache when determining if a span or event is enabled.
+  This significantly improves performance in some use cases (#853)
+- **macros**: Simplified the code generated by macro expansion significantly,
+  which may improve compile times and/or `rustc` optimizatation of surrounding
+  code (#869, #869)
+- **macros**: Macros now check the static max level before checking any runtime
+  filtering, improving performance when a span or event is disabled by a
+  `static_max_level_XXX` feature flag (#868) 
+- `LevelFilter` is now a re-export of the `tracing_core::LevelFilter` type, it
+  can now be used interchangably with the versions in `tracing-core` and
+  `tracing-subscriber` (#853)
+- Significant performance improvements when comparing `LevelFilter`s and
+  `Level`s (#853)
+- Updated the minimum `tracing-core` dependency to 0.1.12 (#853)
+
+### Added
+
+- **macros**: Quoted string literals may now be used as field names, to allow
+  fields whose names are not valid Rust identifiers (#790)
+- **docs**: Several documentation improvements (#850, #857, #841)
+- `LevelFilter::current()` function, which returns the highest level that any
+  subscriber will enable (#853)
+- `Subscriber::max_level_hint` optional trait method, for setting the value
+  returned by `LevelFilter::current()` (#853)
+  
+Thanks to new contributors @cuviper, @ethanboxx, @ben0x539, @dignati,
+@colelawrence, and @rbtcollins for helping out with this release!
+
+# 0.1.17 (July 22, 2020)
+
+### Changed
+
+- **log**: Moved verbose span enter/exit log records to "tracing::span::active"
+  target, allowing them to be filtered separately (#833)
+- **log**: All span lifecycle log records without fields now have the `Trace`
+  log filter, to guard against `log` users enabling them by default with blanket
+  level filtering (#833)
+  
+### Fixed
+
+- **log**/**macros**: Fixed missing implicit imports of the
+  `tracing::field::debug` and `tracing::field::display` functions inside the
+  macros when the "log" feature is enabled (#835)
+
+# 0.1.16 (July 8, 2020)
+
+### Added
+
+- **attributes**: Support for arbitrary expressions as fields in `#[instrument]` (#672)
+- **attributes**: `#[instrument]` now emits a compiler warning when ignoring unrecognized
+  input (#672, #786)
+- Improved documentation on using `tracing` in async code (#769)
+
+### Changed
+
+- Updated `tracing-core` dependency to 0.1.11
+
+### Fixed
+
+- **macros**: Excessive monomorphization in macros, which could lead to
+  longer compilation times (#787) 
+- **log**: Compiler warnings in macros when `log` or `log-always` features
+  are enabled (#753)
+- Compiler error when `tracing-core/std` feature is enabled but `tracing/std` is
+  not (#760)
+
+Thanks to @nagisa for contributing to this release!
+
+# 0.1.15 (June 2, 2020)
+
+### Changed
+
+- **macros**: Replaced use of legacy `local_inner_macros` with `$crate::` (#740)
+
+### Added
+
+- Docs fixes and improvements (#742, #731, #730)
+
+Thanks to @bnjjj, @blaenk, and @LukeMathWalker for contributing to this release!
+
+# 0.1.14 (May 14, 2020)
+
+### Added
+
+- **log**: When using the [`log`] compatibility feature alongside a `tracing`
+  `Subscriber`, log records for spans now include span IDs (#613)
+- **attributes**: Support for using `#[instrument]` on methods that are part of
+  [`async-trait`] trait implementations (#711)
+- **attributes**: Optional `#[instrument(err)]` argument to automatically emit
+  an event if an instrumented function returns `Err` (#637) 
+- Added `#[must_use]` attribute to the guard returned by
+  `subscriber::set_default` (#685)
+  
+### Changed
+
+- **log**: Made [`log`] records emitted by spans much less noisy when span IDs are
+ not available (#613)
+ 
+### Fixed
+
+- Several typos in the documentation (#656, #710, #715)
+
+Thanks to @FintanH, @shepmaster, @inanna-malick, @zekisharif, @bkchr, @majecty,
+@ilana and @nightmared for contributing to this release! 
+
+[`async-trait`]: https://crates.io/crates/async-trait 
+[`log`]: https://crates.io/crates/log
+
+# 0.1.13 (February 26, 2019)
+
+### Added
+
+- **field**: `field::Empty` type for declaring empty fields whose values will be
+  recorded later (#548)
+- **field**: `field::Value` implementations for `Wrapping` and `NonZero*`
+  numbers (#538)
+- **attributes**: Support for adding arbitrary literal fields to spans generated
+  by `#[instrument]` (#569)
+- **attributes**: `#[instrument]` now emits a helpful compiler error when
+  attempting to skip a function parameter (#600)
+
+### Changed
+
+- **attributes**: The `#[instrument]` attribute was placed under an on-by-default
+  feature flag "attributes" (#603)
+
+### Fixed
+
+- Broken and unresolvable links in RustDoc (#595)
+
+Thanks to @oli-cosmian and @Kobzol for contributing to this release!
+
+# 0.1.12 (January 11, 2019)
+
+### Added
+
+- `Span::with_subscriber` method to access the subscriber that tracks a `Span`
+  (#503)
+- API documentation now shows which features are required by feature-flagged
+  items (#523)
+- Improved README examples (#496)
+- Documentation links to related crates (#507)
+
+# 0.1.11 (December 20, 2019)
+
+### Added
+
+- `Span::is_none` method (#475)
+- `LevelFilter::into_level` method (#470)
+- `LevelFilter::from_level` function and `From<Level>` impl (#471)
+- Documented minimum supported Rust version (#482)
+
+### Fixed
+
+- Incorrect parameter type to `Span::follows_from` that made it impossible to
+  call (#467)
+- Missing whitespace in `log` records generated when enabling the `log` feature
+  flag (#484)
+- Typos and missing links in documentation (#405, #423, #439)
+
+# 0.1.10 (October 23, 2019)
+
+### Added
+
+- Support for destructuring in arguments to `#[instrument]`ed functions (#397)
+- Generated field for `self` parameters when `#[instrument]`ing methods (#397)
+- Optional `skip` argument to `#[instrument]` for excluding function parameters
+  from generated spans (#359)
+- Added `dispatcher::set_default` and `subscriber::set_default` APIs, which
+  return a drop guard (#388)
+
+### Fixed
+
+- Some minor documentation errors (#356, #370)
+
+# 0.1.9 (September 13, 2019)
+
+### Fixed
+
+- Fixed `#[instrument]`ed async functions not compiling on `nightly-2019-09-11`
+  or newer (#342)
+
+### Changed
+
+- Significantly reduced performance impact of skipped spans and events when a
+  `Subscriber` is not in use (#326)
+- The `log` feature will now only cause `tracing` spans and events to emit log
+  records when a `Subscriber` is not in use (#346)
+
+### Added
+
+- Added support for overriding the name of the span generated by `#[instrument]`
+  (#330)
+- `log-always` feature flag to emit log records even when a `Subscriber` is set
+  (#346)
+
+# 0.1.8 (September 3, 2019)
+
+### Changed
+
+- Reorganized and improved API documentation (#317)
+
+### Removed
+
+- Dev-dependencies on `ansi_term` and `humantime` crates, which were used only
+  for examples (#316)
+
+# 0.1.7 (August 30, 2019)
+
+### Changed
+
+- New (curly-brace free) event message syntax to place the message in the first
+  field rather than the last (#309)
+
+### Fixed
+
+- Fixed a regression causing macro stack exhaustion when the `log` feature flag
+  is enabled (#304)
+
+# 0.1.6 (August 20, 2019)
+
+### Added
+
+- `std::error::Error` as a new primitive type (#277)
+- Support for mixing key-value fields and `format_args` messages without curly
+  braces as delimiters (#288)
+
+### Changed
+
+- `tracing-core` dependency to 0.1.5 (#294)
+- `tracing-attributes` dependency to 0.1.2 (#297)
+
+# 0.1.5 (August 9, 2019)
+
+### Added
+
+- Support for `no-std` + `liballoc` (#263)
+
+### Changed
+
+- Using the `#[instrument]` attribute on `async fn`s no longer requires a
+  feature flag (#258)
+
+### Fixed
+
+- The `#[instrument]` macro now works on generic functions (#262)
+
+# 0.1.4 (August 8, 2019)
+
+### Added
+
+- `#[instrument]` attribute for automatically adding spans to functions (#253)
+
+# 0.1.3 (July 11, 2019)
+
+### Added
+
+- Log messages when a subscriber indicates that a span has closed, when the
+  `log` feature flag is enabled (#180).
+
+### Changed
+
+- `tracing-core` minimum dependency version to 0.1.2 (#174).
+
+### Fixed
+
+- Fixed an issue where event macro invocations with a single field, using local
+  variable shorthand, would recur infinitely (#166).
+- Fixed uses of deprecated `tracing-core` APIs (#174).
+
+# 0.1.2 (July 6, 2019)
+
+### Added
+
+- `Span::none()` constructor, which does not require metadata and
+  returns a completely empty span (#147).
+- `Span::current()` function, returning the current span if it is
+  known to the subscriber (#148).
+
+### Fixed
+
+- Broken macro imports when used prefixed with `tracing::` (#152).
+
+# 0.1.1 (July 3, 2019)
+
+### Changed
+
+- `cfg_if` dependency to 0.1.9.
+
+### Fixed
+
+- Compilation errors when the `log` feature is enabled (#131).
+- Unclear wording and typos in documentation (#124, #128, #142).
+
+# 0.1.0 (June 27, 2019)
+
+- Initial release
diff --git a/third_party/rust/tracing/Cargo.toml b/third_party/rust/tracing/Cargo.toml
new file mode 100644
index 0000000000..94b5916458
--- /dev/null
+++ b/third_party/rust/tracing/Cargo.toml
@@ -0,0 +1,88 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies
+#
+# If you believe there's an error in this file please file an
+# issue against the rust-lang/cargo repository. If you're
+# editing this file be aware that the upstream Cargo.toml
+# will likely look very different (and much more reasonable)
+
+[package]
+edition = "2018"
+name = "tracing"
+version = "0.1.21"
+authors = ["Eliza Weisman <eliza@buoyant.io>", "Tokio Contributors <team@tokio.rs>"]
+description = "Application-level tracing for Rust.\n"
+homepage = "https://tokio.rs"
+readme = "README.md"
+keywords = ["logging", "tracing", "metrics", "async"]
+categories = ["development-tools::debugging", "development-tools::profiling", "asynchronous", "no-std"]
+license = "MIT"
+repository = "https://github.com/tokio-rs/tracing"
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]
+
+[[bench]]
+name = "subscriber"
+harness = false
+
+[[bench]]
+name = "no_subscriber"
+harness = false
+[dependencies.cfg-if]
+version = "0.1.10"
+
+[dependencies.log]
+version = "0.4"
+optional = true
+
+[dependencies.pin-project-lite]
+version = "0.1"
+
+[dependencies.tracing-attributes]
+version = "0.1.10"
+optional = true
+
+[dependencies.tracing-core]
+version = "0.1.17"
+default-features = false
+[dev-dependencies.criterion]
+version = "0.3"
+default_features = false
+
+[dev-dependencies.futures]
+version = "0.1"
+
+[dev-dependencies.log]
+version = "0.4"
+
+[dev-dependencies.tokio]
+version = "0.2.21"
+features = ["rt-core"]
+
+[features]
+async-await = []
+attributes = ["tracing-attributes"]
+default = ["std", "attributes"]
+log-always = ["log"]
+max_level_debug = []
+max_level_error = []
+max_level_info = []
+max_level_off = []
+max_level_trace = []
+max_level_warn = []
+release_max_level_debug = []
+release_max_level_error = []
+release_max_level_info = []
+release_max_level_off = []
+release_max_level_trace = []
+release_max_level_warn = []
+std = ["tracing-core/std"]
+[target."cfg(target_arch = \"wasm32\")".dev-dependencies.wasm-bindgen-test]
+version = "^0.3"
+[badges.maintenance]
+status = "actively-developed"
diff --git a/third_party/rust/tracing/LICENSE b/third_party/rust/tracing/LICENSE
new file mode 100644
index 0000000000..cdb28b4b56
--- /dev/null
+++ b/third_party/rust/tracing/LICENSE
@@ -0,0 +1,25 @@
+Copyright (c) 2019 Tokio Contributors
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/third_party/rust/tracing/README.md b/third_party/rust/tracing/README.md
new file mode 100644
index 0000000000..e524235feb
--- /dev/null
+++ b/third_party/rust/tracing/README.md
@@ -0,0 +1,444 @@
+![Tracing — Structured, application-level diagnostics][splash]
+
+[splash]: https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/splash.svg
+
+# tracing
+
+Application-level tracing for Rust.
+
+[![Crates.io][crates-badge]][crates-url]
+[![Documentation][docs-badge]][docs-url]
+[![Documentation (master)][docs-master-badge]][docs-master-url]
+[![MIT licensed][mit-badge]][mit-url]
+[![Build Status][actions-badge]][actions-url]
+[![Discord chat][discord-badge]][discord-url]
+
+[Documentation][docs-url] | [Chat][discord-url]
+
+[crates-badge]: https://img.shields.io/crates/v/tracing.svg
+[crates-url]: https://crates.io/crates/tracing/0.1.21
+[docs-badge]: https://docs.rs/tracing/badge.svg
+[docs-url]: https://docs.rs/tracing/0.1.21
+[docs-master-badge]: https://img.shields.io/badge/docs-master-blue
+[docs-master-url]: https://tracing-rs.netlify.com/tracing
+[mit-badge]: https://img.shields.io/badge/license-MIT-blue.svg
+[mit-url]: LICENSE
+[actions-badge]: https://github.com/tokio-rs/tracing/workflows/CI/badge.svg
+[actions-url]:https://github.com/tokio-rs/tracing/actions?query=workflow%3ACI
+[discord-badge]: https://img.shields.io/discord/500028886025895936?logo=discord&label=discord&logoColor=white
+[discord-url]: https://discord.gg/EeF3cQw
+
+## Overview
+
+`tracing` is a framework for instrumenting Rust programs to collect
+structured, event-based diagnostic information.
+
+In asynchronous systems like Tokio, interpreting traditional log messages can
+often be quite challenging. Since individual tasks are multiplexed on the same
+thread, associated events and log lines are intermixed making it difficult to
+trace the logic flow. `tracing` expands upon logging-style diagnostics by
+allowing libraries and applications to record structured events with additional
+information about *temporality* and *causality* — unlike a log message, a span
+in `tracing` has a beginning and end time, may be entered and exited by the
+flow of execution, and may exist within a nested tree of similar spans. In
+addition, `tracing` spans are *structured*, with the ability to record typed
+data as well as textual messages.
+
+The `tracing` crate provides the APIs necessary for instrumenting libraries
+and applications to emit trace data.
+
+*Compiler support: [requires `rustc` 1.40+][msrv]*
+
+[msrv]: #supported-rust-versions
+
+## Usage
+
+(The examples below are borrowed from the `log` crate's yak-shaving
+[example](https://docs.rs/log/0.4.10/log/index.html#examples), modified to
+idiomatic `tracing`.)
+
+### In Applications
+
+In order to record trace events, executables have to use a `Subscriber`
+implementation compatible with `tracing`. A `Subscriber` implements a way of
+collecting trace data, such as by logging it to standard output. [`tracing_subscriber`](https://docs.rs/tracing-subscriber/)'s
+[`fmt` module](https://docs.rs/tracing-subscriber/0.2.0-alpha.2/tracing_subscriber/fmt/index.html) provides reasonable defaults.
+Additionally, `tracing-subscriber` is able to consume messages emitted by `log`-instrumented libraries and modules.
+
+The simplest way to use a subscriber is to call the `set_global_default` function.
+
+```rust
+use tracing::{info, Level};
+use tracing_subscriber::FmtSubscriber;
+
+fn main() {
+    // a builder for `FmtSubscriber`.
+    let subscriber = FmtSubscriber::builder()
+        // all spans/events with a level higher than TRACE (e.g, debug, info, warn, etc.)
+        // will be written to stdout.
+        .with_max_level(Level::TRACE)
+        // completes the builder.
+        .finish();
+
+    tracing::subscriber::set_global_default(subscriber)
+        .expect("setting default subscriber failed");
+
+    let number_of_yaks = 3;
+    // this creates a new event, outside of any spans.
+    info!(number_of_yaks, "preparing to shave yaks");
+
+    let number_shaved = yak_shave::shave_all(number_of_yaks);
+    info!(
+        all_yaks_shaved = number_shaved == number_of_yaks,
+        "yak shaving completed."
+    );
+}
+```
+
+```toml
+[dependencies]
+tracing = "0.1"
+tracing-subscriber = "0.2.0"
+```
+
+This subscriber will be used as the default in all threads for the remainder of the duration
+of the program, similar to how loggers work in the `log` crate.
+
+In addition, you can locally override the default subscriber. For example:
+
+```rust
+use tracing::{info, Level};
+use tracing_subscruber::FmtSubscriber;
+
+fn main() {
+    let subscriber = tracing_subscriber::FmtSubscriber::builder()
+        // all spans/events with a level higher than TRACE (e.g, debug, info, warn, etc.)
+        // will be written to stdout.
+        .with_max_level(Level::TRACE)
+        // builds the subscriber.
+        .finish();
+
+    tracing::subscriber::with_default(subscriber, || {
+        info!("This will be logged to stdout");
+    });
+    info!("This will _not_ be logged to stdout");
+}
+```
+
+This approach allows trace data to be collected by multiple subscribers
+within different contexts in the program. Note that the override only applies to the
+currently executing thread; other threads will not see the change from with_default.
+
+Any trace events generated outside the context of a subscriber will not be collected.
+
+Once a subscriber has been set, instrumentation points may be added to the
+executable using the `tracing` crate's macros.
+
+### In Libraries
+
+Libraries should only rely on the `tracing` crate and use the provided macros
+and types to collect whatever information might be useful to downstream consumers.
+
+```rust
+use std::{error::Error, io};
+use tracing::{debug, error, info, span, warn, Level};
+
+// the `#[tracing::instrument]` attribute creates and enters a span
+// every time the instrumented function is called. The span is named after the
+// the function or method. Paramaters passed to the function are recorded as fields.
+#[tracing::instrument]
+pub fn shave(yak: usize) -> Result<(), Box<dyn Error + 'static>> {
+    // this creates an event at the DEBUG level with two fields:
+    // - `excitement`, with the key "excitement" and the value "yay!"
+    // - `message`, with the key "message" and the value "hello! I'm gonna shave a yak."
+    //
+    // unlike other fields, `message`'s shorthand initialization is just the string itself.
+    debug!(excitement = "yay!", "hello! I'm gonna shave a yak.");
+    if yak == 3 {
+        warn!("could not locate yak!");
+        // note that this is intended to demonstrate `tracing`'s features, not idiomatic
+        // error handling! in a library or application, you should consider returning
+        // a dedicated `YakError`. libraries like snafu or thiserror make this easy.
+        return Err(io::Error::new(io::ErrorKind::Other, "shaving yak failed!").into());
+    } else {
+        debug!("yak shaved successfully");
+    }
+    Ok(())
+}
+
+pub fn shave_all(yaks: usize) -> usize {
+    // Constructs a new span named "shaving_yaks" at the TRACE level,
+    // and a field whose key is "yaks". This is equivalent to writing:
+    //
+    // let span = span!(Level::TRACE, "shaving_yaks", yaks = yaks);
+    //
+    // local variables (`yaks`) can be used as field values
+    // without an assignment, similar to struct initializers.
+    let span = span!(Level::TRACE, "shaving_yaks", yaks);
+    let _enter = span.enter();
+
+    info!("shaving yaks");
+
+    let mut yaks_shaved = 0;
+    for yak in 1..=yaks {
+        let res = shave(yak);
+        debug!(yak, shaved = res.is_ok());
+
+        if let Err(ref error) = res {
+            // Like spans, events can also use the field initialization shorthand.
+            // In this instance, `yak` is the field being initalized.
+            error!(yak, error = error.as_ref(), "failed to shave yak!");
+        } else {
+            yaks_shaved += 1;
+        }
+        debug!(yaks_shaved);
+    }
+
+    yaks_shaved
+}
+```
+
+```toml
+[dependencies]
+tracing = "0.1"
+```
+
+Note: Libraries should *NOT* call `set_global_default()`, as this will cause
+conflicts when executables try to set the default later.
+
+### In Asynchronous Code
+
+If you are instrumenting code that make use of
+[`std::future::Future`](https://doc.rust-lang.org/stable/std/future/trait.Future.html)
+or async/await, be sure to use the
+[`tracing-futures`](https://docs.rs/tracing-futures) crate. This is needed
+because the following example _will not_ work:
+
+```rust
+async {
+    let _s = span.enter();
+    // ...
+}
+```
+
+The span guard `_s` will not exit until the future generated by the `async` block is complete.
+Since futures and spans can be entered and exited _multiple_ times without them completing,
+the span remains entered for as long as the future exists, rather than being entered only when
+it is polled, leading to very confusing and incorrect output.
+For more details, see [the documentation on closing spans](https://tracing.rs/tracing/span/index.html#closing-spans).
+
+There are two ways to instrument asynchronous code. The first is through the
+[`Future::instrument`](https://docs.rs/tracing/latest/tracing/trait.Instrument.html#method.instrument) combinator:
+
+```rust
+use tracing::Instrument;
+
+let my_future = async {
+    // ...
+};
+
+my_future
+    .instrument(tracing::info_span!("my_future"))
+    .await
+```
+
+`Future::instrument` attaches a span to the future, ensuring that the span's lifetime
+is as long as the future's.
+
+The second, and preferred, option is through the
+[`#[instrument]`](https://docs.rs/tracing/0.1.21/tracing/attr.instrument.html)
+attribute:
+
+```rust
+use tracing::{info, instrument};
+use tokio::{io::AsyncWriteExt, net::TcpStream};
+use std::io;
+
+#[instrument]
+async fn write(stream: &mut TcpStream) -> io::Result<usize> {
+    let result = stream.write(b"hello world\n").await;
+    info!("wrote to stream; success={:?}", result.is_ok());
+    result
+}
+```
+
+Under the hood, the `#[instrument]` macro performs the same explicit span
+attachment that `Future::instrument` does.
+
+### Concepts
+
+This crate provides macros for creating `Span`s and `Event`s, which represent
+periods of time and momentary events within the execution of a program,
+respectively.
+
+As a rule of thumb, _spans_ should be used to represent discrete units of work
+(e.g., a given request's lifetime in a server) or periods of time spent in a
+given context (e.g., time spent interacting with an instance of an external
+system, such as a database). In contrast, _events_ should be used to represent
+points in time within a span — a request returned with a given status code,
+_n_ new items were taken from a queue, and so on.
+
+`Span`s are constructed using the `span!` macro, and then _entered_
+to indicate that some code takes place within the context of that `Span`:
+
+```rust
+use tracing::{span, Level};
+
+// Construct a new span named "my span".
+let mut span = span!(Level::INFO, "my span");
+span.in_scope(|| {
+    // Any trace events in this closure or code called by it will occur within
+    // the span.
+});
+// Dropping the span will close it, indicating that it has ended.
+```
+
+The [`#[instrument]`](https://docs.rs/tracing/0.1.21/tracing/attr.instrument.html) attribute macro
+can reduce some of this boilerplate:
+
+```rust
+use tracing::{instrument};
+
+#[instrument]
+pub fn my_function(my_arg: usize) {
+    // This event will be recorded inside a span named `my_function` with the
+    // field `my_arg`.
+    tracing::info!("inside my_function!");
+    // ...
+}
+```
+
+The `Event` type represent an event that occurs instantaneously, and is
+essentially a `Span` that cannot be entered. They are created using the `event!`
+macro:
+
+```rust
+use tracing::{event, Level};
+
+event!(Level::INFO, "something has happened!");
+```
+
+Users of the [`log`] crate should note that `tracing` exposes a set of macros for
+creating `Event`s (`trace!`, `debug!`, `info!`, `warn!`, and `error!`) which may
+be invoked with the same syntax as the similarly-named macros from the `log`
+crate. Often, the process of converting a project to use `tracing` can begin
+with a simple drop-in replacement.
+
+## Supported Rust Versions
+
+Tracing is built against the latest stable release. The minimum supported
+version is 1.40. The current Tracing version is not guaranteed to build on Rust
+versions earlier than the minimum supported version.
+
+Tracing follows the same compiler support policies as the rest of the Tokio
+project. The current stable Rust compiler and the three most recent minor
+versions before it will always be supported. For example, if the current stable
+compiler version is 1.45, the minimum supported version will not be increased
+past 1.42, three minor versions prior. Increasing the minimum supported compiler
+version is not considered a semver breaking change as long as doing so complies
+with this policy.
+
+## Ecosystem
+
+### Related Crates
+
+In addition to `tracing` and `tracing-core`, the [`tokio-rs/tracing`] repository
+contains several additional crates designed to be used with the `tracing` ecosystem.
+This includes a collection of `Subscriber` implementations, as well as utility
+and adapter crates to assist in writing `Subscriber`s and instrumenting
+applications.
+
+In particular, the following crates are likely to be of interest:
+
+- [`tracing-futures`] provides a compatibility layer with the `futures`
+  crate, allowing spans to be attached to `Future`s, `Stream`s, and `Executor`s.
+- [`tracing-subscriber`] provides `Subscriber` implementations and
+  utilities for working with `Subscriber`s. This includes a [`FmtSubscriber`]
+  `FmtSubscriber` for logging formatted trace data to stdout, with similar
+  filtering and formatting to the [`env_logger`] crate.
+- [`tracing-log`] provides a compatibility layer with the [`log`] crate,
+  allowing log messages to be recorded as `tracing` `Event`s within the
+  trace tree. This is useful when a project using `tracing` have
+  dependencies which use `log`. Note that if you're using
+  `tracing-subscriber`'s `FmtSubscriber`, you don't need to depend on
+  `tracing-log` directly.
+
+Additionally, there are also several third-party crates which are not
+maintained by the `tokio` project. These include:
+
+- [`tracing-timing`] implements inter-event timing metrics on top of `tracing`.
+  It provides a subscriber that records the time elapsed between pairs of
+  `tracing` events and generates histograms.
+- [`tracing-opentelemetry`] provides a subscriber for emitting traces to
+  [OpenTelemetry]-compatible distributed tracing systems.
+- [`tracing-honeycomb`] Provides a layer that reports traces spanning multiple machines to [honeycomb.io]. Backed by [`tracing-distributed`].
+- [`tracing-distributed`] Provides a generic implementation of a layer that reports traces spanning multiple machines to some backend.
+- [`tracing-actix`] provides `tracing` integration for the `actix` actor
+  framework.
+- [`tracing-gelf`] implements a subscriber for exporting traces in Greylog
+  GELF format.
+- [`tracing-coz`] provides integration with the [coz] causal profiler
+  (Linux-only).
+- [`test-env-log`] takes care of initializing `tracing` for tests, based on
+  environment variables with an `env_logger` compatible syntax.
+- [`tracing-unwrap`] provides convenience methods to report failed unwraps on `Result` or `Option` types to a `Subscriber`.
+- [`diesel-tracing`] provides integration with [`diesel`] database connections.
+- [`tracing-tracy`] provides a way to collect [Tracy] profiles in instrumented
+  applications.
+
+If you're the maintainer of a `tracing` ecosystem crate not listed above,
+please let us know! We'd love to add your project to the list!
+
+[`tracing-timing`]: https://crates.io/crates/tracing-timing
+[`tracing-opentelemetry`]: https://crates.io/crates/tracing-opentelemetry
+[OpenTelemetry]: https://opentelemetry.io/
+[`tracing-honeycomb`]: https://crates.io/crates/tracing-honeycomb
+[`tracing-distributed`]: https://crates.io/crates/tracing-distributed
+[honeycomb.io]: https://www.honeycomb.io/
+[`tracing-actix`]: https://crates.io/crates/tracing-actix
+[`tracing-gelf`]: https://crates.io/crates/tracing-gelf
+[`tracing-coz`]: https://crates.io/crates/tracing-coz
+[coz]: https://github.com/plasma-umass/coz
+[`test-env-log`]: https://crates.io/crates/test-env-log
+[`tracing-unwrap`]: https://docs.rs/tracing-unwrap
+[`diesel`]: https://crates.io/crates/diesel
+[`diesel-tracing`]: https://crates.io/crates/diesel-tracing
+[`tracing-tracy`]: https://crates.io/crates/tracing-tracy
+[Tracy]: https://github.com/wolfpld/tracy
+
+**Note:** that some of the ecosystem crates are currently unreleased and
+undergoing active development. They may be less stable than `tracing` and
+`tracing-core`.
+
+[`log`]: https://docs.rs/log/0.4.6/log/
+[`tokio-rs/tracing`]: https://github.com/tokio-rs/tracing
+[`tracing-futures`]: https://github.com/tokio-rs/tracing/tree/master/tracing-futures
+[`tracing-subscriber`]: https://github.com/tokio-rs/tracing/tree/master/tracing-subscriber
+[`tracing-log`]: https://github.com/tokio-rs/tracing/tree/master/tracing-log
+[`env_logger`]: https://crates.io/crates/env_logger
+[`FmtSubscriber`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/struct.Subscriber.html
+[`examples`]: https://github.com/tokio-rs/tracing/tree/master/examples
+
+## Supported Rust Versions
+
+Tracing is built against the latest stable release. The minimum supported
+version is 1.40. The current Tracing version is not guaranteed to build on Rust
+versions earlier than the minimum supported version.
+
+Tracing follows the same compiler support policies as the rest of the Tokio
+project. The current stable Rust compiler and the three most recent minor
+versions before it will always be supported. For example, if the current stable
+compiler version is 1.45, the minimum supported version will not be increased
+past 1.42, three minor versions prior. Increasing the minimum supported compiler
+version is not considered a semver breaking change as long as doing so complies
+with this policy.
+
+## License
+
+This project is licensed under the [MIT license](LICENSE).
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in Tokio by you, shall be licensed as MIT, without any additional
+terms or conditions.
diff --git a/third_party/rust/tracing/benches/no_subscriber.rs b/third_party/rust/tracing/benches/no_subscriber.rs
new file mode 100644
index 0000000000..797f258c6b
--- /dev/null
+++ b/third_party/rust/tracing/benches/no_subscriber.rs
@@ -0,0 +1,61 @@
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use tracing::Level;
+
+fn bench_no_subscriber(c: &mut Criterion) {
+    use std::sync::atomic::{AtomicUsize, Ordering};
+
+    let mut group = c.benchmark_group("no_subscriber");
+
+    group.bench_function("span", |b| {
+        b.iter(|| {
+            black_box(tracing::span!(Level::TRACE, "span"));
+        })
+    });
+    group.bench_function("event", |b| {
+        b.iter(|| {
+            tracing::event!(Level::TRACE, "hello");
+        })
+    });
+    group.bench_function("relaxed_load", |b| {
+        let foo = AtomicUsize::new(1);
+        b.iter(|| black_box(foo.load(Ordering::Relaxed)));
+    });
+    group.bench_function("acquire_load", |b| {
+        let foo = AtomicUsize::new(1);
+        b.iter(|| black_box(foo.load(Ordering::Acquire)))
+    });
+    group.bench_function("log", |b| {
+        b.iter(|| {
+            log::log!(log::Level::Info, "log");
+        })
+    });
+    group.finish();
+}
+
+fn bench_fields(c: &mut Criterion) {
+    let mut group = c.benchmark_group("no_subscriber_field");
+    group.bench_function("span", |b| {
+        b.iter(|| {
+            black_box(tracing::span!(
+                Level::TRACE,
+                "span",
+                foo = tracing::field::display(format!("bar {:?}", 2))
+            ));
+        })
+    });
+    group.bench_function("event", |b| {
+        b.iter(|| {
+            tracing::event!(
+                Level::TRACE,
+                foo = tracing::field::display(format!("bar {:?}", 2))
+            );
+        })
+    });
+    group.bench_function("log", |b| {
+        b.iter(|| log::log!(log::Level::Trace, "{}", format!("bar {:?}", 2)))
+    });
+    group.finish();
+}
+
+criterion_group!(benches, bench_no_subscriber, bench_fields);
+criterion_main!(benches);
diff --git a/third_party/rust/tracing/benches/subscriber.rs b/third_party/rust/tracing/benches/subscriber.rs
new file mode 100644
index 0000000000..0595d64222
--- /dev/null
+++ b/third_party/rust/tracing/benches/subscriber.rs
@@ -0,0 +1,192 @@
+extern crate criterion;
+extern crate tracing;
+
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use tracing::Level;
+
+use std::{
+    fmt,
+    sync::{Mutex, MutexGuard},
+};
+use tracing::{field, span, Event, Id, Metadata};
+
+/// A subscriber that is enabled but otherwise does nothing.
+struct EnabledSubscriber;
+
+impl tracing::Subscriber for EnabledSubscriber {
+    fn new_span(&self, span: &span::Attributes<'_>) -> Id {
+        let _ = span;
+        Id::from_u64(0xDEAD_FACE)
+    }
+
+    fn event(&self, event: &Event<'_>) {
+        let _ = event;
+    }
+
+    fn record(&self, span: &Id, values: &span::Record<'_>) {
+        let _ = (span, values);
+    }
+
+    fn record_follows_from(&self, span: &Id, follows: &Id) {
+        let _ = (span, follows);
+    }
+
+    fn enabled(&self, metadata: &Metadata<'_>) -> bool {
+        let _ = metadata;
+        true
+    }
+
+    fn enter(&self, span: &Id) {
+        let _ = span;
+    }
+
+    fn exit(&self, span: &Id) {
+        let _ = span;
+    }
+}
+
+/// Simulates a subscriber that records span data.
