Simplify HashStableContext.

`derive(HashStable_Generic)` generates impls like this:
```
impl<__CTX> HashStable<__CTX> for ExpnKind
where
    __CTX: crate::HashStableContext
{
    fn hash_stable(&self, hcx : &mut __CTX, __hasher: &mut StableHasher) {
        ...
    }
}
```
This is used for crates that are upstream of `rustc_middle`.

The `crate::HashStableContext` bound means every crate that uses
`derive(HashStable_Generic)` must provide (or import) a trait
`HashStableContext` which `rustc_middle` then impls. In `rustc_span`
this trait is sensible, with three methods. In other crates, this trait
is empty, and there is the following trait hierarchy:
```
rustc_session::HashStableContext
  |              |
  |   rustc_hir::HashStableContext
  |         /                   \
rustc_ast::HashStableContext   rustc_abi::HashStableContext
  |
rustc_span::HashStableContext
```
All very strange and unnecessary. This commit changes
`derive(HashStable_Generic)` to use `rustc_span::HashStableContext`
instead of `crate::HashStableContext`. This eliminates the need for all
the empty `HashStableContext` traits and impls. Much better.

Author: nnethercote

compiler/rustc_abi/src/lib.rs

@@ -67,12 +67,6 @@ pub use layout::{FIRST_VARIANT, FieldIdx, LayoutCalculator, LayoutCalculatorErro
 #[cfg(feature = "nightly")]
 pub use layout::{Layout, TyAbiInterface, TyAndLayout};
 
-/// Requirements for a `StableHashingContext` to be used in this crate.
-/// This is a hack to allow using the `HashStable_Generic` derive macro
-/// instead of implementing everything in `rustc_middle`.
-#[cfg(feature = "nightly")]
-pub trait HashStableContext {}
-
 #[derive(Clone, Copy, PartialEq, Eq, Default)]
 #[cfg_attr(
     feature = "nightly",

compiler/rustc_ast/src/ast.rs

@@ -31,7 +31,8 @@ use rustc_data_structures::tagged_ptr::Tag;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic, Walkable};
 pub use rustc_span::AttrId;
 use rustc_span::{
-    ByteSymbol, DUMMY_SP, ErrorGuaranteed, Ident, Span, Spanned, Symbol, kw, respan, sym,
+    ByteSymbol, DUMMY_SP, ErrorGuaranteed, HashStableContext, Ident, Span, Spanned, Symbol, kw,
+    respan, sym,
 };
 use thin_vec::{ThinVec, thin_vec};
 
@@ -120,7 +121,7 @@ impl PartialEq<&[Symbol]> for Path {
     }
 }
 
-impl<Hcx: rustc_span::HashStableContext> HashStable<Hcx> for Path {
+impl<Hcx: HashStableContext> HashStable<Hcx> for Path {
     fn hash_stable(&self, hcx: &mut Hcx, hasher: &mut StableHasher) {
         self.segments.len().hash_stable(hcx, hasher);
         for segment in &self.segments {

compiler/rustc_ast/src/lib.rs

@@ -36,8 +36,3 @@ pub mod visit;
 
 pub use self::ast::*;
 pub use self::ast_traits::{AstNodeWrapper, HasAttrs, HasNodeId, HasTokens};
-
-/// Requirements for a `StableHashingContext` to be used in this crate.
-/// This is a hack to allow using the `HashStable_Generic` derive macro
-/// instead of implementing everything in `rustc_middle`.
-pub trait HashStableContext: rustc_span::HashStableContext {}

compiler/rustc_ast/src/tokenstream.rs

@@ -14,7 +14,7 @@ use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync;
 use rustc_macros::{Decodable, Encodable, HashStable_Generic, Walkable};
 use rustc_serialize::{Decodable, Encodable};
-use rustc_span::{DUMMY_SP, Span, SpanDecoder, SpanEncoder, Symbol, sym};
+use rustc_span::{DUMMY_SP, HashStableContext, Span, SpanDecoder, SpanEncoder, Symbol, sym};
 use thin_vec::ThinVec;
 
