-Z allow-features does not apply to -Z flags

[jyn514]

I tried this code: rustc -Z allow-features= -Z always-encode-mir

I expected to see this happen: rustc gives a hard error that always-encode-mir was not listed in allow-features.

Instead, this happened: rustc silently accepts the -Z flag.

It feels very natural to me that this should apply to flags, not just source code, by the same logic you'd want it anywhere: it gives the person running the build control over what unstable features they use, instead of unknowingly enabling features from the myriad sources that cargo picks up rustflags.

Meta

rustc --version --verbose:

rustc 1.87.0-nightly (43f0014ef 2025-03-25)

[jyn514]

small problem: https://rust-lang.zulipchat.com/#narrow/channel/233931-t-compiler.2Fmajor-changes/topic/Policy.20change.20around.20adding.20new.20unstable.20.E2.80.A6.20compiler-team.23787

although apparently that was never accepted

[jyn514]

an alternative is to make this an argument to -Z unstable-options, rather than having that be a boolean yes/no.