Rollup merge of #64824 - Mark-Simulacrum:no-stable-hasher-result-everywhere, r=michaelwoerister

No StableHasherResult everywhere

This removes the generic parameter on `StableHasher`, instead moving it to the call to `finish`. This has the side-effect of making all `HashStable` impls nicer, since we no longer need the verbose `<W: StableHasherResult>` that previously existed -- often forcing line wrapping.

This is done for two reasons:
 * we should avoid false "generic" dependency on the result of StableHasher
     * we don't need to codegen two/three copies of all the HashStable impls when they're transitively used to produce a fingerprint, u64, or u128. I haven't measured, but this might actually make our artifacts somewhat smaller too.
 * Easier to understand/read/write code -- the result of the stable hasher is irrelevant when writing a hash impl.

src/librustc_data_structures/fingerprint.rs

@@ -76,7 +76,7 @@ fn fmt(&self, formatter: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
 
 impl stable_hasher::StableHasherResult for Fingerprint {
     #[inline]
-    fn finish(hasher: stable_hasher::StableHasher<Self>) -> Self {
+    fn finish(hasher: stable_hasher::StableHasher) -> Self {
         let (_0, _1) = hasher.finalize();
         Fingerprint(_0, _1)
     }

src/librustc_data_structures/stable_hasher.rs

@@ -1,5 +1,4 @@
 use std::hash::{Hash, Hasher, BuildHasher};
-use std::marker::PhantomData;
 use std::mem;
 use smallvec::SmallVec;
 use crate::sip128::SipHasher128;
@@ -13,55 +12,53 @@
 /// To that end we always convert integers to little-endian format before
 /// hashing and the architecture dependent `isize` and `usize` types are
 /// extended to 64 bits if needed.
-pub struct StableHasher<W> {
+pub struct StableHasher {
     state: SipHasher128,
-    width: PhantomData<W>,
 }
 
-impl<W: StableHasherResult> ::std::fmt::Debug for StableHasher<W> {
+impl ::std::fmt::Debug for StableHasher {
     fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
         write!(f, "{:?}", self.state)
     }
 }
 
 pub trait StableHasherResult: Sized {
-    fn finish(hasher: StableHasher<Self>) -> Self;
+    fn finish(hasher: StableHasher) -> Self;
 }
 
-impl<W: StableHasherResult> StableHasher<W> {
+impl StableHasher {
     pub fn new() -> Self {
         StableHasher {
             state: SipHasher128::new_with_keys(0, 0),
-            width: PhantomData,
         }
     }
 
-    pub fn finish(self) -> W {
+    pub fn finish<W: StableHasherResult>(self) -> W {
         W::finish(self)
     }
 }
 
 impl StableHasherResult for u128 {
-    fn finish(hasher: StableHasher<Self>) -> Self {
+    fn finish(hasher: StableHasher) -> Self {
         let (_0, _1) = hasher.finalize();
         u128::from(_0) | (u128::from(_1) << 64)
     }
 }
 
 impl StableHasherResult for u64 {
-    fn finish(hasher: StableHasher<Self>) -> Self {
+    fn finish(hasher: StableHasher) -> Self {
         hasher.finalize().0
     }
 }
 
-impl<W> StableHasher<W> {
+impl StableHasher {
     #[inline]
     pub fn finalize(self) -> (u64, u64) {
         self.state.finish128()
     }
 }
 
-impl<W> Hasher for StableHasher<W> {
+impl Hasher for StableHasher {
     fn finish(&self) -> u64 {
         panic!("use StableHasher::finalize instead");
     }
@@ -165,9 +162,7 @@ fn write_isize(&mut self, i: isize) {
 ///   `StableHasher` takes care of endianness and `isize`/`usize` platform
 ///   differences.
 pub trait HashStable<CTX> {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut CTX,
-                                          hasher: &mut StableHasher<W>);
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher);
 }
 
