librustc: Fix the issue with labels shadowing variable names by making

the leading quote part of the identifier for the purposes of hygiene.

This adopts @jbclements' solution to #14539.

I'm not sure if this is a breaking change or not.

Closes #12512.

[breaking-change]

src/libsyntax/ext/build.rs

@@ -85,6 +85,7 @@ pub trait AstBuilder {
                       typ: P<ast::Ty>,
                       ex: Gc<ast::Expr>)
                       -> Gc<ast::Stmt>;
+    fn stmt_item(&self, sp: Span, item: Gc<ast::Item>) -> Gc<ast::Stmt>;
 
     // blocks
     fn block(&self, span: Span, stmts: Vec<Gc<ast::Stmt>>,
@@ -239,6 +240,14 @@ pub trait AstBuilder {
                 vi: Vec<ast::ViewItem>,
                 items: Vec<Gc<ast::Item>>) -> Gc<ast::Item>;
 
+    fn item_static(&self,
+                   span: Span,
+                   name: Ident,
+                   ty: P<ast::Ty>,
+                   mutbl: ast::Mutability,
+                   expr: Gc<ast::Expr>)
+                   -> Gc<ast::Item>;
+
     fn item_ty_poly(&self,
                     span: Span,
                     name: Ident,
@@ -484,11 +493,19 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         box(GC) respan(sp, ast::StmtDecl(box(GC) decl, ast::DUMMY_NODE_ID))
     }
 
-    fn block(&self, span: Span, stmts: Vec<Gc<ast::Stmt>>,
-             expr: Option<Gc<Expr>>) -> P<ast::Block> {
+    fn block(&self,
+             span: Span,
+             stmts: Vec<Gc<ast::Stmt>>,
+             expr: Option<Gc<Expr>>)
+             -> P<ast::Block> {
         self.block_all(span, Vec::new(), stmts, expr)
     }
 
+    fn stmt_item(&self, sp: Span, item: Gc<ast::Item>) -> Gc<ast::Stmt> {
+        let decl = respan(sp, ast::DeclItem(item));
+        box(GC) respan(sp, ast::StmtDecl(box(GC) decl, ast::DUMMY_NODE_ID))
+    }
+
     fn block_expr(&self, expr: Gc<ast::Expr>) -> P<ast::Block> {
         self.block_all(expr.span, Vec::new(), Vec::new(), Some(expr))
     }
@@ -942,6 +959,16 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         )
     }
 
+    fn item_static(&self,
+                   span: Span,
+                   name: Ident,
+                   ty: P<ast::Ty>,
+                   mutbl: ast::Mutability,
+                   expr: Gc<ast::Expr>)
+                   -> Gc<ast::Item> {
+        self.item(span, name, Vec::new(), ast::ItemStatic(ty, mutbl, expr))
+    }
+
     fn item_ty_poly(&self, span: Span, name: Ident, ty: P<ast::Ty>,
                     generics: Generics) -> Gc<ast::Item> {
         self.item(span, name, Vec::new(), ast::ItemTy(ty, generics))