From 6bf0a5cb5034a7e684dcc3500e841785237ce2dd Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sun, 7 Apr 2024 19:32:43 +0200
Subject: Adding upstream version 1:115.7.0.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 third_party/rust/darling_core/.cargo-checksum.json |   1 +
 third_party/rust/darling_core/Cargo.toml           |  49 ++
 third_party/rust/darling_core/LICENSE              |  21 +
 third_party/rust/darling_core/src/ast/data.rs      | 477 ++++++++++++
 third_party/rust/darling_core/src/ast/generics.rs  | 192 +++++
 third_party/rust/darling_core/src/ast/mod.rs       |   7 +
 .../darling_core/src/codegen/attr_extractor.rs     | 116 +++
 .../rust/darling_core/src/codegen/default_expr.rs  |  53 ++
 third_party/rust/darling_core/src/codegen/error.rs |  46 ++
 third_party/rust/darling_core/src/codegen/field.rs | 199 +++++
 .../src/codegen/from_attributes_impl.rs            | 109 +++
 .../darling_core/src/codegen/from_derive_impl.rs   | 147 ++++
 .../rust/darling_core/src/codegen/from_field.rs    | 114 +++
 .../darling_core/src/codegen/from_meta_impl.rs     | 135 ++++
 .../darling_core/src/codegen/from_type_param.rs    | 113 +++
 .../darling_core/src/codegen/from_variant_impl.rs  | 139 ++++
 third_party/rust/darling_core/src/codegen/mod.rs   |  30 +
 .../darling_core/src/codegen/outer_from_impl.rs    |  60 ++
 .../darling_core/src/codegen/postfix_transform.rs  |  30 +
 .../rust/darling_core/src/codegen/trait_impl.rs    | 136 ++++
 .../rust/darling_core/src/codegen/variant.rs       | 158 ++++
 .../rust/darling_core/src/codegen/variant_data.rs  |  99 +++
 third_party/rust/darling_core/src/derive.rs        |  61 ++
 third_party/rust/darling_core/src/error/kind.rs    | 211 +++++
 third_party/rust/darling_core/src/error/mod.rs     | 831 ++++++++++++++++++++
 .../rust/darling_core/src/from_attributes.rs       |  27 +
 .../rust/darling_core/src/from_derive_input.rs     |  26 +
 third_party/rust/darling_core/src/from_field.rs    |  38 +
 .../rust/darling_core/src/from_generic_param.rs    |  19 +
 third_party/rust/darling_core/src/from_generics.rs |  27 +
 third_party/rust/darling_core/src/from_meta.rs     | 853 +++++++++++++++++++++
 .../rust/darling_core/src/from_type_param.rs       |  32 +
 third_party/rust/darling_core/src/from_variant.rs  |  33 +
 third_party/rust/darling_core/src/lib.rs           |  42 +
 .../rust/darling_core/src/macros_private.rs        |   6 +
 third_party/rust/darling_core/src/macros_public.rs |  96 +++
 third_party/rust/darling_core/src/options/core.rs  | 173 +++++
 .../rust/darling_core/src/options/forward_attrs.rs |  31 +
 .../darling_core/src/options/from_attributes.rs    |  69 ++
 .../rust/darling_core/src/options/from_derive.rs   | 100 +++
 .../rust/darling_core/src/options/from_field.rs    |  79 ++
 .../rust/darling_core/src/options/from_meta.rs     |  50 ++
 .../darling_core/src/options/from_type_param.rs    |  79 ++
 .../rust/darling_core/src/options/from_variant.rs  |  85 ++
 .../rust/darling_core/src/options/input_field.rs   | 180 +++++
 .../rust/darling_core/src/options/input_variant.rs | 104 +++
 third_party/rust/darling_core/src/options/mod.rs   | 145 ++++
 .../rust/darling_core/src/options/outer_from.rs    |  87 +++
 third_party/rust/darling_core/src/options/shape.rs | 258 +++++++
 .../rust/darling_core/src/usage/generics_ext.rs    |  24 +
 .../rust/darling_core/src/usage/ident_set.rs       |   8 +
 .../rust/darling_core/src/usage/lifetimes.rs       | 327 ++++++++
 third_party/rust/darling_core/src/usage/mod.rs     | 111 +++
 third_party/rust/darling_core/src/usage/options.rs |  58 ++
 .../rust/darling_core/src/usage/type_params.rs     | 357 +++++++++
 third_party/rust/darling_core/src/util/flag.rs     | 100 +++
 .../rust/darling_core/src/util/ident_string.rs     | 157 ++++
 third_party/rust/darling_core/src/util/ignored.rs  |  50 ++
 third_party/rust/darling_core/src/util/mod.rs      |  23 +
 .../rust/darling_core/src/util/over_ride.rs        | 147 ++++
 .../rust/darling_core/src/util/parse_attribute.rs  |  73 ++
 .../rust/darling_core/src/util/path_list.rs        | 104 +++
 .../rust/darling_core/src/util/path_to_string.rs   |  36 +
 third_party/rust/darling_core/src/util/shape.rs    | 285 +++++++
 .../rust/darling_core/src/util/spanned_value.rs    | 133 ++++
 .../rust/darling_core/src/util/with_original.rs    |  35 +
 66 files changed, 8201 insertions(+)
 create mode 100644 third_party/rust/darling_core/.cargo-checksum.json
 create mode 100644 third_party/rust/darling_core/Cargo.toml
 create mode 100644 third_party/rust/darling_core/LICENSE
 create mode 100644 third_party/rust/darling_core/src/ast/data.rs
 create mode 100644 third_party/rust/darling_core/src/ast/generics.rs
 create mode 100644 third_party/rust/darling_core/src/ast/mod.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/attr_extractor.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/default_expr.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/error.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/field.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/from_attributes_impl.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/from_derive_impl.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/from_field.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/from_meta_impl.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/from_type_param.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/from_variant_impl.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/mod.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/outer_from_impl.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/postfix_transform.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/trait_impl.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/variant.rs
 create mode 100644 third_party/rust/darling_core/src/codegen/variant_data.rs
 create mode 100644 third_party/rust/darling_core/src/derive.rs
 create mode 100644 third_party/rust/darling_core/src/error/kind.rs
 create mode 100644 third_party/rust/darling_core/src/error/mod.rs
 create mode 100644 third_party/rust/darling_core/src/from_attributes.rs
 create mode 100644 third_party/rust/darling_core/src/from_derive_input.rs
 create mode 100644 third_party/rust/darling_core/src/from_field.rs
 create mode 100644 third_party/rust/darling_core/src/from_generic_param.rs
 create mode 100644 third_party/rust/darling_core/src/from_generics.rs
 create mode 100644 third_party/rust/darling_core/src/from_meta.rs
 create mode 100644 third_party/rust/darling_core/src/from_type_param.rs
 create mode 100644 third_party/rust/darling_core/src/from_variant.rs
 create mode 100644 third_party/rust/darling_core/src/lib.rs
 create mode 100644 third_party/rust/darling_core/src/macros_private.rs
 create mode 100644 third_party/rust/darling_core/src/macros_public.rs
 create mode 100644 third_party/rust/darling_core/src/options/core.rs
 create mode 100644 third_party/rust/darling_core/src/options/forward_attrs.rs
 create mode 100644 third_party/rust/darling_core/src/options/from_attributes.rs
 create mode 100644 third_party/rust/darling_core/src/options/from_derive.rs
 create mode 100644 third_party/rust/darling_core/src/options/from_field.rs
 create mode 100644 third_party/rust/darling_core/src/options/from_meta.rs
 create mode 100644 third_party/rust/darling_core/src/options/from_type_param.rs
 create mode 100644 third_party/rust/darling_core/src/options/from_variant.rs
 create mode 100644 third_party/rust/darling_core/src/options/input_field.rs
 create mode 100644 third_party/rust/darling_core/src/options/input_variant.rs
 create mode 100644 third_party/rust/darling_core/src/options/mod.rs
 create mode 100644 third_party/rust/darling_core/src/options/outer_from.rs
 create mode 100644 third_party/rust/darling_core/src/options/shape.rs
 create mode 100644 third_party/rust/darling_core/src/usage/generics_ext.rs
 create mode 100644 third_party/rust/darling_core/src/usage/ident_set.rs
 create mode 100644 third_party/rust/darling_core/src/usage/lifetimes.rs
 create mode 100644 third_party/rust/darling_core/src/usage/mod.rs
 create mode 100644 third_party/rust/darling_core/src/usage/options.rs
 create mode 100644 third_party/rust/darling_core/src/usage/type_params.rs
 create mode 100644 third_party/rust/darling_core/src/util/flag.rs
 create mode 100644 third_party/rust/darling_core/src/util/ident_string.rs
 create mode 100644 third_party/rust/darling_core/src/util/ignored.rs
 create mode 100644 third_party/rust/darling_core/src/util/mod.rs
 create mode 100644 third_party/rust/darling_core/src/util/over_ride.rs
 create mode 100644 third_party/rust/darling_core/src/util/parse_attribute.rs
 create mode 100644 third_party/rust/darling_core/src/util/path_list.rs
 create mode 100644 third_party/rust/darling_core/src/util/path_to_string.rs
 create mode 100644 third_party/rust/darling_core/src/util/shape.rs
 create mode 100644 third_party/rust/darling_core/src/util/spanned_value.rs
 create mode 100644 third_party/rust/darling_core/src/util/with_original.rs

(limited to 'third_party/rust/darling_core')

diff --git a/third_party/rust/darling_core/.cargo-checksum.json b/third_party/rust/darling_core/.cargo-checksum.json
new file mode 100644
index 0000000000..fbb4b2461e
--- /dev/null
+++ b/third_party/rust/darling_core/.cargo-checksum.json
@@ -0,0 +1 @@
+{"files":{"Cargo.toml":"26ffc472d7fcaf591b6e2783dd724dd5952a3a4d3e49fabcef50de7163fadd0c","LICENSE":"8ea93490d74a5a1b1af3ff71d786271b3f1e5f0bea79ac16e02ec533cef040d6","src/ast/data.rs":"67360d2688d92be168e12e990792ea598091e29b47804b9919e38d75df8ac9e6","src/ast/generics.rs":"82d542307508f9053804511bc331131b037ec64d1ad50d8551a3f991dbbdfef8","src/ast/mod.rs":"58a58eeb1db7682e5994732cc8eb04088f6ca662805460ef443cf089e5c24e2c","src/codegen/attr_extractor.rs":"d801966bfdff0cd15c1db339384879115bd41b3d2298d612ff5429fbcd1763a8","src/codegen/default_expr.rs":"5486c8dc264e3bb8be2a871f3258f56ed2812ae39b1eb9f0d16006605407027f","src/codegen/error.rs":"4ef086317234a161bec493413ba38d842cbbecd0b9610dad0a25fe8883c75d3b","src/codegen/field.rs":"40d3f61507440ee62bb1c2098aca39be84ec74e2cecbd11340e925bd68a3cbfa","src/codegen/from_attributes_impl.rs":"f404e32f68bca0097157a23a2d098365d66ba0c3fad6cfdf10ec50d367916415","src/codegen/from_derive_impl.rs":"0f62e61566202f224bbfb8d251141ca64ace5022c5e85e312739da08f3ef51e2","src/codegen/from_field.rs":"10f18dfdae9e9cc295121585862b2f30ed2f7f1124675fe39ac0dd7921ede62a","src/codegen/from_meta_impl.rs":"15bfc28a3053d6750a2153943a951fcb8558b540ac6593529e4b1f2ec521002f","src/codegen/from_type_param.rs":"d301fabd5b0024b1267a6c7887f6214fb86783fd4c0bb9a15867d5f3f58fc4b8","src/codegen/from_variant_impl.rs":"e2cc68ad5d0a45989830d144760d55b3fb62297c5aea097fa085dabb22e3897f","src/codegen/mod.rs":"77b194876541e8ced86efebb534fc8a4f70c06d92c8b493aa90ef936bcc14273","src/codegen/outer_from_impl.rs":"c6ef8aad7f9c817005e236b3bebf56319455f578b109bdb47b23c0a1dc1149f5","src/codegen/postfix_transform.rs":"41c84a239c15671e5a9cb8ce9fbb1b907f20aebe30d5dea4f04f004e3aa584c1","src/codegen/trait_impl.rs":"22d307450c56abf29dc2fdf1ced8de2220cc5f544f5297452e829a30093e906e","src/codegen/variant.rs":"2a52652c623fc27bf0c3c4706b678a7226a65eb9be6967b656ed3755339b26c6","src/codegen/variant_data.rs":"27c2ef03521254fc071af9586ff75c3b4f263d6f75ff737f472ee376f9ea059b","src/derive.rs":"d35f35233eea28bc66bf6f738358415d650741560ca56f8deaee9465b3e88b2a","src/error/kind.rs":"862fb877500ceb95e89d5b53bef987a0836e8754bdcc5c30d074c0f53e94d715","src/error/mod.rs":"010cc51eedfca91fa63f9def97a42bb552e503448a875b7d29f63af465e67967","src/from_attributes.rs":"9c98a34eccdb66526711793d6d00bd5216de1d0e9d3755133996242a27efe272","src/from_derive_input.rs":"a60aba72ebd4deddd6bb9919995652f90aafdf88f6109c00c1a1c4459e439049","src/from_field.rs":"1e22b7750843b6a018fe75ae1510fb9252343be99ab02d2093881421e87d867d","src/from_generic_param.rs":"bdabc80f49212501e6e3574c7e6c03b4e453d0fe241ac46d465a5f004381f4c9","src/from_generics.rs":"b0a156179b2d1d88467183649afc97405403b01a7fdae39e4df8392f6b9f500c","src/from_meta.rs":"f0975b219d3baea104b2a307f7a850b2a2ce9786f033348e83dff0bec70ca996","src/from_type_param.rs":"0c8930a42e1f0720429aa968f97f938e207460232dd2169cbf8efec12e7c6f5a","src/from_variant.rs":"dd27ffef65ceea0e68809afde7df82d3751cdedc248773062637aa8c146da6e7","src/lib.rs":"b066b0747dcec5150d6962a80850a8fb7690b24700f62537893ebb2d4c88a4a8","src/macros_private.rs":"4373c36e75d516c340fc9b13c52b471d823fff9997cb7faeeb5f93b11d40d0b4","src/macros_public.rs":"7d2ce0c5026227ef7854db11d7a885ad891255438b2e49bbdfda56fa2f92feec","src/options/core.rs":"cd84b2405f4c43bfef720123dbefecb65220313b79a3f3d617392ce909382609","src/options/forward_attrs.rs":"c6a960f74957854eabd5a5c87a15dbee897ea9a2f48d0094fbaec110f3a785cf","src/options/from_attributes.rs":"f73c4dc53cced30e60812a4247f3252ea0630618b1d162058cce0bd1666c43c9","src/options/from_derive.rs":"00e9c5bbbe9682ef2a3283ccf59818c348e3fd9d85e8ef75913028ac99c1e459","src/options/from_field.rs":"006e127a648d9ad095fa92ffc6f9e106fdc1a784dd82be88dc73ed80198fd429","src/options/from_meta.rs":"1a543110dc4ab2e716a914bafced9e1f4035794613117708fc9f37a1036bcf75","src/options/from_type_param.rs":"c6f907d3bceef3a918822b75e019f520650d66c523acc2ffac37d03016234659","src/options/from_variant.rs":"6b426a453710f6e3d40d8fbdce4dbb8aa6470c98f8d385654f0eae98ab7e4be3","src/options/input_field.rs":"93000e7705a0a5f0c2ed3354d6585be1987f0a1ef297bb82ffd140440e26808a","src/options/input_variant.rs":"1ebc013018abb07670f13c71d5c819d079a4e824235ede43fb568239efcbf1f7","src/options/mod.rs":"719373f25a35c3bfab670d0669b1e0f41e7a7a58a98d957e4b34331e8dac1f9e","src/options/outer_from.rs":"96a0abb1e324f7e9295cababf29aef63f1cad9e5fddac08768d7042cc62fd73c","src/options/shape.rs":"75b8fc47a2f8b2b50b40b4c578c2d17fc999b42ad403ef036f24d0b37da25623","src/usage/generics_ext.rs":"b2d15e2ff75c47f43f3ce305b2108e800dd3d89e3a21fadf82bda455837b8090","src/usage/ident_set.rs":"30edb2f0a599284967e3c6b579da31e5f9b15f3dd67bc9a82d6335eb44133df0","src/usage/lifetimes.rs":"4fe4009e9ff901e12aee204270adaad250e15f4a023a0bf497006977ee1a0ddf","src/usage/mod.rs":"e49adadfa8ffed27299d5bbf3c33e68e2c8c7411f89aef655f77e158be9dd642","src/usage/options.rs":"0491c995aad0d55783b24cce8a4e40f0f4435988c54ce2ded34763ac9b199fcf","src/usage/type_params.rs":"4817447aa2eddcc6c5fe24d13623152cac2dfc0763524ca6111750f3b512f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\ No newline at end of file
diff --git a/third_party/rust/darling_core/Cargo.toml b/third_party/rust/darling_core/Cargo.toml
new file mode 100644
index 0000000000..9e5b5365d5
--- /dev/null
+++ b/third_party/rust/darling_core/Cargo.toml
@@ -0,0 +1,49 @@
+# 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 are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2018"
+name = "darling_core"
+version = "0.14.3"
+authors = ["Ted Driggs <ted.driggs@outlook.com>"]
+description = """
+Helper crate for proc-macro library for reading attributes into structs when
+implementing custom derives. Use https://crates.io/crates/darling in your code.
+"""
+license = "MIT"
+repository = "https://github.com/TedDriggs/darling"
+
+[dependencies.fnv]
+version = "1.0.7"
+
+[dependencies.ident_case]
+version = "1.0.1"
+
+[dependencies.proc-macro2]
+version = "1.0.37"
+
+[dependencies.quote]
+version = "1.0.18"
+
+[dependencies.strsim]
+version = "0.10.0"
+optional = true
+
+[dependencies.syn]
+version = "1.0.91"
+features = [
+    "full",
+    "extra-traits",
+]
+
+[features]
+diagnostics = []
+suggestions = ["strsim"]
diff --git a/third_party/rust/darling_core/LICENSE b/third_party/rust/darling_core/LICENSE
new file mode 100644
index 0000000000..0b48eadc9d
--- /dev/null
+++ b/third_party/rust/darling_core/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2017 Ted Driggs
+
+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/darling_core/src/ast/data.rs b/third_party/rust/darling_core/src/ast/data.rs
new file mode 100644
index 0000000000..958c7be9ab
--- /dev/null
+++ b/third_party/rust/darling_core/src/ast/data.rs
@@ -0,0 +1,477 @@
+use std::{slice, vec};
+
+use proc_macro2::{Span, TokenStream};
+use quote::{quote, quote_spanned, ToTokens};
+use syn::spanned::Spanned;
+
+use crate::usage::{
+    self, IdentRefSet, IdentSet, LifetimeRefSet, LifetimeSet, UsesLifetimes, UsesTypeParams,
+};
+use crate::{Error, FromField, FromVariant, Result};
+
+/// A struct or enum body.
+///
+/// `V` is the type which receives any encountered variants, and `F` receives struct fields.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Data<V, F> {
+    Enum(Vec<V>),
+    Struct(Fields<F>),
+}
+
+impl<V, F> Data<V, F> {
+    /// Creates an empty body of the same shape as the passed-in body.
+    ///
+    /// # Panics
+    /// This function will panic if passed `syn::Data::Union`.
+    pub fn empty_from(src: &syn::Data) -> Self {
+        match *src {
+            syn::Data::Enum(_) => Data::Enum(vec![]),
+            syn::Data::Struct(ref vd) => Data::Struct(Fields::empty_from(&vd.fields)),
+            syn::Data::Union(_) => panic!("Unions are not supported"),
+        }
+    }
+
+    /// Creates an empty body of the same shape as the passed-in body.
+    ///
+    /// `darling` does not support unions; calling this function with a union body will return an error.
+    pub fn try_empty_from(src: &syn::Data) -> Result<Self> {
+        match *src {
+            syn::Data::Enum(_) => Ok(Data::Enum(vec![])),
+            syn::Data::Struct(ref vd) => Ok(Data::Struct(Fields::empty_from(&vd.fields))),
+            // This deliberately doesn't set a span on the error message, as the error is most useful if
+            // applied to the call site of the offending macro. Given that the message is very generic,
+            // putting it on the union keyword ends up being confusing.
+            syn::Data::Union(_) => Err(Error::custom("Unions are not supported")),
+        }
+    }
+
+    /// Creates a new `Data<&'a V, &'a F>` instance from `Data<V, F>`.
+    pub fn as_ref(&self) -> Data<&V, &F> {
+        match *self {
+            Data::Enum(ref variants) => Data::Enum(variants.iter().collect()),
+            Data::Struct(ref data) => Data::Struct(data.as_ref()),
+        }
+    }
+
+    /// Applies a function `V -> U` on enum variants, if this is an enum.
+    pub fn map_enum_variants<T, U>(self, map: T) -> Data<U, F>
+    where
+        T: FnMut(V) -> U,
+    {
+        match self {
+            Data::Enum(v) => Data::Enum(v.into_iter().map(map).collect()),
+            Data::Struct(f) => Data::Struct(f),
+        }
+    }
+
+    /// Applies a function `F -> U` on struct fields, if this is a struct.
+    pub fn map_struct_fields<T, U>(self, map: T) -> Data<V, U>
+    where
+        T: FnMut(F) -> U,
+    {
+        match self {
+            Data::Enum(v) => Data::Enum(v),
+            Data::Struct(f) => Data::Struct(f.map(map)),
+        }
+    }
+
+    /// Applies a function to the `Fields` if this is a struct.
+    pub fn map_struct<T, U>(self, mut map: T) -> Data<V, U>
+    where
+        T: FnMut(Fields<F>) -> Fields<U>,
+    {
+        match self {
+            Data::Enum(v) => Data::Enum(v),
+            Data::Struct(f) => Data::Struct(map(f)),
+        }
+    }
+
+    /// Consumes the `Data`, returning `Fields<F>` if it was a struct.
+    pub fn take_struct(self) -> Option<Fields<F>> {
+        match self {
+            Data::Enum(_) => None,
+            Data::Struct(f) => Some(f),
+        }
+    }
+
+    /// Consumes the `Data`, returning `Vec<V>` if it was an enum.
+    pub fn take_enum(self) -> Option<Vec<V>> {
+        match self {
+            Data::Enum(v) => Some(v),
+            Data::Struct(_) => None,
+        }
+    }
+
+    /// Returns `true` if this instance is `Data::Enum`.
+    pub fn is_enum(&self) -> bool {
+        match *self {
+            Data::Enum(_) => true,
+            Data::Struct(_) => false,
+        }
+    }
+
+    /// Returns `true` if this instance is `Data::Struct`.
+    pub fn is_struct(&self) -> bool {
+        !self.is_enum()
+    }
+}
+
+impl<V: FromVariant, F: FromField> Data<V, F> {
+    /// Attempt to convert from a `syn::Data` instance.
+    pub fn try_from(body: &syn::Data) -> Result<Self> {
+        match *body {
+            syn::Data::Enum(ref data) => {
+                let mut errors = Error::accumulator();
+                let items = data
+                    .variants
+                    .iter()
+                    .filter_map(|v| errors.handle(FromVariant::from_variant(v)))
+                    .collect();
+
+                errors.finish_with(Data::Enum(items))
+            }
+            syn::Data::Struct(ref data) => Ok(Data::Struct(Fields::try_from(&data.fields)?)),
+            // This deliberately doesn't set a span on the error message, as the error is most useful if
+            // applied to the call site of the offending macro. Given that the message is very generic,
+            // putting it on the union keyword ends up being confusing.
+            syn::Data::Union(_) => Err(Error::custom("Unions are not supported")),
+        }
+    }
+}
+
+impl<V: UsesTypeParams, F: UsesTypeParams> UsesTypeParams for Data<V, F> {
+    fn uses_type_params<'a>(
+        &self,
+        options: &usage::Options,
+        type_set: &'a IdentSet,
+    ) -> IdentRefSet<'a> {
+        match *self {
+            Data::Struct(ref v) => v.uses_type_params(options, type_set),
+            Data::Enum(ref v) => v.uses_type_params(options, type_set),
+        }
+    }
+}
+
+impl<V: UsesLifetimes, F: UsesLifetimes> UsesLifetimes for Data<V, F> {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &usage::Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        match *self {
+            Data::Struct(ref v) => v.uses_lifetimes(options, lifetimes),
+            Data::Enum(ref v) => v.uses_lifetimes(options, lifetimes),
+        }
+    }
+}
+
+/// Equivalent to `syn::Fields`, but replaces the AST element with a generic.
+#[derive(Debug, Clone)]
+pub struct Fields<T> {
+    pub style: Style,
+    pub fields: Vec<T>,
+    span: Option<Span>,
+    __nonexhaustive: (),
+}
+
+impl<T> Fields<T> {
+    /// Creates a new [`Fields`] struct.
+    pub fn new(style: Style, fields: Vec<T>) -> Self {
+        Self {
+            style,
+            fields,
+            span: None,
+            __nonexhaustive: (),
+        }
+    }
+
+    /// Adds a [`Span`] to [`Fields`].
+    pub fn with_span(mut self, span: Span) -> Self {
+        if self.span.is_none() {
+            self.span = Some(span);
+        }
+        self
+    }
+
+    pub fn empty_from(vd: &syn::Fields) -> Self {
+        Self::new(vd.into(), Vec::new())
+    }
+
+    /// Splits the `Fields` into its style and fields for further processing.
+    /// Returns an empty `Vec` for `Unit` data.
+    pub fn split(self) -> (Style, Vec<T>) {
+        (self.style, self.fields)
+    }
+
+    /// Returns true if this variant's data makes it a newtype.
+    pub fn is_newtype(&self) -> bool {
+        self.style == Style::Tuple && self.len() == 1
+    }
+
+    pub fn is_unit(&self) -> bool {
+        self.style.is_unit()
+    }
+
+    pub fn is_tuple(&self) -> bool {
+        self.style.is_tuple()
+    }
+
+    pub fn is_struct(&self) -> bool {
+        self.style.is_struct()
+    }
+
+    pub fn as_ref(&self) -> Fields<&T> {
+        Fields {
+            style: self.style,
+            fields: self.fields.iter().collect(),
+            span: self.span,
+            __nonexhaustive: (),
+        }
+    }
+
+    pub fn map<F, U>(self, map: F) -> Fields<U>
+    where
+        F: FnMut(T) -> U,
+    {
+        Fields {
+            style: self.style,
+            fields: self.fields.into_iter().map(map).collect(),
+            span: self.span,
+            __nonexhaustive: (),
+        }
+    }
+
+    pub fn iter(&self) -> slice::Iter<T> {
+        self.fields.iter()
+    }
+
+    /// Returns the number of fields in the structure.
+    pub fn len(&self) -> usize {
+        self.fields.len()
+    }
+
+    /// Returns `true` if the `Fields` contains no fields.
+    pub fn is_empty(&self) -> bool {
+        self.fields.is_empty()
+    }
+}
+
+impl<F: FromField> Fields<F> {
+    pub fn try_from(fields: &syn::Fields) -> Result<Self> {
+        let mut errors = Error::accumulator();
+        let items = {
+            match &fields {
+                syn::Fields::Named(fields) => fields
+                    .named
+                    .iter()
+                    .filter_map(|field| {
+                        errors.handle(FromField::from_field(field).map_err(|err| {
+                            // There should always be an ident here, since this is a collection
+                            // of named fields, but `syn` doesn't prevent someone from manually
+                            // constructing an invalid collection so a guard is still warranted.
+                            if let Some(ident) = &field.ident {
+                                err.at(ident)
+                            } else {
+                                err
+                            }
+                        }))
+                    })
+                    .collect(),
+                syn::Fields::Unnamed(fields) => fields
+                    .unnamed
+                    .iter()
+                    .filter_map(|field| errors.handle(FromField::from_field(field)))
+                    .collect(),
+                syn::Fields::Unit => vec![],
+            }
+        };
+
+        errors.finish()?;
+
+        Ok(Self::new(fields.into(), items).with_span(fields.span()))
+    }
+}
+
+impl<T: ToTokens> ToTokens for Fields<T> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let fields = &self.fields;
+        // An unknown Span should be `Span::call_site()`;
+        // https://docs.rs/syn/1.0.12/syn/spanned/trait.Spanned.html#tymethod.span
+        let span = self.span.unwrap_or_else(Span::call_site);
+
+        match self.style {
+            Style::Struct => {
+                let trailing_comma = {
+                    if fields.is_empty() {
+                        quote!()
+                    } else {
+                        quote!(,)
+                    }
+                };
+
+                tokens.extend(quote_spanned![span => { #(#fields),* #trailing_comma }]);
+            }
+            Style::Tuple => {
+                tokens.extend(quote_spanned![span => ( #(#fields),* )]);
+            }
+            Style::Unit => {}
+        }
+    }
+}
+
+impl<T: PartialEq> PartialEq for Fields<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.style == other.style && self.fields == other.fields
+    }
+}
+
+impl<T: Eq> Eq for Fields<T> {}
+
+impl<T> IntoIterator for Fields<T> {
+    type Item = T;
+    type IntoIter = vec::IntoIter<T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.fields.into_iter()
+    }
+}
+
+impl<T> From<Style> for Fields<T> {
+    fn from(style: Style) -> Self {
+        Self::new(style, Vec::new())
+    }
+}
+
+impl<T, U: Into<Vec<T>>> From<(Style, U)> for Fields<T> {
+    fn from((style, fields): (Style, U)) -> Self {
+        style.with_fields(fields)
+    }
+}
+
+impl<T: UsesTypeParams> UsesTypeParams for Fields<T> {
+    fn uses_type_params<'a>(
+        &self,
+        options: &usage::Options,
+        type_set: &'a IdentSet,
+    ) -> IdentRefSet<'a> {
+        self.fields.uses_type_params(options, type_set)
+    }
+}
+
+impl<T: UsesLifetimes> UsesLifetimes for Fields<T> {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &usage::Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        self.fields.uses_lifetimes(options, lifetimes)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Style {
+    Tuple,
+    Struct,
+    Unit,
+}
+
+impl Style {
+    pub fn is_unit(self) -> bool {
+        self == Style::Unit
+    }
+
+    pub fn is_tuple(self) -> bool {
+        self == Style::Tuple
+    }
+
+    pub fn is_struct(self) -> bool {
+        self == Style::Struct
+    }
+
+    /// Creates a new `Fields` of the specified style with the passed-in fields.
+    fn with_fields<T, U: Into<Vec<T>>>(self, fields: U) -> Fields<T> {
+        Fields::new(self, fields.into())
+    }
+}
+
+impl From<syn::Fields> for Style {
+    fn from(vd: syn::Fields) -> Self {
+        (&vd).into()
+    }
+}
+
+impl<'a> From<&'a syn::Fields> for Style {
+    fn from(vd: &syn::Fields) -> Self {
+        match *vd {
+            syn::Fields::Named(_) => Style::Struct,
+            syn::Fields::Unnamed(_) => Style::Tuple,
+            syn::Fields::Unit => Style::Unit,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // it is not possible to directly convert a TokenStream into syn::Fields, so you have
+    // to convert the TokenStream into DeriveInput first and then pass the syn::Fields to
+    // Fields::try_from.
+    fn token_stream_to_fields(input: TokenStream) -> Fields<syn::Field> {
+        Fields::try_from(&{
+            if let syn::Data::Struct(s) = syn::parse2::<syn::DeriveInput>(input).unwrap().data {
+                s.fields
+            } else {
+                panic!();
+            }
+        })
+        .unwrap()
+    }
+
+    #[test]
+    fn test_style_eq() {
+        // `Fields` implements `Eq` manually, so it has to be ensured, that all fields of `Fields`
+        // implement `Eq`, this test would fail, if someone accidentally removed the Eq
+        // implementation from `Style`.
+        struct _AssertEq
+        where
+            Style: Eq;
+    }
+
+    #[test]
+    fn test_fields_to_tokens_struct() {
+        let reference = quote!(
+            {
+                executable: String,
+                args: Vec<String>,
+                env: Vec<String>,
+                index: usize,
+                optional: Option<String>,
+                current_dir: String,
+            }
+        );
+        let input = quote!(
+            struct ExampleTest #reference
+        );
+
+        let fields = token_stream_to_fields(input);
+
+        let mut result = quote!();
+        fields.to_tokens(&mut result);
+        assert_eq!(result.to_string(), reference.to_string());
+    }
+
+    #[test]
+    fn test_fields_to_tokens_tuple() {
+        let reference = quote!((u64, usize, &'a T));
+        let input = quote!(
+            struct ExampleTest #reference;
+        );
+
+        let fields = token_stream_to_fields(input);
+
+        let mut result = quote!();
+        fields.to_tokens(&mut result);
+        assert_eq!(result.to_string(), reference.to_string());
+    }
+}
diff --git a/third_party/rust/darling_core/src/ast/generics.rs b/third_party/rust/darling_core/src/ast/generics.rs
new file mode 100644
index 0000000000..3de3c5794e
--- /dev/null
+++ b/third_party/rust/darling_core/src/ast/generics.rs
@@ -0,0 +1,192 @@
+//! Types for working with generics
+
+use std::iter::Iterator;
+use std::slice::Iter;
+
+use crate::{FromGenericParam, FromGenerics, FromTypeParam, Result};
+
+/// Extension trait for `GenericParam` to support getting values by variant.
+///
+/// # Usage
+/// `darling::ast::Generics` needs a way to test its params array in order to iterate over type params.
+/// Rather than require callers to use `darling::ast::GenericParam` in all cases, this trait makes that
+/// polymorphic.
+pub trait GenericParamExt {
+    /// The type this GenericParam uses to represent type params and their bounds
+    type TypeParam;
+    type LifetimeDef;
+    type ConstParam;
+
+    /// If this GenericParam is a type param, get the underlying value.
+    fn as_type_param(&self) -> Option<&Self::TypeParam> {
+        None
+    }
+
+    /// If this GenericParam is a lifetime, get the underlying value.
+    fn as_lifetime_def(&self) -> Option<&Self::LifetimeDef> {
+        None
+    }
+
+    /// If this GenericParam is a const param, get the underlying value.
+    fn as_const_param(&self) -> Option<&Self::ConstParam> {
+        None
+    }
+}
+
+impl GenericParamExt for syn::GenericParam {
+    type TypeParam = syn::TypeParam;
+    type LifetimeDef = syn::LifetimeDef;
+    type ConstParam = syn::ConstParam;
+
+    fn as_type_param(&self) -> Option<&Self::TypeParam> {
+        if let syn::GenericParam::Type(ref val) = *self {
+            Some(val)
+        } else {
+            None
+        }
+    }
+
+    fn as_lifetime_def(&self) -> Option<&Self::LifetimeDef> {
+        if let syn::GenericParam::Lifetime(ref val) = *self {
+            Some(val)
+        } else {
+            None
+        }
+    }
+
+    fn as_const_param(&self) -> Option<&Self::ConstParam> {
+        if let syn::GenericParam::Const(ref val) = *self {
+            Some(val)
+        } else {
+            None
+        }
+    }
+}
+
+impl GenericParamExt for syn::TypeParam {
+    type TypeParam = syn::TypeParam;
+    type LifetimeDef = ();
+    type ConstParam = ();
+
+    fn as_type_param(&self) -> Option<&Self::TypeParam> {
+        Some(self)
+    }
+}
+
+/// A mirror of `syn::GenericParam` which is generic over all its contents.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum GenericParam<T = syn::TypeParam, L = syn::LifetimeDef, C = syn::ConstParam> {
+    Type(T),
+    Lifetime(L),
+    Const(C),
+}
+
+impl<T: FromTypeParam> FromTypeParam for GenericParam<T> {
+    fn from_type_param(type_param: &syn::TypeParam) -> Result<Self> {
+        Ok(GenericParam::Type(FromTypeParam::from_type_param(
+            type_param,
+        )?))
+    }
+}
+
+impl<T: FromTypeParam> FromGenericParam for GenericParam<T> {
+    fn from_generic_param(param: &syn::GenericParam) -> Result<Self> {
+        Ok(match *param {
+            syn::GenericParam::Type(ref ty) => {
+                GenericParam::Type(FromTypeParam::from_type_param(ty)?)
