stop leaking inference variables from snapshots

[lcnr]

a revival of #100745. Right now we can very easily ICE when trying to use the returned values from probe and commit_if_ok.

I closed that PR as this check detected issues but we didn't get any unexpected ICEs from this, so I thought:

so afaict inference variables actually remain valid, we just drop any unifications from inside of the snapshot 🤔 so i guess while still weird, this isn't a soundness issue?

nah, we just tend to not use the leaked inference vars in any way that causes ICEs. A slight change to coercion which I am working on separately did result in such an ICE.

r? @compiler-errors

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

GITHUB_ENV=/home/runner/work/_temp/_runner_file_commands/set_env_c0bfd535-ed39-4f93-a7ce-fb15d7701d2c
GITHUB_EVENT_NAME=pull_request
GITHUB_EVENT_PATH=/home/runner/work/_temp/_github_workflow/event.json
GITHUB_GRAPHQL_URL=https://api.github.com/graphql
GITHUB_HEAD_REF=commit_if_ok-leaks
GITHUB_JOB=pr
GITHUB_PATH=/home/runner/work/_temp/_runner_file_commands/add_path_c0bfd535-ed39-4f93-a7ce-fb15d7701d2c
GITHUB_REF=refs/pull/121365/merge
GITHUB_REF_NAME=121365/merge
GITHUB_REF_PROTECTED=false
---
#13 4.035 Building wheels for collected packages: reuse
#13 4.036   Building wheel for reuse (pyproject.toml): started
#13 4.387   Building wheel for reuse (pyproject.toml): finished with status 'done'
#13 4.388   Created wheel for reuse: filename=reuse-1.1.0-cp310-cp310-manylinux_2_35_x86_64.whl size=181117 sha256=f5f58750481f69515c2c0d1d503daf565e2565c370d07fc6aeb95fe3498b4269
#13 4.389   Stored in directory: /tmp/pip-ephem-wheel-cache-ydipxqsy/wheels/c2/3c/b9/1120c2ab4bd82694f7e6f0537dc5b9a085c13e2c69a8d0c76d
#13 4.391 Installing collected packages: boolean-py, binaryornot, setuptools, reuse, python-debian, markupsafe, license-expression, jinja2, chardet
#13 4.415   Attempting uninstall: setuptools
#13 4.415     Found existing installation: setuptools 59.6.0
#13 4.417     Not uninstalling setuptools at /usr/lib/python3/dist-packages, outside environment /usr
---
    Checking rustc_incremental v0.0.0 (/checkout/compiler/rustc_incremental)
error[E0308]: mismatched types
   --> compiler/rustc_infer/src/infer/mod.rs:902:17
    |
891 |     pub fn commit_if_ok<T, E, F>(&self, f: F) -> Result<T, E>
    |                                                  ------------ expected `Result<T, E>` because of return type
902 |                 Some(value)
902 |                 Some(value)
    |                 ^^^^^^^^^^^ expected `Result<T, E>`, found `Option<T>`
    = note: expected enum `Result<T, E>`
               found enum `std::option::Option<T>`

For more information about this error, try `rustc --explain E0308`.

[compiler-errors]

@rustbot author

[bors]

☔ The latest upstream changes (presumably #120730) made this pull request unmergeable. Please resolve the merge conflicts.

[lcnr]

closing in favor of #122189