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, 2 insertions, 2 deletions

diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index d3213b1263c..2af25bfd3aa 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -35,10 +35,10 @@ pub type MakeBackendFn = fn() -> Box<dyn CodegenBackend>;
 pub fn add_configuration(cfg: &mut Cfg, sess: &mut Session, codegen_backend: &dyn CodegenBackend) {
     let tf = sym::target_feature;
 
-    let unstable_target_features = codegen_backend.target_features(sess, true);
+    let unstable_target_features = codegen_backend.target_features_cfg(sess, true);
     sess.unstable_target_features.extend(unstable_target_features.iter().cloned());
 
-    let target_features = codegen_backend.target_features(sess, false);
+    let target_features = codegen_backend.target_features_cfg(sess, false);
     sess.target_features.extend(target_features.iter().cloned());
 
     cfg.extend(target_features.into_iter().map(|feat| (tf, Some(feat))));