Shrink Expr_::ExprStruct.

On 64-bit platforms this reduces the size of `Expr_` from  64 bytes to
56 bytes, and reduces the size of `Expr` from 88 bytes to 80 bytes.

Author: nnethercote

Diff for: src/librustc/hir/lowering.rs

@@ -1239,7 +1239,7 @@ impl<'a> LoweringContext<'a> {
                 }), outputs.iter().map(|out| self.lower_expr(&out.expr)).collect(),
                    inputs.iter().map(|&(_, ref input)| self.lower_expr(input)).collect()),
                 ExprKind::Struct(ref path, ref fields, ref maybe_expr) => {
-                    hir::ExprStruct(self.lower_path(path),
+                    hir::ExprStruct(P(self.lower_path(path)),
                                     fields.iter().map(|x| self.lower_field(x)).collect(),
                                     maybe_expr.as_ref().map(|x| self.lower_expr(x)))
                 }
@@ -1743,7 +1743,7 @@ impl<'a> LoweringContext<'a> {
                    e: Option<P<hir::Expr>>,
                    attrs: ThinVec<Attribute>) -> P<hir::Expr> {
         let def = self.resolver.resolve_generated_global_path(&path, false);
-        let expr = self.expr(sp, hir::ExprStruct(path, fields, e), attrs);
+        let expr = self.expr(sp, hir::ExprStruct(P(path), fields, e), attrs);
         self.resolver.record_resolution(expr.id, def);
         expr
     }

Diff for: src/librustc/hir/mod.rs

@@ -946,7 +946,7 @@ pub enum Expr_ {
     ///
     /// For example, `Foo {x: 1, y: 2}`, or
     /// `Foo {x: 1, .. base}`, where `base` is the `Option<Expr>`.
-    ExprStruct(Path, HirVec<Field>, Option<P<Expr>>),
+    ExprStruct(P<Path>, HirVec<Field>, Option<P<Expr>>),
 
     /// An array literal constructed from one repeated element.
     ///

Diff for: src/librustc_const_eval/eval.rs

@@ -314,7 +314,7 @@ pub fn const_expr_to_pat<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                           },
                       }))
                       .collect::<Result<_, _>>()?;
-            PatKind::Struct(path.clone(), field_pats, false)
+            PatKind::Struct((**path).clone(), field_pats, false)
         }
 
         hir::ExprArray(ref exprs) => {