Lint redundant lifetimes in impl header

Author: compiler-errors

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -2028,19 +2028,20 @@ fn lint_redundant_lifetimes<'tcx>(
         | DefKind::TraitAlias
         | DefKind::Fn
         | DefKind::Const
-        | DefKind::Impl { of_trait: false } => {
+        | DefKind::Impl { of_trait: _ } => {
             // Proceed
         }
         DefKind::AssocFn | DefKind::AssocTy | DefKind::AssocConst => {
             let parent_def_id = tcx.local_parent(owner_id);
             if matches!(tcx.def_kind(parent_def_id), DefKind::Impl { of_trait: true }) {
-                // Don't check for redundant lifetimes for trait implementations,
-                // since the signature is required to be compatible with the trait.
+                // Don't check for redundant lifetimes for associated items of trait
+                // implementations, since the signature is required to be compatible
+                // with the trait, even if the implementation implies some lifetimes
+                // are redundant.
                 return;
             }
         }
-        DefKind::Impl { of_trait: true }
-        | DefKind::Mod
+        DefKind::Mod
         | DefKind::Variant
         | DefKind::TyAlias
         | DefKind::ForeignTy

tests/ui/regions/transitively-redundant-lifetimes.rs

@@ -15,4 +15,7 @@ impl<'a> Bar<'a> {
 
 fn ok(x: &'static &()) {}
 
+trait Tr<'a> {}
+impl<'a: 'static> Tr<'a> for () {} //~ ERROR unnecessary lifetime parameter `'a`
+
 fn main() {}

tests/ui/regions/transitively-redundant-lifetimes.stderr

@@ -27,6 +27,14 @@ LL | fn c<'a>(_: Foo<&'a ()>) {}
    |
    = note: you can use the `'static` lifetime directly, in place of `'a`
 
+error: unnecessary lifetime parameter `'a`
+  --> $DIR/transitively-redundant-lifetimes.rs:19:6
+   |
+LL | impl<'a: 'static> Tr<'a> for () {}
+   |      ^^
+   |
+   = note: you can use the `'static` lifetime directly, in place of `'a`
+
 error: unnecessary lifetime parameter `'b`
   --> $DIR/transitively-redundant-lifetimes.rs:13:10
    |
@@ -35,5 +43,5 @@ LL |     fn d<'b: 'a>(&'b self) {}
    |
    = note: you can use the `'a` lifetime directly, in place of `'b`
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors