```
error[E0637]: `&` without an explicit lifetime name cannot be used here
  --> $DIR/E0637.rs:13:13
   |
LL |     T: Into<&u32>,
   |             ^ explicit lifetime name needed here
   |
help: consider introducing a higher-ranked lifetime here
   |
LL |     T: for<'a> Into<&'a u32>,
   |        +++++++       ++
```

…lnay

Stabilize `const_cstr_methods`

This PR seeks to stabilize `const_cstr_methods`. Fixes most of rust-lang#101719

## New const stable API

```rust
impl CStr {
    // depends: memchr
    pub const fn from_bytes_with_nul(bytes: &[u8]) -> Result<&Self, FromBytesWithNulError> {...}
    // depends: const_slice_index
    pub const fn to_bytes(&self) -> &[u8] {}
    // depends: pointer casts
    pub const fn to_bytes_with_nul(&self) -> &[u8] {}
    // depends: str::from_utf8
    pub const fn to_str(&self) -> Result<&str, str::Utf8Error> {}
}
```

I don't think any of these methods will have any issue when `CStr` becomes a thin pointer as long as `memchr` is const  (which also allows for const `strlen`) .

## Notes

- `from_bytes_until_nul` relies on `const_slice_index`, which relies on `const_trait_impls`, and generally this should be avoided. After talking with Oli, it should be OK in this case because we could replace the ranges with pointer tricks if needed (worst case being those feature gates disappear). rust-lang#107624 (comment)
- Making `from_ptr` const is deferred because it depends on `const_eval_select`. I have moved this under the new flag `const_cstr_from_ptr` rust-lang#107624 (comment)

cc ``@oli-obk`` I think you're the const expert

``@rustbot`` modify labels: +T-libs-api +needs-fcp

…-errors

suggest `slice::swap` for `mem::swap(&mut x[0], &mut x[1])` borrowck error

Recently saw someone ask why this code (example slightly modified):
```rs
fn main() {
  let mut foo = [1, 2];
  std::mem::swap(&mut foo[0], &mut foo[1]);
}
```
triggers this error and how to fix it:
```
error[E0499]: cannot borrow `foo[_]` as mutable more than once at a time
 --> src/main.rs:4:33
  |
4 |     std::mem::swap(&mut foo[0], &mut foo[1]);
  |     -------------- -----------  ^^^^^^^^^^^ second mutable borrow occurs here
  |     |              |
  |     |              first mutable borrow occurs here
  |     first borrow later used by call
  |
  = help: consider using `.split_at_mut(position)` or similar method to obtain two mutable non-overlapping sub-slices
```
The current help message is nice and goes in the right direction, but I think we can do better for this specific instance and suggest `slice::swap`, which makes this compile

…time-args-in-apit, r=eholk

Account for late-bound vars from parent arg-position impl trait

We should be reporting an error like we do for late-bound args coming from a parent APIT.

Fixes rust-lang#113016

…unds, r=spastorino

Encode item bounds for `DefKind::ImplTraitPlaceholder`

This was lost in a refactoring -- `hir::ItemKind::OpaqueTy` doesn't always map to `DefKind::Opaque`, specifically for RPITITs, so the check was migrated subtly wrong, and unfortunately I never had a test for this 🙃

Fixes rust-lang#113155

r? ``@cjgillot``

…er-errors

Properly implement variances_of for RPITIT GAT

This fixes some of the issues found by crater run in rust-lang#112988 (comment)

r? ``@compiler-errors``

Use structured suggestion when telling user about `for<'a>`

```
error[E0637]: `&` without an explicit lifetime name cannot be used here
  --> $DIR/E0637.rs:13:13
   |
LL |     T: Into<&u32>,
   |             ^ explicit lifetime name needed here
   |
help: consider introducing a higher-ranked lifetime here
   |
LL |     T: for<'a> Into<&'a u32>,
   |        +++++++       ++
```