+            }
+            syn::GenericParam::Lifetime(ref val) => GenericParam::Lifetime(val.clone()),
+            syn::GenericParam::Const(ref val) => GenericParam::Const(val.clone()),
+        })
+    }
+}
+
+impl<T, L, C> GenericParamExt for GenericParam<T, L, C> {
+    type TypeParam = T;
+    type LifetimeDef = L;
+    type ConstParam = C;
+
+    fn as_type_param(&self) -> Option<&T> {
+        if let GenericParam::Type(ref val) = *self {
+            Some(val)
+        } else {
+            None
+        }
+    }
+
+    fn as_lifetime_def(&self) -> Option<&L> {
+        if let GenericParam::Lifetime(ref val) = *self {
+            Some(val)
+        } else {
+            None
+        }
+    }
+
+    fn as_const_param(&self) -> Option<&C> {
+        if let GenericParam::Const(ref val) = *self {
+            Some(val)
+        } else {
+            None
+        }
+    }
+}
+
+/// A mirror of the `syn::Generics` type which can contain arbitrary representations
+/// of params and where clauses.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Generics<P, W = syn::WhereClause> {
+    pub params: Vec<P>,
+    pub where_clause: Option<W>,
+}
+
+impl<P, W> Generics<P, W> {
+    pub fn type_params(&self) -> TypeParams<'_, P> {
+        TypeParams(self.params.iter())
+    }
+}
+
+impl<P: FromGenericParam> FromGenerics for Generics<P> {
+    fn from_generics(generics: &syn::Generics) -> Result<Self> {
+        Ok(Generics {
+            params: generics
+                .params
+                .iter()
+                .map(FromGenericParam::from_generic_param)
+                .collect::<Result<Vec<P>>>()?,
+            where_clause: generics.where_clause.clone(),
+        })
+    }
+}
+
+pub struct TypeParams<'a, P: 'a>(Iter<'a, P>);
+
+impl<'a, P: GenericParamExt> Iterator for TypeParams<'a, P> {
+    type Item = &'a <P as GenericParamExt>::TypeParam;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let next = self.0.next();
+        match next {
+            None => None,
+            Some(v) => match v.as_type_param() {
+                Some(val) => Some(val),
+                None => self.next(),
+            },
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use syn::parse_quote;
+
+    use super::{GenericParam, Generics};
+    use crate::FromGenerics;
+
+    #[test]
+    fn generics() {
+        let g: syn::Generics = parse_quote!(<T>);
+        let deified: Generics<GenericParam<syn::Ident>> = FromGenerics::from_generics(&g).unwrap();
+        assert!(deified.params.len() == 1);
+        assert!(deified.where_clause.is_none());
+    }
+}
diff --git a/third_party/rust/darling_core/src/ast/mod.rs b/third_party/rust/darling_core/src/ast/mod.rs
new file mode 100644
index 0000000000..492b26f182
--- /dev/null
+++ b/third_party/rust/darling_core/src/ast/mod.rs
@@ -0,0 +1,7 @@
+//! Utility types for working with the AST.
+
+mod data;
+mod generics;
+
+pub use self::data::*;
+pub use self::generics::{GenericParam, GenericParamExt, Generics};
diff --git a/third_party/rust/darling_core/src/codegen/attr_extractor.rs b/third_party/rust/darling_core/src/codegen/attr_extractor.rs
new file mode 100644
index 0000000000..521d01c54c
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/attr_extractor.rs
@@ -0,0 +1,116 @@
+use proc_macro2::TokenStream;
+use quote::quote;
+
+use crate::options::ForwardAttrs;
+use crate::util::PathList;
+
+/// Infrastructure for generating an attribute extractor.
+pub trait ExtractAttribute {
+    /// A set of mutable declarations for all members of the implementing type.
+    fn local_declarations(&self) -> TokenStream;
+
+    /// Gets the list of attribute names that should be parsed by the extractor.
+    fn attr_names(&self) -> &PathList;
+
+    fn forwarded_attrs(&self) -> Option<&ForwardAttrs>;
+
+    /// Gets the name used by the generated impl to return to the `syn` item passed as input.
+    fn param_name(&self) -> TokenStream;
+
+    /// Get the tokens to access a borrowed list of attributes where extraction will take place.
+    ///
+    /// By default, this will be `&#input.attrs` where `#input` is `self.param_name()`.
+    fn attrs_accessor(&self) -> TokenStream {
+        let input = self.param_name();
+        quote!(&#input.attrs)
+    }
+
+    /// Gets the core from-meta-item loop that should be used on matching attributes.
+    fn core_loop(&self) -> TokenStream;
+
+    /// Generates the main extraction loop.
+    fn extractor(&self) -> TokenStream {
+        let declarations = self.local_declarations();
+
+        let will_parse_any = !self.attr_names().is_empty();
+        let will_fwd_any = self
+            .forwarded_attrs()
+            .map(|fa| !fa.is_empty())
+            .unwrap_or_default();
+
+        if !(will_parse_any || will_fwd_any) {
+            return quote! {
+                #declarations
+            };
+        }
+
+        let attrs_accessor = self.attrs_accessor();
+
+        // The block for parsing attributes whose names have been claimed by the target
+        // struct. If no attributes were claimed, this is a pass-through.
+        let parse_handled = if will_parse_any {
+            let attr_names = self.attr_names().to_strings();
+            let core_loop = self.core_loop();
+            quote!(
+                #(#attr_names)|* => {
+                    match ::darling::util::parse_attribute_to_meta_list(__attr) {
+                        ::darling::export::Ok(__data) => {
+                            if __data.nested.is_empty() {
+                                continue;
+                            }
+
+                            let __items = &__data.nested;
+
+                            #core_loop
+                        }
+                        // darling was asked to handle this attribute name, but the actual attribute
+                        // isn't one that darling can work with. This either indicates a typing error
+                        // or some misunderstanding of the meta attribute syntax; in either case, the
+                        // caller should get a useful error.
+                        ::darling::export::Err(__err) => {
+                            __errors.push(__err);
+                        }
+                    }
+                }
+            )
+        } else {
+            quote!()
+        };
+
+        // Specifies the behavior for unhandled attributes. They will either be silently ignored or
+        // forwarded to the inner struct for later analysis.
+        let forward_unhandled = if will_fwd_any {
+            forwards_to_local(self.forwarded_attrs().unwrap())
+        } else {
+            quote!(_ => continue)
+        };
+
+        quote!(
+            #declarations
+            use ::darling::ToTokens;
+            let mut __fwd_attrs: ::darling::export::Vec<::darling::export::syn::Attribute> = vec![];
+
+            for __attr in #attrs_accessor {
+                // Filter attributes based on name
+                match  ::darling::export::ToString::to_string(&__attr.path.clone().into_token_stream()).as_str() {
+                    #parse_handled
+                    #forward_unhandled
+                }
+            }
+        )
+    }
+}
+
+fn forwards_to_local(behavior: &ForwardAttrs) -> TokenStream {
+    let push_command = quote!(__fwd_attrs.push(__attr.clone()));
+    match *behavior {
+        ForwardAttrs::All => quote!(_ => #push_command),
+        ForwardAttrs::Only(ref idents) => {
+            let names = idents.to_strings();
+            quote!(
+                #(#names)|* => #push_command,
+                _ => continue,
+            )
+        }
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/default_expr.rs b/third_party/rust/darling_core/src/codegen/default_expr.rs
new file mode 100644
index 0000000000..fa8b159897
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/default_expr.rs
@@ -0,0 +1,53 @@
+use proc_macro2::{Span, TokenStream};
+use quote::{quote, quote_spanned, ToTokens, TokenStreamExt};
+use syn::{spanned::Spanned, Ident, Path};
+
+/// This will be in scope during struct initialization after option parsing.
+const DEFAULT_STRUCT_NAME: &str = "__default";
+
+/// The fallback value for a field or container.
+#[derive(Debug, Clone)]
+pub enum DefaultExpression<'a> {
+    /// Only valid on fields, `Inherit` indicates that the value should be taken from a pre-constructed
+    /// fallback object. The value in the variant is the ident of the field.
+    Inherit(&'a Ident),
+    Explicit(&'a Path),
+    Trait {
+        span: Span,
+    },
+}
+
+impl<'a> DefaultExpression<'a> {
+    pub fn as_declaration(&'a self) -> DefaultDeclaration<'a> {
+        DefaultDeclaration(self)
+    }
+}
+
+impl<'a> ToTokens for DefaultExpression<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        tokens.append_all(match *self {
+            DefaultExpression::Inherit(ident) => {
+                let dsn = Ident::new(DEFAULT_STRUCT_NAME, ::proc_macro2::Span::call_site());
+                quote!(#dsn.#ident)
+            }
+            DefaultExpression::Explicit(path) => {
+                // Use quote_spanned to properly set the span of the parentheses
+                quote_spanned!(path.span()=>#path())
+            }
+            DefaultExpression::Trait { span } => {
+                quote_spanned!(span=> ::darling::export::Default::default())
+            }
+        });
+    }
+}
+
+/// Used only by containers, this wrapper type generates code to declare the fallback instance.
+pub struct DefaultDeclaration<'a>(&'a DefaultExpression<'a>);
+
+impl<'a> ToTokens for DefaultDeclaration<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let name = Ident::new(DEFAULT_STRUCT_NAME, ::proc_macro2::Span::call_site());
+        let expr = self.0;
+        tokens.append_all(quote!(let #name: Self = #expr;));
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/error.rs b/third_party/rust/darling_core/src/codegen/error.rs
new file mode 100644
index 0000000000..f93a7404e0
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/error.rs
@@ -0,0 +1,46 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens, TokenStreamExt};
+
+/// Declares the local variable into which errors will be accumulated.
+#[derive(Default)]
+pub struct ErrorDeclaration {
+    __hidden: (),
+}
+
+impl ToTokens for ErrorDeclaration {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        tokens.append_all(quote! {
+            let mut __errors = ::darling::Error::accumulator();
+        })
+    }
+}
+
+/// Returns early if attribute or body parsing has caused any errors.
+#[derive(Default)]
+pub struct ErrorCheck<'a> {
+    location: Option<&'a str>,
+    __hidden: (),
+}
+
+impl<'a> ErrorCheck<'a> {
+    pub fn with_location(location: &'a str) -> Self {
+        ErrorCheck {
+            location: Some(location),
+            __hidden: (),
+        }
+    }
+}
+
+impl<'a> ToTokens for ErrorCheck<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let at_call = if let Some(ref s) = self.location {
+            quote!(.map_err(|e| e.at(#s)))
+        } else {
+            quote!()
+        };
+
+        tokens.append_all(quote! {
+            __errors.finish() #at_call?;
+        })
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/field.rs b/third_party/rust/darling_core/src/codegen/field.rs
new file mode 100644
index 0000000000..03467f0094
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/field.rs
@@ -0,0 +1,199 @@
+use std::borrow::Cow;
+
+use proc_macro2::TokenStream;
+use quote::{quote, quote_spanned, ToTokens, TokenStreamExt};
+use syn::{spanned::Spanned, Ident, Path, Type};
+
+use crate::codegen::{DefaultExpression, PostfixTransform};
+use crate::usage::{self, IdentRefSet, IdentSet, UsesTypeParams};
+
+/// Properties needed to generate code for a field in all the contexts
+/// where one may appear.
+#[derive(Debug, Clone)]
+pub struct Field<'a> {
+    /// The name presented to the user of the library. This will appear
+    /// in error messages and will be looked when parsing names.
+    pub name_in_attr: Cow<'a, String>,
+
+    /// The name presented to the author of the library. This will appear
+    /// in the setters or temporary variables which contain the values.
+    pub ident: &'a Ident,
+
+    /// The type of the field in the input.
+    pub ty: &'a Type,
+    pub default_expression: Option<DefaultExpression<'a>>,
+    pub with_path: Cow<'a, Path>,
+    pub post_transform: Option<&'a PostfixTransform>,
+    pub skip: bool,
+    pub multiple: bool,
+}
+
+impl<'a> Field<'a> {
+    pub fn as_name(&'a self) -> &'a str {
+        &self.name_in_attr
+    }
+
+    pub fn as_declaration(&'a self) -> Declaration<'a> {
+        Declaration(self)
+    }
+
+    pub fn as_match(&'a self) -> MatchArm<'a> {
+        MatchArm(self)
+    }
+
+    pub fn as_initializer(&'a self) -> Initializer<'a> {
+        Initializer(self)
+    }
+
+    pub fn as_presence_check(&'a self) -> CheckMissing<'a> {
+        CheckMissing(self)
+    }
+}
+
+impl<'a> UsesTypeParams for Field<'a> {
+    fn uses_type_params<'b>(
+        &self,
+        options: &usage::Options,
+        type_set: &'b IdentSet,
+    ) -> IdentRefSet<'b> {
+        self.ty.uses_type_params(options, type_set)
+    }
+}
+
+/// An individual field during variable declaration in the generated parsing method.
+pub struct Declaration<'a>(&'a Field<'a>);
+
+impl<'a> ToTokens for Declaration<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let field: &Field = self.0;
+        let ident = field.ident;
+        let ty = field.ty;
+
+        tokens.append_all(if field.multiple {
+            // This is NOT mutable, as it will be declared mutable only temporarily.
+            quote!(let mut #ident: #ty = ::darling::export::Default::default();)
+        } else {
+            quote!(let mut #ident: (bool, ::darling::export::Option<#ty>) = (false, None);)
+        });
+    }
+}
+
+/// Represents an individual field in the match.
+pub struct MatchArm<'a>(&'a Field<'a>);
+
+impl<'a> ToTokens for MatchArm<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let field: &Field = self.0;
+        if !field.skip {
+            let name_str = &field.name_in_attr;
+            let ident = field.ident;
+            let with_path = &field.with_path;
+            let post_transform = field.post_transform.as_ref();
+
+            // Errors include the location of the bad input, so we compute that here.
+            // Fields that take multiple values add the index of the error for convenience,
+            // while single-value fields only expose the name in the input attribute.
+            let location = if field.multiple {
+                // we use the local variable `len` here because location is accessed via
+                // a closure, and the borrow checker gets very unhappy if we try to immutably
+                // borrow `#ident` in that closure when it was declared `mut` outside.
+                quote!(&format!("{}[{}]", #name_str, __len))
+            } else {
+                quote!(#name_str)
+            };
+
+            // Give darling's generated code the span of the `with_path` so that if the target
+            // type doesn't impl FromMeta, darling's immediate user gets a properly-spanned error.
+            //
+            // Within the generated code, add the span immediately on extraction failure, so that it's
+            // as specific as possible.
+            // The behavior of `with_span` makes this safe to do; if the child applied an
+            // even-more-specific span, our attempt here will not overwrite that and will only cost
+            // us one `if` check.
+            let extractor = quote_spanned!(with_path.span()=>#with_path(__inner)#post_transform.map_err(|e| e.with_span(&__inner).at(#location)));
+
+            tokens.append_all(if field.multiple {
+                quote!(
+                    #name_str => {
+                        // Store the index of the name we're assessing in case we need
+                        // it for error reporting.
+                        let __len = #ident.len();
+                        if let ::darling::export::Some(__val) = __errors.handle(#extractor) {
+                            #ident.push(__val)
+                        }
+                    }
+                )
+            } else {
+                quote!(
+                    #name_str => {
+                        if !#ident.0 {
+                            #ident = (true, __errors.handle(#extractor));
+                        } else {
+                            __errors.push(::darling::Error::duplicate_field(#name_str).with_span(&__inner));
+                        }
+                    }
+                )
+            });
+        }
+    }
+}
+
+/// Wrapper to generate initialization code for a field.
+pub struct Initializer<'a>(&'a Field<'a>);
+
+impl<'a> ToTokens for Initializer<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let field: &Field = self.0;
+        let ident = field.ident;
+        tokens.append_all(if field.multiple {
+            if let Some(ref expr) = field.default_expression {
+                quote_spanned!(expr.span()=> #ident: if !#ident.is_empty() {
+                    #ident
+                } else {
+                    #expr
+                })
+            } else {
+                quote!(#ident: #ident)
+            }
+        } else if let Some(ref expr) = field.default_expression {
+            quote_spanned!(expr.span()=> #ident: if let Some(__val) = #ident.1 {
+                __val
+            } else {
+                #expr
+            })
+        } else {
+            quote!(#ident: #ident.1.expect("Uninitialized fields without defaults were already checked"))
+        });
+    }
+}
+
+/// Creates an error if a field has no value and no default.
+pub struct CheckMissing<'a>(&'a Field<'a>);
+
+impl<'a> ToTokens for CheckMissing<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        if !self.0.multiple && self.0.default_expression.is_none() {
+            let ident = self.0.ident;
+            let ty = self.0.ty;
+            let name_in_attr = &self.0.name_in_attr;
+
+            // If `ty` does not impl FromMeta, the compiler error should point
+            // at the offending type rather than at the derive-macro call site.
+            let from_none_call =
+                quote_spanned!(ty.span()=> <#ty as ::darling::FromMeta>::from_none());
+
+            tokens.append_all(quote! {
+                if !#ident.0 {
+                    match #from_none_call {
+                        ::darling::export::Some(__type_fallback) => {
+                            #ident.1 = ::darling::export::Some(__type_fallback);
+                        }
+                        ::darling::export::None => {
+                            __errors.push(::darling::Error::missing_field(#name_in_attr))
+                        }
+                    }
+                }
+            })
+        }
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/from_attributes_impl.rs b/third_party/rust/darling_core/src/codegen/from_attributes_impl.rs
new file mode 100644
index 0000000000..49bbf02c0a
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/from_attributes_impl.rs
@@ -0,0 +1,109 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+
+use crate::{
+    ast::Data,
+    codegen::{ExtractAttribute, OuterFromImpl, TraitImpl},
+    options::ForwardAttrs,
+    util::PathList,
+};
+
+pub struct FromAttributesImpl<'a> {
+    pub base: TraitImpl<'a>,
+    pub attr_names: &'a PathList,
+}
+
+impl ToTokens for FromAttributesImpl<'_> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let ty_ident = self.base.ident;
+        let input = self.param_name();
+        let post_transform = self.base.post_transform_call();
+
+        if let Data::Struct(ref data) = self.base.data {
+            if data.is_newtype() {
+                self.wrap(
+                    quote! {
+                        fn from_attributes(#input: &[::darling::export::syn::Attribute]) -> ::darling::Result<Self> {
+                            ::darling::export::Ok(
+                                #ty_ident(::darling::FromAttributes::from_attributes(#input)?)
+                            ) #post_transform
+                        }
+                    },
+                    tokens,
+                );
+
+                return;
+            }
+        }
+
+        let inits = self.base.initializers();
+        let default = self.base.fallback_decl();
+
+        let grab_attrs = self.extractor();
+
+        let declare_errors = self.base.declare_errors();
+        let require_fields = self.base.require_fields();
+        let check_errors = self.base.check_errors();
+
+        self.wrap(
+            quote! {
+                fn from_attributes(#input: &[::darling::export::syn::Attribute]) -> ::darling::Result<Self> {
+                    #declare_errors
+
+                    #grab_attrs
+
+                    #require_fields
+
+                    #check_errors
+
+                    #default
+
+                    ::darling::export::Ok(#ty_ident {
+                        #inits
+                    }) #post_transform
+                }
+            },
+            tokens,
+        );
+    }
+}
+
+impl<'a> ExtractAttribute for FromAttributesImpl<'a> {
+    fn local_declarations(&self) -> TokenStream {
+        self.base.local_declarations()
+    }
+
+    fn attr_names(&self) -> &PathList {
+        self.attr_names
+    }
+
+    fn forwarded_attrs(&self) -> Option<&ForwardAttrs> {
+        None
+    }
+
+    fn param_name(&self) -> TokenStream {
+        quote!(__di)
+    }
+
+    fn attrs_accessor(&self) -> TokenStream {
+        self.param_name()
+    }
+
+    fn core_loop(&self) -> TokenStream {
+        self.base.core_loop()
+    }
+}
+
+impl<'a> OuterFromImpl<'a> for FromAttributesImpl<'a> {
+    fn trait_path(&self) -> syn::Path {
+        path!(::darling::FromAttributes)
+    }
+
+    fn trait_bound(&self) -> syn::Path {
+        path!(::darling::FromMeta)
+    }
+
+    fn base(&'a self) -> &'a TraitImpl<'a> {
+        &self.base
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/from_derive_impl.rs b/third_party/rust/darling_core/src/codegen/from_derive_impl.rs
new file mode 100644
index 0000000000..1c1df63d95
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/from_derive_impl.rs
@@ -0,0 +1,147 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+use syn::Ident;
+
+use crate::{
+    ast::Data,
+    codegen::{ExtractAttribute, OuterFromImpl, TraitImpl},
+    options::{DeriveInputShapeSet, ForwardAttrs},
+    util::PathList,
+};
+
+pub struct FromDeriveInputImpl<'a> {
+    pub ident: Option<&'a Ident>,
+    pub generics: Option<&'a Ident>,
+    pub vis: Option<&'a Ident>,
+    pub attrs: Option<&'a Ident>,
+    pub data: Option<&'a Ident>,
+    pub base: TraitImpl<'a>,
+    pub attr_names: &'a PathList,
+    pub forward_attrs: Option<&'a ForwardAttrs>,
+    pub from_ident: bool,
+    pub supports: Option<&'a DeriveInputShapeSet>,
+}
+
+impl<'a> ToTokens for FromDeriveInputImpl<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let ty_ident = self.base.ident;
+        let input = self.param_name();
+        let post_transform = self.base.post_transform_call();
+
+        if let Data::Struct(ref data) = self.base.data {
+            if data.is_newtype() {
+                self.wrap(
+                    quote!{
+                        fn from_derive_input(#input: &::darling::export::syn::DeriveInput) -> ::darling::Result<Self> {
+                            ::darling::export::Ok(
+                                #ty_ident(::darling::FromDeriveInput::from_derive_input(#input)?)
+                            ) #post_transform
+                        }
+                    },
+                    tokens,
+                );
+
+                return;
+            }
+        }
+
+        let passed_ident = self
+            .ident
+            .as_ref()
+            .map(|i| quote!(#i: #input.ident.clone(),));
+        let passed_vis = self.vis.as_ref().map(|i| quote!(#i: #input.vis.clone(),));
+        let passed_generics = self
+            .generics
+            .as_ref()
+            .map(|i| quote!(#i: ::darling::FromGenerics::from_generics(&#input.generics)?,));
+        let passed_attrs = self.attrs.as_ref().map(|i| quote!(#i: __fwd_attrs,));
+        let passed_body = self
+            .data
+            .as_ref()
+            .map(|i| quote!(#i: ::darling::ast::Data::try_from(&#input.data)?,));
+
+        let supports = self.supports.map(|i| {
+            quote! {
+                #i
+                __errors.handle(__validate_body(&#input.data));
+            }
+        });
+
+        let inits = self.base.initializers();
+        let default = if self.from_ident {
+            quote!(let __default: Self = ::darling::export::From::from(#input.ident.clone());)
+        } else {
+            self.base.fallback_decl()
+        };
+
+        let grab_attrs = self.extractor();
+
+        let declare_errors = self.base.declare_errors();
+        let require_fields = self.base.require_fields();
+        let check_errors = self.base.check_errors();
+
+        self.wrap(
+            quote! {
+                fn from_derive_input(#input: &::darling::export::syn::DeriveInput) -> ::darling::Result<Self> {
+                    #declare_errors
+
+                    #grab_attrs
+
+                    #supports
+
+                    #require_fields
+
+                    #check_errors
+
+                    #default
+
+                    ::darling::export::Ok(#ty_ident {
+                        #passed_ident
+                        #passed_generics
+                        #passed_vis
+                        #passed_attrs
+                        #passed_body
+                        #inits
+                    }) #post_transform
+                }
+            },
+            tokens,
+        );
+    }
+}
+
+impl<'a> ExtractAttribute for FromDeriveInputImpl<'a> {
+    fn attr_names(&self) -> &PathList {
+        self.attr_names
+    }
+
+    fn forwarded_attrs(&self) -> Option<&ForwardAttrs> {
+        self.forward_attrs
+    }
+
+    fn param_name(&self) -> TokenStream {
+        quote!(__di)
+    }
+
+    fn core_loop(&self) -> TokenStream {
+        self.base.core_loop()
+    }
+
+    fn local_declarations(&self) -> TokenStream {
+        self.base.local_declarations()
+    }
+}
+
+impl<'a> OuterFromImpl<'a> for FromDeriveInputImpl<'a> {
+    fn trait_path(&self) -> syn::Path {
+        path!(::darling::FromDeriveInput)
+    }
+
+    fn trait_bound(&self) -> syn::Path {
+        path!(::darling::FromMeta)
+    }
+
+    fn base(&'a self) -> &'a TraitImpl<'a> {
+        &self.base
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/from_field.rs b/third_party/rust/darling_core/src/codegen/from_field.rs
new file mode 100644
index 0000000000..74f340607c
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/from_field.rs
@@ -0,0 +1,114 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+use syn::Ident;
+
+use crate::{
+    codegen::{ExtractAttribute, OuterFromImpl, TraitImpl},
+    options::ForwardAttrs,
+    util::PathList,
+};
+
+/// `impl FromField` generator. This is used for parsing an individual
+/// field and its attributes.
+pub struct FromFieldImpl<'a> {
+    pub ident: Option<&'a Ident>,
+    pub vis: Option<&'a Ident>,
+    pub ty: Option<&'a Ident>,
+    pub attrs: Option<&'a Ident>,
+    pub base: TraitImpl<'a>,
+    pub attr_names: &'a PathList,
+    pub forward_attrs: Option<&'a ForwardAttrs>,
+    pub from_ident: bool,
+}
+
+impl<'a> ToTokens for FromFieldImpl<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let input = self.param_name();
+
+        let error_declaration = self.base.declare_errors();
+        let require_fields = self.base.require_fields();
+        let error_check = self.base.check_errors();
+
+        let initializers = self.base.initializers();
+
+        let default = if self.from_ident {
+            quote!(let __default: Self = ::darling::export::From::from(#input.ident.clone());)
+        } else {
+            self.base.fallback_decl()
+        };
+
+        let passed_ident = self
+            .ident
+            .as_ref()
+            .map(|i| quote!(#i: #input.ident.clone(),));
+        let passed_vis = self.vis.as_ref().map(|i| quote!(#i: #input.vis.clone(),));
+        let passed_ty = self.ty.as_ref().map(|i| quote!(#i: #input.ty.clone(),));
+        let passed_attrs = self.attrs.as_ref().map(|i| quote!(#i: __fwd_attrs,));
+
+        // Determine which attributes to forward (if any).
+        let grab_attrs = self.extractor();
+        let post_transform = self.base.post_transform_call();
+
+        self.wrap(
+            quote! {
+                fn from_field(#input: &::darling::export::syn::Field) -> ::darling::Result<Self> {
+                    #error_declaration
+
+                    #grab_attrs
+
+                    #require_fields
+
+                    #error_check
+
+                    #default
+
+                    ::darling::export::Ok(Self {
+                        #passed_ident
+                        #passed_ty
+                        #passed_vis
+                        #passed_attrs
+                        #initializers
+                    }) #post_transform
+
+                }
+            },
+            tokens,
+        );
+    }
+}
+
+impl<'a> ExtractAttribute for FromFieldImpl<'a> {
+    fn attr_names(&self) -> &PathList {
+        self.attr_names
+    }
+
+    fn forwarded_attrs(&self) -> Option<&ForwardAttrs> {
+        self.forward_attrs
+    }
+
+    fn param_name(&self) -> TokenStream {
+        quote!(__field)
+    }
+
+    fn core_loop(&self) -> TokenStream {
+        self.base.core_loop()
+    }
+
+    fn local_declarations(&self) -> TokenStream {
+        self.base.local_declarations()
+    }
+}
+
+impl<'a> OuterFromImpl<'a> for FromFieldImpl<'a> {
+    fn trait_path(&self) -> syn::Path {
+        path!(::darling::FromField)
+    }
+
+    fn trait_bound(&self) -> syn::Path {
+        path!(::darling::FromMeta)
+    }
+
+    fn base(&'a self) -> &'a TraitImpl<'a> {
+        &self.base
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/from_meta_impl.rs b/third_party/rust/darling_core/src/codegen/from_meta_impl.rs
new file mode 100644
index 0000000000..af59f3b1c7
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/from_meta_impl.rs
@@ -0,0 +1,135 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+
+use crate::ast::{Data, Fields, Style};
+use crate::codegen::{Field, OuterFromImpl, TraitImpl, Variant};
+
+pub struct FromMetaImpl<'a> {
+    pub base: TraitImpl<'a>,
+}
+
+impl<'a> ToTokens for FromMetaImpl<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let base = &self.base;
+
+        let impl_block = match base.data {
+            // Unit structs allow empty bodies only.
+            Data::Struct(ref vd) if vd.style.is_unit() => {
+                let ty_ident = base.ident;
+                quote!(
+                    fn from_word() -> ::darling::Result<Self> {
+                        ::darling::export::Ok(#ty_ident)
+                    }
+                )
+            }
+
+            // Newtype structs proxy to the sole value they contain.
+            Data::Struct(Fields {
+                ref fields,
+                style: Style::Tuple,
+                ..
+            }) if fields.len() == 1 => {
+                let ty_ident = base.ident;
+                quote!(
+                    fn from_meta(__item: &::darling::export::syn::Meta) -> ::darling::Result<Self> {
+                        ::darling::FromMeta::from_meta(__item)
+                            .map_err(|e| e.with_span(&__item))
+                            .map(#ty_ident)
+                    }
+                )
+            }
+            Data::Struct(Fields {
+                style: Style::Tuple,
+                ..
+            }) => {
+                panic!("Multi-field tuples are not supported");
+            }
+            Data::Struct(ref data) => {
+                let inits = data.fields.iter().map(Field::as_initializer);
+                let declare_errors = base.declare_errors();
+                let require_fields = base.require_fields();
+                let check_errors = base.check_errors();
+                let decls = base.local_declarations();
+                let core_loop = base.core_loop();
+                let default = base.fallback_decl();
+                let post_transform = base.post_transform_call();
+
+                quote!(
+                    fn from_list(__items: &[::darling::export::syn::NestedMeta]) -> ::darling::Result<Self> {
+
+                        #decls
+
+                        #declare_errors
+
+                        #core_loop
+
+                        #require_fields
+
+                        #check_errors
+
+                        #default
+
+                        ::darling::export::Ok(Self {
+                            #(#inits),*
+                        }) #post_transform
+                    }
+                )
+            }
+            Data::Enum(ref variants) => {
+                let unit_arms = variants.iter().map(Variant::as_unit_match_arm);
+                let struct_arms = variants.iter().map(Variant::as_data_match_arm);
+
+                let unknown_variant_err = if !variants.is_empty() {
+                    let names = variants.iter().map(Variant::as_name);
+                    quote! {
+                        unknown_field_with_alts(__other, &[#(#names),*])
+                    }
+                } else {
+                    quote! {
+                        unknown_field(__other)
+                    }
+                };
+
+                quote!(
+                    fn from_list(__outer: &[::darling::export::syn::NestedMeta]) -> ::darling::Result<Self> {
+                        // An enum must have exactly one value inside the parentheses if it's not a unit
+                        // match arm
+                        match __outer.len() {
+                            0 => ::darling::export::Err(::darling::Error::too_few_items(1)),
+                            1 => {
+                                if let ::darling::export::syn::NestedMeta::Meta(ref __nested) = __outer[0] {
+                                    match ::darling::util::path_to_string(__nested.path()).as_ref() {
+                                        #(#struct_arms)*
+                                        __other => ::darling::export::Err(::darling::Error::#unknown_variant_err.with_span(__nested))
+                                    }
+                                } else {
+                                    ::darling::export::Err(::darling::Error::unsupported_format("literal"))
+                                }
+                            }
+                            _ => ::darling::export::Err(::darling::Error::too_many_items(1)),
+                        }
+                    }
+
+                    fn from_string(lit: &str) -> ::darling::Result<Self> {
+                        match lit {
+                            #(#unit_arms)*
+                            __other => ::darling::export::Err(::darling::Error::unknown_value(__other))
+                        }
+                    }
+                )
+            }
+        };
+
+        self.wrap(impl_block, tokens);
+    }
+}
+
+impl<'a> OuterFromImpl<'a> for FromMetaImpl<'a> {
+    fn trait_path(&self) -> syn::Path {
+        path!(::darling::FromMeta)
+    }
+
+    fn base(&'a self) -> &'a TraitImpl<'a> {
+        &self.base
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/from_type_param.rs b/third_party/rust/darling_core/src/codegen/from_type_param.rs
new file mode 100644
index 0000000000..1cce0cad4e
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/from_type_param.rs
@@ -0,0 +1,113 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+use syn::Ident;
+
+use crate::codegen::{ExtractAttribute, OuterFromImpl, TraitImpl};
+use crate::options::ForwardAttrs;
+use crate::util::PathList;
+
+pub struct FromTypeParamImpl<'a> {
+    pub base: TraitImpl<'a>,
+    pub ident: Option<&'a Ident>,
+    pub attrs: Option<&'a Ident>,
+    pub bounds: Option<&'a Ident>,
+    pub default: Option<&'a Ident>,
+    pub attr_names: &'a PathList,
+    pub forward_attrs: Option<&'a ForwardAttrs>,
+    pub from_ident: bool,
+}
+
+impl<'a> ToTokens for FromTypeParamImpl<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let input = self.param_name();
+
+        let error_declaration = self.base.declare_errors();
+        let grab_attrs = self.extractor();
+        let require_fields = self.base.require_fields();
+        let error_check = self.base.check_errors();
+
+        let default = if self.from_ident {
+            quote!(let __default: Self = ::darling::export::From::from(#input.ident.clone());)
+        } else {
+            self.base.fallback_decl()
+        };
+
+        let passed_ident = self
+            .ident
+            .as_ref()
+            .map(|i| quote!(#i: #input.ident.clone(),));
+        let passed_attrs = self.attrs.as_ref().map(|i| quote!(#i: __fwd_attrs,));
+        let passed_bounds = self
+            .bounds
+            .as_ref()
+            .map(|i| quote!(#i: #input.bounds.clone().into_iter().collect::<Vec<_>>(),));
+        let passed_default = self
+            .default
+            .as_ref()
+            .map(|i| quote!(#i: #input.default.clone(),));
+        let initializers = self.base.initializers();
+
+        let post_transform = self.base.post_transform_call();
+
+        self.wrap(
+            quote! {
+                fn from_type_param(#input: &::darling::export::syn::TypeParam) -> ::darling::Result<Self> {
+                    #error_declaration
+
+                    #grab_attrs
+
+                    #require_fields
+
+                    #error_check
+
+                    #default
+
+                    ::darling::export::Ok(Self {
+                        #passed_ident
+                        #passed_bounds
+                        #passed_default
+                        #passed_attrs
+                        #initializers
+                    }) #post_transform
+                }
+            },
+            tokens,
+        );
+    }
+}
+
+impl<'a> ExtractAttribute for FromTypeParamImpl<'a> {
+    fn attr_names(&self) -> &PathList {
+        self.attr_names
+    }
+
+    fn forwarded_attrs(&self) -> Option<&ForwardAttrs> {
+        self.forward_attrs
+    }
+
+    fn param_name(&self) -> TokenStream {
+        quote!(__type_param)
+    }
+
+    fn core_loop(&self) -> TokenStream {
+        self.base.core_loop()
+    }
+
+    fn local_declarations(&self) -> TokenStream {
+        self.base.local_declarations()
+    }
+}
+
+impl<'a> OuterFromImpl<'a> for FromTypeParamImpl<'a> {
+    fn trait_path(&self) -> syn::Path {
+        path!(::darling::FromTypeParam)
+    }
+
+    fn trait_bound(&self) -> syn::Path {
+        path!(::darling::FromMeta)
+    }
+
+    fn base(&'a self) -> &'a TraitImpl<'a> {
+        &self.base
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/from_variant_impl.rs b/third_party/rust/darling_core/src/codegen/from_variant_impl.rs
new file mode 100644
index 0000000000..1598e6f2b0
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/from_variant_impl.rs
@@ -0,0 +1,139 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+use syn::Ident;
+
+use crate::codegen::{ExtractAttribute, OuterFromImpl, TraitImpl};
+use crate::options::{DataShape, ForwardAttrs};
+use crate::util::PathList;
+
+pub struct FromVariantImpl<'a> {
+    pub base: TraitImpl<'a>,
+    /// If set, the ident of the field into which the variant ident should be placed.
+    ///
+    /// This is one of `darling`'s "magic fields", which allow a type deriving a `darling`
+    /// trait to get fields from the input `syn` element added to the deriving struct
+    /// automatically.
