rustc_data_structures/
stable_hasher.rs

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1use std::hash::{BuildHasher, Hash, Hasher};
2use std::marker::PhantomData;
3use std::mem;
4use std::num::NonZero;
5
6use rustc_index::bit_set::{self, DenseBitSet};
7use rustc_index::{Idx, IndexSlice, IndexVec};
8use smallvec::SmallVec;
9
10#[cfg(test)]
11mod tests;
12
13use rustc_hashes::{Hash64, Hash128};
14pub use rustc_stable_hash::{
15    FromStableHash, SipHasher128Hash as StableHasherHash, StableSipHasher128 as StableHasher,
16};
17
18/// Something that implements `HashStable<CTX>` can be hashed in a way that is
19/// stable across multiple compilation sessions.
20///
21/// Note that `HashStable` imposes rather more strict requirements than usual
22/// hash functions:
23///
24/// - Stable hashes are sometimes used as identifiers. Therefore they must
25///   conform to the corresponding `PartialEq` implementations:
26///
27///     - `x == y` implies `hash_stable(x) == hash_stable(y)`, and
28///     - `x != y` implies `hash_stable(x) != hash_stable(y)`.
29///
30///   That second condition is usually not required for hash functions
31///   (e.g. `Hash`). In practice this means that `hash_stable` must feed any
32///   information into the hasher that a `PartialEq` comparison takes into
33///   account. See [#49300](https://github.com/rust-lang/rust/issues/49300)
34///   for an example where violating this invariant has caused trouble in the
35///   past.
36///
37/// - `hash_stable()` must be independent of the current
38///    compilation session. E.g. they must not hash memory addresses or other
39///    things that are "randomly" assigned per compilation session.
40///
41/// - `hash_stable()` must be independent of the host architecture. The
42///   `StableHasher` takes care of endianness and `isize`/`usize` platform
43///   differences.
44pub trait HashStable<CTX> {
45    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher);
46}
47
48/// Implement this for types that can be turned into stable keys like, for
49/// example, for DefId that can be converted to a DefPathHash. This is used for
50/// bringing maps into a predictable order before hashing them.
51pub trait ToStableHashKey<HCX> {
52    type KeyType: Ord + Sized + HashStable<HCX>;
53    fn to_stable_hash_key(&self, hcx: &HCX) -> Self::KeyType;
54}
55
56/// Trait for marking a type as having a sort order that is
57/// stable across compilation session boundaries. More formally:
58///
59/// ```txt
60/// Ord::cmp(a1, b1) == Ord::cmp(a2, b2)
61///    where a2 = decode(encode(a1, context1), context2)
62///          b2 = decode(encode(b1, context1), context2)
63/// ```
64///
65/// i.e. the result of `Ord::cmp` is not influenced by encoding
66/// the values in one session and then decoding them in another
67/// session.
68///
69/// This is trivially true for types where encoding and decoding
70/// don't change the bytes of the values that are used during
71/// comparison and comparison only depends on these bytes (as
72/// opposed to some non-local state). Examples are u32, String,
73/// Path, etc.
74///
75/// But it is not true for:
76///  - `*const T` and `*mut T` because the values of these pointers
77///    will change between sessions.
78///  - `DefIndex`, `CrateNum`, `LocalDefId`, because their concrete
79///    values depend on state that might be different between
80///    compilation sessions.
81///
82/// The associated constant `CAN_USE_UNSTABLE_SORT` denotes whether
83/// unstable sorting can be used for this type. Set to true if and
84/// only if `a == b` implies `a` and `b` are fully indistinguishable.
85pub trait StableOrd: Ord {
86    const CAN_USE_UNSTABLE_SORT: bool;
87
88    /// Marker to ensure that implementors have carefully considered
89    /// whether their `Ord` implementation obeys this trait's contract.
90    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: ();
91}
92
93impl<T: StableOrd> StableOrd for &T {
94    const CAN_USE_UNSTABLE_SORT: bool = T::CAN_USE_UNSTABLE_SORT;
95
96    // Ordering of a reference is exactly that of the referent, and since
97    // the ordering of the referet is stable so must be the ordering of the
98    // reference.
99    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
100}
101
102/// This is a companion trait to `StableOrd`. Some types like `Symbol` can be
103/// compared in a cross-session stable way, but their `Ord` implementation is
104/// not stable. In such cases, a `StableOrd` implementation can be provided
105/// to offer a lightweight way for stable sorting. (The more heavyweight option
106/// is to sort via `ToStableHashKey`, but then sorting needs to have access to
107/// a stable hashing context and `ToStableHashKey` can also be expensive as in
108/// the case of `Symbol` where it has to allocate a `String`.)
