…k, r=jyn514

Support non-stage0 check

Seems to work locally - a full stage 1 check succeeds, building std (because we can't get away with checking it), and then checking the compiler and other tools. This ran into the problem that a unconditional x.py check in stage 1 *both* checks and builds stage 1 std, and then has to clean up because for some reason the rmeta and rlib artifacts conflict (though I'm not actually entirely sure why, but it doesn't seem worth digging in in too much detail).

Ideally we wouldn't be building and checking like that but it's a minor worry as checking std is pretty fast and you can avoid it if you're aiming for speed by passing the compiler (e.g., compiler/rustc) explicitly.

r? `@jyn514`

…k, r=jyn514

Support non-stage0 check

Seems to work locally - a full stage 1 check succeeds, building std (because we can't get away with checking it), and then checking the compiler and other tools. This ran into the problem that a unconditional x.py check in stage 1 *both* checks and builds stage 1 std, and then has to clean up because for some reason the rmeta and rlib artifacts conflict (though I'm not actually entirely sure why, but it doesn't seem worth digging in in too much detail).

Ideally we wouldn't be building and checking like that but it's a minor worry as checking std is pretty fast and you can avoid it if you're aiming for speed by passing the compiler (e.g., compiler/rustc) explicitly.

r? ``@jyn514``

Rollup of 13 pull requests

Successful merges:

 - rust-lang#79298 (correctly deal with late-bound lifetimes in anon consts)
 - rust-lang#80031 (resolve: Reject ambiguity built-in attr vs different built-in attr)
 - rust-lang#80201 (Add benchmark and fast path for BufReader::read_exact)
 - rust-lang#80635 (Improve diagnostics when closure doesn't meet trait bound)
 - rust-lang#80765 (resolve: Simplify collection of traits in scope)
 - rust-lang#80932 (Allow downloading LLVM on Windows and MacOS)
 - rust-lang#80983 (Remove is_dllimport_foreign_item definition from cg_ssa)
 - rust-lang#81064 (Support non-stage0 check)
 - rust-lang#81080 (Force vec![] to expression position only)
 - rust-lang#81082 (BTreeMap: clean up a few more comments)
 - rust-lang#81084 (Use Option::map instead of open-coding it)
 - rust-lang#81095 (Use Option::unwrap_or instead of open-coding it)
 - rust-lang#81107 (Add NonZeroUn::is_power_of_two)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

Stage 1 check has been supported since rust-lang#81064.
 rust-lang#81064 changed the error message for this, but I don't think there's any reason we should prevent using it.
I tested locally and `keep-stage` works fine. Don't give a hard error when trying to use it.

…ulacrum

Support `x --keep-stage 0 check`

Stage 1 check has been supported since rust-lang#81064.
 rust-lang#81064 changed the error message for this, but I don't think there's any reason we should prevent using it.
I tested locally and `keep-stage` works fine.

r? `@Mark-Simulacrum`