Auto merge of #133099 - RalfJung:forbidden-hardfloat-features, r=workingjubilee

forbid toggling x87 and fpregs on hard-float targets

Part of https://github.com/rust-lang/rust/issues/116344, follow-up to https://github.com/rust-lang/rust/pull/129884:

The `x87`  target feature on x86 and the `fpregs` target feature on ARM must not be disabled on a hardfloat target, as that would change the float ABI. However, *enabling* `fpregs` on ARM is [explicitly requested](https://github.com/rust-lang/rust/issues/130988) as it seems to be useful. Therefore, we need to refine the distinction of "forbidden" target features and "allowed" target features: all (un)stable target features can determine on a per-target basis whether they should be allowed to be toggled or not. `fpregs` then checks whether the current target has the `soft-float` feature, and if yes, `fpregs` is permitted -- otherwise, it is not. (Same for `x87` on x86).

Also fixes https://github.com/rust-lang/rust/issues/132351. Since `fpregs` and `x87` can be enabled on some builds and disabled on others, it would make sense that one can query it via `cfg`. Therefore, I made them behave in `cfg` like any other unstable target feature.

The first commit prepares the infrastructure, but does not change behavior. The second commit then wires up `fpregs` and `x87` with that new infrastructure.

r? `@workingjubilee`

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/rustc_session/src/config/cfg.rs b/compiler/rustc_session/src/config/cfg.rs
index 99d9d5b7665..43cf1324edd 100644
--- a/compiler/rustc_session/src/config/cfg.rs
+++ b/compiler/rustc_session/src/config/cfg.rs
@@ -371,7 +371,7 @@ impl CheckCfg {
 
         ins!(sym::target_feature, empty_values).extend(
             rustc_target::target_features::all_rust_features()
-                .filter(|(_, s)| s.is_supported())
+                .filter(|(_, s)| s.in_cfg())
                 .map(|(f, _s)| f)
                 .chain(rustc_target::target_features::RUSTC_SPECIFIC_FEATURES.iter().cloned())
                 .map(Symbol::intern),