 /// Implement this for types that can be turned into stable keys like, for
@@ -185,10 +180,10 @@ macro_rules! impl_stable_hash_via_hash {
     ($t:ty) => (
         impl<CTX> $crate::stable_hasher::HashStable<CTX> for $t {
             #[inline]
-            fn hash_stable<W: $crate::stable_hasher::StableHasherResult>(
+            fn hash_stable(
                 &self,
                 _: &mut CTX,
-                hasher: &mut $crate::stable_hasher::StableHasher<W>
+                hasher: &mut $crate::stable_hasher::StableHasher
             ) {
                 ::std::hash::Hash::hash(self, hasher);
             }
@@ -215,17 +210,13 @@ fn hash_stable<W: $crate::stable_hasher::StableHasherResult>(
 impl_stable_hash_via_hash!(());
 
 impl<CTX> HashStable<CTX> for ::std::num::NonZeroU32 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         self.get().hash_stable(ctx, hasher)
     }
 }
 
 impl<CTX> HashStable<CTX> for f32 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         let val: u32 = unsafe {
             ::std::mem::transmute(*self)
         };
@@ -234,9 +225,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 }
 
 impl<CTX> HashStable<CTX> for f64 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         let val: u64 = unsafe {
             ::std::mem::transmute(*self)
         };
@@ -245,26 +234,20 @@ fn hash_stable<W: StableHasherResult>(&self,
 }
 
 impl<CTX> HashStable<CTX> for ::std::cmp::Ordering {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         (*self as i8).hash_stable(ctx, hasher);
     }
 }
 
 impl<T1: HashStable<CTX>, CTX> HashStable<CTX> for (T1,) {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         let (ref _0,) = *self;
         _0.hash_stable(ctx, hasher);
     }
 }
 
 impl<T1: HashStable<CTX>, T2: HashStable<CTX>, CTX> HashStable<CTX> for (T1, T2) {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         let (ref _0, ref _1) = *self;
         _0.hash_stable(ctx, hasher);
         _1.hash_stable(ctx, hasher);
@@ -276,9 +259,7 @@ impl<T1, T2, T3, CTX> HashStable<CTX> for (T1, T2, T3)
            T2: HashStable<CTX>,
            T3: HashStable<CTX>,
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         let (ref _0, ref _1, ref _2) = *self;
         _0.hash_stable(ctx, hasher);
         _1.hash_stable(ctx, hasher);
@@ -292,9 +273,7 @@ impl<T1, T2, T3, T4, CTX> HashStable<CTX> for (T1, T2, T3, T4)
            T3: HashStable<CTX>,
            T4: HashStable<CTX>,
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         let (ref _0, ref _1, ref _2, ref _3) = *self;
         _0.hash_stable(ctx, hasher);
         _1.hash_stable(ctx, hasher);
@@ -304,9 +283,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 }
 
 impl<T: HashStable<CTX>, CTX> HashStable<CTX> for [T] {
-    default fn hash_stable<W: StableHasherResult>(&self,
-                                                  ctx: &mut CTX,
-                                                  hasher: &mut StableHasher<W>) {
+    default fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         self.len().hash_stable(ctx, hasher);
         for item in self {
             item.hash_stable(ctx, hasher);
@@ -316,9 +293,7 @@ impl<T: HashStable<CTX>, CTX> HashStable<CTX> for [T] {
 
 impl<T: HashStable<CTX>, CTX> HashStable<CTX> for Vec<T> {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         (&self[..]).hash_stable(ctx, hasher);
     }
 }
@@ -329,9 +304,7 @@ impl<K, V, R, CTX> HashStable<CTX> for indexmap::IndexMap<K, V, R>
           R: BuildHasher,
 {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         self.len().hash_stable(ctx, hasher);
         for kv in self {
             kv.hash_stable(ctx, hasher);
@@ -344,9 +317,7 @@ impl<K, R, CTX> HashStable<CTX> for indexmap::IndexSet<K, R>
           R: BuildHasher,
 {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         self.len().hash_stable(ctx, hasher);
         for key in self {
             key.hash_stable(ctx, hasher);
@@ -356,45 +327,35 @@ fn hash_stable<W: StableHasherResult>(&self,
 
 impl<A, CTX> HashStable<CTX> for SmallVec<[A; 1]> where A: HashStable<CTX> {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         (&self[..]).hash_stable(ctx, hasher);
     }
 }
 
 impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for Box<T> {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         (**self).hash_stable(ctx, hasher);
     }
 }
 
 impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for ::std::rc::Rc<T> {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         (**self).hash_stable(ctx, hasher);
     }
 }
 
 impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for ::std::sync::Arc<T> {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         (**self).hash_stable(ctx, hasher);
     }
 }
 
 impl<CTX> HashStable<CTX> for str {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          _: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, _: &mut CTX, hasher: &mut StableHasher) {
         self.len().hash(hasher);
         self.as_bytes().hash(hasher);
     }
@@ -403,9 +364,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 
 impl<CTX> HashStable<CTX> for String {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
         (&self[..]).hash_stable(hcx, hasher);
     }
 }
@@ -420,9 +379,7 @@ fn to_stable_hash_key(&self, _: &HCX) -> Self::KeyType {
 
 impl<CTX> HashStable<CTX> for bool {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         (if *self { 1u8 } else { 0u8 }).hash_stable(ctx, hasher);
     }
 }
@@ -432,9 +389,7 @@ impl<T, CTX> HashStable<CTX> for Option<T>
     where T: HashStable<CTX>
 {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         if let Some(ref value) = *self {
             1u8.hash_stable(ctx, hasher);
             value.hash_stable(ctx, hasher);
@@ -449,9 +404,7 @@ impl<T1, T2, CTX> HashStable<CTX> for Result<T1, T2>
           T2: HashStable<CTX>,
 {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         mem::discriminant(self).hash_stable(ctx, hasher);
         match *self {
             Ok(ref x) => x.hash_stable(ctx, hasher),
@@ -464,18 +417,14 @@ impl<'a, T, CTX> HashStable<CTX> for &'a T
     where T: HashStable<CTX> + ?Sized
 {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         (**self).hash_stable(ctx, hasher);
     }
 }
 
 impl<T, CTX> HashStable<CTX> for ::std::mem::Discriminant<T> {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          _: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, _: &mut CTX, hasher: &mut StableHasher) {
         ::std::hash::Hash::hash(self, hasher);
     }
 }
@@ -483,9 +432,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 impl<I: indexed_vec::Idx, T, CTX> HashStable<CTX> for indexed_vec::IndexVec<I, T>
     where T: HashStable<CTX>,
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         self.len().hash_stable(ctx, hasher);
         for v in &self.raw {
             v.hash_stable(ctx, hasher);
@@ -496,9 +443,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 
 impl<I: indexed_vec::Idx, CTX> HashStable<CTX> for bit_set::BitSet<I>
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         self.words().hash_stable(ctx, hasher);
     }
 }
@@ -506,9 +451,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 impl<R: indexed_vec::Idx, C: indexed_vec::Idx, CTX> HashStable<CTX>
 for bit_set::BitMatrix<R, C>
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          ctx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
         self.words().hash_stable(ctx, hasher);
     }
 }
@@ -522,9 +465,7 @@ impl<K, V, R, HCX> HashStable<HCX> for ::std::collections::HashMap<K, V, R>
           R: BuildHasher,
 {
     #[inline]
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut HCX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
         hash_stable_hashmap(hcx, hasher, self, ToStableHashKey::to_stable_hash_key);
     }
 }
@@ -533,9 +474,7 @@ impl<K, R, HCX> HashStable<HCX> for ::std::collections::HashSet<K, R>
     where K: ToStableHashKey<HCX> + Eq + Hash,
           R: BuildHasher,
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut HCX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
         let mut keys: Vec<_> = self.iter()
                                    .map(|k| k.to_stable_hash_key(hcx))
                                    .collect();
@@ -548,9 +487,7 @@ impl<K, V, HCX> HashStable<HCX> for ::std::collections::BTreeMap<K, V>
     where K: ToStableHashKey<HCX>,
           V: HashStable<HCX>,
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut HCX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
         let mut entries: Vec<_> = self.iter()
                                       .map(|(k, v)| (k.to_stable_hash_key(hcx), v))
                                       .collect();
@@ -562,9 +499,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 impl<K, HCX> HashStable<HCX> for ::std::collections::BTreeSet<K>
     where K: ToStableHashKey<HCX>,
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut HCX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
         let mut keys: Vec<_> = self.iter()
                                    .map(|k| k.to_stable_hash_key(hcx))
                                    .collect();
@@ -573,9 +508,9 @@ fn hash_stable<W: StableHasherResult>(&self,
     }
 }
 