 use crate::ast::AttrStyle;
@@ -826,7 +826,7 @@ impl FromIterator<TokenTree> for TokenStream {
 
 impl<Hcx> HashStable<Hcx> for TokenStream
 where
-    Hcx: crate::HashStableContext,
+    Hcx: HashStableContext,
 {
     fn hash_stable(&self, hcx: &mut Hcx, hasher: &mut StableHasher) {
         for sub_tt in self.iter() {

compiler/rustc_hir/src/def.rs

@@ -8,9 +8,9 @@ use rustc_data_structures::stable_hasher::ToStableHashKey;
 use rustc_data_structures::unord::UnordMap;
 use rustc_error_messages::{DiagArgValue, IntoDiagArg};
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
-use rustc_span::Symbol;
 use rustc_span::def_id::{DefId, LocalDefId};
 use rustc_span::hygiene::MacroKind;
+use rustc_span::{HashStableContext, Symbol};
 
 use crate::definitions::DefPathData;
 use crate::hir;
@@ -712,7 +712,7 @@ impl IntoDiagArg for Namespace {
     }
 }
 
-impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for Namespace {
+impl<Hcx: HashStableContext> ToStableHashKey<Hcx> for Namespace {
     type KeyType = Namespace;
 
     #[inline]

compiler/rustc_hir/src/lib.rs

@@ -43,7 +43,6 @@ pub use hir::*;
 pub use lang_items::{LangItem, LanguageItems};
 pub use rustc_ast::attr::version::*;
 pub use stability::*;
-pub use stable_hash_impls::HashStableContext;
 pub use target::{MethodKind, Target};
 
 arena_types!(rustc_arena::declare_arena);

compiler/rustc_hir/src/stable_hash_impls.rs

@@ -1,4 +1,5 @@
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher, ToStableHashKey};
+use rustc_span::HashStableContext;
 use rustc_span::def_id::DefPathHash;
 
 use crate::HashIgnoredAttrId;
@@ -8,12 +9,7 @@ use crate::hir::{
 use crate::hir_id::ItemLocalId;
 use crate::lints::DelayedLints;
 
-/// Requirements for a `StableHashingContext` to be used in this crate.
-/// This is a hack to allow using the `HashStable_Generic` derive macro
-/// instead of implementing everything in `rustc_middle`.
-pub trait HashStableContext: rustc_ast::HashStableContext + rustc_abi::HashStableContext {}
-
-impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for BodyId {
+impl<Hcx: HashStableContext> ToStableHashKey<Hcx> for BodyId {
     type KeyType = (DefPathHash, ItemLocalId);
 
     #[inline]
@@ -23,7 +19,7 @@ impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for BodyId {
     }
 }
 
-impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for ItemId {
+impl<Hcx: HashStableContext> ToStableHashKey<Hcx> for ItemId {
     type KeyType = DefPathHash;
 
     #[inline]
@@ -32,7 +28,7 @@ impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for ItemId {
     }
 }
 
-impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for TraitItemId {
+impl<Hcx: HashStableContext> ToStableHashKey<Hcx> for TraitItemId {
     type KeyType = DefPathHash;
 
     #[inline]
@@ -41,7 +37,7 @@ impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for TraitItemId {
     }
 }
 
-impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for ImplItemId {
+impl<Hcx: HashStableContext> ToStableHashKey<Hcx> for ImplItemId {
     type KeyType = DefPathHash;
 
     #[inline]
@@ -50,7 +46,7 @@ impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for ImplItemId {
     }
 }
 
-impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for ForeignItemId {
+impl<Hcx: HashStableContext> ToStableHashKey<Hcx> for ForeignItemId {
     type KeyType = DefPathHash;
 
     #[inline]
@@ -66,7 +62,7 @@ impl<Hcx: crate::HashStableContext> ToStableHashKey<Hcx> for ForeignItemId {
 // want to pick up on a reference changing its target, so we hash the NodeIds
 // in "DefPath Mode".
 
