Correctly infer types in guard expressions

The root cause was that we forgot to add bindings from the arm to the
guard expression

closes #3980

crates/ra_hir_def/src/body/scope.rs

@@ -157,6 +157,10 @@ fn compute_expr_scopes(expr: ExprId, body: &Body, scopes: &mut ExprScopes, scope
             for arm in arms {
                 let scope = scopes.new_scope(scope);
                 scopes.add_bindings(body, scope, arm.pat);
+                if let Some(guard) = arm.guard {
+                    scopes.set_scope(guard, scope);
+                    compute_expr_scopes(guard, body, scopes, scope);
+                }
                 scopes.set_scope(arm.expr, scope);
                 compute_expr_scopes(arm.expr, body, scopes, scope);
             }

crates/ra_hir_ty/src/tests/patterns.rs

@@ -455,3 +455,29 @@ fn test() {
     "###
     );
 }
+
+#[test]
+fn infer_guard() {
+    assert_snapshot!(
+        infer(r#"
+struct S;
+impl S { fn foo(&self) -> bool { false } }
+
+fn main() {
+    match S {
+        s if s.foo() => (),
+    }
+}
+    "#), @"
+        [28; 32) 'self': &S
+        [42; 51) '{ false }': bool
+        [44; 49) 'false': bool
+        [65; 116) '{     ...   } }': ()
+        [71; 114) 'match ...     }': ()
+        [77; 78) 'S': S
+        [89; 90) 's': S
+        [94; 95) 's': S
+        [94; 101) 's.foo()': bool
+        [105; 107) '()': ()
+    ")
+}