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+Parser for Rust source code
+===========================
+
+[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/syn-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/syn)
+[<img alt="crates.io" src="https://img.shields.io/crates/v/syn.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/syn)
+[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-syn-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/syn)
+[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/syn/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/syn/actions?query=branch%3Amaster)
+
+Syn is a parsing library for parsing a stream of Rust tokens into a syntax tree
+of Rust source code.
+
+Currently this library is geared toward use in Rust procedural macros, but
+contains some APIs that may be useful more generally.
+
+- **Data structures** — Syn provides a complete syntax tree that can represent
+  any valid Rust source code. The syntax tree is rooted at [`syn::File`] which
+  represents a full source file, but there are other entry points that may be
+  useful to procedural macros including [`syn::Item`], [`syn::Expr`] and
+  [`syn::Type`].
+
+- **Derives** — Of particular interest to derive macros is [`syn::DeriveInput`]
+  which is any of the three legal input items to a derive macro. An example
+  below shows using this type in a library that can derive implementations of a
+  user-defined trait.
+
+- **Parsing** — Parsing in Syn is built around [parser functions] with the
+  signature `fn(ParseStream) -> Result<T>`. Every syntax tree node defined by
+  Syn is individually parsable and may be used as a building block for custom
+  syntaxes, or you may dream up your own brand new syntax without involving any
+  of our syntax tree types.
+
+- **Location information** — Every token parsed by Syn is associated with a
+  `Span` that tracks line and column information back to the source of that
+  token. These spans allow a procedural macro to display detailed error messages
+  pointing to all the right places in the user's code. There is an example of
+  this below.
+
+- **Feature flags** — Functionality is aggressively feature gated so your
+  procedural macros enable only what they need, and do not pay in compile time
+  for all the rest.
+
+[`syn::File`]: https://docs.rs/syn/2.0/syn/struct.File.html
+[`syn::Item`]: https://docs.rs/syn/2.0/syn/enum.Item.html
+[`syn::Expr`]: https://docs.rs/syn/2.0/syn/enum.Expr.html
+[`syn::Type`]: https://docs.rs/syn/2.0/syn/enum.Type.html
+[`syn::DeriveInput`]: https://docs.rs/syn/2.0/syn/struct.DeriveInput.html
+[parser functions]: https://docs.rs/syn/2.0/syn/parse/index.html
+
+*Version requirement: Syn supports rustc 1.56 and up.*
+
+[*Release notes*](https://github.com/dtolnay/syn/releases)
+
+<br>
+
+## Resources
+
+The best way to learn about procedural macros is by writing some. Consider
+working through [this procedural macro workshop][workshop] to get familiar with
+the different types of procedural macros. The workshop contains relevant links
+into the Syn documentation as you work through each project.
+
+[workshop]: https://github.com/dtolnay/proc-macro-workshop
+
+<br>
+
+## Example of a derive macro
+
+The canonical derive macro using Syn looks like this. We write an ordinary Rust
+function tagged with a `proc_macro_derive` attribute and the name of the trait
+we are deriving. Any time that derive appears in the user's code, the Rust
+compiler passes their data structure as tokens into our macro. We get to execute
+arbitrary Rust code to figure out what to do with those tokens, then hand some
+tokens back to the compiler to compile into the user's crate.
+
+[`TokenStream`]: https://doc.rust-lang.org/proc_macro/struct.TokenStream.html
+
+```toml
+[dependencies]
+syn = "2.0"
+quote = "1.0"
+
+[lib]
+proc-macro = true
+```
+
+```rust
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+#[proc_macro_derive(MyMacro)]
+pub fn my_macro(input: TokenStream) -> TokenStream {
+    // Parse the input tokens into a syntax tree
+    let input = parse_macro_input!(input as DeriveInput);
+
+    // Build the output, possibly using quasi-quotation
+    let expanded = quote! {
+        // ...
+    };
+
+    // Hand the output tokens back to the compiler
+    TokenStream::from(expanded)
+}
+```
+
+The [`heapsize`] example directory shows a complete working implementation of a
+derive macro. The example derives a `HeapSize` trait which computes an estimate
+of the amount of heap memory owned by a value.
+
+[`heapsize`]: examples/heapsize
+
+```rust
+pub trait HeapSize {
+    /// Total number of bytes of heap memory owned by `self`.
+    fn heap_size_of_children(&self) -> usize;
+}
+```
+
+The derive macro allows users to write `#[derive(HeapSize)]` on data structures
+in their program.
+
+```rust
+#[derive(HeapSize)]
+struct Demo<'a, T: ?Sized> {
+    a: Box<T>,
+    b: u8,
+    c: &'a str,
+    d: String,
+}
+```
+
+<br>
+
+## Spans and error reporting
+
+The token-based procedural macro API provides great control over where the
+compiler's error messages are displayed in user code. Consider the error the
+user sees if one of their field types does not implement `HeapSize`.