+    pub ident: Option<&'a Ident>,
+    /// If set, the ident of the field into which the transformed output of the input
+    /// variant's fields should be placed.
+    ///
+    /// This is one of `darling`'s "magic fields".
+    pub fields: Option<&'a Ident>,
+    /// If set, the ident of the field into which the forwarded attributes of the input
+    /// variant should be placed.
+    ///
+    /// This is one of `darling`'s "magic fields".
+    pub attrs: Option<&'a Ident>,
+    /// If set, the ident of the field into which the discriminant of the input variant
+    /// should be placed. The receiving field must be an `Option` as not all enums have
+    /// discriminants.
+    ///
+    /// This is one of `darling`'s "magic fields".
+    pub discriminant: Option<&'a Ident>,
+    pub attr_names: &'a PathList,
+    pub forward_attrs: Option<&'a ForwardAttrs>,
+    pub from_ident: bool,
+    pub supports: Option<&'a DataShape>,
+}
+
+impl<'a> ToTokens for FromVariantImpl<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let input = self.param_name();
+        let extractor = self.extractor();
+        let passed_ident = self
+            .ident
+            .as_ref()
+            .map(|i| quote!(#i: #input.ident.clone(),));
+        let passed_discriminant = self
+            .discriminant
+            .as_ref()
+            .map(|i| quote!(#i: #input.discriminant.as_ref().map(|(_, expr)| expr.clone()),));
+        let passed_attrs = self.attrs.as_ref().map(|i| quote!(#i: __fwd_attrs,));
+        let passed_fields = self
+            .fields
+            .as_ref()
+            .map(|i| quote!(#i: ::darling::ast::Fields::try_from(&#input.fields)?,));
+
+        let inits = self.base.initializers();
+        let post_transform = self.base.post_transform_call();
+
+        let default = if self.from_ident {
+            quote!(let __default: Self = ::darling::export::From::from(#input.ident.clone());)
+        } else {
+            self.base.fallback_decl()
+        };
+
+        let supports = self.supports.map(|i| {
+            quote! {
+                __errors.handle(#i.check(&#input.fields));
+            }
+        });
+
+        let error_declaration = self.base.declare_errors();
+        let require_fields = self.base.require_fields();
+        let error_check = self.base.check_errors();
+
+        self.wrap(
+            quote!(
+                fn from_variant(#input: &::darling::export::syn::Variant) -> ::darling::Result<Self> {
+                    #error_declaration
+
+                    #extractor
+
+                    #supports
+
+                    #require_fields
+
+                    #error_check
+
+                    #default
+
+                    ::darling::export::Ok(Self {
+                        #passed_ident
+                        #passed_discriminant
+                        #passed_attrs
+                        #passed_fields
+                        #inits
+                    }) #post_transform
+                }
+            ),
+            tokens,
+        );
+    }
+}
+
+impl<'a> ExtractAttribute for FromVariantImpl<'a> {
+    fn local_declarations(&self) -> TokenStream {
+        self.base.local_declarations()
+    }
+
+    fn attr_names(&self) -> &PathList {
+        self.attr_names
+    }
+
+    fn forwarded_attrs(&self) -> Option<&ForwardAttrs> {
+        self.forward_attrs
+    }
+
+    fn param_name(&self) -> TokenStream {
+        quote!(__variant)
+    }
+
+    fn core_loop(&self) -> TokenStream {
+        self.base.core_loop()
+    }
+}
+
+impl<'a> OuterFromImpl<'a> for FromVariantImpl<'a> {
+    fn trait_path(&self) -> syn::Path {
+        path!(::darling::FromVariant)
+    }
+
+    fn trait_bound(&self) -> syn::Path {
+        path!(::darling::FromMeta)
+    }
+
+    fn base(&'a self) -> &'a TraitImpl<'a> {
+        &self.base
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/mod.rs b/third_party/rust/darling_core/src/codegen/mod.rs
new file mode 100644
index 0000000000..d2a1546e05
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/mod.rs
@@ -0,0 +1,30 @@
+mod attr_extractor;
+mod default_expr;
+mod error;
+mod field;
+mod from_attributes_impl;
+mod from_derive_impl;
+mod from_field;
+mod from_meta_impl;
+mod from_type_param;
+mod from_variant_impl;
+mod outer_from_impl;
+mod postfix_transform;
+mod trait_impl;
+mod variant;
+mod variant_data;
+
+pub(in crate::codegen) use self::attr_extractor::ExtractAttribute;
+pub use self::default_expr::DefaultExpression;
+pub use self::field::Field;
+pub use self::from_attributes_impl::FromAttributesImpl;
+pub use self::from_derive_impl::FromDeriveInputImpl;
+pub use self::from_field::FromFieldImpl;
+pub use self::from_meta_impl::FromMetaImpl;
+pub use self::from_type_param::FromTypeParamImpl;
+pub use self::from_variant_impl::FromVariantImpl;
+pub use self::outer_from_impl::OuterFromImpl;
+pub use self::postfix_transform::PostfixTransform;
+pub use self::trait_impl::TraitImpl;
+pub use self::variant::Variant;
+pub use self::variant_data::FieldsGen;
diff --git a/third_party/rust/darling_core/src/codegen/outer_from_impl.rs b/third_party/rust/darling_core/src/codegen/outer_from_impl.rs
new file mode 100644
index 0000000000..c2939a6f13
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/outer_from_impl.rs
@@ -0,0 +1,60 @@
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens, TokenStreamExt};
+use syn::{GenericParam, Generics, Path, TraitBound, TraitBoundModifier, TypeParamBound};
+
+use crate::codegen::TraitImpl;
+use crate::usage::IdentSet;
+
+/// Wrapper for "outer From" traits, such as `FromDeriveInput`, `FromVariant`, and `FromField`.
+pub trait OuterFromImpl<'a> {
+    /// Gets the path of the trait being implemented.
+    fn trait_path(&self) -> Path;
+
+    fn base(&'a self) -> &'a TraitImpl<'a>;
+
+    fn trait_bound(&self) -> Path {
+        self.trait_path()
+    }
+
+    fn wrap<T: ToTokens>(&'a self, body: T, tokens: &mut TokenStream) {
+        let base = self.base();
+        let trayt = self.trait_path();
+        let ty_ident = base.ident;
+        // The type parameters used in non-skipped, non-magic fields.
+        // These must impl `FromMeta` unless they have custom bounds.
+        let used = base.used_type_params();
+        let generics = compute_impl_bounds(self.trait_bound(), base.generics.clone(), &used);
+        let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+        tokens.append_all(quote!(
+            impl #impl_generics #trayt for #ty_ident #ty_generics
+                #where_clause
+            {
+                #body
+            }
+        ));
+    }
+}
+
+fn compute_impl_bounds(bound: Path, mut generics: Generics, applies_to: &IdentSet) -> Generics {
+    if generics.params.is_empty() {
+        return generics;
+    }
+
+    let added_bound = TypeParamBound::Trait(TraitBound {
+        paren_token: None,
+        modifier: TraitBoundModifier::None,
+        lifetimes: None,
+        path: bound,
+    });
+
+    for param in generics.params.iter_mut() {
+        if let GenericParam::Type(ref mut typ) = *param {
+            if applies_to.contains(&typ.ident) {
+                typ.bounds.push(added_bound.clone());
+            }
+        }
+    }
+
+    generics
+}
diff --git a/third_party/rust/darling_core/src/codegen/postfix_transform.rs b/third_party/rust/darling_core/src/codegen/postfix_transform.rs
new file mode 100644
index 0000000000..e631702cae
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/postfix_transform.rs
@@ -0,0 +1,30 @@
+use quote::{quote, ToTokens, TokenStreamExt};
+use syn::{Ident, Path};
+
+/// A method invocation applied to a value.
+///
+/// This is used for `map` and `and_then` transforms in derivations.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct PostfixTransform {
+    pub(crate) transformer: Ident,
+    pub(crate) function: Path,
+}
+
+impl PostfixTransform {
+    pub fn new(transformer: Ident, function: Path) -> Self {
+        Self {
+            transformer,
+            function,
+        }
+    }
+}
+
+impl ToTokens for PostfixTransform {
+    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
+        let Self {
+            transformer,
+            function,
+        } = self;
+        tokens.append_all(quote!(.#transformer(#function)))
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/trait_impl.rs b/third_party/rust/darling_core/src/codegen/trait_impl.rs
new file mode 100644
index 0000000000..0bc7f7eb20
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/trait_impl.rs
@@ -0,0 +1,136 @@
+use proc_macro2::TokenStream;
+use quote::quote;
+use syn::{Generics, Ident, WherePredicate};
+
+use crate::ast::{Data, Fields};
+use crate::codegen::{
+    error::{ErrorCheck, ErrorDeclaration},
+    DefaultExpression, Field, FieldsGen, PostfixTransform, Variant,
+};
+use crate::usage::{CollectTypeParams, IdentSet, Purpose};
+
+#[derive(Debug)]
+pub struct TraitImpl<'a> {
+    pub ident: &'a Ident,
+    pub generics: &'a Generics,
+    pub data: Data<Variant<'a>, Field<'a>>,
+    pub default: Option<DefaultExpression<'a>>,
+    pub post_transform: Option<&'a PostfixTransform>,
+    pub bound: Option<&'a [WherePredicate]>,
+    pub allow_unknown_fields: bool,
+}
+
+impl<'a> TraitImpl<'a> {
+    /// Get all declared type parameters.
+    pub fn declared_type_params(&self) -> IdentSet {
+        self.generics
+            .type_params()
+            .map(|tp| tp.ident.clone())
+            .collect()
+    }
+
+    /// Get the type parameters which are used by non-skipped, non-magic fields.
+    /// These type parameters will have a `FromMeta` bound applied to them in emitted
+    /// code.
+    pub fn used_type_params(&self) -> IdentSet {
+        self.type_params_matching(|f| !f.skip, |v| !v.skip)
+    }
+
+    fn type_params_matching<F, V>(&self, field_filter: F, variant_filter: V) -> IdentSet
+    where
+        F: Fn(&&Field) -> bool,
+        V: Fn(&&Variant) -> bool,
+    {
+        let declared = self.declared_type_params();
+        match self.data {
+            Data::Struct(ref v) => self.type_params_in_fields(v, &field_filter, &declared),
+            Data::Enum(ref v) => {
+                v.iter()
+                    .filter(variant_filter)
+                    .fold(Default::default(), |mut state, variant| {
+                        state.extend(self.type_params_in_fields(
+                            &variant.data,
+                            &field_filter,
+                            &declared,
+                        ));
+                        state
+                    })
+            }
+        }
+    }
+
+    /// Get the type parameters of all fields in a set matching some filter
+    fn type_params_in_fields<'b, F>(
+        &'b self,
+        fields: &'b Fields<Field<'a>>,
+        field_filter: F,
+        declared: &IdentSet,
+    ) -> IdentSet
+    where
+        F: Fn(&&'b Field) -> bool,
+    {
+        fields
+            .iter()
+            .filter(field_filter)
+            .collect_type_params_cloned(&Purpose::BoundImpl.into(), declared)
+    }
+}
+
+impl<'a> TraitImpl<'a> {
+    /// Gets the `let` declaration for errors accumulated during parsing.
+    pub fn declare_errors(&self) -> ErrorDeclaration {
+        ErrorDeclaration::default()
+    }
+
+    /// Gets the check which performs an early return if errors occurred during parsing.
+    pub fn check_errors(&self) -> ErrorCheck {
+        ErrorCheck::default()
+    }
+
+    /// Generate local variable declarations for all fields.
+    pub(in crate::codegen) fn local_declarations(&self) -> TokenStream {
+        if let Data::Struct(ref vd) = self.data {
+            let vdr = vd.as_ref().map(Field::as_declaration);
+            let decls = vdr.fields.as_slice();
+            quote!(#(#decls)*)
+        } else {
+            quote!()
+        }
+    }
+
+    pub(in crate::codegen) fn post_transform_call(&self) -> Option<TokenStream> {
+        self.post_transform.map(|pt| quote!(#pt))
+    }
+
+    /// Generate local variable declaration and initialization for instance from which missing fields will be taken.
+    pub(in crate::codegen) fn fallback_decl(&self) -> TokenStream {
+        let default = self.default.as_ref().map(DefaultExpression::as_declaration);
+        quote!(#default)
+    }
+
+    pub fn require_fields(&self) -> TokenStream {
+        if let Data::Struct(ref vd) = self.data {
+            let check_nones = vd.as_ref().map(Field::as_presence_check);
+            let checks = check_nones.fields.as_slice();
+            quote!(#(#checks)*)
+        } else {
+            quote!()
+        }
+    }
+
+    pub(in crate::codegen) fn initializers(&self) -> TokenStream {
+        self.make_field_ctx().initializers()
+    }
+
+    /// Generate the loop which walks meta items looking for property matches.
+    pub(in crate::codegen) fn core_loop(&self) -> TokenStream {
+        self.make_field_ctx().core_loop()
+    }
+
+    fn make_field_ctx(&'a self) -> FieldsGen<'a> {
+        match self.data {
+            Data::Enum(_) => panic!("Core loop on enums isn't supported"),
+            Data::Struct(ref data) => FieldsGen::new(data, self.allow_unknown_fields),
+        }
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/variant.rs b/third_party/rust/darling_core/src/codegen/variant.rs
new file mode 100644
index 0000000000..9f5902ba3b
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/variant.rs
@@ -0,0 +1,158 @@
+use std::borrow::Cow;
+
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens, TokenStreamExt};
+use syn::Ident;
+
+use crate::ast::Fields;
+use crate::codegen::error::{ErrorCheck, ErrorDeclaration};
+use crate::codegen::{Field, FieldsGen};
+use crate::usage::{self, IdentRefSet, IdentSet, UsesTypeParams};
+
+/// A variant of the enum which is deriving `FromMeta`.
+#[derive(Debug, Clone)]
+pub struct Variant<'a> {
+    /// The name which will appear in code passed to the `FromMeta` input.
+    pub name_in_attr: Cow<'a, String>,
+
+    /// The name of the variant which will be returned for a given `name_in_attr`.
+    pub variant_ident: &'a Ident,
+
+    /// The name of the parent enum type.
+    pub ty_ident: &'a Ident,
+
+    pub data: Fields<Field<'a>>,
+
+    /// Whether or not the variant should be skipped in the generated code.
+    pub skip: bool,
+
+    pub allow_unknown_fields: bool,
+}
+
+impl<'a> Variant<'a> {
+    pub fn as_name(&'a self) -> &'a str {
+        &self.name_in_attr
+    }
+
+    pub fn as_unit_match_arm(&'a self) -> UnitMatchArm<'a> {
+        UnitMatchArm(self)
+    }
+
+    pub fn as_data_match_arm(&'a self) -> DataMatchArm<'a> {
+        DataMatchArm(self)
+    }
+}
+
+impl<'a> UsesTypeParams for Variant<'a> {
+    fn uses_type_params<'b>(
+        &self,
+        options: &usage::Options,
+        type_set: &'b IdentSet,
+    ) -> IdentRefSet<'b> {
+        self.data.uses_type_params(options, type_set)
+    }
+}
+
+/// Code generator for an enum variant in a unit match position.
+/// This is placed in generated `from_string` calls for the parent enum.
+/// Value-carrying variants wrapped in this type will emit code to produce an "unsupported format" error.
+pub struct UnitMatchArm<'a>(&'a Variant<'a>);
+
+impl<'a> ToTokens for UnitMatchArm<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let val: &Variant<'a> = self.0;
+
+        if val.skip {
+            return;
+        }
+
+        let name_in_attr = &val.name_in_attr;
+
+        if val.data.is_unit() {
+            let variant_ident = val.variant_ident;
+            let ty_ident = val.ty_ident;
+
+            tokens.append_all(quote!(
+                #name_in_attr => ::darling::export::Ok(#ty_ident::#variant_ident),
+            ));
+        } else {
+            tokens.append_all(quote!(
+                #name_in_attr => ::darling::export::Err(::darling::Error::unsupported_format("literal")),
+            ));
+        }
+    }
+}
+
+/// Code generator for an enum variant in a data-carrying match position.
+/// This is placed in generated `from_list` calls for the parent enum.
+/// Unit variants wrapped in this type will emit code to produce an "unsupported format" error.
+pub struct DataMatchArm<'a>(&'a Variant<'a>);
+
+impl<'a> ToTokens for DataMatchArm<'a> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let val: &Variant<'a> = self.0;
+
+        if val.skip {
+            return;
+        }
+
+        let name_in_attr = &val.name_in_attr;
+        let variant_ident = val.variant_ident;
+        let ty_ident = val.ty_ident;
+
+        if val.data.is_unit() {
+            tokens.append_all(quote!(
+                #name_in_attr => ::darling::export::Err(::darling::Error::unsupported_format("list")),
+            ));
+
+            return;
+        }
+
+        let vdg = FieldsGen::new(&val.data, val.allow_unknown_fields);
+
+        if val.data.is_struct() {
+            let declare_errors = ErrorDeclaration::default();
+            let check_errors = ErrorCheck::with_location(name_in_attr);
+            let require_fields = vdg.require_fields();
+            let decls = vdg.declarations();
+            let core_loop = vdg.core_loop();
+            let inits = vdg.initializers();
+
+            tokens.append_all(quote!(
+                #name_in_attr => {
+                    if let ::darling::export::syn::Meta::List(ref __data) = *__nested {
+                        let __items = &__data.nested;
+
+                        #declare_errors
+
+                        #decls
+
+                        #core_loop
+
+                        #require_fields
+
+                        #check_errors
+
+                        ::darling::export::Ok(#ty_ident::#variant_ident {
+                            #inits
+                        })
+                    } else {
+                        ::darling::export::Err(::darling::Error::unsupported_format("non-list"))
+                    }
+                }
+            ));
+        } else if val.data.is_newtype() {
+            tokens.append_all(quote!(
+                #name_in_attr => {
+                    ::darling::export::Ok(
+                        #ty_ident::#variant_ident(
+                            ::darling::FromMeta::from_meta(__nested)
+                                .map_err(|e| e.at(#name_in_attr))?)
+                    )
+                }
+            ));
+        } else {
+            panic!("Match arms aren't supported for tuple variants yet");
+        }
+    }
+}
diff --git a/third_party/rust/darling_core/src/codegen/variant_data.rs b/third_party/rust/darling_core/src/codegen/variant_data.rs
new file mode 100644
index 0000000000..d1c878e2bd
--- /dev/null
+++ b/third_party/rust/darling_core/src/codegen/variant_data.rs
@@ -0,0 +1,99 @@
+use proc_macro2::TokenStream;
+use quote::quote;
+
+use crate::ast::{Fields, Style};
+use crate::codegen::Field;
+
+pub struct FieldsGen<'a> {
+    fields: &'a Fields<Field<'a>>,
+    allow_unknown_fields: bool,
+}
+
+impl<'a> FieldsGen<'a> {
+    pub fn new(fields: &'a Fields<Field<'a>>, allow_unknown_fields: bool) -> Self {
+        Self {
+            fields,
+            allow_unknown_fields,
+        }
+    }
+
+    /// Create declarations for all the fields in the struct.
+    pub(in crate::codegen) fn declarations(&self) -> TokenStream {
+        match *self.fields {
+            Fields {
+                style: Style::Struct,
+                ref fields,
+                ..
+            } => {
+                let vdr = fields.iter().map(Field::as_declaration);
+                quote!(#(#vdr)*)
+            }
+            _ => panic!("FieldsGen doesn't support tuples yet"),
+        }
+    }
+
+    /// Generate the loop which walks meta items looking for property matches.
+    pub(in crate::codegen) fn core_loop(&self) -> TokenStream {
+        let arms = self.fields.as_ref().map(Field::as_match);
+
+        // If we're allowing unknown fields, then handling one is a no-op.
+        // Otherwise, we're going to push a new spanned error pointing at the field.
+        let handle_unknown = if self.allow_unknown_fields {
+            quote!()
+        } else {
+            // We can't call `unknown_field_with_alts` with an empty slice, or else it fails to
+            // infer the type of the slice item.
+            let err_fn = if arms.is_empty() {
+                quote!(unknown_field(__other))
+            } else {
+                let names = self.fields.as_ref().map(Field::as_name);
+                let names = names.iter();
+                quote!(unknown_field_with_alts(__other, &[#(#names),*]))
+            };
+
+            quote! {
+                __errors.push(::darling::Error::#err_fn.with_span(__inner));
+            }
+        };
+        let arms = arms.iter();
+
+        quote!(
+            for __item in __items {
+                match *__item {
+                    ::darling::export::syn::NestedMeta::Meta(ref __inner) => {
+                        let __name = ::darling::util::path_to_string(__inner.path());
+                        match __name.as_str() {
+                            #(#arms)*
+                            __other => { #handle_unknown }
+                        }
+                    }
+                    ::darling::export::syn::NestedMeta::Lit(ref __inner) => {
+                        __errors.push(::darling::Error::unsupported_format("literal")
+                            .with_span(__inner));
+                    }
+                }
+            }
+        )
+    }
+
+    pub fn require_fields(&self) -> TokenStream {
+        match *self.fields {
+            Fields {
+                style: Style::Struct,
+                ref fields,
+                ..
+            } => {
+                let checks = fields.iter().map(Field::as_presence_check);
+                quote!(#(#checks)*)
+            }
+            _ => panic!("FieldsGen doesn't support tuples for requirement checks"),
+        }
+    }
+
+    pub(in crate::codegen) fn initializers(&self) -> TokenStream {
+        let inits = self.fields.as_ref().map(Field::as_initializer);
+        let inits = inits.iter();
+
+        quote!(#(#inits),*)
+    }
+}
diff --git a/third_party/rust/darling_core/src/derive.rs b/third_party/rust/darling_core/src/derive.rs
new file mode 100644
index 0000000000..43674aae6b
--- /dev/null
+++ b/third_party/rust/darling_core/src/derive.rs
@@ -0,0 +1,61 @@
+//! Functions to derive `darling`'s traits from well-formed input, without directly depending
+//! on `proc_macro`.
+
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+use syn::DeriveInput;
+
+use crate::options;
+
+/// Run an expression which returns a `darling::Result`, then either return the tokenized
+/// representation of the `Ok` value, or the tokens of the compiler errors in the `Err` case.
+macro_rules! emit_impl_or_error {
+    ($e:expr) => {
+        match $e {
+            Ok(val) => val.into_token_stream(),
+            Err(err) => err.write_errors(),
+        }
+    };
+}
+
+/// Create tokens for a `darling::FromMeta` impl from a `DeriveInput`. If
+/// the input cannot produce a valid impl, the returned tokens will contain
+/// compile errors instead.
+pub fn from_meta(input: &DeriveInput) -> TokenStream {
+    emit_impl_or_error!(options::FromMetaOptions::new(input))
+}
+
+/// Create tokens for a `darling::FromAttributes` impl from a `DeriveInput`. If
+/// the input cannot produce a valid impl, the returned tokens will contain
+/// compile errors instead.
+pub fn from_attributes(input: &DeriveInput) -> TokenStream {
+    emit_impl_or_error!(options::FromAttributesOptions::new(input))
+}
+
+/// Create tokens for a `darling::FromDeriveInput` impl from a `DeriveInput`. If
+/// the input cannot produce a valid impl, the returned tokens will contain
+/// compile errors instead.
+pub fn from_derive_input(input: &DeriveInput) -> TokenStream {
+    emit_impl_or_error!(options::FdiOptions::new(input))
+}
+
+/// Create tokens for a `darling::FromField` impl from a `DeriveInput`. If
+/// the input cannot produce a valid impl, the returned tokens will contain
+/// compile errors instead.
+pub fn from_field(input: &DeriveInput) -> TokenStream {
+    emit_impl_or_error!(options::FromFieldOptions::new(input))
+}
+
+/// Create tokens for a `darling::FromTypeParam` impl from a `DeriveInput`. If
+/// the input cannot produce a valid impl, the returned tokens will contain
+/// compile errors instead.
+pub fn from_type_param(input: &DeriveInput) -> TokenStream {
+    emit_impl_or_error!(options::FromTypeParamOptions::new(input))
+}
+
+/// Create tokens for a `darling::FromVariant` impl from a `DeriveInput`. If
+/// the input cannot produce a valid impl, the returned tokens will contain
+/// compile errors instead.
+pub fn from_variant(input: &DeriveInput) -> TokenStream {
+    emit_impl_or_error!(options::FromVariantOptions::new(input))
+}
diff --git a/third_party/rust/darling_core/src/error/kind.rs b/third_party/rust/darling_core/src/error/kind.rs
new file mode 100644
index 0000000000..2b4eda6765
--- /dev/null
+++ b/third_party/rust/darling_core/src/error/kind.rs
@@ -0,0 +1,211 @@
+use std::fmt;
+
+use crate::error::Error;
+
+type DeriveInputShape = String;
+type FieldName = String;
+type MetaFormat = String;
+
+#[derive(Debug, Clone)]
+// Don't want to publicly commit to ErrorKind supporting equality yet, but
+// not having it makes testing very difficult.
+#[cfg_attr(test, derive(PartialEq, Eq))]
+pub(in crate::error) enum ErrorKind {
+    /// An arbitrary error message.
+    Custom(String),
+    DuplicateField(FieldName),
+    MissingField(FieldName),
+    UnsupportedShape {
+        observed: DeriveInputShape,
+        expected: Option<String>,
+    },
+    UnknownField(ErrorUnknownField),
+    UnexpectedFormat(MetaFormat),
+    UnexpectedType(String),
+    UnknownValue(String),
+    TooFewItems(usize),
+    TooManyItems(usize),
+    /// A set of errors.
+    Multiple(Vec<Error>),
+
+    // TODO make this variant take `!` so it can't exist
+    #[doc(hidden)]
+    __NonExhaustive,
+}
+
+impl ErrorKind {
+    pub fn description(&self) -> &str {
+        use self::ErrorKind::*;
+
+        match *self {
+            Custom(ref s) => s,
+            DuplicateField(_) => "Duplicate field",
+            MissingField(_) => "Missing field",
+            UnknownField(_) => "Unexpected field",
+            UnsupportedShape { .. } => "Unsupported shape",
+            UnexpectedFormat(_) => "Unexpected meta-item format",
+            UnexpectedType(_) => "Unexpected literal type",
+            UnknownValue(_) => "Unknown literal value",
+            TooFewItems(_) => "Too few items",
+            TooManyItems(_) => "Too many items",
+            Multiple(_) => "Multiple errors",
+            __NonExhaustive => unreachable!(),
+        }
+    }
+
+    /// Deeply counts the number of errors this item represents.
+    pub fn len(&self) -> usize {
+        if let ErrorKind::Multiple(ref items) = *self {
+            items.iter().map(Error::len).sum()
+        } else {
+            1
+        }
+    }
+}
+
+impl fmt::Display for ErrorKind {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use self::ErrorKind::*;
+
+        match *self {
+            Custom(ref s) => s.fmt(f),
+            DuplicateField(ref field) => write!(f, "Duplicate field `{}`", field),
+            MissingField(ref field) => write!(f, "Missing field `{}`", field),
+            UnknownField(ref field) => field.fmt(f),
+            UnsupportedShape {
+                ref observed,
+                ref expected,
+            } => {
+                write!(f, "Unsupported shape `{}`", observed)?;
+                if let Some(expected) = &expected {
+                    write!(f, ". Expected {}.", expected)?;
+                }
+
+                Ok(())
+            }
+            UnexpectedFormat(ref format) => write!(f, "Unexpected meta-item format `{}`", format),
+            UnexpectedType(ref ty) => write!(f, "Unexpected literal type `{}`", ty),
+            UnknownValue(ref val) => write!(f, "Unknown literal value `{}`", val),
+            TooFewItems(ref min) => write!(f, "Too few items: Expected at least {}", min),
+            TooManyItems(ref max) => write!(f, "Too many items: Expected no more than {}", max),
+            Multiple(ref items) if items.len() == 1 => items[0].fmt(f),
+            Multiple(ref items) => {
+                write!(f, "Multiple errors: (")?;
+                let mut first = true;
+                for item in items {
+                    if !first {
+                        write!(f, ", ")?;
+                    } else {
+                        first = false;
+                    }
+
+                    item.fmt(f)?;
+                }
+
+                write!(f, ")")
+            }
+            __NonExhaustive => unreachable!(),
+        }
+    }
+}
+
+impl From<ErrorUnknownField> for ErrorKind {
+    fn from(err: ErrorUnknownField) -> Self {
+        ErrorKind::UnknownField(err)
+    }
+}
+
+/// An error for an unknown field, with a possible "did-you-mean" suggestion to get
+/// the user back on the right track.
+#[derive(Clone, Debug)]
+// Don't want to publicly commit to ErrorKind supporting equality yet, but
+// not having it makes testing very difficult.
+#[cfg_attr(test, derive(PartialEq, Eq))]
+pub(in crate::error) struct ErrorUnknownField {
+    name: String,
+    did_you_mean: Option<String>,
+}
+
+impl ErrorUnknownField {
+    pub fn new<I: Into<String>>(name: I, did_you_mean: Option<String>) -> Self {
+        ErrorUnknownField {
+            name: name.into(),
+            did_you_mean,
+        }
+    }
+
+    pub fn with_alts<'a, T, I>(field: &str, alternates: I) -> Self
+    where
+        T: AsRef<str> + 'a,
+        I: IntoIterator<Item = &'a T>,
+    {
+        ErrorUnknownField::new(field, did_you_mean(field, alternates))
+    }
+
+    #[cfg(feature = "diagnostics")]
+    pub fn into_diagnostic(self, span: Option<::proc_macro2::Span>) -> ::proc_macro::Diagnostic {
+        let base = span
+            .unwrap_or_else(::proc_macro2::Span::call_site)
+            .unwrap()
+            .error(self.top_line());
+        match self.did_you_mean {
+            Some(alt_name) => base.help(format!("did you mean `{}`?", alt_name)),
+            None => base,
+        }
+    }
+
+    #[cfg(feature = "diagnostics")]
+    fn top_line(&self) -> String {
+        format!("Unknown field: `{}`", self.name)
+    }
+}
+
+impl From<String> for ErrorUnknownField {
+    fn from(name: String) -> Self {
+        ErrorUnknownField::new(name, None)
+    }
+}
+
+impl<'a> From<&'a str> for ErrorUnknownField {
+    fn from(name: &'a str) -> Self {
+        ErrorUnknownField::new(name, None)
+    }
+}
+
+impl fmt::Display for ErrorUnknownField {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "Unknown field: `{}`", self.name)?;
+
+        if let Some(ref did_you_mean) = self.did_you_mean {
+            write!(f, ". Did you mean `{}`?", did_you_mean)?;
+        }
+
+        Ok(())
+    }
+}
+
+#[cfg(feature = "suggestions")]
+fn did_you_mean<'a, T, I>(field: &str, alternates: I) -> Option<String>
+where
+    T: AsRef<str> + 'a,
+    I: IntoIterator<Item = &'a T>,
+{
+    let mut candidate: Option<(f64, &str)> = None;
+    for pv in alternates {
+        let confidence = ::strsim::jaro_winkler(field, pv.as_ref());
+        if confidence > 0.8 && (candidate.is_none() || (candidate.as_ref().unwrap().0 < confidence))
+        {
+            candidate = Some((confidence, pv.as_ref()));
+        }
+    }
+    candidate.map(|(_, candidate)| candidate.into())
+}
+
+#[cfg(not(feature = "suggestions"))]
+fn did_you_mean<'a, T, I>(_field: &str, _alternates: I) -> Option<String>
+where
+    T: AsRef<str> + 'a,
+    I: IntoIterator<Item = &'a T>,
+{
+    None
+}
diff --git a/third_party/rust/darling_core/src/error/mod.rs b/third_party/rust/darling_core/src/error/mod.rs
new file mode 100644
index 0000000000..f86d58e815
--- /dev/null
+++ b/third_party/rust/darling_core/src/error/mod.rs
@@ -0,0 +1,831 @@
+//! The `darling::Error` type, the multiple error `Accumulator`, and their internals.
+//!
+//! Error handling is one of the core values of `darling`; creating great errors is hard and
+//! never the reason that a proc-macro author started writing their crate. As a result, the
+//! `Error` type in `darling` tries to make adding span information, suggestions, and other
+//! help content easy when manually implementing `darling` traits, and automatic when deriving
+//! them.
+
+use proc_macro2::{Span, TokenStream};
+use std::error::Error as StdError;
+use std::fmt;
+use std::iter::{self, Iterator};
+use std::string::ToString;
+use std::vec;
+use syn::spanned::Spanned;
+use syn::{Lit, LitStr, Path};
+
+mod kind;
+
+use crate::util::path_to_string;
+
+use self::kind::{ErrorKind, ErrorUnknownField};
+
+/// An alias of `Result` specific to attribute parsing.
+pub type Result<T> = ::std::result::Result<T, Error>;
+
+/// An error encountered during attribute parsing.
+///
+/// Given that most errors darling encounters represent code bugs in dependent crates,
+/// the internal structure of the error is deliberately opaque.
+///
+/// # Usage
+/// Proc-macro expansion happens very infrequently compared to runtime tasks such as
+/// deserialization, and it happens in the context of an expensive compilation taks.
+/// For that reason, darling prefers not to fail on the first error it encounters, instead
+/// doing as much work as it can, accumulating errors into a single report.
+///
+/// As a result, `darling::Error` is more of guaranteed-non-empty error collection
+/// than a single problem. These errors also have some notion of hierarchy, stemming from
+/// the hierarchical nature of darling's input.
+///
+/// These characteristics make for great experiences when using darling-powered crates,
+/// provided crates using darling adhere to some best practices:
+///
+/// 1. Do not attempt to simplify a `darling::Error` into some other error type, such as
+///    `syn::Error`. To surface compile errors, instead use `darling::Error::write_errors`.
+///    This preserves all span information, suggestions, etc. Wrapping a `darling::Error` in
+///    a custom error enum works as-expected and does not force any loss of fidelity.
+/// 2. Do not use early return (e.g. the `?` operator) for custom validations. Instead,
+///    create an [`error::Accumulator`](Accumulator) to collect errors as they are encountered.  Then use
+///    [`Accumulator::finish`] to return your validated result; it will give `Ok` if and only if
+///    no errors were encountered.  This can create very complex custom validation functions;
+///    in those cases, split independent "validation chains" out into their own functions to
+///    keep the main validator manageable.
+/// 3. Use `darling::Error::custom` to create additional errors as-needed, then call `with_span`
+///    to ensure those errors appear in the right place. Use `darling::util::SpannedValue` to keep
+///    span information around on parsed fields so that custom diagnostics can point to the correct
+///    parts of the input AST.
+#[derive(Debug, Clone)]
+pub struct Error {
+    kind: ErrorKind,
+    locations: Vec<String>,
+    /// The span to highlight in the emitted diagnostic.
+    span: Option<Span>,
+}
+
+/// Error creation functions
+impl Error {
+    pub(in crate::error) fn new(kind: ErrorKind) -> Self {
+        Error {
+            kind,
+            locations: Vec::new(),
+            span: None,
+        }
+    }
+
+    /// Creates a new error with a custom message.
+    pub fn custom<T: fmt::Display>(msg: T) -> Self {
+        Error::new(ErrorKind::Custom(msg.to_string()))
+    }
+
+    /// Creates a new error for a field that appears twice in the input.
+    pub fn duplicate_field(name: &str) -> Self {
+        Error::new(ErrorKind::DuplicateField(name.into()))
+    }
+
+    /// Creates a new error for a field that appears twice in the input. Helper to avoid repeating
+    /// the syn::Path to String conversion.
+    pub fn duplicate_field_path(path: &Path) -> Self {
+        Error::duplicate_field(&path_to_string(path))
+    }
+
+    /// Creates a new error for a non-optional field that does not appear in the input.
+    pub fn missing_field(name: &str) -> Self {
+        Error::new(ErrorKind::MissingField(name.into()))
+    }
+
+    /// Creates a new error for a field name that appears in the input but does not correspond
+    /// to a known field.
+    pub fn unknown_field(name: &str) -> Self {
+        Error::new(ErrorKind::UnknownField(name.into()))
+    }
+
+    /// Creates a new error for a field name that appears in the input but does not correspond
+    /// to a known field. Helper to avoid repeating the syn::Path to String conversion.
+    pub fn unknown_field_path(path: &Path) -> Self {
+        Error::unknown_field(&path_to_string(path))
+    }
+
+    /// Creates a new error for a field name that appears in the input but does not correspond to
+    /// a known attribute. The second argument is the list of known attributes; if a similar name
+    /// is found that will be shown in the emitted error message.
+    pub fn unknown_field_with_alts<'a, T, I>(field: &str, alternates: I) -> Self
+    where
+        T: AsRef<str> + 'a,
+        I: IntoIterator<Item = &'a T>,
+    {
+        Error::new(ErrorUnknownField::with_alts(field, alternates).into())
+    }
+
+    /// Creates a new error for a struct or variant that does not adhere to the supported shape.
+    pub fn unsupported_shape(shape: &str) -> Self {
+        Error::new(ErrorKind::UnsupportedShape {
+            observed: shape.into(),
+            expected: None,
+        })
+    }
+
+    pub fn unsupported_shape_with_expected<T: fmt::Display>(shape: &str, expected: &T) -> Self {
+        Error::new(ErrorKind::UnsupportedShape {
+            observed: shape.into(),
+            expected: Some(expected.to_string()),
+        })
+    }
+
+    pub fn unsupported_format(format: &str) -> Self {
+        Error::new(ErrorKind::UnexpectedFormat(format.into()))
+    }
+
+    /// Creates a new error for a field which has an unexpected literal type.
+    pub fn unexpected_type(ty: &str) -> Self {
+        Error::new(ErrorKind::UnexpectedType(ty.into()))
+    }
+
+    /// Creates a new error for a field which has an unexpected literal type. This will automatically
+    /// extract the literal type name from the passed-in `Lit` and set the span to encompass only the
+    /// literal value.
+    ///
+    /// # Usage
+    /// This is most frequently used in overrides of the `FromMeta::from_value` method.
+    ///
+    /// ```rust
+    /// # // pretend darling_core is darling so the doc example looks correct.
+    /// # extern crate darling_core as darling;
+    /// # extern crate syn;
+    ///
+    /// use darling::{FromMeta, Error, Result};
+    /// use syn::{Lit, LitStr};
+    ///
+    /// pub struct Foo(String);
+    ///
+    /// impl FromMeta for Foo {
+    ///     fn from_value(value: &Lit) -> Result<Self> {
+    ///         if let Lit::Str(ref lit_str) = *value {
+    ///             Ok(Foo(lit_str.value()))
+    ///         } else {
+    ///             Err(Error::unexpected_lit_type(value))
+    ///         }
+    ///     }
+    /// }
+    ///
+    /// # fn main() {}
+    /// ```
+    pub fn unexpected_lit_type(lit: &Lit) -> Self {
+        Error::unexpected_type(match *lit {
+            Lit::Str(_) => "string",
+            Lit::ByteStr(_) => "byte string",
+            Lit::Byte(_) => "byte",
+            Lit::Char(_) => "char",
+            Lit::Int(_) => "int",
+            Lit::Float(_) => "float",
+            Lit::Bool(_) => "bool",
+            Lit::Verbatim(_) => "verbatim",
+        })
+        .with_span(lit)
+    }
+
+    /// Creates a new error for a value which doesn't match a set of expected literals.
+    pub fn unknown_value(value: &str) -> Self {
+        Error::new(ErrorKind::UnknownValue(value.into()))
+    }
+
+    /// Creates a new error for a list which did not get enough items to proceed.
+    pub fn too_few_items(min: usize) -> Self {
+        Error::new(ErrorKind::TooFewItems(min))
+    }
+
+    /// Creates a new error when a list got more items than it supports. The `max` argument
+    /// is the largest number of items the receiver could accept.
+    pub fn too_many_items(max: usize) -> Self {
+        Error::new(ErrorKind::TooManyItems(max))
+    }
+
+    /// Bundle a set of multiple errors into a single `Error` instance.
+    ///
+    /// Usually it will be more convenient to use an [`error::Accumulator`](Accumulator).
+    ///
+    /// # Panics
+    /// This function will panic if `errors.is_empty() == true`.
+    pub fn multiple(mut errors: Vec<Error>) -> Self {
+        match errors.len() {
+            1 => errors
+                .pop()
+                .expect("Error array of length 1 has a first item"),
+            0 => panic!("Can't deal with 0 errors"),
+            _ => Error::new(ErrorKind::Multiple(errors)),
+        }
+    }
+
+    /// Creates an error collector, for aggregating multiple errors
+    ///
+    /// See [`Accumulator`] for details.
+    pub fn accumulator() -> Accumulator {
+        Default::default()
+    }
+}
+
+impl Error {
+    /// Create a new error about a literal string that doesn't match a set of known
+    /// or permissible values. This function can be made public if the API proves useful
+    /// beyond impls for `syn` types.
+    pub(crate) fn unknown_lit_str_value(value: &LitStr) -> Self {
+        Error::unknown_value(&value.value()).with_span(value)
+    }
+}
+
+/// Error instance methods
+#[allow(clippy::len_without_is_empty)] // Error can never be empty
+impl Error {
+    /// Check if this error is associated with a span in the token stream.
+    pub fn has_span(&self) -> bool {
+        self.span.is_some()
+    }
+
+    /// Tie a span to the error if none is already present. This is used in `darling::FromMeta`
+    /// and other traits to attach errors to the most specific possible location in the input
+    /// source code.
+    ///
+    /// All `darling`-built impls, either from the crate or from the proc macro, will call this
+    /// when appropriate during parsing, so it should not be necessary to call this unless you have
+    /// overridden:
+    ///
+    /// * `FromMeta::from_meta`
+    /// * `FromMeta::from_nested_meta`
+    /// * `FromMeta::from_value`
+    pub fn with_span<T: Spanned>(mut self, node: &T) -> Self {
+        if !self.has_span() {
+            self.span = Some(node.span());
+        }
+
+        self
+    }
+
+    /// Get a span for the error.
+    ///
+    /// # Return Value
+    /// This function will return [`Span::call_site()`](proc_macro2::Span) if [`Self::has_span`] is `false`.
+    /// To get the span only if one has been explicitly set for `self`, instead use [`Error::explicit_span`].
+    pub fn span(&self) -> Span {
+        self.span.unwrap_or_else(Span::call_site)
+    }
+
+    /// Get the span for `self`, if one has been set.
+    pub fn explicit_span(&self) -> Option<Span> {
+        self.span
+    }
+
+    /// Recursively converts a tree of errors to a flattened list.
+    pub fn flatten(self) -> Self {
+        Error::multiple(self.into_vec())
+    }
+
+    fn into_vec(self) -> Vec<Self> {
+        if let ErrorKind::Multiple(errors) = self.kind {
+            let mut flat = Vec::new();
+            for error in errors {
+                flat.extend(error.prepend_at(self.locations.clone()).into_vec());
+            }
+
+            flat
+        } else {
+            vec![self]
+        }
+    }
+
+    /// Adds a location to the error, such as a field or variant.
+    /// Locations must be added in reverse order of specificity.
+    pub fn at<T: fmt::Display>(mut self, location: T) -> Self {
+        self.locations.insert(0, location.to_string());
+        self
+    }
+
+    /// Adds a location to the error, such as a field or variant.
+    /// Locations must be added in reverse order of specificity. This is a helper function to avoid
+    /// repeating path to string logic.
+    pub fn at_path(self, path: &Path) -> Self {
+        self.at(path_to_string(path))
+    }
+
+    /// Gets the number of individual errors in this error.
+    ///
+    /// This function never returns `0`, as it's impossible to construct
+    /// a multi-error from an empty `Vec`.
+    pub fn len(&self) -> usize {
+        self.kind.len()
+    }
+
+    /// Adds a location chain to the head of the error's existing locations.
+    fn prepend_at(mut self, mut locations: Vec<String>) -> Self {
+        if !locations.is_empty() {
+            locations.extend(self.locations);
+            self.locations = locations;
+        }
+
+        self
+    }
+
+    /// Gets the location slice.
+    #[cfg(test)]
+    pub(crate) fn location(&self) -> Vec<&str> {
+        self.locations.iter().map(|i| i.as_str()).collect()
+    }
+
+    /// Write this error and any children as compile errors into a `TokenStream` to
+    /// be returned by the proc-macro.
+    ///
+    /// The behavior of this method will be slightly different if the `diagnostics` feature
+    /// is enabled: In that case, the diagnostics will be emitted immediately by this call,
+    /// and an empty `TokenStream` will be returned.
+    ///
+    /// Return these tokens unmodified to avoid disturbing the attached span information.
+    ///
+    /// # Usage
+    /// ```rust,ignore
+    /// // in your proc-macro function
+    /// let opts = match MyOptions::from_derive_input(&ast) {
+    ///     Ok(val) => val,
+    ///     Err(err) => {
+    ///         return err.write_errors();
+    ///     }
+    /// }
+    /// ```
+    pub fn write_errors(self) -> TokenStream {
+        #[cfg(feature = "diagnostics")]
+        {
+            self.emit();
+            TokenStream::default()
+        }
+
+        #[cfg(not(feature = "diagnostics"))]
+        {
+            syn::Error::from(self).into_compile_error()
+        }
+    }
+
+    #[cfg(feature = "diagnostics")]
+    fn single_to_diagnostic(self) -> ::proc_macro::Diagnostic {
+        use proc_macro::{Diagnostic, Level};
+
+        // Delegate to dedicated error formatters when applicable.
+        //
+        // If span information is available, don't include the error property path
+        // since it's redundant and not consistent with native compiler diagnostics.
+        match self.kind {
+            ErrorKind::UnknownField(euf) => euf.into_diagnostic(self.span),
+            _ => match self.span {
+                Some(span) => span.unwrap().error(self.kind.to_string()),
+                None => Diagnostic::new(Level::Error, self.to_string()),
+            },
+        }
+    }
+
+    /// Transform this error and its children into a list of compiler diagnostics
+    /// and emit them. If the `Error` has associated span information, the diagnostics
+    /// will identify the correct location in source code automatically.
+    ///
+    /// # Stability
+    /// This is only available on `nightly` until the compiler `proc_macro_diagnostic`
+    /// feature stabilizes. Until then, it may break at any time.
+    #[cfg(feature = "diagnostics")]
+    pub fn emit(self) {
+        for error in self.flatten() {
+            error.single_to_diagnostic().emit()
+        }
+    }
+
+    /// Transform the error into a compiler diagnostic and - if the diagnostic points to
+    /// a specific code location - add a spanned help child diagnostic that points to the
+    /// parent derived trait.
+    ///
+    /// This is experimental and therefore not exposed outside the crate.
+    #[cfg(feature = "diagnostics")]
+    #[allow(dead_code)]
+    fn emit_with_macro_help_span(self) {
+        use proc_macro::Diagnostic;
+
+        for error in self.flatten() {
+            let needs_help = error.has_span();
+            let diagnostic = error.single_to_diagnostic();
+            Diagnostic::emit(if needs_help {
+                diagnostic.span_help(
+                    Span::call_site().unwrap(),
+                    "Encountered as part of this derive-mode-macro",
+                )
+            } else {
+                diagnostic
+            })
+        }
+    }
+}
+
+impl StdError for Error {
+    fn description(&self) -> &str {
+        self.kind.description()
+    }
+
+    fn cause(&self) -> Option<&dyn StdError> {
+        None
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.kind)?;
+        if !self.locations.is_empty() {
+            write!(f, " at {}", self.locations.join("/"))?;
+        }
+
+        Ok(())
+    }
+}
+
+impl From<syn::Error> for Error {
+    fn from(e: syn::Error) -> Self {
+        // This impl assumes there is nothing but the message and span that needs to be preserved
+        // from the passed-in error. If this changes at some point, a new ErrorKind should be made
+        // to hold the syn::Error, and this impl should preserve it unmodified while setting its own
+        // span to be a copy of the passed-in error.
+        Self {
+            span: Some(e.span()),
+            ..Self::custom(e)
+        }
+    }
+}
+
+impl From<Error> for syn::Error {
+    fn from(e: Error) -> Self {
+        if e.len() == 1 {
+            if let Some(span) = e.explicit_span() {
+                // Don't include the location path if the error has an explicit span,
+                // since it will be redundant and isn't consistent with how rustc
+                // exposes errors.
+                syn::Error::new(span, e.kind)
+            } else {
+                // If the error's span is going to be the macro call site, include
+                // the location information to try and help the user pinpoint the issue.
+                syn::Error::new(e.span(), e)
+            }
+        } else {
+            let mut syn_errors = e.flatten().into_iter().map(syn::Error::from);
+            let mut error = syn_errors
+                .next()
+                .expect("darling::Error can never be empty");
+
+            for next_error in syn_errors {
+                error.combine(next_error);
+            }
+
+            error
+        }
+    }
+}
+
+// Don't want to publicly commit to Error supporting equality yet, but
+// not having it makes testing very difficult. Note that spans are not
+// considered for equality since that would break testing in most cases.
+#[cfg(test)]
+impl PartialEq for Error {
+    fn eq(&self, other: &Self) -> bool {
+        self.kind == other.kind && self.locations == other.locations
+    }
+}
+
+#[cfg(test)]
+impl Eq for Error {}
+
+impl IntoIterator for Error {
+    type Item = Error;
+    type IntoIter = IntoIter;
+
+    fn into_iter(self) -> IntoIter {
+        if let ErrorKind::Multiple(errors) = self.kind {
+            IntoIter {
+                inner: IntoIterEnum::Multiple(errors.into_iter()),
+            }
+        } else {
+            IntoIter {
+                inner: IntoIterEnum::Single(iter::once(self)),
+            }
+        }
+    }
+}
+
+enum IntoIterEnum {
+    Single(iter::Once<Error>),
+    Multiple(vec::IntoIter<Error>),
+}
+
+impl Iterator for IntoIterEnum {
+    type Item = Error;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match *self {
+            IntoIterEnum::Single(ref mut content) => content.next(),
+            IntoIterEnum::Multiple(ref mut content) => content.next(),
+        }
+    }
+}
+
+/// An iterator that moves out of an `Error`.
+pub struct IntoIter {
+    inner: IntoIterEnum,
+}
+
+impl Iterator for IntoIter {
+    type Item = Error;
+
+    fn next(&mut self) -> Option<Error> {
+        self.inner.next()
+    }
+}
+
+/// Accumulator for errors, for helping call [`Error::multiple`].
+///
+/// See the docs for [`darling::Error`](Error) for more discussion of error handling with darling.
+///
+/// # Panics
+///
+/// `Accumulator` panics on drop unless [`finish`](Self::finish), [`finish_with`](Self::finish_with),
+/// or [`into_inner`](Self::into_inner) has been called, **even if it contains no errors**.
+/// If you want to discard an `Accumulator` that you know to be empty, use `accumulator.finish().unwrap()`.
+///
+/// # Example
+///
+/// ```
+/// # extern crate darling_core as darling;
+/// # struct Thing;
+/// # struct Output;
+/// # impl Thing { fn validate(self) -> darling::Result<Output> { Ok(Output) } }
+/// fn validate_things(inputs: Vec<Thing>) -> darling::Result<Vec<Output>> {
+///     let mut errors = darling::Error::accumulator();
+///
+///     let outputs = inputs
+///         .into_iter()
+///         .filter_map(|thing| errors.handle_in(|| thing.validate()))
+///         .collect::<Vec<_>>();
+///
+///     errors.finish()?;
+///     Ok(outputs)
+/// }
+/// ```
+#[derive(Debug)]
+#[must_use = "Accumulator will panic on drop if not defused."]
+pub struct Accumulator(Option<Vec<Error>>);
+
+impl Accumulator {
+    /// Runs a closure, returning the successful value as `Some`, or collecting the error
+    ///
+    /// The closure's return type is `darling::Result`, so inside it one can use `?`.
+    pub fn handle_in<T, F: FnOnce() -> Result<T>>(&mut self, f: F) -> Option<T> {
+        self.handle(f())
+    }
+
+    /// Handles a possible error.
+    ///
+    /// Returns a successful value as `Some`, or collects the error and returns `None`.
+    pub fn handle<T>(&mut self, result: Result<T>) -> Option<T> {
+        match result {
+            Ok(y) => Some(y),
+            Err(e) => {
+                self.push(e);
+                None
+            }
+        }
+    }
+
+    /// Stop accumulating errors, producing `Ok` if there are no errors or producing
+    /// an error with all those encountered by the accumulator.
+    pub fn finish(self) -> Result<()> {
+        self.finish_with(())
+    }
+
+    /// Bundles the collected errors if there were any, or returns the success value
+    ///
+    /// Call this at the end of your input processing.
+    ///
+    /// If there were no errors recorded, returns `Ok(success)`.
+    /// Otherwise calls [`Error::multiple`] and returns the result as an `Err`.
+    pub fn finish_with<T>(self, success: T) -> Result<T> {
+        let errors = self.into_inner();
+        if errors.is_empty() {
+            Ok(success)
+        } else {
+            Err(Error::multiple(errors))
+        }
+    }
+
+    fn errors(&mut self) -> &mut Vec<Error> {
+        match &mut self.0 {
+            Some(errors) => errors,
+            None => panic!("darling internal error: Accumulator accessed after defuse"),
+        }
+    }
+
+    /// Returns the accumulated errors as a `Vec`.
+    ///
+    /// This function defuses the drop bomb.
+    #[must_use = "Accumulated errors should be handled or propagated to the caller"]
+    pub fn into_inner(mut self) -> Vec<Error> {
+        match std::mem::replace(&mut self.0, None) {
+            Some(errors) => errors,
+            None => panic!("darling internal error: Accumulator accessed after defuse"),
+        }
+    }
+
+    /// Add one error to the collection.
+    pub fn push(&mut self, error: Error) {
+        self.errors().push(error)
+    }
+
+    /// Finish the current accumulation, and if there are no errors create a new `Self` so processing may continue.
+    ///
+    /// This is shorthand for:
+    ///
+    /// ```rust,ignore
+    /// errors.finish()?;
+    /// errors = Error::accumulator();
+    /// ```
+    ///
+    /// # Drop Behavior
+    /// This function returns a new [`Accumulator`] in the success case.
+    /// This new accumulator is "armed" and will detonate if dropped without being finished.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # extern crate darling_core as darling;
+    /// # struct Thing;
+    /// # struct Output;
+    /// # impl Thing { fn validate(&self) -> darling::Result<Output> { Ok(Output) } }
+    /// fn validate(lorem_inputs: &[Thing], ipsum_inputs: &[Thing])
+    ///             -> darling::Result<(Vec<Output>, Vec<Output>)> {
+    ///     let mut errors = darling::Error::accumulator();
+    ///
+    ///     let lorems = lorem_inputs.iter().filter_map(|l| {
+    ///         errors.handle(l.validate())
+    ///     }).collect();
+    ///
+    ///     errors = errors.checkpoint()?;
+    ///
+    ///     let ipsums = ipsum_inputs.iter().filter_map(|l| {
+    ///         errors.handle(l.validate())
+    ///     }).collect();
+    ///
+    ///     errors.finish_with((lorems, ipsums))
+    /// }
+    /// # validate(&[], &[]).unwrap();
+    /// ```
+    pub fn checkpoint(self) -> Result<Accumulator> {
+        // The doc comment says on success we "return the Accumulator for future use".
+        // Actually, we have consumed it by feeding it to finish so we make a fresh one.
+        // This is OK since by definition of the success path, it was empty on entry.
+        self.finish()?;
+        Ok(Self::default())
+    }
+}
+
+impl Default for Accumulator {
+    fn default() -> Self {
+        Accumulator(Some(vec![]))
+    }
+}
+
+impl Extend<Error> for Accumulator {
+    fn extend<I>(&mut self, iter: I)
+    where
+        I: IntoIterator<Item = Error>,
+    {
+        self.errors().extend(iter)
+    }
+}
+
+impl Drop for Accumulator {
+    fn drop(&mut self) {
+        // don't try to panic if we are currently unwinding a panic
+        // otherwise we end up with an unhelful "thread panicked while panicking. aborting." message
+        if !std::thread::panicking() {
+            if let Some(errors) = &mut self.0 {
+                match errors.len() {
+                    0 => panic!("darling::error::Accumulator dropped without being finished"),
+                    error_count => panic!("darling::error::Accumulator dropped without being finished. {} errors were lost.", error_count)
+                }
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Error;
+
+    #[test]
+    fn flatten_noop() {
+        let err = Error::duplicate_field("hello").at("world");
+        assert_eq!(err.clone().flatten(), err);
+    }
+
+    #[test]
+    fn flatten_simple() {
+        let err = Error::multiple(vec![
+            Error::unknown_field("hello").at("world"),
+            Error::missing_field("hell_no").at("world"),
+        ])
+        .at("foo")
+        .flatten();
+
+        assert!(err.location().is_empty());
+
+        let mut err_iter = err.into_iter();
+
+        let first = err_iter.next();
+        assert!(first.is_some());
+        assert_eq!(first.unwrap().location(), vec!["foo", "world"]);
+
+        let second = err_iter.next();
+        assert!(second.is_some());
+
+        assert_eq!(second.unwrap().location(), vec!["foo", "world"]);
+
+        assert!(err_iter.next().is_none());
+    }
+
+    #[test]
+    fn len_single() {
+        let err = Error::duplicate_field("hello");
+        assert_eq!(1, err.len());
+    }
+
+    #[test]
+    fn len_multiple() {
+        let err = Error::multiple(vec![
+            Error::duplicate_field("hello"),
+            Error::missing_field("hell_no"),
+        ]);
+        assert_eq!(2, err.len());
+    }
+
+    #[test]
+    fn len_nested() {
+        let err = Error::multiple(vec![
+            Error::duplicate_field("hello"),
+            Error::multiple(vec![
+                Error::duplicate_field("hi"),
+                Error::missing_field("bye"),
+                Error::multiple(vec![Error::duplicate_field("whatsup")]),
+            ]),
+        ]);
+
+        assert_eq!(4, err.len());
+    }
+
+    #[test]
+    fn accum_ok() {
+        let errs = Error::accumulator();
+        assert_eq!("test", errs.finish_with("test").unwrap());
+    }
+
+    #[test]
+    fn accum_errr() {
+        let mut errs = Error::accumulator();
+        errs.push(Error::custom("foo!"));
+        errs.finish().unwrap_err();
+    }
+
+    #[test]
+    fn accum_into_inner() {
+        let mut errs = Error::accumulator();
+        errs.push(Error::custom("foo!"));
+        let errs: Vec<_> = errs.into_inner();
+        assert_eq!(errs.len(), 1);
+    }
+
+    #[test]
+    #[should_panic(expected = "Accumulator dropped")]
+    fn accum_drop_panic() {
+        let _errs = Error::accumulator();
+    }
+
+    #[test]
+    #[should_panic(expected = "2 errors")]
+    fn accum_drop_panic_with_error_count() {
+        let mut errors = Error::accumulator();
+        errors.push(Error::custom("first"));
+        errors.push(Error::custom("second"));
+    }
+
+    #[test]
+    fn accum_checkpoint_error() {
+        let mut errs = Error::accumulator();
+        errs.push(Error::custom("foo!"));
+        errs.checkpoint().unwrap_err();
+    }
+
+    #[test]
+    #[should_panic(expected = "Accumulator dropped")]
+    fn accum_checkpoint_drop_panic() {
+        let mut errs = Error::accumulator();
+        errs = errs.checkpoint().unwrap();
+        let _ = errs;
+    }
+}
diff --git a/third_party/rust/darling_core/src/from_attributes.rs b/third_party/rust/darling_core/src/from_attributes.rs
new file mode 100644
index 0000000000..c73f0f2cf4
--- /dev/null
+++ b/third_party/rust/darling_core/src/from_attributes.rs
@@ -0,0 +1,27 @@
+use syn::Attribute;
+
+use crate::Result;
+
+/// Create an instance by parsing a list of attributes.
+///
+/// This trait is useful when dealing with items such as traits on traits and impl blocks,
+/// for which `darling` does not provide dedicated traits.
+pub trait FromAttributes: Sized {
+    /// Create an instance by parsing a list of attributes.
+    ///
+    /// By convention, `FromAttributes` implementations should merge item
+    /// declarations across attributes, so that the following forms are
+    /// equivalent:
+    ///
+    /// ```rust,ignore
+    /// #[derive(Serialize)]
+    /// #[serde(rename_all = "camel_case")]
+    /// #[serde(borrow)]
+    /// pub struct SplitExample {}
+    ///
+    /// #[derive(Serialize)]
+    /// #[serde(borrow, rename_all = "camel_case")]
+    /// pub struct JoinedExample {}
+    /// ```
+    fn from_attributes(attrs: &[Attribute]) -> Result<Self>;
+}
diff --git a/third_party/rust/darling_core/src/from_derive_input.rs b/third_party/rust/darling_core/src/from_derive_input.rs
new file mode 100644
index 0000000000..b5bb86f37e
--- /dev/null
+++ b/third_party/rust/darling_core/src/from_derive_input.rs
@@ -0,0 +1,26 @@
+use syn::DeriveInput;
+
+use crate::Result;
+
+/// Creates an instance by parsing an entire proc-macro `derive` input,
+/// including the, identity, generics, and visibility of the type.
+///
+/// This trait should either be derived or manually implemented by a type
+/// in the proc macro crate which is directly using `darling`. It is unlikely
+/// that these implementations will be reusable across crates.
+pub trait FromDeriveInput: Sized {
+    /// Create an instance from `syn::DeriveInput`, or return an error.
+    fn from_derive_input(input: &DeriveInput) -> Result<Self>;
+}
+
+impl FromDeriveInput for () {
+    fn from_derive_input(_: &DeriveInput) -> Result<Self> {
+        Ok(())
+    }
+}
+
+impl FromDeriveInput for DeriveInput {
+    fn from_derive_input(input: &DeriveInput) -> Result<Self> {
+        Ok(input.clone())
+    }
+}
diff --git a/third_party/rust/darling_core/src/from_field.rs b/third_party/rust/darling_core/src/from_field.rs
new file mode 100644
index 0000000000..7ce4e55ca2
--- /dev/null
+++ b/third_party/rust/darling_core/src/from_field.rs
@@ -0,0 +1,38 @@
+use syn::Field;
+
+use crate::Result;
+
+/// Creates an instance by parsing an individual field and its attributes.
+pub trait FromField: Sized {
+    fn from_field(field: &Field) -> Result<Self>;
+}
+
+impl FromField for () {
+    fn from_field(_: &Field) -> Result<Self> {
+        Ok(())
+    }
+}
+
+impl FromField for Field {
+    fn from_field(field: &Field) -> Result<Self> {
+        Ok(field.clone())
+    }
+}
+
+impl FromField for syn::Type {
+    fn from_field(field: &Field) -> Result<Self> {
+        Ok(field.ty.clone())
+    }
+}
+
+impl FromField for syn::Visibility {
+    fn from_field(field: &Field) -> Result<Self> {
+        Ok(field.vis.clone())
+    }
+}
+
+impl FromField for Vec<syn::Attribute> {
+    fn from_field(field: &Field) -> Result<Self> {
+        Ok(field.attrs.clone())
+    }
+}
diff --git a/third_party/rust/darling_core/src/from_generic_param.rs b/third_party/rust/darling_core/src/from_generic_param.rs
new file mode 100644
index 0000000000..757ad768a8
--- /dev/null
+++ b/third_party/rust/darling_core/src/from_generic_param.rs
@@ -0,0 +1,19 @@
+use crate::Result;
+
+/// Creates an instance by parsing a specific `syn::GenericParam`.
+/// This can be a type param, a lifetime, or a const param.
+pub trait FromGenericParam: Sized {
+    fn from_generic_param(param: &syn::GenericParam) -> Result<Self>;
+}
+
+impl FromGenericParam for () {
+    fn from_generic_param(_param: &syn::GenericParam) -> Result<Self> {
+        Ok(())
+    }
+}
+
+impl FromGenericParam for syn::GenericParam {
+    fn from_generic_param(param: &syn::GenericParam) -> Result<Self> {
+        Ok(param.clone())
+    }
+}
diff --git a/third_party/rust/darling_core/src/from_generics.rs b/third_party/rust/darling_core/src/from_generics.rs
new file mode 100644
index 0000000000..8f687106bb
--- /dev/null
+++ b/third_party/rust/darling_core/src/from_generics.rs
@@ -0,0 +1,27 @@
+use syn::Generics;
+
+use crate::Result;
+
+/// Creates an instance by parsing an entire generics declaration, including the
+/// `where` clause.
+pub trait FromGenerics: Sized {
+    fn from_generics(generics: &Generics) -> Result<Self>;
+}
+
+impl FromGenerics for () {
+    fn from_generics(_generics: &Generics) -> Result<Self> {
+        Ok(())
+    }
+}
+
+impl FromGenerics for Generics {
+    fn from_generics(generics: &Generics) -> Result<Self> {
+        Ok(generics.clone())
+    }
+}
+
+impl<T: FromGenerics> FromGenerics for Result<T> {
+    fn from_generics(generics: &Generics) -> Result<Self> {
+        Ok(FromGenerics::from_generics(generics))
+    }
+}
diff --git a/third_party/rust/darling_core/src/from_meta.rs b/third_party/rust/darling_core/src/from_meta.rs
new file mode 100644
index 0000000000..ac4f06c3e3
--- /dev/null
+++ b/third_party/rust/darling_core/src/from_meta.rs
@@ -0,0 +1,853 @@
+use std::borrow::Cow;
+use std::cell::RefCell;
+use std::collections::hash_map::HashMap;
+use std::collections::HashSet;
+use std::hash::BuildHasher;
+use std::rc::Rc;
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+
+use syn::{Expr, Lit, Meta, NestedMeta};
+
+use crate::{util::path_to_string, Error, Result};
+
+/// Create an instance from an item in an attribute declaration.
+///
+/// # Implementing `FromMeta`
+/// * Do not take a dependency on the `ident` of the passed-in meta item. The ident will be set by the field name of the containing struct.
+/// * Implement only the `from_*` methods that you intend to support. The default implementations will return useful errors.
+///
+/// # Provided Implementations
+/// ## bool
+///
+/// * Word with no value specified - becomes `true`.
+/// * As a boolean literal, e.g. `foo = true`.
+/// * As a string literal, e.g. `foo = "true"`.
+///
+/// ## char
+/// * As a char literal, e.g. `foo = '#'`.
+/// * As a string literal consisting of a single character, e.g. `foo = "#"`.
+///
+/// ## String
+/// * As a string literal, e.g. `foo = "hello"`.
+/// * As a raw string literal, e.g. `foo = r#"hello "world""#`.
+///
+/// ## Number
+/// * As a string literal, e.g. `foo = "-25"`.
+/// * As an unquoted positive value, e.g. `foo = 404`. Negative numbers must be in quotation marks.
+///
+/// ## ()
+/// * Word with no value specified, e.g. `foo`. This is best used with `Option`.
+///   See `darling::util::Flag` for a more strongly-typed alternative.
+///
+/// ## Option
+/// * Any format produces `Some`.
+///
+/// ## `Result<T, darling::Error>`
+/// * Allows for fallible parsing; will populate the target field with the result of the
+///   parse attempt.
+pub trait FromMeta: Sized {
+    fn from_nested_meta(item: &NestedMeta) -> Result<Self> {
+        (match *item {
+            NestedMeta::Lit(ref lit) => Self::from_value(lit),
+            NestedMeta::Meta(ref mi) => Self::from_meta(mi),
+        })
+        .map_err(|e| e.with_span(item))
+    }
+
+    /// Create an instance from a `syn::Meta` by dispatching to the format-appropriate
+    /// trait function. This generally should not be overridden by implementers.
+    ///
+    /// # Error Spans
+    /// If this method is overridden and can introduce errors that weren't passed up from
+    /// other `from_meta` calls, the override must call `with_span` on the error using the
+    /// `item` to make sure that the emitted diagnostic points to the correct location in
+    /// source code.
+    fn from_meta(item: &Meta) -> Result<Self> {
+        (match *item {
+            Meta::Path(_) => Self::from_word(),
+            Meta::List(ref value) => Self::from_list(
+                &value
+                    .nested
+                    .iter()
+                    .cloned()
+                    .collect::<Vec<syn::NestedMeta>>()[..],
+            ),
+            Meta::NameValue(ref value) => Self::from_value(&value.lit),
+        })
+        .map_err(|e| e.with_span(item))
+    }
+
+    /// When a field is omitted from a parent meta-item, `from_none` is used to attempt
+    /// recovery before a missing field error is generated.
+    ///
+    /// **Most types should not override this method.** `darling` already allows field-level
+    /// missing-field recovery using `#[darling(default)]` and `#[darling(default = "...")]`,
+    /// and users who add a `String` field to their `FromMeta`-deriving struct would be surprised
+    /// if they get back `""` instead of a missing field error when that field is omitted.
+    ///
+    /// The primary use-case for this is `Option<T>` fields gracefully handlling absence without
+    /// needing `#[darling(default)]`.
+    fn from_none() -> Option<Self> {
+        None
+    }
+
+    /// Create an instance from the presence of the word in the attribute with no
+    /// additional options specified.
+    fn from_word() -> Result<Self> {
+        Err(Error::unsupported_format("word"))
+    }
+
+    /// Create an instance from a list of nested meta items.
+    #[allow(unused_variables)]
+    fn from_list(items: &[NestedMeta]) -> Result<Self> {
+        Err(Error::unsupported_format("list"))
+    }
+
+    /// Create an instance from a literal value of either `foo = "bar"` or `foo("bar")`.
+    /// This dispatches to the appropriate method based on the type of literal encountered,
+    /// and generally should not be overridden by implementers.
+    ///
+    /// # Error Spans
+    /// If this method is overridden, the override must make sure to add `value`'s span
+    /// information to the returned error by calling `with_span(value)` on the `Error` instance.
+    fn from_value(value: &Lit) -> Result<Self> {
+        (match *value {
+            Lit::Bool(ref b) => Self::from_bool(b.value),
+            Lit::Str(ref s) => Self::from_string(&s.value()),
+            Lit::Char(ref ch) => Self::from_char(ch.value()),
+            _ => Err(Error::unexpected_lit_type(value)),
+        })
+        .map_err(|e| e.with_span(value))
+    }
+
+    /// Create an instance from a char literal in a value position.
+    #[allow(unused_variables)]
+    fn from_char(value: char) -> Result<Self> {
+        Err(Error::unexpected_type("char"))
+    }
+
+    /// Create an instance from a string literal in a value position.
+    #[allow(unused_variables)]
+    fn from_string(value: &str) -> Result<Self> {
+        Err(Error::unexpected_type("string"))
+    }
+
+    /// Create an instance from a bool literal in a value position.
+    #[allow(unused_variables)]
+    fn from_bool(value: bool) -> Result<Self> {
+        Err(Error::unexpected_type("bool"))
+    }
+}
+
+// FromMeta impls for std and syn types.
+
+impl FromMeta for () {
+    fn from_word() -> Result<Self> {
+        Ok(())
+    }
+}
+
+impl FromMeta for bool {
+    fn from_word() -> Result<Self> {
+        Ok(true)
+    }
+
+    #[allow(clippy::wrong_self_convention)] // false positive
+    fn from_bool(value: bool) -> Result<Self> {
+        Ok(value)
+    }
+
+    fn from_string(value: &str) -> Result<Self> {
+        value.parse().map_err(|_| Error::unknown_value(value))
+    }
+}
+
+impl FromMeta for AtomicBool {
+    fn from_meta(mi: &Meta) -> Result<Self> {
+        FromMeta::from_meta(mi)
+            .map(AtomicBool::new)
+            .map_err(|e| e.with_span(mi))
+    }
+}
+
+impl FromMeta for char {
+    #[allow(clippy::wrong_self_convention)] // false positive
+    fn from_char(value: char) -> Result<Self> {
+        Ok(value)
+    }
+
+    fn from_string(s: &str) -> Result<Self> {
+        let mut chars = s.chars();
+        let char1 = chars.next();
+        let char2 = chars.next();
+
+        if let (Some(char), None) = (char1, char2) {
+            Ok(char)
+        } else {
+            Err(Error::unexpected_type("string"))
+        }
+    }
+}
+
+impl FromMeta for String {
+    fn from_string(s: &str) -> Result<Self> {
+        Ok(s.to_string())
+    }
+}
+
+/// Generate an impl of `FromMeta` that will accept strings which parse to numbers or
+/// integer literals.
