Fix unused_parens false positive

Author: benschulz

compiler/rustc_lint/src/unused.rs

@@ -1055,6 +1055,7 @@ pub(crate) struct UnusedParens {
 /// ```
 /// type Example = Box<dyn Fn() -> &'static dyn Send>;
 /// ```
+#[derive(Copy, Clone)]
 enum NoBoundsException {
     /// The type must be parenthesized.
     None,
@@ -1333,7 +1334,11 @@ 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).copied();
+                let constraint = own_constraint.unwrap_or(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() {

tests/ui/lint/unused/unused-parens-false-positive-issue-143653.rs

@@ -0,0 +1,9 @@
+//@ check-pass
+
+#![deny(unused_parens)]
+#![allow(warnings)]
+trait MyTrait {}
+
+fn foo(_: Box<dyn FnMut(&mut u32) -> &mut (dyn MyTrait) + Send + Sync>) {}
+
+fn main() {}