Print a friendly error for the if-let construct without an else block

Fixes #19991.

2 files changed, 4 insertions, 2 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index be8f32bc4d5..ab338da63bf 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -754,7 +754,7 @@ pub struct QPath {
 #[deriving(Clone, Copy, PartialEq, Eq, Encodable, Decodable, Hash, Show)]
 pub enum MatchSource {
     MatchNormal,
-    MatchIfLetDesugar,
+    MatchIfLetDesugar(bool /* contains_else_arm */),
     MatchWhileLetDesugar,
 }
 
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 20c8ff20b71..63bd38de8a0 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -170,7 +170,9 @@ pub fn expand_expr(e: P<ast::Expr>, fld: &mut MacroExpander) -> P<ast::Expr> {
             arms.extend(else_if_arms.into_iter());
             arms.push(else_arm);
 
-            let match_expr = fld.cx.expr(span, ast::ExprMatch(expr, arms, ast::MatchIfLetDesugar));
+            let match_expr = fld.cx.expr(span, ast::ExprMatch(expr,
+                                                              arms,
+                                                    ast::MatchIfLetDesugar(elseopt.is_some())));
             fld.fold_expr(match_expr)
         }