+macro_rules! from_meta_num {
+    ($ty:ident) => {
+        impl FromMeta for $ty {
+            fn from_string(s: &str) -> Result<Self> {
+                s.parse().map_err(|_| Error::unknown_value(s))
+            }
+
+            fn from_value(value: &Lit) -> Result<Self> {
+                (match *value {
+                    Lit::Str(ref s) => Self::from_string(&s.value()),
+                    Lit::Int(ref s) => Ok(s.base10_parse::<$ty>().unwrap()),
+                    _ => Err(Error::unexpected_lit_type(value)),
+                })
+                .map_err(|e| e.with_span(value))
+            }
+        }
+    };
+}
+
+from_meta_num!(u8);
+from_meta_num!(u16);
+from_meta_num!(u32);
+from_meta_num!(u64);
+from_meta_num!(usize);
+from_meta_num!(i8);
+from_meta_num!(i16);
+from_meta_num!(i32);
+from_meta_num!(i64);
+from_meta_num!(isize);
+
+/// Generate an impl of `FromMeta` that will accept strings which parse to floats or
+/// float literals.
+macro_rules! from_meta_float {
+    ($ty:ident) => {
+        impl FromMeta for $ty {
+            fn from_string(s: &str) -> Result<Self> {
+                s.parse().map_err(|_| Error::unknown_value(s))
+            }
+
+            fn from_value(value: &Lit) -> Result<Self> {
+                (match *value {
+                    Lit::Str(ref s) => Self::from_string(&s.value()),
+                    Lit::Float(ref s) => Ok(s.base10_parse::<$ty>().unwrap()),
+                    _ => Err(Error::unexpected_lit_type(value)),
+                })
+                .map_err(|e| e.with_span(value))
+            }
+        }
+    };
+}
+
+from_meta_float!(f32);
+from_meta_float!(f64);
+
+/// Parsing support for punctuated. This attempts to preserve span information
+/// when available, but also supports parsing strings with the call site as the
+/// emitted span.
+impl<T: syn::parse::Parse, P: syn::parse::Parse> FromMeta for syn::punctuated::Punctuated<T, P> {
+    fn from_value(value: &Lit) -> Result<Self> {
+        if let Lit::Str(ref ident) = *value {
+            ident
+                .parse_with(syn::punctuated::Punctuated::parse_terminated)
+                .map_err(|_| Error::unknown_lit_str_value(ident))
+        } else {
+            Err(Error::unexpected_lit_type(value))
+        }
+    }
+}
+
+/// Adapter from `syn::parse::Parse` to `FromMeta`.
+///
+/// This cannot be a blanket impl, due to the `syn::Lit` family's need to handle non-string values.
+/// Therefore, we use a macro and a lot of impls.
+macro_rules! from_syn_parse {
+    ($ty:path) => {
+        impl FromMeta for $ty {
+            fn from_string(value: &str) -> Result<Self> {
+                syn::parse_str(value).map_err(|_| Error::unknown_value(value))
+            }
+
+            fn from_value(value: &::syn::Lit) -> Result<Self> {
+                if let ::syn::Lit::Str(ref v) = *value {
+                    v.parse::<$ty>()
+                        .map_err(|_| Error::unknown_lit_str_value(v))
+                } else {
+                    Err(Error::unexpected_lit_type(value))
+                }
+            }
+        }
+    };
+}
+
+from_syn_parse!(syn::Ident);
+from_syn_parse!(syn::Expr);
+from_syn_parse!(syn::ExprArray);
+from_syn_parse!(syn::ExprPath);
+from_syn_parse!(syn::Path);
+from_syn_parse!(syn::Type);
+from_syn_parse!(syn::TypeArray);
+from_syn_parse!(syn::TypeBareFn);
+from_syn_parse!(syn::TypeGroup);
+from_syn_parse!(syn::TypeImplTrait);
+from_syn_parse!(syn::TypeInfer);
+from_syn_parse!(syn::TypeMacro);
+from_syn_parse!(syn::TypeNever);
+from_syn_parse!(syn::TypeParam);
+from_syn_parse!(syn::TypeParen);
+from_syn_parse!(syn::TypePath);
+from_syn_parse!(syn::TypePtr);
+from_syn_parse!(syn::TypeReference);
+from_syn_parse!(syn::TypeSlice);
+from_syn_parse!(syn::TypeTraitObject);
+from_syn_parse!(syn::TypeTuple);
+from_syn_parse!(syn::Visibility);
+from_syn_parse!(syn::WhereClause);
+
+macro_rules! from_numeric_array {
+    ($ty:ident) => {
+        /// Parsing an unsigned integer array, i.e. `example = "[1, 2, 3, 4]"`.
+        impl FromMeta for Vec<$ty> {
+            fn from_value(value: &Lit) -> Result<Self> {
+                let expr_array = syn::ExprArray::from_value(value)?;
+                // To meet rust <1.36 borrow checker rules on expr_array.elems
+                let v =
+                    expr_array
+                        .elems
+                        .iter()
+                        .map(|expr| match expr {
+                            Expr::Lit(lit) => $ty::from_value(&lit.lit),
+                            _ => Err(Error::custom("Expected array of unsigned integers")
+                                .with_span(expr)),
+                        })
+                        .collect::<Result<Vec<$ty>>>();
+                v
+            }
+        }
+    };
+}
+
+from_numeric_array!(u8);
+from_numeric_array!(u16);
+from_numeric_array!(u32);
+from_numeric_array!(u64);
+from_numeric_array!(usize);
+
+impl FromMeta for syn::Lit {
+    fn from_value(value: &Lit) -> Result<Self> {
+        Ok(value.clone())
+    }
+}
+
+macro_rules! from_meta_lit {
+    ($impl_ty:path, $lit_variant:path) => {
+        impl FromMeta for $impl_ty {
+            fn from_value(value: &Lit) -> Result<Self> {
+                if let $lit_variant(ref value) = *value {
+                    Ok(value.clone())
+                } else {
+                    Err(Error::unexpected_lit_type(value))
+                }
+            }
+        }
+    };
+}
+
+from_meta_lit!(syn::LitInt, Lit::Int);
+from_meta_lit!(syn::LitFloat, Lit::Float);
+from_meta_lit!(syn::LitStr, Lit::Str);
+from_meta_lit!(syn::LitByte, Lit::Byte);
+from_meta_lit!(syn::LitByteStr, Lit::ByteStr);
+from_meta_lit!(syn::LitChar, Lit::Char);
+from_meta_lit!(syn::LitBool, Lit::Bool);
+from_meta_lit!(proc_macro2::Literal, Lit::Verbatim);
+
+impl FromMeta for syn::Meta {
+    fn from_meta(value: &syn::Meta) -> Result<Self> {
+        Ok(value.clone())
+    }
+}
+
+impl FromMeta for Vec<syn::WherePredicate> {
+    fn from_string(value: &str) -> Result<Self> {
+        syn::WhereClause::from_string(&format!("where {}", value))
+            .map(|c| c.predicates.into_iter().collect())
+    }
+
+    fn from_value(value: &Lit) -> Result<Self> {
+        if let syn::Lit::Str(s) = value {
+            syn::WhereClause::from_value(&syn::Lit::Str(syn::LitStr::new(
+                &format!("where {}", s.value()),
+                value.span(),
+            )))
+            .map(|c| c.predicates.into_iter().collect())
+        } else {
+            Err(Error::unexpected_lit_type(value))
+        }
+    }
+}
+
+impl FromMeta for ident_case::RenameRule {
+    fn from_string(value: &str) -> Result<Self> {
+        value.parse().map_err(|_| Error::unknown_value(value))
+    }
+}
+
+impl<T: FromMeta> FromMeta for Option<T> {
+    fn from_none() -> Option<Self> {
+        Some(None)
+    }
+
+    fn from_meta(item: &Meta) -> Result<Self> {
+        FromMeta::from_meta(item).map(Some)
+    }
+}
+
+impl<T: FromMeta> FromMeta for Box<T> {
+    fn from_none() -> Option<Self> {
+        T::from_none().map(Box::new)
+    }
+
+    fn from_meta(item: &Meta) -> Result<Self> {
+        FromMeta::from_meta(item).map(Box::new)
+    }
+}
+
+impl<T: FromMeta> FromMeta for Result<T> {
+    fn from_none() -> Option<Self> {
+        T::from_none().map(Ok)
+    }
+
+    fn from_meta(item: &Meta) -> Result<Self> {
+        Ok(FromMeta::from_meta(item))
+    }
+}
+
+/// Parses the meta-item, and in case of error preserves a copy of the input for
+/// later analysis.
+impl<T: FromMeta> FromMeta for ::std::result::Result<T, Meta> {
+    fn from_meta(item: &Meta) -> Result<Self> {
+        T::from_meta(item)
+            .map(Ok)
+            .or_else(|_| Ok(Err(item.clone())))
+    }
+}
+
+impl<T: FromMeta> FromMeta for Rc<T> {
+    fn from_none() -> Option<Self> {
+        T::from_none().map(Rc::new)
+    }
+
+    fn from_meta(item: &Meta) -> Result<Self> {
+        FromMeta::from_meta(item).map(Rc::new)
+    }
+}
+
+impl<T: FromMeta> FromMeta for Arc<T> {
+    fn from_none() -> Option<Self> {
+        T::from_none().map(Arc::new)
+    }
+
+    fn from_meta(item: &Meta) -> Result<Self> {
+        FromMeta::from_meta(item).map(Arc::new)
+    }
+}
+
+impl<T: FromMeta> FromMeta for RefCell<T> {
+    fn from_none() -> Option<Self> {
+        T::from_none().map(RefCell::new)
+    }
+
+    fn from_meta(item: &Meta) -> Result<Self> {
+        FromMeta::from_meta(item).map(RefCell::new)
+    }
+}
+
+/// Trait to convert from a path into an owned key for a map.
+trait KeyFromPath: Sized {
+    fn from_path(path: &syn::Path) -> Result<Self>;
+    fn to_display(&self) -> Cow<'_, str>;
+}
+
+impl KeyFromPath for String {
+    fn from_path(path: &syn::Path) -> Result<Self> {
+        Ok(path_to_string(path))
+    }
+
+    fn to_display(&self) -> Cow<'_, str> {
+        Cow::Borrowed(self)
+    }
+}
+
+impl KeyFromPath for syn::Path {
+    fn from_path(path: &syn::Path) -> Result<Self> {
+        Ok(path.clone())
+    }
+
+    fn to_display(&self) -> Cow<'_, str> {
+        Cow::Owned(path_to_string(self))
+    }
+}
+
+impl KeyFromPath for syn::Ident {
+    fn from_path(path: &syn::Path) -> Result<Self> {
+        if path.segments.len() == 1
+            && path.leading_colon.is_none()
+            && path.segments[0].arguments.is_empty()
+        {
+            Ok(path.segments[0].ident.clone())
+        } else {
+            Err(Error::custom("Key must be an identifier").with_span(path))
+        }
+    }
+
+    fn to_display(&self) -> Cow<'_, str> {
+        Cow::Owned(self.to_string())
+    }
+}
+
+macro_rules! hash_map {
+    ($key:ty) => {
+        impl<V: FromMeta, S: BuildHasher + Default> FromMeta for HashMap<$key, V, S> {
+            fn from_list(nested: &[syn::NestedMeta]) -> Result<Self> {
+                // Convert the nested meta items into a sequence of (path, value result) result tuples.
+                // An outer Err means no (key, value) structured could be found, while an Err in the
+                // second position of the tuple means that value was rejected by FromMeta.
+                //
+                // We defer key conversion into $key so that we don't lose span information in the case
+                // of String keys; we'll need it for good duplicate key errors later.
+                let pairs = nested
+                    .iter()
+                    .map(|item| -> Result<(&syn::Path, Result<V>)> {
+                        match *item {
+                            syn::NestedMeta::Meta(ref inner) => {
+                                let path = inner.path();
+                                Ok((
+                                    path,
+                                    FromMeta::from_meta(inner).map_err(|e| e.at_path(&path)),
+                                ))
+                            }
+                            syn::NestedMeta::Lit(_) => Err(Error::unsupported_format("literal")),
+                        }
+                    });
+
+                let mut errors = Error::accumulator();
+                // We need to track seen keys separately from the final map, since a seen key with an
+                // Err value won't go into the final map but should trigger a duplicate field error.
+                //
+                // This is a set of $key rather than Path to avoid the possibility that a key type
+                // parses two paths of different values to the same key value.
+                let mut seen_keys = HashSet::with_capacity(nested.len());
+
+                // The map to return in the Ok case. Its size will always be exactly nested.len(),
+                // since otherwise ≥1 field had a problem and the entire map is dropped immediately
+                // when the function returns `Err`.
+                let mut map = HashMap::with_capacity_and_hasher(nested.len(), Default::default());
+
+                for item in pairs {
+                    if let Some((path, value)) = errors.handle(item) {
+                        let key: $key = match KeyFromPath::from_path(path) {
+                            Ok(k) => k,
+                            Err(e) => {
+                                errors.push(e);
+
+                                // Surface value errors even under invalid keys
+                                errors.handle(value);
+
+                                continue;
+                            }
+                        };
+
+                        let already_seen = seen_keys.contains(&key);
+
+                        if already_seen {
+                            errors.push(Error::duplicate_field(&key.to_display()).with_span(path));
+                        }
+
+                        match value {
+                            Ok(_) if already_seen => {}
+                            Ok(val) => {
+                                map.insert(key.clone(), val);
+                            }
+                            Err(e) => {
+                                errors.push(e);
+                            }
+                        }
+
+                        seen_keys.insert(key);
+                    }
+                }
+
+                errors.finish_with(map)
+            }
+        }
+    };
+}
+
+// This is done as a macro rather than a blanket impl to avoid breaking backwards compatibility
+// with 0.12.x, while still sharing the same impl.
+hash_map!(String);
+hash_map!(syn::Ident);
+hash_map!(syn::Path);
+
+/// Tests for `FromMeta` implementations. Wherever the word `ignore` appears in test input,
+/// it should not be considered by the parsing.
+#[cfg(test)]
+mod tests {
+    use proc_macro2::TokenStream;
+    use quote::quote;
+    use syn::parse_quote;
+
+    use crate::{Error, FromMeta, Result};
+
+    /// parse a string as a syn::Meta instance.
+    fn pm(tokens: TokenStream) -> ::std::result::Result<syn::Meta, String> {
+        let attribute: syn::Attribute = parse_quote!(#[#tokens]);
+        attribute.parse_meta().map_err(|_| "Unable to parse".into())
+    }
+
+    fn fm<T: FromMeta>(tokens: TokenStream) -> T {
+        FromMeta::from_meta(&pm(tokens).expect("Tests should pass well-formed input"))
+            .expect("Tests should pass valid input")
+    }
+
+    #[test]
+    fn unit_succeeds() {
+        fm::<()>(quote!(ignore));
+    }
+
+    #[test]
+    #[allow(clippy::bool_assert_comparison)]
+    fn bool_succeeds() {
+        // word format
+        assert_eq!(fm::<bool>(quote!(ignore)), true);
+
+        // bool literal
+        assert_eq!(fm::<bool>(quote!(ignore = true)), true);
+        assert_eq!(fm::<bool>(quote!(ignore = false)), false);
+
+        // string literals
+        assert_eq!(fm::<bool>(quote!(ignore = "true")), true);
+        assert_eq!(fm::<bool>(quote!(ignore = "false")), false);
+    }
+
+    #[test]
+    fn char_succeeds() {
+        // char literal
+        assert_eq!(fm::<char>(quote!(ignore = '😬')), '😬');
+
+        // string literal
+        assert_eq!(fm::<char>(quote!(ignore = "😬")), '😬');
+    }
+
+    #[test]
+    fn string_succeeds() {
+        // cooked form
+        assert_eq!(&fm::<String>(quote!(ignore = "world")), "world");
+
+        // raw form
+        assert_eq!(&fm::<String>(quote!(ignore = r#"world"#)), "world");
+    }
+
+    #[test]
+    #[allow(clippy::float_cmp)] // we want exact equality
+    fn number_succeeds() {
+        assert_eq!(fm::<u8>(quote!(ignore = "2")), 2u8);
+        assert_eq!(fm::<i16>(quote!(ignore = "-25")), -25i16);
+        assert_eq!(fm::<f64>(quote!(ignore = "1.4e10")), 1.4e10);
+    }
+
+    #[test]
+    fn int_without_quotes() {
+        assert_eq!(fm::<u8>(quote!(ignore = 2)), 2u8);
+        assert_eq!(fm::<u16>(quote!(ignore = 255)), 255u16);
+        assert_eq!(fm::<u32>(quote!(ignore = 5000)), 5000u32);
+
+        // Check that we aren't tripped up by incorrect suffixes
+        assert_eq!(fm::<u32>(quote!(ignore = 5000i32)), 5000u32);
+    }
+
+    #[test]
+    #[allow(clippy::float_cmp)] // we want exact equality
+    fn float_without_quotes() {
+        assert_eq!(fm::<f32>(quote!(ignore = 2.)), 2.0f32);
+        assert_eq!(fm::<f32>(quote!(ignore = 2.0)), 2.0f32);
+        assert_eq!(fm::<f64>(quote!(ignore = 1.4e10)), 1.4e10f64);
+    }
+
+    #[test]
+    fn meta_succeeds() {
+        use syn::Meta;
+
+        assert_eq!(
+            fm::<Meta>(quote!(hello(world, today))),
+            pm(quote!(hello(world, today))).unwrap()
+        );
+    }
+
+    #[test]
+    fn hash_map_succeeds() {
+        use std::collections::HashMap;
+
+        let comparison = {
+            let mut c = HashMap::new();
+            c.insert("hello".to_string(), true);
+            c.insert("world".to_string(), false);
+            c.insert("there".to_string(), true);
+            c
+        };
+
+        assert_eq!(
+            fm::<HashMap<String, bool>>(quote!(ignore(hello, world = false, there = "true"))),
+            comparison
+        );
+    }
+
+    /// Check that a `HashMap` cannot have duplicate keys, and that the generated error
+    /// is assigned a span to correctly target the diagnostic message.
+    #[test]
+    fn hash_map_duplicate() {
+        use std::collections::HashMap;
+
+        let err: Result<HashMap<String, bool>> =
+            FromMeta::from_meta(&pm(quote!(ignore(hello, hello = false))).unwrap());
+
+        let err = err.expect_err("Duplicate keys in HashMap should error");
+
+        assert!(err.has_span());
+        assert_eq!(err.to_string(), Error::duplicate_field("hello").to_string());
+    }
+
+    #[test]
+    fn hash_map_multiple_errors() {
+        use std::collections::HashMap;
+
+        let err = HashMap::<String, bool>::from_meta(
+            &pm(quote!(ignore(hello, hello = 3, hello = false))).unwrap(),
+        )
+        .expect_err("Duplicates and bad values should error");
+
+        assert_eq!(err.len(), 3);
+        let errors = err.into_iter().collect::<Vec<_>>();
+        assert!(errors[0].has_span());
+        assert!(errors[1].has_span());
+        assert!(errors[2].has_span());
+    }
+
+    #[test]
+    fn hash_map_ident_succeeds() {
+        use std::collections::HashMap;
+        use syn::parse_quote;
+
+        let comparison = {
+            let mut c = HashMap::<syn::Ident, bool>::new();
+            c.insert(parse_quote!(first), true);
+            c.insert(parse_quote!(second), false);
+            c
+        };
+
+        assert_eq!(
+            fm::<HashMap<syn::Ident, bool>>(quote!(ignore(first, second = false))),
+            comparison
+        );
+    }
+
+    #[test]
+    fn hash_map_ident_rejects_non_idents() {
+        use std::collections::HashMap;
+
+        let err: Result<HashMap<syn::Ident, bool>> =
+            FromMeta::from_meta(&pm(quote!(ignore(first, the::second))).unwrap());
+
+        err.unwrap_err();
+    }
+
+    #[test]
+    fn hash_map_path_succeeds() {
+        use std::collections::HashMap;
+        use syn::parse_quote;
+
+        let comparison = {
+            let mut c = HashMap::<syn::Path, bool>::new();
+            c.insert(parse_quote!(first), true);
+            c.insert(parse_quote!(the::second), false);
+            c
+        };
+
+        assert_eq!(
+            fm::<HashMap<syn::Path, bool>>(quote!(ignore(first, the::second = false))),
+            comparison
+        );
+    }
+
+    /// Tests that fallible parsing will always produce an outer `Ok` (from `fm`),
+    /// and will accurately preserve the inner contents.
+    #[test]
+    fn darling_result_succeeds() {
+        fm::<Result<()>>(quote!(ignore)).unwrap();
+        fm::<Result<()>>(quote!(ignore(world))).unwrap_err();
+    }
+
+    /// Test punctuated
+    #[test]
+    fn test_punctuated() {
+        fm::<syn::punctuated::Punctuated<syn::FnArg, syn::token::Comma>>(quote!(
+            ignore = "a: u8, b: Type"
+        ));
+        fm::<syn::punctuated::Punctuated<syn::Expr, syn::token::Comma>>(quote!(ignore = "a, b, c"));
+    }
+
+    #[test]
+    fn test_expr_array() {
+        fm::<syn::ExprArray>(quote!(ignore = "[0x1, 0x2]"));
+        fm::<syn::ExprArray>(quote!(ignore = "[\"Hello World\", \"Test Array\"]"));
+    }
+
+    #[test]
+    fn test_expr() {
+        fm::<syn::Expr>(quote!(ignore = "x + y"));
+        fm::<syn::Expr>(quote!(ignore = "an_object.method_call()"));
+        fm::<syn::Expr>(quote!(ignore = "{ a_statement(); in_a_block }"));
+    }
+
+    #[test]
+    fn test_expr_path() {
+        fm::<syn::ExprPath>(quote!(ignore = "std::mem::replace"));
+        fm::<syn::ExprPath>(quote!(ignore = "x"));
+        fm::<syn::ExprPath>(quote!(ignore = "example::<Test>"));
+    }
+
+    #[test]
+    fn test_number_array() {
+        assert_eq!(
+            fm::<Vec<u8>>(quote!(ignore = "[16, 0xff]")),
+            vec![0x10, 0xff]
+        );
+        assert_eq!(
+            fm::<Vec<u16>>(quote!(ignore = "[32, 0xffff]")),
+            vec![0x20, 0xffff]
+        );
+        assert_eq!(
+            fm::<Vec<u32>>(quote!(ignore = "[48, 0xffffffff]")),
+            vec![0x30, 0xffffffff]
+        );
+        assert_eq!(
+            fm::<Vec<u64>>(quote!(ignore = "[64, 0xffffffffffffffff]")),
+            vec![0x40, 0xffffffffffffffff]
+        );
+        assert_eq!(
+            fm::<Vec<usize>>(quote!(ignore = "[80, 0xffffffff]")),
+            vec![0x50, 0xffffffff]
+        );
+    }
+}
diff --git a/third_party/rust/darling_core/src/from_type_param.rs b/third_party/rust/darling_core/src/from_type_param.rs
new file mode 100644
index 0000000000..bea9796c26
--- /dev/null
+++ b/third_party/rust/darling_core/src/from_type_param.rs
@@ -0,0 +1,32 @@
+use syn::TypeParam;
+
+use crate::Result;
+
+/// Creates an instance by parsing an individual type_param and its attributes.
+pub trait FromTypeParam: Sized {
+    fn from_type_param(type_param: &TypeParam) -> Result<Self>;
+}
+
+impl FromTypeParam for () {
+    fn from_type_param(_: &TypeParam) -> Result<Self> {
+        Ok(())
+    }
+}
+
+impl FromTypeParam for TypeParam {
+    fn from_type_param(type_param: &TypeParam) -> Result<Self> {
+        Ok(type_param.clone())
+    }
+}
+
+impl FromTypeParam for Vec<syn::Attribute> {
+    fn from_type_param(type_param: &TypeParam) -> Result<Self> {
+        Ok(type_param.attrs.clone())
+    }
+}
+
+impl FromTypeParam for syn::Ident {
+    fn from_type_param(type_param: &TypeParam) -> Result<Self> {
+        Ok(type_param.ident.clone())
+    }
+}
diff --git a/third_party/rust/darling_core/src/from_variant.rs b/third_party/rust/darling_core/src/from_variant.rs
new file mode 100644
index 0000000000..50bf6359b6
--- /dev/null
+++ b/third_party/rust/darling_core/src/from_variant.rs
@@ -0,0 +1,33 @@
+use syn::Variant;
+
+use crate::Result;
+
+/// Creates an instance from a specified `syn::Variant`.
+pub trait FromVariant: Sized {
+    /// Create an instance from `syn::Variant`, or return an error.
+    fn from_variant(variant: &Variant) -> Result<Self>;
+}
+
+impl FromVariant for () {
+    fn from_variant(_: &Variant) -> Result<Self> {
+        Ok(())
+    }
+}
+
+impl FromVariant for Variant {
+    fn from_variant(variant: &Variant) -> Result<Self> {
+        Ok(variant.clone())
+    }
+}
+
+impl FromVariant for syn::Ident {
+    fn from_variant(variant: &Variant) -> Result<Self> {
+        Ok(variant.ident.clone())
+    }
+}
+
+impl FromVariant for Vec<syn::Attribute> {
+    fn from_variant(variant: &Variant) -> Result<Self> {
+        Ok(variant.attrs.clone())
+    }
+}
diff --git a/third_party/rust/darling_core/src/lib.rs b/third_party/rust/darling_core/src/lib.rs
new file mode 100644
index 0000000000..bf2050c646
--- /dev/null
+++ b/third_party/rust/darling_core/src/lib.rs
@@ -0,0 +1,42 @@
+#![recursion_limit = "256"]
+#![cfg_attr(feature = "diagnostics", feature(proc_macro_diagnostic))]
+
+#[cfg(feature = "diagnostics")]
+extern crate proc_macro;
+
+#[macro_use]
+mod macros_private;
+#[macro_use]
+mod macros_public;
+
+pub mod ast;
+pub(crate) mod codegen;
+pub mod derive;
+pub mod error;
+mod from_attributes;
+mod from_derive_input;
+mod from_field;
+mod from_generic_param;
+mod from_generics;
+mod from_meta;
+mod from_type_param;
+mod from_variant;
+pub(crate) mod options;
+pub mod usage;
+pub mod util;
+
+pub use self::error::{Error, Result};
+pub use self::from_attributes::FromAttributes;
+pub use self::from_derive_input::FromDeriveInput;
+pub use self::from_field::FromField;
+pub use self::from_generic_param::FromGenericParam;
+pub use self::from_generics::FromGenerics;
+pub use self::from_meta::FromMeta;
+pub use self::from_type_param::FromTypeParam;
+pub use self::from_variant::FromVariant;
+
+// Re-exports
+#[doc(hidden)]
+pub use quote::ToTokens;
+#[doc(hidden)]
+pub use syn;
diff --git a/third_party/rust/darling_core/src/macros_private.rs b/third_party/rust/darling_core/src/macros_private.rs
new file mode 100644
index 0000000000..61edf360a1
--- /dev/null
+++ b/third_party/rust/darling_core/src/macros_private.rs
@@ -0,0 +1,6 @@
+macro_rules! path {
+    ($($path:tt)+) => {
+        ::syn::parse_quote!($($path)+)
+        //stringify!($($path)+).parse().unwrap()
+    };
+}
diff --git a/third_party/rust/darling_core/src/macros_public.rs b/third_party/rust/darling_core/src/macros_public.rs
new file mode 100644
index 0000000000..c264fcc2a8
--- /dev/null
+++ b/third_party/rust/darling_core/src/macros_public.rs
@@ -0,0 +1,96 @@
+//! Macros that should be exported from both `darling_core` and `darling`.
+//! Note that these are **sym-linked** into the main code, and so cannot declare on items that are exported differently
+//! in `darling_core` vs. `darling`.
+
+/// Generator for `UsesTypeParam` impls that unions the used type parameters of the selected fields.
+///
+/// # Usage
+/// The macro takes the type implementing the trait as the first argument, then a comma-separated list of
+/// fields for the rest of its arguments.
+///
+/// The type of each passed-in field must implement `UsesTypeParams`, or the resulting code won't compile.
+///
+/// ```rust
+/// # extern crate syn;
+/// # use darling_core::uses_type_params;
+/// #
+/// struct MyField {
+///     ty: syn::Type,
+/// }
+///
+/// uses_type_params!(MyField, ty);
+///
+/// fn main() {
+///     // no test run
+/// }
+/// ```
+///
+/// `darling` cannot derive this trait automatically, as it doesn't know which information extracted from
+/// proc-macro input is meant to constitute "using" the type parameter, but crate consumers should
+/// implement it by hand or using the macro.
+#[macro_export]
+macro_rules! uses_type_params {
+    ($impl_type:ty, $accessor:ident) => {
+        impl $crate::usage::UsesTypeParams for $impl_type {
+            fn uses_type_params<'gen>(
+                &self,
+                options: &$crate::usage::Options,
+                type_set: &'gen $crate::usage::IdentSet
+            ) -> $crate::usage::IdentRefSet<'gen> {
+                self.$accessor.uses_type_params(options, type_set)
+            }
+        }
+    };
+    ($impl_type:ty, $first:ident, $($field:ident),+) => {
+        impl $crate::usage::UsesTypeParams for $impl_type {
+            fn uses_type_params<'gen>(
+                &self,
+                options: &$crate::usage::Options,
+                type_set: &'gen $crate::usage::IdentSet
+            ) -> $crate::usage::IdentRefSet<'gen> {
+                let mut hits = self.$first.uses_type_params(options, type_set);
+                $(
+                    hits.extend(self.$field.uses_type_params(options, type_set));
+                )*
+                hits
+            }
+        }
+    };
+}
+
+/// Generator for `UsesLifetimes` impls that unions the used lifetimes of the selected fields.
+///
+/// # Usage
+/// The macro takes the type implementing the trait as the first argument, then a comma-separated list of
+/// fields for the rest of its arguments.
+///
+/// The type of each passed-in field must implement `UsesLifetimes`, or the resulting code won't compile.
+#[macro_export]
+macro_rules! uses_lifetimes {
+    ($impl_type:ty, $accessor:ident) => {
+        impl $crate::usage::UsesLifetimes for $impl_type {
+            fn uses_lifetimes<'gen>(
+                &self,
+                options: &$crate::usage::Options,
+                type_set: &'gen $crate::usage::LifetimeSet
+            ) -> $crate::usage::LifetimeRefSet<'gen> {
+                self.$accessor.uses_lifetimes(options, type_set)
+            }
+        }
+    };
+    ($impl_type:ty, $first:ident, $($field:ident),+) => {
+        impl $crate::usage::UsesLifetimes for $impl_type {
+            fn uses_lifetimes<'gen>(
+                &self,
+                options: &$crate::usage::Options,
+                type_set: &'gen $crate::usage::LifetimeSet
+            ) -> $crate::usage::LifetimeRefSet<'gen> {
+                let mut hits = self.$first.uses_lifetimes(options, type_set);
+                $(
+                    hits.extend(self.$field.uses_lifetimes(options, type_set));
+                )*
+                hits
+            }
+        }
+    };
+}
diff --git a/third_party/rust/darling_core/src/options/core.rs b/third_party/rust/darling_core/src/options/core.rs
new file mode 100644
index 0000000000..c79f7d5da3
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/core.rs
@@ -0,0 +1,173 @@
+use ident_case::RenameRule;
+
+use crate::ast::{Data, Fields, Style};
+use crate::codegen;
+use crate::codegen::PostfixTransform;
+use crate::options::{DefaultExpression, InputField, InputVariant, ParseAttribute, ParseData};
+use crate::{Error, FromMeta, Result};
+
+/// A struct or enum which should have `FromMeta` or `FromDeriveInput` implementations
+/// generated.
+#[derive(Debug, Clone)]
+pub struct Core {
+    /// The type identifier.
+    pub ident: syn::Ident,
+
+    /// The type's generics. If the type does not use any generics, this will
+    /// be an empty instance.
+    pub generics: syn::Generics,
+
+    /// Controls whether missing properties should cause errors or should be filled by
+    /// the result of a function call. This can be overridden at the field level.
+    pub default: Option<DefaultExpression>,
+
+    /// The rule that should be used to rename all fields/variants in the container.
+    pub rename_rule: RenameRule,
+
+    /// A transform which will be called on `darling::Result<Self>`. It must either be
+    /// an `FnOnce(T) -> T` when `map` is used, or `FnOnce(T) -> darling::Result<T>` when
+    /// `and_then` is used.
+    ///
+    /// `map` and `and_then` are mutually-exclusive to avoid confusion about the order in
+    /// which the two are applied.
+    pub post_transform: Option<codegen::PostfixTransform>,
+
+    /// The body of the _deriving_ type.
+    pub data: Data<InputVariant, InputField>,
+
+    /// The custom bound to apply to the generated impl
+    pub bound: Option<Vec<syn::WherePredicate>>,
+
+    /// Whether or not unknown fields should produce an error at compilation time.
+    pub allow_unknown_fields: Option<bool>,
+}
+
+impl Core {
+    /// Partially initializes `Core` by reading the identity, generics, and body shape.
+    pub fn start(di: &syn::DeriveInput) -> Result<Self> {
+        Ok(Core {
+            ident: di.ident.clone(),
+            generics: di.generics.clone(),
+            data: Data::try_empty_from(&di.data)?,
+            default: Default::default(),
+            // See https://github.com/TedDriggs/darling/issues/10: We default to snake_case
+            // for enums to help authors produce more idiomatic APIs.
+            rename_rule: if let syn::Data::Enum(_) = di.data {
+                RenameRule::SnakeCase
+            } else {
+                Default::default()
+            },
+            post_transform: Default::default(),
+            bound: Default::default(),
+            allow_unknown_fields: Default::default(),
+        })
+    }
+
+    fn as_codegen_default(&self) -> Option<codegen::DefaultExpression<'_>> {
+        self.default.as_ref().map(|expr| match *expr {
+            DefaultExpression::Explicit(ref path) => codegen::DefaultExpression::Explicit(path),
+            DefaultExpression::Inherit => {
+                // It should be impossible for any input to get here,
+                // so panic rather than returning an error or pretending
+                // everything is fine.
+                panic!("DefaultExpression::Inherit is not valid at container level")
+            }
+            DefaultExpression::Trait { span } => codegen::DefaultExpression::Trait { span },
+        })
+    }
+}
+
+impl ParseAttribute for Core {
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()> {
+        let path = mi.path();
+
+        if path.is_ident("default") {
+            if self.default.is_some() {
+                return Err(Error::duplicate_field("default").with_span(mi));
+            }
+
+            self.default = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("rename_all") {
+            // WARNING: This may have been set based on body shape previously,
+            // so an overwrite may be permissible.
+            self.rename_rule = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("map") || path.is_ident("and_then") {
+            // This unwrap is safe because we just called is_ident above
+            let transformer = path.get_ident().unwrap().clone();
+
+            if let Some(post_transform) = &self.post_transform {
+                if transformer == post_transform.transformer {
+                    return Err(Error::duplicate_field(&transformer.to_string()).with_span(mi));
+                } else {
+                    return Err(Error::custom(format!(
+                        "Options `{}` and `{}` are mutually exclusive",
+                        transformer, post_transform.transformer
+                    ))
+                    .with_span(mi));
+                }
+            }
+
+            self.post_transform =
+                Some(PostfixTransform::new(transformer, FromMeta::from_meta(mi)?));
+        } else if path.is_ident("bound") {
+            self.bound = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("allow_unknown_fields") {
+            if self.allow_unknown_fields.is_some() {
+                return Err(Error::duplicate_field("allow_unknown_fields").with_span(mi));
+            }
+
+            self.allow_unknown_fields = FromMeta::from_meta(mi)?;
+        } else {
+            return Err(Error::unknown_field_path(path).with_span(mi));
+        }
+
+        Ok(())
+    }
+}
+
+impl ParseData for Core {
+    fn parse_variant(&mut self, variant: &syn::Variant) -> Result<()> {
+        let v = InputVariant::from_variant(variant, Some(self))?;
+
+        match self.data {
+            Data::Enum(ref mut variants) => {
+                variants.push(v);
+                Ok(())
+            }
+            Data::Struct(_) => panic!("Core::parse_variant should never be called for a struct"),
+        }
+    }
+
+    fn parse_field(&mut self, field: &syn::Field) -> Result<()> {
+        let f = InputField::from_field(field, Some(self))?;
+
+        match self.data {
+            Data::Struct(Fields {
+                style: Style::Unit, ..
