advice against negative features in target specs

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

Author: RalfJung

compiler/rustc_target/src/spec/mod.rs

@@ -2213,6 +2213,10 @@ pub struct TargetOptions {
     /// `-Ctarget-cpu` but can be overwritten with `-Ctarget-features`.
     /// Corresponds to `llc -mattr=$features`.
     /// Note that these are LLVM feature names, not Rust feature names!
+    ///
+    /// Generally it is a bad idea to use negative features here since they
+    /// interact poorly with `-Ctarget-cpu`. Instead, try to use a lower
+    /// "base CPU" and enable the features you want to use.
     pub features: StaticCow<str>,
     /// Direct or use GOT indirect to reference external data symbols
     pub direct_access_external_data: Option<bool>,
@@ -2608,6 +2612,9 @@ 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))
     }