Migrate away from ad-hoc snapshot testing for rustfix test suite

[weihanglo]

Problem

When dealing with rustfix snapshot test fixes, I found it a bit cumbersome to figure out the actual failure. You need to pass RUST_LOG=parse_and_replace=info to see the log, and RUSTFIX_TEST_BLESS=test-name.rs to update snapshots.

Proposed Solution

In the main Cargo crate, we've already integrated snapbox for UI tests. We could migrate rustfix test suite to that, and maybe share some common infra, like nightly channel detection from cargo-test-macro.

Notes

ehuss has a proposal that we don't run rustc if JSON snapshot exists: #13890 (comment).
This could be a good enhancement, and I think we can leave it to follow-ups.

[yatinmaan]

@rustbot claim

[epage]

FYI I had looked into this in the past and it was a bit messy because we only care about updating the rustc-output snapshots if the rustfix snapshots fail.

[Pyr0de]

Hi,
I'd like to like to try to solve this issue if @yatinmaan is not working on it.

Are all the RUSTFIX_TEST_* environment variables to be replaced by SNAPSHOTS=overwrite?
If so then on what conditions should the .json or .fixed.rs file be updated?

When SNAPSHOTS=overwrite,
If the json file is different from the rustc-output, then update the json file
If the applied changes are different, then update the fixed.rs file

[yatinmaan]

@Pyr0de sure, go ahead