cgen: fix struct field name generation in auto free (#15440)

vlib/v/gen/c/auto_free_methods.v

@@ -72,6 +72,7 @@ fn (mut g Gen) gen_free_for_struct(info ast.Struct, styp string, fn_name string)
 	}
 	fn_builder.writeln('void ${fn_name}($styp* it) {')
 	for field in info.fields {
+		field_name := c_name(field.name)
 		sym := g.table.sym(g.unwrap_generic(field.typ))
 
 		if sym.kind !in [.string, .array, .map, .struct_] {
@@ -88,9 +89,9 @@ fn (mut g Gen) gen_free_for_struct(info ast.Struct, styp string, fn_name string)
 			g.gen_free_method(field.typ)
 		}
 		if is_shared {
-			fn_builder.writeln('\t${field_styp_fn_name}(&(it->$field.name->val));')
+			fn_builder.writeln('\t${field_styp_fn_name}(&(it->$field_name->val));')
 		} else {
-			fn_builder.writeln('\t${field_styp_fn_name}(&(it->$field.name));')
+			fn_builder.writeln('\t${field_styp_fn_name}(&(it->$field_name));')
 		}
 	}
 	fn_builder.writeln('}')

vlib/v/gen/c/testdata/struct_field_free.out

@@ -0,0 +1,6 @@
+&Class{
+    class: 'abc'
+    register: '123'
+    namespace: '456'
+    normal: 'xyz'
+}

vlib/v/gen/c/testdata/struct_field_free.vv

@@ -0,0 +1,15 @@
+// Test, that the autogenerated `free` method compiles,
+// on a struct that has field names that are C/C++ keywords.
+
+struct Class {
+	class     string
+	register  string
+	namespace string
+	normal    string
+}
+
+fn main() {
+	class := &Class{'abc', '123', '456', 'xyz'}
+	println(class)
+	class.free()
+}