Lintcheck never reports ICEs

[y21]

Description

The log file that lintcheck creates after a run has an ### ICE section at the end. However, I've never actually seen any entries in there.
To confirm this, I added a panic!(); to some random check_expr function and it indeed doesn't report them.
It does however print WARNING: bad exit status after checking ..., which I've seen many times but have always ignored because I wasn't sure what that's about, but I suspect that that's the ICE.

Version

99423e8

Additional Labels

@rustbot label +C-bug

[Jacherr]

@rustbot claim

[Jacherr]

After doing some digging around in the lintcheck code, I've boiled this down to the following issues:

• Lintcheck collects information from stdout in JSON format about what Clippy reported for a particular crate (https://github.com/rust-lang/rust-clippy/blob/master/lintcheck/src/main.rs#L431-L436). However, ICEs are not sent to stdout, but to stderr, which is not used for this purpose.
• ICEs are not reported in JSON format (and I'm not sure it's possible to report them as such). As a result, it is not possible to parse them using Message::parse_stream(...) the same way stdout is parsed.
• There is the potential to manually check for ICEs in stderr through string matching (i.e., stdout.contains("panicked at")) - however, constructing a ClippyWarning from this is not possible due to Diagnostics being marked #[non_exhaustive]. We would need to first convert them to a JSON format and deserialize them from that.
• Lastly, it's not possible to return the ICEs without first turning them into a diagnostic, because the run_clippy_lints function only returns a Vec<ClippyWarning> which is the output of the parsed stream (https://github.com/rust-lang/rust-clippy/blob/master/lintcheck/src/main.rs#L314).

Either we need to rethink how ICEs are detected in lintcheck entirely, or somehow convert them to a format that can be parsed into a ClippyWarning.