adjust docs of CopyForDeref and DerefTemp

Author: beepster4096

compiler/rustc_middle/src/mir/mod.rs

@@ -1109,7 +1109,11 @@ pub enum LocalInfo<'tcx> {
     /// A temporary created during evaluating `if` predicate, possibly for pattern matching for `let`s,
     /// and subject to Edition 2024 temporary lifetime rules
     IfThenRescopeTemp { if_then: HirId },
-    /// A temporary created during the pass `Derefer` to avoid it's retagging
+    /// A temporary created during the pass `Derefer` treated as a transparent alias
+    /// for the place its copied from by analysis passes such as `AddRetag` and `ElaborateDrops`.
+    ///
+    /// It may only be written to by a `CopyForDeref` and otherwise only accessed through a deref.
+    /// In runtime MIR, it is replaced with a normal `Boring` local.
     DerefTemp,
     /// A temporary created for borrow checking.
     FakeBorrow,

compiler/rustc_middle/src/mir/syntax.rs

@@ -1487,11 +1487,13 @@ pub enum Rvalue<'tcx> {
     /// A CopyForDeref is equivalent to a read from a place at the
     /// codegen level, but is treated specially by drop elaboration. When such a read happens, it
     /// is guaranteed (via nature of the mir_opt `Derefer` in rustc_mir_transform/src/deref_separator)
-    /// that the only use of the returned value is a deref operation, immediately
-    /// followed by one or more projections. Drop elaboration treats this rvalue as if the
+    /// that the returned value is written into a `DerefTemp` local and that its only use is a deref operation,
+    /// immediately followed by one or more projections. Drop elaboration treats this rvalue as if the
     /// read never happened and just projects further. This allows simplifying various MIR
     /// optimizations and codegen backends that previously had to handle deref operations anywhere
     /// in a place.
+    ///
+    /// Disallowed in runtime MIR and is replaced by normal copies.
     CopyForDeref(Place<'tcx>),
 
     /// Wraps a value in an unsafe binder.