Rollup merge of rust-lang#117385 - RalfJung:deduce_param_attrs, r=oli-obk

deduce_param_attrs: explain a read-only case

This takes the discussion [here](https://github.com/rust-lang/rust/pull/111517/files#r1243443625) and adds it as comment in the code.

Cc `@lukas-code`

Author: fmease

Diff for: compiler/rustc_mir_transform/src/deduce_param_attrs.rs

@@ -44,6 +44,7 @@ impl<'tcx> Visitor<'tcx> for DeduceReadOnly {
                 // 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`.
                 !place.is_indirect()
             }
             PlaceContext::NonMutatingUse(..) | PlaceContext::NonUse(..) => {