Recover parse gracefully from <const N>

compiler/rustc_parse/src/parser/generics.rs

@@ -109,7 +109,31 @@ impl<'a> Parser<'a> {
 
         self.expect_keyword(exp!(Const))?;
         let ident = self.parse_ident()?;
-        self.expect(exp!(Colon))?;
+        if let Err(mut err) = self.expect(exp!(Colon)) {
+            return if self.token.kind == token::Comma || self.token.kind == token::Gt {
+                // Recover parse from `<const N>` where the type is missing.
+                let span = const_span.to(ident.span);
+                err.span_suggestion_verbose(
+                    ident.span.shrink_to_hi(),
+                    "you likely meant to write the type of the const parameter here",
+                    ": /* Type */".to_string(),
+                    Applicability::HasPlaceholders,
+                );
+                let kind = TyKind::Err(err.emit());
+                let ty = self.mk_ty(span, kind);
+                Ok(GenericParam {
+                    ident,
+                    id: ast::DUMMY_NODE_ID,
+                    attrs: preceding_attrs,
+                    bounds: Vec::new(),
+                    kind: GenericParamKind::Const { ty, span, default: None },
+                    is_placeholder: false,
+                    colon_span: None,
+                })
+            } else {
+                Err(err)
+            };
+        }
         let ty = self.parse_ty()?;
 
         // Parse optional const generics default value.

tests/ui/const-generics/incorrect-const-param.rs

@@ -0,0 +1,45 @@
+// #84327
+
+struct VecWrapper<T>(Vec<T>);
+
+// Correct
+impl<T, const N: usize> From<[T; N]> for VecWrapper<T>
+where
+    T: Clone,
+{
+    fn from(slice: [T; N]) -> Self {
+        VecWrapper(slice.to_vec())
+    }
+}
+
+// Forgot const
+impl<T, N: usize> From<[T; N]> for VecWrapper<T> //~ ERROR expected value, found type parameter `N`
+where //~^ ERROR expected trait, found builtin type `usize`
+    T: Clone,
+{
+    fn from(slice: [T; N]) -> Self { //~ ERROR expected value, found type parameter `N`
+        VecWrapper(slice.to_vec())
+    }
+}
+
+// Forgot type
+impl<T, const N> From<[T; N]> for VecWrapper<T> //~ ERROR expected `:`, found `>`
+where
+    T: Clone,
+{
+    fn from(slice: [T; N]) -> Self {
+        VecWrapper(slice.to_vec())
+    }
+}
+
+// Forgot const and type
+impl<T, N> From<[T; N]> for VecWrapper<T> //~ ERROR expected value, found type parameter `N`
+where
+    T: Clone,
+{
+    fn from(slice: [T; N]) -> Self { //~ ERROR expected value, found type parameter `N`
+        VecWrapper(slice.to_vec())
+    }
+}
+
+fn main() {}

tests/ui/const-generics/incorrect-const-param.stderr

@@ -0,0 +1,70 @@
+error: expected `:`, found `>`
+  --> $DIR/incorrect-const-param.rs:26:16
+   |
+LL | impl<T, const N> From<[T; N]> for VecWrapper<T>
+   |                ^ expected `:`
+   |
+help: you likely meant to write the type of the const parameter here
+   |
+LL | impl<T, const N: /* Type */> From<[T; N]> for VecWrapper<T>
+   |                ++++++++++++
+
+error[E0423]: expected value, found type parameter `N`
+  --> $DIR/incorrect-const-param.rs:16:28
+   |
+LL | impl<T, N: usize> From<[T; N]> for VecWrapper<T>
+   |         -                  ^ not a value
+   |         |
+   |         found this type parameter
+
+error[E0404]: expected trait, found builtin type `usize`
+  --> $DIR/incorrect-const-param.rs:16:12
+   |
+LL | impl<T, N: usize> From<[T; N]> for VecWrapper<T>
+   |            ^^^^^ not a trait
+   |
+help: you might have meant to write a const parameter here
+   |
+LL | impl<T, const N: usize> From<[T; N]> for VecWrapper<T>
+   |         +++++
+
+error[E0423]: expected value, found type parameter `N`
+  --> $DIR/incorrect-const-param.rs:20:24
+   |
+LL | impl<T, N: usize> From<[T; N]> for VecWrapper<T>
+   |         - found this type parameter
+...
+LL |     fn from(slice: [T; N]) -> Self {
+   |                        ^ not a value
+
+error[E0423]: expected value, found type parameter `N`
+  --> $DIR/incorrect-const-param.rs:36:21
+   |
+LL | impl<T, N> From<[T; N]> for VecWrapper<T>
+   |         -           ^ not a value
+   |         |
+   |         found this type parameter
+   |
+help: you might have meant to write a const parameter here
+   |
+LL | impl<T, const N: /* Type */> From<[T; N]> for VecWrapper<T>
+   |         +++++  ++++++++++++
+
+error[E0423]: expected value, found type parameter `N`
+  --> $DIR/incorrect-const-param.rs:40:24
+   |
+LL | impl<T, N> From<[T; N]> for VecWrapper<T>
+   |         - found this type parameter
+...
+LL |     fn from(slice: [T; N]) -> Self {
+   |                        ^ not a value
+   |
+help: you might have meant to write a const parameter here
+   |
+LL | impl<T, const N: /* Type */> From<[T; N]> for VecWrapper<T>
+   |         +++++  ++++++++++++
+
+error: aborting due to 6 previous errors
+
+Some errors have detailed explanations: E0404, E0423.
+For more information about an error, try `rustc --explain E0404`.