Match std RUSTFLAGS for host and target for mir-opt test suite to fix double std build/rebuilds

[jieyouxu]

Previously the bootstrap compiletest Step::run flow had:

// ensure that `libproc_macro` is available on the host.
builder.ensure(compile::Std::new(compiler, compiler.host));

// ...

if suite == "mir-opt" {
    builder.ensure(compile::Std::new_for_mir_opt_tests(compiler, target));
} else {
    builder.ensure(compile::Std::new(compiler, target));
}

This can cause unnecessary std rebuilds (even on the same invocation) because if host == target then builder.ensure(compile::Std::new_for_mir_opt_tests(compiler, target)) will have different RUSTFLAGS than builder.ensure(compile::Std::new(compiler, compiler.host)).

This PR fixes that by matching up std RUSTFLAGS if the test suite is mir-opt:

if suite == "mir-opt" {
    builder.ensure(compile::Std::new_for_mir_opt_tests(compiler, compiler.host));
} else {
    builder.ensure(compile::Std::new(compiler, compiler.host));
}

This is a short-term fix, the better fix is to enforce how RUSTFLAGS are handled as described in #131437 (comment).

Fixes #131437.

[rustbot]

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[jieyouxu]

Remark: the compiletest Step::run is some gigantic convoluted build step, lol. I might revisit that in the future and see if I can try to make it uhhh less insane.

[onur-ozkan]

@bors r+

[bors]

📌 Commit becf664 has been approved by onur-ozkan

It is now in the queue for this repository.