Auto merge of #145513 - beepster4096:erasedereftemps, r=saethlin,cjgillot

Validate CopyForDeref and DerefTemps better and remove them from runtime MIR

(split from my WIP #145344)

This PR:
- Removes `Rvalue::CopyForDeref` and `LocalInfo::DerefTemp` from runtime MIR
    - Using a new mir pass `EraseDerefTemps`
    - `CopyForDeref(x)` is turned into `Use(Copy(x))`
    - `DerefTemp` is turned into `Boring`
        - Not sure if this part is actually necessary, it made more sense in #145344 with `DerefTemp` storing actual data that I wanted to keep from having to be kept in sync with the rest of the body in runtime MIR
- Checks in validation that `CopyForDeref` and `DerefTemp` are only used together
- Removes special handling for `CopyForDeref` from many places
- Removes `CopyForDeref` from `custom_mir` reverting #111587
    - In runtime MIR simple copies can be used instead
    - In post cleanup analysis MIR it was already wrong to use due to the lack of support for creating `DerefTemp` locals
    - Possibly this should be its own PR?
 - Adds an argument to `deref_finder` to avoid creating new `DerefTemp`s and `CopyForDeref` in runtime MIR.
     - Ideally we would just avoid making intermediate derefs instead of fixing it at the end of a pass / during shim building
 - Removes some usages of `deref_finder` that I found out don't actually do anything

r? oli-obk

Author: bors

compiler/rustc_borrowck/src/lib.rs

@@ -1538,24 +1538,6 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 self.consume_operand(location, (operand, span), state)
             }
 
-            &Rvalue::CopyForDeref(place) => {
-                self.access_place(
-                    location,
-                    (place, span),
-                    (Deep, Read(ReadKind::Copy)),
-                    LocalMutationIsAllowed::No,
-                    state,
-                );
-
-                // Finally, check if path was already moved.
-                self.check_if_path_or_subpath_is_moved(
-                    location,
-                    InitializationRequiringAction::Use,
-                    (place.as_ref(), span),
-                    state,
-                );
-            }
-
             &Rvalue::Discriminant(place) => {
                 let af = match *rvalue {
                     Rvalue::Discriminant(..) => None,
@@ -1617,6 +1599,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
             Rvalue::WrapUnsafeBinder(op, _) => {
                 self.consume_operand(location, (op, span), state);
             }
+
+            Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in borrowck"),
         }
     }
 

compiler/rustc_borrowck/src/polonius/legacy/loan_invalidations.rs

@@ -300,11 +300,6 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
             | Rvalue::Cast(_ /*cast_kind*/, operand, _ /*ty*/)
             | Rvalue::ShallowInitBox(operand, _ /*ty*/) => self.consume_operand(location, operand),
 
-            &Rvalue::CopyForDeref(place) => {
-                let op = &Operand::Copy(place);
-                self.consume_operand(location, op);
-            }
-
             &Rvalue::Discriminant(place) => {
                 self.access_place(
                     location,
@@ -330,6 +325,8 @@ impl<'a, 'tcx> LoanInvalidationsGenerator<'a, 'tcx> {
             Rvalue::WrapUnsafeBinder(op, _) => {
                 self.consume_operand(location, op);
             }
+
+            Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in borrowck"),
         }
     }
 

compiler/rustc_codegen_cranelift/src/base.rs

@@ -600,11 +600,6 @@ fn codegen_stmt<'tcx>(fx: &mut FunctionCx<'_, '_, 'tcx>, cur_block: Block, stmt:
                     let val = codegen_operand(fx, operand);
                     lval.write_cvalue(fx, val);
                 }
-                Rvalue::CopyForDeref(place) => {
-                    let cplace = codegen_place(fx, place);
-                    let val = cplace.to_cvalue(fx);
-                    lval.write_cvalue(fx, val)
-                }
                 Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) => {
                     let place = codegen_place(fx, place);
                     let ref_ = place.place_ref(fx, lval.layout());
@@ -928,6 +923,7 @@ fn codegen_stmt<'tcx>(fx: &mut FunctionCx<'_, '_, 'tcx>, cur_block: Block, stmt:
                     let operand = codegen_operand(fx, operand);
                     lval.write_cvalue_transmute(fx, operand);
                 }
+                Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in codegen"),
             }
         }
         StatementKind::StorageLive(_)

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -504,9 +504,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 self.codegen_place_to_pointer(bx, place, mk_ref)
             }
 
