advice against negative features in target specs

Co-authored-by: Jubilee <[email protected]>

Author: RalfJung

compiler/rustc_target/src/spec/mod.rs

@@ -2608,6 +2608,9 @@ impl TargetOptions {
         self.features.split(',').any(|f| f.strip_prefix('+').is_some_and(|f| f == search_feature))
     }
 
+    /// We should try to minimize the number of negative features defined for any target
+    /// because they often interact very poorly with how `-Ctarget-cpu` works.
+    /// However, their mere existence is still a possibility, so!
     pub(crate) fn has_neg_feature(&self, search_feature: &str) -> bool {
         self.features.split(',').any(|f| f.strip_prefix('-').is_some_and(|f| f == search_feature))
     }