better diagnostics for impl<..> impl Trait for Type

compiler/rustc_parse/messages.ftl

@@ -478,6 +478,11 @@ parse_missing_for_in_trait_impl = missing `for` in a trait impl
 
 parse_expected_trait_in_trait_impl_found_type = expected a trait, found type
 
+parse_extra_impl_keyword_in_trait_impl = unexpected `impl` keyword
+    .suggestion = remove the extra `impl`
+    .note = this is parsed as an `impl Trait` type, but a trait is expected at this position
+
+
 parse_non_item_in_item_list = non-item in item list
     .suggestion_use_const_not_let = consider using `const` instead of `let` for associated const
     .label_list_start = item list starts here

compiler/rustc_parse/src/errors.rs

@@ -1519,6 +1519,16 @@ pub(crate) struct ExpectedTraitInTraitImplFoundType {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_extra_impl_keyword_in_trait_impl)]
+pub(crate) struct ExtraImplKeywordInTraitImpl {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    pub extra_impl_kw: Span,
+    #[note]
+    pub impl_trait_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_bounds_not_allowed_on_trait_aliases)]
 pub(crate) struct BoundsNotAllowedOnTraitAliases {

compiler/rustc_parse/src/parser/item.rs

@@ -603,10 +603,24 @@ impl<'a> Parser<'a> {
                 let path = match ty_first.kind {
                     // This notably includes paths passed through `ty` macro fragments (#46438).
                     TyKind::Path(None, path) => path,
-                    _ => {
-                        self.sess.emit_err(errors::ExpectedTraitInTraitImplFoundType {
-                            span: ty_first.span,
-                        });
+                    other => {
+                        if let TyKind::ImplTrait(_, bounds) = other
+                            && let [bound] = bounds.as_slice()
+                        {
+                            // Suggest removing extra `impl` keyword:
+                            // `impl<T: Default> impl Default for Wrapper<T>`
+                            //                   ^^^^^
+                            let extra_impl_kw = ty_first.span.until(bound.span());
+                            self.sess
+                                .emit_err(errors::ExtraImplKeywordInTraitImpl {
+                                    extra_impl_kw,
+                                    impl_trait_span: ty_first.span
+                                });
+                        } else {
+                            self.sess.emit_err(errors::ExpectedTraitInTraitImplFoundType {
+                                span: ty_first.span,
+                            });
+                        }
                         err_path(ty_first.span)
                     }
                 };

tests/ui/impl-trait/extra-impl-in-trait-impl.fixed

@@ -0,0 +1,19 @@
+// run-rustfix
+
+struct S<T>(T);
+struct S2;
+
+impl<T: Default> Default for S<T> {
+    //~^ ERROR: unexpected `impl` keyword
+    //~| HELP: remove the extra `impl`
+    fn default() -> Self { todo!() }
+}
+
+impl Default for S2 {
+    //~^ ERROR: unexpected `impl` keyword
+    //~| HELP: remove the extra `impl`
+    fn default() -> Self { todo!() }
+}
+
+
+fn main() {}

tests/ui/impl-trait/extra-impl-in-trait-impl.rs

@@ -0,0 +1,19 @@
+// run-rustfix
+
+struct S<T>(T);
+struct S2;
+
+impl<T: Default> impl Default for S<T> {
+    //~^ ERROR: unexpected `impl` keyword
+    //~| HELP: remove the extra `impl`
+    fn default() -> Self { todo!() }
+}
+
+impl impl Default for S2 {
+    //~^ ERROR: unexpected `impl` keyword
+    //~| HELP: remove the extra `impl`
+    fn default() -> Self { todo!() }
+}
+
+
+fn main() {}

tests/ui/impl-trait/extra-impl-in-trait-impl.stderr

@@ -0,0 +1,26 @@
+error: unexpected `impl` keyword
+  --> $DIR/extra-impl-in-trait-impl.rs:6:18
+   |
+LL | impl<T: Default> impl Default for S<T> {
+   |                  ^^^^^ help: remove the extra `impl`
+   |
+note: this is parsed as an `impl Trait` type, but a trait is expected at this position
+  --> $DIR/extra-impl-in-trait-impl.rs:6:18
+   |
+LL | impl<T: Default> impl Default for S<T> {
+   |                  ^^^^^^^^^^^^
+
+error: unexpected `impl` keyword
+  --> $DIR/extra-impl-in-trait-impl.rs:12:6
+   |
+LL | impl impl Default for S2 {
+   |      ^^^^^ help: remove the extra `impl`
+   |
+note: this is parsed as an `impl Trait` type, but a trait is expected at this position
+  --> $DIR/extra-impl-in-trait-impl.rs:12:6
+   |
+LL | impl impl Default for S2 {
+   |      ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+