rustc: target_features: adapt compile errors and warnings for cfg-only stable features

compiler/rustc_codegen_ssa/src/errors.rs

@@ -1194,9 +1194,10 @@ pub(crate) struct UnknownCTargetFeature<'a> {
 
 #[derive(Diagnostic)]
 #[diag("unstable feature specified for `-Ctarget-feature`: `{$feature}`")]
-#[note("this feature is not stably supported; its behavior can change in the future")]
+#[note("{$note}; its behavior can change in the future")]
 pub(crate) struct UnstableCTargetFeature<'a> {
     pub feature: &'a str,
+    pub note: &'a str,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -65,13 +65,12 @@ pub(crate) fn from_target_feature_attr(
         } else if let Some(nightly_feature) = stability.requires_nightly(/* in_cfg */ false)
             && !rust_features.enabled(nightly_feature)
         {
-            feature_err(
-                &tcx.sess,
-                nightly_feature,
-                feature_span,
-                format!("the target feature `{feature}` is currently unstable"),
-            )
-            .emit();
+            let explain = if stability.is_cfg_stable_toggle_unstable() {
+                format!("the target feature `{feature}` is allowed in cfg but unstable otherwise")
+            } else {
+                format!("the target feature `{feature}` is currently unstable")
+            };
+            feature_err(&tcx.sess, nightly_feature, feature_span, explain).emit();
         } else {
             // Add this and the implied features.
             for &name in tcx.implied_target_features(feature) {
@@ -319,8 +318,15 @@ pub fn cfg_target_feature<'a, const N: usize>(
                         // 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(errors::UnstableCTargetFeature { feature: base_feature });
+                        let note = if stability.is_cfg_stable_toggle_unstable() {
+                            "this feature is allowed in cfg but unstable otherwise"
+                        } else {
+                            "this feature is not stably supported"
+                        };
+                        sess.dcx().emit_warn(errors::UnstableCTargetFeature {
+                            feature: base_feature,
+                            note,
+                        });
                     }
                 }
             }

compiler/rustc_target/src/target_features.rs

@@ -78,6 +78,12 @@ impl Stability {
         }
     }
 
+    /// Returns whether the feature is cfg-stable but still requires a nightly feature gate to
+    /// be used in `#[target_feature]`/`-Ctarget-feature`.
+    pub fn is_cfg_stable_toggle_unstable(&self) -> bool {
+        matches!(self, Stability::CfgStableToggleUnstable { .. })
+    }
+
     /// Returns whether the feature may be toggled via `#[target_feature]` or `-Ctarget-feature`.
     /// (It might still be nightly-only even if this returns `true`, so make sure to also check
     /// `requires_nightly`.)