109///
110/// See the documentation of [StableOrd] for how stable sort order is defined.
111/// The same definition applies here. Be careful when implementing this trait.
112pub trait StableCompare {
113    const CAN_USE_UNSTABLE_SORT: bool;
114
115    fn stable_cmp(&self, other: &Self) -> std::cmp::Ordering;
116}
117
118/// `StableOrd` denotes that the type's `Ord` implementation is stable, so
119/// we can implement `StableCompare` by just delegating to `Ord`.
120impl<T: StableOrd> StableCompare for T {
121    const CAN_USE_UNSTABLE_SORT: bool = T::CAN_USE_UNSTABLE_SORT;
122
123    fn stable_cmp(&self, other: &Self) -> std::cmp::Ordering {
124        self.cmp(other)
125    }
126}
127
128/// Implement HashStable by just calling `Hash::hash()`. Also implement `StableOrd` for the type since
129/// that has the same requirements.
130///
131/// **WARNING** This is only valid for types that *really* don't need any context for fingerprinting.
132/// But it is easy to misuse this macro (see [#96013](https://github.com/rust-lang/rust/issues/96013)
133/// for examples). Therefore this macro is not exported and should only be used in the limited cases
134/// here in this module.
135///
136/// Use `#[derive(HashStable_Generic)]` instead.
137macro_rules! impl_stable_traits_for_trivial_type {
138    ($t:ty) => {
139        impl<CTX> $crate::stable_hasher::HashStable<CTX> for $t {
140            #[inline]
141            fn hash_stable(&self, _: &mut CTX, hasher: &mut $crate::stable_hasher::StableHasher) {
142                ::std::hash::Hash::hash(self, hasher);
143            }
144        }
145
146        impl $crate::stable_hasher::StableOrd for $t {
147            const CAN_USE_UNSTABLE_SORT: bool = true;
148
149            // Encoding and decoding doesn't change the bytes of trivial types
150            // and `Ord::cmp` depends only on those bytes.
151            const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
152        }
153    };
154}
155
156pub(crate) use impl_stable_traits_for_trivial_type;
157
158impl_stable_traits_for_trivial_type!(i8);
159impl_stable_traits_for_trivial_type!(i16);
160impl_stable_traits_for_trivial_type!(i32);
161impl_stable_traits_for_trivial_type!(i64);
162impl_stable_traits_for_trivial_type!(isize);
163
164impl_stable_traits_for_trivial_type!(u8);
165impl_stable_traits_for_trivial_type!(u16);
166impl_stable_traits_for_trivial_type!(u32);
167impl_stable_traits_for_trivial_type!(u64);
168impl_stable_traits_for_trivial_type!(usize);
169
170impl_stable_traits_for_trivial_type!(u128);
171impl_stable_traits_for_trivial_type!(i128);
172
173impl_stable_traits_for_trivial_type!(char);
174impl_stable_traits_for_trivial_type!(());
175
176impl_stable_traits_for_trivial_type!(Hash64);
177
178// We need a custom impl as the default hash function will only hash half the bits. For stable
179// hashing we want to hash the full 128-bit hash.
180impl<CTX> HashStable<CTX> for Hash128 {
181    #[inline]
182    fn hash_stable(&self, _: &mut CTX, hasher: &mut StableHasher) {
183        self.as_u128().hash(hasher);
184    }
185}
186
187impl StableOrd for Hash128 {
188    const CAN_USE_UNSTABLE_SORT: bool = true;
189
190    // Encoding and decoding doesn't change the bytes of `Hash128`
191    // and `Ord::cmp` depends only on those bytes.
