Remove implicit Sized bound from PhantomData's impls

Since PhantomData doesn't contain any data it doesn't matter if T is
Sized or not. There's an implicit Sized bound here that we have to
override to remove it.

This tweaks the (public) `impl_borrow_decode!(...)` and
`impl_borrow_decode_with_context!(...)` macros to support type bounds,
not just comma-separated token trees. I think this change should be
backwards-compatible.

Author: bgw

src/de/impls.rs

@@ -540,12 +540,12 @@ impl<Context> Decode<Context> for () {
 }
 impl_borrow_decode!(());
 
-impl<Context, T> Decode<Context> for core::marker::PhantomData<T> {
+impl<Context, T: ?Sized> Decode<Context> for core::marker::PhantomData<T> {
     fn decode<D: Decoder<Context = Context>>(_: &mut D) -> Result<Self, DecodeError> {
         Ok(core::marker::PhantomData)
     }
 }
-impl_borrow_decode!(core::marker::PhantomData<T>, T);
+impl_borrow_decode!(core::marker::PhantomData<T>, T: ?Sized);
 
 impl<Context, T> Decode<Context> for Option<T>
 where

src/de/mod.rs

@@ -120,8 +120,8 @@ pub trait BorrowDecode<'de, Context>: Sized {
 /// Helper macro to implement `BorrowDecode` for any type that implements `Decode`.
 #[macro_export]
 macro_rules! impl_borrow_decode {
-    ($ty:ty $(, $param:tt)*) => {
-        impl<'de $(, $param)*, __Context> $crate::BorrowDecode<'de, __Context> for $ty {
+    ($ty:ty $(, $($params:tt)*)?) => {
+        impl<'de $(, $($params)*)?, __Context> $crate::BorrowDecode<'de, __Context> for $ty {
             fn borrow_decode<D: $crate::de::BorrowDecoder<'de, Context = __Context>>(
                 decoder: &mut D,
             ) -> core::result::Result<Self, $crate::error::DecodeError> {
@@ -134,8 +134,8 @@ macro_rules! impl_borrow_decode {
 /// Helper macro to implement `BorrowDecode` for any type that implements `Decode`.
 #[macro_export]
 macro_rules! impl_borrow_decode_with_context {
-    ($ty:ty, $context:ty $(, $param:tt)*) => {
-        impl<'de $(, $param)*> $crate::BorrowDecode<'de, $context> for $ty {
+    ($ty:ty, $context:ty $(, $($params:tt)*)?) => {
+        impl<'de $(, $($params)*)?> $crate::BorrowDecode<'de, $context> for $ty {
             fn borrow_decode<D: $crate::de::BorrowDecoder<'de, Context = $context>>(
                 decoder: &mut D,
             ) -> core::result::Result<Self, $crate::error::DecodeError> {

src/enc/impls.rs

@@ -21,7 +21,7 @@ impl Encode for () {
     }
 }
 
-impl<T> Encode for PhantomData<T> {
+impl<T: ?Sized> Encode for PhantomData<T> {
     fn encode<E: Encoder>(&self, _: &mut E) -> Result<(), EncodeError> {
         Ok(())
     }