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`

2 files changed, 26 insertions, 31 deletions

diff --git a/compiler/rustc_codegen_ssa/src/target_features.rs b/compiler/rustc_codegen_ssa/src/target_features.rs
index eee7cc75400..fa600ec7166 100644
--- a/compiler/rustc_codegen_ssa/src/target_features.rs
+++ b/compiler/rustc_codegen_ssa/src/target_features.rs
@@ -11,7 +11,7 @@ use rustc_middle::ty::TyCtxt;
 use rustc_session::parse::feature_err;
 use rustc_span::Span;
 use rustc_span::symbol::{Symbol, sym};
-use rustc_target::target_features::{self, Stability};
+use rustc_target::target_features;
 
 use crate::errors;
 
@@ -20,7 +20,7 @@ use crate::errors;
 pub(crate) fn from_target_feature_attr(
     tcx: TyCtxt<'_>,
     attr: &ast::Attribute,
-    rust_target_features: &UnordMap<String, target_features::Stability>,
+    rust_target_features: &UnordMap<String, target_features::StabilityComputed>,
     target_features: &mut Vec<TargetFeature>,
 ) {
     let Some(list) = attr.meta_item_list() else { return };
@@ -63,32 +63,24 @@ pub(crate) fn from_target_feature_attr(
                 return None;
             };
 
-            // Only allow target features whose feature gates have been enabled.
-            let allowed = match stability {
-                Stability::Forbidden { .. } => false,
-                Stability::Stable => true,
-                Stability::Unstable(name) => rust_features.enabled(*name),
-            };
-            if !allowed {
-                match stability {
-                    Stability::Stable => unreachable!(),
-                    &Stability::Unstable(lang_feature_name) => {
-                        feature_err(
-                            &tcx.sess,
-                            lang_feature_name,
-                            item.span(),
-                            format!("the target feature `{feature}` is currently unstable"),
-                        )
-                        .emit();
-                    }
-                    Stability::Forbidden { reason } => {
-                        tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
-                            span: item.span(),
-                            feature,
-                            reason,
-                        });
-                    }
-                }
+            // Only allow target features whose feature gates have been enabled
+            // and which are permitted to be toggled.
+            if let Err(reason) = stability.allow_toggle() {
+                tcx.dcx().emit_err(errors::ForbiddenTargetFeatureAttr {
+                    span: item.span(),
+                    feature,
+                    reason,
+                });
+            } else if let Some(nightly_feature) = stability.requires_nightly()
+                && !rust_features.enabled(nightly_feature)
+            {
+                feature_err(
+                    &tcx.sess,
+                    nightly_feature,
+                    item.span(),
+                    format!("the target feature `{feature}` is currently unstable"),
+                )
+                .emit();
             }
             Some(Symbol::intern(feature))
         }));
@@ -156,18 +148,19 @@ pub(crate) fn provide(providers: &mut Providers) {
     *providers = Providers {
         rust_target_features: |tcx, cnum| {
             assert_eq!(cnum, LOCAL_CRATE);
+            let target = &tcx.sess.target;
             if tcx.sess.opts.actually_rustdoc {
                 // rustdoc needs to be able to document functions that use all the features, so
                 // whitelist them all
                 rustc_target::target_features::all_rust_features()
-                    .map(|(a, b)| (a.to_string(), b))
+                    .map(|(a, b)| (a.to_string(), b.compute_toggleability(target)))
                     .collect()
             } else {
                 tcx.sess
                     .target
                     .rust_target_features()
                     .iter()
-                    .map(|&(a, b, _)| (a.to_string(), b))
+                    .map(|&(a, b, _)| (a.to_string(), b.compute_toggleability(target)))
                     .collect()
             }
         },
diff --git a/compiler/rustc_codegen_ssa/src/traits/backend.rs b/compiler/rustc_codegen_ssa/src/traits/backend.rs
index 5b4a51fc301..4b17db2c49e 100644
--- a/compiler/rustc_codegen_ssa/src/traits/backend.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/backend.rs
@@ -45,7 +45,9 @@ pub trait CodegenBackend {
 
     fn print(&self, _req: &PrintRequest, _out: &mut String, _sess: &Session) {}
 
-    fn target_features(&self, _sess: &Session, _allow_unstable: bool) -> Vec<Symbol> {
+    /// Returns the features that should be set in `cfg(target_features)`.
+    /// RUSTC_SPECIFIC_FEATURES should be skipped here, those are handled outside codegen.
+    fn target_features_cfg(&self, _sess: &Session, _allow_unstable: bool) -> Vec<Symbol> {
         vec![]
     }