Fix tracking issue numbers for some unstable features
And also remove deprecated unstable `#[panic_implementation]` attribute that was superseded by stable `#[panic_handler]` and doesn't have an open tracking issue.
Add filtering option to `rustc_on_unimplemented` and reword `Iterator` E0277 errors
- Add more targetting filters for arrays to `rustc_on_unimplemented` (Fix#53766)
- Detect one element array of `Range` type, which is potentially a typo:
`for _ in [0..10] {}` where iterating between `0` and `10` was intended.
(Fix#23141)
- Suggest `.bytes()` and `.chars()` for `String`.
- Suggest borrowing or `.iter()` on arrays (Fix#36391)
- Suggest using range literal when iterating on integers (Fix#34353)
- Do not suggest `.iter()` by default (Fix#50773, fix#46806)
- Add regression test (Fix#22872)
- Detect one element array of `Range` type, which is potentially a typo:
`for _ in [0..10] {}` where iterating between `0` and `10` was intended.
(#23141)
- Suggest `.bytes()` and `.chars()` for `String`.
- Suggest borrowing or `.iter()` on arrays (#36391)
- Suggest using range literal when iterating on integers (#34353)
- Do not suggest `.iter()` by default (#50773, #46806)
Stabilize pattern_parentheses feature
Addresses #51087 .
Stabilizes the previously unstable feature `pattern_parentheses` which enables the use of `()` in match patterns.
`impl trait` in bindings (feature: impl-trait-existential-types)
This PR enables `impl Trait` syntax (opaque types) to be used in bindings, e.g.
* `let foo: impl Clone = 1;`
* `static foo: impl Clone = 2;`
* `const foo: impl Clone = 3;`
This is part of [RFC 2071](https://github.com/rust-lang/rfcs/blob/master/text/2071-impl-trait-existential-types.md) ([tracking issue](https://github.com/rust-lang/rust/issues/34511)), but exists behind the separate feature gate `impl_trait_in_bindings`.
CC @cramertj @oli-obk @eddyb @Centril @varkor
Support an explicit annotation for marker traits
From the tracking issue for rust-lang/rfcs#1268:
> It seems obvious that we should make a `#[marker]` annotation. ~ https://github.com/rust-lang/rust/issues/29864#issuecomment-368959441
This PR allows you to put `#[marker]` on a trait, at which point:
- [x] The trait must not have any items ~~All of the trait's items must have defaults~~
- [x] Any impl of the trait must be empty (not override any items)
- [x] But impls of the trait are allowed to overlap
r? @nikomatsakis
