mark some target features as 'forbidden' so they cannot be (un)set

For now, this is just a warning, but should become a hard error in the future

messages.ftl

@@ -8,6 +8,9 @@ codegen_gcc_invalid_minimum_alignment =
 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`
+
 codegen_gcc_unwinding_inline_asm =
     GCC backend does not support unwinding from inline asm
 
@@ -24,11 +27,15 @@ codegen_gcc_lto_dylib = lto cannot be used for `dylib` crate type without `-Zdyl
 codegen_gcc_lto_bitcode_from_rlib = failed to get bitcode from object file for LTO ({$gcc_err})
 
 codegen_gcc_unknown_ctarget_feature =
-    unknown feature specified for `-Ctarget-feature`: `{$feature}`
+    unknown and unstable feature specified for `-Ctarget-feature`: `{$feature}`
-    .note = it is still passed through to the codegen backend
+    .note = it is still passed through to the codegen backend, but use of this feature might be unsound and the behavior of this feature can change in the future
     .possible_feature = you might have meant: `{$rust_feature}`
     .consider_filing_feature_request = consider filing a feature request
 
+codegen_gcc_unstable_ctarget_feature =
+    unstable feature specified for `-Ctarget-feature`: `{$feature}`
+    .note = this feature is not stably supported; its behavior can change in the future
+
 codegen_gcc_missing_features =
     add the missing features in a `target_feature` attribute
 

src/errors.rs

@@ -17,6 +17,19 @@ pub(crate) struct UnknownCTargetFeature<'a> {
     pub rust_feature: PossibleFeature<'a>,
 }
 
+#[derive(Diagnostic)]
+#[diag(codegen_gcc_unstable_ctarget_feature)]
+#[note]
+pub(crate) struct UnstableCTargetFeature<'a> {
+    pub feature: &'a str,
+}
+
+#[derive(Diagnostic)]
+#[diag(codegen_gcc_forbidden_ctarget_feature)]
+pub(crate) struct ForbiddenCTargetFeature<'a> {
+    pub feature: &'a str,
+}
+
 #[derive(Subdiagnostic)]
 pub(crate) enum PossibleFeature<'a> {
     #[help(codegen_gcc_possible_feature)]

src/gcc_util.rs

@@ -5,10 +5,13 @@ 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;
+use rustc_target::target_features::{RUSTC_SPECIFIC_FEATURES, Stability};
 use smallvec::{SmallVec, smallvec};
 
-use crate::errors::{PossibleFeature, UnknownCTargetFeature, UnknownCTargetFeaturePrefix};
+use crate::errors::{
+    ForbiddenCTargetFeature, PossibleFeature, UnknownCTargetFeature, UnknownCTargetFeaturePrefix,
+    UnstableCTargetFeature,
+};
 
 /// The list of GCC features computed from CLI flags (`-Ctarget-cpu`, `-Ctarget-feature`,
 /// `--target` and similar).
@@ -43,7 +46,7 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
     );
 
     // -Ctarget-features
-    let supported_features = sess.target.supported_target_features();
+    let known_features = sess.target.rust_target_features();
     let mut featsmap = FxHashMap::default();
     let feats = sess
         .opts
@@ -62,37 +65,49 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
                 }
             };
 
+            // Get the backend feature name, if any.
+            // This excludes rustc-specific features, that do not get passed down to GCC.
             let feature = backend_feature_name(s)?;
             // Warn against use of GCC specific feature names on the CLI.
-            if diagnostics && !supported_features.iter().any(|&(v, _, _)| v == feature) {
-                let rust_feature = supported_features.iter().find_map(|&(rust_feature, _, _)| {
-                    let gcc_features = to_gcc_features(sess, rust_feature);
-                    if gcc_features.contains(&feature) && !gcc_features.contains(&rust_feature) {
-                        Some(rust_feature)
-                    } else {
-                        None
-                    }
-                });
-                let unknown_feature = if let Some(rust_feature) = rust_feature {
-                    UnknownCTargetFeature {
-                        feature,
-                        rust_feature: PossibleFeature::Some { rust_feature },
-                    }
-                } else {
-                    UnknownCTargetFeature { feature, rust_feature: PossibleFeature::None }
-                };
-                sess.dcx().emit_warn(unknown_feature);
-            }
-
             if diagnostics {
+                let feature_state = known_features.iter().find(|&&(v, _, _)| v == feature);
+                match feature_state {
+                    None => {
+                        let rust_feature =
+                            known_features.iter().find_map(|&(rust_feature, _, _)| {
+                                let gcc_features = to_gcc_features(sess, rust_feature);
+                                if gcc_features.contains(&feature)
+                                    && !gcc_features.contains(&rust_feature)
+                                {
+                                    Some(rust_feature)
+                                } else {
+                                    None
+                                }
+                            });
+                        let unknown_feature = if let Some(rust_feature) = rust_feature {
+                            UnknownCTargetFeature {
+                                feature,
+                                rust_feature: PossibleFeature::Some { rust_feature },
+                            }
+                        } else {
+                            UnknownCTargetFeature { feature, rust_feature: PossibleFeature::None }
+                        };
+                        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 });
+                    }
+                }
+
                 // FIXME(nagisa): figure out how to not allocate a full hashset here.
                 featsmap.insert(feature, enable_disable == '+');
             }
 
-            // rustc-specific features do not get passed down to GCC…
-            if RUSTC_SPECIFIC_FEATURES.contains(&feature) {
-                return None;
-            }
             // ... otherwise though we run through `to_gcc_features` when
             // passing requests down to GCC. This means that all in-language
             // features also work on the command line instead of having two

src/lib.rs

@@ -491,8 +491,9 @@ pub fn target_features(
 ) -> Vec<Symbol> {
     // TODO(antoyo): use global_gcc_features.
     sess.target
-        .supported_target_features()
+        .rust_target_features()
         .iter()
+        .filter(|(_, gate, _)| gate.is_supported())
         .filter_map(|&(feature, gate, _)| {
             if sess.is_nightly_build() || allow_unstable || gate.is_stable() {
                 Some(feature)