Rename hir::ExprKind::Use to ::DropTemps and improve docs.

src/librustc/cfg/construct.rs

@@ -369,7 +369,7 @@ impl<'a, 'tcx> CFGBuilder<'a, 'tcx> {
             hir::ExprKind::AddrOf(_, ref e) |
             hir::ExprKind::Cast(ref e, _) |
             hir::ExprKind::Type(ref e, _) |
-            hir::ExprKind::Use(ref e) |
+            hir::ExprKind::DropTemps(ref e) |
             hir::ExprKind::Unary(_, ref e) |
             hir::ExprKind::Field(ref e, _) |
             hir::ExprKind::Yield(ref e) |

src/librustc/hir/intravisit.rs

@@ -1029,7 +1029,7 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr) {
             visitor.visit_expr(subexpression);
             visitor.visit_ty(typ)
         }
-        ExprKind::Use(ref subexpression) => {
+        ExprKind::DropTemps(ref subexpression) => {
             visitor.visit_expr(subexpression);
         }
         ExprKind::If(ref head_expression, ref if_block, ref optional_else) => {

src/librustc/hir/lowering.rs

@@ -4671,7 +4671,7 @@ impl<'a> LoweringContext<'a> {
                 // The construct was introduced in #21984.
                 // FIXME(60253): Is this still necessary?
                 // Also, add the attributes to the outer returned expr node.
-                return self.expr_use(head_sp, match_expr, e.attrs.clone())
+                return self.expr_drop_temps(head_sp, match_expr, e.attrs.clone())
             }
 
             // Desugar `ExprKind::Try`
@@ -5030,15 +5030,19 @@ impl<'a> LoweringContext<'a> {
         )
     }
 
-    /// Wrap the given `expr` in `hir::ExprKind::Use`.
+    /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
     ///
-    /// In terms of drop order, it has the same effect as
-    /// wrapping `expr` in `{ let _t = $expr; _t }` but
-    /// should provide better compile-time performance.
+    /// In terms of drop order, it has the same effect as wrapping `expr` in
+    /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
     ///
     /// The drop order can be important in e.g. `if expr { .. }`.
-    fn expr_use(&mut self, span: Span, expr: P<hir::Expr>, attrs: ThinVec<Attribute>) -> hir::Expr {
-        self.expr(span, hir::ExprKind::Use(expr), attrs)
+    fn expr_drop_temps(
+        &mut self,
+        span: Span,
+        expr: P<hir::Expr>,
+        attrs: ThinVec<Attribute>
+    ) -> hir::Expr {
+        self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
     }
 
     fn expr_match(

src/librustc/hir/mod.rs

@@ -1366,7 +1366,7 @@ impl Expr {
             ExprKind::Unary(..) => ExprPrecedence::Unary,
             ExprKind::Lit(_) => ExprPrecedence::Lit,
             ExprKind::Type(..) | ExprKind::Cast(..) => ExprPrecedence::Cast,
-            ExprKind::Use(ref expr, ..) => expr.precedence(),
+            ExprKind::DropTemps(ref expr, ..) => expr.precedence(),
             ExprKind::If(..) => ExprPrecedence::If,
             ExprKind::While(..) => ExprPrecedence::While,
             ExprKind::Loop(..) => ExprPrecedence::Loop,
@@ -1438,7 +1438,7 @@ impl Expr {
             ExprKind::Binary(..) |
             ExprKind::Yield(..) |
             ExprKind::Cast(..) |
-            ExprKind::Use(..) |
+            ExprKind::DropTemps(..) |
             ExprKind::Err => {
                 false
             }
@@ -1488,10 +1488,12 @@ pub enum ExprKind {
     Cast(P<Expr>, P<Ty>),
     /// A type reference (e.g., `Foo`).
     Type(P<Expr>, P<Ty>),
-    /// Semantically equivalent to `{ let _t = expr; _t }`.
-    /// Maps directly to `hair::ExprKind::Use`.
-    /// Only exists to tweak the drop order in HIR.
-    Use(P<Expr>),
+    /// Wraps the expression in a terminating scope.
+    /// This makes it semantically equivalent to `{ let _t = expr; _t }`.
+    ///
+    /// This construct only exists to tweak the drop order in HIR lowering.
+    /// An example of that is the desugaring of `for` loops.
+    DropTemps(P<Expr>),
     /// An `if` block, with an optional else block.
     ///
     /// I.e., `if <expr> { <expr> } else { <expr> }`.

src/librustc/hir/print.rs

@@ -1388,7 +1388,7 @@ impl<'a> State<'a> {
                 self.word_space(":")?;
                 self.print_type(&ty)?;
             }
-            hir::ExprKind::Use(ref init) => {
+            hir::ExprKind::DropTemps(ref init) => {
                 // Print `{`:
                 self.cbox(indent_unit)?;
                 self.ibox(0)?;

src/librustc/middle/expr_use_visitor.rs

@@ -520,7 +520,7 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
                 self.consume_expr(&base);
             }
 