-pub fn hash_stable_hashmap<HCX, K, V, R, SK, F, W>(
+pub fn hash_stable_hashmap<HCX, K, V, R, SK, F>(
     hcx: &mut HCX,
-    hasher: &mut StableHasher<W>,
+    hasher: &mut StableHasher,
     map: &::std::collections::HashMap<K, V, R>,
     to_stable_hash_key: F)
     where K: Eq + Hash,
@@ -583,7 +518,6 @@ pub fn hash_stable_hashmap<HCX, K, V, R, SK, F, W>(
           R: BuildHasher,
           SK: HashStable<HCX> + Ord + Clone,
           F: Fn(&K, &HCX) -> SK,
-          W: StableHasherResult,
 {
     let mut entries: Vec<_> = map.iter()
                                   .map(|(k, v)| (to_stable_hash_key(k, hcx), v))
@@ -614,9 +548,7 @@ fn deref(&self) -> &Vec<T> {
 impl<T, HCX> HashStable<HCX> for StableVec<T>
     where T: HashStable<HCX> + ToStableHashKey<HCX>
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut HCX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
         let StableVec(ref v) = *self;
 
         let mut sorted: Vec<_> = v.iter()

src/librustc_data_structures/svh.rs

@@ -61,11 +61,7 @@ fn decode<D: Decoder>(d: &mut D) -> Result<Svh, D::Error> {
 
 impl<T> stable_hasher::HashStable<T> for Svh {
     #[inline]
-    fn hash_stable<W: stable_hasher::StableHasherResult>(
-        &self,
-        ctx: &mut T,
-        hasher: &mut stable_hasher::StableHasher<W>
-    ) {
+    fn hash_stable(&self, ctx: &mut T, hasher: &mut stable_hasher::StableHasher) {
         let Svh {
             hash
         } = *self;

src/librustc_data_structures/thin_vec.rs

@@ -1,4 +1,4 @@
-use crate::stable_hasher::{StableHasher, StableHasherResult, HashStable};
+use crate::stable_hasher::{StableHasher, HashStable};
 
 /// A vector type optimized for cases where this size is usually 0 (cf. `SmallVector`).
 /// The `Option<Box<..>>` wrapping allows us to represent a zero sized vector with `None`,
@@ -60,9 +60,7 @@ fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
 }
 
 impl<T: HashStable<CTX>, CTX> HashStable<CTX> for ThinVec<T> {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
         (**self).hash_stable(hcx, hasher)
     }
 }

src/librustc_data_structures/transitive_relation.rs

@@ -1,6 +1,6 @@
 use crate::bit_set::BitMatrix;
 use crate::fx::FxHashMap;
-use crate::stable_hasher::{HashStable, StableHasher, StableHasherResult};
+use crate::stable_hasher::{HashStable, StableHasher};
 use crate::sync::Lock;
 use rustc_serialize::{Encodable, Encoder, Decodable, Decoder};
 use std::fmt::Debug;
@@ -442,9 +442,7 @@ fn decode<D: Decoder>(d: &mut D) -> Result<Self, D::Error> {
 impl<CTX, T> HashStable<CTX> for TransitiveRelation<T>
     where T: HashStable<CTX> + Eq + Debug + Clone + Hash
 {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
         // We are assuming here that the relation graph has been built in a
         // deterministic way and we can just hash it the way it is.
         let TransitiveRelation {
@@ -462,9 +460,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 }
 
 impl<CTX> HashStable<CTX> for Edge {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
         let Edge {
             ref source,
             ref target,
@@ -476,9 +472,7 @@ fn hash_stable<W: StableHasherResult>(&self,
 }
 
 impl<CTX> HashStable<CTX> for Index {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut CTX,
-                                          hasher: &mut StableHasher<W>) {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
         let Index(idx) = *self;
         idx.hash_stable(hcx, hasher);
     }