Remove where clause from derived TransparentWrapper impls. (#146)

* Remove where clause from derived TransparentWrapper impls.

* Add test for TransparentWrapper trait bound regression.

derive/src/lib.rs

@@ -339,6 +339,12 @@ fn derive_marker_trait_inner<Trait: Derivable>(
     quote!()
   };
 
+  let where_clause = if Trait::requires_where_clause() {
+    where_clause
+  } else {
+    None
+  };
+
   Ok(quote! {
     #asserts
 

derive/src/traits.rs

@@ -32,6 +32,9 @@ pub trait Derivable {
   fn trait_impl(_input: &DeriveInput) -> Result<(TokenStream, TokenStream)> {
     Ok((quote!(), quote!()))
   }
+  fn requires_where_clause() -> bool {
+    true
+  }
 }
 
 pub struct Pod;
@@ -300,6 +303,10 @@ impl Derivable for TransparentWrapper {
       }
     }
   }
+
+  fn requires_where_clause() -> bool {
+    false
+  }
 }
 
 pub struct Contiguous;

tests/derive.rs

@@ -23,3 +23,29 @@ struct TransparentWithZeroSized {
   a: u16,
   b: (),
 }
+
+#[derive(TransparentWrapper)]
+#[repr(transparent)]
+struct TransparentWithGeneric<T> {
+  a: T,
+}
+
+/// Ensuring that no additional bounds are emitted.
+/// See https://github.com/Lokathor/bytemuck/issues/145
+fn test_generic<T>(x: T) -> TransparentWithGeneric<T> {
+  TransparentWithGeneric::wrap(x)
+}
+
+#[derive(TransparentWrapper)]
+#[repr(transparent)]
+#[transparent(T)]
+struct TransparentWithGenericAndZeroSized<T> {
+  a: T,
+  b: ()
+}
+
+/// Ensuring that no additional bounds are emitted.
+/// See https://github.com/Lokathor/bytemuck/issues/145
+fn test_generic_with_zst<T>(x: T) -> TransparentWithGenericAndZeroSized<T> {
+  TransparentWithGenericAndZeroSized::wrap(x)
+}