Infer type of match guard

crates/ra_syntax/src/ast/generated.rs

@@ -1981,7 +1981,11 @@ impl ToOwned for MatchGuard {
 }
 
 
-impl MatchGuard {}
+impl MatchGuard {
+    pub fn expr(&self) -> Option<&Expr> {
+        super::child_opt(self)
+    }
+}
 
 // MethodCallExpr
 #[derive(Debug, PartialEq, Eq, Hash)]

crates/ra_syntax/src/grammar.ron

@@ -418,7 +418,7 @@ Grammar(
             ],
             collections: [ [ "pats", "Pat" ] ]
         ),
-        "MatchGuard": (),
+        "MatchGuard": (options: ["Expr"]),
         "StructLit": (options: ["Path", "NamedFieldList", ["spread", "Expr"]]),
         "NamedFieldList": (collections: [ ["fields", "NamedField"] ]),
         "NamedField": (options: ["NameRef", "Expr"]),

crates/ra_syntax/src/grammar/expressions/atom.rs

@@ -360,8 +360,8 @@ fn match_arm(p: &mut Parser) -> BlockLike {
     while p.eat(PIPE) {
         patterns::pattern(p);
     }
-    if p.eat(IF_KW) {
-        expr(p);
+    if p.at(IF_KW) {
+        match_guard(p);
     }
     p.expect(FAT_ARROW);
     let ret = expr_stmt(p);
@@ -369,6 +369,20 @@ fn match_arm(p: &mut Parser) -> BlockLike {
     ret
 }
 
+// test match_guard
+// fn foo() {
+//     match () {
+//         _ if foo => (),
+//     }
+// }
+fn match_guard(p: &mut Parser) -> CompletedMarker {
+    assert!(p.at(IF_KW));
+    let m = p.start();
+    p.bump();
+    expr(p);
+    m.complete(p, MATCH_GUARD)
+}
+
 // test block_expr
 // fn foo() {
 //     {};