Add VariableLengthSlice to work around a lifetime problem in users

of `PrfExpand` (e.g. the `zip32` crate).

Signed-off-by: Daira-Emma Hopwood <daira@jacaranda.org>

CHANGELOG.md

@@ -10,10 +10,12 @@ and this library adheres to Rust's notion of
 - `zcash_spec::PrfExpand::REGISTERED_ZIP32_CHILD` (for tagged ZIP 32 child
   derivation).
 - `zcash_spec::PrfExpand::ADHOC_ZIP32_CHILD` (renamed and retyped per ZIP 32).
+- `zcash_spec::VariableLengthSlice`
 
 ### Changed
 - `zcash_spec::PrfExpand::ORCHARD_ZIP32_CHILD` now has type
-  `PrfExpand<([u8; 32], [u8; 4], Option<(u8, &[u8])>)>` due to ZIP 32 changes.
+  `PrfExpand<([u8; 32], [u8; 4], Option<(u8, VariableLengthSlice)>)>` due to
+  ZIP 32 changes.
 
 ### Removed
 - `zcash_spec::PrfExpand::ARBITRARY_ZIP32_CHILD` (use `ADHOC_ZIP32_CHILD`

src/prf_expand.rs

@@ -48,6 +48,9 @@ impl<T> PrfExpand<T> {
     }
 }
 
+/// Marker type used for `PrfExpand` instantiations that permit a variable-length input.
+pub struct VariableLengthSlice;
+
 macro_rules! with_inputs {
     ($($arr:ident, $arrlen:ident),*) => {
         #[allow(unused_parens)]
@@ -107,7 +110,7 @@ impl PrfExpand<([u8; 96], [u8; 32], [u8; 4])> {
 }
 with_inputs!(a, A, b, B, c, C);
 
-impl PrfExpand<([u8; 32], [u8; 4], Option<(u8, &[u8])>)> {
+impl PrfExpand<([u8; 32], [u8; 4], Option<(u8, VariableLengthSlice)>)> {
     pub const ORCHARD_ZIP32_CHILD: Self = Self::new(0x81);
     pub const ADHOC_ZIP32_CHILD: Self = Self::new(0xAB);
     pub const REGISTERED_ZIP32_CHILD: Self = Self::new(0xAC);