+struct VisitingSubscriber(Mutex<String>);
+
+struct Visitor<'a>(MutexGuard<'a, String>);
+
+impl<'a> field::Visit for Visitor<'a> {
+    fn record_debug(&mut self, _field: &field::Field, value: &dyn fmt::Debug) {
+        use std::fmt::Write;
+        let _ = write!(&mut *self.0, "{:?}", value);
+    }
+}
+
+impl tracing::Subscriber for VisitingSubscriber {
+    fn new_span(&self, span: &span::Attributes<'_>) -> Id {
+        let mut visitor = Visitor(self.0.lock().unwrap());
+        span.record(&mut visitor);
+        Id::from_u64(0xDEAD_FACE)
+    }
+
+    fn record(&self, _span: &Id, values: &span::Record<'_>) {
+        let mut visitor = Visitor(self.0.lock().unwrap());
+        values.record(&mut visitor);
+    }
+
+    fn event(&self, event: &Event<'_>) {
+        let mut visitor = Visitor(self.0.lock().unwrap());
+        event.record(&mut visitor);
+    }
+
+    fn record_follows_from(&self, span: &Id, follows: &Id) {
+        let _ = (span, follows);
+    }
+
+    fn enabled(&self, metadata: &Metadata<'_>) -> bool {
+        let _ = metadata;
+        true
+    }
+
+    fn enter(&self, span: &Id) {
+        let _ = span;
+    }
+
+    fn exit(&self, span: &Id) {
+        let _ = span;
+    }
+}
+
+const N_SPANS: usize = 100;
+
+fn criterion_benchmark(c: &mut Criterion) {
+    c.bench_function("span_no_fields", |b| {
+        tracing::subscriber::with_default(EnabledSubscriber, || {
+            b.iter(|| span!(Level::TRACE, "span"))
+        });
+    });
+
+    c.bench_function("enter_span", |b| {
+        tracing::subscriber::with_default(EnabledSubscriber, || {
+            let span = span!(Level::TRACE, "span");
+            #[allow(clippy::unit_arg)]
+            b.iter(|| black_box(span.in_scope(|| {})))
+        });
+    });
+
+    c.bench_function("span_repeatedly", |b| {
+        #[inline]
+        fn mk_span(i: u64) -> tracing::Span {
+            span!(Level::TRACE, "span", i = i)
+        }
+
+        let n = black_box(N_SPANS);
+        tracing::subscriber::with_default(EnabledSubscriber, || {
+            b.iter(|| (0..n).fold(mk_span(0), |_, i| mk_span(i as u64)))
+        });
+    });
+
+    c.bench_function("span_with_fields", |b| {
+        tracing::subscriber::with_default(EnabledSubscriber, || {
+            b.iter(|| {
+                span!(
+                    Level::TRACE,
+                    "span",
+                    foo = "foo",
+                    bar = "bar",
+                    baz = 3,
+                    quuux = tracing::field::debug(0.99)
+                )
+            })
+        });
+    });
+
+    c.bench_function("span_with_fields_record", |b| {
+        let subscriber = VisitingSubscriber(Mutex::new(String::from("")));
+        tracing::subscriber::with_default(subscriber, || {
+            b.iter(|| {
+                span!(
+                    Level::TRACE,
+                    "span",
+                    foo = "foo",
+                    bar = "bar",
+                    baz = 3,
+                    quuux = tracing::field::debug(0.99)
+                )
+            })
+        });
+    });
+}
+
+fn bench_dispatch(c: &mut Criterion) {
+    let mut group = c.benchmark_group("dispatch");
+    group.bench_function("no_dispatch_get_ref", |b| {
+        b.iter(|| {
+            tracing::dispatcher::get_default(|current| {
+                black_box(&current);
+            })
+        })
+    });
+    group.bench_function("no_dispatch_get_clone", |b| {
+        b.iter(|| {
+            let current = tracing::dispatcher::get_default(|current| current.clone());
+            black_box(current);
+        })
+    });
+    group.bench_function("get_ref", |b| {
+        tracing::subscriber::with_default(EnabledSubscriber, || {
+            b.iter(|| {
+                tracing::dispatcher::get_default(|current| {
+                    black_box(&current);
+                })
+            })
+        })
+    });
+    group.bench_function("get_clone", |b| {
+        tracing::subscriber::with_default(EnabledSubscriber, || {
+            b.iter(|| {
+                let current = tracing::dispatcher::get_default(|current| current.clone());
+                black_box(current);
+            })
+        })
+    });
+    group.finish();
+}
+
+criterion_group!(benches, criterion_benchmark, bench_dispatch);
+criterion_main!(benches);
diff --git a/third_party/rust/tracing/src/dispatcher.rs b/third_party/rust/tracing/src/dispatcher.rs
new file mode 100644
index 0000000000..4041c994fb
--- /dev/null
+++ b/third_party/rust/tracing/src/dispatcher.rs
@@ -0,0 +1,152 @@
+//! Dispatches trace events to [`Subscriber`]s.
+//!
+//! The _dispatcher_ is the component of the tracing system which is responsible
+//! for forwarding trace data from the instrumentation points that generate it
+//! to the subscriber that collects it.
+//!
+//! # Using the Trace Dispatcher
+//!
+//! Every thread in a program using `tracing` has a _default subscriber_. When
+//! events occur, or spans are created, they are dispatched to the thread's
+//! current subscriber.
+//!
+//! ## Setting the Default Subscriber
+//!
+//! By default, the current subscriber is an empty implementation that does
+//! nothing. To use a subscriber implementation, it must be set as the default.
+//! There are two methods for doing so: [`with_default`] and
+//! [`set_global_default`]. `with_default` sets the default subscriber for the
+//! duration of a scope, while `set_global_default` sets a default subscriber
+//! for the entire process.
+//!
+//! To use either of these functions, we must first wrap our subscriber in a
+//! [`Dispatch`], a cloneable, type-erased reference to a subscriber. For
+//! example:
+//! ```rust
+//! # pub struct FooSubscriber;
+//! # use tracing_core::{
+//! #   dispatcher, Event, Metadata,
+//! #   span::{Attributes, Id, Record}
+//! # };
+//! # impl tracing_core::Subscriber for FooSubscriber {
+//! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
+//! #   fn record(&self, _: &Id, _: &Record) {}
+//! #   fn event(&self, _: &Event) {}
+//! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
+//! #   fn enabled(&self, _: &Metadata) -> bool { false }
+//! #   fn enter(&self, _: &Id) {}
+//! #   fn exit(&self, _: &Id) {}
+//! # }
+//! # impl FooSubscriber { fn new() -> Self { FooSubscriber } }
+//! use dispatcher::Dispatch;
+//!
+//! let my_subscriber = FooSubscriber::new();
+//! let my_dispatch = Dispatch::new(my_subscriber);
+//! ```
+//! Then, we can use [`with_default`] to set our `Dispatch` as the default for
+//! the duration of a block:
+//! ```rust
+//! # pub struct FooSubscriber;
+//! # use tracing_core::{
+//! #   dispatcher, Event, Metadata,
+//! #   span::{Attributes, Id, Record}
+//! # };
+//! # impl tracing_core::Subscriber for FooSubscriber {
+//! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
+//! #   fn record(&self, _: &Id, _: &Record) {}
+//! #   fn event(&self, _: &Event) {}
+//! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
+//! #   fn enabled(&self, _: &Metadata) -> bool { false }
+//! #   fn enter(&self, _: &Id) {}
+//! #   fn exit(&self, _: &Id) {}
+//! # }
+//! # impl FooSubscriber { fn new() -> Self { FooSubscriber } }
+//! # let my_subscriber = FooSubscriber::new();
+//! # let my_dispatch = dispatcher::Dispatch::new(my_subscriber);
+//! // no default subscriber
+//!
+//! # #[cfg(feature = "std")]
+//! dispatcher::with_default(&my_dispatch, || {
+//!     // my_subscriber is the default
+//! });
+//!
+//! // no default subscriber again
+//! ```
+//! It's important to note that `with_default` will not propagate the current
+//! thread's default subscriber to any threads spawned within the `with_default`
+//! block. To propagate the default subscriber to new threads, either use
+//! `with_default` from the new thread, or use `set_global_default`.
+//!
+//! As an alternative to `with_default`, we can use [`set_global_default`] to
+//! set a `Dispatch` as the default for all threads, for the lifetime of the
+//! program. For example:
+//! ```rust
+//! # pub struct FooSubscriber;
+//! # use tracing_core::{
+//! #   dispatcher, Event, Metadata,
+//! #   span::{Attributes, Id, Record}
+//! # };
+//! # impl tracing_core::Subscriber for FooSubscriber {
+//! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
+//! #   fn record(&self, _: &Id, _: &Record) {}
+//! #   fn event(&self, _: &Event) {}
+//! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
+//! #   fn enabled(&self, _: &Metadata) -> bool { false }
+//! #   fn enter(&self, _: &Id) {}
+//! #   fn exit(&self, _: &Id) {}
+//! # }
+//! # impl FooSubscriber { fn new() -> Self { FooSubscriber } }
+//! # let my_subscriber = FooSubscriber::new();
+//! # let my_dispatch = dispatcher::Dispatch::new(my_subscriber);
+//! // no default subscriber
+//!
+//! dispatcher::set_global_default(my_dispatch)
+//!     // `set_global_default` will return an error if the global default
+//!     // subscriber has already been set.
+//!     .expect("global default was already set!");
+//!
+//! // `my_subscriber` is now the default
+//! ```
+//! <div class="information">
+//!     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
+//! </div>
+//! <div class="example-wrap" style="display:inline-block">
+//! <pre class="ignore" style="white-space:normal;font:inherit;">
+//! <strong>Note</strong>: The thread-local scoped dispatcher (<code>with_default</code>)
+//! requires the Rust standard library. <code>no_std</code> users should
+//! use <a href="fn.set_global_default.html"><code>set_global_default</code></a>
+//! instead.
+//! </pre></div>
+//!
+//! ## Accessing the Default Subscriber
+//!
+//! A thread's current default subscriber can be accessed using the
+//! [`get_default`] function, which executes a closure with a reference to the
+//! currently default `Dispatch`. This is used primarily by `tracing`
+//! instrumentation.
+//!
+//! [`Subscriber`]: struct.Subscriber.html
+//! [`with_default`]: fn.with_default.html
+//! [`set_global_default`]: fn.set_global_default.html
+//! [`get_default`]: fn.get_default.html
+//! [`Dispatch`]: struct.Dispatch.html
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+pub use tracing_core::dispatcher::set_default;
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+pub use tracing_core::dispatcher::with_default;
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+pub use tracing_core::dispatcher::DefaultGuard;
+pub use tracing_core::dispatcher::{
+    get_default, set_global_default, Dispatch, SetGlobalDefaultError,
+};
+
+/// Private API for internal use by tracing's macros.
+///
+/// This function is *not* considered part of `tracing`'s public API, and has no
+/// stability guarantees. If you use it, and it breaks or disappears entirely,
+/// don't say we didn;'t warn you.
+#[doc(hidden)]
+pub use tracing_core::dispatcher::has_been_set;
diff --git a/third_party/rust/tracing/src/field.rs b/third_party/rust/tracing/src/field.rs
new file mode 100644
index 0000000000..2514336585
--- /dev/null
+++ b/third_party/rust/tracing/src/field.rs
@@ -0,0 +1,62 @@
+//! Structured data associated with `Span`s and `Event`s.
+pub use tracing_core::field::*;
+
+use crate::Metadata;
+
+/// Trait implemented to allow a type to be used as a field key.
+///
+/// <div class="information">
+///     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
+/// </div>
+/// <div class="example-wrap" style="display:inline-block">
+/// <pre class="ignore" style="white-space:normal;font:inherit;">
+/// <strong>Note</strong>: Although this is implemented for both the
+/// <a href="./struct.Field.html"><code>Field</code></a> type <em>and</em> any
+/// type that can be borrowed as an <code>&str</code>, only <code>Field</code>
+/// allows <em>O</em>(1) access.
+/// Indexing a field with a string results in an iterative search that performs
+/// string comparisons. Thus, if possible, once the key for a field is known, it
+/// should be used whenever possible.
+/// </pre>
+pub trait AsField: crate::sealed::Sealed {
+    /// Attempts to convert `&self` into a `Field` with the specified `metadata`.
+    ///
+    /// If `metadata` defines this field, then the field is returned. Otherwise,
+    /// this returns `None`.
+    fn as_field(&self, metadata: &Metadata<'_>) -> Option<Field>;
+}
+
+// ===== impl AsField =====
+
+impl AsField for Field {
+    #[inline]
+    fn as_field(&self, metadata: &Metadata<'_>) -> Option<Field> {
+        if self.callsite() == metadata.callsite() {
+            Some(self.clone())
+        } else {
+            None
+        }
+    }
+}
+
+impl<'a> AsField for &'a Field {
+    #[inline]
+    fn as_field(&self, metadata: &Metadata<'_>) -> Option<Field> {
+        if self.callsite() == metadata.callsite() {
+            Some((*self).clone())
+        } else {
+            None
+        }
+    }
+}
+
+impl AsField for str {
+    #[inline]
+    fn as_field(&self, metadata: &Metadata<'_>) -> Option<Field> {
+        metadata.fields().field(&self)
+    }
+}
+
+impl crate::sealed::Sealed for Field {}
+impl<'a> crate::sealed::Sealed for &'a Field {}
+impl crate::sealed::Sealed for str {}
diff --git a/third_party/rust/tracing/src/instrument.rs b/third_party/rust/tracing/src/instrument.rs
new file mode 100644
index 0000000000..d1070ba8e2
--- /dev/null
+++ b/third_party/rust/tracing/src/instrument.rs
@@ -0,0 +1,194 @@
+use crate::stdlib::pin::Pin;
+use crate::stdlib::task::{Context, Poll};
+use crate::stdlib::{future::Future, marker::Sized};
+use crate::{dispatcher, span::Span, Dispatch};
+use pin_project_lite::pin_project;
+
+/// Attaches spans to a `std::future::Future`.
+///
+/// Extension trait allowing futures to be
+/// instrumented with a `tracing` [span].
+///
+/// [span]:  ../struct.Span.html
+pub trait Instrument: Sized {
+    /// Instruments this type with the provided `Span`, returning an
+    /// `Instrumented` wrapper.
+    ///
+    /// The attached `Span` will be [entered] every time the instrumented `Future` is polled.
+    ///
+    /// # Examples
+    ///
+    /// Instrumenting a future:
+    ///
+    /// ```rust
+    /// use tracing::Instrument;
+    ///
+    /// # async fn doc() {
+    /// let my_future = async {
+    ///     // ...
+    /// };
+    ///
+    /// my_future
+    ///     .instrument(tracing::info_span!("my_future"))
+    ///     .await
+    /// # }
+    /// ```
+    ///
+    /// [entered]: ../struct.Span.html#method.enter
+    fn instrument(self, span: Span) -> Instrumented<Self> {
+        Instrumented { inner: self, span }
+    }
+
+    /// Instruments this type with the [current] `Span`, returning an
+    /// `Instrumented` wrapper.
+    ///
+    /// If the instrumented type is a future, stream, or sink, the attached `Span`
+    /// will be [entered] every time it is polled. If the instrumented type
+    /// is a future executor, every future spawned on that executor will be
+    /// instrumented by the attached `Span`.
+    ///
+    /// This can be used to propagate the current span when spawning a new future.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use tracing::Instrument;
+    ///
+    /// # async fn doc() {
+    /// let span = tracing::info_span!("my_span");
+    /// let _enter = span.enter();
+    ///
+    /// // ...
+    ///
+    /// let future = async {
+    ///     tracing::debug!("this event will occur inside `my_span`");
+    ///     // ...
+    /// };
+    /// tokio::spawn(future.in_current_span());
+    /// # }
+    /// ```
+    ///
+    /// [current]: ../struct.Span.html#method.current
+    /// [entered]: ../struct.Span.html#method.enter
+    #[inline]
+    fn in_current_span(self) -> Instrumented<Self> {
+        self.instrument(Span::current())
+    }
+}
+
+/// Extension trait allowing futures to be instrumented with
+/// a `tracing` [`Subscriber`].
+///
+/// [`Subscriber`]: ../trait.Subscriber.html
+pub trait WithSubscriber: Sized {
+    /// Attaches the provided [`Subscriber`] to this type, returning a
+    /// `WithDispatch` wrapper.
+    ///
+    /// The attached subscriber will be set as the [default] when the returned `Future` is polled.
+    ///
+    /// [`Subscriber`]: ../trait.Subscriber.html
+    /// [default]: https://docs.rs/tracing/latest/tracing/dispatcher/index.html#setting-the-default-subscriber
+    fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
+    where
+        S: Into<Dispatch>,
+    {
+        WithDispatch {
+            inner: self,
+            dispatch: subscriber.into(),
+        }
+    }
+
+    /// Attaches the current [default] [`Subscriber`] to this type, returning a
+    /// `WithDispatch` wrapper.
+    ///
+    /// When the wrapped type is a future, stream, or sink, the attached
+    /// subscriber will be set as the [default] while it is being polled.
+    /// When the wrapped type is an executor, the subscriber will be set as the
+    /// default for any futures spawned on that executor.
+    ///
+    /// This can be used to propagate the current dispatcher context when
+    /// spawning a new future.
+    ///
+    /// [`Subscriber`]: ../trait.Subscriber.html
+    /// [default]: https://docs.rs/tracing/latest/tracing/dispatcher/index.html#setting-the-default-subscriber
+    #[inline]
+    fn with_current_subscriber(self) -> WithDispatch<Self> {
+        WithDispatch {
+            inner: self,
+            dispatch: dispatcher::get_default(|default| default.clone()),
+        }
+    }
+}
+
+pin_project! {
+    /// A future that has been instrumented with a `tracing` subscriber.
+    #[derive(Clone, Debug)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct WithDispatch<T> {
+        #[pin]
+        inner: T,
+        dispatch: Dispatch,
+    }
+}
+
+pin_project! {
+    /// A future that has been instrumented with a `tracing` span.
+    #[derive(Debug, Clone)]
+    #[must_use = "futures do nothing unless you `.await` or poll them"]
+    pub struct Instrumented<T> {
+        #[pin]
+        inner: T,
+        span: Span,
+    }
+}
+
+impl<T: Future> Future for Instrumented<T> {
+    type Output = T::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let this = self.project();
+        let _enter = this.span.enter();
+        this.inner.poll(cx)
+    }
+}
+
+impl<T: Sized> Instrument for T {}
+
+impl<T> Instrumented<T> {
+    /// Borrows the `Span` that this type is instrumented by.
+    pub fn span(&self) -> &Span {
+        &self.span
+    }
+
+    /// Mutably borrows the `Span` that this type is instrumented by.
+    pub fn span_mut(&mut self) -> &mut Span {
+        &mut self.span
+    }
+
+    /// Borrows the wrapped type.
+    pub fn inner(&self) -> &T {
+        &self.inner
+    }
+
+    /// Mutably borrows the wrapped type.
+    pub fn inner_mut(&mut self) -> &mut T {
+        &mut self.inner
+    }
+
+    /// Get a pinned reference to the wrapped type.
+    pub fn inner_pin_ref(self: Pin<&Self>) -> Pin<&T> {
+        self.project_ref().inner
+    }
+
+    /// Get a pinned mutable reference to the wrapped type.
+    pub fn inner_pin_mut(self: Pin<&mut Self>) -> Pin<&mut T> {
+        self.project().inner
+    }
+
+    /// Consumes the `Instrumented`, returning the wrapped type.
+    ///
+    /// Note that this drops the span.
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+}
diff --git a/third_party/rust/tracing/src/level_filters.rs b/third_party/rust/tracing/src/level_filters.rs
new file mode 100644
index 0000000000..df9405b5e4
--- /dev/null
+++ b/third_party/rust/tracing/src/level_filters.rs
@@ -0,0 +1,81 @@
+//! Trace verbosity level filtering.
+//!
+//! # Compile time filters
+//!
+//! Trace verbosity levels can be statically disabled at compile time via Cargo
+//! features, similar to the [`log` crate]. Trace instrumentation at disabled
+//! levels will be skipped and will not even be present in the resulting binary
+//! unless the verbosity level is specified dynamically. This level is
+//! configured separately for release and debug builds. The features are:
+//!
+//! * `max_level_off`
+//! * `max_level_error`
+//! * `max_level_warn`
+//! * `max_level_info`
+//! * `max_level_debug`
+//! * `max_level_trace`
+//! * `release_max_level_off`
+//! * `release_max_level_error`
+//! * `release_max_level_warn`
+//! * `release_max_level_info`
+//! * `release_max_level_debug`
+//! * `release_max_level_trace`
+//!
+//! These features control the value of the `STATIC_MAX_LEVEL` constant. The
+//! instrumentation macros macros check this value before recording an event or
+//! constructing a span. By default, no levels are disabled.
+//!
+//! For example, a crate can disable trace level instrumentation in debug builds
+//! and trace, debug, and info level instrumentation in release builds with the
+//! following configuration:
+//!
+//! ```toml
+//! [dependencies]
+//! tracing = { version = "0.1", features = ["max_level_debug", "release_max_level_warn"] }
+//! ```
+//!
+//! *Compiler support: requires rustc 1.39+*
+//!
+//! [`log` crate]: https://docs.rs/log/0.4.6/log/#compile-time-filters
+pub use tracing_core::{metadata::ParseLevelFilterError, LevelFilter};
+
+/// The statically configured maximum trace level.
+///
+/// See the [module-level documentation] for information on how to configure
+/// this.
+///
+/// This value is checked by the `event!` and `span!` macros. Code that
+/// manually constructs events or spans via the `Event::record` function or
+/// `Span` constructors should compare the level against this value to
+/// determine if those spans or events are enabled.
+///
+/// [module-level documentation]: ../index.html#compile-time-filters
+pub const STATIC_MAX_LEVEL: LevelFilter = MAX_LEVEL;
+
+cfg_if! {
+    if #[cfg(all(not(debug_assertions), feature = "release_max_level_off"))] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::OFF;
+    } else if #[cfg(all(not(debug_assertions), feature = "release_max_level_error"))] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::ERROR;
+    } else if #[cfg(all(not(debug_assertions), feature = "release_max_level_warn"))] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::WARN;
+    } else if #[cfg(all(not(debug_assertions), feature = "release_max_level_info"))] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::INFO;
+    } else if #[cfg(all(not(debug_assertions), feature = "release_max_level_debug"))] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::DEBUG;
+    } else if #[cfg(all(not(debug_assertions), feature = "release_max_level_trace"))] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::TRACE;
+    } else if #[cfg(feature = "max_level_off")] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::OFF;
+    } else if #[cfg(feature = "max_level_error")] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::ERROR;
+    } else if #[cfg(feature = "max_level_warn")] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::WARN;
+    } else if #[cfg(feature = "max_level_info")] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::INFO;
+    } else if #[cfg(feature = "max_level_debug")] {
+        const MAX_LEVEL: LevelFilter = LevelFilter::DEBUG;
+    } else {
+        const MAX_LEVEL: LevelFilter = LevelFilter::TRACE;
+    }
+}
diff --git a/third_party/rust/tracing/src/lib.rs b/third_party/rust/tracing/src/lib.rs
new file mode 100644
index 0000000000..5fc5382642
--- /dev/null
+++ b/third_party/rust/tracing/src/lib.rs
@@ -0,0 +1,1035 @@
+//! A scoped, structured logging and diagnostics system.
+//!
+//! # Overview
+//!
+//! `tracing` is a framework for instrumenting Rust programs to collect
+//! structured, event-based diagnostic information.
+//!
+//! In asynchronous systems like Tokio, interpreting traditional log messages can
+//! often be quite challenging. Since individual tasks are multiplexed on the same
+//! thread, associated events and log lines are intermixed making it difficult to
+//! trace the logic flow. `tracing` expands upon logging-style diagnostics by
+//! allowing libraries and applications to record structured events with additional
+//! information about *temporality* and *causality* — unlike a log message, a span
+//! in `tracing` has a beginning and end time, may be entered and exited by the
+//! flow of execution, and may exist within a nested tree of similar spans. In
+//! addition, `tracing` spans are *structured*, with the ability to record typed
+//! data as well as textual messages.
+//!
+//! The `tracing` crate provides the APIs necessary for instrumenting libraries
+//! and applications to emit trace data.
+//!
+//! *Compiler support: [requires `rustc` 1.40+][msrv]*
+//!
+//! [msrv]: #supported-rust-versions
+//! # Core Concepts
+//!
+//! The core of `tracing`'s API is composed of _spans_, _events_ and
+//! _subscribers_. We'll cover these in turn.
+//!
+//! ## Spans
+//!
+//! To record the flow of execution through a program, `tracing` introduces the
+//! concept of [spans]. Unlike a log line that represents a _moment in
+//! time_, a span represents a _period of time_ with a beginning and an end. When a
+//! program begins executing in a context or performing a unit of work, it
+//! _enters_ that context's span, and when it stops executing in that context,
+//! it _exits_ the span. The span in which a thread is currently executing is
+//! referred to as that thread's _current_ span.
+//!
+//! For example:
+//! ```
+//! use tracing::{span, Level};
+//! # fn main() {
+//! let span = span!(Level::TRACE, "my_span");
+//! // `enter` returns a RAII guard which, when dropped, exits the span. this
+//! // indicates that we are in the span for the current lexical scope.
+//! let _enter = span.enter();
+//! // perform some work in the context of `my_span`...
+//! # }
+//!```
+//!
+//! The [`span` module][span]'s documentation provides further details on how to
+//! use spans.
+//!
+//! <div class="information">
+//!     <div class="tooltip compile_fail" style="">&#x26a0; &#xfe0f;<span class="tooltiptext">Warning</span></div>
+//! </div><div class="example-wrap" style="display:inline-block"><pre class="compile_fail" style="white-space:normal;font:inherit;">
+//!     <strong>Warning</strong>: In asynchronous code that uses async/await syntax,
+//!     <code>Span::enter</code> may produce incorrect traces if the returned drop
+//!     guard is held across an await point. See
+//!     <a href="span/struct.Span.html#in-asynchronous-code">the method documentation</a>
+//!     for details.
+//! </pre></div>
+//!
+//! ## Events
+//!
+//! An [`Event`] represents a _moment_ in time. It signifies something that
+//! happened while a trace was being recorded. `Event`s are comparable to the log
+//! records emitted by unstructured logging code, but unlike a typical log line,
+//! an `Event` may occur within the context of a span.
+//!
+//! For example:
+//! ```
+//! use tracing::{event, span, Level};
+//!
+//! # fn main() {
+//! // records an event outside of any span context:
+//! event!(Level::INFO, "something happened");
+//!
+//! let span = span!(Level::INFO, "my_span");
+//! let _guard = span.enter();
+//!
+//! // records an event within "my_span".
+//! event!(Level::DEBUG, "something happened inside my_span");
+//! # }
+//!```
+//!
+//! In general, events should be used to represent points in time _within_ a
+//! span — a request returned with a given status code, _n_ new items were
+//! taken from a queue, and so on.
+//!
+//! The [`Event` struct][`Event`] documentation provides further details on using
+//! events.
+//!
+//! ## Subscribers
+//!
+//! As `Span`s and `Event`s occur, they are recorded or aggregated by
+//! implementations of the [`Subscriber`] trait. `Subscriber`s are notified
+//! when an `Event` takes place and when a `Span` is entered or exited. These
+//! notifications are represented by the following `Subscriber` trait methods:
+//!
+//! + [`event`][Subscriber::event], called when an `Event` takes place,
+//! + [`enter`], called when execution enters a `Span`,
+//! + [`exit`], called when execution exits a `Span`
+//!
+//! In addition, subscribers may implement the [`enabled`] function to _filter_
+//! the notifications they receive based on [metadata] describing each `Span`
+//! or `Event`. If a call to `Subscriber::enabled` returns `false` for a given
+//! set of metadata, that `Subscriber` will *not* be notified about the
+//! corresponding `Span` or `Event`. For performance reasons, if no currently
+//! active subscribers express interest in a given set of metadata by returning
+//! `true`, then the corresponding `Span` or `Event` will never be constructed.
+//!
+//! # Usage
+//!
+//! First, add this to your `Cargo.toml`:
+//!
+//! ```toml
+//! [dependencies]
+//! tracing = "0.1"
+//! ```
+//!
+//! *Compiler support: requires rustc 1.39+*
+//!
+//! ## Recording Spans and Events
+//!
+//! Spans and events are recorded using macros.
+//!
+//! ### Spans
+//!
+//! The [`span!`] macro expands to a [`Span` struct][`Span`] which is used to
+//! record a span. The [`Span::enter`] method on that struct records that the
+//! span has been entered, and returns a [RAII] guard object, which will exit
+//! the span when dropped.
+//!
+//! For example:
+//!
+//! ```rust
+//! use tracing::{span, Level};
+//! # fn main() {
+//! // Construct a new span named "my span" with trace log level.
+//! let span = span!(Level::TRACE, "my span");
+//!
+//! // Enter the span, returning a guard object.
+//! let _enter = span.enter();
+//!
+//! // Any trace events that occur before the guard is dropped will occur
+//! // within the span.
+//!
+//! // Dropping the guard will exit the span.
+//! # }
+//! ```
+//!
+//! The [`#[instrument]`][instrument] attribute provides an easy way to
+//! add `tracing` spans to functions. A function annotated with `#[instrument]`
+//! will create and enter a span with that function's name every time the
+//! function is called, with arguments to that function will be recorded as
+//! fields using `fmt::Debug`.
+//!
+//! For example:
+//! ```ignore
+//! # // this doctest is ignored because we don't have a way to say
+//! # // that it should only be run with cfg(feature = "attributes")
+//! use tracing::{Level, event, instrument};
+//!
+//! #[instrument]
+//! pub fn my_function(my_arg: usize) {
+//!     // This event will be recorded inside a span named `my_function` with the
+//!     // field `my_arg`.
+//!     event!(Level::INFO, "inside my_function!");
+//!     // ...
+//! }
+//! # fn main() {}
+//! ```
+//!
+//!
+//! You can find more examples showing how to use this crate [here][examples].
+//!
+//! [RAII]: https://github.com/rust-unofficial/patterns/blob/master/patterns/RAII.md
+//! [examples]: https://github.com/tokio-rs/tracing/tree/master/examples
+//!
+//! ### Events
+//!
+//! [`Event`]s are recorded using the [`event!`] macro:
+//!
+//! ```rust
+//! # fn main() {
+//! use tracing::{event, Level};
+//! event!(Level::INFO, "something has happened!");
+//! # }
+//! ```
+//!
+//! ## Using the Macros
+//!
+//! The [`span!`] and [`event!`] macros use fairly similar syntax, with some
+//! exceptions.
+//!
+//! ### Configuring Attributes
+//!
+//! Both macros require a [`Level`] specifying the verbosity of the span or
+//! event. Optionally, the [target] and [parent span] may be overridden. If the
+//! target and parent span are not overridden, they will default to the
+//! module path where the macro was invoked and the current span (as determined
+//! by the subscriber), respectively.
+//!
+//! For example:
+//!
+//! ```
+//! # use tracing::{span, event, Level};
+//! # fn main() {
+//! span!(target: "app_spans", Level::TRACE, "my span");
+//! event!(target: "app_events", Level::INFO, "something has happened!");
+//! # }
+//! ```
+//! ```
+//! # use tracing::{span, event, Level};
+//! # fn main() {
+//! let span = span!(Level::TRACE, "my span");
+//! event!(parent: &span, Level::INFO, "something has happened!");
+//! # }
+//! ```
+//!
+//! The span macros also take a string literal after the level, to set the name
+//! of the span.
+//!
+//! ### Recording Fields
+//!
+//! Structured fields on spans and events are specified using the syntax
+//! `field_name = field_value`. Fields are separated by commas.
+//!
+//! ```
+//! # use tracing::{event, Level};
+//! # fn main() {
+//! // records an event with two fields:
+//! //  - "answer", with the value 42
+//! //  - "question", with the value "life, the universe and everything"
+//! event!(Level::INFO, answer = 42, question = "life, the universe, and everything");
+//! # }
+//! ```
+//!
+//! As shorthand, local variables may be used as field values without an
+//! assignment, similar to [struct initializers]. For example:
+//!
+//! ```
+//! # use tracing::{span, Level};
+//! # fn main() {
+//! let user = "ferris";
+//!
+//! span!(Level::TRACE, "login", user);
+//! // is equivalent to:
+//! span!(Level::TRACE, "login", user = user);
+//! # }
+//!```
+//!
+//! Field names can include dots, but should not be terminated by them:
+//! ```
+//! # use tracing::{span, Level};
+//! # fn main() {
+//! let user = "ferris";
+//! let email = "ferris@rust-lang.org";
+//! span!(Level::TRACE, "login", user, user.email = email);
+//! # }
+//!```
+//!
+//! Since field names can include dots, fields on local structs can be used
+//! using the local variable shorthand:
+//! ```
+//! # use tracing::{span, Level};
+//! # fn main() {
+//! # struct User {
+//! #    name: &'static str,
+//! #    email: &'static str,
+//! # }
+//! let user = User {
+//!     name: "ferris",
+//!     email: "ferris@rust-lang.org",
+//! };
+//! // the span will have the fields `user.name = "ferris"` and
+//! // `user.email = "ferris@rust-lang.org"`.
+//! span!(Level::TRACE, "login", user.name, user.email);
+//! # }
+//!```
+//!
+//! Fields with names that are not Rust identifiers, or with names that are Rust reserved words,
+//! may be created using quoted string literals. However, this may not be used with the local
+//! variable shorthand.
+//! ```
+//! # use tracing::{span, Level};
+//! # fn main() {
+//! // records an event with fields whose names are not Rust identifiers
+//! //  - "guid:x-request-id", containing a `:`, with the value "abcdef"
+//! //  - "type", which is a reserved word, with the value "request"
+//! span!(Level::TRACE, "api", "guid:x-request-id" = "abcdef", "type" = "request");
+//! # }
+//!```
+//!
+//! The `?` sigil is shorthand that specifies a field should be recorded using
+//! its [`fmt::Debug`] implementation:
+//! ```
+//! # use tracing::{event, Level};
+//! # fn main() {
+//! #[derive(Debug)]
+//! struct MyStruct {
+//!     field: &'static str,
+//! }
+//!
+//! let my_struct = MyStruct {
+//!     field: "Hello world!"
+//! };
+//!
+//! // `my_struct` will be recorded using its `fmt::Debug` implementation.
+//! event!(Level::TRACE, greeting = ?my_struct);
+//! // is equivalent to:
+//! event!(Level::TRACE, greeting = tracing::field::debug(&my_struct));
+//! # }
+//! ```
+//!
+//! The `%` sigil operates similarly, but indicates that the value should be
+//! recorded using its [`fmt::Display`] implementation:
+//! ```
+//! # use tracing::{event, Level};
+//! # fn main() {
+//! # #[derive(Debug)]
+//! # struct MyStruct {
+//! #     field: &'static str,
+//! # }
+//! #
+//! # let my_struct = MyStruct {
+//! #     field: "Hello world!"
+//! # };
+//! // `my_struct.field` will be recorded using its `fmt::Display` implementation.
+//! event!(Level::TRACE, greeting = %my_struct.field);
+//! // is equivalent to:
+//! event!(Level::TRACE, greeting = tracing::field::display(&my_struct.field));
+//! # }
+//! ```
+//!
+//! The `%` and `?` sigils may also be used with local variable shorthand:
+//!
+//! ```
+//! # use tracing::{event, Level};
+//! # fn main() {
+//! # #[derive(Debug)]
+//! # struct MyStruct {
+//! #     field: &'static str,
+//! # }
+//! #
+//! # let my_struct = MyStruct {
+//! #     field: "Hello world!"
+//! # };
+//! // `my_struct.field` will be recorded using its `fmt::Display` implementation.
+//! event!(Level::TRACE, %my_struct.field);
+//! # }
+//! ```
+//!
+//! Additionally, a span may declare fields with the special value [`Empty`],
+//! which indicates that that the value for that field does not currently exist
+//! but may be recorded later. For example:
+//!
+//! ```
+//! use tracing::{trace_span, field};
+//!
+//! // Create a span with two fields: `greeting`, with the value "hello world", and
+//! // `parting`, without a value.
+//! let span = trace_span!("my_span", greeting = "hello world", parting = field::Empty);
+//!
+//! // ...
+//!
+//! // Now, record a value for parting as well.
+//! span.record("parting", &"goodbye world!");
+//! ```
+//!
+//! Note that a span may have up to 32 fields. The following will not compile:
+//!
+//! ```rust,compile_fail
+//! # use tracing::Level;
+//! # fn main() {
+//! let bad_span = span!(
+//!     Level::TRACE,
+//!     "too many fields!",
+//!     a = 1, b = 2, c = 3, d = 4, e = 5, f = 6, g = 7, h = 8, i = 9,
+//!     j = 10, k = 11, l = 12, m = 13, n = 14, o = 15, p = 16, q = 17,
+//!     r = 18, s = 19, t = 20, u = 21, v = 22, w = 23, x = 24, y = 25,
+//!     z = 26, aa = 27, bb = 28, cc = 29, dd = 30, ee = 31, ff = 32, gg = 33
+//! );
+//! # }
+//! ```
+//!
+//! Finally, events may also include human-readable messages, in the form of a
+//! [format string][fmt] and (optional) arguments, **after** the event's
+//! key-value fields. If a format string and arguments are provided,
+//! they will implicitly create a new field named `message` whose value is the
+//! provided set of format arguments.
+//!
+//! For example:
+//!
+//! ```
+//! # use tracing::{event, Level};
+//! # fn main() {
+//! let question = "the answer to the ultimate question of life, the universe, and everything";
+//! let answer = 42;
+//! // records an event with the following fields:
+//! // - `question.answer` with the value 42,
+//! // - `question.tricky` with the value `true`,
+//! // - "message", with the value "the answer to the ultimate question of life, the
+//! //    universe, and everything is 42."
+//! event!(
+//!     Level::DEBUG,
+//!     question.answer = answer,
+//!     question.tricky = true,
+//!     "the answer to {} is {}.", question, answer
+//! );
+//! # }
+//! ```
+//!
+//! Specifying a formatted message in this manner does not allocate by default.
+//!
