Evaluate hair::ExprKind::Use in into

This avoids some unnecessary moves

src/librustc_mir/build/expr/as_rvalue.rs

@@ -150,10 +150,6 @@ fn expr_as_rvalue(
                 let source = unpack!(block = this.as_operand(block, scope, source));
                 block.and(Rvalue::Cast(CastKind::Misc, source, expr.ty))
             }
-            ExprKind::Use { source } => {
-                let source = unpack!(block = this.as_operand(block, scope, source));
-                block.and(Rvalue::Use(source))
-            }
             ExprKind::Pointer { cast, source } => {
                 let source = unpack!(block = this.as_operand(block, scope, source));
                 block.and(Rvalue::Cast(CastKind::Pointer(cast), source, expr.ty))
@@ -363,6 +359,7 @@ fn expr_as_rvalue(
             | ExprKind::Match { .. }
             | ExprKind::If { .. }
             | ExprKind::NeverToAny { .. }
+            | ExprKind::Use { .. }
             | ExprKind::Loop { .. }
             | ExprKind::LogicalOp { .. }
             | ExprKind::Call { .. }

src/librustc_mir/build/expr/category.rs

@@ -48,6 +48,7 @@ pub fn of<'tcx>(ek: &ExprKind<'tcx>) -> Option<Category> {
             | ExprKind::If { .. }
             | ExprKind::Match { .. }
             | ExprKind::NeverToAny { .. }
+            | ExprKind::Use { .. }
             | ExprKind::Call { .. } => Some(Category::Rvalue(RvalueFunc::Into)),
 
             ExprKind::Array { .. }
@@ -58,7 +59,6 @@ pub fn of<'tcx>(ek: &ExprKind<'tcx>) -> Option<Category> {
             | ExprKind::Binary { .. }
             | ExprKind::Box { .. }
             | ExprKind::Cast { .. }
-            | ExprKind::Use { .. }
             | ExprKind::Pointer { .. }
             | ExprKind::Repeat { .. }
             | ExprKind::Borrow { .. }

src/librustc_mir/build/expr/into.rs

@@ -327,6 +327,9 @@ pub fn into_expr(
                     success.unit()
                 }
             }
+            ExprKind::Use { source } => {
+                this.into(destination, block, source)
+            }
 
             // These cases don't actually need a destination
             ExprKind::Assign { .. }
@@ -379,7 +382,6 @@ pub fn into_expr(
             | ExprKind::Binary { .. }
             | ExprKind::Box { .. }
             | ExprKind::Cast { .. }
-            | ExprKind::Use { .. }
             | ExprKind::Pointer { .. }
             | ExprKind::Repeat { .. }
             | ExprKind::Borrow { .. }

src/test/mir-opt/array-index-is-temporary.rs

@@ -18,25 +18,24 @@ fn main() {
 // START rustc.main.EraseRegions.after.mir
 //     bb0: {
 //         ...
-//         _6 = &mut _2;
-//         _5 = &mut (*_6);
-//         _4 = move _5 as *mut usize (Misc);
-//         _3 = move _4;
+//         _5 = &mut _2;
+//         _4 = &mut (*_5);
+//         _3 = move _4 as *mut usize (Misc);
 //         ...
-//         _8 = _3;
-//         _7 = const foo(move _8) -> bb1;
+//         _7 = _3;
+//         _6 = const foo(move _7) -> bb1;
 //     }
 //
 //     bb1: {
 //         ...
-//         _9 = _2;
-//         _10 = Len(_1);
-//         _11 = Lt(_9, _10);
-//         assert(move _11, "index out of bounds: the len is move _10 but the index is _9") -> bb2;
+//         _8 = _2;
+//         _9 = Len(_1);
+//         _10 = Lt(_8, _9);
+//         assert(move _10, "index out of bounds: the len is move _9 but the index is _8") -> bb2;
 //     }
 //
 //     bb2: {
-//         _1[_9] = move _7;
+//         _1[_8] = move _6;
 //         ...
 //         return;
 //     }

src/test/mir-opt/retag.rs

@@ -75,18 +75,18 @@ fn main() {
 //         _10 = move _8;
 //         Retag(_10);
 //         ...
-//         _15 = &mut (*_10);
-//         Retag(_15);
-//         _14 = move _15 as *mut i32 (Misc);
-//         Retag([raw] _14);
+//         _13 = &mut (*_10);
+//         Retag(_13);
+//         _12 = move _13 as *mut i32 (Misc);
+//         Retag([raw] _12);
 //         ...
-//         _18 = move _19(move _20) -> bb2;
+//         _16 = move _17(move _18) -> bb2;
 //     }
 //
 //     bb2: {
-//         Retag(_18);
+//         Retag(_16);
 //         ...
-//         _22 = const Test::foo_shr(move _23, move _25) -> bb3;
+//         _20 = const Test::foo_shr(move _21, move _23) -> bb3;
 //     }
 //
 //     bb3: {