192    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
193}
194
195impl<CTX> HashStable<CTX> for ! {
196    fn hash_stable(&self, _ctx: &mut CTX, _hasher: &mut StableHasher) {
197        unreachable!()
198    }
199}
200
201impl<CTX, T> HashStable<CTX> for PhantomData<T> {
202    fn hash_stable(&self, _ctx: &mut CTX, _hasher: &mut StableHasher) {}
203}
204
205impl<CTX> HashStable<CTX> for NonZero<u32> {
206    #[inline]
207    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
208        self.get().hash_stable(ctx, hasher)
209    }
210}
211
212impl<CTX> HashStable<CTX> for NonZero<usize> {
213    #[inline]
214    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
215        self.get().hash_stable(ctx, hasher)
216    }
217}
218
219impl<CTX> HashStable<CTX> for f32 {
220    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
221        let val: u32 = self.to_bits();
222        val.hash_stable(ctx, hasher);
223    }
224}
225
226impl<CTX> HashStable<CTX> for f64 {
227    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
228        let val: u64 = self.to_bits();
229        val.hash_stable(ctx, hasher);
230    }
231}
232
233impl<CTX> HashStable<CTX> for ::std::cmp::Ordering {
234    #[inline]
235    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
236        (*self as i8).hash_stable(ctx, hasher);
237    }
238}
239
240impl<T1: HashStable<CTX>, CTX> HashStable<CTX> for (T1,) {
241    #[inline]
242    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
243        let (ref _0,) = *self;
244        _0.hash_stable(ctx, hasher);
245    }
246}
247
248impl<T1: HashStable<CTX>, T2: HashStable<CTX>, CTX> HashStable<CTX> for (T1, T2) {
249    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
250        let (ref _0, ref _1) = *self;
251        _0.hash_stable(ctx, hasher);
252        _1.hash_stable(ctx, hasher);
253    }
254}
255
256impl<T1: StableOrd, T2: StableOrd> StableOrd for (T1, T2) {
257    const CAN_USE_UNSTABLE_SORT: bool = T1::CAN_USE_UNSTABLE_SORT && T2::CAN_USE_UNSTABLE_SORT;
258
259    // Ordering of tuples is a pure function of their elements' ordering, and since
260    // the ordering of each element is stable so must be the ordering of the tuple.
261    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
262}
263
264impl<T1, T2, T3, CTX> HashStable<CTX> for (T1, T2, T3)
265where
266    T1: HashStable<CTX>,
267    T2: HashStable<CTX>,
268    T3: HashStable<CTX>,
269{
270    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
271        let (ref _0, ref _1, ref _2) = *self;
272        _0.hash_stable(ctx, hasher);
273        _1.hash_stable(ctx, hasher);
274        _2.hash_stable(ctx, hasher);
275    }
276}
277
278impl<T1: StableOrd, T2: StableOrd, T3: StableOrd> StableOrd for (T1, T2, T3) {
279    const CAN_USE_UNSTABLE_SORT: bool =
280        T1::CAN_USE_UNSTABLE_SORT && T2::CAN_USE_UNSTABLE_SORT && T3::CAN_USE_UNSTABLE_SORT;
281
282    // Ordering of tuples is a pure function of their elements' ordering, and since
283    // the ordering of each element is stable so must be the ordering of the tuple.
284    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
285}
286
287impl<T1, T2, T3, T4, CTX> HashStable<CTX> for (T1, T2, T3, T4)
288where
289    T1: HashStable<CTX>,
290    T2: HashStable<CTX>,
291    T3: HashStable<CTX>,
292    T4: HashStable<CTX>,
293{
294    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
295        let (ref _0, ref _1, ref _2, ref _3) = *self;
296        _0.hash_stable(ctx, hasher);
297        _1.hash_stable(ctx, hasher);
298        _2.hash_stable(ctx, hasher);
299        _3.hash_stable(ctx, hasher);
300    }
301}
302
303impl<T1: StableOrd, T2: StableOrd, T3: StableOrd, T4: StableOrd> StableOrd for (T1, T2, T3, T4) {
304    const CAN_USE_UNSTABLE_SORT: bool = T1::CAN_USE_UNSTABLE_SORT
305        && T2::CAN_USE_UNSTABLE_SORT
306        && T3::CAN_USE_UNSTABLE_SORT
307        && T4::CAN_USE_UNSTABLE_SORT;
308
309    // Ordering of tuples is a pure function of their elements' ordering, and since
310    // the ordering of each element is stable so must be the ordering of the tuple.