+            }) => panic!("Core::parse_field should not be called on unit"),
+            Data::Struct(Fields { ref mut fields, .. }) => {
+                fields.push(f);
+                Ok(())
+            }
+            Data::Enum(_) => panic!("Core::parse_field should never be called for an enum"),
+        }
+    }
+}
+
+impl<'a> From<&'a Core> for codegen::TraitImpl<'a> {
+    fn from(v: &'a Core) -> Self {
+        codegen::TraitImpl {
+            ident: &v.ident,
+            generics: &v.generics,
+            data: v
+                .data
+                .as_ref()
+                .map_struct_fields(InputField::as_codegen_field)
+                .map_enum_variants(|variant| variant.as_codegen_variant(&v.ident)),
+            default: v.as_codegen_default(),
+            post_transform: v.post_transform.as_ref(),
+            bound: v.bound.as_ref().map(|i| i.as_slice()),
+            allow_unknown_fields: v.allow_unknown_fields.unwrap_or_default(),
+        }
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/forward_attrs.rs b/third_party/rust/darling_core/src/options/forward_attrs.rs
new file mode 100644
index 0000000000..c72d009d59
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/forward_attrs.rs
@@ -0,0 +1,31 @@
+use syn::NestedMeta;
+
+use crate::util::PathList;
+use crate::{FromMeta, Result};
+
+/// A rule about which attributes to forward to the generated struct.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ForwardAttrs {
+    All,
+    Only(PathList),
+}
+
+impl ForwardAttrs {
+    /// Returns `true` if this will not forward any attributes.
+    pub fn is_empty(&self) -> bool {
+        match *self {
+            ForwardAttrs::All => false,
+            ForwardAttrs::Only(ref list) => list.is_empty(),
+        }
+    }
+}
+
+impl FromMeta for ForwardAttrs {
+    fn from_word() -> Result<Self> {
+        Ok(ForwardAttrs::All)
+    }
+
+    fn from_list(nested: &[NestedMeta]) -> Result<Self> {
+        Ok(ForwardAttrs::Only(PathList::from_list(nested)?))
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/from_attributes.rs b/third_party/rust/darling_core/src/options/from_attributes.rs
new file mode 100644
index 0000000000..0a133d1b24
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/from_attributes.rs
@@ -0,0 +1,69 @@
+use quote::ToTokens;
+
+use crate::{ast::Data, codegen::FromAttributesImpl, Error, Result};
+
+use super::{OuterFrom, ParseAttribute, ParseData};
+
+/// Receiver for derived `FromAttributes` impls.
+pub struct FromAttributesOptions {
+    // Note: FromAttributes has no behaviors beyond those common
+    // to all the `OuterFrom` traits.
+    pub base: OuterFrom,
+}
+
+impl FromAttributesOptions {
+    pub fn new(di: &syn::DeriveInput) -> Result<Self> {
+        let opts = (Self {
+            base: OuterFrom::start(di)?,
+        })
+        .parse_attributes(&di.attrs)?
+        .parse_body(&di.data)?;
+
+        if !opts.is_newtype() && opts.base.attr_names.is_empty() {
+            Err(Error::custom(
+                "FromAttributes without attributes collects nothing",
+            ))
+        } else {
+            Ok(opts)
+        }
+    }
+
+    fn is_newtype(&self) -> bool {
+        if let Data::Struct(ref data) = self.base.container.data {
+            data.is_newtype()
+        } else {
+            false
+        }
+    }
+}
+
+impl ParseAttribute for FromAttributesOptions {
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()> {
+        self.base.parse_nested(mi)
+    }
+}
+
+impl ParseData for FromAttributesOptions {
+    fn parse_variant(&mut self, variant: &syn::Variant) -> Result<()> {
+        self.base.parse_variant(variant)
+    }
+
+    fn parse_field(&mut self, field: &syn::Field) -> Result<()> {
+        self.base.parse_field(field)
+    }
+}
+
+impl<'a> From<&'a FromAttributesOptions> for FromAttributesImpl<'a> {
+    fn from(v: &'a FromAttributesOptions) -> Self {
+        FromAttributesImpl {
+            base: (&v.base.container).into(),
+            attr_names: &v.base.attr_names,
+        }
+    }
+}
+
+impl ToTokens for FromAttributesOptions {
+    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
+        FromAttributesImpl::from(self).to_tokens(tokens)
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/from_derive.rs b/third_party/rust/darling_core/src/options/from_derive.rs
new file mode 100644
index 0000000000..0f1e1c4d84
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/from_derive.rs
@@ -0,0 +1,100 @@
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+use syn::Ident;
+
+use crate::codegen::FromDeriveInputImpl;
+use crate::options::{DeriveInputShapeSet, OuterFrom, ParseAttribute, ParseData};
+use crate::{FromMeta, Result};
+
+#[derive(Debug)]
+pub struct FdiOptions {
+    pub base: OuterFrom,
+
+    /// The field on the target struct which should receive the type visibility, if any.
+    pub vis: Option<Ident>,
+
+    /// The field on the target struct which should receive the type generics, if any.
+    pub generics: Option<Ident>,
+
+    pub data: Option<Ident>,
+
+    pub supports: Option<DeriveInputShapeSet>,
+}
+
+impl FdiOptions {
+    pub fn new(di: &syn::DeriveInput) -> Result<Self> {
+        (FdiOptions {
+            base: OuterFrom::start(di)?,
+            vis: Default::default(),
+            generics: Default::default(),
+            data: Default::default(),
+            supports: Default::default(),
+        })
+        .parse_attributes(&di.attrs)?
+        .parse_body(&di.data)
+    }
+}
+
+impl ParseAttribute for FdiOptions {
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()> {
+        if mi.path().is_ident("supports") {
+            self.supports = FromMeta::from_meta(mi)?;
+            Ok(())
+        } else {
+            self.base.parse_nested(mi)
+        }
+    }
+}
+
+impl ParseData for FdiOptions {
+    fn parse_variant(&mut self, variant: &syn::Variant) -> Result<()> {
+        self.base.parse_variant(variant)
+    }
+
+    fn parse_field(&mut self, field: &syn::Field) -> Result<()> {
+        match field
+            .ident
+            .as_ref()
+            .map(|v| v.to_string())
+            .as_ref()
+            .map(|v| v.as_str())
+        {
+            Some("vis") => {
+                self.vis = field.ident.clone();
+                Ok(())
+            }
+            Some("data") => {
+                self.data = field.ident.clone();
+                Ok(())
+            }
+            Some("generics") => {
+                self.generics = field.ident.clone();
+                Ok(())
+            }
+            _ => self.base.parse_field(field),
+        }
+    }
+}
+
+impl<'a> From<&'a FdiOptions> for FromDeriveInputImpl<'a> {
+    fn from(v: &'a FdiOptions) -> Self {
+        FromDeriveInputImpl {
+            base: (&v.base.container).into(),
+            attr_names: &v.base.attr_names,
+            from_ident: v.base.from_ident,
+            ident: v.base.ident.as_ref(),
+            vis: v.vis.as_ref(),
+            data: v.data.as_ref(),
+            generics: v.generics.as_ref(),
+            attrs: v.base.attrs.as_ref(),
+            forward_attrs: v.base.forward_attrs.as_ref(),
+            supports: v.supports.as_ref(),
+        }
+    }
+}
+
+impl ToTokens for FdiOptions {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        FromDeriveInputImpl::from(self).to_tokens(tokens)
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/from_field.rs b/third_party/rust/darling_core/src/options/from_field.rs
new file mode 100644
index 0000000000..fcfab75592
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/from_field.rs
@@ -0,0 +1,79 @@
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+use syn::Ident;
+
+use crate::codegen::FromFieldImpl;
+use crate::options::{OuterFrom, ParseAttribute, ParseData};
+use crate::Result;
+
+#[derive(Debug)]
+pub struct FromFieldOptions {
+    pub base: OuterFrom,
+    pub vis: Option<Ident>,
+    pub ty: Option<Ident>,
+}
+
+impl FromFieldOptions {
+    pub fn new(di: &syn::DeriveInput) -> Result<Self> {
+        (FromFieldOptions {
+            base: OuterFrom::start(di)?,
+            vis: Default::default(),
+            ty: Default::default(),
+        })
+        .parse_attributes(&di.attrs)?
+        .parse_body(&di.data)
+    }
+}
+
+impl ParseAttribute for FromFieldOptions {
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()> {
+        self.base.parse_nested(mi)
+    }
+}
+
+impl ParseData for FromFieldOptions {
+    fn parse_variant(&mut self, variant: &syn::Variant) -> Result<()> {
+        self.base.parse_variant(variant)
+    }
+
+    fn parse_field(&mut self, field: &syn::Field) -> Result<()> {
+        match field
+            .ident
+            .as_ref()
+            .map(|v| v.to_string())
+            .as_ref()
+            .map(|v| v.as_str())
+        {
+            Some("vis") => {
+                self.vis = field.ident.clone();
+                Ok(())
+            }
+            Some("ty") => {
+                self.ty = field.ident.clone();
+                Ok(())
+            }
+            _ => self.base.parse_field(field),
+        }
+    }
+}
+
+impl<'a> From<&'a FromFieldOptions> for FromFieldImpl<'a> {
+    fn from(v: &'a FromFieldOptions) -> Self {
+        FromFieldImpl {
+            ident: v.base.ident.as_ref(),
+            vis: v.vis.as_ref(),
+            ty: v.ty.as_ref(),
+            attrs: v.base.attrs.as_ref(),
+            base: (&v.base.container).into(),
+            attr_names: &v.base.attr_names,
+            forward_attrs: v.base.forward_attrs.as_ref(),
+            from_ident: v.base.from_ident,
+        }
+    }
+}
+
+impl ToTokens for FromFieldOptions {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        FromFieldImpl::from(self).to_tokens(tokens)
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/from_meta.rs b/third_party/rust/darling_core/src/options/from_meta.rs
new file mode 100644
index 0000000000..18c2bfa676
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/from_meta.rs
@@ -0,0 +1,50 @@
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+
+use crate::codegen::FromMetaImpl;
+use crate::options::{Core, ParseAttribute, ParseData};
+use crate::Result;
+
+pub struct FromMetaOptions {
+    base: Core,
+}
+
+impl FromMetaOptions {
+    pub fn new(di: &syn::DeriveInput) -> Result<Self> {
+        (FromMetaOptions {
+            base: Core::start(di)?,
+        })
+        .parse_attributes(&di.attrs)?
+        .parse_body(&di.data)
+    }
+}
+
+impl ParseAttribute for FromMetaOptions {
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()> {
+        self.base.parse_nested(mi)
+    }
+}
+
+impl ParseData for FromMetaOptions {
+    fn parse_variant(&mut self, variant: &syn::Variant) -> Result<()> {
+        self.base.parse_variant(variant)
+    }
+
+    fn parse_field(&mut self, field: &syn::Field) -> Result<()> {
+        self.base.parse_field(field)
+    }
+}
+
+impl<'a> From<&'a FromMetaOptions> for FromMetaImpl<'a> {
+    fn from(v: &'a FromMetaOptions) -> Self {
+        FromMetaImpl {
+            base: (&v.base).into(),
+        }
+    }
+}
+
+impl ToTokens for FromMetaOptions {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        FromMetaImpl::from(self).to_tokens(tokens)
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/from_type_param.rs b/third_party/rust/darling_core/src/options/from_type_param.rs
new file mode 100644
index 0000000000..5ab3241e25
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/from_type_param.rs
@@ -0,0 +1,79 @@
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+use syn::Ident;
+
+use crate::codegen::FromTypeParamImpl;
+use crate::options::{OuterFrom, ParseAttribute, ParseData};
+use crate::Result;
+
+#[derive(Debug)]
+pub struct FromTypeParamOptions {
+    pub base: OuterFrom,
+    pub bounds: Option<Ident>,
+    pub default: Option<Ident>,
+}
+
+impl FromTypeParamOptions {
+    pub fn new(di: &syn::DeriveInput) -> Result<Self> {
+        (FromTypeParamOptions {
+            base: OuterFrom::start(di)?,
+            bounds: None,
+            default: None,
+        })
+        .parse_attributes(&di.attrs)?
+        .parse_body(&di.data)
+    }
+}
+
+impl ParseAttribute for FromTypeParamOptions {
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()> {
+        self.base.parse_nested(mi)
+    }
+}
+
+impl ParseData for FromTypeParamOptions {
+    fn parse_variant(&mut self, variant: &syn::Variant) -> Result<()> {
+        self.base.parse_variant(variant)
+    }
+
+    fn parse_field(&mut self, field: &syn::Field) -> Result<()> {
+        match field
+            .ident
+            .as_ref()
+            .map(|v| v.to_string())
+            .as_ref()
+            .map(|v| v.as_str())
+        {
+            Some("bounds") => {
+                self.bounds = field.ident.clone();
+                Ok(())
+            }
+            Some("default") => {
+                self.default = field.ident.clone();
+                Ok(())
+            }
+            _ => self.base.parse_field(field),
+        }
+    }
+}
+
+impl<'a> From<&'a FromTypeParamOptions> for FromTypeParamImpl<'a> {
+    fn from(v: &'a FromTypeParamOptions) -> Self {
+        FromTypeParamImpl {
+            base: (&v.base.container).into(),
+            ident: v.base.ident.as_ref(),
+            attrs: v.base.attrs.as_ref(),
+            bounds: v.bounds.as_ref(),
+            default: v.default.as_ref(),
+            attr_names: &v.base.attr_names,
+            forward_attrs: v.base.forward_attrs.as_ref(),
+            from_ident: v.base.from_ident,
+        }
+    }
+}
+
+impl ToTokens for FromTypeParamOptions {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        FromTypeParamImpl::from(self).to_tokens(tokens)
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/from_variant.rs b/third_party/rust/darling_core/src/options/from_variant.rs
new file mode 100644
index 0000000000..fabdae17f7
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/from_variant.rs
@@ -0,0 +1,85 @@
+use proc_macro2::TokenStream;
+use quote::ToTokens;
+use syn::{DeriveInput, Field, Ident, Meta};
+
+use crate::codegen::FromVariantImpl;
+use crate::options::{DataShape, OuterFrom, ParseAttribute, ParseData};
+use crate::{FromMeta, Result};
+
+#[derive(Debug, Clone)]
+pub struct FromVariantOptions {
+    pub base: OuterFrom,
+    /// The field on the deriving struct into which the discriminant expression
+    /// should be placed by the derived `FromVariant` impl.
+    pub discriminant: Option<Ident>,
+    pub fields: Option<Ident>,
+    pub supports: Option<DataShape>,
+}
+
+impl FromVariantOptions {
+    pub fn new(di: &DeriveInput) -> Result<Self> {
+        (FromVariantOptions {
+            base: OuterFrom::start(di)?,
+            discriminant: Default::default(),
+            fields: Default::default(),
+            supports: Default::default(),
+        })
+        .parse_attributes(&di.attrs)?
+        .parse_body(&di.data)
+    }
+}
+
+impl<'a> From<&'a FromVariantOptions> for FromVariantImpl<'a> {
+    fn from(v: &'a FromVariantOptions) -> Self {
+        FromVariantImpl {
+            base: (&v.base.container).into(),
+            ident: v.base.ident.as_ref(),
+            discriminant: v.discriminant.as_ref(),
+            fields: v.fields.as_ref(),
+            attrs: v.base.attrs.as_ref(),
+            attr_names: &v.base.attr_names,
+            forward_attrs: v.base.forward_attrs.as_ref(),
+            from_ident: v.base.from_ident,
+            supports: v.supports.as_ref(),
+        }
+    }
+}
+
+impl ParseAttribute for FromVariantOptions {
+    fn parse_nested(&mut self, mi: &Meta) -> Result<()> {
+        if mi.path().is_ident("supports") {
+            self.supports = FromMeta::from_meta(mi)?;
+            Ok(())
+        } else {
+            self.base.parse_nested(mi)
+        }
+    }
+}
+
+impl ParseData for FromVariantOptions {
+    fn parse_field(&mut self, field: &Field) -> Result<()> {
+        match field
+            .ident
+            .as_ref()
+            .map(|v| v.to_string())
+            .as_ref()
+            .map(|v| v.as_str())
+        {
+            Some("discriminant") => {
+                self.discriminant = field.ident.clone();
+                Ok(())
+            }
+            Some("fields") => {
+                self.fields = field.ident.clone();
+                Ok(())
+            }
+            _ => self.base.parse_field(field),
+        }
+    }
+}
+
+impl ToTokens for FromVariantOptions {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        FromVariantImpl::from(self).to_tokens(tokens)
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/input_field.rs b/third_party/rust/darling_core/src/options/input_field.rs
new file mode 100644
index 0000000000..27fe805f98
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/input_field.rs
@@ -0,0 +1,180 @@
+use std::borrow::Cow;
+
+use syn::{parse_quote_spanned, spanned::Spanned};
+
+use crate::codegen;
+use crate::options::{Core, DefaultExpression, ParseAttribute};
+use crate::util::SpannedValue;
+use crate::{Error, FromMeta, Result};
+
+#[derive(Debug, Clone)]
+pub struct InputField {
+    pub ident: syn::Ident,
+    pub attr_name: Option<String>,
+    pub ty: syn::Type,
+    pub default: Option<DefaultExpression>,
+    pub with: Option<syn::Path>,
+
+    /// If `true`, generated code will not look for this field in the input meta item,
+    /// instead always falling back to either `InputField::default` or `Default::default`.
+    pub skip: Option<SpannedValue<bool>>,
+    pub post_transform: Option<codegen::PostfixTransform>,
+    pub multiple: Option<bool>,
+}
+
+impl InputField {
+    /// Generate a view into this field that can be used for code generation.
+    pub fn as_codegen_field(&self) -> codegen::Field<'_> {
+        codegen::Field {
+            ident: &self.ident,
+            name_in_attr: self
+                .attr_name
+                .as_ref()
+                .map_or_else(|| Cow::Owned(self.ident.to_string()), Cow::Borrowed),
+            ty: &self.ty,
+            default_expression: self.as_codegen_default(),
+            with_path: self.with.as_ref().map_or_else(
+                || {
+                    Cow::Owned(
+                        parse_quote_spanned!(self.ty.span()=> ::darling::FromMeta::from_meta),
+                    )
+                },
+                Cow::Borrowed,
+            ),
+            skip: *self.skip.unwrap_or_default(),
+            post_transform: self.post_transform.as_ref(),
+            multiple: self.multiple.unwrap_or_default(),
+        }
+    }
+
+    /// Generate a codegen::DefaultExpression for this field. This requires the field name
+    /// in the `Inherit` case.
+    fn as_codegen_default(&self) -> Option<codegen::DefaultExpression<'_>> {
+        self.default.as_ref().map(|expr| match *expr {
+            DefaultExpression::Explicit(ref path) => codegen::DefaultExpression::Explicit(path),
+            DefaultExpression::Inherit => codegen::DefaultExpression::Inherit(&self.ident),
+            DefaultExpression::Trait { span } => codegen::DefaultExpression::Trait { span },
+        })
+    }
+
+    fn new(ident: syn::Ident, ty: syn::Type) -> Self {
+        InputField {
+            ident,
+            ty,
+            attr_name: None,
+            default: None,
+            with: None,
+            skip: None,
+            post_transform: Default::default(),
+            multiple: None,
+        }
+    }
+
+    pub fn from_field(f: &syn::Field, parent: Option<&Core>) -> Result<Self> {
+        let ident = f
+            .ident
+            .clone()
+            .unwrap_or_else(|| syn::Ident::new("__unnamed", ::proc_macro2::Span::call_site()));
+        let ty = f.ty.clone();
+        let base = Self::new(ident, ty).parse_attributes(&f.attrs)?;
+
+        Ok(if let Some(container) = parent {
+            base.with_inherited(container)
+        } else {
+            base
+        })
+    }
+
+    /// Apply inherited settings from the container. This is done _after_ parsing
+    /// to ensure deference to explicit field-level settings.
+    fn with_inherited(mut self, parent: &Core) -> Self {
+        // explicit renamings take precedence over rename rules on the container,
+        // but in the absence of an explicit name we apply the rule.
+        if self.attr_name.is_none() {
+            self.attr_name = Some(parent.rename_rule.apply_to_field(self.ident.to_string()));
+        }
+
+        // Determine the default expression for this field, based on three pieces of information:
+        // 1. Will we look for this field in the attribute?
+        // 1. Is there a locally-defined default?
+        // 1. Did the parent define a default?
+        self.default = match (&self.skip, self.default.is_some(), parent.default.is_some()) {
+            // If we have a default, use it.
+            (_, true, _) => self.default,
+
+            // If there isn't an explicit default but the struct sets a default, we'll
+            // inherit from that.
+            (_, false, true) => Some(DefaultExpression::Inherit),
+
+            // If we're skipping the field and no defaults have been expressed then we should
+            // use the ::darling::export::Default trait, and set the span to the skip keyword
+            // so that an error caused by the skipped field's type not implementing `Default`
+            // will correctly identify why darling is trying to use `Default`.
+            (Some(v), false, false) if **v => Some(DefaultExpression::Trait { span: v.span() }),
+
+            // If we don't have or need a default, then leave it blank.
+            (_, false, false) => None,
+        };
+
+        self
+    }
+}
+
+impl ParseAttribute for InputField {
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()> {
+        let path = mi.path();
+
+        if path.is_ident("rename") {
+            if self.attr_name.is_some() {
+                return Err(Error::duplicate_field_path(path).with_span(mi));
+            }
+
+            self.attr_name = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("default") {
+            if self.default.is_some() {
+                return Err(Error::duplicate_field_path(path).with_span(mi));
+            }
+            self.default = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("with") {
+            if self.with.is_some() {
+                return Err(Error::duplicate_field_path(path).with_span(mi));
+            }
+
+            self.with = Some(FromMeta::from_meta(mi)?);
+        } else if path.is_ident("skip") {
+            if self.skip.is_some() {
+                return Err(Error::duplicate_field_path(path).with_span(mi));
+            }
+
+            self.skip = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("map") || path.is_ident("and_then") {
+            let transformer = path.get_ident().unwrap().clone();
+            if let Some(post_transform) = &self.post_transform {
+                if transformer == post_transform.transformer {
+                    return Err(Error::duplicate_field_path(path).with_span(mi));
+                } else {
+                    return Err(Error::custom(format!(
+                        "Options `{}` and `{}` are mutually exclusive",
+                        transformer, post_transform.transformer
+                    ))
+                    .with_span(mi));
+                }
+            }
+
+            self.post_transform = Some(codegen::PostfixTransform::new(
+                transformer,
+                FromMeta::from_meta(mi)?,
+            ));
+        } else if path.is_ident("multiple") {
+            if self.multiple.is_some() {
+                return Err(Error::duplicate_field_path(path).with_span(mi));
+            }
+
+            self.multiple = FromMeta::from_meta(mi)?;
+        } else {
+            return Err(Error::unknown_field_path(path).with_span(mi));
+        }
+
+        Ok(())
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/input_variant.rs b/third_party/rust/darling_core/src/options/input_variant.rs
new file mode 100644
index 0000000000..5fab28ffac
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/input_variant.rs
@@ -0,0 +1,104 @@
+use std::borrow::Cow;
+
+use crate::ast::Fields;
+use crate::codegen;
+use crate::options::{Core, InputField, ParseAttribute};
+use crate::{Error, FromMeta, Result};
+
+#[derive(Debug, Clone)]
+pub struct InputVariant {
+    ident: syn::Ident,
+    attr_name: Option<String>,
+    data: Fields<InputField>,
+    skip: Option<bool>,
+    /// Whether or not unknown fields are acceptable in this
+    allow_unknown_fields: Option<bool>,
+}
+
+impl InputVariant {
+    pub fn as_codegen_variant<'a>(&'a self, ty_ident: &'a syn::Ident) -> codegen::Variant<'a> {
+        codegen::Variant {
+            ty_ident,
+            variant_ident: &self.ident,
+            name_in_attr: self
+                .attr_name
+                .as_ref()
+                .map_or_else(|| Cow::Owned(self.ident.to_string()), Cow::Borrowed),
+            data: self.data.as_ref().map(InputField::as_codegen_field),
+            skip: self.skip.unwrap_or_default(),
+            allow_unknown_fields: self.allow_unknown_fields.unwrap_or_default(),
+        }
+    }
+
+    pub fn from_variant(v: &syn::Variant, parent: Option<&Core>) -> Result<Self> {
+        let mut starter = (InputVariant {
+            ident: v.ident.clone(),
+            attr_name: Default::default(),
+            data: Fields::empty_from(&v.fields),
+            skip: Default::default(),
+            allow_unknown_fields: None,
+        })
+        .parse_attributes(&v.attrs)?;
+
+        starter.data.fields = match v.fields {
+            syn::Fields::Unit => vec![],
+            syn::Fields::Unnamed(ref fields) => {
+                let mut items = Vec::with_capacity(fields.unnamed.len());
+                for item in &fields.unnamed {
+                    items.push(InputField::from_field(item, parent)?);
+                }
+
+                items
+            }
+            syn::Fields::Named(ref fields) => {
+                let mut items = Vec::with_capacity(fields.named.len());
+                for item in &fields.named {
+                    items.push(InputField::from_field(item, parent)?);
+                }
+
+                items
+            }
+        };
+
+        Ok(if let Some(p) = parent {
+            starter.with_inherited(p)
+        } else {
+            starter
+        })
+    }
+
+    fn with_inherited(mut self, parent: &Core) -> Self {
+        if self.attr_name.is_none() {
+            self.attr_name = Some(parent.rename_rule.apply_to_variant(self.ident.to_string()));
+        }
+
+        if self.allow_unknown_fields.is_none() {
+            self.allow_unknown_fields = Some(parent.allow_unknown_fields.unwrap_or_default());
+        }
+
+        self
+    }
+}
+
+impl ParseAttribute for InputVariant {
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()> {
+        let path = mi.path();
+        if path.is_ident("rename") {
+            if self.attr_name.is_some() {
+                return Err(Error::duplicate_field_path(path).with_span(mi));
+            }
+
+            self.attr_name = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("skip") {
+            if self.skip.is_some() {
+                return Err(Error::duplicate_field_path(path).with_span(mi));
+            }
+
+            self.skip = FromMeta::from_meta(mi)?;
+        } else {
+            return Err(Error::unknown_field_path(path).with_span(mi));
+        }
+
+        Ok(())
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/mod.rs b/third_party/rust/darling_core/src/options/mod.rs
new file mode 100644
index 0000000000..d8a147d8ed
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/mod.rs
@@ -0,0 +1,145 @@
+use proc_macro2::Span;
+use syn::{parse_quote, spanned::Spanned};
+
+use crate::{Error, FromMeta, Result};
+
+mod core;
+mod forward_attrs;
+mod from_attributes;
+mod from_derive;
+mod from_field;
+mod from_meta;
+mod from_type_param;
+mod from_variant;
+mod input_field;
+mod input_variant;
+mod outer_from;
+mod shape;
+
+pub use self::core::Core;
+pub use self::forward_attrs::ForwardAttrs;
+pub use self::from_attributes::FromAttributesOptions;
+pub use self::from_derive::FdiOptions;
+pub use self::from_field::FromFieldOptions;
+pub use self::from_meta::FromMetaOptions;
+pub use self::from_type_param::FromTypeParamOptions;
+pub use self::from_variant::FromVariantOptions;
+pub use self::input_field::InputField;
+pub use self::input_variant::InputVariant;
+pub use self::outer_from::OuterFrom;
+pub use self::shape::{DataShape, DeriveInputShapeSet};
+
+/// A default/fallback expression encountered in attributes during parsing.
+#[derive(Debug, Clone)]
+pub enum DefaultExpression {
+    /// The value should be taken from the `default` instance of the containing struct.
+    /// This is not valid in container options.
+    Inherit,
+    Explicit(syn::Path),
+    Trait {
+        /// The input span that is responsible for the use of `Default::default`.
+        span: Span,
+    },
+}
+
+#[doc(hidden)]
+impl FromMeta for DefaultExpression {
+    // Note: This cannot use `from_word` as it needs to capture the span
+    // in the `Meta::Path` case.
+    fn from_meta(item: &syn::Meta) -> Result<Self> {
+        match item {
+            syn::Meta::Path(_) => Ok(DefaultExpression::Trait { span: item.span() }),
+            syn::Meta::List(nm) => Err(Error::unsupported_format("list").with_span(nm)),
+            syn::Meta::NameValue(nv) => Self::from_value(&nv.lit),
+        }
+    }
+
+    fn from_value(value: &syn::Lit) -> Result<Self> {
+        syn::Path::from_value(value).map(DefaultExpression::Explicit)
+    }
+}
+
+/// Middleware for extracting attribute values. Implementers are expected to override
+/// `parse_nested` so they can apply individual items to themselves, while `parse_attributes`
+/// is responsible for looping through distinct outer attributes and collecting errors.
+pub trait ParseAttribute: Sized {
+    fn parse_attributes(mut self, attrs: &[syn::Attribute]) -> Result<Self> {
+        let mut errors = Error::accumulator();
+        for attr in attrs {
+            if attr.path == parse_quote!(darling) {
+                errors.handle(parse_attr(attr, &mut self));
+            }
+        }
+
+        errors.finish_with(self)
+    }
+
+    /// Read a meta-item, and apply its values to the current instance.
+    fn parse_nested(&mut self, mi: &syn::Meta) -> Result<()>;
+}
+
+fn parse_attr<T: ParseAttribute>(attr: &syn::Attribute, target: &mut T) -> Result<()> {
+    let mut errors = Error::accumulator();
+    match attr.parse_meta().ok() {
+        Some(syn::Meta::List(data)) => {
+            for item in data.nested {
+                if let syn::NestedMeta::Meta(ref mi) = item {
+                    errors.handle(target.parse_nested(mi));
+                } else {
+                    panic!("Wasn't able to parse: `{:?}`", item);
+                }
+            }
+
+            errors.finish()
+        }
+        Some(ref item) => panic!("Wasn't able to parse: `{:?}`", item),
+        None => panic!("Unable to parse {:?}", attr),
+    }
+}
+
+/// Middleware for extracting values from the body of the derive input. Implementers are
+/// expected to override `parse_field` or `parse_variant` as appropriate for their use-case,
+/// while `parse_body` dispatches to the appropriate methods and handles error collection.
+pub trait ParseData: Sized {
+    fn parse_body(mut self, body: &syn::Data) -> Result<Self> {
+        use syn::{Data, Fields};
+
+        let mut errors = Error::accumulator();
+
+        match *body {
+            Data::Struct(ref data) => match data.fields {
+                Fields::Unit => {}
+                Fields::Named(ref fields) => {
+                    for field in &fields.named {
+                        errors.handle(self.parse_field(field));
+                    }
+                }
+                Fields::Unnamed(ref fields) => {
+                    for field in &fields.unnamed {
+                        errors.handle(self.parse_field(field));
+                    }
+                }
+            },
+            Data::Enum(ref data) => {
+                for variant in &data.variants {
+                    errors.handle(self.parse_variant(variant));
+                }
+            }
+            Data::Union(_) => unreachable!(),
+        };
+
+        errors.finish_with(self)
+    }
+
+    /// Apply the next found variant to the object, returning an error
+    /// if parsing goes wrong.
+    fn parse_variant(&mut self, variant: &syn::Variant) -> Result<()> {
+        Err(Error::unsupported_format("enum variant").with_span(variant))
+    }
+
+    /// Apply the next found struct field to the object, returning an error
+    /// if parsing goes wrong.
+    fn parse_field(&mut self, field: &syn::Field) -> Result<()> {
+        Err(Error::unsupported_format("struct field").with_span(field))
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/outer_from.rs b/third_party/rust/darling_core/src/options/outer_from.rs
new file mode 100644
index 0000000000..0a8088a096
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/outer_from.rs
@@ -0,0 +1,87 @@
+use syn::spanned::Spanned;
+use syn::{Field, Ident, Meta};
+
+use crate::options::{Core, DefaultExpression, ForwardAttrs, ParseAttribute, ParseData};
+use crate::util::PathList;
+use crate::{FromMeta, Result};
+
+/// Reusable base for `FromDeriveInput`, `FromVariant`, `FromField`, and other top-level
+/// `From*` traits.
+#[derive(Debug, Clone)]
+pub struct OuterFrom {
+    /// The field on the target struct which should receive the type identifier, if any.
+    pub ident: Option<Ident>,
+
+    /// The field on the target struct which should receive the type attributes, if any.
+    pub attrs: Option<Ident>,
+
+    pub container: Core,
+
+    /// The attribute names that should be searched.
+    pub attr_names: PathList,
+
+    /// The attribute names that should be forwarded. The presence of the word with no additional
+    /// filtering will cause _all_ attributes to be cloned and exposed to the struct after parsing.
+    pub forward_attrs: Option<ForwardAttrs>,
+
+    /// Whether or not the container can be made through conversion from the type `Ident`.
+    pub from_ident: bool,
+}
+
+impl OuterFrom {
+    pub fn start(di: &syn::DeriveInput) -> Result<Self> {
+        Ok(OuterFrom {
+            container: Core::start(di)?,
+            attrs: Default::default(),
+            ident: Default::default(),
+            attr_names: Default::default(),
+            forward_attrs: Default::default(),
+            from_ident: Default::default(),
+        })
+    }
+}
+
+impl ParseAttribute for OuterFrom {
+    fn parse_nested(&mut self, mi: &Meta) -> Result<()> {
+        let path = mi.path();
+        if path.is_ident("attributes") {
+            self.attr_names = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("forward_attrs") {
+            self.forward_attrs = FromMeta::from_meta(mi)?;
+        } else if path.is_ident("from_ident") {
+            // HACK: Declaring that a default is present will cause fields to
+            // generate correct code, but control flow isn't that obvious.
+            self.container.default = Some(DefaultExpression::Trait {
+                // Use the span of the `from_ident` keyword so that errors in generated code
+                // caused by this will point back to the correct location.
+                span: path.span(),
+            });
+            self.from_ident = true;
+        } else {
+            return self.container.parse_nested(mi);
+        }
+        Ok(())
+    }
+}
+
+impl ParseData for OuterFrom {
+    fn parse_field(&mut self, field: &Field) -> Result<()> {
+        match field
+            .ident
+            .as_ref()
+            .map(|v| v.to_string())
+            .as_ref()
+            .map(|v| v.as_str())
+        {
+            Some("ident") => {
+                self.ident = field.ident.clone();
+                Ok(())
+            }
+            Some("attrs") => {
+                self.attrs = field.ident.clone();
+                Ok(())
+            }
+            _ => self.container.parse_field(field),
+        }
+    }
+}
diff --git a/third_party/rust/darling_core/src/options/shape.rs b/third_party/rust/darling_core/src/options/shape.rs
new file mode 100644
index 0000000000..5245a00d93
--- /dev/null
+++ b/third_party/rust/darling_core/src/options/shape.rs
@@ -0,0 +1,258 @@
+//! Types for "shape" validation. This allows types deriving `FromDeriveInput` etc. to declare
+//! that they only work on - for example - structs with named fields, or newtype enum variants.
+
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens, TokenStreamExt};
+use syn::{parse_quote, Meta, NestedMeta};
+
+use crate::{Error, FromMeta, Result};
+
+/// Receiver struct for shape validation. Shape validation allows a deriving type
+/// to declare that it only accepts - for example - named structs, or newtype enum
+/// variants.
