From f801ef5e1733d53d011b6456fadb254af5caceb4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lucas=20Jen=C3=9F?= <public@x3ro.de>
Date: Fri, 29 Oct 2021 11:03:05 +0200
Subject: [PATCH] checker: disallow `mut` keyword in right-hand side of
 assignment (#12318)

---
 vlib/v/checker/checker.v                     | 5 +++++
 vlib/v/checker/tests/right_hand_side_mut.out | 7 +++++++
 vlib/v/checker/tests/right_hand_side_mut.vv  | 6 ++++++
 3 files changed, 18 insertions(+)
 create mode 100644 vlib/v/checker/tests/right_hand_side_mut.out
 create mode 100644 vlib/v/checker/tests/right_hand_side_mut.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 12ebdb888a..f4fe75e070 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3717,6 +3717,11 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 					right.pos)
 			}
 		}
+		if right is ast.Ident {
+			if right.is_mut {
+				c.error('unexpected `mut` on right-hand side of assignment', right.mut_pos)
+			}
+		}
 	}
 	if node.left.len != right_len {
 		if right_first is ast.CallExpr {
diff --git a/vlib/v/checker/tests/right_hand_side_mut.out b/vlib/v/checker/tests/right_hand_side_mut.out
new file mode 100644
index 0000000000..c52488c03a
--- /dev/null
+++ b/vlib/v/checker/tests/right_hand_side_mut.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/right_hand_side_mut.vv:4:19: error: unexpected `mut` on right-hand side of assignment
+    2 |     mut flubbel := 123
+    3 |     mut wubbel := 234
+    4 |     _, _ := flubbel, mut wubbel
+      |                      ~~~
+    5 |     print(wubbel)
+    6 | }
diff --git a/vlib/v/checker/tests/right_hand_side_mut.vv b/vlib/v/checker/tests/right_hand_side_mut.vv
new file mode 100644
index 0000000000..919efd63ce
--- /dev/null
+++ b/vlib/v/checker/tests/right_hand_side_mut.vv
@@ -0,0 +1,6 @@
+fn main() {
+	mut flubbel := 123
+	mut wubbel := 234
+	_, _ := flubbel, mut wubbel
+	print(wubbel)
+}