Produce a better error for irrefutable if let patterns

Modify ast::ExprMatch to include a new value of type ast::MatchSource,
making it easy to tell whether the match was written literally or
produced via desugaring. This allows us to customize error messages
appropriately.

src/libsyntax/ast.rs

@@ -529,7 +529,7 @@ pub enum Expr_ {
     // Conditionless loop (can be exited with break, cont, or ret)
     // FIXME #6993: change to Option<Name> ... or not, if these are hygienic.
     ExprLoop(P<Block>, Option<Ident>),
-    ExprMatch(P<Expr>, Vec<Arm>),
+    ExprMatch(P<Expr>, Vec<Arm>, MatchSource),
     ExprFnBlock(CaptureClause, P<FnDecl>, P<Block>),
     ExprProc(P<FnDecl>, P<Block>),
     ExprUnboxedFn(CaptureClause, UnboxedClosureKind, P<FnDecl>, P<Block>),
@@ -577,6 +577,12 @@ pub struct QPath {
     pub item_name: Ident,
 }
 
+#[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
+pub enum MatchSource {
+    MatchNormal,
+    MatchIfLetDesugar
+}
+
 #[deriving(Clone, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub enum CaptureClause {
     CaptureByValue,