+
+```rust
+#[derive(HeapSize)]
+struct Broken {
+    ok: String,
+    bad: std::thread::Thread,
+}
+```
+
+By tracking span information all the way through the expansion of a procedural
+macro as shown in the `heapsize` example, token-based macros in Syn are able to
+trigger errors that directly pinpoint the source of the problem.
+
+```console
+error[E0277]: the trait bound `std::thread::Thread: HeapSize` is not satisfied
+ --> src/main.rs:7:5
+  |
+7 |     bad: std::thread::Thread,
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `HeapSize` is not implemented for `std::thread::Thread`
+```
+
+<br>
+
+## Parsing a custom syntax
+
+The [`lazy-static`] example directory shows the implementation of a
+`functionlike!(...)` procedural macro in which the input tokens are parsed using
+Syn's parsing API.
+
+[`lazy-static`]: examples/lazy-static
+
+The example reimplements the popular `lazy_static` crate from crates.io as a
+procedural macro.
+
+```rust
+lazy_static! {
+    static ref USERNAME: Regex = Regex::new("^[a-z0-9_-]{3,16}$").unwrap();
+}
+```
+
+The implementation shows how to trigger custom warnings and error messages on
+the macro input.
+
+```console
+warning: come on, pick a more creative name
+  --> src/main.rs:10:16
+   |
+10 |     static ref FOO: String = "lazy_static".to_owned();
+   |                ^^^
+```
+
+<br>
+
+## Testing
+
+When testing macros, we often care not just that the macro can be used
+successfully but also that when the macro is provided with invalid input it
+produces maximally helpful error messages. Consider using the [`trybuild`] crate
+to write tests for errors that are emitted by your macro or errors detected by
+the Rust compiler in the expanded code following misuse of the macro. Such tests
+help avoid regressions from later refactors that mistakenly make an error no
+longer trigger or be less helpful than it used to be.
+
+[`trybuild`]: https://github.com/dtolnay/trybuild
+
+<br>
+
+## Debugging
+
+When developing a procedural macro it can be helpful to look at what the
+generated code looks like. Use `cargo rustc -- -Zunstable-options
+--pretty=expanded` or the [`cargo expand`] subcommand.
+
+[`cargo expand`]: https://github.com/dtolnay/cargo-expand
+
+To show the expanded code for some crate that uses your procedural macro, run
+`cargo expand` from that crate. To show the expanded code for one of your own
+test cases, run `cargo expand --test the_test_case` where the last argument is
+the name of the test file without the `.rs` extension.
+
+This write-up by Brandon W Maister discusses debugging in more detail:
+[Debugging Rust's new Custom Derive system][debugging].
+
+[debugging]: https://quodlibetor.github.io/posts/debugging-rusts-new-custom-derive-system/
+
+<br>
+
+## Optional features
+
+Syn puts a lot of functionality behind optional features in order to optimize
+compile time for the most common use cases. The following features are
+available.
+
+- **`derive`** *(enabled by default)* — Data structures for representing the
+  possible input to a derive macro, including structs and enums and types.
+- **`full`** — Data structures for representing the syntax tree of all valid
+  Rust source code, including items and expressions.
+- **`parsing`** *(enabled by default)* — Ability to parse input tokens into a
+  syntax tree node of a chosen type.
+- **`printing`** *(enabled by default)* — Ability to print a syntax tree node as
+  tokens of Rust source code.
+- **`visit`** — Trait for traversing a syntax tree.
+- **`visit-mut`** — Trait for traversing and mutating in place a syntax tree.
+- **`fold`** — Trait for transforming an owned syntax tree.
+- **`clone-impls`** *(enabled by default)* — Clone impls for all syntax tree
+  types.
+- **`extra-traits`** — Debug, Eq, PartialEq, Hash impls for all syntax tree
+  types.
+- **`proc-macro`** *(enabled by default)* — Runtime dependency on the dynamic
+  library libproc_macro from rustc toolchain.
+
+<br>
+
+## Proc macro shim
+
+Syn operates on the token representation provided by the [proc-macro2] crate
+from crates.io rather than using the compiler's built in proc-macro crate
+directly. This enables code using Syn to execute outside of the context of a
+procedural macro, such as in unit tests or build.rs, and we avoid needing
+incompatible ecosystems for proc macros vs non-macro use cases.
+
+In general all of your code should be written against proc-macro2 rather than
+proc-macro. The one exception is in the signatures of procedural macro entry
+points, which are required by the language to use `proc_macro::TokenStream`.
+
+The proc-macro2 crate will automatically detect and use the compiler's data
+structures when a procedural macro is active.
+
+[proc-macro2]: https://docs.rs/proc-macro2/1.0/proc_macro2/
+
+<br>
+
+#### License
+
+<sup>
+Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
+2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
+</sup>
+
+<br>
+
+<sub>
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
+be dual licensed as above, without any additional terms or conditions.
+</sub>