+///
+/// ```rust,ignore
+/// #[ignore(any, struct_named, enum_newtype)]
+/// ```
+#[derive(Debug, Clone)]
+pub struct DeriveInputShapeSet {
+    enum_values: DataShape,
+    struct_values: DataShape,
+    any: bool,
+}
+
+impl Default for DeriveInputShapeSet {
+    fn default() -> Self {
+        DeriveInputShapeSet {
+            enum_values: DataShape::new("enum_"),
+            struct_values: DataShape::new("struct_"),
+            any: Default::default(),
+        }
+    }
+}
+
+impl FromMeta for DeriveInputShapeSet {
+    fn from_list(items: &[NestedMeta]) -> Result<Self> {
+        let mut new = DeriveInputShapeSet::default();
+        for item in items {
+            if let NestedMeta::Meta(Meta::Path(ref path)) = *item {
+                let ident = &path.segments.first().unwrap().ident;
+                let word = ident.to_string();
+                if word == "any" {
+                    new.any = true;
+                } else if word.starts_with("enum_") {
+                    new.enum_values
+                        .set_word(&word)
+                        .map_err(|e| e.with_span(&ident))?;
+                } else if word.starts_with("struct_") {
+                    new.struct_values
+                        .set_word(&word)
+                        .map_err(|e| e.with_span(&ident))?;
+                } else {
+                    return Err(Error::unknown_value(&word).with_span(&ident));
+                }
+            } else {
+                return Err(Error::unsupported_format("non-word").with_span(item));
+            }
+        }
+
+        Ok(new)
+    }
+}
+
+impl ToTokens for DeriveInputShapeSet {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let fn_body = if self.any {
+            quote!(::darling::export::Ok(()))
+        } else {
+            let en = &self.enum_values;
+            let st = &self.struct_values;
+
+            quote! {
+                {
+                    let struct_check = #st;
+                    let enum_check = #en;
+
+                    match *__body {
+                        ::darling::export::syn::Data::Enum(ref data) => {
+                            if enum_check.is_empty() {
+                                return ::darling::export::Err(
+                                    ::darling::Error::unsupported_shape_with_expected("enum", &format!("struct with {}", struct_check))
+                                );
+                            }
+
+                            let mut variant_errors = ::darling::Error::accumulator();
+                            for variant in &data.variants {
+                                variant_errors.handle(enum_check.check(variant));
+                            }
+
+                            variant_errors.finish()
+                        }
+                        ::darling::export::syn::Data::Struct(ref struct_data) => {
+                            if struct_check.is_empty() {
+                                return ::darling::export::Err(
+                                    ::darling::Error::unsupported_shape_with_expected("struct", &format!("enum with {}", enum_check))
+                                );
+                            }
+
+                            struct_check.check(struct_data)
+                        }
+                        ::darling::export::syn::Data::Union(_) => unreachable!(),
+                    }
+                }
+            }
+        };
+
+        tokens.append_all(quote! {
+            #[allow(unused_variables)]
+            fn __validate_body(__body: &::darling::export::syn::Data) -> ::darling::Result<()> {
+                #fn_body
+            }
+        });
+    }
+}
+
+/// Receiver for shape information within a struct or enum context. See `Shape` for more information
+/// on valid uses of shape validation.
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct DataShape {
+    /// The kind of shape being described. This can be `struct_` or `enum_`.
+    prefix: &'static str,
+    newtype: bool,
+    named: bool,
+    tuple: bool,
+    unit: bool,
+    any: bool,
+}
+
+impl DataShape {
+    fn new(prefix: &'static str) -> Self {
+        DataShape {
+            prefix,
+            ..Default::default()
+        }
+    }
+
+    fn set_word(&mut self, word: &str) -> Result<()> {
+        match word.trim_start_matches(self.prefix) {
+            "newtype" => {
+                self.newtype = true;
+                Ok(())
+            }
+            "named" => {
+                self.named = true;
+                Ok(())
+            }
+            "tuple" => {
+                self.tuple = true;
+                Ok(())
+            }
+            "unit" => {
+                self.unit = true;
+                Ok(())
+            }
+            "any" => {
+                self.any = true;
+                Ok(())
+            }
+            _ => Err(Error::unknown_value(word)),
+        }
+    }
+}
+
+impl FromMeta for DataShape {
+    fn from_list(items: &[NestedMeta]) -> Result<Self> {
+        let mut errors = Error::accumulator();
+        let mut new = DataShape::default();
+
+        for item in items {
+            if let NestedMeta::Meta(Meta::Path(ref path)) = *item {
+                errors.handle(new.set_word(&path.segments.first().unwrap().ident.to_string()));
+            } else {
+                errors.push(Error::unsupported_format("non-word").with_span(item));
+            }
+        }
+
+        errors.finish_with(new)
+    }
+}
+
+impl ToTokens for DataShape {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let Self {
+            any,
+            named,
+            tuple,
+            unit,
+            newtype,
+            ..
+        } = *self;
+
+        let shape_path: syn::Path = parse_quote!(::darling::util::Shape);
+
+        let mut shapes = vec![];
+        if any || named {
+            shapes.push(quote!(#shape_path::Named));
+        }
+
+        if any || tuple {
+            shapes.push(quote!(#shape_path::Tuple));
+        }
+
+        if any || newtype {
+            shapes.push(quote!(#shape_path::Newtype));
+        }
+
+        if any || unit {
+            shapes.push(quote!(#shape_path::Unit));
+        }
+
+        tokens.append_all(quote! {
+            ::darling::util::ShapeSet::new(vec![#(#shapes),*])
+        });
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use proc_macro2::TokenStream;
+    use quote::quote;
+    use syn::parse_quote;
+
+    use super::DeriveInputShapeSet;
+    use crate::FromMeta;
+
+    /// parse a string as a syn::Meta instance.
+    fn pm(tokens: TokenStream) -> ::std::result::Result<syn::Meta, String> {
+        let attribute: syn::Attribute = parse_quote!(#[#tokens]);
+        attribute.parse_meta().map_err(|_| "Unable to parse".into())
+    }
+
+    fn fm<T: FromMeta>(tokens: TokenStream) -> T {
+        FromMeta::from_meta(&pm(tokens).expect("Tests should pass well-formed input"))
+            .expect("Tests should pass valid input")
+    }
+
+    #[test]
+    fn supports_any() {
+        let decl = fm::<DeriveInputShapeSet>(quote!(ignore(any)));
+        assert!(decl.any);
+    }
+
+    #[test]
+    fn supports_struct() {
+        let decl = fm::<DeriveInputShapeSet>(quote!(ignore(struct_any, struct_newtype)));
+        assert!(decl.struct_values.any);
+        assert!(decl.struct_values.newtype);
+    }
+
+    #[test]
+    fn supports_mixed() {
+        let decl =
+            fm::<DeriveInputShapeSet>(quote!(ignore(struct_newtype, enum_newtype, enum_tuple)));
+        assert!(decl.struct_values.newtype);
+        assert!(decl.enum_values.newtype);
+        assert!(decl.enum_values.tuple);
+        assert!(!decl.struct_values.any);
+    }
+}
diff --git a/third_party/rust/darling_core/src/usage/generics_ext.rs b/third_party/rust/darling_core/src/usage/generics_ext.rs
new file mode 100644
index 0000000000..74e514215c
--- /dev/null
+++ b/third_party/rust/darling_core/src/usage/generics_ext.rs
@@ -0,0 +1,24 @@
+use syn::Generics;
+
+use crate::usage::{IdentSet, LifetimeSet};
+
+/// Extension trait for pulling specific generics data from a generics AST representation.
+pub trait GenericsExt {
+    /// Get the set of all lifetimes declared by the syntax element.
+    /// This does not look for usage of the lifetime; see `UsesLifetimes` for that.
+    fn declared_lifetimes(&self) -> LifetimeSet;
+
+    /// Get the set of all type parameters declared by the syntax element.
+    /// This does not look for usage of the type parameter; see `UsesTypeParams` for that.
+    fn declared_type_params(&self) -> IdentSet;
+}
+
+impl GenericsExt for Generics {
+    fn declared_lifetimes(&self) -> LifetimeSet {
+        self.lifetimes().map(|lt| lt.lifetime.clone()).collect()
+    }
+
+    fn declared_type_params(&self) -> IdentSet {
+        self.type_params().map(|tp| tp.ident.clone()).collect()
+    }
+}
diff --git a/third_party/rust/darling_core/src/usage/ident_set.rs b/third_party/rust/darling_core/src/usage/ident_set.rs
new file mode 100644
index 0000000000..e38d332dd0
--- /dev/null
+++ b/third_party/rust/darling_core/src/usage/ident_set.rs
@@ -0,0 +1,8 @@
+use fnv::FnvHashSet;
+use syn::Ident;
+
+/// A set of idents.
+pub type IdentSet = FnvHashSet<Ident>;
+
+/// A set of references to idents.
+pub type IdentRefSet<'a> = FnvHashSet<&'a Ident>;
diff --git a/third_party/rust/darling_core/src/usage/lifetimes.rs b/third_party/rust/darling_core/src/usage/lifetimes.rs
new file mode 100644
index 0000000000..b940173cae
--- /dev/null
+++ b/third_party/rust/darling_core/src/usage/lifetimes.rs
@@ -0,0 +1,327 @@
+use fnv::FnvHashSet;
+use syn::punctuated::Punctuated;
+use syn::{Lifetime, Type};
+
+use crate::usage::Options;
+
+/// A set of lifetimes.
+pub type LifetimeSet = FnvHashSet<Lifetime>;
+
+/// A set of references to lifetimes.
+pub type LifetimeRefSet<'a> = FnvHashSet<&'a Lifetime>;
+
+/// Searcher for finding lifetimes in a syntax tree.
+/// This can be used to determine which lifetimes must be emitted in generated code.
+pub trait UsesLifetimes {
+    /// Returns the subset of the queried lifetimes that are used by the implementing syntax element.
+    ///
+    /// This method only accounts for direct usage by the element; indirect usage via bounds or `where`
+    /// predicates are not detected.
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a>;
+
+    /// Find all used lifetimes, then clone them and return that set.
+    fn uses_lifetimes_cloned(&self, options: &Options, lifetimes: &LifetimeSet) -> LifetimeSet {
+        self.uses_lifetimes(options, lifetimes)
+            .into_iter()
+            .cloned()
+            .collect()
+    }
+}
+
+/// Searcher for finding lifetimes in an iterator.
+///
+/// This trait extends iterators, providing a way to turn a filtered list of fields or variants into a set
+/// of lifetimes.
+pub trait CollectLifetimes {
+    /// Consume an iterator, accumulating all lifetimes in the elements which occur in `lifetimes`.
+    fn collect_lifetimes<'a>(
+        self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a>;
+
+    /// Consume an iterator using `collect_lifetimes`, then clone all found lifetimes and return that set.
+    fn collect_lifetimes_cloned(self, options: &Options, lifetimes: &LifetimeSet) -> LifetimeSet;
+}
+
+impl<'i, I, T> CollectLifetimes for T
+where
+    T: IntoIterator<Item = &'i I>,
+    I: 'i + UsesLifetimes,
+{
+    fn collect_lifetimes<'a>(
+        self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        self.into_iter()
+            .fold(Default::default(), |mut state, value| {
+                state.extend(value.uses_lifetimes(options, lifetimes));
+                state
+            })
+    }
+
+    fn collect_lifetimes_cloned(self, options: &Options, lifetimes: &LifetimeSet) -> LifetimeSet {
+        self.collect_lifetimes(options, lifetimes)
+            .into_iter()
+            .cloned()
+            .collect()
+    }
+}
+
+impl<T: UsesLifetimes> UsesLifetimes for Vec<T> {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        self.collect_lifetimes(options, lifetimes)
+    }
+}
+
+impl<T: UsesLifetimes, U> UsesLifetimes for Punctuated<T, U> {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        self.collect_lifetimes(options, lifetimes)
+    }
+}
+
+impl<T: UsesLifetimes> UsesLifetimes for Option<T> {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        self.as_ref()
+            .map(|v| v.uses_lifetimes(options, lifetimes))
+            .unwrap_or_default()
+    }
+}
+
+impl UsesLifetimes for Lifetime {
+    fn uses_lifetimes<'a>(&self, _: &Options, lifetimes: &'a LifetimeSet) -> LifetimeRefSet<'a> {
+        lifetimes.iter().filter(|lt| *lt == self).collect()
+    }
+}
+
+uses_lifetimes!(syn::AngleBracketedGenericArguments, args);
+uses_lifetimes!(syn::BareFnArg, ty);
+uses_lifetimes!(syn::Binding, ty);
+uses_lifetimes!(syn::BoundLifetimes, lifetimes);
+uses_lifetimes!(syn::Constraint, bounds);
+uses_lifetimes!(syn::DataEnum, variants);
+uses_lifetimes!(syn::DataStruct, fields);
+uses_lifetimes!(syn::DataUnion, fields);
+uses_lifetimes!(syn::Field, ty);
+uses_lifetimes!(syn::FieldsNamed, named);
+uses_lifetimes!(syn::LifetimeDef, lifetime, bounds);
+uses_lifetimes!(syn::ParenthesizedGenericArguments, inputs, output);
+uses_lifetimes!(syn::Path, segments);
+uses_lifetimes!(syn::PathSegment, arguments);
+uses_lifetimes!(syn::PredicateEq, lhs_ty, rhs_ty);
+uses_lifetimes!(syn::PredicateLifetime, lifetime, bounds);
+uses_lifetimes!(syn::PredicateType, lifetimes, bounded_ty, bounds);
+uses_lifetimes!(syn::QSelf, ty);
+uses_lifetimes!(syn::TraitBound, path, lifetimes);
+uses_lifetimes!(syn::TypeArray, elem);
+uses_lifetimes!(syn::TypeBareFn, inputs, output);
+uses_lifetimes!(syn::TypeGroup, elem);
+uses_lifetimes!(syn::TypeImplTrait, bounds);
+uses_lifetimes!(syn::TypeParen, elem);
+uses_lifetimes!(syn::TypePtr, elem);
+uses_lifetimes!(syn::TypeReference, lifetime, elem);
+uses_lifetimes!(syn::TypeSlice, elem);
+uses_lifetimes!(syn::TypeTuple, elems);
+uses_lifetimes!(syn::TypeTraitObject, bounds);
+uses_lifetimes!(syn::Variant, fields);
+
+impl UsesLifetimes for syn::Data {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        match *self {
+            syn::Data::Struct(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::Data::Enum(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::Data::Union(ref v) => v.uses_lifetimes(options, lifetimes),
+        }
+    }
+}
+
+impl UsesLifetimes for Type {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        match *self {
+            Type::Slice(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::Array(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::Ptr(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::Reference(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::BareFn(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::Tuple(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::Path(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::Paren(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::Group(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::TraitObject(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::ImplTrait(ref v) => v.uses_lifetimes(options, lifetimes),
+            Type::Macro(_) | Type::Verbatim(_) | Type::Infer(_) | Type::Never(_) => {
+                Default::default()
+            }
+            _ => panic!("Unknown syn::Type: {:?}", self),
+        }
+    }
+}
+
+impl UsesLifetimes for syn::Fields {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        self.collect_lifetimes(options, lifetimes)
+    }
+}
+
+impl UsesLifetimes for syn::TypePath {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        let mut hits = self.path.uses_lifetimes(options, lifetimes);
+
+        if options.include_type_path_qself() {
+            hits.extend(self.qself.uses_lifetimes(options, lifetimes));
+        }
+
+        hits
+    }
+}
+
+impl UsesLifetimes for syn::ReturnType {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        if let syn::ReturnType::Type(_, ref ty) = *self {
+            ty.uses_lifetimes(options, lifetimes)
+        } else {
+            Default::default()
+        }
+    }
+}
+
+impl UsesLifetimes for syn::PathArguments {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        match *self {
+            syn::PathArguments::None => Default::default(),
+            syn::PathArguments::AngleBracketed(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::PathArguments::Parenthesized(ref v) => v.uses_lifetimes(options, lifetimes),
+        }
+    }
+}
+
+impl UsesLifetimes for syn::WherePredicate {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        match *self {
+            syn::WherePredicate::Type(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::WherePredicate::Lifetime(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::WherePredicate::Eq(ref v) => v.uses_lifetimes(options, lifetimes),
+        }
+    }
+}
+
+impl UsesLifetimes for syn::GenericArgument {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        match *self {
+            syn::GenericArgument::Type(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::GenericArgument::Binding(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::GenericArgument::Lifetime(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::GenericArgument::Constraint(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::GenericArgument::Const(_) => Default::default(),
+        }
+    }
+}
+
+impl UsesLifetimes for syn::TypeParamBound {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        match *self {
+            syn::TypeParamBound::Trait(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::TypeParamBound::Lifetime(ref v) => v.uses_lifetimes(options, lifetimes),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use proc_macro2::Span;
+    use syn::parse_quote;
+    use syn::DeriveInput;
+
+    use super::UsesLifetimes;
+    use crate::usage::GenericsExt;
+    use crate::usage::Purpose::*;
+
+    #[test]
+    fn struct_named() {
+        let input: DeriveInput = parse_quote! {
+            struct Foo<'a, 'b: 'a> {
+                parent: &'b Bar,
+                child: &'a Baz,
+            }
+        };
+        let omitted = syn::Lifetime::new("'c", Span::call_site());
+
+        let lifetimes = {
+            let mut lt = input.generics.declared_lifetimes();
+            lt.insert(omitted);
+            lt
+        };
+
+        let matches = input.data.uses_lifetimes(&BoundImpl.into(), &lifetimes);
+        assert_eq!(matches.len(), 2);
+    }
+
+    #[test]
+    fn qself() {
+        let input: DeriveInput = parse_quote! {
+            struct Foo<'a, 'b: 'a> {
+                parent: &'b Bar,
+                child: <Bar<'a> as MyIterator>::Item,
+            }
+        };
+        let lifetimes = input.generics.declared_lifetimes();
+        let matches = input.data.uses_lifetimes(&BoundImpl.into(), &lifetimes);
+        assert_eq!(matches.len(), 1);
+
+        let decl_matches = input.data.uses_lifetimes(&Declare.into(), &lifetimes);
+        assert_eq!(decl_matches.len(), 2);
+    }
+}
diff --git a/third_party/rust/darling_core/src/usage/mod.rs b/third_party/rust/darling_core/src/usage/mod.rs
new file mode 100644
index 0000000000..fe8f12fb98
--- /dev/null
+++ b/third_party/rust/darling_core/src/usage/mod.rs
@@ -0,0 +1,111 @@
+//! Traits and types used for tracking the usage of generic parameters through a proc-macro input.
+//!
+//! When generating trait impls, libraries often want to automatically figure out which type parameters
+//! are used in which fields, and then emit bounds that will produce the most permissive compilable
+//! code.
+//!
+//! # Usage
+//!
+//! ## Example 1: Filtering
+//! This example accepts a proc-macro input, then finds all lifetimes and type parameters used
+//! by private fields.
+//!
+//! ```rust
+//! # extern crate darling_core;
+//! # extern crate syn;
+//! #
+//! # // in real-world usage, import from `darling`
+//! # use darling_core::usage::{self, CollectLifetimes, CollectTypeParams, GenericsExt, Purpose};
+//! # use syn::{Data, DeriveInput, GenericParam, Generics, Visibility};
+//! #
+//! # #[allow(dead_code)]
+//! fn process(input: &DeriveInput) -> Generics {
+//!     let type_params = input.generics.declared_type_params();
+//!     let lifetimes = input.generics.declared_lifetimes();
+//!
+//!     let mut ret_generics = input.generics.clone();
+//!
+//!     if let Data::Struct(ref body) = input.data {
+//!         let internal_fields = body
+//!             .fields
+//!             .iter()
+//!             .filter(|field| field.vis == Visibility::Inherited)
+//!             .collect::<Vec<_>>();
+//!
+//!         let int_type_params = internal_fields
+//!             .collect_type_params(&Purpose::BoundImpl.into(), &type_params);
+//!
+//!         // We could reuse the vec from above, but here we'll instead
+//!         // directly consume the chained iterator.
+//!         let int_lifetimes = body
+//!             .fields
+//!             .iter()
+//!             .filter(|field| field.vis == Visibility::Inherited)
+//!             .collect_lifetimes(&Purpose::BoundImpl.into(), &lifetimes);
+//!
+//!
+//!         ret_generics.params = ret_generics
+//!             .params
+//!             .into_iter()
+//!             .filter(|gp| {
+//!                 match *gp {
+//!                     GenericParam::Type(ref ty) => int_type_params.contains(&ty.ident),
+//!                     GenericParam::Lifetime(ref lt) => int_lifetimes.contains(&lt.lifetime),
+//!                     _ => true,
+//!                 }
+//!             })
+//!             .collect();
+//!     }
+//!
+//!     ret_generics
+//! }
+//!
+//! # fn main() {}
+//! ```
+//!
+//! ## Example 2: Integrating with `FromDeriveInput`
+//! It is possible to use `darling`'s magic fields feature in tandem with the `usage` feature set.
+//! While there is no custom derive for `UsesTypeParams` or `UsesLifetimes`, there are macros to
+//! generate impls.
+//!
+//! ```rust,ignore
+//! #![allow(dead_code)]
+//!
+//! #[derive(FromField)]
+//! #[darling(attributes(speak))]
+//! struct SpeakerField {
+//!     ident: Option<syn::Ident>,
+//!     ty: syn::Type,
+//!     #[darling(default)]
+//!     volume: Option<u32>,
+//! }
+//!
+//! uses_type_params!(SpeakerField, ty);
+//! uses_lifetimes!(SpeakerField, ty);
+//!
+//! #[derive(FromDeriveInput)]
+//! struct SpeakerOptions {
+//!     generics: syn::Generics,
+//!     data: darling::ast::Data<darling::util::Ignored, SpeakerField>,
+//! }
+//! ```
+//!
+//! At this point, you are able to call `uses_type_params` on `SpeakerOptions.data`, or any filtered
+//! view of it. `darling` internally uses this in conjunction with the `skip` meta-item to determine
+//! which type parameters don't require the `FromMeta` bound in generated impls.
+//!
+//! **Note:** If you are performing operations referencing generic params in meta-items parsed by `darling`,
+//! you should determine if those impact the emitted code and wire up `UsesTypeParams` accordingly for
+//! your field/variant.
+
+mod generics_ext;
+mod ident_set;
+mod lifetimes;
+mod options;
+mod type_params;
+
+pub use self::generics_ext::GenericsExt;
+pub use self::ident_set::{IdentRefSet, IdentSet};
+pub use self::lifetimes::{CollectLifetimes, LifetimeRefSet, LifetimeSet, UsesLifetimes};
+pub use self::options::{Options, Purpose};
+pub use self::type_params::{CollectTypeParams, UsesTypeParams};
diff --git a/third_party/rust/darling_core/src/usage/options.rs b/third_party/rust/darling_core/src/usage/options.rs
new file mode 100644
index 0000000000..f154881be9
--- /dev/null
+++ b/third_party/rust/darling_core/src/usage/options.rs
@@ -0,0 +1,58 @@
+/// The goal of tracing generic parameter usage.
+///
+/// Not all uses of type parameters imply a need to add bounds to a generated trait impl.
+/// For example, a field of type `<Vec<T> as a::b::Trait>::Associated` does not need a
+/// `where T: Serialize` bound in `serde`.
+/// However, a proc macro that is attempting to generate a helper struct _would_ need to
+/// know about this usage, or else the generated code would reference an unknown type `T`
+/// and fail to compile.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum Purpose {
+    /// The tracing is being used to generate an `impl` block.
+    ///
+    /// Uses such as `syn::TypePath.qself` will _not_ be returned.
+    BoundImpl,
+    /// The tracing is being used to generate a new struct or enum.
+    ///
+    /// All uses will be returned.
+    Declare,
+}
+
+/// Control struct for searching type parameters.
+///
+/// This acts as the search context, preserving information that might have been
+/// kept on a visitor in a different implementation.
+/// Trait implementers are required to pass this through on any invocations they make.
+///
+/// # Usage
+/// For extensibility, `Options` hides all of its fields from consumers.
+/// To create an instance, use the `From<Purpose>` trait implementation:
+///
+/// ```rust
+/// # use darling_core::usage::{Options, Purpose};
+/// let opts: Options = Purpose::BoundImpl.into();
+/// assert!(!opts.include_type_path_qself());
+/// ```
+#[derive(Debug, Clone)]
+pub struct Options {
+    purpose: Purpose,
+    #[doc(hidden)]
+    __nonexhaustive: (),
+}
+
+impl From<Purpose> for Options {
+    fn from(purpose: Purpose) -> Self {
+        Self {
+            purpose,
+            __nonexhaustive: (),
+        }
+    }
+}
+
+impl Options {
+    /// Returns `true` if the implementer of `UseTypeParams` should search
+    /// `<___ as ...>::...` when looking for type parameter uses.
+    pub fn include_type_path_qself(&self) -> bool {
+        self.purpose == Purpose::Declare
+    }
+}
diff --git a/third_party/rust/darling_core/src/usage/type_params.rs b/third_party/rust/darling_core/src/usage/type_params.rs
new file mode 100644
index 0000000000..152d218eb5
--- /dev/null
+++ b/third_party/rust/darling_core/src/usage/type_params.rs
@@ -0,0 +1,357 @@
+use syn::punctuated::Punctuated;
+use syn::{Ident, Type};
+
+use crate::usage::{IdentRefSet, IdentSet, Options};
+
+/// Searcher for finding type params in a syntax tree.
+/// This can be used to determine if a given type parameter needs to be bounded in a generated impl.
+pub trait UsesTypeParams {
+    /// Returns the subset of the queried type parameters that are used by the implementing syntax element.
+    ///
+    /// This method only accounts for direct usage by the element; indirect usage via bounds or `where`
+    /// predicates are not detected.
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a>;
+
+    /// Find all type params using `uses_type_params`, then clone the found values and return the set.
+    fn uses_type_params_cloned(&self, options: &Options, type_set: &IdentSet) -> IdentSet {
+        self.uses_type_params(options, type_set)
+            .into_iter()
+            .cloned()
+            .collect()
+    }
+}
+
+/// Searcher for finding type params in an iterator.
+///
+/// This trait extends iterators, providing a way to turn a filtered list of fields or variants into a set
+/// of type parameter idents.
+pub trait CollectTypeParams {
+    /// Consume an iterator, accumulating all type parameters in the elements which occur in `type_set`.
+    fn collect_type_params<'a>(self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a>;
+
+    /// Consume an iterator using `collect_type_params`, then clone all found type params and return that set.
+    fn collect_type_params_cloned(self, options: &Options, type_set: &IdentSet) -> IdentSet;
+}
+
+impl<'i, T, I> CollectTypeParams for T
+where
+    T: IntoIterator<Item = &'i I>,
+    I: 'i + UsesTypeParams,
+{
+    fn collect_type_params<'a>(self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        self.into_iter().fold(
+            IdentRefSet::with_capacity_and_hasher(type_set.len(), Default::default()),
+            |state, value| union_in_place(state, value.uses_type_params(options, type_set)),
+        )
+    }
+
+    fn collect_type_params_cloned(self, options: &Options, type_set: &IdentSet) -> IdentSet {
+        self.collect_type_params(options, type_set)
+            .into_iter()
+            .cloned()
+            .collect()
+    }
+}
+
+/// Insert the contents of `right` into `left`.
+fn union_in_place<'a>(mut left: IdentRefSet<'a>, right: IdentRefSet<'a>) -> IdentRefSet<'a> {
+    left.extend(right);
+
+    left
+}
+
+impl UsesTypeParams for () {
+    fn uses_type_params<'a>(&self, _options: &Options, _type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        Default::default()
+    }
+}
+
+impl<T: UsesTypeParams> UsesTypeParams for Option<T> {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        self.as_ref()
+            .map(|v| v.uses_type_params(options, type_set))
+            .unwrap_or_default()
+    }
+}
+
+impl<T: UsesTypeParams> UsesTypeParams for Vec<T> {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        self.collect_type_params(options, type_set)
+    }
+}
+
+impl<T: UsesTypeParams, U> UsesTypeParams for Punctuated<T, U> {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        self.collect_type_params(options, type_set)
+    }
+}
+
+uses_type_params!(syn::AngleBracketedGenericArguments, args);
+uses_type_params!(syn::BareFnArg, ty);
+uses_type_params!(syn::Binding, ty);
+uses_type_params!(syn::Constraint, bounds);
+uses_type_params!(syn::DataEnum, variants);
+uses_type_params!(syn::DataStruct, fields);
+uses_type_params!(syn::DataUnion, fields);
+uses_type_params!(syn::Field, ty);
+uses_type_params!(syn::FieldsNamed, named);
+uses_type_params!(syn::ParenthesizedGenericArguments, inputs, output);
+uses_type_params!(syn::PredicateEq, lhs_ty, rhs_ty);
+uses_type_params!(syn::PredicateType, bounded_ty, bounds);
+uses_type_params!(syn::QSelf, ty);
+uses_type_params!(syn::TraitBound, path);
+uses_type_params!(syn::TypeArray, elem);
+uses_type_params!(syn::TypeBareFn, inputs, output);
+uses_type_params!(syn::TypeGroup, elem);
+uses_type_params!(syn::TypeImplTrait, bounds);
+uses_type_params!(syn::TypeParen, elem);
+uses_type_params!(syn::TypePtr, elem);
+uses_type_params!(syn::TypeReference, elem);
+uses_type_params!(syn::TypeSlice, elem);
+uses_type_params!(syn::TypeTuple, elems);
+uses_type_params!(syn::TypeTraitObject, bounds);
+uses_type_params!(syn::Variant, fields);
+
+impl UsesTypeParams for syn::Data {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        match *self {
+            syn::Data::Struct(ref v) => v.uses_type_params(options, type_set),
+            syn::Data::Enum(ref v) => v.uses_type_params(options, type_set),
+            syn::Data::Union(ref v) => v.uses_type_params(options, type_set),
+        }
+    }
+}
+
+impl UsesTypeParams for syn::Fields {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        self.collect_type_params(options, type_set)
+    }
+}
+
+/// Check if an Ident exactly matches one of the sought-after type parameters.
+impl UsesTypeParams for Ident {
+    fn uses_type_params<'a>(&self, _options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        type_set.iter().filter(|v| *v == self).collect()
+    }
+}
+
+impl UsesTypeParams for syn::ReturnType {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        if let syn::ReturnType::Type(_, ref ty) = *self {
+            ty.uses_type_params(options, type_set)
+        } else {
+            Default::default()
+        }
+    }
+}
+
+impl UsesTypeParams for Type {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        match *self {
+            Type::Slice(ref v) => v.uses_type_params(options, type_set),
+            Type::Array(ref v) => v.uses_type_params(options, type_set),
+            Type::Ptr(ref v) => v.uses_type_params(options, type_set),
+            Type::Reference(ref v) => v.uses_type_params(options, type_set),
+            Type::BareFn(ref v) => v.uses_type_params(options, type_set),
+            Type::Tuple(ref v) => v.uses_type_params(options, type_set),
+            Type::Path(ref v) => v.uses_type_params(options, type_set),
+            Type::Paren(ref v) => v.uses_type_params(options, type_set),
+            Type::Group(ref v) => v.uses_type_params(options, type_set),
+            Type::TraitObject(ref v) => v.uses_type_params(options, type_set),
+            Type::ImplTrait(ref v) => v.uses_type_params(options, type_set),
+            Type::Macro(_) | Type::Verbatim(_) | Type::Infer(_) | Type::Never(_) => {
+                Default::default()
+            }
+            _ => panic!("Unknown syn::Type: {:?}", self),
+        }
+    }
+}
+
+impl UsesTypeParams for syn::TypePath {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        let hits = self.path.uses_type_params(options, type_set);
+
+        if options.include_type_path_qself() {
+            union_in_place(hits, self.qself.uses_type_params(options, type_set))
+        } else {
+            hits
+        }
+    }
+}
+
+impl UsesTypeParams for syn::Path {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        // Not sure if this is even possible, but a path with no segments definitely
+        // can't use type parameters.
+        if self.segments.is_empty() {
+            return Default::default();
+        }
+
+        // A path segment ident can only match if it is not global and it is the first segment
+        // in the path.
+        let ident_hits = if self.leading_colon.is_none() {
+            self.segments[0].ident.uses_type_params(options, type_set)
+        } else {
+            Default::default()
+        };
+
+        // Merge ident hit, if any, with all hits from path arguments
+        self.segments.iter().fold(ident_hits, |state, segment| {
+            union_in_place(state, segment.arguments.uses_type_params(options, type_set))
+        })
+    }
+}
+
+impl UsesTypeParams for syn::PathArguments {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        match *self {
+            syn::PathArguments::None => Default::default(),
+            syn::PathArguments::AngleBracketed(ref v) => v.uses_type_params(options, type_set),
+            syn::PathArguments::Parenthesized(ref v) => v.uses_type_params(options, type_set),
+        }
+    }
+}
+
+impl UsesTypeParams for syn::WherePredicate {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        match *self {
+            syn::WherePredicate::Lifetime(_) => Default::default(),
+            syn::WherePredicate::Type(ref v) => v.uses_type_params(options, type_set),
+            syn::WherePredicate::Eq(ref v) => v.uses_type_params(options, type_set),
+        }
+    }
+}
+
+impl UsesTypeParams for syn::GenericArgument {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        match *self {
+            syn::GenericArgument::Type(ref v) => v.uses_type_params(options, type_set),
+            syn::GenericArgument::Binding(ref v) => v.uses_type_params(options, type_set),
+            syn::GenericArgument::Constraint(ref v) => v.uses_type_params(options, type_set),
+            syn::GenericArgument::Const(_) | syn::GenericArgument::Lifetime(_) => {
+                Default::default()
+            }
+        }
+    }
+}
+
+impl UsesTypeParams for syn::TypeParamBound {
+    fn uses_type_params<'a>(&self, options: &Options, type_set: &'a IdentSet) -> IdentRefSet<'a> {
+        match *self {
+            syn::TypeParamBound::Trait(ref v) => v.uses_type_params(options, type_set),
+            syn::TypeParamBound::Lifetime(_) => Default::default(),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use proc_macro2::Span;
+    use syn::{parse_quote, DeriveInput, Ident};
+
+    use super::UsesTypeParams;
+    use crate::usage::IdentSet;
+    use crate::usage::Purpose::*;
+
+    fn ident_set(idents: Vec<&str>) -> IdentSet {
+        idents
+            .into_iter()
+            .map(|s| Ident::new(s, Span::call_site()))
+            .collect()
+    }
+
+    #[test]
+    fn finds_simple() {
+        let input: DeriveInput = parse_quote! { struct Foo<T, U>(T, i32, A, U); };
+        let generics = ident_set(vec!["T", "U", "X"]);
+        let matches = input.data.uses_type_params(&BoundImpl.into(), &generics);
+        assert_eq!(matches.len(), 2);
+        assert!(matches.contains::<Ident>(&parse_quote!(T)));
+        assert!(matches.contains::<Ident>(&parse_quote!(U)));
+        assert!(!matches.contains::<Ident>(&parse_quote!(X)));
+        assert!(!matches.contains::<Ident>(&parse_quote!(A)));
+    }
+
+    #[test]
+    fn finds_named() {
+        let input: DeriveInput = parse_quote! {
+            struct Foo<T, U = usize> {
+                bar: T,
+                world: U,
+            }
+        };
+
+        let generics = ident_set(vec!["T", "U", "X"]);
+
+        let matches = input.data.uses_type_params(&BoundImpl.into(), &generics);
+
+        assert_eq!(matches.len(), 2);
+        assert!(matches.contains::<Ident>(&parse_quote!(T)));
+        assert!(matches.contains::<Ident>(&parse_quote!(U)));
+        assert!(!matches.contains::<Ident>(&parse_quote!(X)));
+        assert!(!matches.contains::<Ident>(&parse_quote!(A)));
+    }
+
+    #[test]
+    fn finds_as_type_arg() {
+        let input: DeriveInput = parse_quote! {
+            struct Foo<T, U> {
+                bar: T,
+                world: Vec<U>,
+            }
+        };
+
+        let generics = ident_set(vec!["T", "U", "X"]);
+
+        let matches = input.data.uses_type_params(&BoundImpl.into(), &generics);
+
+        assert_eq!(matches.len(), 2);
+        assert!(matches.contains::<Ident>(&parse_quote!(T)));
+        assert!(matches.contains::<Ident>(&parse_quote!(U)));
+        assert!(!matches.contains::<Ident>(&parse_quote!(X)));
+        assert!(!matches.contains::<Ident>(&parse_quote!(A)));
+    }
+
+    #[test]
+    fn associated_type() {
+        let input: DeriveInput =
+            parse_quote! { struct Foo<'a, T> where T: Iterator { peek: T::Item } };
+        let generics = ident_set(vec!["T", "INTO"]);
+        let matches = input.data.uses_type_params(&BoundImpl.into(), &generics);
+        assert_eq!(matches.len(), 1);
+    }
+
+    #[test]
+    fn box_fn_output() {
+        let input: DeriveInput = parse_quote! { struct Foo<T>(Box<Fn() -> T>); };
+        let generics = ident_set(vec!["T"]);
+        let matches = input.data.uses_type_params(&BoundImpl.into(), &generics);
+        assert_eq!(matches.len(), 1);
+        assert!(matches.contains::<Ident>(&parse_quote!(T)));
+    }
+
+    #[test]
+    fn box_fn_input() {
+        let input: DeriveInput = parse_quote! { struct Foo<T>(Box<Fn(&T) -> ()>); };
+        let generics = ident_set(vec!["T"]);
+        let matches = input.data.uses_type_params(&BoundImpl.into(), &generics);
+        assert_eq!(matches.len(), 1);
+        assert!(matches.contains::<Ident>(&parse_quote!(T)));
+    }
+
+    /// Test that `syn::TypePath` is correctly honoring the different modes a
+    /// search can execute in.
