Extend const_convert to rest of blanket core::convert impls
This adds constness to all the blanket impls in `core::convert` under the existing `const_convert` feature, tracked by #88674.
Existing impls under that feature:
```rust
impl<T> const From<T> for T;
impl<T, U> const Into<U> for T where U: ~const From<T>;
impl<T> const ops::Try for Option<T>;
impl<T> const ops::FromResidual for Option<T>;
impl<T, E> const ops::Try for Result<T, E>;
impl<T, E, F> const ops::FromResidual<Result<convert::Infallible, E>> for Result<T, F> where F: ~const From<E>;
```
Additional impls:
```rust
impl<T: ?Sized, U: ?Sized> const AsRef<U> for &T where T: ~const AsRef<U>;
impl<T: ?Sized, U: ?Sized> const AsRef<U> for &mut T where T: ~const AsRef<U>;
impl<T: ?Sized, U: ?Sized> const AsMut<U> for &mut T where T: ~const AsMut<U>;
impl<T, U> const TryInto<U> for T where U: ~const TryFrom<T>;
impl<T, U> const TryFrom<U> for T where U: ~const Into<T>;
```
Implement missing `AsMut<str>` for `str`
Allows `&mut str` to be taken by a Generic which requires `T` such that `T: AsMut<str>`. Motivating example:
```rust
impl<'i, T> From<T> for StructImmut<'i> where
T: AsRef<str> + 'i,
{
fn from(asref: T) -> Self {
let string: &str = asref.as_ref();
// ...
}
}
impl<'i, T> From<T> for StructMut<'i> where
T: AsMut<str> + 'i,
{
fn from(mut asmut: T) -> Self {
let string: &mut str = asmut.as_mut();
// ...
}
}
```
The Immutable form of this structure can be constructed by `StructImmut::from(s)` where `s` may be a `&String` or a `&str`, because `AsRef<str>` is implemented for `str`. However, the mutable form of the structure can be constructed in the same way **only** with a `&mut String`, and **not** with a `&mut str`.
This change does have some precedent, because as can be seen in [the Implementors](https://doc.rust-lang.org/std/convert/trait.AsMut.html#implementors), `AsMut<[T]>` is implemented for `[T]` as well as for `Vec<T>`, but `AsMut<str>` is implemented only for `String`. This would complete the symmetry.
As a trait implementation, this should be immediately stable.
Following traits are now diagnostic items:
- `From` (unchanged)
- `Into`
- `TryFrom`
- `TryInto`
This also adds symbols for those items:
- `into_trait`
- `try_from_trait`
- `try_into_trait`
Implement TryFrom between NonZero types.
This will instantly be stable, as trait implementations for stable types and traits can not be `#[unstable]`.
Closes#77258.
@rustbot modify labels: +T-libs
This commit adds new lang items which will be used in AST lowering once
`QPath::LangItem` is introduced.
Co-authored-by: Matthew Jasper <mjjasper1@gmail.com>
Signed-off-by: David Wood <david@davidtw.co>