+//! [struct initializers]: https://doc.rust-lang.org/book/ch05-01-defining-structs.html#using-the-field-init-shorthand-when-variables-and-fields-have-the-same-name
+//! [target]: struct.Metadata.html#method.target
+//! [parent span]: span/struct.Attributes.html#method.parent
+//! [determined contextually]: span/struct.Attributes.html#method.is_contextual
+//! [`fmt::Debug`]: https://doc.rust-lang.org/std/fmt/trait.Debug.html
+//! [`fmt::Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html
+//! [fmt]: https://doc.rust-lang.org/std/fmt/#usage
+//! [`Empty`]: field/struct.Empty.html
+//!
+//! ### Shorthand Macros
+//!
+//! `tracing` also offers a number of macros with preset verbosity levels.
+//! The [`trace!`], [`debug!`], [`info!`], [`warn!`], and [`error!`] behave
+//! similarly to the [`event!`] macro, but with the [`Level`] argument already
+//! specified, while the corresponding [`trace_span!`], [`debug_span!`],
+//! [`info_span!`], [`warn_span!`], and [`error_span!`] macros are the same,
+//! but for the [`span!`] macro.
+//!
+//! These are intended both as a shorthand, and for compatibility with the [`log`]
+//! crate (see the next section).
+//!
+//! [`span!`]: macro.span.html
+//! [`event!`]: macro.event.html
+//! [`trace!`]: macro.trace.html
+//! [`debug!`]: macro.debug.html
+//! [`info!`]: macro.info.html
+//! [`warn!`]: macro.warn.html
+//! [`error!`]: macro.error.html
+//! [`trace_span!`]: macro.trace_span.html
+//! [`debug_span!`]: macro.debug_span.html
+//! [`info_span!`]: macro.info_span.html
+//! [`warn_span!`]: macro.warn_span.html
+//! [`error_span!`]: macro.error_span.html
+//! [`Level`]: struct.Level.html
+//!
+//! ### For `log` Users
+//!
+//! Users of the [`log`] crate should note that `tracing` exposes a set of
+//! macros for creating `Event`s (`trace!`, `debug!`, `info!`, `warn!`, and
+//! `error!`) which may be invoked with the same syntax as the similarly-named
+//! macros from the `log` crate. Often, the process of converting a project to
+//! use `tracing` can begin with a simple drop-in replacement.
+//!
+//! Let's consider the `log` crate's yak-shaving example:
+//!
+//! ```rust,ignore
+//! use std::{error::Error, io};
+//! use tracing::{debug, error, info, span, warn, Level};
+//!
+//! // the `#[tracing::instrument]` attribute creates and enters a span
+//! // every time the instrumented function is called. The span is named after the
+//! // the function or method. Parameters passed to the function are recorded as fields.
+//! #[tracing::instrument]
+//! pub fn shave(yak: usize) -> Result<(), Box<dyn Error + 'static>> {
+//!     // this creates an event at the DEBUG level with two fields:
+//!     // - `excitement`, with the key "excitement" and the value "yay!"
+//!     // - `message`, with the key "message" and the value "hello! I'm gonna shave a yak."
+//!     //
+//!     // unlike other fields, `message`'s shorthand initialization is just the string itself.
+//!     debug!(excitement = "yay!", "hello! I'm gonna shave a yak.");
+//!     if yak == 3 {
+//!         warn!("could not locate yak!");
+//!         // note that this is intended to demonstrate `tracing`'s features, not idiomatic
+//!         // error handling! in a library or application, you should consider returning
+//!         // a dedicated `YakError`. libraries like snafu or thiserror make this easy.
+//!         return Err(io::Error::new(io::ErrorKind::Other, "shaving yak failed!").into());
+//!     } else {
+//!         debug!("yak shaved successfully");
+//!     }
+//!     Ok(())
+//! }
+//!
+//! pub fn shave_all(yaks: usize) -> usize {
+//!     // Constructs a new span named "shaving_yaks" at the TRACE level,
+//!     // and a field whose key is "yaks". This is equivalent to writing:
+//!     //
+//!     // let span = span!(Level::TRACE, "shaving_yaks", yaks = yaks);
+//!     //
+//!     // local variables (`yaks`) can be used as field values
+//!     // without an assignment, similar to struct initializers.
+//!     let span = span!(Level::TRACE, "shaving_yaks", yaks);
+//!     let _enter = span.enter();
+//!
+//!     info!("shaving yaks");
+//!
+//!     let mut yaks_shaved = 0;
+//!     for yak in 1..=yaks {
+//!         let res = shave(yak);
+//!         debug!(yak, shaved = res.is_ok());
+//!
+//!         if let Err(ref error) = res {
+//!             // Like spans, events can also use the field initialization shorthand.
+//!             // In this instance, `yak` is the field being initalized.
+//!             error!(yak, error = error.as_ref(), "failed to shave yak!");
+//!         } else {
+//!             yaks_shaved += 1;
+//!         }
+//!         debug!(yaks_shaved);
+//!     }
+//!
+//!     yaks_shaved
+//! }
+//! ```
+//!
+//! ## In libraries
+//!
+//! Libraries should link only to the `tracing` crate, and use the provided
+//! macros to record whatever information will be useful to downstream
+//! consumers.
+//!
+//! ## In executables
+//!
+//! In order to record trace events, executables have to use a `Subscriber`
+//! implementation compatible with `tracing`. A `Subscriber` implements a
+//! way of collecting trace data, such as by logging it to standard output.
+//!
+//! This library does not contain any `Subscriber` implementations; these are
+//! provided by [other crates](#related-crates).
+//!
+//! The simplest way to use a subscriber is to call the [`set_global_default`]
+//! function:
+//!
+//! ```
+//! extern crate tracing;
+//! # pub struct FooSubscriber;
+//! # use tracing::{span::{Id, Attributes, Record}, Metadata};
+//! # impl tracing::Subscriber for FooSubscriber {
+//! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
+//! #   fn record(&self, _: &Id, _: &Record) {}
+//! #   fn event(&self, _: &tracing::Event) {}
+//! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
+//! #   fn enabled(&self, _: &Metadata) -> bool { false }
+//! #   fn enter(&self, _: &Id) {}
+//! #   fn exit(&self, _: &Id) {}
+//! # }
+//! # impl FooSubscriber {
+//! #   fn new() -> Self { FooSubscriber }
+//! # }
+//! # fn main() {
+//!
+//! let my_subscriber = FooSubscriber::new();
+//! tracing::subscriber::set_global_default(my_subscriber)
+//!     .expect("setting tracing default failed");
+//! # }
+//! ```
+//!
+//! <div class="information">
+//!     <div class="tooltip compile_fail" style="">&#x26a0; &#xfe0f;<span class="tooltiptext">Warning</span></div>
+//! </div><div class="example-wrap" style="display:inline-block"><pre class="compile_fail" style="white-space:normal;font:inherit;">
+//! <strong>Warning</strong>: In general, libraries should <em>not</em> call
+//! <code>set_global_default()</code>! Doing so will cause conflicts when
+//! executables that depend on the library try to set the default later.
+//! </pre></div>
+//!
+//! This subscriber will be used as the default in all threads for the
+//! remainder of the duration of the program, similar to setting the logger
+//! in the `log` crate.
+//!
+//! In addition, the default subscriber can be set through using the
+//! [`with_default`] function. This follows the `tokio` pattern of using
+//! closures to represent executing code in a context that is exited at the end
+//! of the closure. For example:
+//!
+//! ```rust
+//! # pub struct FooSubscriber;
+//! # use tracing::{span::{Id, Attributes, Record}, Metadata};
+//! # impl tracing::Subscriber for FooSubscriber {
+//! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
+//! #   fn record(&self, _: &Id, _: &Record) {}
+//! #   fn event(&self, _: &tracing::Event) {}
+//! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
+//! #   fn enabled(&self, _: &Metadata) -> bool { false }
+//! #   fn enter(&self, _: &Id) {}
+//! #   fn exit(&self, _: &Id) {}
+//! # }
+//! # impl FooSubscriber {
+//! #   fn new() -> Self { FooSubscriber }
+//! # }
+//! # fn main() {
+//!
+//! let my_subscriber = FooSubscriber::new();
+//! # #[cfg(feature = "std")]
+//! tracing::subscriber::with_default(my_subscriber, || {
+//!     // Any trace events generated in this closure or by functions it calls
+//!     // will be collected by `my_subscriber`.
+//! })
+//! # }
+//! ```
+//!
+//! This approach allows trace data to be collected by multiple subscribers
+//! within different contexts in the program. Note that the override only applies to the
+//! currently executing thread; other threads will not see the change from with_default.
+//!
+//! Any trace events generated outside the context of a subscriber will not be collected.
+//!
+//! Once a subscriber has been set, instrumentation points may be added to the
+//! executable using the `tracing` crate's macros.
+//!
+//! ## `log` Compatibility
+//!
+//! The [`log`] crate provides a simple, lightweight logging facade for Rust.
+//! While `tracing` builds upon `log`'s foundation with richer structured
+//! diagnostic data, `log`'s simplicity and ubiquity make it the "lowest common
+//! denominator" for text-based logging in Rust — a vast majority of Rust
+//! libraries and applications either emit or consume `log` records. Therefore,
+//! `tracing` provides multiple forms of interoperability with `log`: `tracing`
+//! instrumentation can emit `log` records, and a compatibility layer enables
+//! `tracing` [`Subscriber`]s to consume `log` records as `tracing` [`Event`]s.
+//!
+//! ### Emitting `log` Records
+//!
+//! This crate provides two feature flags, "log" and "log-always", which will
+//! cause [spans] and [events] to emit `log` records. When the "log" feature is
+//! enabled, if no `tracing` `Subscriber` is active, invoking an event macro or
+//! creating a span with fields will emit a `log` record. This is intended
+//! primarily for use in libraries which wish to emit diagnostics that can be
+//! consumed by applications using `tracing` *or* `log`, without paying the
+//! additional overhead of emitting both forms of diagnostics when `tracing` is
+//! in use.
+//!
+//! Enabling the "log-always" feature will cause `log` records to be emitted
+//! even if a `tracing` `Subscriber` _is_ set. This is intended to be used in
+//! applications where a `log` `Logger` is being used to record a textual log,
+//! and `tracing` is used only to record other forms of diagnostics (such as
+//! metrics, profiling, or distributed tracing data). Unlike the "log" feature,
+//! libraries generally should **not** enable the "log-always" feature, as doing
+//! so will prevent applications from being able to opt out of the `log` records.
+//!
+//! See [here][flags] for more details on this crate's feature flags.
+//!
+//! The generated `log` records' messages will be a string representation of the
+//! span or event's fields, and all additional information recorded by `log`
+//! (target, verbosity level, module path, file, and line number) will also be
+//! populated. Additionally, `log` records are also generated when spans are
+//! entered, exited, and closed. Since these additional span lifecycle logs have
+//! the potential to be very verbose, and don't include additional fields, they
+//! will always be emitted at the `Trace` level, rather than inheriting the
+//! level of the span that generated them. Furthermore, they are are categorized
+//! under a separate `log` target, "tracing::span" (and its sub-target,
+//! "tracing::span::active", for the logs on entering and exiting a span), which
+//! may be enabled or disabled separately from other `log` records emitted by
+//! `tracing`.
+//!
+//! ### Consuming `log` Records
+//!
+//! The [`tracing-log`] crate provides a compatibility layer which
+//! allows a `tracing` [`Subscriber`] to consume `log` records as though they
+//! were `tracing` [events]. This allows applications using `tracing` to record
+//! the logs emitted by dependencies using `log` as events within the context of
+//! the application's trace tree. See [that crate's documentation][log-tracer]
+//! for details.
+//!
+//! [log-tracer]: https://docs.rs/tracing-log/latest/tracing_log/#convert-log-records-to-tracing-events
+//!
+//! ## Related Crates
+//!
+//! In addition to `tracing` and `tracing-core`, the [`tokio-rs/tracing`] repository
+//! contains several additional crates designed to be used with the `tracing` ecosystem.
+//! This includes a collection of `Subscriber` implementations, as well as utility
+//! and adapter crates to assist in writing `Subscriber`s and instrumenting
+//! applications.
+//!
+//! In particular, the following crates are likely to be of interest:
+//!
+//!  - [`tracing-futures`] provides a compatibility layer with the `futures`
+//!    crate, allowing spans to be attached to `Future`s, `Stream`s, and `Executor`s.
+//!  - [`tracing-subscriber`] provides `Subscriber` implementations and
+//!    utilities for working with `Subscriber`s. This includes a [`FmtSubscriber`]
+//!    `FmtSubscriber` for logging formatted trace data to stdout, with similar
+//!    filtering and formatting to the [`env_logger`] crate.
+//!  - [`tracing-log`] provides a compatibility layer with the [`log`] crate,
+//!    allowing log messages to be recorded as `tracing` `Event`s within the
+//!    trace tree. This is useful when a project using `tracing` have
+//!    dependencies which use `log`. Note that if you're using
+//!    `tracing-subscriber`'s `FmtSubscriber`, you don't need to depend on
+//!    `tracing-log` directly.
+//!  - [`tracing-appender`] provides utilities for outputting tracing data,
+//!     including a file appender and non blocking writer.
+//!
+//! Additionally, there are also several third-party crates which are not
+//! maintained by the `tokio` project. These include:
+//!
+//!  - [`tracing-timing`] implements inter-event timing metrics on top of `tracing`.
+//!    It provides a subscriber that records the time elapsed between pairs of
+//!    `tracing` events and generates histograms.
+//!  - [`tracing-opentelemetry`] provides a subscriber for emitting traces to
+//!    [OpenTelemetry]-compatible distributed tracing systems.
+//!  - [`tracing-honeycomb`] Provides a layer that reports traces spanning multiple machines to [honeycomb.io]. Backed by [`tracing-distributed`].
+//!  - [`tracing-distributed`] Provides a generic implementation of a layer that reports traces spanning multiple machines to some backend.
+//!  - [`tracing-actix`] provides `tracing` integration for the `actix` actor
+//!    framework.
+//!  - [`tracing-gelf`] implements a subscriber for exporting traces in Greylog
+//!    GELF format.
+//!  - [`tracing-coz`] provides integration with the [coz] causal profiler
+//!    (Linux-only).
+//!  - [`tracing-bunyan-formatter`] provides a layer implementation that reports events and spans
+//!    in [bunyan] format, enriched with timing information.
+//!  - [`tracing-wasm`] provides a `Subscriber`/`Layer` implementation that reports
+//!    events and spans via browser `console.log` and [User Timing API (`window.performance`)].
+//!  - [`tide-tracing`] provides a [tide] middleware to trace all incoming requests and responses.
+//!  - [`test-env-log`] takes care of initializing `tracing` for tests, based on
+//!    environment variables with an `env_logger` compatible syntax.
+//!  - [`tracing-unwrap`] provides convenience methods to report failed unwraps
+//!    on `Result` or `Option` types to a `Subscriber`.
+//!  - [`diesel-tracing`] provides integration with [`diesel`] database connections.
+//!  - [`tracing-tracy`] provides a way to collect [Tracy] profiles in instrumented
+//!    applications.
+//!
+//! If you're the maintainer of a `tracing` ecosystem crate not listed above,
+//! please let us know! We'd love to add your project to the list!
+//!
+//! [`tracing-opentelemetry`]: https://crates.io/crates/tracing-opentelemetry
+//! [OpenTelemetry]: https://opentelemetry.io/
+//! [`tracing-honeycomb`]: https://crates.io/crates/tracing-honeycomb
+//! [`tracing-distributed`]: https://crates.io/crates/tracing-distributed
+//! [honeycomb.io]: https://www.honeycomb.io/
+//! [`tracing-actix`]: https://crates.io/crates/tracing-actix
+//! [`tracing-gelf`]: https://crates.io/crates/tracing-gelf
+//! [`tracing-coz`]: https://crates.io/crates/tracing-coz
+//! [coz]: https://github.com/plasma-umass/coz
+//! [`tracing-bunyan-formatter`]: https://crates.io/crates/tracing-bunyan-formatter
+//! [bunyan]: https://github.com/trentm/node-bunyan
+//! [`tracing-wasm`]: https://docs.rs/tracing-wasm
+//! [User Timing API (`window.performance`)]: https://developer.mozilla.org/en-US/docs/Web/API/User_Timing_API
+//! [`tide-tracing`]: https://crates.io/crates/tide-tracing
+//! [tide]: https://crates.io/crates/tide
+//! [`test-env-log`]: https://crates.io/crates/test-env-log
+//! [`tracing-unwrap`]: https://docs.rs/tracing-unwrap
+//! [`diesel`]: https://crates.io/crates/diesel
+//! [`diesel-tracing`]: https://crates.io/crates/diesel-tracing
+//! [`tracing-tracy`]: https://crates.io/crates/tracing-tracy
+//! [Tracy]: https://github.com/wolfpld/tracy
+//!
+//! <div class="information">
+//!     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
+//! </div>
+//! <div class="example-wrap" style="display:inline-block">
+//! <pre class="ignore" style="white-space:normal;font:inherit;">
+//! <strong>Note</strong>: Some of these ecosystem crates are currently
+//! unreleased and/or in earlier stages of development. They may be less stable
+//! than <code>tracing</code> and <code>tracing-core</code>.
+//! </pre></div>
+//!
+//! ## Crate Feature Flags
+//!
+//! The following crate feature flags are available:
+//!
+//! * A set of features controlling the [static verbosity level].
+//! * `log`: causes trace instrumentation points to emit [`log`] records as well
+//!   as trace events, if a default `tracing` subscriber has not been set. This
+//!   is intended for use in libraries whose users may be using either `tracing`
+//!   or `log`.
+//! * `log-always`: Emit `log` records from all `tracing` spans and events, even
+//!   if a `tracing` subscriber has been set. This should be set only by
+//!   applications which intend to collect traces and logs separately; if an
+//!   adapter is used to convert `log` records into `tracing` events, this will
+//!   cause duplicate events to occur.
+//! * `attributes`: Includes support for the `#[instrument]` attribute.
+//!   This is on by default, but does bring in the `syn` crate as a dependency,
+//!   which may add to the compile time of crates that do not already use it.
+//! * `std`: Depend on the Rust standard library (enabled by default).
+//!
+//!   `no_std` users may disable this feature with `default-features = false`:
+//!
+//!   ```toml
+//!   [dependencies]
+//!   tracing = { version = "0.1.21", default-features = false }
+//!   ```
+//!
+//! <div class="information">
+//!     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
+//! </div>
+//! <div class="example-wrap" style="display:inline-block">
+//! <pre class="ignore" style="white-space:normal;font:inherit;">
+//! <strong>Note</strong>: <code>tracing</code>'s <code>no_std</code> support
+//! requires <code>liballoc</code>.
+//! </pre></div>
+//!
+//! ## Supported Rust Versions
+//!
+//! Tracing is built against the latest stable release. The minimum supported
+//! version is 1.40. The current Tracing version is not guaranteed to build on
+//! Rust versions earlier than the minimum supported version.
+//!
+//! Tracing follows the same compiler support policies as the rest of the Tokio
+//! project. The current stable Rust compiler and the three most recent minor
+//! versions before it will always be supported. For example, if the current
+//! stable compiler version is 1.45, the minimum supported version will not be
+//! increased past 1.42, three minor versions prior. Increasing the minimum
+//! supported compiler version is not considered a semver breaking change as
+//! long as doing so complies with this policy.
+//!
+//! [`log`]: https://docs.rs/log/0.4.6/log/
+//! [span]: mod@span
+//! [spans]: mod@span
+//! [`Span`]: span::Span
+//! [`in_scope`]: span::Span::in_scope
+//! [event]: Event
+//! [events]: Event
+//! [`Subscriber`]: subscriber::Subscriber
+//! [Subscriber::event]: subscriber::Subscriber::event
+//! [`enter`]: subscriber::Subscriber::enter
+//! [`exit`]: subscriber::Subscriber::exit
+//! [`enabled`]: subscriber::Subscriber::enabled
+//! [metadata]: Metadata
+//! [`field::display`]: field::display
+//! [`field::debug`]: field::debug
+//! [`set_global_default`]: subscriber::set_global_default
+//! [`with_default`]: subscriber::with_default
+//! [`tokio-rs/tracing`]: https://github.com/tokio-rs/tracing
+//! [`tracing-futures`]: https://crates.io/crates/tracing-futures
+//! [`tracing-subscriber`]: https://crates.io/crates/tracing-subscriber
+//! [`tracing-log`]: https://crates.io/crates/tracing-log
+//! [`tracing-timing`]: https://crates.io/crates/tracing-timing
+//! [`tracing-appender`]: https://crates.io/crates/tracing-appender
+//! [`env_logger`]: https://crates.io/crates/env_logger
+//! [`FmtSubscriber`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/struct.Subscriber.html
+//! [static verbosity level]: level_filters/index.html#compile-time-filters
+//! [instrument]: https://docs.rs/tracing-attributes/latest/tracing_attributes/attr.instrument.html
+//! [flags]: #crate-feature-flags
+#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(docsrs, feature(doc_cfg), deny(broken_intra_doc_links))]
+#![doc(html_root_url = "https://docs.rs/tracing/0.1.21")]
+#![doc(
+    html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png",
+    issue_tracker_base_url = "https://github.com/tokio-rs/tracing/issues/"
+)]
+#![warn(
+    missing_debug_implementations,
+    missing_docs,
+    rust_2018_idioms,
+    unreachable_pub,
+    bad_style,
+    const_err,
+    dead_code,
+    improper_ctypes,
+    non_shorthand_field_patterns,
+    no_mangle_generic_items,
+    overflowing_literals,
+    path_statements,
+    patterns_in_fns_without_body,
+    private_in_public,
+    unconditional_recursion,
+    unused,
+    unused_allocation,
+    unused_comparisons,
+    unused_parens,
+    while_true
+)]
+
+#[cfg(not(feature = "std"))]
+extern crate alloc;
+
+#[macro_use]
+extern crate cfg_if;
+
+#[cfg(feature = "log")]
+#[doc(hidden)]
+pub use log;
+
+// Somehow this `use` statement is necessary for us to re-export the `core`
+// macros on Rust 1.26.0. I'm not sure how this makes it work, but it does.
+#[allow(unused_imports)]
+#[doc(hidden)]
+use tracing_core::*;
+
+#[doc(inline)]
+pub use self::instrument::Instrument;
+pub use self::{dispatcher::Dispatch, event::Event, field::Value, subscriber::Subscriber};
+
+#[doc(hidden)]
+pub use self::span::Id;
+
+#[doc(hidden)]
+pub use tracing_core::{
+    callsite::{self, Callsite},
+    metadata,
+};
+pub use tracing_core::{event, Level, Metadata};
+
+#[doc(inline)]
+pub use self::span::Span;
+#[cfg(feature = "attributes")]
+#[cfg_attr(docsrs, doc(cfg(feature = "attributes")))]
+#[doc(inline)]
+pub use tracing_attributes::instrument;
+
+#[macro_use]
+mod macros;
+
+pub mod dispatcher;
+pub mod field;
+/// Attach a span to a `std::future::Future`.
+pub mod instrument;
+pub mod level_filters;
+pub mod span;
+pub(crate) mod stdlib;
+pub mod subscriber;
+
+#[doc(hidden)]
+pub mod __macro_support {
+    pub use crate::callsite::Callsite;
+    use crate::stdlib::sync::atomic::{AtomicUsize, Ordering};
+    use crate::{subscriber::Interest, Metadata};
+    use tracing_core::Once;
+
+    /// Callsite implementation used by macro-generated code.
+    ///
+    /// /!\ WARNING: This is *not* a stable API! /!\
+    /// This type, and all code contained in the `__macro_support` module, is
+    /// a *private* API of `tracing`. It is exposed publicly because it is used
+    /// by the `tracing` macros, but it is not part of the stable versioned API.
+    /// Breaking changes to this module may occur in small-numbered versions
+    /// without warning.
+    #[derive(Debug)]
+    pub struct MacroCallsite {
+        interest: AtomicUsize,
+        meta: &'static Metadata<'static>,
+        registration: Once,
+    }
+
+    impl MacroCallsite {
+        /// Returns a new `MacroCallsite` with the specified `Metadata`.
+        ///
+        /// /!\ WARNING: This is *not* a stable API! /!\
+        /// This method, and all code contained in the `__macro_support` module, is
+        /// a *private* API of `tracing`. It is exposed publicly because it is used
+        /// by the `tracing` macros, but it is not part of the stable versioned API.
+        /// Breaking changes to this module may occur in small-numbered versions
+        /// without warning.
+        pub const fn new(meta: &'static Metadata<'static>) -> Self {
+            Self {
+                interest: AtomicUsize::new(0xDEADFACED),
+                meta,
+                registration: Once::new(),
+            }
+        }
+
+        /// Registers this callsite with the global callsite registry.
+        ///
+        /// If the callsite is already registered, this does nothing.
+        ///
+        /// /!\ WARNING: This is *not* a stable API! /!\
+        /// This method, and all code contained in the `__macro_support` module, is
+        /// a *private* API of `tracing`. It is exposed publicly because it is used
+        /// by the `tracing` macros, but it is not part of the stable versioned API.
+        /// Breaking changes to this module may occur in small-numbered versions
+        /// without warning.
+        #[inline(never)]
+        // This only happens once (or if the cached interest value was corrupted).
+        #[cold]
+        pub fn register(&'static self) -> Interest {
+            self.registration
+                .call_once(|| crate::callsite::register(self));
+            match self.interest.load(Ordering::Relaxed) {
+                0 => Interest::never(),
+                2 => Interest::always(),
+                _ => Interest::sometimes(),
+            }
+        }
+
+        /// Returns the callsite's cached Interest, or registers it for the
+        /// first time if it has not yet been registered.
+        ///
+        /// /!\ WARNING: This is *not* a stable API! /!\
+        /// This method, and all code contained in the `__macro_support` module, is
+        /// a *private* API of `tracing`. It is exposed publicly because it is used
+        /// by the `tracing` macros, but it is not part of the stable versioned API.
+        /// Breaking changes to this module may occur in small-numbered versions
+        /// without warning.
+        #[inline]
+        pub fn interest(&'static self) -> Interest {
+            match self.interest.load(Ordering::Relaxed) {
+                0 => Interest::never(),
+                1 => Interest::sometimes(),
+                2 => Interest::always(),
+                _ => self.register(),
+            }
+        }
+
+        pub fn is_enabled(&self, interest: Interest) -> bool {
+            interest.is_always()
+                || crate::dispatcher::get_default(|default| default.enabled(self.meta))
+        }
+
+        #[inline]
+        #[cfg(feature = "log")]
+        pub fn disabled_span(&self) -> crate::Span {
+            crate::Span::new_disabled(self.meta)
+        }
+
+        #[inline]
+        #[cfg(not(feature = "log"))]
+        pub fn disabled_span(&self) -> crate::Span {
+            crate::Span::none()
+        }
+    }
+
+    impl Callsite for MacroCallsite {
+        fn set_interest(&self, interest: Interest) {
+            let interest = match () {
+                _ if interest.is_never() => 0,
+                _ if interest.is_always() => 2,
+                _ => 1,
+            };
+            self.interest.store(interest, Ordering::SeqCst);
+        }
+
+        #[inline(always)]
+        fn metadata(&self) -> &Metadata<'static> {
+            &self.meta
+        }
+    }
+}
+
+mod sealed {
+    pub trait Sealed {}
+}
diff --git a/third_party/rust/tracing/src/macros.rs b/third_party/rust/tracing/src/macros.rs
new file mode 100644
index 0000000000..3222abb741
--- /dev/null
+++ b/third_party/rust/tracing/src/macros.rs
@@ -0,0 +1,2369 @@
+/// Constructs a new span.
+///
+/// See [the top-level documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [lib]: index.html#using-the-macros
+///
+/// # Examples
+///
+/// Creating a new span:
+/// ```
+/// # use tracing::{span, Level};
+/// # fn main() {
+/// let span = span!(Level::TRACE, "my span");
+/// let _enter = span.enter();
+/// // do work inside the span...
+/// # }
+/// ```
+#[macro_export]
+macro_rules! span {
+    (target: $target:expr, parent: $parent:expr, $lvl:expr, $name:expr) => {
+        $crate::span!(target: $target, parent: $parent, $lvl, $name,)
+    };
+    (target: $target:expr, parent: $parent:expr, $lvl:expr, $name:expr, $($fields:tt)*) => {
+        {
+            use $crate::__macro_support::Callsite as _;
+            static CALLSITE: $crate::__macro_support::MacroCallsite = $crate::callsite2! {
+                name: $name,
+                kind: $crate::metadata::Kind::SPAN,
+                target: $target,
+                level: $lvl,
+                fields: $($fields)*
+            };
+            let mut interest = $crate::subscriber::Interest::never();
+            if $crate::level_enabled!($lvl)
+                && { interest = CALLSITE.interest(); !interest.is_never() }
+                && CALLSITE.is_enabled(interest)
+            {
+                let meta = CALLSITE.metadata();
+                // span with explicit parent
+                $crate::Span::child_of(
+                    $parent,
+                    meta,
+                    &$crate::valueset!(meta.fields(), $($fields)*),
+                )
+            } else {
+                let span = CALLSITE.disabled_span();
+                $crate::if_log_enabled! {{
+                    span.record_all(&$crate::valueset!(CALLSITE.metadata().fields(), $($fields)*));
+                }};
+                span
+            }
+        }
+    };
+    (target: $target:expr, $lvl:expr, $name:expr, $($fields:tt)*) => {
+        {
+            use $crate::__macro_support::Callsite as _;
+            static CALLSITE: $crate::__macro_support::MacroCallsite = $crate::callsite2! {
+                name: $name,
+                kind: $crate::metadata::Kind::SPAN,
+                target: $target,
+                level: $lvl,
+                fields: $($fields)*
+            };
+            let mut interest = $crate::subscriber::Interest::never();
+            if $crate::level_enabled!($lvl)
+                && { interest = CALLSITE.interest(); !interest.is_never() }
+                && CALLSITE.is_enabled(interest)
+            {
+                let meta = CALLSITE.metadata();
+                // span with contextual parent
+                $crate::Span::new(
+                    meta,
+                    &$crate::valueset!(meta.fields(), $($fields)*),
+                )
+            } else {
+                let span = CALLSITE.disabled_span();
+                $crate::if_log_enabled! {{
+                    span.record_all(&$crate::valueset!(CALLSITE.metadata().fields(), $($fields)*));
+                }};
+                span
+            }
+        }
+    };
+    (target: $target:expr, parent: $parent:expr, $lvl:expr, $name:expr) => {
+        $crate::span!(target: $target, parent: $parent, $lvl, $name,)
+    };
+    (parent: $parent:expr, $lvl:expr, $name:expr, $($fields:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            $name,
+            $($fields)*
+        )
+    };
+    (parent: $parent:expr, $lvl:expr, $name:expr) => {
+        $crate::span!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            $name,
+        )
+    };
+    (target: $target:expr, $lvl:expr, $name:expr, $($fields:tt)*) => {
+        $crate::span!(
+            target: $target,
+            $lvl,
+            $name,
+            $($fields)*
+        )
+    };
+    (target: $target:expr, $lvl:expr, $name:expr) => {
+        $crate::span!(target: $target, $lvl, $name,)
+    };
+    ($lvl:expr, $name:expr, $($fields:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            $lvl,
+            $name,
+            $($fields)*
+        )
+    };
+    ($lvl:expr, $name:expr) => {
+        $crate::span!(
+            target: module_path!(),
+            $lvl,
+            $name,
+        )
+    };
+}
+
+/// Constructs a span at the trace level.
+///
+/// [Fields] and [attributes] are set using the same syntax as the [`span!`]
+/// macro.
+///
+/// See [the top-level documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [lib]: index.html#using-the-macros
+/// [attributes]: index.html#configuring-attributes
+/// [Fields]: index.html#recording-fields
+/// [`span!`]: macro.span.html
+///
+/// # Examples
+///
+/// ```rust
+/// # use tracing::{trace_span, span, Level};
+/// # fn main() {
+/// trace_span!("my_span");
+/// // is equivalent to:
+/// span!(Level::TRACE, "my_span");
+/// # }
+/// ```
+///
+/// ```rust
+/// # use tracing::{trace_span, span, Level};
+/// # fn main() {
+/// let span = trace_span!("my span");
+/// span.in_scope(|| {
+///     // do work inside the span...
+/// });
+/// # }
+/// ```
+#[macro_export]
+macro_rules! trace_span {
+    (target: $target:expr, parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            parent: $parent,
+            $crate::Level::TRACE,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, parent: $parent:expr, $name:expr) => {
+        $crate::trace_span!(target: $target, parent: $parent, $name,)
+    };
+    (parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            $name,
+            $($field)*
+        )
+    };
+    (parent: $parent:expr, $name:expr) => {
+        $crate::trace_span!(parent: $parent, $name,)
+    };
+    (target: $target:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            $crate::Level::TRACE,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, $name:expr) => {
+        $crate::trace_span!(target: $target, $name,)
+    };
+    ($name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            $name,
+            $($field)*
+        )
+    };
+    ($name:expr) => { $crate::trace_span!($name,) };
+}
+
+/// Constructs a span at the debug level.
+///
+/// [Fields] and [attributes] are set using the same syntax as the [`span!`]
+/// macro.
+///
+/// See [the top-level documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [lib]: index.html#using-the-macros
+/// [attributes]: index.html#configuring-attributes
+/// [Fields]: index.html#recording-fields
+/// [`span!`]: macro.span.html
+///
+/// # Examples
+///
+/// ```rust
+/// # use tracing::{debug_span, span, Level};
+/// # fn main() {
+/// debug_span!("my_span");
+/// // is equivalent to:
+/// span!(Level::DEBUG, "my_span");
+/// # }
+/// ```
+///
+/// ```rust
+/// # use tracing::debug_span;
+/// # fn main() {
+/// let span = debug_span!("my span");
+/// span.in_scope(|| {
+///     // do work inside the span...
+/// });
+/// # }
+/// ```
+#[macro_export]
+macro_rules! debug_span {
+    (target: $target:expr, parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            parent: $parent,
+            $crate::Level::DEBUG,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, parent: $parent:expr, $name:expr) => {
+        $crate::debug_span!(target: $target, parent: $parent, $name,)
+    };
+    (parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            $name,
+            $($field)*
+        )
+    };
+    (parent: $parent:expr, $name:expr) => {
+        $crate::debug_span!(parent: $parent, $name,)
+    };
+    (target: $target:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            $crate::Level::DEBUG,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, $name:expr) => {
+        $crate::debug_span!(target: $target, $name,)
+    };
+    ($name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            $name,
+            $($field)*
+        )
+    };
+    ($name:expr) => {$crate::debug_span!($name,)};
+}
+
+/// Constructs a span at the info level.
+///
+/// [Fields] and [attributes] are set using the same syntax as the [`span!`]
+/// macro.
+///
+/// See [the top-level documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [lib]: index.html#using-the-macros
+/// [attributes]: index.html#configuring-attributes
+/// [Fields]: index.html#recording-fields
+/// [`span!`]: macro.span.html
+///
+/// # Examples
+///
+/// ```rust
+/// # use tracing::{span, info_span, Level};
+/// # fn main() {
+/// info_span!("my_span");
+/// // is equivalent to:
+/// span!(Level::INFO, "my_span");
+/// # }
+/// ```
+///
+/// ```rust
+/// # use tracing::info_span;
+/// # fn main() {
+/// let span = info_span!("my span");
+/// span.in_scope(|| {
+///     // do work inside the span...
+/// });
+/// # }
+/// ```
+#[macro_export]
+macro_rules! info_span {
+    (target: $target:expr, parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            parent: $parent,
+            $crate::Level::INFO,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, parent: $parent:expr, $name:expr) => {
+        $crate::info_span!(target: $target, parent: $parent, $name,)
+    };
+    (parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            $name,
+            $($field)*
+        )
+    };
+    (parent: $parent:expr, $name:expr) => {
+        $crate::info_span!(parent: $parent, $name,)
+    };
+    (target: $target:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            $crate::Level::INFO,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, $name:expr) => {
+        $crate::info_span!(target: $target, $name,)
+    };
+    ($name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            $name,
+            $($field)*
+        )
+    };
+    ($name:expr) => {$crate::info_span!($name,)};
+}
+
+/// Constructs a span at the warn level.
