r? @nikomatsakis

I went ahead and replaced repeat count with a `Constant`, because it cannot be non-constant to the best of my knowledge.

Closes https://github.com/rust-lang/rust/issues/29746

- Successful merges: #29677, #29772, #29775, #29777
- Failed merges:

None

It's a bit strange to expect users of `libstd` to require the use of an external crates.io crate to work with standard types. This commit encourages the use `os::raw::c_char` instead, although users are certainly free to use `libc::c_char` if they wish; the test still exists to ensure the two types are identical (though the reported bug only exists on platforms that are not officially tested).

Fixes #29774

r? @Manishearth
cc @AlisdairO

Part of #24407

Fixes #7526

The older algorithm was pretty inefficient for big matches. Fixes #29227. (On my computer, MIR construction on this test case goes from 9.9s to 0.025s.) Whereas before we had a loop like:

- for all outcomes of the test we are performing
    - for all candidates
        - check whether candidate is relevant to outcome

We now do:

- for all candidates
    - determine which outcomes the candidate is relevant to

Since the number of outcomes in this case is proportional to the number of candidates, the original algorithm turned out to be O(n^2), and the newer one is just O(n).

This PR also does some minor speedups by eagerly mirroring all patterns, so that we can just pass around `&Pattern<'tcx>`, which makes cloning cheaper. We could probably go a bit further in this direction.

r? @Aatch

Hi Rustaceans!

This is the secong take on running latest rustfmt on librustc_driver!

All fixups made in https://github.com/rust-lang/rust/pull/29033 were done (also rustfmt got better).

//cc @nrc

`Vec` should not be resized.

Moved from #29753. r? @steveklabnik

None

Fixes #29751.

Fix #16410.

before we iterated over the test and each outcome thereof, and then
checked processed every candidate against this outcome, we now organize
the walk differently. Instead, we visit each candidate and say "Here is
the test being performed. Figure out the resulting candidates for each
possible outcome and add yourself into the appropriate places."