checker: don't disallow method call chains (#7128)

vlib/v/checker/checker.v

@@ -994,8 +994,6 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) (string, token.Position) {
 					// No automatic lock for array slicing (yet(?))
 					explicit_lock_needed = true
 				}
-			} else {
-				c.error('cannot use function call as mut', expr.pos)
 			}
 		}
 		ast.ArrayInit {

vlib/v/tests/method_call_chain_test.v

@@ -0,0 +1,16 @@
+struct Test {
+mut:
+	val int
+}
+
+// this must return a reference, or else you'll get a C error
+// TODO: add a proper checker check for that case
+fn new(x int) &Test { return &Test{ x } }
+fn (mut t Test) inc() &Test { t.val++ return t }
+fn (mut t Test) add(x int) &Test { t.val += x return t }
+fn (mut t Test) div(x int) &Test { t.val /= x return t }
+
+fn test_method_call_chains() {
+	mut x := new(4).inc().inc().inc().inc().add(4).div(2).inc()
+	assert x.val == 7
+}