+///
+/// [Fields] and [attributes] are set using the same syntax as the [`span!`]
+/// macro.
+///
+/// See [the top-level documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [lib]: index.html#using-the-macros
+/// [attributes]: index.html#configuring-attributes
+/// [Fields]: index.html#recording-fields
+/// [`span!`]: macro.span.html
+///
+/// # Examples
+///
+/// ```rust
+/// # use tracing::{warn_span, span, Level};
+/// # fn main() {
+/// warn_span!("my_span");
+/// // is equivalent to:
+/// span!(Level::WARN, "my_span");
+/// # }
+/// ```
+///
+/// ```rust
+/// use tracing::warn_span;
+/// # fn main() {
+/// let span = warn_span!("my span");
+/// span.in_scope(|| {
+///     // do work inside the span...
+/// });
+/// # }
+/// ```
+#[macro_export]
+macro_rules! warn_span {
+    (target: $target:expr, parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            parent: $parent,
+            $crate::Level::WARN,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, parent: $parent:expr, $name:expr) => {
+        $crate::warn_span!(target: $target, parent: $parent, $name,)
+    };
+    (parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            $name,
+            $($field)*
+        )
+    };
+    (parent: $parent:expr, $name:expr) => {
+        $crate::warn_span!(parent: $parent, $name,)
+    };
+    (target: $target:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            $crate::Level::WARN,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, $name:expr) => {
+        $crate::warn_span!(target: $target, $name,)
+    };
+    ($name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            $name,
+            $($field)*
+        )
+    };
+    ($name:expr) => {$crate::warn_span!($name,)};
+}
+/// Constructs a span at the error level.
+///
+/// [Fields] and [attributes] are set using the same syntax as the [`span!`]
+/// macro.
+///
+/// See [the top-level documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [lib]: index.html#using-the-macros
+/// [attributes]: index.html#configuring-attributes
+/// [Fields]: index.html#recording-fields
+/// [`span!`]: macro.span.html
+///
+/// # Examples
+///
+/// ```rust
+/// # use tracing::{span, error_span, Level};
+/// # fn main() {
+/// error_span!("my_span");
+/// // is equivalent to:
+/// span!(Level::ERROR, "my_span");
+/// # }
+/// ```
+///
+/// ```rust
+/// # use tracing::error_span;
+/// # fn main() {
+/// let span = error_span!("my span");
+/// span.in_scope(|| {
+///     // do work inside the span...
+/// });
+/// # }
+/// ```
+#[macro_export]
+macro_rules! error_span {
+    (target: $target:expr, parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            parent: $parent,
+            $crate::Level::ERROR,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, parent: $parent:expr, $name:expr) => {
+        $crate::error_span!(target: $target, parent: $parent, $name,)
+    };
+    (parent: $parent:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            $name,
+            $($field)*
+        )
+    };
+    (parent: $parent:expr, $name:expr) => {
+        $crate::error_span!(parent: $parent, $name,)
+    };
+    (target: $target:expr, $name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: $target,
+            $crate::Level::ERROR,
+            $name,
+            $($field)*
+        )
+    };
+    (target: $target:expr, $name:expr) => {
+        $crate::error_span!(target: $target, $name,)
+    };
+    ($name:expr, $($field:tt)*) => {
+        $crate::span!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            $name,
+            $($field)*
+        )
+    };
+    ($name:expr) => {$crate::error_span!($name,)};
+}
+
+/// Constructs a new `Event`.
+///
+/// The event macro is invoked with a `Level` and up to 32 key-value fields.
+/// Optionally, a format string and arguments may follow the fields; this will
+/// be used to construct an implicit field named "message".
+///
+/// See [the top-level documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [lib]: index.html#using-the-macros
+///
+/// # Examples
+///
+/// ```rust
+/// use tracing::{event, Level};
+///
+/// # fn main() {
+/// let data = (42, "forty-two");
+/// let private_data = "private";
+/// let error = "a bad error";
+///
+/// event!(Level::ERROR, %error, "Received error");
+/// event!(
+///     target: "app_events",
+///     Level::WARN,
+///     private_data,
+///     ?data,
+///     "App warning: {}",
+///     error
+/// );
+/// event!(Level::INFO, the_answer = data.0);
+/// # }
+/// ```
+///
+// /// Note that *unlike `span!`*, `event!` requires a value for all fields. As
+// /// events are recorded immediately when the macro is invoked, there is no
+// /// opportunity for fields to be recorded later. A trailing comma on the final
+// /// field is valid.
+// ///
+// /// For example, the following does not compile:
+// /// ```rust,compile_fail
+// /// # #[macro_use]
+// /// # extern crate tracing;
+// /// # use tracing::Level;
+// /// # fn main() {
+// /// event!(Level::INFO, foo = 5, bad_field, bar = "hello")
+// /// #}
+// /// ```
+#[macro_export]
+macro_rules! event {
+    (target: $target:expr, parent: $parent:expr, $lvl:expr, { $($fields:tt)* } )=> (
+        $crate::__tracing_log!(
+            target: $target,
+            $lvl,
+            $($fields)*
+        );
+
+        if $crate::level_enabled!($lvl) {
+            use $crate::__macro_support::*;
+            static CALLSITE: $crate::__macro_support::MacroCallsite = $crate::callsite2! {
+                name: concat!(
+                    "event ",
+                    file!(),
+                    ":",
+                    line!()
+                ),
+                kind: $crate::metadata::Kind::EVENT,
+                target: $target,
+                level: $lvl,
+                fields: $($fields)*
+            };
+            let interest = CALLSITE.interest();
+            if !interest.is_never() && CALLSITE.is_enabled(interest)  {
+                let meta = CALLSITE.metadata();
+                // event with explicit parent
+                $crate::Event::child_of(
+                    $parent,
+                    meta,
+                    &$crate::valueset!(meta.fields(), $($fields)*)
+                );
+            }
+        }
+    );
+
+    (target: $target:expr, parent: $parent:expr, $lvl:expr, { $($fields:tt)* }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: $target,
+            parent: $parent,
+            $lvl,
+            { message = format_args!($($arg)+), $($fields)* }
+        )
+    );
+    (target: $target:expr, parent: $parent:expr, $lvl:expr, $($k:ident).+ = $($fields:tt)* ) => (
+        $crate::event!(target: $target, parent: $parent, $lvl, { $($k).+ = $($fields)* })
+    );
+    (target: $target:expr, parent: $parent:expr, $lvl:expr, $($arg:tt)+) => (
+        $crate::event!(target: $target, parent: $parent, $lvl, { $($arg)+ })
+    );
+    (target: $target:expr, $lvl:expr, { $($fields:tt)* } )=> ({
+        $crate::__tracing_log!(
+            target: $target,
+            $lvl,
+            $($fields)*
+        );
+        if $crate::level_enabled!($lvl) {
+            use $crate::__macro_support::*;
+            static CALLSITE: $crate::__macro_support::MacroCallsite = $crate::callsite2! {
+                name: concat!(
+                    "event ",
+                    file!(),
+                    ":",
+                    line!()
+                ),
+                kind: $crate::metadata::Kind::EVENT,
+                target: $target,
+                level: $lvl,
+                fields: $($fields)*
+            };
+            let interest = CALLSITE.interest();
+            if !interest.is_never() && CALLSITE.is_enabled(interest)  {
+                let meta = CALLSITE.metadata();
+                // event with contextual parent
+                $crate::Event::dispatch(
+                    meta,
+                    &$crate::valueset!(meta.fields(), $($fields)*)
+                );
+            }
+        }
+    });
+    (target: $target:expr, $lvl:expr, { $($fields:tt)* }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: $target,
+            $lvl,
+            { message = format_args!($($arg)+), $($fields)* }
+        )
+    );
+    (target: $target:expr, $lvl:expr, $($k:ident).+ = $($fields:tt)* ) => (
+        $crate::event!(target: $target, $lvl, { $($k).+ = $($fields)* })
+    );
+    (target: $target:expr, $lvl:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, $lvl, { $($arg)+ })
+    );
+    (parent: $parent:expr, $lvl:expr, { $($fields:tt)* }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            { message = format_args!($($arg)+), $($fields)* }
+        )
+    );
+    (parent: $parent:expr, $lvl:expr, { $($fields:tt)* }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            $lvl,
+            parent: $parent,
+            { message = format_args!($($arg)+), $($fields)* }
+        )
+    );
+    (parent: $parent:expr, $lvl:expr, $($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, $lvl:expr, ?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, $lvl:expr, %$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, $lvl:expr, $($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            { $($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, $lvl:expr, %$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, $lvl:expr, ?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $lvl,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, $lvl:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: module_path!(), parent: $parent, $lvl, { $($arg)+ })
+    );
+    ( $lvl:expr, { $($fields:tt)* }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            $lvl,
+            { message = format_args!($($arg)+), $($fields)* }
+        )
+    );
+    ( $lvl:expr, { $($fields:tt)* }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            $lvl,
+            { message = format_args!($($arg)+), $($fields)* }
+        )
+    );
+    ($lvl:expr, $($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $lvl,
+            { $($k).+ = $($field)*}
+        )
+    );
+    ($lvl:expr, $($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $lvl,
+            { $($k).+, $($field)*}
+        )
+    );
+    ($lvl:expr, ?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $lvl,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    ($lvl:expr, %$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $lvl,
+            { %$($k).+, $($field)*}
+        )
+    );
+    ($lvl:expr, ?$($k:ident).+) => (
+        $crate::event!($lvl, ?$($k).+,)
+    );
+    ($lvl:expr, %$($k:ident).+) => (
+        $crate::event!($lvl, %$($k).+,)
+    );
+    ($lvl:expr, $($k:ident).+) => (
+        $crate::event!($lvl, $($k).+,)
+    );
+    ( $lvl:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: module_path!(), $lvl, { $($arg)+ })
+    );
+}
+
+/// Constructs an event at the trace level.
+///
+/// This functions similarly to the [`event!`] macro. See [the top-level
+/// documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [`event!`]: macro.event.html
+/// [lib]: index.html#using-the-macros
+///
+/// # Examples
+///
+/// ```rust
+/// use tracing::trace;
+/// # #[derive(Debug, Copy, Clone)] struct Position { x: f32, y: f32 }
+/// # impl Position {
+/// # const ORIGIN: Self = Self { x: 0.0, y: 0.0 };
+/// # fn dist(&self, other: Position) -> f32 {
+/// #    let x = (other.x - self.x).exp2(); let y = (self.y - other.y).exp2();
+/// #    (x + y).sqrt()
+/// # }
+/// # }
+/// # fn main() {
+/// let pos = Position { x: 3.234, y: -1.223 };
+/// let origin_dist = pos.dist(Position::ORIGIN);
+///
+/// trace!(position = ?pos, ?origin_dist);
+/// trace!(
+///     target: "app_events",
+///     position = ?pos,
+///     "x is {} and y is {}",
+///     if pos.x >= 0.0 { "positive" } else { "negative" },
+///     if pos.y >= 0.0 { "positive" } else { "negative" }
+/// );
+/// # }
+/// ```
+#[macro_export]
+macro_rules! trace {
+    (target: $target:expr, parent: $parent:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::TRACE, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, parent: $parent:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::TRACE, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::TRACE, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::TRACE, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::TRACE, {}, $($arg)+)
+    );
+    (parent: $parent:expr, { $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            { $($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::TRACE,
+            {},
+            $($arg)+
+        )
+    );
+    (target: $target:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, $crate::Level::TRACE, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::TRACE, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::TRACE, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::TRACE, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::TRACE, {}, $($arg)+)
+    );
+    ({ $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    ($($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { $($k).+ = $($field)*}
+        )
+    );
+    ($($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { $($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (%$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { ?$($k).+ }
+        )
+    );
+    (%$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { %$($k).+ }
+        )
+    );
+    ($($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { $($k).+ }
+        )
+    );
+    ($($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            {},
+            $($arg)+
+        )
+    );
+}
+
+/// Constructs an event at the debug level.
+///
+/// This functions similarly to the [`event!`] macro. See [the top-level
+/// documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [`event!`]: macro.event.html
+/// [lib]: index.html#using-the-macros
+///
+/// # Examples
+///
+/// ```rust
+/// use tracing::debug;
+/// # fn main() {
+/// # #[derive(Debug)] struct Position { x: f32, y: f32 }
+///
+/// let pos = Position { x: 3.234, y: -1.223 };
+///
+/// debug!(?pos.x, ?pos.y);
+/// debug!(target: "app_events", position = ?pos, "New position");
+/// # }
+/// ```
+#[macro_export]
+macro_rules! debug {
+    (target: $target:expr, parent: $parent:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::DEBUG, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, parent: $parent:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::DEBUG, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::DEBUG, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::DEBUG, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::DEBUG, {}, $($arg)+)
+    );
+    (parent: $parent:expr, { $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            { $($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::DEBUG,
+            {},
+            $($arg)+
+        )
+    );
+    (target: $target:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, $crate::Level::DEBUG, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::DEBUG, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::DEBUG, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::DEBUG, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::DEBUG, {}, $($arg)+)
+    );
+    ({ $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    ($($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (%$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    ($($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { $($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (%$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { ?$($k).+ }
+        )
+    );
+    (%$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { %$($k).+ }
+        )
+    );
+    ($($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            { $($k).+ }
+        )
+    );
+    ($($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::DEBUG,
+            {},
+            $($arg)+
+        )
+    );
+}
+
+/// Constructs an event at the info level.
+///
+/// This functions similarly to the [`event!`] macro. See [the top-level
+/// documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [`event!`]: macro.event.html
+/// [lib]: index.html#using-the-macros
+///
+/// # Examples
+///
+/// ```rust
+/// use tracing::info;
+/// # // this is so the test will still work in no-std mode
+/// # #[derive(Debug)]
+/// # pub struct Ipv4Addr;
+/// # impl Ipv4Addr { fn new(o1: u8, o2: u8, o3: u8, o4: u8) -> Self { Self } }
+/// # fn main() {
+/// # struct Connection { port: u32, speed: f32 }
+/// use tracing::field;
+///
+/// let addr = Ipv4Addr::new(127, 0, 0, 1);
+/// let conn = Connection { port: 40, speed: 3.20 };
+///
+/// info!(conn.port, "connected to {:?}", addr);
+/// info!(
+///     target: "connection_events",
+///     ip = ?addr,
+///     conn.port,
+///     ?conn.speed,
+/// );
+/// # }
+/// ```
+#[macro_export]
+macro_rules! info {
+     (target: $target:expr, parent: $parent:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::INFO, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, parent: $parent:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::INFO, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::INFO, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::INFO, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::INFO, {}, $($arg)+)
+    );
+    (parent: $parent:expr, { $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            { $($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::INFO,
+            {},
+            $($arg)+
+        )
+    );
+    (target: $target:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, $crate::Level::INFO, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::INFO, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::INFO, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::INFO, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::INFO, {}, $($arg)+)
+    );
+    ({ $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    ($($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (%$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    ($($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { $($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (%$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { ?$($k).+ }
+        )
+    );
+    (%$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { %$($k).+ }
+        )
+    );
+    ($($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            { $($k).+ }
+        )
+    );
+    ($($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::INFO,
+            {},
+            $($arg)+
+        )
+    );
+}
+
+/// Constructs an event at the warn level.
+///
+/// This functions similarly to the [`event!`] macro. See [the top-level
+/// documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [`event!`]: macro.event.html
+/// [lib]: index.html#using-the-macros
+///
+/// # Examples
+///
+/// ```rust
+/// use tracing::warn;
+/// # fn main() {
+///
+/// let warn_description = "Invalid Input";
+/// let input = &[0x27, 0x45];
+///
+/// warn!(?input, warning = warn_description);
+/// warn!(
+///     target: "input_events",
+///     warning = warn_description,
+///     "Received warning for input: {:?}", input,
+/// );
+/// # }
+/// ```
+#[macro_export]
+macro_rules! warn {
+     (target: $target:expr, parent: $parent:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::WARN, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, parent: $parent:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::WARN, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::WARN, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::WARN, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::WARN, {}, $($arg)+)
+    );
+    (parent: $parent:expr, { $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            { $($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::WARN,
+            {},
+            $($arg)+
+        )
+    );
+    (target: $target:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, $crate::Level::WARN, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::WARN, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::WARN, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::WARN, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::WARN, {}, $($arg)+)
+    );
+    ({ $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    ($($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (%$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    ($($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { $($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (%$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { ?$($k).+ }
+        )
+    );
+    (%$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { %$($k).+ }
+        )
+    );
+    ($($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            { $($k).+ }
+        )
+    );
+    ($($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::WARN,
+            {},
+            $($arg)+
+        )
+    );
+}
+
+/// Constructs an event at the error level.
+///
+/// This functions similarly to the [`event!`] macro. See [the top-level
+/// documentation][lib] for details on the syntax accepted by
+/// this macro.
+///
+/// [`event!`]: macro.event.html
+/// [lib]: index.html#using-the-macros
+///
+/// # Examples
+///
+/// ```rust
+/// use tracing::error;
+/// # fn main() {
+///
+/// let (err_info, port) = ("No connection", 22);
+///
+/// error!(port, error = %err_info);
+/// error!(target: "app_events", "App Error: {}", err_info);
+/// error!({ info = err_info }, "error on port: {}", port);
+/// # }
+/// ```
+#[macro_export]
+macro_rules! error {
+     (target: $target:expr, parent: $parent:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::ERROR, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, parent: $parent:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::ERROR, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::ERROR, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::ERROR, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, parent: $parent:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, parent: $parent, $crate::Level::ERROR, {}, $($arg)+)
+    );
+    (parent: $parent:expr, { $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            { $($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, ?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, %$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (parent: $parent:expr, $($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            parent: $parent,
+            $crate::Level::ERROR,
+            {},
+            $($arg)+
+        )
+    );
+    (target: $target:expr, { $($field:tt)* }, $($arg:tt)* ) => (
+        $crate::event!(target: $target, $crate::Level::ERROR, { $($field)* }, $($arg)*)
+    );
+    (target: $target:expr, $($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::ERROR, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, ?$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::ERROR, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, %$($k:ident).+ $($field:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::ERROR, { $($k).+ $($field)+ })
+    );
+    (target: $target:expr, $($arg:tt)+ ) => (
+        $crate::event!(target: $target, $crate::Level::ERROR, {}, $($arg)+)
+    );
+    ({ $($field:tt)+ }, $($arg:tt)+ ) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { $($field)+ },
+            $($arg)+
+        )
+    );
+    ($($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { $($k).+ = $($field)*}
+        )
+    );
+    (?$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { ?$($k).+ = $($field)*}
+        )
+    );
+    (%$($k:ident).+ = $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::TRACE,
+            { %$($k).+ = $($field)*}
+        )
+    );
+    ($($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { $($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { ?$($k).+, $($field)*}
+        )
+    );
+    (%$($k:ident).+, $($field:tt)*) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { %$($k).+, $($field)*}
+        )
+    );
+    (?$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { ?$($k).+ }
+        )
+    );
+    (%$($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { %$($k).+ }
+        )
+    );
+    ($($k:ident).+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            { $($k).+ }
+        )
+    );
+    ($($arg:tt)+) => (
+        $crate::event!(
+            target: module_path!(),
+            $crate::Level::ERROR,
+            {},
+            $($arg)+
+        )
+    );
+}
+
+/// Constructs a new static callsite for a span or event.
+#[doc(hidden)]
+#[macro_export]
+macro_rules! callsite {
+    (name: $name:expr, kind: $kind:expr, fields: $($fields:tt)*) => {{
+        $crate::callsite! {
+            name: $name,
+            kind: $kind,
+            target: module_path!(),
+            level: $crate::Level::TRACE,
+            fields: $($fields)*
+        }
+    }};
+    (
+        name: $name:expr,
+        kind: $kind:expr,
+        level: $lvl:expr,
+        fields: $($fields:tt)*
+    ) => {{
+        $crate::callsite! {
+            name: $name,
+            kind: $kind,
+            target: module_path!(),
+            level: $lvl,
+            fields: $($fields)*
+        }
+    }};
+    (
+        name: $name:expr,
+        kind: $kind:expr,
+        target: $target:expr,
+        level: $lvl:expr,
+        fields: $($fields:tt)*
+    ) => {{
+        use $crate::__macro_support::MacroCallsite;
+        static META: $crate::Metadata<'static> = {
+            $crate::metadata! {
+                name: $name,
+                target: $target,
+                level: $lvl,
+                fields: $crate::fieldset!( $($fields)* ),
+                callsite: &CALLSITE,
+                kind: $kind,
+            }
+        };
+        static CALLSITE: MacroCallsite = MacroCallsite::new(&META);
+        CALLSITE.register();
+        &CALLSITE
+    }};
+}
+
+/// Constructs a new static callsite for a span or event.
+#[doc(hidden)]
+#[macro_export]
+macro_rules! callsite2 {
+    (name: $name:expr, kind: $kind:expr, fields: $($fields:tt)*) => {{
+        $crate::callsite2! {
+            name: $name,
+            kind: $kind,
+            target: module_path!(),
+            level: $crate::Level::TRACE,
+            fields: $($fields)*
+        }
+    }};
+    (
+        name: $name:expr,
+        kind: $kind:expr,
+        level: $lvl:expr,
+        fields: $($fields:tt)*
+    ) => {{
+        $crate::callsite2! {
+            name: $name,
+            kind: $kind,
+            target: module_path!(),
+            level: $lvl,
+            fields: $($fields)*
+        }
+    }};
+    (
+        name: $name:expr,
+        kind: $kind:expr,
+        target: $target:expr,
+        level: $lvl:expr,
+        fields: $($fields:tt)*
+    ) => {{
+        use $crate::__macro_support::MacroCallsite;
+        static META: $crate::Metadata<'static> = {
+            $crate::metadata! {
+                name: $name,
+                target: $target,
+                level: $lvl,
+                fields: $crate::fieldset!( $($fields)* ),
+                callsite: &CALLSITE,
+                kind: $kind,
+            }
+        };
+        MacroCallsite::new(&META)
+    }};
+}
+
+#[macro_export]
+// TODO: determine if this ought to be public API?`
+#[doc(hidden)]
+macro_rules! level_enabled {
+    ($lvl:expr) => {
+        $lvl <= $crate::level_filters::STATIC_MAX_LEVEL
+            && $lvl <= $crate::level_filters::LevelFilter::current()
+    };
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! valueset {
+
+    // === base case ===
+    (@ { $(,)* $($val:expr),* $(,)* }, $next:expr $(,)*) => {
+        &[ $($val),* ]
+    };
+
+    // === recursive case (more tts) ===
+
+    // TODO(#1138): determine a new syntax for uninitialized span fields, and
+    // re-enable this.
+    // (@{ $(,)* $($out:expr),* }, $next:expr, $($k:ident).+ = _, $($rest:tt)*) => {
+    //     $crate::valueset!(@ { $($out),*, (&$next, None) }, $next, $($rest)*)
+    // };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($k:ident).+ = ?$val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&debug(&$val) as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($k:ident).+ = %$val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&display(&$val) as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($k:ident).+ = $val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&$val as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($k:ident).+, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&$($k).+ as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, ?$($k:ident).+, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&debug(&$($k).+) as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, %$($k:ident).+, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&display(&$($k).+) as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($k:ident).+ = ?$val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&debug(&$val) as &Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($k:ident).+ = %$val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&display(&$val) as &Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($k:ident).+ = $val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&$val as &Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($k:ident).+) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&$($k).+ as &Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, ?$($k:ident).+) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&debug(&$($k).+) as &Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, %$($k:ident).+) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&display(&$($k).+) as &Value)) },
+            $next,
+        )
+    };
+
+    // Handle literal names
+    (@ { $(,)* $($out:expr),* }, $next:expr, $k:literal = ?$val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&debug(&$val) as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $k:literal = %$val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&display(&$val) as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $k:literal = $val:expr, $($rest:tt)*) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&$val as &Value)) },
+            $next,
+            $($rest)*
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $k:literal = ?$val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&debug(&$val) as &Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $k:literal = %$val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&display(&$val) as &Value)) },
+            $next,
+        )
+    };
+    (@ { $(,)* $($out:expr),* }, $next:expr, $k:literal = $val:expr) => {
+        $crate::valueset!(
+            @ { $($out),*, (&$next, Some(&$val as &Value)) },
+            $next,
+        )
+    };
+
+    // Remainder is unparseable, but exists --- must be format args!
+    (@ { $(,)* $($out:expr),* }, $next:expr, $($rest:tt)+) => {
+        $crate::valueset!(@ { (&$next, Some(&format_args!($($rest)+) as &Value)), $($out),* }, $next, )
+    };
+
+    // === entry ===
+    ($fields:expr, $($kvs:tt)+) => {
+        {
+            #[allow(unused_imports)]
+            use $crate::field::{debug, display, Value};
+            let mut iter = $fields.iter();
+            $fields.value_set($crate::valueset!(
+                @ { },
+                iter.next().expect("FieldSet corrupted (this is a bug)"),
+                $($kvs)+
+            ))
+        }
+    };
+    ($fields:expr,) => {
+        {
+            $fields.value_set(&[])
+        }
+    };
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! fieldset {
+    // == base case ==
+    (@ { $(,)* $($out:expr),* $(,)* } $(,)*) => {
+        &[ $($out),* ]
+    };
+
+    // == recursive cases (more tts) ==
+    (@ { $(,)* $($out:expr),* } $($k:ident).+ = ?$val:expr, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $crate::__tracing_stringify!($($k).+) } $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* } $($k:ident).+ = %$val:expr, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $crate::__tracing_stringify!($($k).+) } $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* } $($k:ident).+ = $val:expr, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $crate::__tracing_stringify!($($k).+) } $($rest)*)
+    };
+    // TODO(#1138): determine a new syntax for uninitialized span fields, and
+    // re-enable this.
+    // (@ { $($out:expr),* } $($k:ident).+ = _, $($rest:tt)*) => {
+    //     $crate::fieldset!(@ { $($out),*, $crate::__tracing_stringify!($($k).+) } $($rest)*)
+    // };
+    (@ { $(,)* $($out:expr),* } ?$($k:ident).+, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $crate::__tracing_stringify!($($k).+) } $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* } %$($k:ident).+, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $crate::__tracing_stringify!($($k).+) } $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* } $($k:ident).+, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $crate::__tracing_stringify!($($k).+) } $($rest)*)
+    };
+
+    // Handle literal names
+    (@ { $(,)* $($out:expr),* } $k:literal = ?$val:expr, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $k } $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* } $k:literal = %$val:expr, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $k } $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* } $k:literal = $val:expr, $($rest:tt)*) => {
+        $crate::fieldset!(@ { $($out),*, $k } $($rest)*)
+    };
+
+    // Remainder is unparseable, but exists --- must be format args!
+    (@ { $(,)* $($out:expr),* } $($rest:tt)+) => {
+        $crate::fieldset!(@ { "message", $($out),*, })
+    };
+
+    // == entry ==
+    ($($args:tt)*) => {
+        $crate::fieldset!(@ { } $($args)*,)
+    };
+
+}
+
+#[cfg(feature = "log")]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! level_to_log {
+    ($level:expr) => {
+        match $level {
+            $crate::Level::ERROR => $crate::log::Level::Error,
+            $crate::Level::WARN => $crate::log::Level::Warn,
+            $crate::Level::INFO => $crate::log::Level::Info,
+            $crate::Level::DEBUG => $crate::log::Level::Debug,
+            _ => $crate::log::Level::Trace,
+        }
+    };
+}
+
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __tracing_stringify {
+    ($s:expr) => {
+        stringify!($s)
+    };
+}
+
+#[cfg(not(feature = "log"))]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __tracing_log {
+    (target: $target:expr, $level:expr, $($field:tt)+ ) => {};
+}
+
+#[cfg(feature = "log")]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __mk_format_string {
+    // === base case ===
+    (@ { $(,)* $($out:expr),* $(,)* } $(,)*) => {
+        concat!( $($out),*)
+    };
+
+    // === recursive case (more tts), ===
+    // ====== shorthand field syntax ===
+    (@ { $(,)* $($out:expr),* }, ?$($k:ident).+, $($rest:tt)*) => {
+        $crate::__mk_format_string!(@ { $($out),*, $crate::__tracing_stringify!($($k).+), "={:?} " }, $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* }, %$($k:ident).+, $($rest:tt)*) => {
+        $crate::__mk_format_string!(@ { $($out),*, $crate::__tracing_stringify!($($k).+), "={} " }, $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* }, $($k:ident).+, $($rest:tt)*) => {
+        $crate::__mk_format_string!(@ { $($out),*, $crate::__tracing_stringify!($($k).+), "={:?} " }, $($rest)*)
+    };
+    // ====== kv field syntax ===
+    (@ { $(,)* $($out:expr),* }, message = $val:expr, $($rest:tt)*) => {
+        $crate::__mk_format_string!(@ { $($out),*, "{} " }, $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* }, $($k:ident).+ = ?$val:expr, $($rest:tt)*) => {
+        $crate::__mk_format_string!(@ { $($out),*, $crate::__tracing_stringify!($($k).+), "={:?} " }, $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* }, $($k:ident).+ = %$val:expr, $($rest:tt)*) => {
+        $crate::__mk_format_string!(@ { $($out),*, $crate::__tracing_stringify!($($k).+), "={} " }, $($rest)*)
+    };
+    (@ { $(,)* $($out:expr),* }, $($k:ident).+ = $val:expr, $($rest:tt)*) => {
+        $crate::__mk_format_string!(@ { $($out),*, $crate::__tracing_stringify!($($k).+), "={:?} " }, $($rest)*)
+    };
+
+    // === rest is unparseable --- must be fmt args ===
+    (@ { $(,)* $($out:expr),* }, $($rest:tt)+) => {
+        $crate::__mk_format_string!(@ { "{} ", $($out),* }, )
+    };
+
+    // === entry ===
+    ($($kvs:tt)+) => {
+        $crate::__mk_format_string!(@ { }, $($kvs)+,)
+    };
+    () => {
+        ""
+    }
+}
+
+#[cfg(feature = "log")]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __mk_format_args {
+    // === finished --- called into by base cases ===
+    (@ { $(,)* $($out:expr),* $(,)* }, $fmt:expr, fields: $(,)*) => {
+        format_args!($fmt, $($out),*)
+    };
+
+    // === base case (no more tts) ===
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($k:ident).+ = ?$val:expr $(,)*) => {
+        $crate::__mk_format_args!(@ { $($out),*, $val }, $fmt, fields: )
+    };
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($k:ident).+ = %$val:expr $(,)*) => {
+        $crate::__mk_format_args!(@ { $($out),*, $val }, $fmt, fields: )
+    };
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($k:ident).+ = $val:expr $(,)*) => {
+        $crate::__mk_format_args!(@ { $($out),*, $val }, $fmt, fields: )
+    };
+    // ====== shorthand field syntax ===
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: ?$($k:ident).+ $(,)*) => {
+        $crate::__mk_format_args!(@ { $($out),*, &$($k).+ }, $fmt, fields:)
+    };
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: %$($k:ident).+ $(,)*) => {
+        $crate::__mk_format_args!(@ { $($out),*, &$($k).+ }, $fmt, fields: )
+    };
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($k:ident).+ $(,)*) => {
+        $crate::__mk_format_args!(@ { $($out),*, &$($k).+ }, $fmt, fields: )
+    };
+
+    // === recursive case (more tts) ===
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($k:ident).+ = ?$val:expr, $($rest:tt)+) => {
+        $crate::__mk_format_args!(@ { $($out),*, $val }, $fmt, fields: $($rest)+)
+    };
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($k:ident).+ = %$val:expr, $($rest:tt)+) => {
+        $crate::__mk_format_args!(@ { $($out),*, $val }, $fmt, fields: $($rest)+)
+    };
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($k:ident).+ = $val:expr, $($rest:tt)+) => {
+        $crate::__mk_format_args!(@ { $($out),*, $val }, $fmt, fields: $($rest)+)
+    };
+    // ====== shorthand field syntax ===
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: ?$($k:ident).+, $($rest:tt)+) => {
+        $crate::__mk_format_args!(@ { $($out),*, &$($k).+ }, $fmt, fields: $($rest)+)
+    };
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: %$($k:ident).+, $($rest:tt)+) => {
+        $crate::__mk_format_args!(@ { $($out),*, &$($k).+ }, $fmt, fields: $($rest)+)
+    };
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($k:ident).+, $($rest:tt)+) => {
+        $crate::__mk_format_args!(@ { $($out),*, &$($k).+ }, $fmt, fields: $($rest)+)
+    };
+
+    // === rest is unparseable --- must be fmt args ===
+    (@ { $(,)* $($out:expr),* }, $fmt:expr, fields: $($rest:tt)+) => {
+        $crate::__mk_format_args!(@ { format_args!($($rest)+), $($out),* }, $fmt, fields: )
+    };
+
+    // === entry ===
+    ($($kv:tt)*) => {
+        {
+            #[allow(unused_imports)]
+            use $crate::field::{debug, display};
+            // use $crate::__mk_format_string;
+            $crate::__mk_format_args!(@ { }, $crate::__mk_format_string!($($kv)*), fields: $($kv)*)
+        }
+    };
+}
+
+#[cfg(feature = "log")]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __tracing_log {
+    (target: $target:expr, $level:expr, $($field:tt)+ ) => {
+        $crate::if_log_enabled! {{
+            use $crate::log;
+            let level = $crate::level_to_log!($level);
+            if level <= log::STATIC_MAX_LEVEL && level <= log::max_level() {
+                let log_meta = log::Metadata::builder()
+                    .level(level)
+                    .target($target)
+                    .build();
+                let logger = log::logger();
+                if logger.enabled(&log_meta) {
+                    logger.log(&log::Record::builder()
+                        .file(Some(file!()))
+                        .module_path(Some(module_path!()))
+                        .line(Some(line!()))
+                        .metadata(log_meta)
+                        .args($crate::__mk_format_args!($($field)+))
+                        .build());
+                }
+            }
+        }}
+    };
+}
+
+#[cfg(not(feature = "log"))]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! if_log_enabled {
+    ($e:expr;) => {
+        $crate::if_log_enabled! { $e }
+    };
+    ($if_log:block) => {
+        $crate::if_log_enabled! { $if_log else {} }
+    };
+    ($if_log:block else $else_block:block) => {
+        $else_block
+    };
+}
+
+#[cfg(all(feature = "log", not(feature = "log-always")))]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! if_log_enabled {
+    ($e:expr;) => {
+        $crate::if_log_enabled! { $e }
+    };
+    ($if_log:block) => {
+        $crate::if_log_enabled! { $if_log else {} }
+    };
+    ($if_log:block else $else_block:block) => {
+        if !$crate::dispatcher::has_been_set() {
+            $if_log
+        } else {
+            $else_block
+        }
+    };
+}
+
+#[cfg(all(feature = "log", feature = "log-always"))]
+#[doc(hidden)]
+#[macro_export]
+macro_rules! if_log_enabled {
+    ($e:expr;) => {
+        $crate::if_log_enabled! { $e }
+    };
+    ($if_log:block) => {
+        $crate::if_log_enabled! { $if_log else {} }
+    };
+    ($if_log:block else $else_block:block) => {
+        #[allow(unused_braces)]
+        $if_log
+    };
+}
diff --git a/third_party/rust/tracing/src/span.rs b/third_party/rust/tracing/src/span.rs
new file mode 100644
index 0000000000..c1e0ee1a0c
--- /dev/null
+++ b/third_party/rust/tracing/src/span.rs
@@ -0,0 +1,1283 @@
+//! Spans represent periods of time in which a program was executing in a
+//! particular context.
+//!
+//! A span consists of [fields], user-defined key-value pairs of arbitrary data
+//! that describe the context the span represents, and a set of fixed attributes
+//! that describe all `tracing` spans and events. Attributes describing spans
+//! include:
+//!
+//! - An [`Id`] assigned by the subscriber that uniquely identifies it in relation
+//!   to other spans.
