Add test for failing macros on enum variants

It would have been security risk similar to #1 if that was accepted,
but it is not.

closes #5

Signed-off-by: Ahmed Abdelraoof <ahmed.abdelraoof@huawei.com>

safe-discriminant-derive/src/lib.rs

@@ -155,3 +155,12 @@ pub fn repr(_: TokenStream, item: TokenStream) -> TokenStream {
 pub fn do_nothing(_: TokenStream, item: TokenStream) -> TokenStream {
     item
 }
+
+#[cfg(feature = "test-utils")]
+/// This macro will remove the discriminant of any enum field it is applied to
+#[proc_macro_attribute]
+pub fn remove_disc(_: TokenStream, item: TokenStream) -> TokenStream {
+    let mut variant = parse_macro_input!(item as Variant);
+    variant.discriminant = None;
+    quote!(#variant).into()
+}

safe-discriminant/tests/fail/remove_disc.rs

@@ -0,0 +1,11 @@
+use safe_discriminant::Discriminant;
+use safe_discriminant_derive::remove_disc;
+#[derive(Discriminant)]
+#[repr(u8)]
+pub enum Foo {
+    #[remove_disc]
+    A = 0,
+    B = 1,
+}
+
+fn main() {}

safe-discriminant/tests/fail/remove_disc.stderr

@@ -0,0 +1,5 @@
+error: expected non-macro attribute, found attribute macro `remove_disc`
+ --> tests/fail/remove_disc.rs:6:7
+  |
+6 |     #[remove_disc]
+  |       ^^^^^^^^^^^ not a non-macro attribute