From 4c664961d7769f56b7af75c2d20ab0bd0c348091 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Esteban=20K=C3=BCber?= <esteban@kuber.com.ar>
Date: Tue, 4 Jun 2024 19:09:15 +0000
Subject: [PATCH] When `derive`ing, account for HRTB on `BareFn` fields

When given

```rust
trait SomeTrait {
    type SomeType<'a>;
}

#[derive(Clone)]
struct Foo<T: SomeTrait> {
    x: for<'a> fn(T::SomeType<'a>)
}
```

expand to

```rust
impl<T: ::core::clone::Clone + SomeTrait> ::core::clone::Clone for Foo<T>
    where for<'a> T::SomeType<'a>: ::core::clone::Clone {
    #[inline]
    fn clone(&self) -> Foo<T> {
        Foo { x: ::core::clone::Clone::clone(&self.x) }
    }
}
```

instead of the previous invalid

```
impl<T: ::core::clone::Clone + SomeTrait> ::core::clone::Clone for Foo<T>
    where T::SomeType<'a>: ::core::clone::Clone {
    #[inline]
    fn clone(&self) -> Foo<T> {
        Foo { x: ::core::clone::Clone::clone(&self.x) }
    }
}
```

Fix #122622.
---
 .../src/deriving/generic/mod.rs                    | 12 +++++++++++-
 .../derive-hrtb-for-bare-fn-field-with-lifetime.rs | 14 ++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/derives/derive-hrtb-for-bare-fn-field-with-lifetime.rs

diff --git a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
index 217fa5ff9f1e5..ba289f9552e22 100644
--- a/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/generic/mod.rs
@@ -412,6 +412,15 @@ fn find_type_parameters(
 
     impl<'a, 'b> visit::Visitor<'a> for Visitor<'a, 'b> {
         fn visit_ty(&mut self, ty: &'a ast::Ty) {
+            let stack_len = self.bound_generic_params_stack.len();
+            if let ast::TyKind::BareFn(bare_fn) = &ty.kind
+                && !bare_fn.generic_params.is_empty()
+            {
+                // Given a field `x: for<'a> fn(T::SomeType<'a>)`, we wan't to account for `'a` so
+                // that we generate `where for<'a> T::SomeType<'a>: ::core::clone::Clone`. #122622
+                self.bound_generic_params_stack.extend(bare_fn.generic_params.iter().cloned());
+            }
+
             if let ast::TyKind::Path(_, path) = &ty.kind
                 && let Some(segment) = path.segments.first()
                 && self.ty_param_names.contains(&segment.ident.name)
@@ -422,7 +431,8 @@ fn find_type_parameters(
                 });
             }
 
-            visit::walk_ty(self, ty)
+            visit::walk_ty(self, ty);
+            self.bound_generic_params_stack.truncate(stack_len);
         }
 
         // Place bound generic params on a stack, to extract them when a type is encountered.
diff --git a/tests/ui/derives/derive-hrtb-for-bare-fn-field-with-lifetime.rs b/tests/ui/derives/derive-hrtb-for-bare-fn-field-with-lifetime.rs
new file mode 100644
index 0000000000000..92008f0fa0f7f
--- /dev/null
+++ b/tests/ui/derives/derive-hrtb-for-bare-fn-field-with-lifetime.rs
@@ -0,0 +1,14 @@
+//@ run-pass
+// Issue #122622: `#[derive(Clone)]` should work for HRTB function type taking an associated type
+#122622 
+#![allow(dead_code)]
+trait SomeTrait {
+    type SomeType<'a>;
+}
+
+#[derive(Clone)]
+struct Foo<T: SomeTrait> {
+    x: for<'a> fn(T::SomeType<'a>)
+}
+
+fn main() {}