generalize 'forbidden feature' concept so that even (un)stable feature can be invalid to toggle

Also rename some things for extra clarity

Author: RalfJung

compiler/rustc_codegen_cranelift/src/lib.rs

@@ -185,7 +185,11 @@ impl CodegenBackend for CraneliftCodegenBackend {
         }
     }
 
-    fn target_features(&self, sess: &Session, _allow_unstable: bool) -> Vec<rustc_span::Symbol> {
+    fn target_features_cfg(
+        &self,
+        sess: &Session,
+        _allow_unstable: bool,
+    ) -> Vec<rustc_span::Symbol> {
         // FIXME return the actually used target features. this is necessary for #[cfg(target_feature)]
         if sess.target.arch == "x86_64" && sess.target.os != "none" {
             // x86_64 mandates SSE2 support

compiler/rustc_codegen_gcc/messages.ftl

@@ -9,7 +9,7 @@ codegen_gcc_lto_not_supported =
     LTO is not supported. You may get a linker error.
 
 codegen_gcc_forbidden_ctarget_feature =
-    target feature `{$feature}` cannot be toggled with `-Ctarget-feature`
+    target feature `{$feature}` cannot be toggled with `-Ctarget-feature`: {$reason}
 
 codegen_gcc_unwinding_inline_asm =
     GCC backend does not support unwinding from inline asm

compiler/rustc_codegen_gcc/src/errors.rs

@@ -28,6 +28,7 @@ pub(crate) struct UnstableCTargetFeature<'a> {
 #[diag(codegen_gcc_forbidden_ctarget_feature)]
 pub(crate) struct ForbiddenCTargetFeature<'a> {
     pub feature: &'a str,
+    pub reason: &'a str,
 }
 
 #[derive(Subdiagnostic)]

compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -5,7 +5,7 @@ use rustc_codegen_ssa::errors::TargetFeatureDisableOrEnable;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_middle::bug;
 use rustc_session::Session;
-use rustc_target::target_features::{RUSTC_SPECIFIC_FEATURES, Stability};
+use rustc_target::target_features::RUSTC_SPECIFIC_FEATURES;
 use smallvec::{SmallVec, smallvec};
 
 use crate::errors::{
@@ -94,13 +94,15 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
                         };
                         sess.dcx().emit_warn(unknown_feature);
                     }
-                    Some((_, Stability::Stable, _)) => {}
-                    Some((_, Stability::Unstable(_), _)) => {
-                        // An unstable feature. Warn about using it.
-                        sess.dcx().emit_warn(UnstableCTargetFeature { feature });
-                    }
-                    Some((_, Stability::Forbidden { .. }, _)) => {
-                        sess.dcx().emit_err(ForbiddenCTargetFeature { feature });
+                    Some((_, stability, _)) => {
+                        if let Err(reason) = stability.compute(&sess.target).allow_toggle() {
+                            sess.dcx().emit_warn(ForbiddenCTargetFeature { feature, reason });
+                        } else if stability.requires_nightly().is_some() {
+                            // An unstable feature. Warn about using it. (It makes little sense
+                            // to hard-error here since we just warn about fully unknown
+                            // features above).
+                            sess.dcx().emit_warn(UnstableCTargetFeature { feature });
+                        }
                     }
                 }
 

compiler/rustc_codegen_gcc/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

compiler/rustc_codegen_llvm/src/lib.rs

@@ -28,7 +28,7 @@ use std::mem::ManuallyDrop;
 use back::owned_target_machine::OwnedTargetMachine;
 use back::write::{create_informational_target_machine, create_target_machine};
 use errors::ParseTargetMachineConfig;
-pub use llvm_util::target_features;
+pub use llvm_util::target_features_cfg;
 use rustc_ast::expand::allocator::AllocatorKind;
 use rustc_codegen_ssa::back::lto::{LtoModuleCodegen, SerializedModule, ThinModule};
 use rustc_codegen_ssa::back::write::{
@@ -331,8 +331,8 @@ impl CodegenBackend for LlvmCodegenBackend {
         llvm_util::print_version();
     }
 
-    fn target_features(&self, sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
-        target_features(sess, allow_unstable)
+    fn target_features_cfg(&self, sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
+        target_features_cfg(sess, allow_unstable)
     }
 
     fn codegen_crate<'tcx>(

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -17,7 +17,7 @@ use rustc_session::Session;
 use rustc_session::config::{PrintKind, PrintRequest};
 use rustc_span::symbol::Symbol;
 use rustc_target::spec::{MergeFunctions, PanicStrategy, SmallDataThresholdSupport};
-use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES, Stability};
+use rustc_target::target_features::{RUSTC_SPECIAL_FEATURES, RUSTC_SPECIFIC_FEATURES};
 
 use crate::back::write::create_informational_target_machine;
 use crate::errors::{
@@ -297,7 +297,7 @@ pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFea
 /// Must express features in the way Rust understands them.
 ///
 /// We do not have to worry about RUSTC_SPECIFIC_FEATURES here, those are handled outside codegen.
-pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
+pub fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
     let mut features: FxHashSet<Symbol> = Default::default();
 
     // Add base features for the target.
@@ -313,7 +313,7 @@ pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
         sess.target
             .rust_target_features()
             .iter()
-            .filter(|(_, gate, _)| gate.is_supported())
+            .filter(|(_, gate, _)| gate.in_cfg())
             .filter(|(feature, _, _)| {
                 // skip checking special features, as LLVM may not understand them
                 if RUSTC_SPECIAL_FEATURES.contains(feature) {
@@ -369,9 +369,9 @@ pub fn target_features(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
     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
@@ -490,7 +490,7 @@ fn print_target_features(sess: &Session, tm: &llvm::TargetMachine, out: &mut Str
         .rust_target_features()
         .iter()
         .filter_map(|(feature, gate, _implied)| {
-            if !gate.is_supported() {
+            if !gate.in_cfg() {
                 // Only list (experimentally) supported features.
                 return None;
             }
@@ -713,13 +713,15 @@ pub(crate) fn global_llvm_features(
                             };
                             sess.dcx().emit_warn(unknown_feature);
                         }
-                        Some((_, Stability::Stable, _)) => {}
-                        Some((_, Stability::Unstable(_), _)) => {
-                            // An unstable feature. Warn about using it.
-                            sess.dcx().emit_warn(UnstableCTargetFeature { feature });
-                        }
-                        Some((_, Stability::Forbidden { reason }, _)) => {
-                            sess.dcx().emit_warn(ForbiddenCTargetFeature { feature, reason });
+                        Some((_, stability, _)) => {
+                            if let Err(reason) = stability.compute(&sess.target).allow_toggle() {
+                                sess.dcx().emit_warn(ForbiddenCTargetFeature { feature, reason });
+                            } else if stability.requires_nightly().is_some() {
+                                // An unstable feature. Warn about using it. (It makes little sense
+                                // to hard-error here since we just warn about fully unknown
+                                // features above).
+                                sess.dcx().emit_warn(UnstableCTargetFeature { feature });
+                            }
                         }
                     }
 

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(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(target)))
                     .collect()
             }
         },

compiler/rustc_codegen_ssa/src/traits/backend.rs

@@ -46,7 +46,9 @@ 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![]
     }
 

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))));