From 8543d5e055ff16dfda1a23ce18bfa84f654b4add Mon Sep 17 00:00:00 2001
From: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Date: Sun, 27 Nov 2022 05:07:35 +0100
Subject: [PATCH] checker: improve mut arg error msg (#16540)

---
 vlib/v/checker/fn.v                               | 7 ++++++-
 vlib/v/checker/tests/check_fn_init_as_mutable.out | 6 ++++++
 vlib/v/checker/tests/check_fn_init_as_mutable.vv  | 7 +++++++
 vlib/v/checker/tests/pass_mut_lit.out             | 4 ++--
 4 files changed, 21 insertions(+), 3 deletions(-)
 create mode 100644 vlib/v/checker/tests/check_fn_init_as_mutable.out
 create mode 100644 vlib/v/checker/tests/check_fn_init_as_mutable.vv

diff --git a/vlib/v/checker/fn.v b/vlib/v/checker/fn.v
index b4fc2765d..31e090589 100644
--- a/vlib/v/checker/fn.v
+++ b/vlib/v/checker/fn.v
@@ -1028,7 +1028,12 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 		if call_arg.is_mut {
 			to_lock, pos := c.fail_if_immutable(call_arg.expr)
 			if !call_arg.expr.is_lvalue() {
-				c.error('cannot pass expression as `mut`', call_arg.expr.pos())
+				if call_arg.expr is ast.StructInit {
+					c.error('cannot pass a struct initialization as `mut`, you may want to use a variable `mut var := ${call_arg.expr}`',
+						call_arg.expr.pos())
+				} else {
+					c.error('cannot pass expression as `mut`', call_arg.expr.pos())
+				}
 			}
 			if !param.is_mut {
 				tok := call_arg.share.str()
diff --git a/vlib/v/checker/tests/check_fn_init_as_mutable.out b/vlib/v/checker/tests/check_fn_init_as_mutable.out
new file mode 100644
index 000000000..f18dbcc78
--- /dev/null
+++ b/vlib/v/checker/tests/check_fn_init_as_mutable.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/check_fn_init_as_mutable.vv:6:14: error: cannot pass a struct initialization as `mut`, you may want to use a variable `mut var := MyStruct{....}`
+    4 | 
+    5 | fn main() {
+    6 |     process(mut MyStruct{})
+      |                 ~~~~~~~~~~
+    7 | }
diff --git a/vlib/v/checker/tests/check_fn_init_as_mutable.vv b/vlib/v/checker/tests/check_fn_init_as_mutable.vv
new file mode 100644
index 000000000..3224de2b2
--- /dev/null
+++ b/vlib/v/checker/tests/check_fn_init_as_mutable.vv
@@ -0,0 +1,7 @@
+struct MyStruct {}
+
+fn process(mut ms MyStruct) {}
+
+fn main() {
+	process(mut MyStruct{})
+}
diff --git a/vlib/v/checker/tests/pass_mut_lit.out b/vlib/v/checker/tests/pass_mut_lit.out
index 09a03433a..a9e060af1 100644
--- a/vlib/v/checker/tests/pass_mut_lit.out
+++ b/vlib/v/checker/tests/pass_mut_lit.out
@@ -1,5 +1,5 @@
-vlib/v/checker/tests/pass_mut_lit.vv:10:12: error: cannot pass expression as `mut`
+vlib/v/checker/tests/pass_mut_lit.vv:10:12: error: cannot pass a struct initialization as `mut`, you may want to use a variable `mut var := Box{....}`
     8 | }
-    9 | 
+    9 |
    10 | modify(mut Box{}, 10)
       |            ~~~~~
-- 
2.39.5