Add specific feature gate error for const-unstable features
Before:
```
error: `impl Trait` in const fn is unstable
--> src/lib.rs:7:19
|
7 | const fn foo() -> impl T {
| ^^^^^^
error: aborting due to previous error
```
After:
```
error[E0723]: `impl Trait` in const fn is unstable (see issue #57563)
--> src/lib.rs:7:19
|
7 | const fn foo() -> impl T {
| ^^^^^^
= help: add #![feature(const_fn)] to the crate attributes to enable
error: aborting due to previous error
```
This improves the situation with https://github.com/rust-lang/rust/issues/57563. Fixes https://github.com/rust-lang/rust/issues/57544. Fixes https://github.com/rust-lang/rust/issues/54469.
r? @oli-obk
Stabilize slice_sort_by_cached_key
I was going to ask on the tracking issue (https://github.com/rust-lang/rust/issues/34447), but decided to just send this and hope for an FCP here. The method was added last March by https://github.com/rust-lang/rust/pull/48639.
Signature: https://doc.rust-lang.org/std/primitive.slice.html#method.sort_by_cached_key
```rust
impl [T] {
pub fn sort_by_cached_key<K, F>(&mut self, f: F)
where F: FnMut(&T) -> K, K: Ord;
}
```
That's an identical signature to the existing `sort_by_key`, so I think the questions are just naming, implementation, and the usual "do we want this?".
The implementation seems to have proven its use in rustc at least, which many uses: https://github.com/rust-lang/rust/search?l=Rust&q=sort_by_cached_key
(I'm asking because it's exactly what I just needed the other day:
```rust
all_positions.sort_by_cached_key(|&n|
data::CITIES.iter()
.map(|x| *metric_closure.get_edge(n, x.pos).unwrap())
.sum::<usize>()
);
```
since caching that key is a pretty obviously good idea.)
Closes#34447
In our type inference system, when we "generalize" a type T to become
a suitable value for a type variable V, we sometimes wind up creating
new inference variables. So, for example, if we are making V be some
subtype of `&'X u32`, then we might instantiate V with `&'Y u32`.
This generalized type is then related `&'Y u32 <: &'X u32`, resulting
in a region constriant `'Y: 'X`. Previously, however, we were making
these fresh variables like `'Y` in the "current universe", but they
should be created in the universe of V. Moreover, we sometimes cheat
in an invariant context and avoid creating fresh variables if we know
the result must be equal -- we can only do that when the universes
work out.
rustc_mir: split qualify_consts' "value qualification" bitflags into separate computations.
Prerequisite for computing those bits through a dataflow algorithm ~~(which I might do in this PR later)~~.
This PR should not change behavior overall, other than treating `simd_shuffle*` identically to `#[rustc_args_required_const]` (maybe we should just have `#[rustc_args_required_const]` on the intrinsic imports of `simd_shuffle*`? cc @gnzlbg)
cc @oli-obk @alexreg
