Recover keywords in trait bounds
(_this pr was inspired by [this tweet](https://twitter.com/Azumanga/status/1552982326409367561)_)
Recover keywords in trait bound, motivational example:
```rust
fn f(_: impl fn()) {} // mistyped, meant `Fn`
```
<details><summary>Current nightly (3 needless and confusing errors!)</summary>
<p>
```text
error: expected identifier, found keyword `fn`
--> ./t.rs:1:15
|
1 | fn _f(_: impl fn()) {}
| ^^ expected identifier, found keyword
|
help: escape `fn` to use it as an identifier
|
1 | fn _f(_: impl r#fn()) {}
| ++
error: expected one of `:` or `|`, found `)`
--> ./t.rs:1:19
|
1 | fn _f(_: impl fn()) {}
| ^ expected one of `:` or `|`
error: expected one of `!`, `(`, `)`, `,`, `?`, `for`, `~`, lifetime, or path, found keyword `fn`
--> ./t.rs:1:15
|
1 | fn _f(_: impl fn()) {}
| -^^ expected one of 9 possible tokens
| |
| help: missing `,`
error: at least one trait must be specified
--> ./t.rs:1:10
|
1 | fn _f(_: impl fn()) {}
| ^^^^
```
</p>
</details>
This PR:
```text
error: expected identifier, found keyword `fn`
--> ./t.rs:1:15
|
1 | fn _f(_: impl fn()) {}
| ^^ expected identifier, found keyword
|
help: escape `fn` to use it as an identifier
|
1 | fn _f(_: impl r#fn()) {}
| ++
error[E0405]: cannot find trait `r#fn` in this scope
--> ./t.rs:1:15
|
1 | fn _f(_: impl fn()) {}
| ^^ help: a trait with a similar name exists (notice the capitalization): `Fn`
|
::: /home/waffle/projects/repos/rust/library/core/src/ops/function.rs:74:1
|
74 | pub trait Fn<Args>: FnMut<Args> {
| ------------------------------- similarly named trait `Fn` defined here
```
It would be nice to have suggestion in the first error like "have you meant `Fn` trait", instead of a separate error, but the recovery is deep inside ident parsing, which makes it a lot harder to do.
r? `@compiler-errors`
This alters the diagnostics a bit, as the trait method is still stable.
The only thing this check does is ensure that compilation fails if a
trait implementation is declared const-stable.
Rather than deferring to const eval for checking if a trait is const, we
now check up-front. This allows the error to be emitted earlier, notably
at the same time as other stability checks.
Also included in this commit is a change of the default const stability
level to UNstable. Previously, an item that was `const` but did not
explicitly state it was unstable was implicitly stable.
`s/compiler-flags/compile-flags` in compiletest
Also make compiletest panic so this doesn't happen in the future! I literally always forget which it's called, so I wanted to make my life easier in the future.
Also open to the possibility of parsing both.
Instead of probing for all possible impls that could have caused an
`ImplObligation`, keep track of its `DefId` and obligation spans for
accurate error reporting.
Follow up to #89580. Addresses #89418.
Remove some unnecessary clones.
Tweak output for auto trait impl obligations.
