bootstrap: Allow configuring the opt-level, not just debug vs release

[Noratrieb]

It would be nice if we could speed up stage1 library builds by building it with less opts and no debuginfo.

@jyn514 had the idea to control this with something like rust.stage1.opt-level

[jyn514]

i'm going to use #112679 as a separate more broad issue for rust.stage1 - i'll repurpose this to just be for rust.opt-level (which is a lot simpler to add)

[jyn514]

mentoring instructions: add this option to the config::Rust struct, then around

rust/src/bootstrap/builder.rs

Line 1679 in dcf3571

cargo.env(profile_var("DEBUG"), debuginfo_level.to_string());

, set cargo.env(profile_var("OPT_LEVEL"), config.opt_level)

[Mark-Simulacrum]

Can we confirm before we land this support that this is actually a significant win in practice? I recall historically having issues with optimize = false equivalents where the net effect is broken and slower tests - I would prefer to avoid landing support we don't want to actually support :)

[jyn514]

@Mark-Simulacrum this is especially useful in combination with #112679 - optimizing only rustc and not std should be a speedup even if we include the time taken for tests.

[Rustin170506]

@rustbot claim

[Kobzol]

Maybe we should just generalize the optimize config value to a number/boolean, instead of adding a new attribute?