auto merge of #15929 : pcwalton/rust/by-ref-closures, r=alexcrichton

by-reference upvars.

This partially implements RFC 38. A snapshot will be needed to turn this
on, because stage0 cannot yet parse the keyword.

Part of #12831.

r? @alexcrichton

src/libsyntax/ast.rs

@@ -526,9 +526,9 @@ pub enum Expr_ {
     // FIXME #6993: change to Option<Name> ... or not, if these are hygienic.
     ExprLoop(P<Block>, Option<Ident>),
     ExprMatch(Gc<Expr>, Vec<Arm>),
-    ExprFnBlock(P<FnDecl>, P<Block>),
+    ExprFnBlock(CaptureClause, P<FnDecl>, P<Block>),
     ExprProc(P<FnDecl>, P<Block>),
-    ExprUnboxedFn(P<FnDecl>, P<Block>),
+    ExprUnboxedFn(CaptureClause, P<FnDecl>, P<Block>),
     ExprBlock(P<Block>),
 
     ExprAssign(Gc<Expr>, Gc<Expr>),
@@ -559,6 +559,12 @@ pub enum Expr_ {
     ExprParen(Gc<Expr>)
 }
 
+#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
+pub enum CaptureClause {
+    CaptureByValue,
+    CaptureByRef,
+}
+
 /// When the main rust parser encounters a syntax-extension invocation, it
 /// parses the arguments to the invocation as a token-tree. This is a very
 /// loose structure, such that all sorts of different AST-fragments can