+    #[test]
+    fn qself_vec() {
+        let input: DeriveInput =
+            parse_quote! { struct Foo<T>(<Vec<T> as a::b::Trait>::AssociatedItem); };
+        let generics = ident_set(vec!["T", "U"]);
+
+        let bound_matches = input.data.uses_type_params(&BoundImpl.into(), &generics);
+        assert_eq!(bound_matches.len(), 0);
+
+        let declare_matches = input.data.uses_type_params(&Declare.into(), &generics);
+        assert_eq!(declare_matches.len(), 1);
+        assert!(declare_matches.contains::<Ident>(&parse_quote!(T)));
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/flag.rs b/third_party/rust/darling_core/src/util/flag.rs
new file mode 100644
index 0000000000..ade88988ab
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/flag.rs
@@ -0,0 +1,100 @@
+use proc_macro2::Span;
+use syn::{spanned::Spanned, Meta};
+
+use crate::{FromMeta, Result};
+
+/// A meta-item that can be present as a word - with no value - or absent.
+///
+/// # Defaulting
+/// Like `Option`, `Flag` does not require `#[darling(default)]` to be optional.
+/// If the caller does not include the property, then an absent `Flag` will be included
+/// in the receiver struct.
+///
+/// # Spans
+/// `Flag` keeps the span where its word was seen.
+/// This enables attaching custom error messages to the word, such as in the case of two
+/// conflicting flags being present.
+///
+/// # Example
+/// ```ignore
+/// #[derive(FromMeta)]
+/// #[darling(and_then = "Self::not_both")]
+/// struct Demo {
+///     flag_a: Flag,
+///     flag_b: Flag,
+/// }
+///
+/// impl Demo {
+///     fn not_both(self) -> Result<Self> {
+///         if self.flag_a.is_present() && self.flag_b.is_present() {
+///             Err(Error::custom("Cannot set flag_a and flag_b").with_span(self.flag_b))
+///         } else {
+///             Ok(self)
+///         }
+///     }
+/// }
+/// ```
+///
+/// The above struct would then produce the following error.
+///
+/// ```ignore
+/// #[example(flag_a, flag_b)]
+/// //                ^^^^^^ Cannot set flag_a and flag_b
+/// ```
+#[derive(Debug, Clone, Copy, Default)]
+pub struct Flag(Option<Span>);
+
+impl Flag {
+    /// Creates a new `Flag` which corresponds to the presence of a value.
+    pub fn present() -> Self {
+        Flag(Some(Span::call_site()))
+    }
+
+    /// Check if the flag is present.
+    pub fn is_present(&self) -> bool {
+        self.0.is_some()
+    }
+
+    #[deprecated(since = "0.14.0", note = "Use Flag::is_present")]
+    pub fn is_some(&self) -> bool {
+        self.is_present()
+    }
+}
+
+impl FromMeta for Flag {
+    fn from_none() -> Option<Self> {
+        Some(Flag(None))
+    }
+
+    fn from_meta(mi: &syn::Meta) -> Result<Self> {
+        if let Meta::Path(p) = mi {
+            Ok(Flag(Some(p.span())))
+        } else {
+            // The implementation for () will produce an error for all non-path meta items;
+            // call it to make sure the span behaviors and error messages are the same.
+            Err(<()>::from_meta(mi).unwrap_err())
+        }
+    }
+}
+
+impl Spanned for Flag {
+    fn span(&self) -> Span {
+        self.0.unwrap_or_else(Span::call_site)
+    }
+}
+
+impl From<Flag> for bool {
+    fn from(flag: Flag) -> Self {
+        flag.is_present()
+    }
+}
+
+impl From<bool> for Flag {
+    fn from(v: bool) -> Self {
+        if v {
+            Flag::present()
+        } else {
+            Flag(None)
+        }
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/ident_string.rs b/third_party/rust/darling_core/src/util/ident_string.rs
new file mode 100644
index 0000000000..937d1a6c49
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/ident_string.rs
@@ -0,0 +1,157 @@
+use std::fmt;
+use std::hash::{Hash, Hasher};
+
+use proc_macro2::{Span, TokenStream};
+use quote::ToTokens;
+use syn::{Ident, Meta};
+
+use crate::{FromMeta, Result};
+
+/// A wrapper for an `Ident` which also keeps the value as a string.
+///
+/// This struct can be used to perform string comparisons and operations.
+#[derive(Clone, PartialOrd, Ord)]
+pub struct IdentString {
+    ident: Ident,
+    string: String,
+}
+
+impl IdentString {
+    /// Create a new `IdentString`.
+    pub fn new(ident: Ident) -> Self {
+        IdentString {
+            string: ident.to_string(),
+            ident,
+        }
+    }
+
+    /// Get the ident as a `proc_macro2::Ident`.
+    pub fn as_ident(&self) -> &Ident {
+        &self.ident
+    }
+
+    /// Get the ident as a string.
+    pub fn as_str(&self) -> &str {
+        &self.string
+    }
+
+    /// Get the location of this `Ident` in source.
+    pub fn span(&self) -> Span {
+        self.ident.span()
+    }
+
+    /// Apply some transform to the ident's string representation.
+    ///
+    /// # Panics
+    /// This will panic if the transform produces an invalid ident.
+    pub fn map<F, S>(self, map_fn: F) -> Self
+    where
+        F: FnOnce(String) -> S,
+        S: AsRef<str>,
+    {
+        let span = self.span();
+        let string = map_fn(self.string);
+        Ident::new(string.as_ref(), span).into()
+    }
+}
+
+impl AsRef<Ident> for IdentString {
+    fn as_ref(&self) -> &Ident {
+        self.as_ident()
+    }
+}
+
+impl AsRef<str> for IdentString {
+    fn as_ref(&self) -> &str {
+        self.as_str()
+    }
+}
+
+impl From<Ident> for IdentString {
+    fn from(ident: Ident) -> Self {
+        IdentString::new(ident)
+    }
+}
+
+impl From<IdentString> for Ident {
+    fn from(v: IdentString) -> Ident {
+        v.ident
+    }
+}
+
+impl From<IdentString> for String {
+    fn from(v: IdentString) -> String {
+        v.string
+    }
+}
+
+impl Eq for IdentString {}
+
+impl PartialEq for IdentString {
+    fn eq(&self, rhs: &Self) -> bool {
+        self.ident == rhs.ident
+    }
+}
+
+impl PartialEq<String> for IdentString {
+    fn eq(&self, rhs: &String) -> bool {
+        self.as_str() == rhs
+    }
+}
+
+impl<'a> PartialEq<&'a str> for IdentString {
+    fn eq(&self, rhs: &&str) -> bool {
+        self.as_str() == *rhs
+    }
+}
+
+impl Hash for IdentString {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.ident.hash(state);
+    }
+}
+
+impl ToTokens for IdentString {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        self.ident.to_tokens(tokens);
+    }
+}
+
+impl fmt::Debug for IdentString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{:?}", self.ident)
+    }
+}
+
+impl fmt::Display for IdentString {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.ident)
+    }
+}
+
+impl FromMeta for IdentString {
+    fn from_meta(item: &Meta) -> Result<Self> {
+        Ident::from_meta(item).map(IdentString::from)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use syn::parse_quote;
+
+    use super::IdentString;
+
+    #[test]
+    fn convert() {
+        let i_str = IdentString::new(parse_quote!(t));
+        assert_eq!(i_str.as_str(), "t");
+    }
+
+    #[test]
+    fn map_transform() {
+        let i = IdentString::new(parse_quote!(my));
+        let after = i.map(|v| format!("var_{}", v));
+        assert_eq!(after, "var_my");
+        assert_eq!(after, String::from("var_my"));
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/ignored.rs b/third_party/rust/darling_core/src/util/ignored.rs
new file mode 100644
index 0000000000..b74acfdda8
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/ignored.rs
@@ -0,0 +1,50 @@
+use crate::{
+    usage::{self, UsesLifetimes, UsesTypeParams},
+    FromDeriveInput, FromField, FromGenericParam, FromGenerics, FromMeta, FromTypeParam,
+    FromVariant, Result,
+};
+
+/// An efficient way of discarding data from a syntax element.
+///
+/// All syntax elements will be successfully read into
+/// the `Ignored` struct, with all properties discarded.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Default)]
+pub struct Ignored;
+
+macro_rules! ignored {
+    ($trayt:ident, $method:ident, $syn:path) => {
+        impl $trayt for Ignored {
+            fn $method(_: &$syn) -> Result<Self> {
+                Ok(Ignored)
+            }
+        }
+    };
+}
+
+ignored!(FromGenericParam, from_generic_param, syn::GenericParam);
+ignored!(FromGenerics, from_generics, syn::Generics);
+ignored!(FromTypeParam, from_type_param, syn::TypeParam);
+ignored!(FromMeta, from_meta, syn::Meta);
+ignored!(FromDeriveInput, from_derive_input, syn::DeriveInput);
+ignored!(FromField, from_field, syn::Field);
+ignored!(FromVariant, from_variant, syn::Variant);
+
+impl UsesTypeParams for Ignored {
+    fn uses_type_params<'a>(
+        &self,
+        _opts: &usage::Options,
+        _: &'a usage::IdentSet,
+    ) -> usage::IdentRefSet<'a> {
+        Default::default()
+    }
+}
+
+impl UsesLifetimes for Ignored {
+    fn uses_lifetimes<'a>(
+        &self,
+        _opts: &usage::Options,
+        _: &'a usage::LifetimeSet,
+    ) -> usage::LifetimeRefSet<'a> {
+        Default::default()
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/mod.rs b/third_party/rust/darling_core/src/util/mod.rs
new file mode 100644
index 0000000000..ddc240b7be
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/mod.rs
@@ -0,0 +1,23 @@
+//! Utility types for attribute parsing.
+
+mod flag;
+mod ident_string;
+mod ignored;
+mod over_ride;
+mod parse_attribute;
+mod path_list;
+mod path_to_string;
+mod shape;
+mod spanned_value;
+mod with_original;
+
+pub use self::flag::Flag;
+pub use self::ident_string::IdentString;
+pub use self::ignored::Ignored;
+pub use self::over_ride::Override;
+pub use self::parse_attribute::parse_attribute_to_meta_list;
+pub use self::path_list::PathList;
+pub use self::path_to_string::path_to_string;
+pub use self::shape::{AsShape, Shape, ShapeSet};
+pub use self::spanned_value::SpannedValue;
+pub use self::with_original::WithOriginal;
diff --git a/third_party/rust/darling_core/src/util/over_ride.rs b/third_party/rust/darling_core/src/util/over_ride.rs
new file mode 100644
index 0000000000..92ce25a3b6
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/over_ride.rs
@@ -0,0 +1,147 @@
+use std::fmt;
+
+use syn::{Lit, NestedMeta};
+
+use crate::{FromMeta, Result};
+
+use self::Override::*;
+
+/// A value which can inherit a default value or have an explicit value specified.
+///
+/// # Usage
+/// This type is meant for attributes like `default` in `darling`, which can take the following forms:
+///
+/// * `#[darling(default)]`
+/// * `#[darling(default="path::to::fn")]`
+///
+/// In a struct collecting input for this attribute, that would be written as:
+///
+/// ```rust,ignore
+/// use darling::{util::Override, FromField};
+/// #[derive(FromField)]
+/// #[darling(attributes(darling))]
+/// pub struct Options {
+///    default: Option<Override<syn::Path>>,
+/// }
+///
+/// impl Options {
+///     fn hydrate(self) -> Option<syn::Path> {
+///         self.default.map(|ov| ov.unwrap_or(syn::parse_path("::Default::default").unwrap()))
+///     }
+/// }
+/// ```
+///
+/// The `word` format (with no associated value), would produce `Override::Inherit`, while a list
+/// or value format would produce `Override::Explicit`.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Override<T> {
+    /// Inherit the eventual value from an external source.
+    Inherit,
+
+    /// Explicitly set the value.
+    Explicit(T),
+}
+
+impl<T> Override<T> {
+    /// Converts from `Override<T>` to `Override<&T>`.
+    ///
+    /// Produces a new `Override`, containing a reference into the original, leaving the original in place.
+    pub fn as_ref(&self) -> Override<&T> {
+        match *self {
+            Inherit => Inherit,
+            Explicit(ref val) => Explicit(val),
+        }
+    }
+
+    /// Converts from `Override<T>` to `Override<&mut T>`.
+    ///
+    /// Produces a new `Override`, containing a mutable reference into the original.
+    pub fn as_mut(&mut self) -> Override<&mut T> {
+        match *self {
+            Inherit => Inherit,
+            Explicit(ref mut val) => Explicit(val),
+        }
+    }
+
+    /// Returns `true` if the override is an `Explicit` value.
+    pub fn is_explicit(&self) -> bool {
+        match *self {
+            Inherit => false,
+            Explicit(_) => true,
+        }
+    }
+
+    /// Converts from `Override<T>` to `Option<T>`.
+    pub fn explicit(self) -> Option<T> {
+        match self {
+            Inherit => None,
+            Explicit(val) => Some(val),
+        }
+    }
+
+    /// Unwraps an override, yielding the content of an `Explicit`. Otherwise, it returns `optb`.
+    pub fn unwrap_or(self, optb: T) -> T {
+        match self {
+            Inherit => optb,
+            Explicit(val) => val,
+        }
+    }
+
+    /// Unwraps an override, yielding the content of an `Explicit`. Otherwise, it calls `op`.
+    pub fn unwrap_or_else<F>(self, op: F) -> T
+    where
+        F: FnOnce() -> T,
+    {
+        match self {
+            Inherit => op(),
+            Explicit(val) => val,
+        }
+    }
+}
+
+impl<T: Default> Override<T> {
+    /// Returns the contained value or the default value of `T`.
+    pub fn unwrap_or_default(self) -> T {
+        self.unwrap_or_else(Default::default)
+    }
+}
+
+impl<T> Default for Override<T> {
+    fn default() -> Self {
+        Inherit
+    }
+}
+
+impl<T> From<Option<T>> for Override<T> {
+    fn from(v: Option<T>) -> Self {
+        match v {
+            None => Inherit,
+            Some(val) => Explicit(val),
+        }
+    }
+}
+
+impl<T: fmt::Display> fmt::Display for Override<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Inherit => write!(f, "Inherit"),
+            Explicit(ref val) => write!(f, "Explicit `{}`", val),
+        }
+    }
+}
+
+/// Parses a `Meta`. A bare word will produce `Override::Inherit`, while
+/// any value will be forwarded to `T::from_meta`.
+impl<T: FromMeta> FromMeta for Override<T> {
+    fn from_word() -> Result<Self> {
+        Ok(Inherit)
+    }
+
+    fn from_list(items: &[NestedMeta]) -> Result<Self> {
+        Ok(Explicit(FromMeta::from_list(items)?))
+    }
+
+    fn from_value(lit: &Lit) -> Result<Self> {
+        Ok(Explicit(FromMeta::from_value(lit)?))
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/parse_attribute.rs b/third_party/rust/darling_core/src/util/parse_attribute.rs
new file mode 100644
index 0000000000..8d8be5d7ce
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/parse_attribute.rs
@@ -0,0 +1,73 @@
+use crate::{util::SpannedValue, Error, Result};
+use std::fmt;
+use syn::{punctuated::Pair, spanned::Spanned, token, Attribute, Meta, MetaList, Path};
+
+/// Try to parse an attribute into a meta list. Path-type meta values are accepted and returned
+/// as empty lists with their passed-in path. Name-value meta values and non-meta attributes
+/// will cause errors to be returned.
+pub fn parse_attribute_to_meta_list(attr: &Attribute) -> Result<MetaList> {
+    match attr.parse_meta() {
+        Ok(Meta::List(list)) => Ok(list),
+        Ok(Meta::NameValue(nv)) => Err(Error::custom(format!(
+            "Name-value arguments are not supported. Use #[{}(...)]",
+            DisplayPath(&nv.path)
+        ))
+        .with_span(&nv)),
+        Ok(Meta::Path(path)) => Ok(MetaList {
+            path,
+            paren_token: token::Paren(attr.span()),
+            nested: Default::default(),
+        }),
+        Err(e) => Err(Error::custom(format!("Unable to parse attribute: {}", e))
+            .with_span(&SpannedValue::new((), e.span()))),
+    }
+}
+
+struct DisplayPath<'a>(&'a Path);
+
+impl fmt::Display for DisplayPath<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let path = self.0;
+        if path.leading_colon.is_some() {
+            write!(f, "::")?;
+        }
+        for segment in path.segments.pairs() {
+            match segment {
+                Pair::Punctuated(segment, _) => write!(f, "{}::", segment.ident)?,
+                Pair::End(segment) => segment.ident.fmt(f)?,
+            }
+        }
+
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::parse_attribute_to_meta_list;
+    use syn::{parse_quote, spanned::Spanned, Ident};
+
+    #[test]
+    fn parse_list() {
+        let meta = parse_attribute_to_meta_list(&parse_quote!(#[bar(baz = 4)])).unwrap();
+        assert_eq!(meta.nested.len(), 1);
+    }
+
+    #[test]
+    fn parse_path_returns_empty_list() {
+        let meta = parse_attribute_to_meta_list(&parse_quote!(#[bar])).unwrap();
+        assert!(meta.path.is_ident(&Ident::new("bar", meta.path.span())));
+        assert!(meta.nested.is_empty());
+    }
+
+    #[test]
+    fn parse_name_value_returns_error() {
+        parse_attribute_to_meta_list(&parse_quote!(#[bar = 4])).unwrap_err();
+    }
+
+    #[test]
+    fn parse_name_value_error_includes_example() {
+        let err = parse_attribute_to_meta_list(&parse_quote!(#[bar = 4])).unwrap_err();
+        assert!(err.to_string().contains("#[bar(...)]"));
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/path_list.rs b/third_party/rust/darling_core/src/util/path_list.rs
new file mode 100644
index 0000000000..18b0f630fd
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/path_list.rs
@@ -0,0 +1,104 @@
+use std::ops::Deref;
+
+use syn::{Meta, NestedMeta, Path};
+
+use crate::{Error, FromMeta, Result};
+
+use super::path_to_string;
+
+/// A list of `syn::Path` instances. This type is used to extract a list of paths from an
+/// attribute.
+///
+/// # Usage
+/// An `PathList` field on a struct implementing `FromMeta` will turn `#[builder(derive(serde::Debug, Clone))]` into:
+///
+/// ```rust,ignore
+/// StructOptions {
+///     derive: PathList(vec![syn::Path::new("serde::Debug"), syn::Path::new("Clone")])
+/// }
+/// ```
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
+pub struct PathList(Vec<Path>);
+
+impl PathList {
+    /// Create a new list.
+    pub fn new<T: Into<Path>>(vals: Vec<T>) -> Self {
+        PathList(vals.into_iter().map(T::into).collect())
+    }
+
+    /// Create a new `Vec` containing the string representation of each path.
+    pub fn to_strings(&self) -> Vec<String> {
+        self.0.iter().map(path_to_string).collect()
+    }
+}
+
+impl Deref for PathList {
+    type Target = Vec<Path>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl From<Vec<Path>> for PathList {
+    fn from(v: Vec<Path>) -> Self {
+        PathList(v)
+    }
+}
+
+impl FromMeta for PathList {
+    fn from_list(v: &[NestedMeta]) -> Result<Self> {
+        let mut paths = Vec::with_capacity(v.len());
+        for nmi in v {
+            if let NestedMeta::Meta(Meta::Path(ref path)) = *nmi {
+                paths.push(path.clone());
+            } else {
+                return Err(Error::unexpected_type("non-word").with_span(nmi));
+            }
+        }
+
+        Ok(PathList(paths))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::PathList;
+    use crate::FromMeta;
+    use proc_macro2::TokenStream;
+    use quote::quote;
+    use syn::{parse_quote, Attribute, Meta};
+
+    /// parse a string as a syn::Meta instance.
+    fn pm(tokens: TokenStream) -> ::std::result::Result<Meta, String> {
+        let attribute: Attribute = parse_quote!(#[#tokens]);
+        attribute.parse_meta().map_err(|_| "Unable to parse".into())
+    }
+
+    fn fm<T: FromMeta>(tokens: TokenStream) -> T {
+        FromMeta::from_meta(&pm(tokens).expect("Tests should pass well-formed input"))
+            .expect("Tests should pass valid input")
+    }
+
+    #[test]
+    fn succeeds() {
+        let paths = fm::<PathList>(quote!(ignore(Debug, Clone, Eq)));
+        assert_eq!(
+            paths.to_strings(),
+            vec![
+                String::from("Debug"),
+                String::from("Clone"),
+                String::from("Eq")
+            ]
+        );
+    }
+
+    /// Check that the parser rejects non-word members of the list, and that the error
+    /// has an associated span.
+    #[test]
+    fn fails_non_word() {
+        let input = PathList::from_meta(&pm(quote!(ignore(Debug, Clone = false))).unwrap());
+        let err = input.unwrap_err();
+        assert!(err.has_span());
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/path_to_string.rs b/third_party/rust/darling_core/src/util/path_to_string.rs
new file mode 100644
index 0000000000..0e0ad99c7a
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/path_to_string.rs
@@ -0,0 +1,36 @@
+/// Transform Rust paths to a readable and comparable string.
+///
+/// # Limitations
+/// * Leading colons are ignored.
+/// * Angle brackets and `as` elements are ignored.
+///
+/// # Example
+/// ```rust
+/// # use darling_core::util::path_to_string;
+/// # use syn::parse_quote;
+/// assert_eq!(path_to_string(&parse_quote!(a::b)), "a::b");
+/// ```
+pub fn path_to_string(path: &syn::Path) -> String {
+    path.segments
+        .iter()
+        .map(|s| s.ident.to_string())
+        .collect::<Vec<String>>()
+        .join("::")
+}
+
+#[cfg(test)]
+mod tests {
+    use syn::parse_quote;
+
+    use super::path_to_string;
+
+    #[test]
+    fn simple_ident() {
+        assert_eq!(path_to_string(&parse_quote!(a)), "a");
+    }
+
+    #[test]
+    fn simple_path() {
+        assert_eq!(path_to_string(&parse_quote!(a::b)), "a::b");
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/shape.rs b/third_party/rust/darling_core/src/util/shape.rs
new file mode 100644
index 0000000000..5a90891bfc
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/shape.rs
@@ -0,0 +1,285 @@
+use std::{fmt, iter::FromIterator};
+
+use crate::ast;
+
+/// Get the "shape" of a fields container, such as a struct or variant.
+pub trait AsShape {
+    /// Get the "shape" of a fields container.
+    fn as_shape(&self) -> Shape;
+}
+
+impl<T> AsShape for ast::Fields<T> {
+    fn as_shape(&self) -> Shape {
+        match self.style {
+            ast::Style::Tuple if self.fields.len() == 1 => Shape::Newtype,
+            ast::Style::Tuple => Shape::Tuple,
+            ast::Style::Struct => Shape::Named,
+            ast::Style::Unit => Shape::Unit,
+        }
+    }
+}
+
+impl AsShape for syn::Fields {
+    fn as_shape(&self) -> Shape {
+        match self {
+            syn::Fields::Named(fields) => fields.as_shape(),
+            syn::Fields::Unnamed(fields) => fields.as_shape(),
+            syn::Fields::Unit => Shape::Unit,
+        }
+    }
+}
+
+impl AsShape for syn::FieldsNamed {
+    fn as_shape(&self) -> Shape {
+        Shape::Named
+    }
+}
+
+impl AsShape for syn::FieldsUnnamed {
+    fn as_shape(&self) -> Shape {
+        if self.unnamed.len() == 1 {
+            Shape::Newtype
+        } else {
+            Shape::Tuple
+        }
+    }
+}
+
+impl AsShape for syn::DataStruct {
+    fn as_shape(&self) -> Shape {
+        self.fields.as_shape()
+    }
+}
+
+impl AsShape for syn::Variant {
+    fn as_shape(&self) -> Shape {
+        self.fields.as_shape()
+    }
+}
+
+/// Description of how fields in a struct or variant are syntactically laid out.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Shape {
+    /// A set of named fields, e.g. `{ field: String }`.
+    Named,
+    /// A list of unnamed fields, e.g. `(String, u64)`.
+    Tuple,
+    /// No fields, e.g. `struct Example;`
+    Unit,
+    /// A special case of [`Tuple`](Shape#variant.Tuple) with exactly one field, e.g. `(String)`.
+    Newtype,
+}
+
+impl Shape {
+    pub fn description(&self) -> &'static str {
+        match self {
+            Shape::Named => "named fields",
+            Shape::Tuple => "unnamed fields",
+            Shape::Unit => "no fields",
+            Shape::Newtype => "one unnamed field",
+        }
+    }
+}
+
+impl fmt::Display for Shape {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.description())
+    }
+}
+
+impl AsShape for Shape {
+    fn as_shape(&self) -> Shape {
+        *self
+    }
+}
+
+/// A set of [`Shape`] values, which correctly handles the relationship between
+/// [newtype](Shape#variant.Newtype) and [tuple](Shape#variant.Tuple) shapes.
+///
+/// # Example
+/// ```rust
+/// # use darling_core::util::{Shape, ShapeSet};
+/// let shape_set = ShapeSet::new(vec![Shape::Tuple]);
+///
+/// // This is correct, because all newtypes are single-field tuples.
+/// assert!(shape_set.contains(&Shape::Newtype));
+/// ```
+#[derive(Debug, Clone, Default)]
+pub struct ShapeSet {
+    newtype: bool,
+    named: bool,
+    tuple: bool,
+    unit: bool,
+}
+
+impl ShapeSet {
+    /// Create a new `ShapeSet` which includes the specified items.
+    ///
+    /// # Exampe
+    /// ```rust
+    /// # use darling_core::util::{Shape, ShapeSet};
+    /// let shape_set = ShapeSet::new(vec![Shape::Named, Shape::Newtype]);
+    /// assert!(shape_set.contains(&Shape::Newtype));
+    /// ```
+    pub fn new(items: impl IntoIterator<Item = Shape>) -> Self {
+        items.into_iter().collect()
+    }
+
+    /// Insert all possible shapes into the set.
+    ///
+    /// This is equivalent to calling [`insert`](ShapeSet#method.insert) with every value of [`Shape`].
+    ///
+    /// # Example
+    /// ```rust
+    /// # use darling_core::util::{Shape, ShapeSet};
+    /// let mut shape_set = ShapeSet::default();
+    /// shape_set.insert_all();
+    /// assert!(shape_set.contains(&Shape::Named));
+    /// ```
+    pub fn insert_all(&mut self) {
+        self.insert(Shape::Named);
+        self.insert(Shape::Newtype);
+        self.insert(Shape::Tuple);
+        self.insert(Shape::Unit);
+    }
+
+    /// Insert a shape into the set, so that the set will match that shape
+    pub fn insert(&mut self, shape: Shape) {
+        match shape {
+            Shape::Named => self.named = true,
+            Shape::Tuple => self.tuple = true,
+            Shape::Unit => self.unit = true,
+            Shape::Newtype => self.newtype = true,
+        }
+    }
+
+    /// Whether this set is empty.
+    pub fn is_empty(&self) -> bool {
+        !self.named && !self.newtype && !self.tuple && !self.unit
+    }
+
+    fn contains_shape(&self, shape: Shape) -> bool {
+        match shape {
+            Shape::Named => self.named,
+            Shape::Tuple => self.tuple,
+            Shape::Unit => self.unit,
+            Shape::Newtype => self.newtype || self.tuple,
+        }
+    }
+
+    /// Check if a fields container's shape is in this set.
+    pub fn contains(&self, fields: &impl AsShape) -> bool {
+        self.contains_shape(fields.as_shape())
+    }
+
+    /// Check if a field container's shape is in this set of shapes, and produce
+    /// an [`Error`](crate::Error) if it does not.
+    pub fn check(&self, fields: &impl AsShape) -> crate::Result<()> {
+        let shape = fields.as_shape();
+
+        if self.contains_shape(shape) {
+            Ok(())
+        } else {
+            Err(crate::Error::unsupported_shape_with_expected(
+                shape.description(),
+                self,
+            ))
+        }
+    }
+
+    fn to_vec(&self) -> Vec<Shape> {
+        let mut shapes = Vec::with_capacity(3);
+
+        if self.named {
+            shapes.push(Shape::Named);
+        }
+
+        if self.tuple || self.newtype {
+            shapes.push(if self.tuple {
+                Shape::Tuple
+            } else {
+                Shape::Newtype
+            });
+        }
+
+        if self.unit {
+            shapes.push(Shape::Unit)
+        }
+
+        shapes
+    }
+}
+
+impl fmt::Display for ShapeSet {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let shapes = self.to_vec();
+
+        match shapes.len() {
+            0 => write!(f, "nothing"),
+            1 => write!(f, "{}", shapes[0]),
+            2 => write!(f, "{} or {}", shapes[0], shapes[1]),
+            3 => write!(f, "{}, {}, or {}", shapes[0], shapes[1], shapes[2]),
+            _ => unreachable!(),
+        }
+    }
+}
+
+impl FromIterator<Shape> for ShapeSet {
+    fn from_iter<T: IntoIterator<Item = Shape>>(iter: T) -> Self {
+        let mut output = ShapeSet::default();
+        for shape in iter.into_iter() {
+            output.insert(shape);
+        }
+
+        output
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use syn::parse_quote;
+
+    use super::*;
+
+    #[test]
+    fn any_accepts_anything() {
+        let mut filter = ShapeSet::default();
+        filter.insert_all();
+        let unit_struct: syn::DeriveInput = syn::parse_quote! {
+            struct Example;
+        };
+        if let syn::Data::Struct(data) = unit_struct.data {
+            assert!(filter.contains(&data));
+        } else {
+            panic!("Struct not parsed as struct");
+        };
+    }
+
+    #[test]
+    fn tuple_accepts_newtype() {
+        let filter = ShapeSet::new(vec![Shape::Tuple]);
+        let newtype_struct: syn::DeriveInput = parse_quote! {
+            struct Example(String);
+        };
+
+        if let syn::Data::Struct(data) = newtype_struct.data {
+            assert!(filter.contains(&data));
+        } else {
+            panic!("Struct not parsed as struct");
+        };
+    }
+
+    #[test]
+    fn newtype_rejects_tuple() {
+        let filter = ShapeSet::new(vec![Shape::Newtype]);
+        let tuple_struct: syn::DeriveInput = parse_quote! {
+            struct Example(String, u64);
+        };
+
+        if let syn::Data::Struct(data) = tuple_struct.data {
+            assert!(!filter.contains(&data));
+        } else {
+            panic!("Struct not parsed as struct");
+        };
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/spanned_value.rs b/third_party/rust/darling_core/src/util/spanned_value.rs
new file mode 100644
index 0000000000..a97f9977df
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/spanned_value.rs
@@ -0,0 +1,133 @@
+use proc_macro2::Span;
+use std::ops::{Deref, DerefMut};
+use syn::spanned::Spanned;
+
+use crate::{
+    FromDeriveInput, FromField, FromGenericParam, FromGenerics, FromMeta, FromTypeParam,
+    FromVariant, Result,
+};
+
+/// A value and an associated position in source code. The main use case for this is
+/// to preserve position information to emit warnings from proc macros. You can use
+/// a `SpannedValue<T>` as a field in any struct that implements or derives any of
+/// `darling`'s core traits.
+///
+/// To access the underlying value, use the struct's `Deref` implementation.
+///
+/// # Defaulting
+/// This type is meant to be used in conjunction with attribute-extracted options,
+/// but the user may not always explicitly set those options in their source code.
+/// In this case, using `Default::default()` will create an instance which points
+/// to `Span::call_site()`.
+#[derive(Debug, Clone, Copy)]
+pub struct SpannedValue<T> {
+    value: T,
+    span: Span,
+}
+
+impl<T> SpannedValue<T> {
+    pub fn new(value: T, span: Span) -> Self {
+        SpannedValue { value, span }
+    }
+
+    /// Get the source code location referenced by this struct.
+    pub fn span(&self) -> Span {
+        self.span
+    }
+
+    /// Apply a mapping function to a reference to the spanned value.
+    pub fn map_ref<U>(&self, map_fn: impl FnOnce(&T) -> U) -> SpannedValue<U> {
+        SpannedValue::new(map_fn(&self.value), self.span)
+    }
+}
+
+impl<T: Default> Default for SpannedValue<T> {
+    fn default() -> Self {
+        SpannedValue::new(Default::default(), Span::call_site())
+    }
+}
+
+impl<T> Deref for SpannedValue<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        &self.value
+    }
+}
+
+impl<T> DerefMut for SpannedValue<T> {
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.value
+    }
+}
+
+impl<T> AsRef<T> for SpannedValue<T> {
+    fn as_ref(&self) -> &T {
+        &self.value
+    }
+}
+
+impl<T> Spanned for SpannedValue<T> {
+    fn span(&self) -> Span {
+        self.span
+    }
+}
+
+macro_rules! spanned {
+    ($trayt:ident, $method:ident, $syn:path) => {
+        impl<T: $trayt> $trayt for SpannedValue<T> {
+            fn $method(value: &$syn) -> Result<Self> {
+                Ok(SpannedValue::new(
+                    $trayt::$method(value).map_err(|e| e.with_span(value))?,
+                    value.span(),
+                ))
+            }
+        }
+    };
+}
+
+impl<T: FromMeta> FromMeta for SpannedValue<T> {
+    fn from_meta(item: &syn::Meta) -> Result<Self> {
+        let value = T::from_meta(item).map_err(|e| e.with_span(item))?;
+        let span = match item {
+            // Example: `#[darling(skip)]` as SpannedValue<bool>
+            // should have the span pointing to the word `skip`.
+            syn::Meta::Path(path) => path.span(),
+            // Example: `#[darling(attributes(Value))]` as a SpannedValue<Vec<String>>
+            // should have the span pointing to the list contents.
+            syn::Meta::List(list) => list.nested.span(),
+            // Example: `#[darling(skip = true)]` as SpannedValue<bool>
+            // should have the span pointing to the word `true`.
+            syn::Meta::NameValue(nv) => nv.lit.span(),
+        };
+
+        Ok(Self::new(value, span))
+    }
+}
+
+spanned!(FromGenericParam, from_generic_param, syn::GenericParam);
+spanned!(FromGenerics, from_generics, syn::Generics);
+spanned!(FromTypeParam, from_type_param, syn::TypeParam);
+spanned!(FromDeriveInput, from_derive_input, syn::DeriveInput);
+spanned!(FromField, from_field, syn::Field);
+spanned!(FromVariant, from_variant, syn::Variant);
+
+impl<T: Spanned> From<T> for SpannedValue<T> {
+    fn from(value: T) -> Self {
+        let span = value.span();
+        SpannedValue::new(value, span)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use proc_macro2::Span;
+
+    /// Make sure that `SpannedValue` can be seamlessly used as its underlying type.
+    #[test]
+    fn deref() {
+        let test = SpannedValue::new("hello", Span::call_site());
+        assert_eq!("hello", test.trim());
+    }
+}
diff --git a/third_party/rust/darling_core/src/util/with_original.rs b/third_party/rust/darling_core/src/util/with_original.rs
new file mode 100644
index 0000000000..5aa82f850b
--- /dev/null
+++ b/third_party/rust/darling_core/src/util/with_original.rs
@@ -0,0 +1,35 @@
+use crate::{
+    FromDeriveInput, FromField, FromGenericParam, FromGenerics, FromMeta, FromTypeParam,
+    FromVariant, Result,
+};
+
+/// A container to parse some syntax and retain access to the original.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct WithOriginal<T, O> {
+    pub parsed: T,
+    pub original: O,
+}
+
+impl<T, O> WithOriginal<T, O> {
+    pub fn new(parsed: T, original: O) -> Self {
+        WithOriginal { parsed, original }
+    }
+}
+
+macro_rules! with_original {
+    ($trayt:ident, $func:ident, $syn:path) => {
+        impl<T: $trayt> $trayt for WithOriginal<T, $syn> {
+            fn $func(value: &$syn) -> Result<Self> {
+                Ok(WithOriginal::new($trayt::$func(value)?, value.clone()))
+            }
+        }
+    };
+}
+
+with_original!(FromDeriveInput, from_derive_input, syn::DeriveInput);
+with_original!(FromField, from_field, syn::Field);
+with_original!(FromGenerics, from_generics, syn::Generics);
+with_original!(FromGenericParam, from_generic_param, syn::GenericParam);
+with_original!(FromMeta, from_meta, syn::Meta);
+with_original!(FromTypeParam, from_type_param, syn::TypeParam);
+with_original!(FromVariant, from_variant, syn::Variant);
-- 
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