Auto merge of #129884 - RalfJung:forbidden-target-features, r=workingjubilee

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

The context for this is https://github.com/rust-lang/rust/issues/116344: some target features change the way floats are passed between functions. Changing those target features is unsound as code compiled for the same target may now use different ABIs.

So this introduces a new concept of "forbidden" target features (on top of the existing "stable " and "unstable" categories), and makes it a hard error to (un)set such a target feature. For now, the x86 and ARM feature `soft-float` is on that list. We'll have to make some effort to collect more relevant features, and similar features from other targets, but that can happen after the basic infrastructure for this landed. (These features are being collected in https://github.com/rust-lang/rust/issues/131799.)

I've made this a warning for now to give people some time to speak up if this would break something.

MCP: https://github.com/rust-lang/compiler-team/issues/780

3 files changed, 51 insertions, 26 deletions

diff --git a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
index 8f3592090c6..32e9422e005 100644
--- a/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
+++ b/compiler/rustc_codegen_ssa/src/codegen_attrs.rs
@@ -20,8 +20,8 @@ use rustc_span::symbol::Ident;
 use rustc_span::{Span, sym};
 use rustc_target::spec::{SanitizerSet, abi};
 
-use crate::errors::{self, MissingFeatures, TargetFeatureDisableOrEnable};
-use crate::target_features::{check_target_feature_trait_unsafe, from_target_feature};
+use crate::errors;
+use crate::target_features::{check_target_feature_trait_unsafe, from_target_feature_attr};
 
 fn linkage_by_name(tcx: TyCtxt<'_>, def_id: LocalDefId, name: &str) -> Linkage {
     use rustc_middle::mir::mono::Linkage::*;
@@ -73,7 +73,7 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
         codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_BUILTINS;
     }
 
-    let supported_target_features = tcx.supported_target_features(LOCAL_CRATE);
+    let rust_target_features = tcx.rust_target_features(LOCAL_CRATE);
 
     let mut inline_span = None;
     let mut link_ordinal_span = None;
@@ -281,10 +281,10 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
                         check_target_feature_trait_unsafe(tcx, did, attr.span);
                     }
                 }
-                from_target_feature(
+                from_target_feature_attr(
                     tcx,
                     attr,
-                    supported_target_features,
+                    rust_target_features,
                     &mut codegen_fn_attrs.target_features,
                 );
             }
@@ -676,10 +676,10 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
             .next()
             .map_or_else(|| tcx.def_span(did), |a| a.span);
         tcx.dcx()
-            .create_err(TargetFeatureDisableOrEnable {
+            .create_err(errors::TargetFeatureDisableOrEnable {
                 features,
                 span: Some(span),
-                missing_features: Some(MissingFeatures),
+                missing_features: Some(errors::MissingFeatures),
             })
             .emit();
     }
diff --git a/compiler/rustc_codegen_ssa/src/errors.rs b/compiler/rustc_codegen_ssa/src/errors.rs
index cf8d1cfa0d1..a22a58ef3d7 100644
--- a/compiler/rustc_codegen_ssa/src/errors.rs
+++ b/compiler/rustc_codegen_ssa/src/errors.rs
@@ -1028,6 +1028,15 @@ pub(crate) struct TargetFeatureSafeTrait {
 }
 
 #[derive(Diagnostic)]
+#[diag(codegen_ssa_forbidden_target_feature_attr)]
+pub struct ForbiddenTargetFeatureAttr<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub feature: &'a str,
+    pub reason: &'a str,
+}
+
+#[derive(Diagnostic)]
 #[diag(codegen_ssa_failed_to_get_layout)]
 pub struct FailedToGetLayout<'tcx> {
     #[primary_span]
diff --git a/compiler/rustc_codegen_ssa/src/target_features.rs b/compiler/rustc_codegen_ssa/src/target_features.rs
index 0845bcc5749..eee7cc75400 100644
--- a/compiler/rustc_codegen_ssa/src/target_features.rs
+++ b/compiler/rustc_codegen_ssa/src/target_features.rs
@@ -11,13 +11,16 @@ 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 crate::errors;
 
-pub(crate) fn from_target_feature(
+/// Compute the enabled target features from the `#[target_feature]` function attribute.
+/// Enabled target features are added to `target_features`.
+pub(crate) fn from_target_feature_attr(
     tcx: TyCtxt<'_>,
     attr: &ast::Attribute,
-    supported_target_features: &UnordMap<String, Option<Symbol>>,
+    rust_target_features: &UnordMap<String, target_features::Stability>,
     target_features: &mut Vec<TargetFeature>,
 ) {
     let Some(list) = attr.meta_item_list() else { return };
@@ -46,12 +49,12 @@ pub(crate) fn from_target_feature(
 
         // We allow comma separation to enable multiple features.
         added_target_features.extend(value.as_str().split(',').filter_map(|feature| {
-            let Some(feature_gate) = supported_target_features.get(feature) else {
+            let Some(stability) = rust_target_features.get(feature) else {
                 let msg = format!("the feature named `{feature}` is not valid for this target");
                 let mut err = tcx.dcx().struct_span_err(item.span(), msg);
                 err.span_label(item.span(), format!("`{feature}` is not valid for this target"));
                 if let Some(stripped) = feature.strip_prefix('+') {
-                    let valid = supported_target_features.contains_key(stripped);
+                    let valid = rust_target_features.contains_key(stripped);
                     if valid {
                         err.help("consider removing the leading `+` in the feature name");
                     }
@@ -61,18 +64,31 @@ pub(crate) fn from_target_feature(
             };
 
             // Only allow target features whose feature gates have been enabled.
-            let allowed = match feature_gate.as_ref().copied() {
-                Some(name) => rust_features.enabled(name),
-                None => true,
+            let allowed = match stability {
+                Stability::Forbidden { .. } => false,
+                Stability::Stable => true,
+                Stability::Unstable(name) => rust_features.enabled(*name),
             };
             if !allowed {
-                feature_err(
-                    &tcx.sess,
-                    feature_gate.unwrap(),
-                    item.span(),
-                    format!("the target feature `{feature}` is currently unstable"),
-                )
-                .emit();
+                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,
+                        });
+                    }
+                }
             }
             Some(Symbol::intern(feature))
         }));
@@ -138,20 +154,20 @@ pub(crate) fn check_target_feature_trait_unsafe(tcx: TyCtxt<'_>, id: LocalDefId,
 
 pub(crate) fn provide(providers: &mut Providers) {
     *providers = Providers {
-        supported_target_features: |tcx, cnum| {
+        rust_target_features: |tcx, cnum| {
             assert_eq!(cnum, LOCAL_CRATE);
             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_known_features()
-                    .map(|(a, b)| (a.to_string(), b.as_feature_name()))
+                rustc_target::target_features::all_rust_features()
+                    .map(|(a, b)| (a.to_string(), b))
                     .collect()
             } else {
                 tcx.sess
                     .target
-                    .supported_target_features()
+                    .rust_target_features()
                     .iter()
-                    .map(|&(a, b, _)| (a.to_string(), b.as_feature_name()))
+                    .map(|&(a, b, _)| (a.to_string(), b))
                     .collect()
             }
         },