Allow for opt-level=s,z during bootstrap

[MagaTailor]

Until rustc starts accepting both -O and -C opt-level on the same command line (the last option given should take precedence, other flags are also affected) this change enables using e.g.:

RUSTFLAGS=-Copt-level=z during bootstrap, as multiple opt-level flags happen to be handled correctly.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[brson]

@bors r+ rollup

[bors]

📌 Commit a345842 has been approved by brson

[MagaTailor]

@brson The tests set the -O option again after not trying to unset the -C opt-level which lead to the clash I'd mentioned in the first comment. Maybe the compiler's option parsing should get fixed first and only then I'd revisit this Makefile issue?

[brson]

@bors r-

@petevine Hm, I guess I don't understand what the issue is. Am I correct that this patch fixes the bootstrap when overriding -C opt-level but the test suite is still broken? If that's the case then this does seem like a harmless incremental improvement, but I'm happy to hold off if that's what you want.

[MagaTailor]

Yeah thanks, I inadvertently broke the tests which in their own tests.mk file try to unset just the preexisting -O and then set it back again for release builds.

If you want me to unset all possible opt-levels, will the syntax like:
subst -C opt-level=[123][sz],, work in that Makefile?

[sanxiyn]

FYI, option parsing issue is #32352.

[sanxiyn]

@brson This patch can't be merged as-is because it breaks tests even if you don't override RUSTFLAGS.

[MagaTailor]

I'll close for the time being as it seems environment RUSTFLAGS wouldn't even take precedence over the Makefile's opts.