"echo" is not an application on Windows so `Command` tests could fail even if that's not what's being tested for.

The spans for "trait bound not satisfied" errors in trivial trait bounds referenced the entire item (fn, impl, struct) before.
Now they only reference the obligation itself (`String: Copy`)

Address rust-lang#90869

This should significantly reduce the frequency of merge conflicts.

Co-authored-by: Yuki Okushi <jtitor@2k36.org>

Windows: Resolve `process::Command` program without using the current directory

Currently `std::process::Command` searches many directories for the executable to run, including the current directory. This has lead to a [CVE for `ripgrep`](https://cve.circl.lu/cve/CVE-2021-3013) but presumably other command line utilities could be similarly vulnerable if they run commands. This was [discussed on the internals forum](https://internals.rust-lang.org/t/std-command-resolve-to-avoid-security-issues-on-windows/14800). Also discussed was [which directories should be searched](https://internals.rust-lang.org/t/windows-where-should-command-new-look-for-executables/15015).

EDIT: This PR originally removed all implicit paths. They've now been added back as laid out in the rest of this comment.

## Old Search Strategy

The old search strategy is [documented here][1]. Additionally Rust adds searching the child's paths (see also rust-lang#37519). So the full list of paths that were searched was:

1. The directories that are listed in the child's `PATH` environment variable.
2. The directory from which the application loaded.
3. The current directory for the parent process.
4. The 32-bit Windows system directory.
5. The 16-bit Windows system directory.
6. The Windows directory.
7. The directories that are listed in the PATH environment variable.

## New Search Strategy

The new strategy removes the current directory from the searched paths.

1. The directories that are listed in the child's PATH environment variable.
2. The directory from which the application loaded.
3. The 32-bit Windows system directory.
4. The Windows directory.
5. The directories that are listed in the parent's PATH environment variable.

Note that it also removes the 16-bit system directory, mostly because there isn't a function to get it. I do not anticipate this being an issue in modern Windows.

## Impact

Removing the current directory should fix CVE's like the one linked above. However, it's possible some Windows users of affected Rust CLI applications have come to expect the old behaviour.

This change could also affect small Windows-only script-like programs that assumed the current directory would be used. The user would need to use `.\file.exe` instead of the bare application name.

This PR could break tests, especially those that test the exact output of error messages (e.g. Cargo) as this does change the error messages is some cases.

[1]: https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-createprocessa#parameters

warn on must_use use on async fn's

As referenced in rust-lang#78149

This only works on `async` fn's for now, I can also look into if I can get `Box<dyn Future>` and `impl Future` working at this level (hir)

…shtriplett

Add Vec::retain_mut

This is to continue the discussion started in rust-lang#83218.

Original comment was:

> Take 2 of rust-lang#34265, since I needed this today.

The reason I think why we should add `retain_mut` is for coherency and for discoverability. For example we have `chunks` and `chunks_mut` or `get` and `get_mut` or `iter` and `iter_mut`, etc. When looking for mutable `retain`, I would expect `retain_mut` to exist. It took me a while to find out about `drain_filter`. So even if it provides an API close to `drain_filter`, just for the discoverability, I think it's worth it.

cc ```@m-ou-se``` ```@jonas-schievink``` ```@Mark-Simulacrum```

…r=oli-obk

Add `#[inline]`s to `SortedIndexMultiMap`

They're small enough and good candidates to add `#[inline]` generally.

Print escaped string if char literal has multiple characters, but only one printable character

Fixes rust-lang#90857

I'm not sure about the error message here, it could get rather long and *maybe* using the names of characters would be better? That wouldn't help the length any, though.

…und, r=estebank

Fix span for non-satisfied trivial trait bounds

The spans for "trait bound not satisfied" errors in trivial trait bounds referenced the entire item (fn, impl, struct) before.
Now they only reference the obligation itself (`String: Copy`)

Address rust-lang#90869

Remove workaround for the forward progress handling in LLVM

this workaround was only needed for LLVM < 12 and the minimum LLVM version was updated to 12 in rust-lang#90175

…=lnicola

⬆️ rust-analyzer

r? `@ghost`

…htriplett

Alphabetize language features

This should significantly reduce the frequency of merge conflicts.

r? ```@joshtriplett```

```@rustbot``` label: +A-contributor-roadblock +S-waiting-on-review

update miri

This is needed to fix https://github.com/rust-lang/miri-test-libstd
r? ```@ghost```