add new rval, pull deref early

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 {