parser: add better error for mut variadic fn argument (#21063)

vlib/v/parser/fn.v

@@ -912,6 +912,10 @@ fn (mut p Parser) fn_params() ([]ast.Param, bool, bool) {
 			}
 			if is_mut {
 				if !param_type.has_flag(.generic) {
+					if is_variadic {
+						p.error_with_pos('variadic arguments cannot be `mut`, `shared` or `atomic`',
+							pos)
+					}
 					if is_shared {
 						p.check_fn_shared_arguments(param_type, pos)
 					} else if is_atomic {
@@ -1027,6 +1031,10 @@ fn (mut p Parser) fn_params() ([]ast.Param, bool, bool) {
 			}
 			if is_mut {
 				if !typ.has_flag(.generic) {
+					if is_variadic {
+						p.error_with_pos('variadic arguments cannot be `mut`, `shared` or `atomic`',
+							pos)
+					}
 					if is_shared {
 						p.check_fn_shared_arguments(typ, pos)
 					} else if is_atomic {

vlib/v/parser/tests/fn_alias_arg_variadic_mut_err.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/fn_alias_arg_variadic_mut_err.vv:1:22: error: variadic arguments cannot be `mut`, `shared` or `atomic`
+    1 | type Foo = fn(mut ...string)
+      |                      ~~~~~~

vlib/v/parser/tests/fn_alias_arg_variadic_mut_err.vv

@@ -0,0 +1 @@
+type Foo = fn(mut ...string)

vlib/v/parser/tests/fn_arg_variadic_mut_err.out

@@ -0,0 +1,5 @@
+vlib/v/parser/tests/fn_arg_variadic_mut_err.vv:1:26: error: variadic arguments cannot be `mut`, `shared` or `atomic`
+    1 | fn load(mut filepaths ...string) {
+      |                          ~~~~~~
+    2 |     println(filepaths[0])
+    3 | }

vlib/v/parser/tests/fn_arg_variadic_mut_err.vv

@@ -0,0 +1,3 @@
+fn load(mut filepaths ...string) {
+	println(filepaths[0])
+}