+//! - The span's [parent] in the trace tree.
+//! - [Metadata] that describes static characteristics of all spans
+//!   originating from that callsite, such as its name, source code location,
+//!   [verbosity level], and the names of its fields.
+//!
+//! # Creating Spans
+//!
+//! Spans are created using the [`span!`] macro. This macro is invoked with the
+//! following arguments, in order:
+//!
+//! - The [`target`] and/or [`parent`][parent] attributes, if the user wishes to
+//!   override their default values.
+//! - The span's [verbosity level]
+//! - A string literal providing the span's name.
+//! - Finally, between zero and 32 arbitrary key/value fields.
+//!
+//! [`target`]: ../struct.Metadata.html#method.target
+//!
+//! For example:
+//! ```rust
+//! use tracing::{span, Level};
+//!
+//! /// Construct a new span at the `INFO` level named "my_span", with a single
+//! /// field named answer , with the value `42`.
+//! let my_span = span!(Level::INFO, "my_span", answer = 42);
+//! ```
+//!
+//! The documentation for the [`span!`] macro provides additional examples of
+//! the various options that exist when creating spans.
+//!
+//! The [`trace_span!`], [`debug_span!`], [`info_span!`], [`warn_span!`], and
+//! [`error_span!`] exist as shorthand for constructing spans at various
+//! verbosity levels.
+//!
+//! ## Recording Span Creation
+//!
+//! The [`Attributes`] type contains data associated with a span, and is
+//! provided to the [`Subscriber`] when a new span is created. It contains
+//! the span's metadata, the ID of [the span's parent][parent] if one was
+//! explicitly set, and any fields whose values were recorded when the span was
+//! constructed. The subscriber, which is responsible for recording `tracing`
+//! data, can then store or record these values.
+//!
+//! # The Span Lifecycle
+//!
+//! ## Entering a Span
+//!
+//! A thread of execution is said to _enter_ a span when it begins executing,
+//! and _exit_ the span when it switches to another context. Spans may be
+//! entered through the [`enter`] and [`in_scope`] methods.
+//!
+//! The `enter` method enters a span, returning a [guard] that exits the span
+//! when dropped
+//! ```
+//! # #[macro_use] extern crate tracing;
+//! # use tracing::Level;
+//! let my_var: u64 = 5;
+//! let my_span = span!(Level::TRACE, "my_span", my_var);
+//!
+//! // `my_span` exists but has not been entered.
+//!
+//! // Enter `my_span`...
+//! let _enter = my_span.enter();
+//!
+//! // Perform some work inside of the context of `my_span`...
+//! // Dropping the `_enter` guard will exit the span.
+//!```
+//!
+//! <div class="information">
+//!     <div class="tooltip compile_fail" style="">&#x26a0; &#xfe0f;<span class="tooltiptext">Warning</span></div>
+//! </div><div class="example-wrap" style="display:inline-block"><pre class="compile_fail" style="white-space:normal;font:inherit;">
+//!     <strong>Warning</strong>: In asynchronous code that uses async/await syntax,
+//!     <code>Span::enter</code> may produce incorrect traces if the returned drop
+//!     guard is held across an await point. See
+//!     <a href="struct.Span.html#in-asynchronous-code">the method documentation</a>
+//!     for details.
+//! </pre></div>
+//!
+//! `in_scope` takes a closure or function pointer and executes it inside the
+//! span.
+//! ```
+//! # #[macro_use] extern crate tracing;
+//! # use tracing::Level;
+//! let my_var: u64 = 5;
+//! let my_span = span!(Level::TRACE, "my_span", my_var = &my_var);
+//!
+//! my_span.in_scope(|| {
+//!     // perform some work in the context of `my_span`...
+//! });
+//!
+//! // Perform some work outside of the context of `my_span`...
+//!
+//! my_span.in_scope(|| {
+//!     // Perform some more work in the context of `my_span`.
+//! });
+//! ```
+//!
+//! <div class="information">
+//!     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
+//! </div>
+//! <div class="example-wrap" style="display:inline-block">
+//! <pre class="ignore" style="white-space:normal;font:inherit;">
+//! <strong>Note</strong>: Since entering a span takes <code>&self</code<, and
+//! <code>Span</code>s are <code>Clone</code>, <code>Send</code>, and
+//! <code>Sync</code>, it is entirely valid for multiple threads to enter the
+//! same span concurrently.
+//! </pre></div>
+//!
+//! ## Span Relationships
+//!
+//! Spans form a tree structure — unless it is a root span, all spans have a
+//! _parent_, and may have one or more _children_. When a new span is created,
+//! the current span becomes the new span's parent. The total execution time of
+//! a span consists of the time spent in that span and in the entire subtree
+//! represented by its children. Thus, a parent span always lasts for at least
+//! as long as the longest-executing span in its subtree.
+//!
+//! ```
+//! # #[macro_use] extern crate tracing;
+//! # use tracing::Level;
+//! // this span is considered the "root" of a new trace tree:
+//! span!(Level::INFO, "root").in_scope(|| {
+//!     // since we are now inside "root", this span is considered a child
+//!     // of "root":
+//!     span!(Level::DEBUG, "outer_child").in_scope(|| {
+//!         // this span is a child of "outer_child", which is in turn a
+//!         // child of "root":
+//!         span!(Level::TRACE, "inner_child").in_scope(|| {
+//!             // and so on...
+//!         });
+//!     });
+//!     // another span created here would also be a child of "root".
+//! });
+//!```
+//!
+//! In addition, the parent of a span may be explicitly specified in
+//! the `span!` macro. For example:
+//!
+//! ```rust
+//! # #[macro_use] extern crate tracing;
+//! # use tracing::Level;
+//! // Create, but do not enter, a span called "foo".
+//! let foo = span!(Level::INFO, "foo");
+//!
+//! // Create and enter a span called "bar".
+//! let bar = span!(Level::INFO, "bar");
+//! let _enter = bar.enter();
+//!
+//! // Although we have currently entered "bar", "baz"'s parent span
+//! // will be "foo".
+//! let baz = span!(parent: &foo, Level::INFO, "baz");
+//! ```
+//!
+//! A child span should typically be considered _part_ of its parent. For
+//! example, if a subscriber is recording the length of time spent in various
+//! spans, it should generally include the time spent in a span's children as
+//! part of that span's duration.
+//!
+//! In addition to having zero or one parent, a span may also _follow from_ any
+//! number of other spans. This indicates a causal relationship between the span
+//! and the spans that it follows from, but a follower is *not* typically
+//! considered part of the duration of the span it follows. Unlike the parent, a
+//! span may record that it follows from another span after it is created, using
+//! the [`follows_from`] method.
+//!
+//! As an example, consider a listener task in a server. As the listener accepts
+//! incoming connections, it spawns new tasks that handle those connections. We
+//! might want to have a span representing the listener, and instrument each
+//! spawned handler task with its own span. We would want our instrumentation to
+//! record that the handler tasks were spawned as a result of the listener task.
+//! However, we might not consider the handler tasks to be _part_ of the time
+//! spent in the listener task, so we would not consider those spans children of
+//! the listener span. Instead, we would record that the handler tasks follow
+//! from the listener, recording the causal relationship but treating the spans
+//! as separate durations.
+//!
+//! ## Closing Spans
+//!
+//! Execution may enter and exit a span multiple times before that span is
+//! _closed_. Consider, for example, a future which has an associated
+//! span and enters that span every time it is polled:
+//! ```rust
+//! # extern crate tracing;
+//! # extern crate futures;
+//! # use futures::{Future, Poll, Async};
+//! struct MyFuture {
+//!    // data
+//!    span: tracing::Span,
+//! }
+//!
+//! impl Future for MyFuture {
+//!     type Item = ();
+//!     type Error = ();
+//!
+//!     fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
+//!         let _enter = self.span.enter();
+//!         // Do actual future work...
+//! # Ok(Async::Ready(()))
+//!     }
+//! }
+//! ```
+//!
+//! If this future was spawned on an executor, it might yield one or more times
+//! before `poll` returns `Ok(Async::Ready)`. If the future were to yield, then
+//! the executor would move on to poll the next future, which may _also_ enter
+//! an associated span or series of spans. Therefore, it is valid for a span to
+//! be entered repeatedly before it completes. Only the time when that span or
+//! one of its children was the current span is considered to be time spent in
+//! that span. A span which is not executing and has not yet been closed is said
+//! to be _idle_.
+//!
+//! Because spans may be entered and exited multiple times before they close,
+//! [`Subscriber`]s have separate trait methods which are called to notify them
+//! of span exits and when span handles are dropped. When execution exits a
+//! span, [`exit`] will always be called with that span's ID to notify the
+//! subscriber that the span has been exited. When span handles are dropped, the
+//! [`drop_span`] method is called with that span's ID. The subscriber may use
+//! this to determine whether or not the span will be entered again.
+//!
+//! If there is only a single handle with the capacity to exit a span, dropping
+//! that handle "closes" the span, since the capacity to enter it no longer
+//! exists. For example:
+//! ```
+//! # #[macro_use] extern crate tracing;
+//! # use tracing::Level;
+//! {
+//!     span!(Level::TRACE, "my_span").in_scope(|| {
+//!         // perform some work in the context of `my_span`...
+//!     }); // --> Subscriber::exit(my_span)
+//!
+//!     // The handle to `my_span` only lives inside of this block; when it is
+//!     // dropped, the subscriber will be informed via `drop_span`.
+//!
+//! } // --> Subscriber::drop_span(my_span)
+//! ```
+//!
+//! However, if multiple handles exist, the span can still be re-entered even if
+//! one or more is dropped. For determining when _all_ handles to a span have
+//! been dropped, `Subscriber`s have a [`clone_span`] method, which is called
+//! every time a span handle is cloned. Combined with `drop_span`, this may be
+//! used to track the number of handles to a given span — if `drop_span` has
+//! been called one more time than the number of calls to `clone_span` for a
+//! given ID, then no more handles to the span with that ID exist. The
+//! subscriber may then treat it as closed.
+//!
+//! # When to use spans
+//!
+//! As a rule of thumb, spans should be used to represent discrete units of work
+//! (e.g., a given request's lifetime in a server) or periods of time spent in a
+//! given context (e.g., time spent interacting with an instance of an external
+//! system, such as a database).
+//!
+//! Which scopes in a program correspond to new spans depend somewhat on user
+//! intent. For example, consider the case of a loop in a program. Should we
+//! construct one span and perform the entire loop inside of that span, like:
+//!
+//! ```rust
+//! # #[macro_use] extern crate tracing;
+//! # use tracing::Level;
+//! # let n = 1;
+//! let span = span!(Level::TRACE, "my_loop");
+//! let _enter = span.enter();
+//! for i in 0..n {
+//!     # let _ = i;
+//!     // ...
+//! }
+//! ```
+//! Or, should we create a new span for each iteration of the loop, as in:
+//! ```rust
+//! # #[macro_use] extern crate tracing;
+//! # use tracing::Level;
+//! # let n = 1u64;
+//! for i in 0..n {
+//!     let span = span!(Level::TRACE, "my_loop", iteration = i);
+//!     let _enter = span.enter();
+//!     // ...
+//! }
+//! ```
+//!
+//! Depending on the circumstances, we might want to do either, or both. For
+//! example, if we want to know how long was spent in the loop overall, we would
+//! create a single span around the entire loop; whereas if we wanted to know how
+//! much time was spent in each individual iteration, we would enter a new span
+//! on every iteration.
+//!
+//! [fields]: ../field/index.html
+//! [Metadata]: ../struct.Metadata.html
+//! [`Id`]: struct.Id.html
+//! [verbosity level]: ../struct.Level.html
+//! [`span!`]: ../macro.span.html
+//! [`trace_span!`]: ../macro.trace_span.html
+//! [`debug_span!`]: ../macro.debug_span.html
+//! [`info_span!`]: ../macro.info_span.html
+//! [`warn_span!`]: ../macro.warn_span.html
+//! [`error_span!`]: ../macro.error_span.html
+//! [`clone_span`]: ../subscriber/trait.Subscriber.html#method.clone_span
+//! [`drop_span`]: ../subscriber/trait.Subscriber.html#method.drop_span
+//! [`exit`]: ../subscriber/trait.Subscriber.html#tymethod.exit
+//! [`Subscriber`]: ../subscriber/trait.Subscriber.html
+//! [`Attributes`]: struct.Attributes.html
+//! [`enter`]: struct.Span.html#method.enter
+//! [`in_scope`]: struct.Span.html#method.in_scope
+//! [`follows_from`]: struct.Span.html#method.follows_from
+//! [guard]: struct.Entered.html
+//! [parent]: #span-relationships
+pub use tracing_core::span::{Attributes, Id, Record};
+
+use crate::stdlib::{
+    cmp, fmt,
+    hash::{Hash, Hasher},
+};
+use crate::{
+    dispatcher::{self, Dispatch},
+    field, Metadata,
+};
+
+/// Trait implemented by types which have a span `Id`.
+pub trait AsId: crate::sealed::Sealed {
+    /// Returns the `Id` of the span that `self` corresponds to, or `None` if
+    /// this corresponds to a disabled span.
+    fn as_id(&self) -> Option<&Id>;
+}
+
+/// A handle representing a span, with the capability to enter the span if it
+/// exists.
+///
+/// If the span was rejected by the current `Subscriber`'s filter, entering the
+/// span will silently do nothing. Thus, the handle can be used in the same
+/// manner regardless of whether or not the trace is currently being collected.
+#[derive(Clone)]
+pub struct Span {
+    /// A handle used to enter the span when it is not executing.
+    ///
+    /// If this is `None`, then the span has either closed or was never enabled.
+    inner: Option<Inner>,
+    /// Metadata describing the span.
+    ///
+    /// This might be `Some` even if `inner` is `None`, in the case that the
+    /// span is disabled but the metadata is needed for `log` support.
+    meta: Option<&'static Metadata<'static>>,
+}
+
+/// A handle representing the capacity to enter a span which is known to exist.
+///
+/// Unlike `Span`, this type is only constructed for spans which _have_ been
+/// enabled by the current filter. This type is primarily used for implementing
+/// span handles; users should typically not need to interact with it directly.
+#[derive(Debug)]
+pub(crate) struct Inner {
+    /// The span's ID, as provided by `subscriber`.
+    id: Id,
+
+    /// The subscriber that will receive events relating to this span.
+    ///
+    /// This should be the same subscriber that provided this span with its
+    /// `id`.
+    subscriber: Dispatch,
+}
+
+/// A guard representing a span which has been entered and is currently
+/// executing.
+///
+/// When the guard is dropped, the span will be exited.
+///
+/// This is returned by the [`Span::enter`] function.
+///
+/// [`Span::enter`]: ../struct.Span.html#method.enter
+#[derive(Debug)]
+#[must_use = "once a span has been entered, it should be exited"]
+pub struct Entered<'a> {
+    span: &'a Span,
+}
+
+/// `log` target for all span lifecycle (creation/enter/exit/close) records.
+#[cfg(feature = "log")]
+const LIFECYCLE_LOG_TARGET: &str = "tracing::span";
+/// `log` target for span activity (enter/exit) records.
+#[cfg(feature = "log")]
+const ACTIVITY_LOG_TARGET: &str = "tracing::span::active";
+
+// ===== impl Span =====
+
+impl Span {
+    /// Constructs a new `Span` with the given [metadata] and set of
+    /// [field values].
+    ///
+    /// The new span will be constructed by the currently-active [`Subscriber`],
+    /// with the current span as its parent (if one exists).
+    ///
+    /// After the span is constructed, [field values] and/or [`follows_from`]
+    /// annotations may be added to it.
+    ///
+    /// [metadata]: ../metadata
+    /// [`Subscriber`]: ../subscriber/trait.Subscriber.html
+    /// [field values]: ../field/struct.ValueSet.html
+    /// [`follows_from`]: ../struct.Span.html#method.follows_from
+    pub fn new(meta: &'static Metadata<'static>, values: &field::ValueSet<'_>) -> Span {
+        dispatcher::get_default(|dispatch| Self::new_with(meta, values, dispatch))
+    }
+
+    #[inline]
+    #[doc(hidden)]
+    pub fn new_with(
+        meta: &'static Metadata<'static>,
+        values: &field::ValueSet<'_>,
+        dispatch: &Dispatch,
+    ) -> Span {
+        let new_span = Attributes::new(meta, values);
+        Self::make_with(meta, new_span, dispatch)
+    }
+
+    /// Constructs a new `Span` as the root of its own trace tree, with the
+    /// given [metadata] and set of [field values].
+    ///
+    /// After the span is constructed, [field values] and/or [`follows_from`]
+    /// annotations may be added to it.
+    ///
+    /// [metadata]: ../metadata
+    /// [field values]: ../field/struct.ValueSet.html
+    /// [`follows_from`]: ../struct.Span.html#method.follows_from
+    pub fn new_root(meta: &'static Metadata<'static>, values: &field::ValueSet<'_>) -> Span {
+        dispatcher::get_default(|dispatch| Self::new_root_with(meta, values, dispatch))
+    }
+
+    #[inline]
+    #[doc(hidden)]
+    pub fn new_root_with(
+        meta: &'static Metadata<'static>,
+        values: &field::ValueSet<'_>,
+        dispatch: &Dispatch,
+    ) -> Span {
+        let new_span = Attributes::new_root(meta, values);
+        Self::make_with(meta, new_span, dispatch)
+    }
+
+    /// Constructs a new `Span` as child of the given parent span, with the
+    /// given [metadata] and set of [field values].
+    ///
+    /// After the span is constructed, [field values] and/or [`follows_from`]
+    /// annotations may be added to it.
+    ///
+    /// [metadata]: ../metadata
+    /// [field values]: ../field/struct.ValueSet.html
+    /// [`follows_from`]: ../struct.Span.html#method.follows_from
+    pub fn child_of(
+        parent: impl Into<Option<Id>>,
+        meta: &'static Metadata<'static>,
+        values: &field::ValueSet<'_>,
+    ) -> Span {
+        let mut parent = parent.into();
+        dispatcher::get_default(move |dispatch| {
+            Self::child_of_with(Option::take(&mut parent), meta, values, dispatch)
+        })
+    }
+
+    #[inline]
+    #[doc(hidden)]
+    pub fn child_of_with(
+        parent: impl Into<Option<Id>>,
+        meta: &'static Metadata<'static>,
+        values: &field::ValueSet<'_>,
+        dispatch: &Dispatch,
+    ) -> Span {
+        let new_span = match parent.into() {
+            Some(parent) => Attributes::child_of(parent, meta, values),
+            None => Attributes::new_root(meta, values),
+        };
+        Self::make_with(meta, new_span, dispatch)
+    }
+
+    /// Constructs a new disabled span with the given `Metadata`.
+    ///
+    /// This should be used when a span is constructed from a known callsite,
+    /// but the subscriber indicates that it is disabled.
+    ///
+    /// Entering, exiting, and recording values on this span will not notify the
+    /// `Subscriber` but _may_ record log messages if the `log` feature flag is
+    /// enabled.
+    #[inline(always)]
+    pub fn new_disabled(meta: &'static Metadata<'static>) -> Span {
+        Self {
+            inner: None,
+            meta: Some(meta),
+        }
+    }
+
+    /// Constructs a new span that is *completely disabled*.
+    ///
+    /// This can be used rather than `Option<Span>` to represent cases where a
+    /// span is not present.
+    ///
+    /// Entering, exiting, and recording values on this span will do nothing.
+    #[inline(always)]
+    pub const fn none() -> Span {
+        Self {
+            inner: None,
+            meta: None,
+        }
+    }
+
+    /// Returns a handle to the span [considered by the `Subscriber`] to be the
+    /// current span.
+    ///
+    /// If the subscriber indicates that it does not track the current span, or
+    /// that the thread from which this function is called is not currently
+    /// inside a span, the returned span will be disabled.
+    ///
+    /// [considered by the `Subscriber`]: ../subscriber/trait.Subscriber.html#method.current
+    pub fn current() -> Span {
+        dispatcher::get_default(|dispatch| {
+            if let Some((id, meta)) = dispatch.current_span().into_inner() {
+                let id = dispatch.clone_span(&id);
+                Self {
+                    inner: Some(Inner::new(id, dispatch)),
+                    meta: Some(meta),
+                }
+            } else {
+                Self::none()
+            }
+        })
+    }
+
+    fn make_with(
+        meta: &'static Metadata<'static>,
+        new_span: Attributes<'_>,
+        dispatch: &Dispatch,
+    ) -> Span {
+        let attrs = &new_span;
+        let id = dispatch.new_span(attrs);
+        let inner = Some(Inner::new(id, dispatch));
+
+        let span = Self {
+            inner,
+            meta: Some(meta),
+        };
+
+        if_log_enabled! {{
+            let target = if attrs.is_empty() {
+                LIFECYCLE_LOG_TARGET
+            } else {
+                meta.target()
+            };
+            span.log(target, level_to_log!(*meta.level()), format_args!("++ {}{}", meta.name(), FmtAttrs(attrs)));
+        }}
+
+        span
+    }
+    /// Enters this span, returning a guard that will exit the span when dropped.
+    ///
+    /// If this span is enabled by the current subscriber, then this function will
+    /// call [`Subscriber::enter`] with the span's [`Id`], and dropping the guard
+    /// will call [`Subscriber::exit`]. If the span is disabled, this does
+    /// nothing.
+    ///
+    /// # In Asynchronous Code
+    ///
+    /// **Warning**: in asynchronous code that uses [async/await syntax][syntax],
+    /// `Span::enter` should be used very carefully or avoided entirely. Holding
+    /// the drop guard returned by `Span::enter` across `.await` points will
+    /// result in incorrect traces.
+    ///
+    /// For example,
+    ///
+    /// ```
+    /// # use tracing::info_span;
+    /// # async fn some_other_async_function() {}
+    /// async fn my_async_function() {
+    ///     let span = info_span!("my_async_function");
+    ///
+    ///     // THIS WILL RESULT IN INCORRECT TRACES
+    ///     let _enter = span.enter();
+    ///     some_other_async_function().await;
+    ///
+    ///     // ...
+    /// }
+    /// ```
+    ///
+    /// The drop guard returned by `Span::enter` exits the span when it is
+    /// dropped. When an async function or async block yields at an `.await`
+    /// point, the current scope is _exited_, but values in that scope are
+    /// **not** dropped (because the async block will eventually resume
+    /// execution from that await point). This means that _another_ task will
+    /// begin executing while _remaining_ in the entered span. This results in
+    /// an incorrect trace.
+    ///
+    /// Instead of using `Span::enter` in asynchronous code, prefer the
+    /// following:
+    ///
+    /// * To enter a span for a synchronous section of code within an async
+    ///   block or function, prefer [`Span::in_scope`]. Since `in_scope` takes a
+    ///   synchronous closure and exits the span when the closure returns, the
+    ///   span will always be exited before the next await point. For example:
+    ///   ```
+    ///   # use tracing::info_span;
+    ///   # async fn some_other_async_function(_: ()) {}
+    ///   async fn my_async_function() {
+    ///       let span = info_span!("my_async_function");
+    ///
+    ///       let some_value = span.in_scope(|| {
+    ///           // run some synchronous code inside the span...
+    ///       });
+    ///
+    ///       // This is okay! The span has already been exited before we reach
+    ///       // the await point.
+    ///       some_other_async_function(some_value).await;
+    ///  
+    ///       // ...
+    ///   }
+    ///   ```
+    /// * For instrumenting asynchronous code, `tracing` provides the
+    ///   [`Future::instrument` combinator][instrument] for
+    ///   attaching a span to a future (async function or block). This will
+    ///   enter the span _every_ time the future is polled, and exit it whenever
+    ///   the future yields.
+    ///   
+    ///   `Instrument` can be used with an async block inside an async function:
+    ///   ```ignore
+    ///   # use tracing::info_span;
+    ///   use tracing::Instrument;
+    ///
+    ///   # async fn some_other_async_function() {}
+    ///   async fn my_async_function() {
+    ///       let span = info_span!("my_async_function");
+    ///       async move {
+    ///          // This is correct! If we yield here, the span will be exited,
+    ///          // and re-entered when we resume.
+    ///          some_other_async_function().await;
+    ///
+    ///          //more asynchronous code inside the span...
+    ///
+    ///       }
+    ///         // instrument the async block with the span...
+    ///         .instrument(span)
+    ///         // ...and await it.
+    ///         .await
+    ///   }
+    ///   ```
+    ///
+    ///   It can also be used to instrument calls to async functions at the
+    ///   callsite:
+    ///   ```ignore
+    ///   # use tracing::debug_span;
+    ///   use tracing::Instrument;
+    ///
+    ///   # async fn some_other_async_function() {}
+    ///   async fn my_async_function() {
+    ///       let some_value = some_other_async_function()
+    ///          .instrument(debug_span!("some_other_async_function"))
+    ///          .await;
+    ///
+    ///       // ...
+    ///   }
+    ///   ```
+    ///
+    /// * The [`#[instrument]` attribute macro][attr] can automatically generate
+    ///   correct code when used on an async function:
+    ///
+    ///   ```ignore
+    ///   # async fn some_other_async_function() {}
+    ///   #[tracing::instrument(level = "info")]
+    ///   async fn my_async_function() {
+    ///   
+    ///       // This is correct! If we yield here, the span will be exited,
+    ///       // and re-entered when we resume.
+    ///       some_other_async_function().await;
+    ///
+    ///       // ...
+    ///    
+    ///   }
+    ///   ```
+    ///
+    /// [syntax]: https://rust-lang.github.io/async-book/01_getting_started/04_async_await_primer.html
+    /// [`Span::in_scope`]: #method.in_scope
+    /// [instrument]: https://docs.rs/tracing/latest/tracing/trait.Instrument.html
+    /// [attr]: ../../attr.instrument.html
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #[macro_use] extern crate tracing;
+    /// # use tracing::Level;
+    /// let span = span!(Level::INFO, "my_span");
+    /// let guard = span.enter();
+    ///
+    /// // code here is within the span
+    ///
+    /// drop(guard);
+    ///
+    /// // code here is no longer within the span
+    ///
+    /// ```
+    ///
+    /// Guards need not be explicitly dropped:
+    ///
+    /// ```
+    /// #[macro_use] extern crate tracing;
+    /// fn my_function() -> String {
+    ///     // enter a span for the duration of this function.
+    ///     let span = trace_span!("my_function");
+    ///     let _enter = span.enter();
+    ///
+    ///     // anything happening in functions we call is still inside the span...
+    ///     my_other_function();
+    ///
+    ///     // returning from the function drops the guard, exiting the span.
+    ///     return "Hello world".to_owned();
+    /// }
+    ///
+    /// fn my_other_function() {
+    ///     // ...
+    /// }
+    /// ```
+    ///
+    /// Sub-scopes may be created to limit the duration for which the span is
+    /// entered:
+    ///
+    /// ```
+    /// #[macro_use] extern crate tracing;
+    /// let span = info_span!("my_great_span");
+    ///
+    /// {
+    ///     let _enter = span.enter();
+    ///
+    ///     // this event occurs inside the span.
+    ///     info!("i'm in the span!");
+    ///
+    ///     // exiting the scope drops the guard, exiting the span.
+    /// }
+    ///
+    /// // this event is not inside the span.
+    /// info!("i'm outside the span!")
+    /// ```
+    ///
+    /// [`Subscriber::enter`]: ../subscriber/trait.Subscriber.html#method.enter
+    /// [`Subscriber::exit`]: ../subscriber/trait.Subscriber.html#method.exit
+    /// [`Id`]: ../struct.Id.html
+    pub fn enter(&self) -> Entered<'_> {
+        if let Some(ref inner) = self.inner.as_ref() {
+            inner.subscriber.enter(&inner.id);
+        }
+
+        if_log_enabled! {{
+            if let Some(ref meta) = self.meta {
+                self.log(ACTIVITY_LOG_TARGET, log::Level::Trace, format_args!("-> {}", meta.name()));
+            }
+        }}
+
+        Entered { span: self }
+    }
+
+    /// Executes the given function in the context of this span.
+    ///
+    /// If this span is enabled, then this function enters the span, invokes `f`
+    /// and then exits the span. If the span is disabled, `f` will still be
+    /// invoked, but in the context of the currently-executing span (if there is
+    /// one).
+    ///
+    /// Returns the result of evaluating `f`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[macro_use] extern crate tracing;
+    /// # use tracing::Level;
+    /// let my_span = span!(Level::TRACE, "my_span");
+    ///
+    /// my_span.in_scope(|| {
+    ///     // this event occurs within the span.
+    ///     trace!("i'm in the span!");
+    /// });
+    ///
+    /// // this event occurs outside the span.
+    /// trace!("i'm not in the span!");
+    /// ```
+    ///
+    /// Calling a function and returning the result:
+    /// ```
+    /// # use tracing::{info_span, Level};
+    /// fn hello_world() -> String {
+    ///     "Hello world!".to_owned()
+    /// }
+    ///
+    /// let span = info_span!("hello_world");
+    /// // the span will be entered for the duration of the call to
+    /// // `hello_world`.
+    /// let a_string = span.in_scope(hello_world);
+    ///
+    pub fn in_scope<F: FnOnce() -> T, T>(&self, f: F) -> T {
+        let _enter = self.enter();
+        f()
+    }
+
+    /// Returns a [`Field`](../field/struct.Field.html) for the field with the
+    /// given `name`, if one exists,
+    pub fn field<Q: ?Sized>(&self, field: &Q) -> Option<field::Field>
+    where
+        Q: field::AsField,
+    {
+        self.metadata().and_then(|meta| field.as_field(meta))
+    }
+
+    /// Returns true if this `Span` has a field for the given
+    /// [`Field`](../field/struct.Field.html) or field name.
+    #[inline]
+    pub fn has_field<Q: ?Sized>(&self, field: &Q) -> bool
+    where
+        Q: field::AsField,
+    {
+        self.field(field).is_some()
+    }
+
+    /// Records that the field described by `field` has the value `value`.
+    ///
+    /// This may be used with [`field::Empty`] to declare fields whose values
+    /// are not known when the span is created, and record them later:
+    /// ```
+    /// use tracing::{trace_span, field};
+    ///
+    /// // Create a span with two fields: `greeting`, with the value "hello world", and
+    /// // `parting`, without a value.
+    /// let span = trace_span!("my_span", greeting = "hello world", parting = field::Empty);
+    ///
+    /// // ...
+    ///
+    /// // Now, record a value for parting as well.
+    /// // (note that the field name is passed as a string slice)
+    /// span.record("parting", &"goodbye world!");
+    /// ```
+    /// However, it may also be used to record a _new_ value for a field whose
+    /// value was already recorded:
+    /// ```
+    /// use tracing::info_span;
+    /// # fn do_something() -> Result<(), ()> { Err(()) }
+    ///
+    /// // Initially, let's assume that our attempt to do something is going okay...
+    /// let span = info_span!("doing_something", is_okay = true);
+    /// let _e = span.enter();
+    ///
+    /// match do_something() {
+    ///     Ok(something) => {
+    ///         // ...
+    ///     }
+    ///     Err(_) => {
+    ///         // Things are no longer okay!
+    ///         span.record("is_okay", &false);
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// <div class="information">
+    ///     <div class="tooltip ignore" style="">ⓘ<span class="tooltiptext">Note</span></div>
+    /// </div>
+    /// <div class="example-wrap" style="display:inline-block">
+    /// <pre class="ignore" style="white-space:normal;font:inherit;">
+    /// <strong>Note</strong>: The fields associated with a span are part of its
+    /// <a href="../struct.Metadata.html"><code>Metadata</code></a>.
+    /// The <a href="../struct.Metadata.html"><code>Metadata</code></a>. describing a particular
+    /// span is constructed statically when the span is created and cannot be extended later to
+    /// add new fields. Therefore, you cannot record a value for a field that was not specified
+    /// when the span was created:</pre></div>
+    ///
+    /// ```
+    /// use tracing::{trace_span, field};
+    ///
+    /// // Create a span with two fields: `greeting`, with the value "hello world", and
+    /// // `parting`, without a value.
+    /// let span = trace_span!("my_span", greeting = "hello world", parting = field::Empty);
+    ///
+    /// // ...
+    ///
+    /// // Now, you try to record a value for a new field, `new_field`, which was not
+    /// // declared as `Empty` or populated when you created `span`.
+    /// // You won't get any error, but the assignment will have no effect!
+    /// span.record("new_field", &"interesting_value_you_really_need");
+    ///
+    /// // Instead, all fields that may be recorded after span creation should be declared up front,
+    /// // using field::Empty when a value is not known, as we did for `parting`.
+    /// // This `record` call will indeed replace field::Empty with "you will be remembered".
+    /// span.record("parting", &"you will be remembered");
+    /// ```
+    ///
+    /// [`field::Empty`]: ../field/struct.Empty.html
+    /// [`Metadata`]: ../struct.Metadata.html
+    pub fn record<Q: ?Sized, V>(&self, field: &Q, value: &V) -> &Self
+    where
+        Q: field::AsField,
+        V: field::Value,
+    {
+        if let Some(ref meta) = self.meta {
+            if let Some(field) = field.as_field(meta) {
+                self.record_all(
+                    &meta
+                        .fields()
+                        .value_set(&[(&field, Some(value as &dyn field::Value))]),
+                );
+            }
+        }
+
+        self
+    }
+
+    /// Records all the fields in the provided `ValueSet`.
+    pub fn record_all(&self, values: &field::ValueSet<'_>) -> &Self {
+        let record = Record::new(values);
+        if let Some(ref inner) = self.inner {
+            inner.record(&record);
+        }
+
+        if_log_enabled! {{
+            if let Some(ref meta) = self.meta {
+                let target = if record.is_empty() {
+                    LIFECYCLE_LOG_TARGET
+                } else {
+                    meta.target()
+                };
+                self.log(target, level_to_log!(*meta.level()), format_args!("{}{}", meta.name(), FmtValues(&record)));
+            }
+        }}
+
+        self
+    }
+
+    /// Returns `true` if this span was disabled by the subscriber and does not
+    /// exist.
+    ///
+    /// See also [`is_none`].
+    ///
+    /// [`is_none`]: #method.is_none
+    #[inline]
+    pub fn is_disabled(&self) -> bool {
+        self.inner.is_none()
+    }
+
+    /// Returns `true` if this span was constructed by [`Span::none`] and is
+    /// empty.
+    ///
+    /// If `is_none` returns `true` for a given span, then [`is_disabled`] will
+    /// also return `true`. However, when a span is disabled by the subscriber
+    /// rather than constructed by `Span::none`, this method will return
+    /// `false`, while `is_disabled` will return `true`.
+    ///
+    /// [`Span::none`]: #method.none
+    /// [`is_disabled`]: #method.is_disabled
+    #[inline]
+    pub fn is_none(&self) -> bool {
+        self.is_disabled() && self.meta.is_none()
+    }
+
+    /// Indicates that the span with the given ID has an indirect causal
+    /// relationship with this span.
+    ///
+    /// This relationship differs somewhat from the parent-child relationship: a
+    /// span may have any number of prior spans, rather than a single one; and
+    /// spans are not considered to be executing _inside_ of the spans they
+    /// follow from. This means that a span may close even if subsequent spans
+    /// that follow from it are still open, and time spent inside of a
+    /// subsequent span should not be included in the time its precedents were
+    /// executing. This is used to model causal relationships such as when a
+    /// single future spawns several related background tasks, et cetera.
+    ///
+    /// If this span is disabled, or the resulting follows-from relationship
+    /// would be invalid, this function will do nothing.
