rustc: target_features: allow for cfg-only stable target_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

@@ -62,16 +62,15 @@ pub(crate) fn from_target_feature_attr(
                 feature: feature_str,
                 reason,
             });
-        } else if let Some(nightly_feature) = stability.requires_nightly()
+        } 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) {
@@ -315,12 +314,19 @@ pub fn cfg_target_feature<'a, const N: usize>(
                             enabled: if enable { "enabled" } else { "disabled" },
                             reason,
                         });
-                    } else if stability.requires_nightly().is_some() {
+                    } else if stability.requires_nightly(/* in_cfg */ false).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(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,
+                        });
                     }
                 }
             }
@@ -346,7 +352,8 @@ pub fn cfg_target_feature<'a, const N: usize>(
                 // "forbidden" features.
                 if allow_unstable
                     || (gate.in_cfg()
-                        && (sess.is_nightly_build() || gate.requires_nightly().is_none()))
+                        && (sess.is_nightly_build()
+                            || gate.requires_nightly(/* in_cfg */ true).is_none()))
                 {
                     Some(Symbol::intern(feature))
                 } else {

compiler/rustc_target/src/target_features.rs

@@ -17,6 +17,13 @@ pub enum Stability {
     /// This target feature is stable, it can be used in `#[target_feature]` and
     /// `#[cfg(target_feature)]`.
     Stable,
+    /// This target feature is cfg-stable. It can be used for `#[cfg(target_feature)]` on stable,
+    /// but using it in `#[target_feature]` requires the given nightly feature.
+    CfgStableToggleUnstable(
+        /// This must be a *language* feature, or else rustc will ICE when reporting a missing
+        /// feature gate!
+        Symbol,
+    ),
     /// This target feature is unstable. It is only present in `#[cfg(target_feature)]` on
     /// nightly and using it in `#[target_feature]` requires enabling the given nightly feature.
     Unstable(
@@ -37,7 +44,12 @@ impl Stability {
     /// (It might still be nightly-only even if this returns `true`, so make sure to also check
     /// `requires_nightly`.)
     pub fn in_cfg(&self) -> bool {
-        matches!(self, Stability::Stable | Stability::Unstable { .. })
+        matches!(
+            self,
+            Stability::Stable
+                | Stability::CfgStableToggleUnstable { .. }
+                | Stability::Unstable { .. }
+        )
     }
 
     /// Returns the nightly feature that is required to toggle this target feature via
@@ -48,20 +60,38 @@ impl Stability {
     /// Before calling this, ensure the feature is even permitted for this use:
     /// - for `#[target_feature]`/`-Ctarget-feature`, check `toggle_allowed()`
     /// - for `cfg(target_feature)`, check `in_cfg()`
-    pub fn requires_nightly(&self) -> Option<Symbol> {
+    ///
+    /// The `in_cfg` parameter is used to determine whether it will be used in
+    /// `cfg(target_feature)` (true) or `#[target_feature]`/`-Ctarget-feature` (false)
+    pub fn requires_nightly(&self, in_cfg: bool) -> Option<Symbol> {
         match *self {
             Stability::Unstable(nightly_feature) => Some(nightly_feature),
+            Stability::CfgStableToggleUnstable(nightly_feature) => {
+                if in_cfg {
+                    None
+                } else {
+                    Some(nightly_feature)
+                }
+            }
             Stability::Stable { .. } => None,
             Stability::Forbidden { .. } => panic!("forbidden features should not reach this far"),
         }
     }
 
+    /// 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`.)
     pub fn toggle_allowed(&self) -> Result<(), &'static str> {
         match self {
-            Stability::Unstable(_) | Stability::Stable { .. } => Ok(()),
+            Stability::Unstable(_)
+            | Stability::CfgStableToggleUnstable(_)
+            | Stability::Stable { .. } => Ok(()),
             Stability::Forbidden { reason } => Err(reason),
         }
     }