-            hir::ExprKind::Use(ref expr) => {
+            hir::ExprKind::DropTemps(ref expr) => {
                 self.consume_expr(&expr);
             }
 

src/librustc/middle/liveness.rs

@@ -521,7 +521,7 @@ fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
       hir::ExprKind::Binary(..) |
       hir::ExprKind::AddrOf(..) |
       hir::ExprKind::Cast(..) |
-      hir::ExprKind::Use(..) |
+      hir::ExprKind::DropTemps(..) |
       hir::ExprKind::Unary(..) |
       hir::ExprKind::Break(..) |
       hir::ExprKind::Continue(_) |
@@ -1222,7 +1222,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             hir::ExprKind::AddrOf(_, ref e) |
             hir::ExprKind::Cast(ref e, _) |
             hir::ExprKind::Type(ref e, _) |
-            hir::ExprKind::Use(ref e) |
+            hir::ExprKind::DropTemps(ref e) |
             hir::ExprKind::Unary(_, ref e) |
             hir::ExprKind::Yield(ref e) |
             hir::ExprKind::Repeat(ref e, _) => {
@@ -1526,7 +1526,7 @@ fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
         hir::ExprKind::Match(..) | hir::ExprKind::While(..) | hir::ExprKind::Loop(..) |
         hir::ExprKind::Index(..) | hir::ExprKind::Field(..) |
         hir::ExprKind::Array(..) | hir::ExprKind::Tup(..) | hir::ExprKind::Binary(..) |
-        hir::ExprKind::Cast(..) | hir::ExprKind::Use(..) | hir::ExprKind::Unary(..) |
+        hir::ExprKind::Cast(..) | hir::ExprKind::DropTemps(..) | hir::ExprKind::Unary(..) |
         hir::ExprKind::Ret(..) | hir::ExprKind::Break(..) | hir::ExprKind::Continue(..) |
         hir::ExprKind::Lit(_) | hir::ExprKind::Block(..) | hir::ExprKind::AddrOf(..) |
         hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) |

src/librustc/middle/mem_categorization.rs

@@ -677,7 +677,7 @@ impl<'a, 'gcx, 'tcx> MemCategorizationContext<'a, 'gcx, 'tcx> {
             hir::ExprKind::Assign(..) | hir::ExprKind::AssignOp(..) |
             hir::ExprKind::Closure(..) | hir::ExprKind::Ret(..) |
             hir::ExprKind::Unary(..) | hir::ExprKind::Yield(..) |
-            hir::ExprKind::MethodCall(..) | hir::ExprKind::Cast(..) | hir::ExprKind::Use(..) |
+            hir::ExprKind::MethodCall(..) | hir::ExprKind::Cast(..) | hir::ExprKind::DropTemps(..) |
             hir::ExprKind::Array(..) | hir::ExprKind::Tup(..) | hir::ExprKind::If(..) |
             hir::ExprKind::Binary(..) | hir::ExprKind::While(..) |
             hir::ExprKind::Block(..) | hir::ExprKind::Loop(..) | hir::ExprKind::Match(..) |

src/librustc/middle/region.rs

@@ -908,8 +908,8 @@ fn resolve_expr<'a, 'tcx>(visitor: &mut RegionResolutionVisitor<'a, 'tcx>, expr:
                 visitor.cx.var_parent = visitor.cx.parent;
             }
 
-            hir::ExprKind::Use(ref expr) => {
-                // `Use(expr)` does not denote a conditional scope.
+            hir::ExprKind::DropTemps(ref expr) => {
+                // `DropTemps(expr)` does not denote a conditional scope.
                 // Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
                 terminating(expr.hir_id.local_id);
             }

src/librustc_mir/hair/cx/expr.rs

@@ -759,7 +759,7 @@ fn make_mirror_unadjusted<'a, 'gcx, 'tcx>(cx: &mut Cx<'a, 'gcx, 'tcx>,
                 }
             }
         }
-        hir::ExprKind::Use(ref source) => {
+        hir::ExprKind::DropTemps(ref source) => {
             ExprKind::Use { source: source.to_ref() }
         }
         hir::ExprKind::Box(ref value) => {

src/librustc_passes/rvalue_promotion.rs

@@ -437,7 +437,7 @@ fn check_expr_kind<'a, 'tcx>(
         hir::ExprKind::AddrOf(_, ref expr) |
         hir::ExprKind::Repeat(ref expr, _) |
         hir::ExprKind::Type(ref expr, _) |
-        hir::ExprKind::Use(ref expr) => {
+        hir::ExprKind::DropTemps(ref expr) => {
             v.check_expr(&expr)
         }
 

src/librustc_typeck/check/mod.rs

@@ -4538,7 +4538,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 self.check_expr_eq_type(&e, ty);
                 ty
             }
-            ExprKind::Use(ref e) => {
+            ExprKind::DropTemps(ref e) => {
                 self.check_expr_with_expectation(e, expected)
             }
             ExprKind::Array(ref args) => {