311    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
312}
313
314impl<T: HashStable<CTX>, CTX> HashStable<CTX> for [T] {
315    default fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
316        self.len().hash_stable(ctx, hasher);
317        for item in self {
318            item.hash_stable(ctx, hasher);
319        }
320    }
321}
322
323impl<CTX> HashStable<CTX> for [u8] {
324    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
325        self.len().hash_stable(ctx, hasher);
326        hasher.write(self);
327    }
328}
329
330impl<T: HashStable<CTX>, CTX> HashStable<CTX> for Vec<T> {
331    #[inline]
332    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
333        self[..].hash_stable(ctx, hasher);
334    }
335}
336
337impl<K, V, R, CTX> HashStable<CTX> for indexmap::IndexMap<K, V, R>
338where
339    K: HashStable<CTX> + Eq + Hash,
340    V: HashStable<CTX>,
341    R: BuildHasher,
342{
343    #[inline]
344    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
345        self.len().hash_stable(ctx, hasher);
346        for kv in self {
347            kv.hash_stable(ctx, hasher);
348        }
349    }
350}
351
352impl<K, R, CTX> HashStable<CTX> for indexmap::IndexSet<K, R>
353where
354    K: HashStable<CTX> + Eq + Hash,
355    R: BuildHasher,
356{
357    #[inline]
358    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
359        self.len().hash_stable(ctx, hasher);
360        for key in self {
361            key.hash_stable(ctx, hasher);
362        }
363    }
364}
365
366impl<A, const N: usize, CTX> HashStable<CTX> for SmallVec<[A; N]>
367where
368    A: HashStable<CTX>,
369{
370    #[inline]
371    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
372        self[..].hash_stable(ctx, hasher);
373    }
374}
375
376impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for Box<T> {
377    #[inline]
378    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
379        (**self).hash_stable(ctx, hasher);
380    }
381}
382
383impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for ::std::rc::Rc<T> {
384    #[inline]
385    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
386        (**self).hash_stable(ctx, hasher);
387    }
388}
389
390impl<T: ?Sized + HashStable<CTX>, CTX> HashStable<CTX> for ::std::sync::Arc<T> {
391    #[inline]
392    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
393        (**self).hash_stable(ctx, hasher);
394    }
395}
396
397impl<CTX> HashStable<CTX> for str {
398    #[inline]
399    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
400        self.as_bytes().hash_stable(ctx, hasher);
401    }
402}
403
404impl StableOrd for &str {
405    const CAN_USE_UNSTABLE_SORT: bool = true;
406
407    // Encoding and decoding doesn't change the bytes of string slices
408    // and `Ord::cmp` depends only on those bytes.
409    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
410}
411
412impl<CTX> HashStable<CTX> for String {
413    #[inline]
414    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
415        self[..].hash_stable(hcx, hasher);
416    }
417}
418
419impl StableOrd for String {
420    const CAN_USE_UNSTABLE_SORT: bool = true;
421
422    // String comparison only depends on their contents and the
423    // contents are not changed by (de-)serialization.
424    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
425}
426
427impl<HCX> ToStableHashKey<HCX> for String {
428    type KeyType = String;
429    #[inline]
430    fn to_stable_hash_key(&self, _: &HCX) -> Self::KeyType {
431        self.clone()
432    }
433}
434
435impl<HCX, T1: ToStableHashKey<HCX>, T2: ToStableHashKey<HCX>> ToStableHashKey<HCX> for (T1, T2) {
436    type KeyType = (T1::KeyType, T2::KeyType);
437    #[inline]
438    fn to_stable_hash_key(&self, hcx: &HCX) -> Self::KeyType {
439        (self.0.to_stable_hash_key(hcx), self.1.to_stable_hash_key(hcx))
440    }
441}
442
443impl<CTX> HashStable<CTX> for bool {
444    #[inline]
445    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
446        (if *self { 1u8 } else { 0u8 }).hash_stable(ctx, hasher);
447    }
448}
449
450impl StableOrd for bool {
451    const CAN_USE_UNSTABLE_SORT: bool = true;
452
453    // sort order of bools is not changed by (de-)serialization.
454    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
455}
456
457impl<T, CTX> HashStable<CTX> for Option<T>
458where
459    T: HashStable<CTX>,
460{
461    #[inline]
462    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
463        if let Some(ref value) = *self {
464            1u8.hash_stable(ctx, hasher);
465            value.hash_stable(ctx, hasher);
466        } else {
467            0u8.hash_stable(ctx, hasher);
468        }
469    }
470}
471
472impl<T: StableOrd> StableOrd for Option<T> {
473    const CAN_USE_UNSTABLE_SORT: bool = T::CAN_USE_UNSTABLE_SORT;
474
475    // the Option wrapper does not add instability to comparison.
