Feat: Allow lifetime for bounds in non-binded generic params

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Allow lifetime `for<'a, ...>` bounds in non-bounded generic parameters.
+
 ### Changed
 - Use the `Copy` implementation for `Clone` and the `Ord` implementation for
   `PartialOrd` when custom bounds are present.

src/attr/item.rs

@@ -7,7 +7,7 @@ use syn::{
 	parse::{discouraged::Speculative, Parse, ParseStream},
 	punctuated::Punctuated,
 	spanned::Spanned,
-	Attribute, Data, Ident, Meta, Path, PredicateType, Result, Token, TraitBound,
+	Attribute, BoundLifetimes, Data, Ident, Meta, Path, PredicateType, Result, Token, TraitBound,
 	TraitBoundModifier, Type, TypeParamBound, TypePath, WhereClause, WherePredicate,
 };
 
@@ -241,7 +241,10 @@ impl DeriveWhere {
 	/// Returns `true` if the given generic type parameter if present.
 	pub fn has_type_param(&self, type_param: &Ident) -> bool {
 		self.generics.iter().any(|generic| match generic {
-			Generic::NoBound(Type::Path(TypePath { qself: None, path })) => {
+			Generic::NoBound(GenericNoBound {
+				lifetimes: _,
+				ty: Type::Path(TypePath { qself: None, path }),
+			}) => {
 				if let Some(ident) = path.get_ident() {
 					ident == type_param
 				} else {
@@ -281,9 +284,12 @@ impl DeriveWhere {
 					.predicates
 					.push(WherePredicate::Type(match generic {
 						Generic::CustomBound(type_bound) => type_bound.clone(),
-						Generic::NoBound(path) => PredicateType {
-							lifetimes: None,
-							bounded_ty: path.clone(),
+						Generic::NoBound(GenericNoBound {
+							lifetimes: bound_lifetimes,
+							ty,
+						}) => PredicateType {
+							lifetimes: bound_lifetimes.clone(),
+							bounded_ty: ty.clone(),
 							colon_token: <Token![:]>::default(),
 							bounds: trait_.where_bounds(item),
 						},
@@ -293,22 +299,43 @@ impl DeriveWhere {
 	}
 }
 
-/// Holds a single generic [type](Type) or [type with bound](PredicateType).
+/// Holds the first part of a [`PredicateType`] prior to the `:`. Optionally
+/// contains lifetime `for` bindings.
+#[derive(Eq, PartialEq)]
+pub struct GenericNoBound {
+	/// Any `for<'a, 'b, 'etc>` bindings for the type.
+	lifetimes: Option<BoundLifetimes>,
+	/// The type bound to the [`DeriveTrait`].
+	ty: Type,
+}
+
+impl Parse for GenericNoBound {
+	fn parse(input: ParseStream) -> Result<Self> {
+		Ok(Self {
+			lifetimes: input.parse()?,
+			ty: input.parse()?,
+		})
+	}
+}
+
+/// Holds a single generic [type](GenericNoBound) with optional lifetime bounds
+/// or [type with bound](PredicateType).
 #[derive(Eq, PartialEq)]
 pub enum Generic {
 	/// Generic type with custom [specified bounds](PredicateType).
 	CustomBound(PredicateType),
-	/// Generic [type](Type) which will be bound to the [`DeriveTrait`].
-	NoBound(Type),
+	/// Generic [type](GenericNoBound) which will be bound to the
+	/// [`DeriveTrait`].
+	NoBound(GenericNoBound),
 }
 
 impl Parse for Generic {
 	fn parse(input: ParseStream) -> Result<Self> {
 		let fork = input.fork();
 
 		// Try to parse input as a `WherePredicate`. The problem is, both expressions
-		// start with a Type, so starting with the `WherePredicate` is the easiest way
-		// of differentiating them.
+		// start with an optional lifetime for bound and then Type, so starting with the
+		// `WherePredicate` is the easiest way of differentiating them.
 		if let Ok(where_predicate) = WherePredicate::parse(&fork) {
 			input.advance_to(&fork);
 
@@ -319,8 +346,8 @@ impl Parse for Generic {
 				Err(Error::generic(where_predicate.span()))
 			}
 		} else {
-			match Type::parse(input) {
-				Ok(type_) => Ok(Generic::NoBound(type_)),
+			match GenericNoBound::parse(input) {
+				Ok(no_bound) => Ok(Generic::NoBound(no_bound)),
 				Err(error) => Err(Error::generic_syntax(error.span(), error)),
 			}
 		}

src/test/bound.rs

@@ -111,6 +111,31 @@ fn where_() -> Result<()> {
 	)
 }
 
+#[test]
+fn for_lifetime() -> Result<()> {
+	test_derive(
+		quote! {
+			#[derive_where(Clone; for<'a> T)]
+			struct Test<T, U>(T, std::marker::PhantomData<U>) where T: std::fmt::Debug;
+		},
+		quote! {
+			#[automatically_derived]
+			impl<T, U> ::core::clone::Clone for Test<T, U>
+			where
+				T: std::fmt::Debug,
+				for<'a> T: ::core::clone::Clone
+			{
+				#[inline]
+				fn clone(&self) -> Self {
+					match self {
+						Test(ref __field_0, ref __field_1) => Test(::core::clone::Clone::clone(__field_0), ::core::clone::Clone::clone(__field_1)),
+					}
+				}
+			}
+		},
+	)
+}
+
 #[test]
 fn associated_type() -> Result<()> {
 	test_derive(