chore: checkpoint before Python removal

vendor/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()
+    }
+}

vendor/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>;

vendor/darling_core/src/usage/lifetimes.rs

@@ -0,0 +1,351 @@
+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::AssocType, ty);
+uses_lifetimes!(syn::BareFnArg, ty);
+uses_lifetimes!(syn::BoundLifetimes, lifetimes);
+uses_lifetimes!(syn::ConstParam, ty);
+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::LifetimeParam, lifetime, bounds);
+uses_lifetimes!(syn::ParenthesizedGenericArguments, inputs, output);
+uses_lifetimes!(syn::Path, segments);
+uses_lifetimes!(syn::PathSegment, arguments);
+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::TypeParam, 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),
+            // non-exhaustive enum
+            // TODO: replace panic with failible function
+            _ => panic!("Unknown syn::WherePredicate: {:?}", self),
+        }
+    }
+}
+
+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::AssocType(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::AssocConst(_) | syn::GenericArgument::Const(_) => {
+                Default::default()
+            }
+            // non-exhaustive enum
+            // TODO: replace panic with failible function
+            _ => panic!("Unknown syn::GenericArgument: {:?}", self),
+        }
+    }
+}
+
+impl UsesLifetimes for syn::GenericParam {
+    fn uses_lifetimes<'a>(
+        &self,
+        options: &Options,
+        lifetimes: &'a LifetimeSet,
+    ) -> LifetimeRefSet<'a> {
+        match *self {
+            syn::GenericParam::Lifetime(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::GenericParam::Type(ref v) => v.uses_lifetimes(options, lifetimes),
+            syn::GenericParam::Const(ref v) => v.uses_lifetimes(options, lifetimes),
+        }
+    }
+}
+
+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),
+            // non-exhaustive enum
+            // TODO: replace panic with failible function
+            _ => panic!("Unknown syn::TypeParamBound: {:?}", self),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use proc_macro2::Span;
+    use syn::{parse_quote, 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);
+    }
+}

vendor/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};

vendor/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
+    }
+}

vendor/darling_core/src/usage/type_params.rs

@@ -0,0 +1,364 @@
+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::AssocType, ty);
+uses_type_params!(syn::BareFnArg, 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::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),
+            // non-exhaustive enum
+            // TODO: replace panic with failible function
+            _ => panic!("Unknown syn::WherePredicate: {:?}", self),
+        }
+    }
+}
+
+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::AssocType(ref v) => v.uses_type_params(options, type_set),
+            syn::GenericArgument::Constraint(ref v) => v.uses_type_params(options, type_set),
+            syn::GenericArgument::AssocConst(_)
+            | syn::GenericArgument::Const(_)
+            | syn::GenericArgument::Lifetime(_) => Default::default(),
+            // non-exhaustive enum
+            // TODO: replace panic with failible function
+            _ => panic!("Unknown syn::GenericArgument: {:?}", self),
+        }
+    }
+}
+
+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(),
+            // non-exhaustive enum
+            // TODO: replace panic with failible function
+            _ => panic!("Unknown syn::TypeParamBound: {:?}", self),
+        }
+    }
+}
+
+#[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<dyn 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<dyn 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)));
+    }
+}