+    ///
+    /// # Examples
+    ///
+    /// Setting a `follows_from` relationship with a `Span`:
+    /// ```
+    /// # use tracing::{span, Id, Level, Span};
+    /// let span1 = span!(Level::INFO, "span_1");
+    /// let span2 = span!(Level::DEBUG, "span_2");
+    /// span2.follows_from(span1);
+    /// ```
+    ///
+    /// Setting a `follows_from` relationship with the current span:
+    /// ```
+    /// # use tracing::{span, Id, Level, Span};
+    /// let span = span!(Level::INFO, "hello!");
+    /// span.follows_from(Span::current());
+    /// ```
+    ///
+    /// Setting a `follows_from` relationship with a `Span` reference:
+    /// ```
+    /// # use tracing::{span, Id, Level, Span};
+    /// let span = span!(Level::INFO, "hello!");
+    /// let curr = Span::current();
+    /// span.follows_from(&curr);
+    /// ```
+    ///
+    /// Setting a `follows_from` relationship with an `Id`:
+    /// ```
+    /// # use tracing::{span, Id, Level, Span};
+    /// let span = span!(Level::INFO, "hello!");
+    /// let id = span.id();
+    /// span.follows_from(id);
+    /// ```
+    pub fn follows_from(&self, from: impl Into<Option<Id>>) -> &Self {
+        if let Some(ref inner) = self.inner {
+            if let Some(from) = from.into() {
+                inner.follows_from(&from);
+            }
+        }
+        self
+    }
+
+    /// Returns this span's `Id`, if it is enabled.
+    pub fn id(&self) -> Option<Id> {
+        self.inner.as_ref().map(Inner::id)
+    }
+
+    /// Returns this span's `Metadata`, if it is enabled.
+    pub fn metadata(&self) -> Option<&'static Metadata<'static>> {
+        self.meta
+    }
+
+    #[cfg(feature = "log")]
+    #[inline]
+    fn log(&self, target: &str, level: log::Level, message: fmt::Arguments<'_>) {
+        if let Some(ref meta) = self.meta {
+            if level_to_log!(*meta.level()) <= log::max_level() {
+                let logger = log::logger();
+                let log_meta = log::Metadata::builder().level(level).target(target).build();
+                if logger.enabled(&log_meta) {
+                    if let Some(ref inner) = self.inner {
+                        logger.log(
+                            &log::Record::builder()
+                                .metadata(log_meta)
+                                .module_path(meta.module_path())
+                                .file(meta.file())
+                                .line(meta.line())
+                                .args(format_args!("{}; span={}", message, inner.id.into_u64()))
+                                .build(),
+                        );
+                    } else {
+                        logger.log(
+                            &log::Record::builder()
+                                .metadata(log_meta)
+                                .module_path(meta.module_path())
+                                .file(meta.file())
+                                .line(meta.line())
+                                .args(message)
+                                .build(),
+                        );
+                    }
+                }
+            }
+        }
+    }
+
+    /// Invokes a function with a reference to this span's ID and subscriber.
+    ///
+    /// if this span is enabled, the provided function is called, and the result is returned.
+    /// If the span is disabled, the function is not called, and this method returns `None`
+    /// instead.
+    pub fn with_subscriber<T>(&self, f: impl FnOnce((&Id, &Dispatch)) -> T) -> Option<T> {
+        self.inner
+            .as_ref()
+            .map(|inner| f((&inner.id, &inner.subscriber)))
+    }
+}
+
+impl cmp::PartialEq for Span {
+    fn eq(&self, other: &Self) -> bool {
+        match (&self.meta, &other.meta) {
+            (Some(this), Some(that)) => {
+                this.callsite() == that.callsite() && self.inner == other.inner
+            }
+            _ => false,
+        }
+    }
+}
+
+impl Hash for Span {
+    fn hash<H: Hasher>(&self, hasher: &mut H) {
+        self.inner.hash(hasher);
+    }
+}
+
+impl fmt::Debug for Span {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut span = f.debug_struct("Span");
+        if let Some(ref meta) = self.meta {
+            span.field("name", &meta.name())
+                .field("level", &meta.level())
+                .field("target", &meta.target());
+
+            if let Some(ref inner) = self.inner {
+                span.field("id", &inner.id());
+            } else {
+                span.field("disabled", &true);
+            }
+
+            if let Some(ref path) = meta.module_path() {
+                span.field("module_path", &path);
+            }
+
+            if let Some(ref line) = meta.line() {
+                span.field("line", &line);
+            }
+
+            if let Some(ref file) = meta.file() {
+                span.field("file", &file);
+            }
+        } else {
+            span.field("none", &true);
+        }
+
+        span.finish()
+    }
+}
+
+impl<'a> Into<Option<&'a Id>> for &'a Span {
+    fn into(self) -> Option<&'a Id> {
+        self.inner.as_ref().map(|inner| &inner.id)
+    }
+}
+
+impl<'a> Into<Option<Id>> for &'a Span {
+    fn into(self) -> Option<Id> {
+        self.inner.as_ref().map(Inner::id)
+    }
+}
+
+impl Into<Option<Id>> for Span {
+    fn into(self) -> Option<Id> {
+        self.inner.as_ref().map(Inner::id)
+    }
+}
+
+impl Drop for Span {
+    fn drop(&mut self) {
+        if let Some(Inner {
+            ref id,
+            ref subscriber,
+        }) = self.inner
+        {
+            subscriber.try_close(id.clone());
+        }
+
+        if_log_enabled!({
+            if let Some(ref meta) = self.meta {
+                self.log(
+                    LIFECYCLE_LOG_TARGET,
+                    log::Level::Trace,
+                    format_args!("-- {}", meta.name()),
+                );
+            }
+        })
+    }
+}
+
+// ===== impl Inner =====
+
+impl Inner {
+    /// Indicates that the span with the given ID has an indirect causal
+    /// relationship with this span.
+    ///
+    /// This relationship differs somewhat from the parent-child relationship: a
+    /// span may have any number of prior spans, rather than a single one; and
+    /// spans are not considered to be executing _inside_ of the spans they
+    /// follow from. This means that a span may close even if subsequent spans
+    /// that follow from it are still open, and time spent inside of a
+    /// subsequent span should not be included in the time its precedents were
+    /// executing. This is used to model causal relationships such as when a
+    /// single future spawns several related background tasks, et cetera.
+    ///
+    /// If this span is disabled, this function will do nothing. Otherwise, it
+    /// returns `Ok(())` if the other span was added as a precedent of this
+    /// span, or an error if this was not possible.
+    fn follows_from(&self, from: &Id) {
+        self.subscriber.record_follows_from(&self.id, &from)
+    }
+
+    /// Returns the span's ID.
+    fn id(&self) -> Id {
+        self.id.clone()
+    }
+
+    fn record(&self, values: &Record<'_>) {
+        self.subscriber.record(&self.id, values)
+    }
+
+    fn new(id: Id, subscriber: &Dispatch) -> Self {
+        Inner {
+            id,
+            subscriber: subscriber.clone(),
+        }
+    }
+}
+
+impl cmp::PartialEq for Inner {
+    fn eq(&self, other: &Self) -> bool {
+        self.id == other.id
+    }
+}
+
+impl Hash for Inner {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.id.hash(state);
+    }
+}
+
+impl Clone for Inner {
+    fn clone(&self) -> Self {
+        Inner {
+            id: self.subscriber.clone_span(&self.id),
+            subscriber: self.subscriber.clone(),
+        }
+    }
+}
+
+// ===== impl Entered =====
+
+impl<'a> Drop for Entered<'a> {
+    #[inline]
+    fn drop(&mut self) {
+        // Dropping the guard exits the span.
+        //
+        // Running this behaviour on drop rather than with an explicit function
+        // call means that spans may still be exited when unwinding.
+        if let Some(inner) = self.span.inner.as_ref() {
+            inner.subscriber.exit(&inner.id);
+        }
+
+        if_log_enabled! {{
+            if let Some(ref meta) = self.span.meta {
+                self.span.log(ACTIVITY_LOG_TARGET, log::Level::Trace, format_args!("<- {}", meta.name()));
+            }
+        }}
+    }
+}
+
+#[cfg(feature = "log")]
+struct FmtValues<'a>(&'a Record<'a>);
+
+#[cfg(feature = "log")]
+impl<'a> fmt::Display for FmtValues<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut res = Ok(());
+        let mut is_first = true;
+        self.0.record(&mut |k: &field::Field, v: &dyn fmt::Debug| {
+            res = write!(f, "{} {}={:?}", if is_first { ";" } else { "" }, k, v);
+            is_first = false;
+        });
+        res
+    }
+}
+
+#[cfg(feature = "log")]
+struct FmtAttrs<'a>(&'a Attributes<'a>);
+
+#[cfg(feature = "log")]
+impl<'a> fmt::Display for FmtAttrs<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut res = Ok(());
+        let mut is_first = true;
+        self.0.record(&mut |k: &field::Field, v: &dyn fmt::Debug| {
+            res = write!(f, "{} {}={:?}", if is_first { ";" } else { "" }, k, v);
+            is_first = false;
+        });
+        res
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    trait AssertSend: Send {}
+    impl AssertSend for Span {}
+
+    trait AssertSync: Sync {}
+    impl AssertSync for Span {}
+}
diff --git a/third_party/rust/tracing/src/stdlib.rs b/third_party/rust/tracing/src/stdlib.rs
new file mode 100644
index 0000000000..12b54084d4
--- /dev/null
+++ b/third_party/rust/tracing/src/stdlib.rs
@@ -0,0 +1,55 @@
+//! Re-exports either the Rust `std` library or `core` and `alloc` when `std` is
+//! disabled.
+//!
+//! `crate::stdlib::...` should be used rather than `std::` when adding code that
+//! will be available with the standard library disabled.
+//!
+//! Note that this module is called `stdlib` rather than `std`, as Rust 1.34.0
+//! does not permit redefining the name `stdlib` (although this works on the
+//! latest stable Rust).
+#[cfg(feature = "std")]
+pub(crate) use std::*;
+
+#[cfg(not(feature = "std"))]
+pub(crate) use self::no_std::*;
+
+#[cfg(not(feature = "std"))]
+mod no_std {
+    // We pre-emptively export everything from libcore/liballoc, (even modules
+    // we aren't using currently) to make adding new code easier. Therefore,
+    // some of these imports will be unused.
+    #![allow(unused_imports)]
+
+    pub(crate) use core::{
+        any, array, ascii, cell, char, clone, cmp, convert, default, f32, f64, ffi, future, hash,
+        hint, i128, i16, i8, isize, iter, marker, mem, num, ops, option, pin, ptr, result, task,
+        time, u128, u16, u32, u8, usize,
+    };
+
+    pub(crate) use alloc::{boxed, collections, rc, string, vec};
+
+    pub(crate) mod borrow {
+        pub(crate) use alloc::borrow::*;
+        pub(crate) use core::borrow::*;
+    }
+
+    pub(crate) mod fmt {
+        pub(crate) use alloc::fmt::*;
+        pub(crate) use core::fmt::*;
+    }
+
+    pub(crate) mod slice {
+        pub(crate) use alloc::slice::*;
+        pub(crate) use core::slice::*;
+    }
+
+    pub(crate) mod str {
+        pub(crate) use alloc::str::*;
+        pub(crate) use core::str::*;
+    }
+
+    pub(crate) mod sync {
+        pub(crate) use alloc::sync::*;
+        pub(crate) use core::sync::*;
+    }
+}
diff --git a/third_party/rust/tracing/src/subscriber.rs b/third_party/rust/tracing/src/subscriber.rs
new file mode 100644
index 0000000000..8f35d84e09
--- /dev/null
+++ b/third_party/rust/tracing/src/subscriber.rs
@@ -0,0 +1,68 @@
+//! Collects and records trace data.
+pub use tracing_core::subscriber::*;
+
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+pub use tracing_core::dispatcher::DefaultGuard;
+
+/// Sets this subscriber as the default for the duration of a closure.
+///
+/// The default subscriber is used when creating a new [`Span`] or
+/// [`Event`], _if no span is currently executing_. If a span is currently
+/// executing, new spans or events are dispatched to the subscriber that
+/// tagged that span, instead.
+///
+/// [`Span`]: ../span/struct.Span.html
+/// [`Subscriber`]: ../subscriber/trait.Subscriber.html
+/// [`Event`]: :../event/struct.Event.html
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+pub fn with_default<T, S>(subscriber: S, f: impl FnOnce() -> T) -> T
+where
+    S: Subscriber + Send + Sync + 'static,
+{
+    crate::dispatcher::with_default(&crate::Dispatch::new(subscriber), f)
+}
+
+/// Sets this subscriber as the global default for the duration of the entire program.
+/// Will be used as a fallback if no thread-local subscriber has been set in a thread (using `with_default`.)
+///
+/// Can only be set once; subsequent attempts to set the global default will fail.
+/// Returns whether the initialization was successful.
+///
+/// Note: Libraries should *NOT* call `set_global_default()`! That will cause conflicts when
+/// executables try to set them later.
+///
+/// [span]: ../span/index.html
+/// [`Subscriber`]: ../subscriber/trait.Subscriber.html
+/// [`Event`]: ../event/struct.Event.html
+pub fn set_global_default<S>(subscriber: S) -> Result<(), SetGlobalDefaultError>
+where
+    S: Subscriber + Send + Sync + 'static,
+{
+    crate::dispatcher::set_global_default(crate::Dispatch::new(subscriber))
+}
+
+/// Sets the subscriber as the default for the duration of the lifetime of the
+/// returned [`DefaultGuard`]
+///
+/// The default subscriber is used when creating a new [`Span`] or
+/// [`Event`], _if no span is currently executing_. If a span is currently
+/// executing, new spans or events are dispatched to the subscriber that
+/// tagged that span, instead.
+///
+/// [`Span`]: ../span/struct.Span.html
+/// [`Subscriber`]: ../subscriber/trait.Subscriber.html
+/// [`Event`]: :../event/struct.Event.html
+/// [`DefaultGuard`]: ../dispatcher/struct.DefaultGuard.html
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+#[must_use = "Dropping the guard unregisters the subscriber."]
+pub fn set_default<S>(subscriber: S) -> DefaultGuard
+where
+    S: Subscriber + Send + Sync + 'static,
+{
+    crate::dispatcher::set_default(&crate::Dispatch::new(subscriber))
+}
+
+pub use tracing_core::dispatcher::SetGlobalDefaultError;
diff --git a/third_party/rust/tracing/tests/event.rs b/third_party/rust/tracing/tests/event.rs
new file mode 100644
index 0000000000..d61ef748b8
--- /dev/null
+++ b/third_party/rust/tracing/tests/event.rs
@@ -0,0 +1,375 @@
+// These tests require the thread-local scoped dispatcher, which only works when
+// we have a standard library. The behaviour being tested should be the same
+// with the standard lib disabled.
+//
+// The alternative would be for each of these tests to be defined in a separate
+// file, which is :(
+#![cfg(feature = "std")]
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+
+use tracing::{
+    field::{debug, display},
+    subscriber::with_default,
+    Level,
+};
+
+macro_rules! event_without_message {
+    ($name:ident: $e:expr) => {
+        #[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+        #[test]
+        fn $name() {
+            let (subscriber, handle) = subscriber::mock()
+                .event(
+                    event::mock().with_fields(
+                        field::mock("answer")
+                            .with_value(&42)
+                            .and(
+                                field::mock("to_question")
+                                    .with_value(&"life, the universe, and everything"),
+                            )
+                            .only(),
+                    ),
+                )
+                .done()
+                .run_with_handle();
+
+            with_default(subscriber, || {
+                info!(
+                    answer = $e,
+                    to_question = "life, the universe, and everything"
+                );
+            });
+
+            handle.assert_finished();
+        }
+    };
+}
+
+event_without_message! {event_without_message: 42}
+event_without_message! {wrapping_event_without_message: std::num::Wrapping(42)}
+event_without_message! {nonzeroi32_event_without_message: std::num::NonZeroI32::new(42).unwrap()}
+// needs API breakage
+//event_without_message!{nonzerou128_event_without_message: std::num::NonZeroU128::new(42).unwrap()}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_with_message() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(event::mock().with_fields(field::mock("message").with_value(
+            &tracing::field::debug(format_args!("hello from my event! yak shaved = {:?}", true)),
+        )))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        debug!("hello from my event! yak shaved = {:?}", true);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn message_without_delims() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("answer")
+                    .with_value(&42)
+                    .and(field::mock("question").with_value(&"life, the universe, and everything"))
+                    .and(
+                        field::mock("message").with_value(&tracing::field::debug(format_args!(
+                            "hello from my event! tricky? {:?}!",
+                            true
+                        ))),
+                    )
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let question = "life, the universe, and everything";
+        debug!(answer = 42, question, "hello from {where}! tricky? {:?}!", true, where = "my event");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn string_message_without_delims() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("answer")
+                    .with_value(&42)
+                    .and(field::mock("question").with_value(&"life, the universe, and everything"))
+                    .and(
+                        field::mock("message").with_value(&tracing::field::debug(format_args!(
+                            "hello from my event"
+                        ))),
+                    )
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let question = "life, the universe, and everything";
+        debug!(answer = 42, question, "hello from my event");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn one_with_everything() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock()
+                .with_fields(
+                    field::mock("message")
+                        .with_value(&tracing::field::debug(format_args!(
+                            "{:#x} make me one with{what:.>20}",
+                            4_277_009_102u64,
+                            what = "everything"
+                        )))
+                        .and(field::mock("foo").with_value(&666))
+                        .and(field::mock("bar").with_value(&false))
+                        .only(),
+                )
+                .at_level(Level::ERROR)
+                .with_target("whatever"),
+        )
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        event!(
+            target: "whatever",
+            Level::ERROR,
+            { foo = 666, bar = false },
+             "{:#x} make me one with{what:.>20}", 4_277_009_102u64, what = "everything"
+        );
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn moved_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("foo")
+                    .with_value(&display("hello from my event"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let from = "my event";
+        event!(Level::INFO, foo = display(format!("hello from {}", from)))
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn dotted_field_name() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("foo.bar")
+                    .with_value(&true)
+                    .and(field::mock("foo.baz").with_value(&false))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        event!(Level::INFO, foo.bar = true, foo.baz = false);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn borrowed_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("foo")
+                    .with_value(&display("hello from my event"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let from = "my event";
+        let mut message = format!("hello from {}", from);
+        event!(Level::INFO, foo = display(&message));
+        message.push_str(", which happened!");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+// If emitting log instrumentation, this gets moved anyway, breaking the test.
+#[cfg(not(feature = "log"))]
+fn move_field_out_of_struct() {
+    use tracing::field::debug;
+
+    #[derive(Debug)]
+    struct Position {
+        x: f32,
+        y: f32,
+    }
+
+    let pos = Position {
+        x: 3.234,
+        y: -1.223,
+    };
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("x")
+                    .with_value(&debug(3.234))
+                    .and(field::mock("y").with_value(&debug(-1.223)))
+                    .only(),
+            ),
+        )
+        .event(event::mock().with_fields(field::mock("position").with_value(&debug(&pos))))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let pos = Position {
+            x: 3.234,
+            y: -1.223,
+        };
+        debug!(x = debug(pos.x), y = debug(pos.y));
+        debug!(target: "app_events", { position = debug(pos) }, "New position");
+    });
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn display_shorthand() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("my_field")
+                    .with_value(&display("hello world"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        event!(Level::TRACE, my_field = %"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_shorthand() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("my_field")
+                    .with_value(&debug("hello world"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        event!(Level::TRACE, my_field = ?"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn both_shorthands() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(
+            event::mock().with_fields(
+                field::mock("display_field")
+                    .with_value(&display("hello world"))
+                    .and(field::mock("debug_field").with_value(&debug("hello world")))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        event!(Level::TRACE, display_field = %"hello world", debug_field = ?"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        event!(parent: foo.id(), Level::TRACE, "bar");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child_at_levels() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .event(event::mock().with_explicit_parent(Some("foo")))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        trace!(parent: foo.id(), "a");
+        debug!(parent: foo.id(), "b");
+        info!(parent: foo.id(), "c");
+        warn!(parent: foo.id(), "d");
+        error!(parent: foo.id(), "e");
+    });
+
+    handle.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/filter_caching_is_lexically_scoped.rs b/third_party/rust/tracing/tests/filter_caching_is_lexically_scoped.rs
new file mode 100644
index 0000000000..9b997a0c6b
--- /dev/null
+++ b/third_party/rust/tracing/tests/filter_caching_is_lexically_scoped.rs
@@ -0,0 +1,69 @@
+// Tests that depend on a count of the number of times their filter is evaluated
+// can't exist in the same file with other tests that add subscribers to the
+// registry. The registry was changed so that each time a new dispatcher is
+// added all filters are re-evaluated. The tests being run only in separate
+// threads with shared global state lets them interfere with each other
+
+#[cfg(not(feature = "std"))]
+extern crate std;
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+use std::sync::{
+    atomic::{AtomicUsize, Ordering},
+    Arc,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn filter_caching_is_lexically_scoped() {
+    pub fn my_great_function() -> bool {
+        span!(Level::TRACE, "emily").in_scope(|| true)
+    }
+
+    pub fn my_other_function() -> bool {
+        span!(Level::TRACE, "frank").in_scope(|| true)
+    }
+
+    let count = Arc::new(AtomicUsize::new(0));
+    let count2 = count.clone();
+
+    let subscriber = subscriber::mock()
+        .with_filter(move |meta| match meta.name() {
+            "emily" | "frank" => {
+                count2.fetch_add(1, Ordering::Relaxed);
+                true
+            }
+            _ => false,
+        })
+        .run();
+
+    // Since this test is in its own file anyway, we can do this. Thus, this
+    // test will work even with no-std.
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    // Call the function once. The filter should be re-evaluated.
+    assert!(my_great_function());
+    assert_eq!(count.load(Ordering::Relaxed), 1);
+
+    // Call the function again. The cached result should be used.
+    assert!(my_great_function());
+    assert_eq!(count.load(Ordering::Relaxed), 1);
+
+    assert!(my_other_function());
+    assert_eq!(count.load(Ordering::Relaxed), 2);
+
+    assert!(my_great_function());
+    assert_eq!(count.load(Ordering::Relaxed), 2);
+
+    assert!(my_other_function());
+    assert_eq!(count.load(Ordering::Relaxed), 2);
+
+    assert!(my_great_function());
+    assert_eq!(count.load(Ordering::Relaxed), 2);
+}
diff --git a/third_party/rust/tracing/tests/filters_are_not_reevaluated_for_the_same_span.rs b/third_party/rust/tracing/tests/filters_are_not_reevaluated_for_the_same_span.rs
new file mode 100644
index 0000000000..53e11ef448
--- /dev/null
+++ b/third_party/rust/tracing/tests/filters_are_not_reevaluated_for_the_same_span.rs
@@ -0,0 +1,74 @@
+// Tests that depend on a count of the number of times their filter is evaluated
+// cant exist in the same file with other tests that add subscribers to the
+// registry. The registry was changed so that each time a new dispatcher is
+// added all filters are re-evaluated. The tests being run only in separate
+// threads with shared global state lets them interfere with each other
+#[cfg(not(feature = "std"))]
+extern crate std;
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+use std::sync::{
+    atomic::{AtomicUsize, Ordering},
+    Arc,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn filters_are_not_reevaluated_for_the_same_span() {
+    // Asserts that the `span!` macro caches the result of calling
+    // `Subscriber::enabled` for each span.
+    let alice_count = Arc::new(AtomicUsize::new(0));
+    let bob_count = Arc::new(AtomicUsize::new(0));
+    let alice_count2 = alice_count.clone();
+    let bob_count2 = bob_count.clone();
+
+    let (subscriber, handle) = subscriber::mock()
+        .with_filter(move |meta| match meta.name() {
+            "alice" => {
+                alice_count2.fetch_add(1, Ordering::Relaxed);
+                false
+            }
+            "bob" => {
+                bob_count2.fetch_add(1, Ordering::Relaxed);
+                true
+            }
+            _ => false,
+        })
+        .run_with_handle();
+
+    // Since this test is in its own file anyway, we can do this. Thus, this
+    // test will work even with no-std.
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    // Enter "alice" and then "bob". The dispatcher expects to see "bob" but
+    // not "alice."
+    let alice = span!(Level::TRACE, "alice");
+    let bob = alice.in_scope(|| {
+        let bob = span!(Level::TRACE, "bob");
+        bob.in_scope(|| ());
+        bob
+    });
+
+    // The filter should have seen each span a single time.
+    assert_eq!(alice_count.load(Ordering::Relaxed), 1);
+    assert_eq!(bob_count.load(Ordering::Relaxed), 1);
+
+    alice.in_scope(|| bob.in_scope(|| {}));
+
+    // The subscriber should see "bob" again, but the filter should not have
+    // been called.
+    assert_eq!(alice_count.load(Ordering::Relaxed), 1);
+    assert_eq!(bob_count.load(Ordering::Relaxed), 1);
+
+    bob.in_scope(|| {});
+    assert_eq!(alice_count.load(Ordering::Relaxed), 1);
+    assert_eq!(bob_count.load(Ordering::Relaxed), 1);
+
+    handle.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/filters_are_reevaluated_for_different_call_sites.rs b/third_party/rust/tracing/tests/filters_are_reevaluated_for_different_call_sites.rs
new file mode 100644
index 0000000000..5675e4e678
--- /dev/null
+++ b/third_party/rust/tracing/tests/filters_are_reevaluated_for_different_call_sites.rs
@@ -0,0 +1,84 @@
+// Tests that depend on a count of the number of times their filter is evaluated
+// cant exist in the same file with other tests that add subscribers to the
+// registry. The registry was changed so that each time a new dispatcher is
+// added all filters are re-evaluated. The tests being run only in separate
+// threads with shared global state lets them interfere with each other
+#[cfg(not(feature = "std"))]
+extern crate std;
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+use std::sync::{
+    atomic::{AtomicUsize, Ordering},
+    Arc,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn filters_are_reevaluated_for_different_call_sites() {
+    // Asserts that the `span!` macro caches the result of calling
+    // `Subscriber::enabled` for each span.
+    let charlie_count = Arc::new(AtomicUsize::new(0));
+    let dave_count = Arc::new(AtomicUsize::new(0));
+    let charlie_count2 = charlie_count.clone();
+    let dave_count2 = dave_count.clone();
+
+    let subscriber = subscriber::mock()
+        .with_filter(move |meta| {
+            println!("Filter: {:?}", meta.name());
+            match meta.name() {
+                "charlie" => {
+                    charlie_count2.fetch_add(1, Ordering::Relaxed);
+                    false
+                }
+                "dave" => {
+                    dave_count2.fetch_add(1, Ordering::Relaxed);
+                    true
+                }
+                _ => false,
+            }
+        })
+        .run();
+
+    // Since this test is in its own file anyway, we can do this. Thus, this
+    // test will work even with no-std.
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    // Enter "charlie" and then "dave". The dispatcher expects to see "dave" but
+    // not "charlie."
+    let charlie = span!(Level::TRACE, "charlie");
+    let dave = charlie.in_scope(|| {
+        let dave = span!(Level::TRACE, "dave");
+        dave.in_scope(|| {});
+        dave
+    });
+
+    // The filter should have seen each span a single time.
+    assert_eq!(charlie_count.load(Ordering::Relaxed), 1);
+    assert_eq!(dave_count.load(Ordering::Relaxed), 1);
+
+    charlie.in_scope(|| dave.in_scope(|| {}));
+
+    // The subscriber should see "dave" again, but the filter should not have
+    // been called.
+    assert_eq!(charlie_count.load(Ordering::Relaxed), 1);
+    assert_eq!(dave_count.load(Ordering::Relaxed), 1);
+
+    // A different span with the same name has a different call site, so it
+    // should cause the filter to be reapplied.
+    let charlie2 = span!(Level::TRACE, "charlie");
+    charlie.in_scope(|| {});
+    assert_eq!(charlie_count.load(Ordering::Relaxed), 2);
+    assert_eq!(dave_count.load(Ordering::Relaxed), 1);
+
+    // But, the filter should not be re-evaluated for the new "charlie" span
+    // when it is re-entered.
+    charlie2.in_scope(|| span!(Level::TRACE, "dave").in_scope(|| {}));
+    assert_eq!(charlie_count.load(Ordering::Relaxed), 2);
+    assert_eq!(dave_count.load(Ordering::Relaxed), 2);
+}
diff --git a/third_party/rust/tracing/tests/filters_dont_leak.rs b/third_party/rust/tracing/tests/filters_dont_leak.rs
new file mode 100644
index 0000000000..666335a8a4
--- /dev/null
+++ b/third_party/rust/tracing/tests/filters_dont_leak.rs
@@ -0,0 +1,82 @@
+#![cfg(feature = "std")]
+
+mod support;
+use self::support::*;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn spans_dont_leak() {
+    fn do_span() {
+        let span = tracing::debug_span!("alice");
+        let _e = span.enter();
+    }
+
+    let (subscriber, handle) = subscriber::mock()
+        .named("spans/subscriber1")
+        .with_filter(|_| false)
+        .done()
+        .run_with_handle();
+
+    let _guard = tracing::subscriber::set_default(subscriber);
+
+    do_span();
+
+    let alice = span::mock().named("alice");
+    let (subscriber2, handle2) = subscriber::mock()
+        .named("spans/subscriber2")
+        .with_filter(|_| true)
+        .new_span(alice.clone())
+        .enter(alice.clone())
+        .exit(alice.clone())
+        .drop_span(alice)
+        .done()
+        .run_with_handle();
+
+    tracing::subscriber::with_default(subscriber2, || {
+        println!("--- subscriber 2 is default ---");
+        do_span()
+    });
+
+    println!("--- subscriber 1 is default ---");
+    do_span();
+
+    handle.assert_finished();
+    handle2.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn events_dont_leak() {
+    fn do_event() {
+        tracing::debug!("alice");
+    }
+
+    let (subscriber, handle) = subscriber::mock()
+        .named("events/subscriber1")
+        .with_filter(|_| false)
+        .done()
+        .run_with_handle();
+
+    let _guard = tracing::subscriber::set_default(subscriber);
+
+    do_event();
+
+    let (subscriber2, handle2) = subscriber::mock()
+        .named("events/subscriber2")
+        .with_filter(|_| true)
+        .event(event::mock())
+        .done()
+        .run_with_handle();
+
+    tracing::subscriber::with_default(subscriber2, || {
+        println!("--- subscriber 2 is default ---");
+        do_event()
+    });
+
+    println!("--- subscriber 1 is default ---");
+
+    do_event();
+
+    handle.assert_finished();
+    handle2.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/macro_imports.rs b/third_party/rust/tracing/tests/macro_imports.rs
new file mode 100644
index 0000000000..2d0a9d6528
--- /dev/null
+++ b/third_party/rust/tracing/tests/macro_imports.rs
@@ -0,0 +1,23 @@
+use tracing::Level;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn prefixed_span_macros() {
+    tracing::span!(Level::DEBUG, "foo");
+    tracing::trace_span!("foo");
+    tracing::debug_span!("foo");
+    tracing::info_span!("foo");
+    tracing::warn_span!("foo");
+    tracing::error_span!("foo");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn prefixed_event_macros() {
+    tracing::event!(Level::DEBUG, "foo");
+    tracing::trace!("foo");
+    tracing::debug!("foo");
+    tracing::info!("foo");
+    tracing::warn!("foo");
+    tracing::error!("foo");
+}
diff --git a/third_party/rust/tracing/tests/macros.rs b/third_party/rust/tracing/tests/macros.rs
new file mode 100644
index 0000000000..39642667f7
--- /dev/null
+++ b/third_party/rust/tracing/tests/macros.rs
@@ -0,0 +1,890 @@
+#![deny(warnings)]
+use tracing::Level;
+
+#[macro_use]
+extern crate tracing;
+// Tests that macros work across various invocation syntax.
+//
+// These are quite repetitive, and _could_ be generated by a macro. However,
+// they're compile-time tests, so I want to get line numbers etc out of
+// failures, and producing them with a macro would muddy the waters a bit.
