Pull Derefer before ElaborateDrops

compiler/rustc_borrowck/src/invalidation.rs

@@ -289,6 +289,10 @@ impl<'cx, 'tcx> InvalidationGenerator<'cx, 'tcx> {
             | Rvalue::ShallowInitBox(ref operand, _ /*ty*/) => {
                 self.consume_operand(location, operand)
             }
+            Rvalue::CopyForDeref(ref place) => {
+                let op = &Operand::Copy(*place);
+                self.consume_operand(location, op);
+            }
 
             Rvalue::Len(place) | Rvalue::Discriminant(place) => {
                 let af = match *rvalue {

compiler/rustc_borrowck/src/lib.rs

@@ -1236,6 +1236,23 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
             | Rvalue::ShallowInitBox(ref operand, _ /*ty*/) => {
                 self.consume_operand(location, (operand, span), flow_state)
             }
+            Rvalue::CopyForDeref(place) => {
+                self.access_place(
+                    location,
+                    (place, span),
+                    (Deep, Read(ReadKind::Copy)),
+                    LocalMutationIsAllowed::No,
+                    flow_state,
+                );
+
+                // Finally, check if path was already moved.
+                self.check_if_path_or_subpath_is_moved(
+                    location,
+                    InitializationRequiringAction::Use,
+                    (place.as_ref(), span),
+                    flow_state,
+                );
+            }
 
             Rvalue::Len(place) | Rvalue::Discriminant(place) => {
                 let af = match *rvalue {

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -2269,6 +2269,10 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             Rvalue::Use(operand) | Rvalue::UnaryOp(_, operand) => {
                 self.check_operand(operand, location);
             }
+            Rvalue::CopyForDeref(place) => {
+                let op = &Operand::Copy(*place);
+                self.check_operand(op, location);
+            }
 
             Rvalue::BinaryOp(_, box (left, right))
             | Rvalue::CheckedBinaryOp(_, box (left, right)) => {
@@ -2299,6 +2303,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             | Rvalue::BinaryOp(..)
             | Rvalue::CheckedBinaryOp(..)
             | Rvalue::NullaryOp(..)
+            | Rvalue::CopyForDeref(..)
             | Rvalue::UnaryOp(..)
             | Rvalue::Discriminant(..) => None,
 

compiler/rustc_codegen_cranelift/src/base.rs

@@ -503,6 +503,11 @@ fn codegen_stmt<'tcx>(
                     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::AddressOf(_, place) => {
                     let place = codegen_place(fx, place);
                     let ref_ = place.place_ref(fx, lval.layout());

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -8,6 +8,7 @@ use crate::traits::*;
 use crate::MemFlags;
 
 use rustc_middle::mir;
+use rustc_middle::mir::Operand;
 use rustc_middle::ty::cast::{CastTy, IntTy};
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
 use rustc_middle::ty::{self, adjustment::PointerCast, Instance, Ty, TyCtxt};
@@ -344,6 +345,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 self.codegen_place_to_pointer(bx, place, mk_ref)
             }
 
+            mir::Rvalue::CopyForDeref(place) => {
+                let operand = self.codegen_operand(&mut bx, &Operand::Copy(place));
+                (bx, operand)
+            }
             mir::Rvalue::AddressOf(mutability, place) => {
                 let mk_ptr = move |tcx: TyCtxt<'tcx>, ty: Ty<'tcx>| {
                     tcx.mk_ptr(ty::TypeAndMut { ty, mutbl: mutability })
@@ -698,6 +703,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     pub fn rvalue_creates_operand(&self, rvalue: &mir::Rvalue<'tcx>, span: Span) -> bool {
         match *rvalue {
             mir::Rvalue::Ref(..) |
+            mir::Rvalue::CopyForDeref(..) |
             mir::Rvalue::AddressOf(..) |
             mir::Rvalue::Len(..) |
             mir::Rvalue::Cast(..) | // (*)

compiler/rustc_const_eval/src/interpret/step.rs

@@ -172,6 +172,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                 self.copy_op(&op, &dest, /*allow_transmute*/ false)?;
             }
 
+            CopyForDeref(ref place) => {
+                let op = self.eval_place_to_op(*place, Some(dest.layout))?;
+                self.copy_op(&op, &dest, /* allow_transmute*/ false)?;
+            }
+
             BinaryOp(bin_op, box (ref left, ref right)) => {
                 let layout = binop_left_homogeneous(bin_op).then_some(dest.layout);
                 let left = self.read_immediate(&self.eval_operand(left, layout)?)?;

