Auto merge of #46321 - eddyb:even-mirer-3, r=<try>

 rustc_mir: implement an "lvalue reuse" optimization (aka destination propagation aka NRVO).

Replaces a chain of moves, such as `a = ...; ... b = move a; ... f(&mut b) ... c = move b`, with the final destination, i.e. only `c = ...; ... f(&mut c); ...` remains (note that borrowing is allowed).

**DO NOT MERGE** only for testing atm. Based on #46142.

src/librustc/ich/impls_mir.rs

@@ -420,7 +420,10 @@ impl<'gcx> HashStable<StableHashingContext<'gcx>> for mir::Operand<'gcx> {
         mem::discriminant(self).hash_stable(hcx, hasher);
 
         match *self {
-            mir::Operand::Consume(ref lvalue) => {
+            mir::Operand::Copy(ref lvalue) => {
+                lvalue.hash_stable(hcx, hasher);
+            }
+            mir::Operand::Move(ref lvalue) => {
                 lvalue.hash_stable(hcx, hasher);
             }
             mir::Operand::Constant(ref constant) => {

src/librustc/mir/mod.rs

@@ -1283,7 +1283,17 @@ pub struct VisibilityScopeData {
 /// being nested in one another.
 #[derive(Clone, PartialEq, RustcEncodable, RustcDecodable)]
 pub enum Operand<'tcx> {
-    Consume(Lvalue<'tcx>),
+    /// Copy: The value must be available for use afterwards.
+    ///
+    /// This implies that the type of the lvalue must be `Copy`; this is true
+    /// by construction during build, but also checked by the MIR type checker.
+    Copy(Lvalue<'tcx>),
+    /// Move: The value (including old borrows of it) will not be used again.
+    ///
+    /// Safe for values of all types (modulo future developments towards `?Move`).
+    /// Correct usage patterns are enforced by the borrow checker for safe code.
+    /// `Copy` may be converted to `Move` to enable "last-use" optimizations.
+    Move(Lvalue<'tcx>),
     Constant(Box<Constant<'tcx>>),
 }
 
@@ -1292,7 +1302,8 @@ impl<'tcx> Debug for Operand<'tcx> {
         use self::Operand::*;
         match *self {
             Constant(ref a) => write!(fmt, "{:?}", a),
-            Consume(ref lv) => write!(fmt, "{:?}", lv),
+            Copy(ref lv) => write!(fmt, "{:?}", lv),
+            Move(ref lv) => write!(fmt, "move {:?}", lv),
         }
     }
 }
@@ -2089,14 +2100,16 @@ impl<'tcx> TypeFoldable<'tcx> for Rvalue<'tcx> {
 impl<'tcx> TypeFoldable<'tcx> for Operand<'tcx> {
     fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
         match *self {
-            Operand::Consume(ref lval) => Operand::Consume(lval.fold_with(folder)),
+            Operand::Copy(ref lval) => Operand::Copy(lval.fold_with(folder)),
+            Operand::Move(ref lval) => Operand::Move(lval.fold_with(folder)),
             Operand::Constant(ref c) => Operand::Constant(c.fold_with(folder)),
         }
     }
 
     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
         match *self {
-            Operand::Consume(ref lval) => lval.visit_with(visitor),
+            Operand::Copy(ref lval) |
+            Operand::Move(ref lval) => lval.visit_with(visitor),
             Operand::Constant(ref c) => c.visit_with(visitor)
         }
     }

src/librustc/mir/tcx.rs

@@ -230,7 +230,8 @@ impl<'tcx> Operand<'tcx> {
         where D: HasLocalDecls<'tcx>
     {
         match self {
-            &Operand::Consume(ref l) => l.ty(local_decls, tcx).to_ty(tcx),
+            &Operand::Copy(ref l) |
+            &Operand::Move(ref l) => l.ty(local_decls, tcx).to_ty(tcx),
             &Operand::Constant(ref c) => c.ty,
         }
     }

src/librustc/mir/visit.rs

@@ -611,8 +611,11 @@ macro_rules! make_mir_visitor {
                              operand: & $($mutability)* Operand<'tcx>,
                              location: Location) {
                 match *operand {
-                    Operand::Consume(ref $($mutability)* lvalue) => {
-                        self.visit_lvalue(lvalue, LvalueContext::Consume, location);
+                    Operand::Copy(ref $($mutability)* lvalue) => {
+                        self.visit_lvalue(lvalue, LvalueContext::Copy, location);
+                    }
+                    Operand::Move(ref $($mutability)* lvalue) => {
+                        self.visit_lvalue(lvalue, LvalueContext::Move, location);
                     }
                     Operand::Constant(ref $($mutability)* constant) => {
                         self.visit_constant(constant, location);
@@ -679,7 +682,7 @@ macro_rules! make_mir_visitor {
                         self.visit_ty(ty, TyContext::Location(location));
                     }
                     ProjectionElem::Index(ref $($mutability)* local) => {
-                        self.visit_local(local, LvalueContext::Consume, location);
+                        self.visit_local(local, LvalueContext::Copy, location);
                     }
                     ProjectionElem::ConstantIndex { offset: _,
                                                     min_length: _,
@@ -860,7 +863,8 @@ pub enum LvalueContext<'tcx> {
     Projection(Mutability),
 
     // Consumed as part of an operand
-    Consume,
+    Copy,
+    Move,
 
     // Starting and ending a storage live range
     StorageLive,
@@ -913,7 +917,8 @@ impl<'tcx> LvalueContext<'tcx> {
             LvalueContext::Inspect |
             LvalueContext::Borrow { kind: BorrowKind::Shared, .. } |
             LvalueContext::Borrow { kind: BorrowKind::Unique, .. } |
-            LvalueContext::Projection(Mutability::Not) | LvalueContext::Consume |
+            LvalueContext::Projection(Mutability::Not) |
+            LvalueContext::Copy | LvalueContext::Move |
             LvalueContext::StorageLive | LvalueContext::StorageDead |
             LvalueContext::Validate => false,
         }
@@ -924,7 +929,8 @@ impl<'tcx> LvalueContext<'tcx> {
         match *self {
             LvalueContext::Inspect | LvalueContext::Borrow { kind: BorrowKind::Shared, .. } |
             LvalueContext::Borrow { kind: BorrowKind::Unique, .. } |
-            LvalueContext::Projection(Mutability::Not) | LvalueContext::Consume => true,
+            LvalueContext::Projection(Mutability::Not) |
+            LvalueContext::Copy | LvalueContext::Move => true,
             LvalueContext::Borrow { kind: BorrowKind::Mut, .. } | LvalueContext::Store |
             LvalueContext::Call | LvalueContext::Projection(Mutability::Mut) |
             LvalueContext::Drop | LvalueContext::StorageLive | LvalueContext::StorageDead |