Cut code size for feature hashing

This locally cuts ~32 kB of .text instructions.

Author: Mark-Simulacrum

Diff for: compiler/rustc_feature/src/unstable.rs

@@ -50,6 +50,8 @@ macro_rules! declare_features {
             }),+
         ];
 
+        const NUM_FEATURES: usize = UNSTABLE_FEATURES.len();
+
         /// A set of features to be used by later passes.
         #[derive(Clone, Default, Debug)]
         pub struct Features {
@@ -82,8 +84,14 @@ macro_rules! declare_features {
                 self.declared_features.insert(symbol);
             }
 
-            pub fn walk_feature_fields(&self, mut f: impl FnMut(&str, bool)) {
-                $(f(stringify!($feature), self.$feature);)+
+            /// This is intended for hashing the set of active features.
+            ///
+            /// The expectation is that this produces much smaller code than other alternatives.
+            ///
+            /// Note that the total feature count is pretty small, so this is not a huge array.
+            #[inline]
+            pub fn all_features(&self) -> [u8; NUM_FEATURES] {
+                [$(self.$feature as u8),+]
             }
 
             /// Is the given feature explicitly declared, i.e. named in a

Diff for: compiler/rustc_query_system/src/ich/impls_syntax.rs

@@ -117,9 +117,9 @@ impl<'tcx> HashStable<StableHashingContext<'tcx>> for rustc_feature::Features {
         self.declared_lang_features.hash_stable(hcx, hasher);
         self.declared_lib_features.hash_stable(hcx, hasher);
 
-        self.walk_feature_fields(|feature_name, value| {
-            feature_name.hash_stable(hcx, hasher);
-            value.hash_stable(hcx, hasher);
-        });
+        self.all_features()[..].hash_stable(hcx, hasher);
+        for feature in rustc_feature::UNSTABLE_FEATURES.iter() {
+            feature.feature.name.hash_stable(hcx, hasher);
+        }
     }
 }