-            mir::Rvalue::CopyForDeref(place) => {
-                self.codegen_operand(bx, &mir::Operand::Copy(place))
-            }
             mir::Rvalue::RawPtr(kind, place) => {
                 let mk_ptr = move |tcx: TyCtxt<'tcx>, ty: Ty<'tcx>| {
                     Ty::new_ptr(tcx, ty, kind.to_mutbl_lossy())
@@ -742,6 +739,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 let layout = bx.cx().layout_of(binder_ty);
                 OperandRef { val: operand.val, layout }
             }
+            mir::Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in codegen"),
         }
     }
 

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -234,7 +234,7 @@ where
 
         Rvalue::Discriminant(place) => in_place::<Q, _>(cx, in_local, place.as_ref()),
 
-        Rvalue::CopyForDeref(place) => in_place::<Q, _>(cx, in_local, place.as_ref()),
+        Rvalue::CopyForDeref(_) => bug!("`CopyForDeref` in runtime MIR"),
 
         Rvalue::Use(operand)
         | Rvalue::Repeat(operand, _)

compiler/rustc_const_eval/src/interpret/step.rs

@@ -183,10 +183,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 self.copy_op(&op, &dest)?;
             }
 
-            CopyForDeref(place) => {
-                let op = self.eval_place_to_op(place, Some(dest.layout))?;
-                self.copy_op(&op, &dest)?;
-            }
+            CopyForDeref(_) => bug!("`CopyForDeref` in runtime MIR"),
 
             BinaryOp(bin_op, box (ref left, ref right)) => {
                 let layout = util::binop_left_homogeneous(bin_op).then_some(dest.layout);

compiler/rustc_middle/src/mir/mod.rs

@@ -1065,7 +1065,11 @@ 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

@@ -130,7 +130,9 @@ pub enum RuntimePhase {
     /// * [`TerminatorKind::Yield`]
     /// * [`TerminatorKind::CoroutineDrop`]
     /// * [`Rvalue::Aggregate`] for any `AggregateKind` except `Array`
+    /// * [`Rvalue::CopyForDeref`]
     /// * [`PlaceElem::OpaqueCast`]
+    /// * [`LocalInfo::DerefTemp`](super::LocalInfo::DerefTemp)
     ///
     /// And the following variants are allowed:
     /// * [`StatementKind::Retag`]
@@ -1454,11 +1456,13 @@ 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.

compiler/rustc_mir_build/src/builder/custom/parse/instruction.rs

@@ -250,7 +250,6 @@ impl<'a, 'tcx> ParseCtxt<'a, 'tcx> {
                 Ok(Rvalue::BinaryOp(BinOp::Offset, Box::new((ptr, offset))))
             },
             @call(mir_ptr_metadata, args) => Ok(Rvalue::UnaryOp(UnOp::PtrMetadata, self.parse_operand(args[0])?)),
-            @call(mir_copy_for_deref, args) => Ok(Rvalue::CopyForDeref(self.parse_place(args[0])?)),
             ExprKind::Borrow { borrow_kind, arg } => Ok(
                 Rvalue::Ref(self.tcx.lifetimes.re_erased, *borrow_kind, self.parse_place(*arg)?)
             ),

compiler/rustc_mir_transform/src/copy_prop.rs

@@ -74,9 +74,7 @@ fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> DenseBitSet<Local> {
     let mut fully_moved = DenseBitSet::new_filled(body.local_decls.len());
 
     for (_, rvalue, _) in ssa.assignments(body) {
-        let (Rvalue::Use(Operand::Copy(place) | Operand::Move(place))
-        | Rvalue::CopyForDeref(place)) = rvalue
-        else {
+        let Rvalue::Use(Operand::Copy(place) | Operand::Move(place)) = rvalue else {
             continue;
         };
 
@@ -85,7 +83,7 @@ fn fully_moved_locals(ssa: &SsaLocals, body: &Body<'_>) -> DenseBitSet<Local> {
             continue;
         }
 
-        if let Rvalue::Use(Operand::Copy(_)) | Rvalue::CopyForDeref(_) = rvalue {
+        if let Rvalue::Use(Operand::Copy(_)) = rvalue {
             fully_moved.remove(rhs);
         }
     }
@@ -146,8 +144,7 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
 
         // Do not leave tautological assignments around.
         if let StatementKind::Assign(box (lhs, ref rhs)) = stmt.kind
-            && let Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)) | Rvalue::CopyForDeref(rhs) =
-                *rhs
+            && let Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)) = *rhs
             && lhs == rhs
         {
             stmt.make_nop(true);