Force muliple repr hint to be hard error

The consideraton is this will be unsafe in our environmnt. In rust, it
is possible that users would allow that error, thinking it shouldn't
lead to unsafe behavior.

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

Author: Ahmed Abdelraoof

safe-discriminant-derive/src/lib.rs

@@ -26,12 +26,21 @@ fn get_enum_repr_prim(attrs: &[Attribute], error_span: Span) -> Result<Path> {
         if !attr.path().is_ident("repr") {
             continue;
         }
-        let _ = attr.parse_nested_meta(|meta| {
-            if is_prim(&meta.path) {
-                prim = Some(meta.path);
+        attr.parse_nested_meta(|meta| {
+            if !is_prim(&meta.path) {
+                return Ok(());
             }
+            if prim.is_some() {
+                return Err(Error::new(
+                    // TODO join meta.path span with prim span
+                    // but the function is currently nightly only
+                    meta.path.span(),
+                    "conflicting representation hints",
+                ));
+            }
+            prim = Some(meta.path);
             Ok(())
-        });
+        })?;
     }
     match prim {
         Some(prim) => Ok(prim),

safe-discriminant/tests/fail/multi_repr.stderr

@@ -1,3 +1,9 @@
+error: conflicting representation hints
+ --> tests/fail/multi_repr.rs:4:15
+  |
+4 | #[repr(C, u8, i8)]
+  |               ^^
+
 error[E0566]: conflicting representation hints
  --> tests/fail/multi_repr.rs:4:8
   |