Attempt to compress representation.

Author: cjgillot

compiler/rustc_middle/src/middle/deduced_param_attrs.rs

@@ -34,12 +34,18 @@ pub struct DeducedParamAttrs {
 
 // By default, consider the parameters to be mutated.
 impl Default for DeducedParamAttrs {
+    #[inline]
     fn default() -> DeducedParamAttrs {
         DeducedParamAttrs { read_only: DeducedReadOnlyParam::MUTATED }
     }
 }
 
 impl DeducedParamAttrs {
+    #[inline]
+    pub fn is_default(self) -> bool {
+        self.read_only.contains(DeducedReadOnlyParam::MUTATED)
+    }
+
     pub fn read_only<'tcx>(
         &self,
         tcx: TyCtxt<'tcx>,

compiler/rustc_mir_transform/src/deduce_param_attrs.rs

@@ -170,16 +170,17 @@ pub(super) fn deduced_param_attrs<'tcx>(
     deduce_read_only.visit_body(body);
     tracing::trace!(?deduce_read_only.read_only);
 
-    let mut deduced_param_attrs = tcx.arena.alloc_from_iter(
+    let mut deduced_param_attrs: &[_] = tcx.arena.alloc_from_iter(
         deduce_read_only.read_only.into_iter().map(|read_only| DeducedParamAttrs { read_only }),
     );
 
     // Trailing parameters past the size of the `deduced_param_attrs` array are assumed to have the
     // default set of attributes, so we don't have to store them explicitly. Pop them off to save a
     // few bytes in metadata.
-    while deduced_param_attrs.last() == Some(&DeducedParamAttrs::default()) {
-        let last_index = deduced_param_attrs.len() - 1;
-        deduced_param_attrs = &mut deduced_param_attrs[0..last_index];
+    while let Some((last, rest)) = deduced_param_attrs.split_last()
+        && last.is_default()
+    {
+        deduced_param_attrs = rest;
     }
 
     deduced_param_attrs