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, 6 insertions, 5 deletions

diff --git a/src/lib.rs b/src/lib.rs
index 452e92bffa2..764e84be1fe 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -260,8 +260,8 @@ impl CodegenBackend for GccCodegenBackend {
             .join(sess)
     }
 
-    fn target_features(&self, sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
-        target_features(sess, allow_unstable, &self.target_info)
+    fn target_features_cfg(&self, sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
+        target_features_cfg(sess, allow_unstable, &self.target_info)
     }
 }
 
@@ -472,7 +472,8 @@ fn to_gcc_opt_level(optlevel: Option<OptLevel>) -> OptimizationLevel {
     }
 }
 
-pub fn target_features(
+/// Returns the features that should be set in `cfg(target_feature)`.
+fn target_features_cfg(
     sess: &Session,
     allow_unstable: bool,
     target_info: &LockedTargetInfo,
@@ -481,9 +482,9 @@ pub fn target_features(
     sess.target
         .rust_target_features()
         .iter()
-        .filter(|(_, gate, _)| gate.is_supported())
+        .filter(|(_, gate, _)| gate.in_cfg())
         .filter_map(|&(feature, gate, _)| {
-            if sess.is_nightly_build() || allow_unstable || gate.is_stable() {
+            if sess.is_nightly_build() || allow_unstable || gate.requires_nightly().is_none() {
                 Some(feature)
             } else {
                 None