Don't error when just giving neon as a target feature

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -325,7 +325,7 @@ pub fn from_fn_attrs<'ll, 'tcx>(
 
     if let Some(f) = llvm_util::check_tied_features(
         cx.tcx.sess,
-        &function_features.iter().map(|f| (*f, true)).collect(),
+        &mut function_features.iter().map(|f| (*f, true)).collect(),
     ) {
         let span = cx
             .tcx

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -217,8 +217,15 @@ pub fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> SmallVec<[&'a str; 2]
 // ensure only valid combinations are allowed.
 pub fn check_tied_features(
     sess: &Session,
-    features: &FxHashMap<&str, bool>,
+    features: &mut FxHashMap<&str, bool>,
 ) -> Option<&'static [&'static str]> {
+    // neon implies fp, so we don't need to error in this case.
+    if sess.target.arch == "aarch64" && !features.contains_key("fp") {
+        if let Some(neon) = features.get("neon").copied() {
+            features.insert("fp", neon);
+        }
+    }
+
     for tied in tied_target_features(sess) {
         // Tied features must be set to the same value, or not set at all
         let mut tied_iter = tied.iter();
@@ -487,8 +494,8 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
 
     if diagnostics {
         // FIXME(nagisa): figure out how to not allocate a full hashset here.
-        let featmap = feats.iter().map(|&(flag, feat)| (feat, flag == '+')).collect();
-        if let Some(f) = check_tied_features(sess, &featmap) {
+        let mut featmap = feats.iter().map(|&(flag, feat)| (feat, flag == '+')).collect();
+        if let Some(f) = check_tied_features(sess, &mut featmap) {
             sess.err(&format!(
                 "target features {} must all be enabled or disabled together",
                 f.join(", ")

src/test/ui/target-feature/tied-features-cli.rs

@@ -1,4 +1,4 @@
-// revisions: one two three
+// revisions: one two three four five
 // compile-flags: --crate-type=rlib --target=aarch64-unknown-linux-gnu
 // needs-llvm-components: aarch64
 //
@@ -11,6 +11,8 @@
 // [three] compile-flags: -C target-feature=+paca,+pacg,-paca
 // [four] build-pass
 // [four] compile-flags: -C target-feature=-paca,+pacg -C target-feature=+paca
+// [five] build-pass
+// [five] compile-flags: -C target-feature=+neon
 #![feature(no_core, lang_items)]
 #![no_core]
 

src/test/ui/target-feature/tied-features.rs

@@ -19,17 +19,20 @@ pub fn main() {
         bar();
         baz();
         inner();
+        implicit_fp();
     }
 }
 
 #[target_feature(enable = "paca")]
 //~^ ERROR must all be either enabled or disabled together
 unsafe fn foo() {}
 
-
 #[target_feature(enable = "paca,pacg")]
 unsafe fn bar() {}
 
 #[target_feature(enable = "paca")]
 #[target_feature(enable = "pacg")]
 unsafe fn baz() {}
+
+#[target_feature(enable = "neon")]
+unsafe fn implicit_fp() {}

src/test/ui/target-feature/tied-features.stderr

@@ -7,7 +7,7 @@ LL |     #[target_feature(enable = "pacg")]
    = help: add the missing features in a `target_feature` attribute
 
 error: the target features paca, pacg must all be either enabled or disabled together
-  --> $DIR/tied-features.rs:25:1
+  --> $DIR/tied-features.rs:26:1
    |
 LL | #[target_feature(enable = "paca")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^