Seems worth a try, but note that some formulae builds also set RUSTC_WRAPPER, and it's not clear how those will interact with this.

@carlocab How would you suggest we solve this without using RUSTC_WRAPPER?

It might also be worth adding RUSTFLAGS here too, so that formulae can do

My reading of #18556 is that this would just result in that issue not being fixed because then the formula would be overriding .cargo/config.toml rather than superenv/stdenv.

It sounds instead like we should actively discourage ever setting RUSTFLAGS in a formula (perhaps with an audit/rubocop) for that reason as it will prevent upstream projects from reliably setting RUSTFLAGS

Seems worth a try, but note that some formulae builds also set RUSTC_WRAPPER, and it's not clear how those will interact with this.

@carlocab How would you suggest we solve this without using RUSTC_WRAPPER?

We could try CARGO_ENCODED_RUSTFLAGS, but that's not necessarily guaranteed to be safer.

We may not even need to solve this, as I did say I don't know how the interaction would happen (and it could happen entirely favourably).

My reading of #18556 is that this would just result in that issue not being fixed because then the formula would be overriding .cargo/config.toml rather than superenv/stdenv.

It sounds instead like we should actively discourage ever setting RUSTFLAGS in a formula (perhaps with an audit/rubocop) for that reason as it will prevent upstream projects from reliably setting RUSTFLAGS

Yes, point taken. Formulae will have to work with HOMEBREW_RUSTFLAGS instead. I only suggested this initially because we don't like it when formulae touch HOMEBREW_* environment variables because they're meant to be implementation details. Maybe, instead, formulae can use

ENV.rustflags

instead? With methods like

ENV.rustflags << "..."

for appending, etc.

Thanks for review @carlocab. Adjusted based on your comments and added ENV.append_rustflags for use in formulae. Tested locally with some Rust formulae and all seems to be working as expected. Opening homebrew/core PR after this is merged.