mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature

[RalfJung]

Proposal

Some target features fundamentally change the ABI (such as switching between soft-float and hard-float conventions for passing float values). Enabling those features in a crate while linking against a standard library built without these features (or vice versa) is unsound. An example of this is building x86 code with -Ctarget-feature=+soft-float or -Ctarget-feature=-x87. For more discussion and context, see rust-lang/rust#116344. In rust-lang/lang-team#235, the lang team generally agreed that code built with the same target triple should be API-compatible -- this was not an FCP, just the vibe of the people in the meeting. (This can lead to an explosion of target triples, so long-term we might want to invest in an "ABI variant" system. But that system needs to come with extra checks to ensure soundness, and should IMO be separate from -Ctarget-feature.)

Similar to #779, there's little that can be done here other than just not letting people shoot themselves in the foot. We already often have separate targets for this distinction, e.g. x86_64-unknown-none as a soft-float x86-64 target. So I propose that we make it a hard error to toggle such features via -Ctarget-feature.

The way I think this should be implemented is by extending the feature "database" in compiler/rustc_target/src/target_features.rs. Currently this tracks which features we "know", and whether that classification is stable or not. Stable features can be freely used with -Ctarget-feature, #[target_feature] and cfg(target_feature). Unstable features warn when used with -Ctarget-feature, need a nightly language feature flag to be used with #[target_feature], and cfg(target_feature) is only ever true on nightly. Completely unknown features are still allowed with -Ctarget-feature so that LLVM features can be accessed directly, but we show a warning. I suggest we add a new class of "forbidden" features, which behave mostly like unstable features except they are also rejected in -Ctarget-feature. This also ensures that they will never be added as regular "known" features.

Custom target declarations can still freely use these features.

One question to decide is whether this should become a hard error immediately, or show some sort of warning for a while first.

Mentors or Reviewers

This is implemented in rust-lang/rust#129884 and waiting for review.

Process

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Comments

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[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

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cc @rust-lang/compiler @rust-lang/compiler-contributors

[davidtwco]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted