checker: disallow invalid prefix on left side of assign stmt (#18750)

vlib/v/checker/assign.v

@@ -392,6 +392,11 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 							left.right.obj.is_used = true
 						}
 					}
+				} else if left.op == .amp {
+					c.error('cannot use a reference on the left side of `${node.op}`',
+						left.pos)
+				} else {
+					c.error('cannot use `${left.op}` on the left of `${node.op}`', left.pos)
 				}
 				if is_decl {
 					c.error('non-name on the left side of `:=`', left.pos)

vlib/v/checker/tests/invalid_prefix_left_side_assign_stmt_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/invalid_prefix_left_side_assign_stmt_err.vv:3:2: error: cannot use `~` on the left of `=`
+    1 | fn main() {
+    2 |     mut x := 0
+    3 |     ~x = 34
+      |     ^
+    4 |     &x = unsafe { nil }
+    5 |     println(x)
+vlib/v/checker/tests/invalid_prefix_left_side_assign_stmt_err.vv:4:2: error: cannot use a reference on the left side of `=`
+    2 |     mut x := 0
+    3 |     ~x = 34
+    4 |     &x = unsafe { nil }
+      |     ^
+    5 |     println(x)
+    6 | }

vlib/v/checker/tests/invalid_prefix_left_side_assign_stmt_err.vv

@@ -0,0 +1,6 @@
+fn main() {
+	mut x := 0
+	~x = 34
+	&x = unsafe { nil }
+	println(x)
+}

vlib/v/checker/tests/prefix_expr_decl_assign_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/prefix_expr_decl_assign_err.vv:2:5: error: non-name on the left side of `:=`
+vlib/v/checker/tests/prefix_expr_decl_assign_err.vv:2:5: error: cannot use a reference on the left side of `:=`
     1 | fn main() {
     2 |     &a := 12
       |     ^