rustbuild: --no-fail-fast tries to compile rustc 40+ times (due to crossbeam-utils failure) before eventually failing

[infinity0]

See for example https://buildd.debian.org/status/fetch.php?pkg=rustc&arch=amd64&ver=1.44.1%2Bdfsg1-1%7Eexp1&stamp=1594606550&raw=0 (warning, 30MB HTML), raw text 25MB where the error message "cannot find type AtomicU128" crops up 46 times.

Presumably this is because 46 things depend on crossbeam-utils. But then they should automatically fail when one of their dependencies has recorded a failure, instead of re-running the same command. (In other words --no-fail-fast should only move onto other targets whose dependencies haven't failed.)

It's hard to notice these failures, since the error messages quickly scroll out of view. I only noticed because I eventually realised the build really shouldn't be taking 3+ hours.

[infinity0]

edited the bug title to clarify that it tries to compile the whole of rustc 40+ times, a massive waste of time

[Mark-Simulacrum]

AFAICT, this is not trying to compile rustc 40+ times, it's just the tests for each rustc crate -- building those will always fail if a dependency like crossbeam-utils is failing to compile.

I'm not sure what behavior is expected from no-fail-fast, though -- I don't think we have a good way of differentiating the different kinds of cargo test failures (i.e., unit test failure from "could not build"). Maybe we could get Cargo to return different exit codes or something like that...

[Enselic]

Triage: Is this still a problem?