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

Also rename some things for extra clarity

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

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)]

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 });
+                        }
                     }
                 }
 

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