comments and tweaks for deduce_param_attrs

Author: RalfJung

compiler/rustc_mir_transform/src/deduce_param_attrs.rs

@@ -4,6 +4,10 @@
 //! body of the function instead of just the signature. These can be useful for optimization
 //! purposes on a best-effort basis. We compute them here and store them into the crate metadata so
 //! dependent crates can use them.
+//!
+//! Note that this *crucially* relies on codegen *not* doing any more MIR-level transformations
+//! after `optimized_mir`! We check for things that are *not* guaranteed to be preserved by MIR
+//! transforms, such as which local variables happen to be mutated.
 
 use rustc_hir::def_id::LocalDefId;
 use rustc_index::bit_set::DenseBitSet;
@@ -34,6 +38,11 @@ impl<'tcx> Visitor<'tcx> for DeduceReadOnly {
         if place.local == RETURN_PLACE || place.local.index() > self.mutable_args.domain_size() {
             return;
         }
+        // Mutating an indirect place doesn't mutate the local it is based on, so we can skip those.
+        // We're using runtime MIR so we can use the `first_projection` check.
+        if place.is_indirect_first_projection() {
+            return;
+        }
 
         let mark_as_mutable = match context {
             PlaceContext::MutatingUse(..) => {
@@ -42,13 +51,14 @@ impl<'tcx> Visitor<'tcx> for DeduceReadOnly {
             }
             PlaceContext::NonMutatingUse(NonMutatingUseContext::RawBorrow) => {
                 // Whether mutating though a `&raw const` is allowed is still undecided, so we
-                // disable any sketchy `readonly` optimizations for now. But we only need to do
-                // this if the pointer would point into the argument. IOW: for indirect places,
-                // like `&raw (*local).field`, this surely cannot mutate `local`.
+                // disable any sketchy `readonly` optimizations for now.
                 !place.is_indirect()
             }
             PlaceContext::NonMutatingUse(..) | PlaceContext::NonUse(..) => {
-                // Not mutating, so it's fine.
+                // This could still mutate if it's a shared reference to a `!Freeze` type -- but
+                // we check for `Freeze` below when converting the final `mutable_args` values
+                // into the actually deduced attributes.
+                // The rest is not mutating, so it's fine.
                 false
             }
         };