Recover parse gracefully from `<const N>`
When a const param doesn't have a `: Type`, recover the parser state and provide a structured suggestion. This not only provides guidance on what was missing, but it also makes subsuequent errors to be emitted that would otherwise be silenced.
```
error: expected `:`, found `>`
--> $DIR/incorrect-const-param.rs:26:16
|
LL | impl<T, const N> From<[T; N]> for VecWrapper<T>
| ^ expected `:`
|
help: you might have meant to write the type of the const parameter here
|
LL | impl<T, const N: /* Type */> From<[T; N]> for VecWrapper<T>
| ++++++++++++
```
r? @fmease
Follow up to rust-lang/rust#151077. Fixrust-lang/rust#84327.
00501de1c8