This commit makes the following functions from `core::str` `const fn`:
- `from_utf8[_mut]` (`feature(const_str_from_utf8)`)
- `from_utf8_unchecked_mut` (`feature(const_str_from_utf8_unchecked_mut)`)
- `Utf8Error::{valid_up_to,error_len}` (`feature(const_str_from_utf8)`)

Fixes rust-lang#90974

…_utf8_unchecked_mut` features

…=Aaron1011

Add `-Zassert-incr-state` to assert state of incremental cache

Closes rust-lang#85864.

…=camelid

Clean up mess for --show-coverage documentation

It was somewhat duplicated for some reasons... Anyway, this remove this duplication and clean up a bit.

r? `@camelid`

Mention `Vec::remove` in `Vec::swap_remove`'s docs

Thought this was a nice addition.

add const generics test

cc rust-lang#89829 (comment)

r? rust-lang/project-const-generics

…=oli-obk

Make slice->str conversion and related functions `const`

This PR marks the following APIs as `const`:
```rust
// core::str
pub const fn from_utf8(v: &[u8]) -> Result<&str, Utf8Error>;
pub const fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error>;
pub const unsafe fn from_utf8_unchecked_mut(v: &mut [u8]) -> &mut str;

impl Utf8Error {
    pub const fn valid_up_to(&self) -> usize;
    pub const fn error_len(&self) -> Option<usize>;
}
```

Everything but `from_utf8_unchecked_mut` uses `const_str_from_utf8` feature gate, `from_utf8_unchecked_mut` uses `const_str_from_utf8_unchecked_mut` feature gate.

---

I'm not sure why `from_utf8_unchecked_mut` was left out being  non-`const`, considering that `from_utf8_unchecked` is not only `const`, but **`const` stable**.

---

r? `@oli-obk` (performance-only `const_eval_select` use)

…loat-ending-in-dot, r=sanxiyn

Avoid suggesting literal formatting that turns into member access

Fixes rust-lang#90974