Reject enums with attribute macros

This patch checks if there is any top level attribute macro expansion,
and if there it, we report it as an error.

fixes: #1

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

safe-discriminant-derive/src/lib.rs

@@ -83,13 +83,28 @@ fn validate_all_variants(variants: impl Iterator<Item = Variant>) -> Result<()>
         .unwrap_or(Ok(()))
 }
 
+/// Returns true if there is any #[x] where x is not `repr`
+/// returns false otherwise.
+fn contains_attribute_macros(attrs: &[Attribute]) -> bool {
+    attrs.iter().any(|attr| !attr.path().is_ident("repr"))
+}
 /// Constructs Discriminant trait implementation for given enum.
 /// Returns error in one of two cases:
 /// 1- No valid `#[repr(x)]` is found.
 /// 2- Any of the enum variants is missing discriminant.
+/// 3- contains any additional top level attribute macros.
 fn derive_discriminant_inner(tagged_enum: ItemEnum) -> Result<TokenStream> {
     let prim = get_enum_repr_prim(&tagged_enum.attrs, tagged_enum.ident.span())?;
     validate_all_variants(tagged_enum.variants.into_iter())?;
+    if contains_attribute_macros(&tagged_enum.attrs) {
+        return Err(Error::new(
+            tagged_enum.ident.span(),
+            concat!(
+                "Discriminant is not compatiable with any top ",
+                "level `#[attr]` except `#[repr(_)]`."
+            ),
+        ));
+    }
     let name = tagged_enum.ident;
     let generics = tagged_enum.generics;
     let derive = quote! {

safe-discriminant/tests/fail/remove_repr_disc1.rs

@@ -0,0 +1,12 @@
+use safe_discriminant::Discriminant;
+use safe_discriminant_derive::remove_repr;
+
+#[remove_repr]
+#[derive(Discriminant)]
+#[repr(u8)]
+pub enum Foo {
+    A = 0,
+    B = 1,
+}
+
+fn main() {}

safe-discriminant/tests/fail/remove_repr_disc1.stderr

@@ -0,0 +1,5 @@
+error: Discriminant requires a `#[repr(x)] where x is one of u8, i8, u16, i16, u32, i32, u64, i64, u128, i128.
+ --> tests/fail/remove_repr_disc1.rs:7:10
+  |
+7 | pub enum Foo {
+  |          ^^^

safe-discriminant/tests/fail/remove_repr_disc2.rs

@@ -0,0 +1,12 @@
+use safe_discriminant::Discriminant;
+use safe_discriminant_derive::remove_repr;
+
+#[derive(Discriminant)]
+#[remove_repr]
+#[repr(u8)]
+pub enum Foo {
+    A = 0,
+    B = 1,
+}
+
+fn main() {}

safe-discriminant/tests/fail/remove_repr_disc2.stderr

@@ -0,0 +1,5 @@
+error: Discriminant is not compatiable with any top level `#[attr]` except `#[repr(_)]`.
+ --> tests/fail/remove_repr_disc2.rs:7:10
+  |
+7 | pub enum Foo {
+  |          ^^^