checker: improve mut arg error msg (#16540)

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()

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 | }

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{})
+}

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)
       |            ~~~~~