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -446,6 +446,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             Rvalue::ThreadLocalRef(_) => self.check_op(ops::ThreadLocalAccess),
 
             Rvalue::Use(_)
+            | Rvalue::CopyForDeref(..)
             | Rvalue::Repeat(..)
             | Rvalue::Discriminant(..)
             | Rvalue::Len(_)

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -260,6 +260,8 @@ where
             in_place::<Q, _>(cx, in_local, place.as_ref())
         }
 
+        Rvalue::CopyForDeref(place) => in_place::<Q, _>(cx, in_local, place.as_ref()),
+
         Rvalue::Use(operand)
         | Rvalue::Repeat(operand, _)
         | Rvalue::UnaryOp(_, operand)

compiler/rustc_const_eval/src/transform/check_consts/resolver.rs

@@ -199,6 +199,7 @@ where
             mir::Rvalue::Cast(..)
             | mir::Rvalue::ShallowInitBox(..)
             | mir::Rvalue::Use(..)
+            | mir::Rvalue::CopyForDeref(..)
             | mir::Rvalue::ThreadLocalRef(..)
             | mir::Rvalue::Repeat(..)
             | mir::Rvalue::Len(..)

compiler/rustc_const_eval/src/transform/promote_consts.rs

@@ -494,6 +494,10 @@ impl<'tcx> Validator<'_, 'tcx> {
             Rvalue::Use(operand) | Rvalue::Repeat(operand, _) => {
                 self.validate_operand(operand)?;
             }
+            Rvalue::CopyForDeref(place) => {
+                let op = &Operand::Copy(*place);
+                self.validate_operand(op)?
+            }
 
             Rvalue::Discriminant(place) | Rvalue::Len(place) => {
                 self.validate_place(place.as_ref())?

compiler/rustc_const_eval/src/transform/validate.rs

@@ -382,7 +382,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             };
         }
         match rvalue {
-            Rvalue::Use(_) => {}
+            Rvalue::Use(_) | Rvalue::CopyForDeref(_) => {}
             Rvalue::Aggregate(agg_kind, _) => {
                 let disallowed = match **agg_kind {
                     AggregateKind::Array(..) => false,
@@ -592,6 +592,15 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         ),
                     );
                 }
+                if let Rvalue::CopyForDeref(place) = rvalue {
+                    if !place.ty(&self.body.local_decls, self.tcx).ty.builtin_deref(true).is_some()
+                    {
+                        self.fail(
+                            location,
+                            "`CopyForDeref` should only be used for dereferenceable types",
+                        )
+                    }
+                }
                 // FIXME(JakobDegen): Check this for all rvalues, not just this one.
                 if let Rvalue::Use(Operand::Copy(src) | Operand::Move(src)) = rvalue {
                     // The sides of an assignment must not alias. Currently this just checks whether

compiler/rustc_middle/src/mir/mod.rs

@@ -61,7 +61,7 @@ mod syntax;
 pub use syntax::*;
 mod switch_sources;
 pub mod tcx;
-mod terminator;
+pub mod terminator;
 pub use terminator::*;
 
 pub mod traversal;
@@ -925,6 +925,15 @@ impl<'tcx> LocalDecl<'tcx> {
         }
     }
 
+    /// Returns `true` if this is a DerefTemp
+    pub fn is_deref_temp(&self) -> bool {
+        match self.local_info {
+            Some(box LocalInfo::DerefTemp) => return true,
+            _ => (),
+        }
+        return false;
+    }
+
     /// Returns `true` is the local is from a compiler desugaring, e.g.,
     /// `__next` from a `for` loop.
     #[inline]
@@ -1795,6 +1804,7 @@ impl<'tcx> Rvalue<'tcx> {
             Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => false,
 
             Rvalue::Use(_)
+            | Rvalue::CopyForDeref(_)
             | Rvalue::Repeat(_, _)
             | Rvalue::Ref(_, _, _)
             | Rvalue::ThreadLocalRef(_)
@@ -1889,6 +1899,8 @@ impl<'tcx> Debug for Rvalue<'tcx> {
                 write!(fmt, "&{}{}{:?}", region, kind_str, place)
             }
 
