Suggest adding a type parameter for impls

Author: mibac138

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1600,8 +1600,8 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         if !self.diagnostic_metadata.currently_processing_generics && !single_uppercase_char {
             return None;
         }
-        match (self.diagnostic_metadata.current_item, single_uppercase_char) {
-            (Some(Item { kind: ItemKind::Fn(..), ident, .. }), _) if ident.name == sym::main => {
+        match (self.diagnostic_metadata.current_item, single_uppercase_char, self.diagnostic_metadata.currently_processing_generics) {
+            (Some(Item { kind: ItemKind::Fn(..), ident, .. }), _, _) if ident.name == sym::main => {
                 // Ignore `fn main()` as we don't want to suggest `fn main<T>()`
             }
             (
@@ -1613,9 +1613,11 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                         | kind @ ItemKind::Union(..),
                     ..
                 }),
-                true,
+                true, _
             )
-            | (Some(Item { kind, .. }), false) => {
+            // Without the 2nd `true`, we'd suggest `impl <T>` for `impl T` when a type `T` isn't found
+            | (Some(Item { kind: kind @ ItemKind::Impl(..), .. }), true, true)
+            | (Some(Item { kind, .. }), false, _) => {
                 // Likely missing type parameter.
                 if let Some(generics) = kind.generics() {
                     if span.overlaps(generics.span) {

src/test/ui/const-generics/diagnostics.stderr

@@ -5,7 +5,16 @@ LL | struct A<const N: u8>;
    | ---------------------- similarly named struct `A` defined here
 LL | trait Foo {}
 LL | impl Foo for A<N> {}
-   |                ^ help: a struct with a similar name exists: `A`
+   |                ^
+   |
+help: a struct with a similar name exists
+   |
+LL | impl Foo for A<A> {}
+   |                ^
+help: you might be missing a type parameter
+   |
+LL | impl<N> Foo for A<N> {}
+   |     ^^^
 
 error[E0412]: cannot find type `T` in this scope
   --> $DIR/diagnostics.rs:16:32
@@ -14,7 +23,16 @@ LL | struct A<const N: u8>;
    | ---------------------- similarly named struct `A` defined here
 ...
 LL | impl<const N: u8> Foo for C<N, T> {}
-   |                                ^ help: a struct with a similar name exists: `A`
+   |                                ^
+   |
+help: a struct with a similar name exists
+   |
+LL | impl<const N: u8> Foo for C<N, A> {}
+   |                                ^
+help: you might be missing a type parameter
+   |
+LL | impl<const N, T: u8> Foo for C<N, T> {}
+   |             ^^^
 
 error[E0747]: unresolved item provided when a constant was expected
   --> $DIR/diagnostics.rs:7:16

src/test/ui/traits/issue-75627.stderr

@@ -2,7 +2,9 @@ error[E0412]: cannot find type `T` in this scope
   --> $DIR/issue-75627.rs:3:26
    |
 LL | unsafe impl Send for Foo<T> {}
-   |                          ^ not found in this scope
+   |            -             ^ not found in this scope
+   |            |
+   |            help: you might be missing a type parameter: `<T>`
 
 error: aborting due to previous error
 

src/test/ui/traits/issue-78372.stderr

@@ -13,9 +13,18 @@ error[E0412]: cannot find type `U` in this scope
   --> $DIR/issue-78372.rs:3:31
    |
 LL | impl<T> DispatchFromDyn<Smaht<U, MISC>> for T {}
-   |      -                        ^ help: a type parameter with a similar name exists: `T`
+   |      -                        ^
    |      |
    |      similarly named type parameter `T` defined here
+   |
+help: a type parameter with a similar name exists
+   |
+LL | impl<T> DispatchFromDyn<Smaht<T, MISC>> for T {}
+   |                               ^
+help: you might be missing a type parameter
+   |
+LL | impl<T, U> DispatchFromDyn<Smaht<U, MISC>> for T {}
+   |       ^^^
 
 error[E0412]: cannot find type `MISC` in this scope
   --> $DIR/issue-78372.rs:3:34