Auto merge of #85041 - mibac138:suggest-generics, r=estebank

Suggest adding a type parameter for impls

Add a new suggestion upon encountering an unknown type in a `impl` that suggests adding a new type parameter. This diagnostic suggests to add a new type parameter even though it may be a const parameter, however after adding the parameter and running rustc again a follow up error steers the user to change the type parameter to a const parameter.

```rust
struct X<const C: ()>();
impl X<C> {}
```
suggests
```
error[E0412]: cannot find type `C` in this scope
 --> bar.rs:2:8
  |
1 | struct X<const C: ()>();
  | ------------------------ similarly named struct `X` defined here
2 | impl X<C> {}
  |        ^
  |
help: a struct with a similar name exists
  |
2 | impl X<X> {}
  |        ^
help: you might be missing a type parameter
  |
2 | impl<C> X<C> {}
  |     ^^^
```
After adding a type parameter the code now becomes
```rust
struct X<const C: ()>();
impl<C> X<C> {}
```
and the error now fully steers the user towards the correct code
```
error[E0747]: type provided when a constant was expected
 --> bar.rs:2:11
  |
2 | impl<C> X<C> {}
  |           ^
  |
help: consider changing this type parameter to be a `const` generic
  |
2 | impl<const C: ()> X<C> {}
  |      ^^^^^^^^^^^
```
r? `@estebank`
Somewhat related #84946

1 files changed, 14 insertions, 5 deletions

diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index 13e457507cf..03b578d4ade 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -6,7 +6,10 @@ use crate::{CrateLint, Module, ModuleKind, ModuleOrUniformRoot};
 use crate::{PathResult, PathSource, Segment};
 
 use rustc_ast::visit::FnKind;
-use rustc_ast::{self as ast, Expr, ExprKind, Item, ItemKind, NodeId, Path, Ty, TyKind};
+use rustc_ast::{
+    self as ast, Expr, ExprKind, GenericParam, GenericParamKind, Item, ItemKind, NodeId, Path, Ty,
+    TyKind,
+};
 use rustc_ast_pretty::pprust::path_segment_to_string;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, SuggestionStyle};
@@ -1600,8 +1603,8 @@ 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 +1616,11 @@ 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) {
@@ -1633,6 +1638,10 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                     let (span, sugg) = if let [.., param] = &generics.params[..] {
                         let span = if let [.., bound] = &param.bounds[..] {
                             bound.span()
+                        } else if let GenericParam {
+                            kind: GenericParamKind::Const { ty, kw_span: _, default  }, ..
+                        } = param {
+                            default.as_ref().map(|def| def.value.span).unwrap_or(ty.span)
                         } else {
                             param.ident.span
                         };