+            CopyForDeref(ref place) => write!(fmt, "deref_copy {:#?}", place),
+
             AddressOf(mutability, ref place) => {
                 let kind_str = match mutability {
                     Mutability::Mut => "mut",

compiler/rustc_middle/src/mir/syntax.rs

@@ -52,6 +52,8 @@ pub enum MirPhase {
     /// of the `mir_promoted` query), these promoted elements are available in the `promoted_mir`
     /// query.
     ConstsPromoted = 2,
+    /// After this projections may only contain deref projections as the first element.
+    Derefered = 3,
     /// Beginning with this phase, the following variants are disallowed:
     /// * [`TerminatorKind::DropAndReplace`]
     /// * [`TerminatorKind::FalseUnwind`]
@@ -66,9 +68,7 @@ pub enum MirPhase {
     /// Furthermore, `Drop` now uses explicit drop flags visible in the MIR and reaching a `Drop`
     /// terminator means that the auto-generated drop glue will be invoked. Also, `Copy` operands
     /// are allowed for non-`Copy` types.
-    DropsLowered = 3,
-    /// After this projections may only contain deref projections as the first element.
-    Derefered = 4,
+    DropsLowered = 4,
     /// Beginning with this phase, the following variant is disallowed:
     /// * [`Rvalue::Aggregate`] for any `AggregateKind` except `Array`
     ///
@@ -1051,6 +1051,16 @@ pub enum Rvalue<'tcx> {
     /// initialized but its content as uninitialized. Like other pointer casts, this in general
     /// affects alias analysis.
     ShallowInitBox(Operand<'tcx>, Ty<'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
+    /// 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.
+    CopyForDeref(Place<'tcx>),
 }
 
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]

compiler/rustc_middle/src/mir/tcx.rs

@@ -211,6 +211,7 @@ impl<'tcx> Rvalue<'tcx> {
                 }
             },
             Rvalue::ShallowInitBox(_, ty) => tcx.mk_box(ty),
+            Rvalue::CopyForDeref(ref place) => place.ty(local_decls, tcx).ty,
         }
     }
 

compiler/rustc_middle/src/mir/type_foldable.rs

@@ -122,6 +122,7 @@ impl<'tcx> TypeFoldable<'tcx> for Rvalue<'tcx> {
             Ref(region, bk, place) => {
                 Ref(region.try_fold_with(folder)?, bk, place.try_fold_with(folder)?)
             }
+            CopyForDeref(place) => CopyForDeref(place.try_fold_with(folder)?),
             AddressOf(mutability, place) => AddressOf(mutability, place.try_fold_with(folder)?),
             Len(place) => Len(place.try_fold_with(folder)?),
             Cast(kind, op, ty) => Cast(kind, op.try_fold_with(folder)?, ty.try_fold_with(folder)?),

compiler/rustc_middle/src/mir/type_visitable.rs

@@ -78,6 +78,10 @@ impl<'tcx> TypeVisitable<'tcx> for Rvalue<'tcx> {
         use crate::mir::Rvalue::*;
         match *self {
             Use(ref op) => op.visit_with(visitor),
+            CopyForDeref(ref place) => {
+                let op = &Operand::Copy(*place);
+                op.visit_with(visitor)
+            }
             Repeat(ref op, _) => op.visit_with(visitor),
             ThreadLocalRef(did) => did.visit_with(visitor),
             Ref(region, _, ref place) => {

compiler/rustc_middle/src/mir/visit.rs

@@ -711,6 +711,13 @@ macro_rules! make_mir_visitor {
                         };
                         self.visit_place(path, ctx, location);
                     }
+                    Rvalue::CopyForDeref(place) => {
+                        self.visit_place(
+                            place,
+                            PlaceContext::NonMutatingUse(NonMutatingUseContext::Inspect),
+                            location
+                        );
+                    }
 
                     Rvalue::AddressOf(m, path) => {
                         let ctx = match m {

compiler/rustc_mir_dataflow/src/impls/borrowed_locals.rs

@@ -102,7 +102,8 @@ where
             | mir::Rvalue::NullaryOp(..)
             | mir::Rvalue::UnaryOp(..)
             | mir::Rvalue::Discriminant(..)
-            | mir::Rvalue::Aggregate(..) => {}
+            | mir::Rvalue::Aggregate(..)
+            | mir::Rvalue::CopyForDeref(..) => {}
         }
     }
 

compiler/rustc_mir_dataflow/src/lib.rs

@@ -38,6 +38,7 @@ pub mod impls;
 pub mod move_paths;
 pub mod rustc_peek;
 pub mod storage;
+pub mod un_derefer;
 
 pub(crate) mod indexes {
     pub(crate) use super::move_paths::MovePathIndex;