476    const THIS_IMPLEMENTATION_HAS_BEEN_TRIPLE_CHECKED: () = ();
477}
478
479impl<T1, T2, CTX> HashStable<CTX> for Result<T1, T2>
480where
481    T1: HashStable<CTX>,
482    T2: HashStable<CTX>,
483{
484    #[inline]
485    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
486        mem::discriminant(self).hash_stable(ctx, hasher);
487        match *self {
488            Ok(ref x) => x.hash_stable(ctx, hasher),
489            Err(ref x) => x.hash_stable(ctx, hasher),
490        }
491    }
492}
493
494impl<'a, T, CTX> HashStable<CTX> for &'a T
495where
496    T: HashStable<CTX> + ?Sized,
497{
498    #[inline]
499    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
500        (**self).hash_stable(ctx, hasher);
501    }
502}
503
504impl<T, CTX> HashStable<CTX> for ::std::mem::Discriminant<T> {
505    #[inline]
506    fn hash_stable(&self, _: &mut CTX, hasher: &mut StableHasher) {
507        ::std::hash::Hash::hash(self, hasher);
508    }
509}
510
511impl<T, CTX> HashStable<CTX> for ::std::ops::RangeInclusive<T>
512where
513    T: HashStable<CTX>,
514{
515    #[inline]
516    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
517        self.start().hash_stable(ctx, hasher);
518        self.end().hash_stable(ctx, hasher);
519    }
520}
521
522impl<I: Idx, T, CTX> HashStable<CTX> for IndexSlice<I, T>
523where
524    T: HashStable<CTX>,
525{
526    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
527        self.len().hash_stable(ctx, hasher);
528        for v in &self.raw {
529            v.hash_stable(ctx, hasher);
530        }
531    }
532}
533
534impl<I: Idx, T, CTX> HashStable<CTX> for IndexVec<I, T>
535where
536    T: HashStable<CTX>,
537{
538    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
539        self.len().hash_stable(ctx, hasher);
540        for v in &self.raw {
541            v.hash_stable(ctx, hasher);
542        }
543    }
544}
545
546impl<I: Idx, CTX> HashStable<CTX> for DenseBitSet<I> {
547    fn hash_stable(&self, _ctx: &mut CTX, hasher: &mut StableHasher) {
548        ::std::hash::Hash::hash(self, hasher);
549    }
550}
551
552impl<R: Idx, C: Idx, CTX> HashStable<CTX> for bit_set::BitMatrix<R, C> {
553    fn hash_stable(&self, _ctx: &mut CTX, hasher: &mut StableHasher) {
554        ::std::hash::Hash::hash(self, hasher);
555    }
556}
557
558impl<T, CTX> HashStable<CTX> for bit_set::FiniteBitSet<T>
559where
560    T: HashStable<CTX> + bit_set::FiniteBitSetTy,
561{
562    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
563        self.0.hash_stable(hcx, hasher);
564    }
565}
566
567impl_stable_traits_for_trivial_type!(::std::ffi::OsStr);
568
569impl_stable_traits_for_trivial_type!(::std::path::Path);
570impl_stable_traits_for_trivial_type!(::std::path::PathBuf);
571
572// It is not safe to implement HashStable for HashSet, HashMap or any other collection type
573// with unstable but observable iteration order.
574// See https://github.com/rust-lang/compiler-team/issues/533 for further information.
575impl<V, HCX> !HashStable<HCX> for std::collections::HashSet<V> {}
576impl<K, V, HCX> !HashStable<HCX> for std::collections::HashMap<K, V> {}
577
578impl<K, V, HCX> HashStable<HCX> for ::std::collections::BTreeMap<K, V>
579where
580    K: HashStable<HCX> + StableOrd,
581    V: HashStable<HCX>,
582{
583    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
584        self.len().hash_stable(hcx, hasher);
585        for entry in self.iter() {
586            entry.hash_stable(hcx, hasher);
587        }
588    }
589}
590
591impl<K, HCX> HashStable<HCX> for ::std::collections::BTreeSet<K>
592where
593    K: HashStable<HCX> + StableOrd,
594{
595    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
596        self.len().hash_stable(hcx, hasher);
597        for entry in self.iter() {
598            entry.hash_stable(hcx, hasher);
599        }
600    }
601}
602
603/// Controls what data we do or do not hash.
604/// Whenever a `HashStable` implementation caches its
605/// result, it needs to include `HashingControls` as part
606/// of the key, to ensure that it does not produce an incorrect
607/// result (for example, using a `Fingerprint` produced while
608/// hashing `Span`s when a `Fingerprint` without `Span`s is
609/// being requested)
610#[derive(Clone, Hash, Eq, PartialEq, Debug)]
611pub struct HashingControls {
612    pub hash_spans: bool,
613}
rustc_data_structures/stable_hasher.rs

rustc_data_structures/
stable_hasher.rs