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span() {
+    span!(target: "foo_events", Level::DEBUG, "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    span!(target: "foo_events", Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(target: "foo_events", Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(target: "foo_events", Level::DEBUG, "foo");
+    span!(target: "foo_events", Level::DEBUG, "bar",);
+    span!(Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(Level::DEBUG, "foo", bar.baz = ?2);
+    span!(Level::DEBUG, "foo", bar.baz = %2);
+    span!(Level::DEBUG, "foo");
+    span!(Level::DEBUG, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_span() {
+    trace_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    trace_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    trace_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    trace_span!(target: "foo_events", "foo");
+    trace_span!(target: "foo_events", "bar",);
+    trace_span!("foo", bar.baz = 2, quux = 3);
+    trace_span!("foo", bar.baz = 2, quux = 4,);
+    trace_span!("foo", bar.baz = ?2);
+    trace_span!("foo", bar.baz = %2);
+    trace_span!("bar");
+    trace_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_span() {
+    debug_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    debug_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    debug_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    debug_span!(target: "foo_events", "foo");
+    debug_span!(target: "foo_events", "bar",);
+    debug_span!("foo", bar.baz = 2, quux = 3);
+    debug_span!("foo", bar.baz = 2, quux = 4,);
+    debug_span!("foo", bar.baz = ?2);
+    debug_span!("foo", bar.baz = %2);
+    debug_span!("bar");
+    debug_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_span() {
+    info_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    info_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    info_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    info_span!(target: "foo_events", "foo");
+    info_span!(target: "foo_events", "bar",);
+    info_span!("foo", bar.baz = 2, quux = 3);
+    info_span!("foo", bar.baz = 2, quux = 4,);
+    info_span!("foo", bar.baz = ?2);
+    info_span!("foo", bar.baz = %2);
+    info_span!("bar");
+    info_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_span() {
+    warn_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    warn_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    warn_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    warn_span!(target: "foo_events", "foo");
+    warn_span!(target: "foo_events", "bar",);
+    warn_span!("foo", bar.baz = 2, quux = 3);
+    warn_span!("foo", bar.baz = 2, quux = 4,);
+    warn_span!("foo", bar.baz = ?2);
+    warn_span!("foo", bar.baz = %2);
+    warn_span!("bar");
+    warn_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_span() {
+    error_span!(target: "foo_events", "foo", bar.baz = ?2, quux = %3, quuux = 4);
+    error_span!(target: "foo_events", "foo", bar.baz = 2, quux = 3);
+    error_span!(target: "foo_events", "foo", bar.baz = 2, quux = 4,);
+    error_span!(target: "foo_events", "foo");
+    error_span!(target: "foo_events", "bar",);
+    error_span!("foo", bar.baz = 2, quux = 3);
+    error_span!("foo", bar.baz = 2, quux = 4,);
+    error_span!("foo", bar.baz = ?2);
+    error_span!("foo", bar.baz = %2);
+    error_span!("bar");
+    error_span!("bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_root() {
+    span!(target: "foo_events", parent: None, Level::TRACE, "foo", bar.baz = 2, quux = 3);
+    span!(target: "foo_events", parent: None, Level::TRACE, "foo", bar.baz = 2, quux = 3);
+    span!(target: "foo_events", parent: None, Level::TRACE, "foo", bar.baz = 2, quux = 4,);
+    span!(target: "foo_events", parent: None, Level::TRACE, "foo");
+    span!(target: "foo_events", parent: None, Level::TRACE, "bar",);
+    span!(parent: None, Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(parent: None, Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(parent: None, Level::DEBUG, "foo");
+    span!(parent: None, Level::DEBUG, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_span_root() {
+    trace_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    trace_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    trace_span!(target: "foo_events", parent: None, "foo");
+    trace_span!(target: "foo_events", parent: None, "bar",);
+    trace_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    trace_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    trace_span!(parent: None, "foo");
+    trace_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_span_root() {
+    debug_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    debug_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    debug_span!(target: "foo_events", parent: None, "foo");
+    debug_span!(target: "foo_events", parent: None, "bar",);
+    debug_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    debug_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    debug_span!(parent: None, "foo");
+    debug_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_span_root() {
+    info_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    info_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    info_span!(target: "foo_events", parent: None, "foo");
+    info_span!(target: "foo_events", parent: None, "bar",);
+    info_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    info_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    info_span!(parent: None, "foo");
+    info_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_span_root() {
+    warn_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    warn_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    warn_span!(target: "foo_events", parent: None, "foo");
+    warn_span!(target: "foo_events", parent: None, "bar",);
+    warn_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    warn_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    warn_span!(parent: None, "foo");
+    warn_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_span_root() {
+    error_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 3);
+    error_span!(target: "foo_events", parent: None, "foo", bar.baz = 2, quux = 4,);
+    error_span!(target: "foo_events", parent: None, "foo");
+    error_span!(target: "foo_events", parent: None, "bar",);
+    error_span!(parent: None, "foo", bar.baz = 2, quux = 3);
+    error_span!(parent: None, "foo", bar.baz = 2, quux = 4,);
+    error_span!(parent: None, "foo");
+    error_span!(parent: None, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    span!(target: "foo_events", parent: &p, Level::TRACE, "foo", bar.baz = 2, quux = 3);
+    span!(target: "foo_events", parent: &p, Level::TRACE, "foo", bar.baz = 2, quux = 4,);
+    span!(target: "foo_events", parent: &p, Level::TRACE, "foo");
+    span!(target: "foo_events", parent: &p, Level::TRACE, "bar",);
+    span!(parent: &p, Level::DEBUG, "foo", bar.baz = 2, quux = 3);
+    span!(parent: &p, Level::DEBUG, "foo", bar.baz = 2, quux = 4,);
+    span!(parent: &p, Level::DEBUG, "foo");
+    span!(parent: &p, Level::DEBUG, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    trace_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    trace_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    trace_span!(target: "foo_events", parent: &p, "foo");
+    trace_span!(target: "foo_events", parent: &p, "bar",);
+
+    trace_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    trace_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    trace_span!(parent: &p, "foo");
+    trace_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    debug_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    debug_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    debug_span!(target: "foo_events", parent: &p, "foo");
+    debug_span!(target: "foo_events", parent: &p, "bar",);
+
+    debug_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    debug_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    debug_span!(parent: &p, "foo");
+    debug_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    info_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    info_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    info_span!(target: "foo_events", parent: &p, "foo");
+    info_span!(target: "foo_events", parent: &p, "bar",);
+
+    info_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    info_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    info_span!(parent: &p, "foo");
+    info_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    warn_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    warn_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    warn_span!(target: "foo_events", parent: &p, "foo");
+    warn_span!(target: "foo_events", parent: &p, "bar",);
+
+    warn_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    warn_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    warn_span!(parent: &p, "foo");
+    warn_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_span_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    error_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 3);
+    error_span!(target: "foo_events", parent: &p, "foo", bar.baz = 2, quux = 4,);
+    error_span!(target: "foo_events", parent: &p, "foo");
+    error_span!(target: "foo_events", parent: &p, "bar",);
+
+    error_span!(parent: &p, "foo", bar.baz = 2, quux = 3);
+    error_span!(parent: &p, "foo", bar.baz = 2, quux = 4,);
+
+    error_span!(parent: &p, "foo");
+    error_span!(parent: &p, "bar",);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_with_non_rust_symbol() {
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = ?"abcdef", "more {}", 42);
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = %"abcdef", "more {}", 51);
+    span!(
+        Level::TRACE,
+        "non-rust",
+        "guid:x-request-id" = "abcdef",
+        "more {}",
+        60
+    );
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = ?"abcdef");
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = %"abcdef");
+    span!(Level::TRACE, "non-rust", "guid:x-request-id" = "abcdef");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event() {
+    event!(Level::DEBUG, foo = ?3, bar.baz = %2, quux = false);
+    event!(Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(Level::DEBUG, "foo");
+    event!(Level::DEBUG, "foo: {}", 3);
+    event!(Level::INFO, foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    event!(
+        Level::INFO,
+        foo = 3,
+        bar.baz = 2,
+        quux = false,
+        "hello world {:?}",
+        42
+    );
+    event!(Level::INFO, foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    event!(Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(Level::DEBUG, { foo = ?2, bar.baz = %78 }, "quux");
+    event!(target: "foo_events", Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(target: "foo_events", Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(target: "foo_events", Level::DEBUG, "foo");
+    event!(target: "foo_events", Level::DEBUG, "foo: {}", 3);
+    event!(target: "foo_events", Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(target: "foo_events", Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(target: "foo_events", Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(target: "foo_events", Level::DEBUG, { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    event!(Level::DEBUG, ?foo);
+    event!(Level::DEBUG, %foo);
+    event!(Level::DEBUG, foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn locals_with_message() {
+    let data = (42, "forty-two");
+    let private_data = "private";
+    let error = "a bad error";
+    event!(Level::ERROR, %error, "Received error");
+    event!(
+        target: "app_events",
+        Level::WARN,
+        private_data,
+        ?data,
+        "App warning: {}",
+        error
+    );
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn locals_no_message() {
+    let data = (42, "forty-two");
+    let private_data = "private";
+    let error = "a bad error";
+    event!(
+        target: "app_events",
+        Level::WARN,
+        private_data,
+        ?data,
+    );
+    event!(
+        target: "app_events",
+        Level::WARN,
+        private_data,
+        ?data,
+        error,
+    );
+    event!(
+        target: "app_events",
+        Level::WARN,
+        private_data,
+        ?data,
+        error
+    );
+    event!(Level::WARN, private_data, ?data, error,);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace() {
+    trace!(foo = ?3, bar.baz = %2, quux = false);
+    trace!(foo = 3, bar.baz = 2, quux = false);
+    trace!(foo = 3, bar.baz = 3,);
+    trace!("foo");
+    trace!("foo: {}", 3);
+    trace!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    trace!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    trace!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    trace!({ foo = 3, bar.baz = 80 }, "quux");
+    trace!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!({ foo = 2, bar.baz = 78 }, "quux");
+    trace!({ foo = ?2, bar.baz = %78 }, "quux");
+    trace!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    trace!(target: "foo_events", foo = 3, bar.baz = 3,);
+    trace!(target: "foo_events", "foo");
+    trace!(target: "foo_events", "foo: {}", 3);
+    trace!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    trace!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    trace!(?foo);
+    trace!(%foo);
+    trace!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug() {
+    debug!(foo = ?3, bar.baz = %2, quux = false);
+    debug!(foo = 3, bar.baz = 2, quux = false);
+    debug!(foo = 3, bar.baz = 3,);
+    debug!("foo");
+    debug!("foo: {}", 3);
+    debug!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    debug!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    debug!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    debug!({ foo = 3, bar.baz = 80 }, "quux");
+    debug!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!({ foo = 2, bar.baz = 78 }, "quux");
+    debug!({ foo = ?2, bar.baz = %78 }, "quux");
+    debug!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    debug!(target: "foo_events", foo = 3, bar.baz = 3,);
+    debug!(target: "foo_events", "foo");
+    debug!(target: "foo_events", "foo: {}", 3);
+    debug!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    debug!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    debug!(?foo);
+    debug!(%foo);
+    debug!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info() {
+    info!(foo = ?3, bar.baz = %2, quux = false);
+    info!(foo = 3, bar.baz = 2, quux = false);
+    info!(foo = 3, bar.baz = 3,);
+    info!("foo");
+    info!("foo: {}", 3);
+    info!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    info!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    info!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    info!({ foo = 3, bar.baz = 80 }, "quux");
+    info!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!({ foo = 2, bar.baz = 78 }, "quux");
+    info!({ foo = ?2, bar.baz = %78 }, "quux");
+    info!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    info!(target: "foo_events", foo = 3, bar.baz = 3,);
+    info!(target: "foo_events", "foo");
+    info!(target: "foo_events", "foo: {}", 3);
+    info!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    info!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    info!(?foo);
+    info!(%foo);
+    info!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn() {
+    warn!(foo = ?3, bar.baz = %2, quux = false);
+    warn!(foo = 3, bar.baz = 2, quux = false);
+    warn!(foo = 3, bar.baz = 3,);
+    warn!("foo");
+    warn!("foo: {}", 3);
+    warn!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    warn!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    warn!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    warn!({ foo = 3, bar.baz = 80 }, "quux");
+    warn!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!({ foo = 2, bar.baz = 78 }, "quux");
+    warn!({ foo = ?2, bar.baz = %78 }, "quux");
+    warn!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    warn!(target: "foo_events", foo = 3, bar.baz = 3,);
+    warn!(target: "foo_events", "foo");
+    warn!(target: "foo_events", "foo: {}", 3);
+    warn!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    warn!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    warn!(?foo);
+    warn!(%foo);
+    warn!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error() {
+    error!(foo = ?3, bar.baz = %2, quux = false);
+    error!(foo = 3, bar.baz = 2, quux = false);
+    error!(foo = 3, bar.baz = 3,);
+    error!("foo");
+    error!("foo: {}", 3);
+    error!(foo = ?3, bar.baz = %2, quux = false, "hello world {:?}", 42);
+    error!(foo = 3, bar.baz = 2, quux = false, "hello world {:?}", 42);
+    error!(foo = 3, bar.baz = 3, "hello world {:?}", 42,);
+    error!({ foo = 3, bar.baz = 80 }, "quux");
+    error!({ foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!({ foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!({ foo = 2, bar.baz = 78, }, "quux");
+    error!({ foo = ?2, bar.baz = %78 }, "quux");
+    error!(target: "foo_events", foo = 3, bar.baz = 2, quux = false);
+    error!(target: "foo_events", foo = 3, bar.baz = 3,);
+    error!(target: "foo_events", "foo");
+    error!(target: "foo_events", "foo: {}", 3);
+    error!(target: "foo_events", { foo = 3, bar.baz = 80 }, "quux");
+    error!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(target: "foo_events", { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(target: "foo_events", { foo = 2, bar.baz = 78, }, "quux");
+    let foo = 1;
+    error!(?foo);
+    error!(%foo);
+    error!(foo);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_root() {
+    event!(parent: None, Level::DEBUG, foo = ?3, bar.baz = %2, quux = false);
+    event!(
+        parent: None,
+        Level::DEBUG,
+        foo = 3,
+        bar.baz = 2,
+        quux = false
+    );
+    event!(parent: None, Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(parent: None, Level::DEBUG, "foo");
+    event!(parent: None, Level::DEBUG, "foo: {}", 3);
+    event!(parent: None, Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(parent: None, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(parent: None, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(parent: None, Level::DEBUG, { foo = ?2, bar.baz = %78 }, "quux");
+    event!(target: "foo_events", parent: None, Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(target: "foo_events", parent: None, Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(target: "foo_events", parent: None, Level::DEBUG, "foo");
+    event!(target: "foo_events", parent: None, Level::DEBUG, "foo: {}", 3);
+    event!(target: "foo_events", parent: None, Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(target: "foo_events", parent: None, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(target: "foo_events", parent: None, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(target: "foo_events", parent: None, Level::DEBUG, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_root() {
+    trace!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    trace!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    trace!(parent: None, foo = 3, bar.baz = 3,);
+    trace!(parent: None, "foo");
+    trace!(parent: None, "foo: {}", 3);
+    trace!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    trace!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    trace!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    trace!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    trace!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    trace!(target: "foo_events", parent: None, "foo");
+    trace!(target: "foo_events", parent: None, "foo: {}", 3);
+    trace!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    trace!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_root() {
+    debug!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    debug!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    debug!(parent: None, foo = 3, bar.baz = 3,);
+    debug!(parent: None, "foo");
+    debug!(parent: None, "foo: {}", 3);
+    debug!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    debug!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    debug!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    debug!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    debug!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    debug!(target: "foo_events", parent: None, "foo");
+    debug!(target: "foo_events", parent: None, "foo: {}", 3);
+    debug!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    debug!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_root() {
+    info!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    info!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    info!(parent: None, foo = 3, bar.baz = 3,);
+    info!(parent: None, "foo");
+    info!(parent: None, "foo: {}", 3);
+    info!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    info!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    info!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    info!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    info!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    info!(target: "foo_events", parent: None, "foo");
+    info!(target: "foo_events", parent: None, "foo: {}", 3);
+    info!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    info!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_root() {
+    warn!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    warn!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    warn!(parent: None, foo = 3, bar.baz = 3,);
+    warn!(parent: None, "foo");
+    warn!(parent: None, "foo: {}", 3);
+    warn!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    warn!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    warn!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    warn!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    warn!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    warn!(target: "foo_events", parent: None, "foo");
+    warn!(target: "foo_events", parent: None, "foo: {}", 3);
+    warn!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    warn!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_root() {
+    error!(parent: None, foo = ?3, bar.baz = %2, quux = false);
+    error!(parent: None, foo = 3, bar.baz = 2, quux = false);
+    error!(parent: None, foo = 3, bar.baz = 3,);
+    error!(parent: None, "foo");
+    error!(parent: None, "foo: {}", 3);
+    error!(parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    error!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(parent: None, { foo = 2, bar.baz = 78 }, "quux");
+    error!(parent: None, { foo = ?2, bar.baz = %78 }, "quux");
+    error!(target: "foo_events", parent: None, foo = 3, bar.baz = 2, quux = false);
+    error!(target: "foo_events", parent: None, foo = 3, bar.baz = 3,);
+    error!(target: "foo_events", parent: None, "foo");
+    error!(target: "foo_events", parent: None, "foo: {}", 3);
+    error!(target: "foo_events", parent: None, { foo = 3, bar.baz = 80 }, "quux");
+    error!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(target: "foo_events", parent: None, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(target: "foo_events", parent: None, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    event!(parent: &p, Level::DEBUG, foo = ?3, bar.baz = %2, quux = false);
+    event!(parent: &p, Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(parent: &p, Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(parent: &p, Level::DEBUG, "foo");
+    event!(parent: &p, Level::DEBUG, "foo: {}", 3);
+    event!(parent: &p, Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(parent: &p, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(parent: &p, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(parent: &p, Level::DEBUG, { foo = ?2, bar.baz = %78 }, "quux");
+    event!(target: "foo_events", parent: &p, Level::DEBUG, foo = 3, bar.baz = 2, quux = false);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, foo = 3, bar.baz = 3,);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, "foo");
+    event!(target: "foo_events", parent: &p, Level::DEBUG, "foo: {}", 3);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, { foo = 3, bar.baz = 80 }, "quux");
+    event!(target: "foo_events", parent: &p, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    event!(target: "foo_events", parent: &p, Level::DEBUG, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn trace_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    trace!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    trace!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    trace!(parent: &p, foo = 3, bar.baz = 3,);
+    trace!(parent: &p, "foo");
+    trace!(parent: &p, "foo: {}", 3);
+    trace!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    trace!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    trace!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    trace!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    trace!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    trace!(target: "foo_events", parent: &p, "foo");
+    trace!(target: "foo_events", parent: &p, "foo: {}", 3);
+    trace!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    trace!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    trace!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    trace!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    debug!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    debug!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    debug!(parent: &p, foo = 3, bar.baz = 3,);
+    debug!(parent: &p, "foo");
+    debug!(parent: &p, "foo: {}", 3);
+    debug!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    debug!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    debug!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    debug!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    debug!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    debug!(target: "foo_events", parent: &p, "foo");
+    debug!(target: "foo_events", parent: &p, "foo: {}", 3);
+    debug!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    debug!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    debug!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    debug!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn info_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    info!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    info!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    info!(parent: &p, foo = 3, bar.baz = 3,);
+    info!(parent: &p, "foo");
+    info!(parent: &p, "foo: {}", 3);
+    info!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    info!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    info!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    info!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    info!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    info!(target: "foo_events", parent: &p, "foo");
+    info!(target: "foo_events", parent: &p, "foo: {}", 3);
+    info!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    info!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    info!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    info!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn warn_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    warn!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    warn!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    warn!(parent: &p, foo = 3, bar.baz = 3,);
+    warn!(parent: &p, "foo");
+    warn!(parent: &p, "foo: {}", 3);
+    warn!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    warn!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    warn!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    warn!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    warn!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    warn!(target: "foo_events", parent: &p, "foo");
+    warn!(target: "foo_events", parent: &p, "foo: {}", 3);
+    warn!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    warn!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    warn!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    warn!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn error_with_parent() {
+    let p = span!(Level::TRACE, "im_a_parent!");
+    error!(parent: &p, foo = ?3, bar.baz = %2, quux = false);
+    error!(parent: &p, foo = 3, bar.baz = 2, quux = false);
+    error!(parent: &p, foo = 3, bar.baz = 3,);
+    error!(parent: &p, "foo");
+    error!(parent: &p, "foo: {}", 3);
+    error!(parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    error!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(parent: &p, { foo = 2, bar.baz = 78 }, "quux");
+    error!(parent: &p, { foo = ?2, bar.baz = %78 }, "quux");
+    error!(target: "foo_events", parent: &p, foo = 3, bar.baz = 2, quux = false);
+    error!(target: "foo_events", parent: &p, foo = 3, bar.baz = 3,);
+    error!(target: "foo_events", parent: &p, "foo");
+    error!(target: "foo_events", parent: &p, "foo: {}", 3);
+    error!(target: "foo_events", parent: &p, { foo = 3, bar.baz = 80 }, "quux");
+    error!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}", true);
+    error!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 79 }, "quux {:?}, {quux}", true, quux = false);
+    error!(target: "foo_events", parent: &p, { foo = 2, bar.baz = 78, }, "quux");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn field_shorthand_only() {
+    #[derive(Debug)]
+    struct Position {
+        x: f32,
+        y: f32,
+    }
+    let pos = Position {
+        x: 3.234,
+        y: -1.223,
+    };
+
+    trace!(?pos.x, ?pos.y);
+    debug!(?pos.x, ?pos.y);
+    info!(?pos.x, ?pos.y);
+    warn!(?pos.x, ?pos.y);
+    error!(?pos.x, ?pos.y);
+    event!(Level::TRACE, ?pos.x, ?pos.y);
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn borrow_val_events() {
+    // Reproduces https://github.com/tokio-rs/tracing/issues/954
+    let mut foo = (String::new(), String::new());
+    let zero = &mut foo.0;
+    trace!(one = ?foo.1);
+    debug!(one = ?foo.1);
+    info!(one = ?foo.1);
+    warn!(one = ?foo.1);
+    error!(one = ?foo.1);
+    zero.push_str("hello world");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn borrow_val_spans() {
+    // Reproduces https://github.com/tokio-rs/tracing/issues/954
+    let mut foo = (String::new(), String::new());
+    let zero = &mut foo.0;
+    let _span = trace_span!("span", one = ?foo.1);
+    let _span = debug_span!("span", one = ?foo.1);
+    let _span = info_span!("span", one = ?foo.1);
+    let _span = warn_span!("span", one = ?foo.1);
+    let _span = error_span!("span", one = ?foo.1);
+    zero.push_str("hello world");
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn callsite_macro_api() {
+    // This test should catch any inadvertent breaking changes
+    // caused by changes to the macro.
+    let _callsite = callsite! {
+        name: "test callsite",
+        kind: tracing::metadata::Kind::EVENT,
+        target: "test target",
+        level: tracing::Level::TRACE,
+        fields: foo, bar,
+    };
+    let _callsite = callsite! {
+        name: "test callsite",
+        kind: tracing::metadata::Kind::SPAN,
+        level: tracing::Level::TRACE,
+        fields: foo,
+    };
+    let _callsite = callsite! {
+        name: "test callsite",
+        kind: tracing::metadata::Kind::SPAN,
+        fields: foo,
+    };
+}
diff --git a/third_party/rust/tracing/tests/max_level_hint.rs b/third_party/rust/tracing/tests/max_level_hint.rs
new file mode 100644
index 0000000000..49f14d55fe
--- /dev/null
+++ b/third_party/rust/tracing/tests/max_level_hint.rs
@@ -0,0 +1,39 @@
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn max_level_hints() {
+    // This test asserts that when a subscriber provides us with the global
+    // maximum level that it will enable (by implementing the
+    // `Subscriber::max_level_hint` method), we will never call
+    // `Subscriber::enabled` for events above that maximum level.
+    //
+    // In this case, we test that by making the `enabled` method assert that no
+    // `Metadata` for spans or events at the `TRACE` or `DEBUG` levels.
+    let (subscriber, handle) = subscriber::mock()
+        .with_max_level_hint(Level::INFO)
+        .with_filter(|meta| {
+            assert!(
+                dbg!(meta).level() <= &Level::INFO,
+                "a TRACE or DEBUG event was dynamically filtered: "
+            );
+            true
+        })
+        .event(event::mock().at_level(Level::INFO))
+        .event(event::mock().at_level(Level::WARN))
+        .event(event::mock().at_level(Level::ERROR))
+        .done()
+        .run_with_handle();
+
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+
+    tracing::info!("doing a thing that you might care about");
+    tracing::debug!("charging turboencabulator with interocitor");
+    tracing::warn!("extremely serious warning, pay attention");
+    tracing::trace!("interocitor charge level is 10%");
+    tracing::error!("everything is on fire");
+    handle.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/multiple_max_level_hints.rs b/third_party/rust/tracing/tests/multiple_max_level_hints.rs
new file mode 100644
index 0000000000..b2be6351cf
--- /dev/null
+++ b/third_party/rust/tracing/tests/multiple_max_level_hints.rs
@@ -0,0 +1,70 @@
+#![cfg(feature = "std")]
+mod support;
+
+use self::support::*;
+use tracing::Level;
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn multiple_max_level_hints() {
+    // This test ensures that when multiple subscribers are active, their max
+    // level hints are handled correctly. The global max level should be the
+    // maximum of the level filters returned by the two `Subscriber`'s
+    // `max_level_hint` method.
+    //
+    // In this test, we create a subscriber whose max level is `INFO`, and
+    // another whose max level is `DEBUG`. We then add an assertion to both of
+    // those subscribers' `enabled` method that no metadata for `TRACE` spans or
+    // events are filtered, since they are disabled by the global max filter.
+
+    fn do_events() {
+        tracing::info!("doing a thing that you might care about");
+        tracing::debug!("charging turboencabulator with interocitor");
+        tracing::warn!("extremely serious warning, pay attention");
+        tracing::trace!("interocitor charge level is 10%");
+        tracing::error!("everything is on fire");
+    }
+
+    let (subscriber1, handle1) = subscriber::mock()
+        .named("subscriber1")
+        .with_max_level_hint(Level::INFO)
+        .with_filter(|meta| {
+            let level = dbg!(meta.level());
+            assert!(
+                level <= &Level::DEBUG,
+                "a TRACE event was dynamically filtered by subscriber1"
+            );
+            level <= &Level::INFO
+        })
+        .event(event::mock().at_level(Level::INFO))
+        .event(event::mock().at_level(Level::WARN))
+        .event(event::mock().at_level(Level::ERROR))
+        .done()
+        .run_with_handle();
+    let (subscriber2, handle2) = subscriber::mock()
+        .named("subscriber2")
+        .with_max_level_hint(Level::DEBUG)
+        .with_filter(|meta| {
+            let level = dbg!(meta.level());
+            assert!(
+                level <= &Level::DEBUG,
+                "a TRACE event was dynamically filtered by subscriber2"
+            );
+            level <= &Level::DEBUG
+        })
+        .event(event::mock().at_level(Level::INFO))
+        .event(event::mock().at_level(Level::DEBUG))
+        .event(event::mock().at_level(Level::WARN))
+        .event(event::mock().at_level(Level::ERROR))
+        .done()
+        .run_with_handle();
+
+    let dispatch1 = tracing::Dispatch::new(subscriber1);
+
+    tracing::dispatcher::with_default(&dispatch1, do_events);
+    handle1.assert_finished();
+
+    let dispatch2 = tracing::Dispatch::new(subscriber2);
+    tracing::dispatcher::with_default(&dispatch2, do_events);
+    handle2.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/span.rs b/third_party/rust/tracing/tests/span.rs
new file mode 100644
index 0000000000..79914950c2
--- /dev/null
+++ b/third_party/rust/tracing/tests/span.rs
@@ -0,0 +1,767 @@
+// These tests require the thread-local scoped dispatcher, which only works when
+// we have a standard library. The behaviour being tested should be the same
+// with the standard lib disabled.
+#![cfg(feature = "std")]
+
+#[macro_use]
+extern crate tracing;
+mod support;
+
+use self::support::*;
+use std::thread;
+use tracing::{
+    field::{debug, display},
+    subscriber::with_default,
+    Level, Span,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn handles_to_the_same_span_are_equal() {
+    // Create a mock subscriber that will return `true` on calls to
+    // `Subscriber::enabled`, so that the spans will be constructed. We
+    // won't enter any spans in this test, so the subscriber won't actually
+    // expect to see any spans.
+    with_default(subscriber::mock().run(), || {
+        let foo1 = span!(Level::TRACE, "foo");
+        let foo2 = foo1.clone();
+        // Two handles that point to the same span are equal.
+        assert_eq!(foo1, foo2);
+    });
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn handles_to_different_spans_are_not_equal() {
+    with_default(subscriber::mock().run(), || {
+        // Even though these spans have the same name and fields, they will have
+        // differing metadata, since they were created on different lines.
+        let foo1 = span!(Level::TRACE, "foo", bar = 1u64, baz = false);
+        let foo2 = span!(Level::TRACE, "foo", bar = 1u64, baz = false);
+
+        assert_ne!(foo1, foo2);
+    });
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn handles_to_different_spans_with_the_same_metadata_are_not_equal() {
+    // Every time time this function is called, it will return a _new
+    // instance_ of a span with the same metadata, name, and fields.
+    fn make_span() -> Span {
+        span!(Level::TRACE, "foo", bar = 1u64, baz = false)
+    }
+
+    with_default(subscriber::mock().run(), || {
+        let foo1 = make_span();
+        let foo2 = make_span();
+
+        assert_ne!(foo1, foo2);
+        // assert_ne!(foo1.data(), foo2.data());
+    });
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn spans_always_go_to_the_subscriber_that_tagged_them() {
+    let subscriber1 = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run();
+    let subscriber2 = subscriber::mock().run();
+
+    let foo = with_default(subscriber1, || {
+        let foo = span!(Level::TRACE, "foo");
+        foo.in_scope(|| {});
+        foo
+    });
+    // Even though we enter subscriber 2's context, the subscriber that
+    // tagged the span should see the enter/exit.
+    with_default(subscriber2, move || foo.in_scope(|| {}));
+}
+
+// This gets exempt from testing in wasm because of: `thread::spawn` which is
+// not yet possible to do in WASM. There is work going on see:
+// <https://rustwasm.github.io/2018/10/24/multithreading-rust-and-wasm.html>
+//
+// But for now since it's not possible we don't need to test for it :)
+#[test]
+fn spans_always_go_to_the_subscriber_that_tagged_them_even_across_threads() {
+    let subscriber1 = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run();
+    let foo = with_default(subscriber1, || {
+        let foo = span!(Level::TRACE, "foo");
+        foo.in_scope(|| {});
+        foo
+    });
+
+    // Even though we enter subscriber 2's context, the subscriber that
+    // tagged the span should see the enter/exit.
+    thread::spawn(move || {
+        with_default(subscriber::mock().run(), || {
+            foo.in_scope(|| {});
+        })
+    })
+    .join()
+    .unwrap();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn dropping_a_span_calls_drop_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo");
+        span.in_scope(|| {});
+        drop(span);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_closes_after_event() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .event(event::mock())
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo").in_scope(|| {
+            event!(Level::DEBUG, {}, "my event!");
+        });
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn new_span_after_event() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .event(event::mock())
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .enter(span::mock().named("bar"))
+        .exit(span::mock().named("bar"))
+        .drop_span(span::mock().named("bar"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo").in_scope(|| {
+            event!(Level::DEBUG, {}, "my event!");
+        });
+        span!(Level::TRACE, "bar").in_scope(|| {});
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_outside_of_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .event(event::mock())
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        debug!("my event!");
+        span!(Level::TRACE, "foo").in_scope(|| {});
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn cloning_a_span_calls_clone_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .clone_span(span::mock().named("foo"))
+        .run_with_handle();
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo");
+        // Allow the "redundant" `.clone` since it is used to call into the `.clone_span` hook.
+        #[allow(clippy::redundant_clone)]
+        let _span2 = span.clone();
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn drop_span_when_exiting_dispatchers_context() {
+    let (subscriber, handle) = subscriber::mock()
+        .clone_span(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .run_with_handle();
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo");
+        let _span2 = span.clone();
+        drop(span);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn clone_and_drop_span_always_go_to_the_subscriber_that_tagged_the_span() {
+    let (subscriber1, handle1) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .clone_span(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .run_with_handle();
+    let subscriber2 = subscriber::mock().done().run();
+
+    let foo = with_default(subscriber1, || {
+        let foo = span!(Level::TRACE, "foo");
+        foo.in_scope(|| {});
+        foo
+    });
+    // Even though we enter subscriber 2's context, the subscriber that
+    // tagged the span should see the enter/exit.
+    with_default(subscriber2, move || {
+        let foo2 = foo.clone();
+        foo.in_scope(|| {});
+        drop(foo);
+        drop(foo2);
+    });
+
+    handle1.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn span_closes_when_exited() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+
+        foo.in_scope(|| {});
+
+        drop(foo);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn enter() {
+    let (subscriber, handle) = subscriber::mock()
+        .enter(span::mock().named("foo"))
+        .event(event::mock())
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        let _enter = foo.enter();
+        debug!("dropping guard...");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn moved_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("bar")
+                    .with_value(&display("hello from my span"))
+                    .only(),
+            ),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        let from = "my span";
+        let span = span!(
+            Level::TRACE,
+            "foo",
+            bar = display(format!("hello from {}", from))
+        );
+        span.in_scope(|| {});
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn dotted_field_name() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock()
+                .named("foo")
+                .with_field(field::mock("fields.bar").with_value(&true).only()),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo", fields.bar = true);
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn borrowed_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("bar")
+                    .with_value(&display("hello from my span"))
+                    .only(),
+            ),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let from = "my span";
+        let mut message = format!("hello from {}", from);
+        let span = span!(Level::TRACE, "foo", bar = display(&message));
+        span.in_scope(|| {
+            message.insert_str(10, " inside");
+        });
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+// If emitting log instrumentation, this gets moved anyway, breaking the test.
+#[cfg(not(feature = "log"))]
+fn move_field_out_of_struct() {
+    use tracing::field::debug;
+
+    #[derive(Debug)]
+    struct Position {
+        x: f32,
+        y: f32,
+    }
+
+    let pos = Position {
+        x: 3.234,
+        y: -1.223,
+    };
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("x")
+                    .with_value(&debug(3.234))
+                    .and(field::mock("y").with_value(&debug(-1.223)))
+                    .only(),
+            ),
+        )
+        .new_span(
+            span::mock()
+                .named("bar")
+                .with_field(field::mock("position").with_value(&debug(&pos)).only()),
+        )
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let pos = Position {
+            x: 3.234,
+            y: -1.223,
+        };
+        let foo = span!(Level::TRACE, "foo", x = debug(pos.x), y = debug(pos.y));
+        let bar = span!(Level::TRACE, "bar", position = debug(pos));
+        foo.in_scope(|| {});
+        bar.in_scope(|| {});
+    });
+
+    handle.assert_finished();
+}
+
+// TODO(#1138): determine a new syntax for uninitialized span fields, and
+// re-enable these.
+/*
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn add_field_after_new_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock()
+                .named("foo")
+                .with_field(field::mock("bar").with_value(&5)
+                .and(field::mock("baz").with_value).only()),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("baz").with_value(&true).only(),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo", bar = 5, baz = false);
+        span.record("baz", &true);
+        span.in_scope(|| {})
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn add_fields_only_after_new_span() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .record(
+            span::mock().named("foo"),
+            field::mock("bar").with_value(&5).only(),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("baz").with_value(&true).only(),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo", bar = _, baz = _);
+        span.record("bar", &5);
+        span.record("baz", &true);
+        span.in_scope(|| {})
+    });
+
+    handle.assert_finished();
+}
+*/
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn record_new_value_for_field() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("bar")
+                    .with_value(&5)
+                    .and(field::mock("baz").with_value(&false))
+                    .only(),
+            ),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("baz").with_value(&true).only(),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo", bar = 5, baz = false);
+        span.record("baz", &true);
+        span.in_scope(|| {})
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn record_new_values_for_fields() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("foo").with_field(
+                field::mock("bar")
+                    .with_value(&4)
+                    .and(field::mock("baz").with_value(&false))
+                    .only(),
+            ),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("bar").with_value(&5).only(),
+        )
+        .record(
+            span::mock().named("foo"),
+            field::mock("baz").with_value(&true).only(),
+        )
+        .enter(span::mock().named("foo"))
+        .exit(span::mock().named("foo"))
+        .drop_span(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let span = span!(Level::TRACE, "foo", bar = 4, baz = false);
+        span.record("bar", &5);
+        span.record("baz", &true);
+        span.in_scope(|| {})
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn new_span_with_target_and_log_level() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock()
+                .named("foo")
+                .with_target("app_span")
+                .at_level(Level::DEBUG),
+        )
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(target: "app_span", Level::DEBUG, "foo");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_root_span_is_root() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo").with_explicit_parent(None))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(parent: None, Level::TRACE, "foo");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_root_span_is_root_regardless_of_ctx() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .new_span(span::mock().named("bar").with_explicit_parent(None))
+        .exit(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo").in_scope(|| {
+            span!(parent: None, Level::TRACE, "bar");
+        })
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .new_span(span::mock().named("bar").with_explicit_parent(Some("foo")))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        span!(parent: foo.id(), Level::TRACE, "bar");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child_at_levels() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .new_span(span::mock().named("a").with_explicit_parent(Some("foo")))
+        .new_span(span::mock().named("b").with_explicit_parent(Some("foo")))
+        .new_span(span::mock().named("c").with_explicit_parent(Some("foo")))
+        .new_span(span::mock().named("d").with_explicit_parent(Some("foo")))
+        .new_span(span::mock().named("e").with_explicit_parent(Some("foo")))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        trace_span!(parent: foo.id(), "a");
+        debug_span!(parent: foo.id(), "b");
+        info_span!(parent: foo.id(), "c");
+        warn_span!(parent: foo.id(), "d");
+        error_span!(parent: foo.id(), "e");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn explicit_child_regardless_of_ctx() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .new_span(span::mock().named("bar"))
+        .enter(span::mock().named("bar"))
+        .new_span(span::mock().named("baz").with_explicit_parent(Some("foo")))
+        .exit(span::mock().named("bar"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        let foo = span!(Level::TRACE, "foo");
+        span!(Level::TRACE, "bar").in_scope(|| span!(parent: foo.id(), Level::TRACE, "baz"))
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn contextual_root() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo").with_contextual_parent(None))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn contextual_child() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(span::mock().named("foo"))
+        .enter(span::mock().named("foo"))
+        .new_span(
+            span::mock()
+                .named("bar")
+                .with_contextual_parent(Some("foo")),
+        )
+        .exit(span::mock().named("foo"))
+        .done()
+        .run_with_handle();
+
+    with_default(subscriber, || {
+        span!(Level::TRACE, "foo").in_scope(|| {
+            span!(Level::TRACE, "bar");
+        })
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn display_shorthand() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("my_span").with_field(
+                field::mock("my_field")
+                    .with_value(&display("hello world"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "my_span", my_field = %"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn debug_shorthand() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("my_span").with_field(
+                field::mock("my_field")
+                    .with_value(&debug("hello world"))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "my_span", my_field = ?"hello world");
+    });
+
+    handle.assert_finished();
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn both_shorthands() {
+    let (subscriber, handle) = subscriber::mock()
+        .new_span(
+            span::mock().named("my_span").with_field(
+                field::mock("display_field")
+                    .with_value(&display("hello world"))
+                    .and(field::mock("debug_field").with_value(&debug("hello world")))
+                    .only(),
+            ),
+        )
+        .done()
+        .run_with_handle();
+    with_default(subscriber, || {
+        span!(Level::TRACE, "my_span", display_field = %"hello world", debug_field = ?"hello world");
+    });
+
+    handle.assert_finished();
+}
diff --git a/third_party/rust/tracing/tests/subscriber.rs b/third_party/rust/tracing/tests/subscriber.rs
new file mode 100644
index 0000000000..2a810c9111
--- /dev/null
+++ b/third_party/rust/tracing/tests/subscriber.rs
@@ -0,0 +1,57 @@
+// These tests require the thread-local scoped dispatcher, which only works when
+// we have a standard library. The behaviour being tested should be the same
+// with the standard lib disabled.