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -1,3 +1,4 @@
+use crate::un_derefer::UnDerefer;
 use rustc_index::vec::IndexVec;
 use rustc_middle::mir::tcx::RvalueInitializationState;
 use rustc_middle::mir::*;
@@ -19,6 +20,7 @@ struct MoveDataBuilder<'a, 'tcx> {
     param_env: ty::ParamEnv<'tcx>,
     data: MoveData<'tcx>,
     errors: Vec<(Place<'tcx>, MoveError<'tcx>)>,
+    un_derefer: UnDerefer<'tcx>,
 }
 
 impl<'a, 'tcx> MoveDataBuilder<'a, 'tcx> {
@@ -32,6 +34,7 @@ impl<'a, 'tcx> MoveDataBuilder<'a, 'tcx> {
             tcx,
             param_env,
             errors: Vec::new(),
+            un_derefer: UnDerefer { tcx: tcx, derefer_sidetable: Default::default() },
             data: MoveData {
                 moves: IndexVec::new(),
                 loc_map: LocationMap::new(body),
@@ -94,6 +97,11 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
     ///
     /// Maybe we should have separate "borrowck" and "moveck" modes.
     fn move_path_for(&mut self, place: Place<'tcx>) -> Result<MovePathIndex, MoveError<'tcx>> {
+        if let Some(new_place) = self.builder.un_derefer.derefer(place.as_ref(), self.builder.body)
+        {
+            return self.move_path_for(new_place);
+        }
+
         debug!("lookup({:?})", place);
         let mut base = self.builder.data.rev_lookup.locals[place.local];
 
@@ -276,6 +284,12 @@ struct Gatherer<'b, 'a, 'tcx> {
 impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
     fn gather_statement(&mut self, stmt: &Statement<'tcx>) {
         match &stmt.kind {
+            StatementKind::Assign(box (place, Rvalue::CopyForDeref(reffed))) => {
+                assert!(place.projection.is_empty());
+                if self.builder.body.local_decls[place.local].is_deref_temp() {
+                    self.builder.un_derefer.derefer_sidetable.insert(place.local, *reffed);
+                }
+            }
             StatementKind::Assign(box (place, rval)) => {
                 self.create_move_path(*place);
                 if let RvalueInitializationState::Shallow = rval.initialization_state() {
@@ -294,7 +308,10 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
             }
             StatementKind::StorageLive(_) => {}
             StatementKind::StorageDead(local) => {
-                self.gather_move(Place::from(*local));
+                // DerefTemp locals (results of CopyForDeref) don't actually move anything.
+                if !self.builder.un_derefer.derefer_sidetable.contains_key(&local) {
+                    self.gather_move(Place::from(*local));
+                }
             }
             StatementKind::SetDiscriminant { .. } | StatementKind::Deinit(..) => {
                 span_bug!(
@@ -328,6 +345,7 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
                     self.gather_operand(operand);
                 }
             }
+            Rvalue::CopyForDeref(..) => unreachable!(),
             Rvalue::Ref(..)
             | Rvalue::AddressOf(..)
             | Rvalue::Discriminant(..)
@@ -439,6 +457,11 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
 
     fn gather_move(&mut self, place: Place<'tcx>) {
         debug!("gather_move({:?}, {:?})", self.loc, place);
+        if let Some(new_place) = self.builder.un_derefer.derefer(place.as_ref(), self.builder.body)
+        {
+            self.gather_move(new_place);
+            return;
+        }
 
         if let [ref base @ .., ProjectionElem::Subslice { from, to, from_end: false }] =
             **place.projection
@@ -494,6 +517,11 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
     fn gather_init(&mut self, place: PlaceRef<'tcx>, kind: InitKind) {
         debug!("gather_init({:?}, {:?})", self.loc, place);
 
+        if let Some(new_place) = self.builder.un_derefer.derefer(place, self.builder.body) {
+            self.gather_init(new_place.as_ref(), kind);
+            return;
+        }
+
         let mut place = place;
 
         // Check if we are assigning into a field of a union, if so, lookup the place