Simplify implied_target_features.

Currently its argument is an iterator, but in practice it's always a
singleton.

compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -48,7 +48,7 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
     for feature in sess.opts.cg.target_feature.split(',') {
         if let Some(feature) = feature.strip_prefix('+') {
             all_rust_features.extend(
-                UnordSet::from(sess.target.implied_target_features(std::iter::once(feature)))
+                UnordSet::from(sess.target.implied_target_features(feature))
                     .to_sorted_stable_ord()
                     .iter()
                     .map(|&&s| (true, s)),

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -360,7 +360,7 @@ pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<S
             #[allow(rustc::potential_query_instability)]
             features.extend(
                 sess.target
-                    .implied_target_features(std::iter::once(feature.as_str()))
+                    .implied_target_features(feature.as_str())
                     .iter()
                     .map(|s| Symbol::intern(s)),
             );
@@ -373,7 +373,7 @@ pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<S
             features.retain(|f| {
                 if sess
                     .target
-                    .implied_target_features(std::iter::once(f.as_str()))
+                    .implied_target_features(f.as_str())
                     .contains(&feature.as_str())
                 {
                     // If `f` if implies `feature`, then `!feature` implies `!f`, so we have to
@@ -681,7 +681,7 @@ pub(crate) fn global_llvm_features(
         for feature in sess.opts.cg.target_feature.split(',') {
             if let Some(feature) = feature.strip_prefix('+') {
                 all_rust_features.extend(
-                    UnordSet::from(sess.target.implied_target_features(std::iter::once(feature)))
+                    UnordSet::from(sess.target.implied_target_features(feature))
                         .to_sorted_stable_ord()
                         .iter()
                         .map(|&&s| (true, s)),

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -190,7 +190,7 @@ pub(crate) fn provide(providers: &mut Providers) {
         },
         implied_target_features: |tcx, feature: Symbol| {
             let feature = feature.as_str();
-            UnordSet::from(tcx.sess.target.implied_target_features(std::iter::once(feature)))
+            UnordSet::from(tcx.sess.target.implied_target_features(feature))
                 .into_sorted_stable_ord()
                 .into_iter()
                 .map(|s| Symbol::intern(s))

compiler/rustc_target/src/target_features.rs

@@ -768,17 +768,15 @@ impl Target {
         }
     }
 
-    pub fn implied_target_features<'a>(
+    // Note: the returned set includes `base_feature`.
-        &self,
+    pub fn implied_target_features<'a>(&self, base_feature: &'a str) -> FxHashSet<&'a str> {
-        base_features: impl Iterator<Item = &'a str>,
-    ) -> FxHashSet<&'a str> {
         let implied_features =
             self.rust_target_features().iter().map(|(f, _, i)| (f, i)).collect::<FxHashMap<_, _>>();
 
-        // implied target features have their own implied target features, so we traverse the
+        // Implied target features have their own implied target features, so we traverse the
-        // map until there are no more features to add
+        // map until there are no more features to add.
         let mut features = FxHashSet::default();
-        let mut new_features = base_features.collect::<Vec<&str>>();
+        let mut new_features = vec![base_feature];
         while let Some(new_feature) = new_features.pop() {
             if features.insert(new_feature) {
                 if let Some(implied_features) = implied_features.get(&new_feature) {