`-Zallow-features` is incompatible with `-Zbuild-std`

[Nemo157]

Problem

When using -Zallow-features and -Zbuild-std together I expect that the builds of the standard library crates would bypass the allowed features (as would have to eventually happen with a stabilized build-std), instead currently you have to manually enable all the many features they use (at least, I assume that would work, I haven't gone through the effort of collecting the >150 features required to get just core to build).

Steps

> cargo new foo && cd foo
     Created binary (application) `foo` package

> cargo build -Zbuild-std -Zallow-features=build-std --target x86_64-unknown-linux-gnu
error: failed to load manifest for workspace member `/nix/store/rnlrycs51r4x67hhm3ibaj8f9yif912l-rust-default-1.76.0-nightly-2023-11-12/lib/rustlib/src/rust/library/std`

Caused by:
  failed to parse manifest at `/nix/store/rnlrycs51r4x67hhm3ibaj8f9yif912l-rust-default-1.76.0-nightly-2023-11-12/lib/rustlib/src/rust/library/std/Cargo.toml`

Caused by:
  the feature `public-dependency` is not in the list of allowed features: [build-std]

> cargo build -Zbuild-std -Zallow-features=build-std,public-dependency --target x86_64-unknown-linux-gnu |& tail -n1
error: could not compile `core` (lib) due to 26965 previous errors

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.76.0-nightly (6790a5127 2023-11-10)
release: 1.76.0-nightly
commit-hash: 6790a5127895debec95c24aefaeb18e059270df3
commit-date: 2023-11-10
host: x86_64-unknown-linux-gnu
libgit2: 1.7.1 (sys:0.18.1 vendored)
libcurl: 8.4.0-DEV (sys:0.4.68+curl-8.4.0 vendored ssl:OpenSSL/1.1.1u)
ssl: OpenSSL 1.1.1u  30 May 2023
os: NixOS 23.5.0 [64-bit]

[weihanglo]

It seems nearly impossible to fix? Cargo never knows which feature is involved in a build-std build, so it can only allow all features in order to make -Zallow-features compatible with -Zbuild-std.

[Nemo157]

That seems fine to me, the standard libraries should be treated similar to #[allow_internal_unstable], bypassing the stability of the rest of the build.

[weihanglo]

Sounds like it. Maybe just don't pass -Zallow-features= in when invoking rustc for building standard libraries.

Out of curiosity, could you share the reason using -Zallow-features with -Zbuild-std?

[Nemo157]

I'm using -Zbuild-std globally to get more debug-assertions enabled. I also try to avoid letting dependencies build-scripts do nightly feature detection, -Zallow-features is one of the approaches I take to block them.