+//
+// The alternative would be for each of these tests to be defined in a separate
+// file, which is :(
+#![cfg(feature = "std")]
+
+#[macro_use]
+extern crate tracing;
+use tracing::{
+    span,
+    subscriber::{with_default, Interest, Subscriber},
+    Event, Level, Metadata,
+};
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen_test::wasm_bindgen_test)]
+#[test]
+fn event_macros_dont_infinite_loop() {
+    // This test ensures that an event macro within a subscriber
+    // won't cause an infinite loop of events.
+    struct TestSubscriber;
+    impl Subscriber for TestSubscriber {
+        fn register_callsite(&self, _: &Metadata<'_>) -> Interest {
+            // Always return sometimes so that `enabled` will be called
+            // (which can loop).
+            Interest::sometimes()
+        }
+
+        fn enabled(&self, meta: &Metadata<'_>) -> bool {
+            assert!(meta.fields().iter().any(|f| f.name() == "foo"));
+            event!(Level::TRACE, bar = false);
+            true
+        }
+
+        fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
+            span::Id::from_u64(0xAAAA)
+        }
+
+        fn record(&self, _: &span::Id, _: &span::Record<'_>) {}
+
+        fn record_follows_from(&self, _: &span::Id, _: &span::Id) {}
+
+        fn event(&self, event: &Event<'_>) {
+            assert!(event.metadata().fields().iter().any(|f| f.name() == "foo"));
+            event!(Level::TRACE, baz = false);
+        }
+
+        fn enter(&self, _: &span::Id) {}
+
+        fn exit(&self, _: &span::Id) {}
+    }
+
+    with_default(TestSubscriber, || {
+        event!(Level::TRACE, foo = false);
+    })
+}
diff --git a/third_party/rust/tracing/tests/support/event.rs b/third_party/rust/tracing/tests/support/event.rs
new file mode 100644
index 0000000000..7033d8a134
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/event.rs
@@ -0,0 +1,99 @@
+#![allow(missing_docs)]
+use super::{field, metadata, Parent};
+
+use std::fmt;
+
+/// A mock event.
+///
+/// This is intended for use with the mock subscriber API in the
+/// `subscriber` module.
+#[derive(Debug, Default, Eq, PartialEq)]
+pub struct MockEvent {
+    pub fields: Option<field::Expect>,
+    pub(in crate::support) parent: Option<Parent>,
+    metadata: metadata::Expect,
+}
+
+pub fn mock() -> MockEvent {
+    MockEvent {
+        ..Default::default()
+    }
+}
+
+impl MockEvent {
+    pub fn named<I>(self, name: I) -> Self
+    where
+        I: Into<String>,
+    {
+        Self {
+            metadata: metadata::Expect {
+                name: Some(name.into()),
+                ..self.metadata
+            },
+            ..self
+        }
+    }
+
+    pub fn with_fields<I>(self, fields: I) -> Self
+    where
+        I: Into<field::Expect>,
+    {
+        Self {
+            fields: Some(fields.into()),
+            ..self
+        }
+    }
+
+    pub fn at_level(self, level: tracing::Level) -> Self {
+        Self {
+            metadata: metadata::Expect {
+                level: Some(level),
+                ..self.metadata
+            },
+            ..self
+        }
+    }
+
+    pub fn with_target<I>(self, target: I) -> Self
+    where
+        I: Into<String>,
+    {
+        Self {
+            metadata: metadata::Expect {
+                target: Some(target.into()),
+                ..self.metadata
+            },
+            ..self
+        }
+    }
+
+    pub fn with_explicit_parent(self, parent: Option<&str>) -> MockEvent {
+        let parent = match parent {
+            Some(name) => Parent::Explicit(name.into()),
+            None => Parent::ExplicitRoot,
+        };
+        Self {
+            parent: Some(parent),
+            ..self
+        }
+    }
+
+    pub(in crate::support) fn check(&mut self, event: &tracing::Event<'_>) {
+        let meta = event.metadata();
+        let name = meta.name();
+        self.metadata
+            .check(meta, format_args!("event \"{}\"", name));
+        assert!(meta.is_event(), "expected {}, but got {:?}", self, event);
+        if let Some(ref mut expected_fields) = self.fields {
+            let mut checker = expected_fields.checker(name.to_string());
+            event.record(&mut checker);
+            checker.finish();
+        }
+    }
+}
+
+impl fmt::Display for MockEvent {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "an event{}", self.metadata)
+    }
+}
diff --git a/third_party/rust/tracing/tests/support/field.rs b/third_party/rust/tracing/tests/support/field.rs
new file mode 100644
index 0000000000..3667cf08b4
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/field.rs
@@ -0,0 +1,226 @@
+use tracing::{
+    callsite,
+    callsite::Callsite,
+    field::{self, Field, Value, Visit},
+    metadata::Kind,
+};
+
+use std::{collections::HashMap, fmt};
+
+#[derive(Default, Debug, Eq, PartialEq)]
+pub struct Expect {
+    fields: HashMap<String, MockValue>,
+    only: bool,
+}
+
+#[derive(Debug)]
+pub struct MockField {
+    name: String,
+    value: MockValue,
+}
+
+#[derive(Debug, Eq, PartialEq)]
+pub enum MockValue {
+    I64(i64),
+    U64(u64),
+    Bool(bool),
+    Str(String),
+    Debug(String),
+    Any,
+}
+
+pub fn mock<K>(name: K) -> MockField
+where
+    String: From<K>,
+{
+    MockField {
+        name: name.into(),
+        value: MockValue::Any,
+    }
+}
+
+impl MockField {
+    /// Expect a field with the given name and value.
+    pub fn with_value(self, value: &dyn Value) -> Self {
+        Self {
+            value: MockValue::from(value),
+            ..self
+        }
+    }
+
+    pub fn and(self, other: MockField) -> Expect {
+        Expect {
+            fields: HashMap::new(),
+            only: false,
+        }
+        .and(self)
+        .and(other)
+    }
+
+    pub fn only(self) -> Expect {
+        Expect {
+            fields: HashMap::new(),
+            only: true,
+        }
+        .and(self)
+    }
+}
+
+impl Into<Expect> for MockField {
+    fn into(self) -> Expect {
+        Expect {
+            fields: HashMap::new(),
+            only: false,
+        }
+        .and(self)
+    }
+}
+
+impl Expect {
+    pub fn and(mut self, field: MockField) -> Self {
+        self.fields.insert(field.name, field.value);
+        self
+    }
+
+    /// Indicates that no fields other than those specified should be expected.
+    pub fn only(self) -> Self {
+        Self { only: true, ..self }
+    }
+
+    fn compare_or_panic(&mut self, name: &str, value: &dyn Value, ctx: &str) {
+        let value = value.into();
+        match self.fields.remove(name) {
+            Some(MockValue::Any) => {}
+            Some(expected) => assert!(
+                expected == value,
+                "\nexpected `{}` to contain:\n\t`{}{}`\nbut got:\n\t`{}{}`",
+                ctx,
+                name,
+                expected,
+                name,
+                value
+            ),
+            None if self.only => panic!(
+                "\nexpected `{}` to contain only:\n\t`{}`\nbut got:\n\t`{}{}`",
+                ctx, self, name, value
+            ),
+            _ => {}
+        }
+    }
+
+    pub fn checker(&mut self, ctx: String) -> CheckVisitor<'_> {
+        CheckVisitor { expect: self, ctx }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.fields.is_empty()
+    }
+}
+
+impl fmt::Display for MockValue {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            MockValue::I64(v) => write!(f, "i64 = {:?}", v),
+            MockValue::U64(v) => write!(f, "u64 = {:?}", v),
+            MockValue::Bool(v) => write!(f, "bool = {:?}", v),
+            MockValue::Str(v) => write!(f, "&str = {:?}", v),
+            MockValue::Debug(v) => write!(f, "&fmt::Debug = {:?}", v),
+            MockValue::Any => write!(f, "_ = _"),
+        }
+    }
+}
+
+pub struct CheckVisitor<'a> {
+    expect: &'a mut Expect,
+    ctx: String,
+}
+
+impl<'a> Visit for CheckVisitor<'a> {
+    fn record_i64(&mut self, field: &Field, value: i64) {
+        self.expect
+            .compare_or_panic(field.name(), &value, &self.ctx[..])
+    }
+
+    fn record_u64(&mut self, field: &Field, value: u64) {
+        self.expect
+            .compare_or_panic(field.name(), &value, &self.ctx[..])
+    }
+
+    fn record_bool(&mut self, field: &Field, value: bool) {
+        self.expect
+            .compare_or_panic(field.name(), &value, &self.ctx[..])
+    }
+
+    fn record_str(&mut self, field: &Field, value: &str) {
+        self.expect
+            .compare_or_panic(field.name(), &value, &self.ctx[..])
+    }
+
+    fn record_debug(&mut self, field: &Field, value: &dyn fmt::Debug) {
+        self.expect
+            .compare_or_panic(field.name(), &field::debug(value), &self.ctx)
+    }
+}
+
+impl<'a> CheckVisitor<'a> {
+    pub fn finish(self) {
+        assert!(
+            self.expect.fields.is_empty(),
+            "{}missing {}",
+            self.expect,
+            self.ctx
+        );
+    }
+}
+
+impl<'a> From<&'a dyn Value> for MockValue {
+    fn from(value: &'a dyn Value) -> Self {
+        struct MockValueBuilder {
+            value: Option<MockValue>,
+        }
+
+        impl Visit for MockValueBuilder {
+            fn record_i64(&mut self, _: &Field, value: i64) {
+                self.value = Some(MockValue::I64(value));
+            }
+
+            fn record_u64(&mut self, _: &Field, value: u64) {
+                self.value = Some(MockValue::U64(value));
+            }
+
+            fn record_bool(&mut self, _: &Field, value: bool) {
+                self.value = Some(MockValue::Bool(value));
+            }
+
+            fn record_str(&mut self, _: &Field, value: &str) {
+                self.value = Some(MockValue::Str(value.to_owned()));
+            }
+
+            fn record_debug(&mut self, _: &Field, value: &dyn fmt::Debug) {
+                self.value = Some(MockValue::Debug(format!("{:?}", value)));
+            }
+        }
+
+        let fake_field = callsite!(name: "fake", kind: Kind::EVENT, fields: fake_field)
+            .metadata()
+            .fields()
+            .field("fake_field")
+            .unwrap();
+        let mut builder = MockValueBuilder { value: None };
+        value.record(&fake_field, &mut builder);
+        builder
+            .value
+            .expect("finish called before a value was recorded")
+    }
+}
+
+impl fmt::Display for Expect {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "fields ")?;
+        let entries = self
+            .fields
+            .iter()
+            .map(|(k, v)| (field::display(k), field::display(v)));
+        f.debug_map().entries(entries).finish()
+    }
+}
diff --git a/third_party/rust/tracing/tests/support/metadata.rs b/third_party/rust/tracing/tests/support/metadata.rs
new file mode 100644
index 0000000000..2c3606b05e
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/metadata.rs
@@ -0,0 +1,64 @@
+use std::fmt;
+use tracing::Metadata;
+
+#[derive(Clone, Debug, Eq, PartialEq, Default)]
+pub struct Expect {
+    pub name: Option<String>,
+    pub level: Option<tracing::Level>,
+    pub target: Option<String>,
+}
+
+impl Expect {
+    pub(in crate::support) fn check(&self, actual: &Metadata<'_>, ctx: fmt::Arguments<'_>) {
+        if let Some(ref expected_name) = self.name {
+            let name = actual.name();
+            assert!(
+                expected_name == name,
+                "expected {} to be named `{}`, but got one named `{}`",
+                ctx,
+                expected_name,
+                name
+            )
+        }
+
+        if let Some(ref expected_level) = self.level {
+            let level = actual.level();
+            assert!(
+                expected_level == level,
+                "expected {} to be at level `{:?}`, but it was at level `{:?}` instead",
+                ctx,
+                expected_level,
+                level,
+            )
+        }
+
+        if let Some(ref expected_target) = self.target {
+            let target = actual.target();
+            assert!(
+                expected_target == target,
+                "expected {} to have target `{}`, but it had target `{}` instead",
+                ctx,
+                expected_target,
+                target,
+            )
+        }
+    }
+}
+
+impl fmt::Display for Expect {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(ref name) = self.name {
+            write!(f, " named `{}`", name)?;
+        }
+
+        if let Some(ref level) = self.level {
+            write!(f, " at the `{:?}` level", level)?;
+        }
+
+        if let Some(ref target) = self.target {
+            write!(f, " with target `{}`", target)?;
+        }
+
+        Ok(())
+    }
+}
diff --git a/third_party/rust/tracing/tests/support/mod.rs b/third_party/rust/tracing/tests/support/mod.rs
new file mode 100644
index 0000000000..7900f38c76
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/mod.rs
@@ -0,0 +1,14 @@
+#![allow(dead_code)]
+pub mod event;
+pub mod field;
+mod metadata;
+pub mod span;
+pub mod subscriber;
+
+#[derive(Debug, Eq, PartialEq)]
+pub(in crate::support) enum Parent {
+    ContextualRoot,
+    Contextual(String),
+    ExplicitRoot,
+    Explicit(String),
+}
diff --git a/third_party/rust/tracing/tests/support/span.rs b/third_party/rust/tracing/tests/support/span.rs
new file mode 100644
index 0000000000..023e5b7079
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/span.rs
@@ -0,0 +1,167 @@
+#![allow(missing_docs)]
+use super::{field, metadata, Parent};
+use std::fmt;
+
+/// A mock span.
+///
+/// This is intended for use with the mock subscriber API in the
+/// `subscriber` module.
+#[derive(Clone, Debug, Default, Eq, PartialEq)]
+pub struct MockSpan {
+    pub(in crate::support) metadata: metadata::Expect,
+}
+
+#[derive(Debug, Default, Eq, PartialEq)]
+pub struct NewSpan {
+    pub(in crate::support) span: MockSpan,
+    pub(in crate::support) fields: field::Expect,
+    pub(in crate::support) parent: Option<Parent>,
+}
+
+pub fn mock() -> MockSpan {
+    MockSpan {
+        ..Default::default()
+    }
+}
+
+impl MockSpan {
+    pub fn named<I>(self, name: I) -> Self
+    where
+        I: Into<String>,
+    {
+        Self {
+            metadata: metadata::Expect {
+                name: Some(name.into()),
+                ..self.metadata
+            },
+        }
+    }
+
+    pub fn at_level(self, level: tracing::Level) -> Self {
+        Self {
+            metadata: metadata::Expect {
+                level: Some(level),
+                ..self.metadata
+            },
+        }
+    }
+
+    pub fn with_target<I>(self, target: I) -> Self
+    where
+        I: Into<String>,
+    {
+        Self {
+            metadata: metadata::Expect {
+                target: Some(target.into()),
+                ..self.metadata
+            },
+        }
+    }
+
+    pub fn with_explicit_parent(self, parent: Option<&str>) -> NewSpan {
+        let parent = match parent {
+            Some(name) => Parent::Explicit(name.into()),
+            None => Parent::ExplicitRoot,
+        };
+        NewSpan {
+            parent: Some(parent),
+            span: self,
+            ..Default::default()
+        }
+    }
+
+    pub fn with_contextual_parent(self, parent: Option<&str>) -> NewSpan {
+        let parent = match parent {
+            Some(name) => Parent::Contextual(name.into()),
+            None => Parent::ContextualRoot,
+        };
+        NewSpan {
+            parent: Some(parent),
+            span: self,
+            ..Default::default()
+        }
+    }
+
+    pub fn name(&self) -> Option<&str> {
+        self.metadata.name.as_ref().map(String::as_ref)
+    }
+
+    pub fn with_field<I>(self, fields: I) -> NewSpan
+    where
+        I: Into<field::Expect>,
+    {
+        NewSpan {
+            span: self,
+            fields: fields.into(),
+            ..Default::default()
+        }
+    }
+
+    pub(in crate::support) fn check_metadata(&self, actual: &tracing::Metadata<'_>) {
+        self.metadata.check(actual, format_args!("span {}", self));
+        assert!(actual.is_span(), "expected a span but got {:?}", actual);
+    }
+}
+
+impl fmt::Display for MockSpan {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.metadata.name.is_some() {
+            write!(f, "a span{}", self.metadata)
+        } else {
+            write!(f, "any span{}", self.metadata)
+        }
+    }
+}
+
+impl Into<NewSpan> for MockSpan {
+    fn into(self) -> NewSpan {
+        NewSpan {
+            span: self,
+            ..Default::default()
+        }
+    }
+}
+
+impl NewSpan {
+    pub fn with_explicit_parent(self, parent: Option<&str>) -> NewSpan {
+        let parent = match parent {
+            Some(name) => Parent::Explicit(name.into()),
+            None => Parent::ExplicitRoot,
+        };
+        NewSpan {
+            parent: Some(parent),
+            ..self
+        }
+    }
+
+    pub fn with_contextual_parent(self, parent: Option<&str>) -> NewSpan {
+        let parent = match parent {
+            Some(name) => Parent::Contextual(name.into()),
+            None => Parent::ContextualRoot,
+        };
+        NewSpan {
+            parent: Some(parent),
+            ..self
+        }
+    }
+
+    pub fn with_field<I>(self, fields: I) -> NewSpan
+    where
+        I: Into<field::Expect>,
+    {
+        NewSpan {
+            fields: fields.into(),
+            ..self
+        }
+    }
+}
+
+impl fmt::Display for NewSpan {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "a new span{}", self.span.metadata)?;
+        if !self.fields.is_empty() {
+            write!(f, " with {}", self.fields)?;
+        }
+        Ok(())
+    }
+}
diff --git a/third_party/rust/tracing/tests/support/subscriber.rs b/third_party/rust/tracing/tests/support/subscriber.rs
new file mode 100644
index 0000000000..83f1139849
--- /dev/null
+++ b/third_party/rust/tracing/tests/support/subscriber.rs
@@ -0,0 +1,569 @@
+#![allow(missing_docs)]
+use super::{
+    event::MockEvent,
+    field as mock_field,
+    span::{MockSpan, NewSpan},
+    Parent,
+};
+use std::{
+    collections::{HashMap, VecDeque},
+    fmt,
+    sync::{
+        atomic::{AtomicUsize, Ordering},
+        Arc, Mutex,
+    },
+    thread,
+};
+use tracing::{
+    level_filters::LevelFilter,
+    span::{self, Attributes, Id},
+    subscriber::Interest,
+    Event, Metadata, Subscriber,
+};
+
+#[derive(Debug, Eq, PartialEq)]
+enum Expect {
+    Event(MockEvent),
+    Enter(MockSpan),
+    Exit(MockSpan),
+    CloneSpan(MockSpan),
+    DropSpan(MockSpan),
+    Visit(MockSpan, mock_field::Expect),
+    NewSpan(NewSpan),
+    Nothing,
+}
+
+struct SpanState {
+    name: &'static str,
+    refs: usize,
+}
+
+struct Running<F: Fn(&Metadata<'_>) -> bool> {
+    spans: Mutex<HashMap<Id, SpanState>>,
+    expected: Arc<Mutex<VecDeque<Expect>>>,
+    current: Mutex<Vec<Id>>,
+    ids: AtomicUsize,
+    max_level: Option<LevelFilter>,
+    filter: F,
+    name: String,
+}
+
+pub struct MockSubscriber<F: Fn(&Metadata<'_>) -> bool> {
+    expected: VecDeque<Expect>,
+    max_level: Option<LevelFilter>,
+    filter: F,
+    name: String,
+}
+
+pub struct MockHandle(Arc<Mutex<VecDeque<Expect>>>, String);
+
+pub fn mock() -> MockSubscriber<fn(&Metadata<'_>) -> bool> {
+    MockSubscriber {
+        expected: VecDeque::new(),
+        filter: (|_: &Metadata<'_>| true) as for<'r, 's> fn(&'r Metadata<'s>) -> _,
+        max_level: None,
+        name: thread::current()
+            .name()
+            .unwrap_or("mock_subscriber")
+            .to_string(),
+    }
+}
+
+impl<F> MockSubscriber<F>
+where
+    F: Fn(&Metadata<'_>) -> bool + 'static,
+{
+    /// Overrides the name printed by the mock subscriber's debugging output.
+    ///
+    /// The debugging output is displayed if the test panics, or if the test is
+    /// run with `--nocapture`.
+    ///
+    /// By default, the mock subscriber's name is the  name of the test
+    /// (*technically*, the name of the thread where it was created, which is
+    /// the name of the test unless tests are run with `--test-threads=1`).
+    /// When a test has only one mock subscriber, this is sufficient. However,
+    /// some tests may include multiple subscribers, in order to test
+    /// interactions between multiple subscribers. In that case, it can be
+    /// helpful to give each subscriber a separate name to distinguish where the
+    /// debugging output comes from.
+    pub fn named(self, name: impl ToString) -> Self {
+        Self {
+            name: name.to_string(),
+            ..self
+        }
+    }
+
+    pub fn enter(mut self, span: MockSpan) -> Self {
+        self.expected.push_back(Expect::Enter(span));
+        self
+    }
+
+    pub fn event(mut self, event: MockEvent) -> Self {
+        self.expected.push_back(Expect::Event(event));
+        self
+    }
+
+    pub fn exit(mut self, span: MockSpan) -> Self {
+        self.expected.push_back(Expect::Exit(span));
+        self
+    }
+
+    pub fn clone_span(mut self, span: MockSpan) -> Self {
+        self.expected.push_back(Expect::CloneSpan(span));
+        self
+    }
+
+    #[allow(deprecated)]
+    pub fn drop_span(mut self, span: MockSpan) -> Self {
+        self.expected.push_back(Expect::DropSpan(span));
+        self
+    }
+
+    pub fn done(mut self) -> Self {
+        self.expected.push_back(Expect::Nothing);
+        self
+    }
+
+    pub fn record<I>(mut self, span: MockSpan, fields: I) -> Self
+    where
+        I: Into<mock_field::Expect>,
+    {
+        self.expected.push_back(Expect::Visit(span, fields.into()));
+        self
+    }
+
+    pub fn new_span<I>(mut self, new_span: I) -> Self
+    where
+        I: Into<NewSpan>,
+    {
+        self.expected.push_back(Expect::NewSpan(new_span.into()));
+        self
+    }
+
+    pub fn with_filter<G>(self, filter: G) -> MockSubscriber<G>
+    where
+        G: Fn(&Metadata<'_>) -> bool + 'static,
+    {
+        MockSubscriber {
+            expected: self.expected,
+            filter,
+            max_level: self.max_level,
+            name: self.name,
+        }
+    }
+
+    pub fn with_max_level_hint(self, hint: impl Into<LevelFilter>) -> Self {
+        Self {
+            max_level: Some(hint.into()),
+            ..self
+        }
+    }
+
+    pub fn run(self) -> impl Subscriber {
+        let (subscriber, _) = self.run_with_handle();
+        subscriber
+    }
+
+    pub fn run_with_handle(self) -> (impl Subscriber, MockHandle) {
+        let expected = Arc::new(Mutex::new(self.expected));
+        let handle = MockHandle(expected.clone(), self.name.clone());
+        let subscriber = Running {
+            spans: Mutex::new(HashMap::new()),
+            expected,
+            current: Mutex::new(Vec::new()),
+            ids: AtomicUsize::new(1),
+            filter: self.filter,
+            max_level: self.max_level,
+            name: self.name,
+        };
+        (subscriber, handle)
+    }
+}
+
+impl<F> Subscriber for Running<F>
+where
+    F: Fn(&Metadata<'_>) -> bool + 'static,
+{
+    fn enabled(&self, meta: &Metadata<'_>) -> bool {
+        println!("[{}] enabled: {:#?}", self.name, meta);
+        let enabled = (self.filter)(meta);
+        println!("[{}] enabled -> {}", self.name, enabled);
+        enabled
+    }
+
+    fn register_callsite(&self, meta: &'static Metadata<'static>) -> Interest {
+        println!("[{}] register_callsite: {:#?}", self.name, meta);
+        if self.enabled(meta) {
+            Interest::always()
+        } else {
+            Interest::never()
+        }
+    }
+    fn max_level_hint(&self) -> Option<LevelFilter> {
+        self.max_level
+    }
+
+    fn record(&self, id: &Id, values: &span::Record<'_>) {
+        let spans = self.spans.lock().unwrap();
+        let mut expected = self.expected.lock().unwrap();
+        let span = spans
+            .get(id)
+            .unwrap_or_else(|| panic!("[{}] no span for ID {:?}", self.name, id));
+        println!(
+            "[{}] record: {}; id={:?}; values={:?};",
+            self.name, span.name, id, values
+        );
+        let was_expected = if let Some(Expect::Visit(_, _)) = expected.front() {
+            true
+        } else {
+            false
+        };
+        if was_expected {
+            if let Expect::Visit(expected_span, mut expected_values) = expected.pop_front().unwrap()
+            {
+                if let Some(name) = expected_span.name() {
+                    assert_eq!(name, span.name);
+                }
+                let mut checker = expected_values.checker(format!("span {}: ", span.name));
+                values.record(&mut checker);
+                checker.finish();
+            }
+        }
+    }
+
+    fn event(&self, event: &Event<'_>) {
+        let name = event.metadata().name();
+        println!("[{}] event: {};", self.name, name);
+        match self.expected.lock().unwrap().pop_front() {
+            None => {}
+            Some(Expect::Event(mut expected)) => {
+                let spans = self.spans.lock().unwrap();
+                expected.check(event);
+                match expected.parent {
+                    Some(Parent::ExplicitRoot) => {
+                        assert!(
+                            event.is_root(),
+                            "[{}] expected {:?} to be an explicit root event",
+                            self.name,
+                            name
+                        );
+                    }
+                    Some(Parent::Explicit(expected_parent)) => {
+                        let actual_parent =
+                            event.parent().and_then(|id| spans.get(id)).map(|s| s.name);
+                        assert_eq!(
+                            Some(expected_parent.as_ref()),
+                            actual_parent,
+                            "[{}] expected {:?} to have explicit parent {:?}",
+                            self.name,
+                            name,
+                            expected_parent,
+                        );
+                    }
+                    Some(Parent::ContextualRoot) => {
+                        assert!(
+                            event.is_contextual(),
+                            "[{}] expected {:?} to have a contextual parent",
+                            self.name,
+                            name
+                        );
+                        assert!(
+                            self.current.lock().unwrap().last().is_none(),
+                            "[{}] expected {:?} to be a root, but we were inside a span",
+                            self.name,
+                            name
+                        );
+                    }
+                    Some(Parent::Contextual(expected_parent)) => {
+                        assert!(
+                            event.is_contextual(),
+                            "[{}] expected {:?} to have a contextual parent",
+                            self.name,
+                            name
+                        );
+                        let stack = self.current.lock().unwrap();
+                        let actual_parent =
+                            stack.last().and_then(|id| spans.get(id)).map(|s| s.name);
+                        assert_eq!(
+                            Some(expected_parent.as_ref()),
+                            actual_parent,
+                            "[{}] expected {:?} to have contextual parent {:?}",
+                            self.name,
+                            name,
+                            expected_parent,
+                        );
+                    }
+                    None => {}
+                }
+            }
+            Some(ex) => ex.bad(
+                &self.name,
+                format_args!("[{}] observed event {:?}", self.name, event),
+            ),
+        }
+    }
+
+    fn record_follows_from(&self, _span: &Id, _follows: &Id) {
+        // TODO: it should be possible to expect spans to follow from other spans
+    }
+
+    fn new_span(&self, span: &Attributes<'_>) -> Id {
+        let meta = span.metadata();
+        let id = self.ids.fetch_add(1, Ordering::SeqCst);
+        let id = Id::from_u64(id as u64);
+        println!(
+            "[{}] new_span: name={:?}; target={:?}; id={:?};",
+            self.name,
+            meta.name(),
+            meta.target(),
+            id
+        );
+        let mut expected = self.expected.lock().unwrap();
+        let was_expected = match expected.front() {
+            Some(Expect::NewSpan(_)) => true,
+            _ => false,
+        };
+        let mut spans = self.spans.lock().unwrap();
+        if was_expected {
+            if let Expect::NewSpan(mut expected) = expected.pop_front().unwrap() {
+                let name = meta.name();
+                expected
+                    .span
+                    .metadata
+                    .check(meta, format_args!("span `{}`", name));
+                let mut checker = expected.fields.checker(name.to_string());
+                span.record(&mut checker);
+                checker.finish();
+                match expected.parent {
+                    Some(Parent::ExplicitRoot) => {
+                        assert!(
+                            span.is_root(),
+                            "[{}] expected {:?} to be an explicit root span",
+                            self.name,
+                            name
+                        );
+                    }
+                    Some(Parent::Explicit(expected_parent)) => {
+                        let actual_parent =
+                            span.parent().and_then(|id| spans.get(id)).map(|s| s.name);
+                        assert_eq!(
+                            Some(expected_parent.as_ref()),
+                            actual_parent,
+                            "[{}] expected {:?} to have explicit parent {:?}",
+                            self.name,
+                            name,
+                            expected_parent,
+                        );
+                    }
+                    Some(Parent::ContextualRoot) => {
+                        assert!(
+                            span.is_contextual(),
+                            "[{}] expected {:?} to have a contextual parent",
+                            self.name,
+                            name
+                        );
+                        assert!(
+                            self.current.lock().unwrap().last().is_none(),
+                            "[{}] expected {:?} to be a root, but we were inside a span",
+                            self.name,
+                            name
+                        );
+                    }
+                    Some(Parent::Contextual(expected_parent)) => {
+                        assert!(
+                            span.is_contextual(),
+                            "[{}] expected {:?} to have a contextual parent",
+                            self.name,
+                            name
+                        );
+                        let stack = self.current.lock().unwrap();
+                        let actual_parent =
+                            stack.last().and_then(|id| spans.get(id)).map(|s| s.name);
+                        assert_eq!(
+                            Some(expected_parent.as_ref()),
+                            actual_parent,
+                            "[{}] expected {:?} to have contextual parent {:?}",
+                            self.name,
+                            name,
+                            expected_parent,
+                        );
+                    }
+                    None => {}
+                }
+            }
+        }
+        spans.insert(
+            id.clone(),
+            SpanState {
+                name: meta.name(),
+                refs: 1,
+            },
+        );
+        id
+    }
+
+    fn enter(&self, id: &Id) {
+        let spans = self.spans.lock().unwrap();
+        if let Some(span) = spans.get(id) {
+            println!("[{}] enter: {}; id={:?};", self.name, span.name, id);
+            match self.expected.lock().unwrap().pop_front() {
+                None => {}
+                Some(Expect::Enter(ref expected_span)) => {
+                    if let Some(name) = expected_span.name() {
+                        assert_eq!(name, span.name);
+                    }
+                }
+                Some(ex) => ex.bad(&self.name, format_args!("entered span {:?}", span.name)),
+            }
+        };
+        self.current.lock().unwrap().push(id.clone());
+    }
+
+    fn exit(&self, id: &Id) {
+        if std::thread::panicking() {
+            // `exit()` can be called in `drop` impls, so we must guard against
+            // double panics.
+            println!("[{}] exit {:?} while panicking", self.name, id);
+            return;
+        }
+        let spans = self.spans.lock().unwrap();
+        let span = spans
+            .get(id)
+            .unwrap_or_else(|| panic!("[{}] no span for ID {:?}", self.name, id));
+        println!("[{}] exit: {}; id={:?};", self.name, span.name, id);
+        match self.expected.lock().unwrap().pop_front() {
+            None => {}
+            Some(Expect::Exit(ref expected_span)) => {
+                if let Some(name) = expected_span.name() {
+                    assert_eq!(name, span.name);
+                }
+                let curr = self.current.lock().unwrap().pop();
+                assert_eq!(
+                    Some(id),
+                    curr.as_ref(),
+                    "[{}] exited span {:?}, but the current span was {:?}",
+                    self.name,
+                    span.name,
+                    curr.as_ref().and_then(|id| spans.get(id)).map(|s| s.name)
+                );
+            }
+            Some(ex) => ex.bad(&self.name, format_args!("exited span {:?}", span.name)),
+        };
+    }
+
+    fn clone_span(&self, id: &Id) -> Id {
+        let name = self.spans.lock().unwrap().get_mut(id).map(|span| {
+            let name = span.name;
+            println!(
+                "[{}] clone_span: {}; id={:?}; refs={:?};",
+                self.name, name, id, span.refs
+            );
+            span.refs += 1;
+            name
+        });
+        if name.is_none() {
+            println!("[{}] clone_span: id={:?};", self.name, id);
+        }
+        let mut expected = self.expected.lock().unwrap();
+        let was_expected = if let Some(Expect::CloneSpan(ref span)) = expected.front() {
+            assert_eq!(
+                name,
+                span.name(),
+                "[{}] expected to clone a span named {:?}",
+                self.name,
+                span.name()
+            );
+            true
+        } else {
+            false
+        };
+        if was_expected {
+            expected.pop_front();
+        }
+        id.clone()
+    }
+
+    fn drop_span(&self, id: Id) {
+        let mut is_event = false;
+        let name = if let Ok(mut spans) = self.spans.try_lock() {
+            spans.get_mut(&id).map(|span| {
+                let name = span.name;
+                if name.contains("event") {
+                    is_event = true;
+                }
+                println!(
+                    "[{}] drop_span: {}; id={:?}; refs={:?};",
+                    self.name, name, id, span.refs
+                );
+                span.refs -= 1;
+                name
+            })
+        } else {
+            None
+        };
+        if name.is_none() {
+            println!("[{}] drop_span: id={:?}", self.name, id);
+        }
+        if let Ok(mut expected) = self.expected.try_lock() {
+            let was_expected = match expected.front() {
+                Some(Expect::DropSpan(ref span)) => {
+                    // Don't assert if this function was called while panicking,
+                    // as failing the assertion can cause a double panic.
+                    if !::std::thread::panicking() {
+                        assert_eq!(name, span.name());
+                    }
+                    true
+                }
+                Some(Expect::Event(_)) => {
+                    if !::std::thread::panicking() {
+                        assert!(is_event, "[{}] expected an event", self.name);
+                    }
+                    true
+                }
+                _ => false,
+            };
+            if was_expected {
+                expected.pop_front();
+            }
+        }
+    }
+}
+
+impl MockHandle {
+    pub fn assert_finished(&self) {
+        if let Ok(ref expected) = self.0.lock() {
+            assert!(
+                !expected.iter().any(|thing| thing != &Expect::Nothing),
+                "[{}] more notifications expected: {:?}",
+                self.1,
+                **expected
+            );
+        }
+    }
+}
+
+impl Expect {
+    fn bad<'a>(&self, name: impl AsRef<str>, what: fmt::Arguments<'a>) {
+        let name = name.as_ref();
+        match self {
+            Expect::Event(e) => panic!("[{}] expected event {}, but {} instead", name, e, what,),
+            Expect::Enter(e) => panic!("[{}] expected to enter {} but {} instead", name, e, what,),
+            Expect::Exit(e) => panic!("[{}] expected to exit {} but {} instead", name, e, what,),
+            Expect::CloneSpan(e) => {
+                panic!("[{}] expected to clone {} but {} instead", name, e, what,)
+            }
+            Expect::DropSpan(e) => {
+                panic!("[{}] expected to drop {} but {} instead", name, e, what,)
+            }
+            Expect::Visit(e, fields) => panic!(
+                "[{}] expected {} to record {} but {} instead",
+                name, e, fields, what,
+            ),
+            Expect::NewSpan(e) => panic!("[{}] expected {} but {} instead", name, e, what),
+            Expect::Nothing => panic!(
+                "[{}] expected nothing else to happen, but {} instead",
+                name, what,
+            ),
+        }
+    }
+}
author	Daniel Baumann <daniel.baumann@progress-linux.org>	2024-04-28 14:29:10 +0000
committer	Daniel Baumann <daniel.baumann@progress-linux.org>	2024-04-28 14:29:10 +0000
commit	2aa4a82499d4becd2284cdb482213d541b8804dd (patch)
tree	b80bf8bf13c3766139fbacc530efd0dd9d54394c /third_party/rust/tracing
parent	Initial commit. (diff)
download	firefox-2aa4a82499d4becd2284cdb482213d541b8804dd.tar.xz firefox-2aa4a82499d4becd2284cdb482213d541b8804dd.zip