Rollup merge of #143698 - benschulz:unused-parens-2, r=lcnr,compiler-errors

Fix unused_parens false positive

Resolves rust-lang/rust#143653.

The "no bounds exception" was indiscriminately set to `OneBound` for referents and pointees. However, if the reference or pointer type itself appears in no-bounds position, any constraints it has must be propagated.

```rust
// unused parens: not in no-bounds position
fn foo(_: Box<(dyn Send)>) {}

// unused parens: in no-bounds position, but one-bound exception applies
fn bar(_: Box<dyn Fn(&u32) -> &(dyn Send)>) {}

// *NOT* unused parens: in no-bounds position, but no exceptions to be made
fn baz(_: Box<dyn Fn(&u32) -> &(dyn Send) + Send>) {}
```

1 files changed, 9 insertions, 1 deletions

diff --git a/compiler/rustc_lint/src/unused.rs b/compiler/rustc_lint/src/unused.rs
index a9eb1739f7f..df9f3a500d9 100644
--- a/compiler/rustc_lint/src/unused.rs
+++ b/compiler/rustc_lint/src/unused.rs
@@ -1340,7 +1340,15 @@ impl EarlyLintPass for UnusedParens {
                 self.with_self_ty_parens = false;
             }
             ast::TyKind::Ref(_, mut_ty) | ast::TyKind::Ptr(mut_ty) => {
-                self.in_no_bounds_pos.insert(mut_ty.ty.id, NoBoundsException::OneBound);
+                // If this type itself appears in no-bounds position, we propagate its
+                // potentially tighter constraint or risk a false posive (issue 143653).
+                let own_constraint = self.in_no_bounds_pos.get(&ty.id);
+                let constraint = match own_constraint {
+                    Some(NoBoundsException::None) => NoBoundsException::None,
+                    Some(NoBoundsException::OneBound) => NoBoundsException::OneBound,
+                    None => NoBoundsException::OneBound,
+                };
+                self.in_no_bounds_pos.insert(mut_ty.ty.id, constraint);
             }
             ast::TyKind::TraitObject(bounds, _) | ast::TyKind::ImplTrait(_, bounds) => {
                 for i in 0..bounds.len() {