-impl<'tcx, Hcx: crate::HashStableContext> HashStable<Hcx> for OwnerNodes<'tcx> {
+impl<'tcx, Hcx: HashStableContext> HashStable<Hcx> for OwnerNodes<'tcx> {
     fn hash_stable(&self, hcx: &mut Hcx, hasher: &mut StableHasher) {
         // We ignore the `nodes` and `bodies` fields since these refer to information included in
         // `hash` which is hashed in the collector and used for the crate hash.
@@ -78,14 +74,14 @@ impl<'tcx, Hcx: crate::HashStableContext> HashStable<Hcx> for OwnerNodes<'tcx> {
     }
 }
 
-impl<Hcx: crate::HashStableContext> HashStable<Hcx> for DelayedLints {
+impl<Hcx: HashStableContext> HashStable<Hcx> for DelayedLints {
     fn hash_stable(&self, hcx: &mut Hcx, hasher: &mut StableHasher) {
         let DelayedLints { opt_hash, .. } = *self;
         opt_hash.unwrap().hash_stable(hcx, hasher);
     }
 }
 
-impl<'tcx, Hcx: crate::HashStableContext> HashStable<Hcx> for AttributeMap<'tcx> {
+impl<'tcx, Hcx: HashStableContext> HashStable<Hcx> for AttributeMap<'tcx> {
     fn hash_stable(&self, hcx: &mut Hcx, hasher: &mut StableHasher) {
         // We ignore the `map` since it refers to information included in `opt_hash` which is
         // hashed in the collector and used for the crate hash.
@@ -94,7 +90,7 @@ impl<'tcx, Hcx: crate::HashStableContext> HashStable<Hcx> for AttributeMap<'tcx>
     }
 }
 
-impl<Hcx: crate::HashStableContext> HashStable<Hcx> for HashIgnoredAttrId {
+impl<Hcx: HashStableContext> HashStable<Hcx> for HashIgnoredAttrId {
     fn hash_stable(&self, _hcx: &mut Hcx, _hasher: &mut StableHasher) {
         /* we don't hash HashIgnoredAttrId, we ignore them */
     }

compiler/rustc_hir_id/src/lib.rs

@@ -8,7 +8,7 @@ use std::fmt::{self, Debug};
 
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher, StableOrd, ToStableHashKey};
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
-pub use rustc_span::HashStableContext;
+use rustc_span::HashStableContext;
 use rustc_span::def_id::{CRATE_DEF_ID, DefId, DefIndex, DefPathHash, LocalDefId};
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Encodable, Decodable)]
@@ -176,7 +176,7 @@ pub const CRATE_HIR_ID: HirId =
 
 pub const CRATE_OWNER_ID: OwnerId = OwnerId { def_id: CRATE_DEF_ID };
 
-impl<Hcx: rustc_span::HashStableContext> ToStableHashKey<Hcx> for HirId {
+impl<Hcx: HashStableContext> ToStableHashKey<Hcx> for HirId {
     type KeyType = (DefPathHash, ItemLocalId);
 
     #[inline]

compiler/rustc_lint_defs/src/lib.rs

@@ -8,11 +8,11 @@ use rustc_data_structures::stable_hasher::{
     HashStable, StableCompare, StableHasher, ToStableHashKey,
 };
 use rustc_error_messages::{DiagArgValue, IntoDiagArg};
-use rustc_hir_id::{HashStableContext, HirId, ItemLocalId};
+use rustc_hir_id::{HirId, ItemLocalId};
 use rustc_macros::{Decodable, Encodable, HashStable_Generic};
 use rustc_span::def_id::DefPathHash;
 pub use rustc_span::edition::Edition;
-use rustc_span::{Ident, Span, Symbol, sym};
+use rustc_span::{HashStableContext, Ident, Span, Symbol, sym};
 use serde::{Deserialize, Serialize};
 
 pub use self::Level::*;

compiler/rustc_macros/src/hash_stable.rs

@@ -84,7 +84,7 @@ fn hash_stable_derive_with_mode(
     match mode {
         HashStableMode::Normal => {}
         HashStableMode::Generic => {
-            s.add_where_predicate(parse_quote! { __CTX: crate::HashStableContext });
+            s.add_where_predicate(parse_quote! { __CTX: ::rustc_span::HashStableContext });
         }
         HashStableMode::NoContext => {}
     }