stable_hasher.rs

use crate::sip128::SipHasher128;
use rustc_index::bit_set::{self, BitSet};
use rustc_index::{Idx, IndexSlice, IndexVec};
use smallvec::SmallVec;
use std::fmt;
use std::hash::{BuildHasher, Hash, Hasher};
use std::marker::PhantomData;
use std::mem;

#[cfg(test)]
mod tests;

pub use crate::hashes::{Hash128, Hash64};

/// When hashing something that ends up affecting properties like symbol names,
/// we want these symbol names to be calculated independently of other factors
/// like what architecture you're compiling *from*.
///
/// 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 {
    state: SipHasher128,
}

impl fmt::Debug for StableHasher {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{:?}", self.state)
    }
}

pub trait StableHasherResult: Sized {
    fn finish(hasher: StableHasher) -> Self;
}

impl StableHasher {
    #[inline]
    pub fn new() -> Self {
        StableHasher { state: SipHasher128::new_with_keys(0, 0) }
    }

    #[inline]
    pub fn finish<W: StableHasherResult>(self) -> W {
        W::finish(self)
    }
}

impl StableHasher {
    #[inline]
    pub fn finalize(self) -> (u64, u64) {
        self.state.finish128()
    }
}

impl Hasher for StableHasher {
    fn finish(&self) -> u64 {
        panic!("use StableHasher::finalize instead");
    }

    #[inline]
    fn write(&mut self, bytes: &[u8]) {
        self.state.write(bytes);
    }

    #[inline]
    fn write_str(&mut self, s: &str) {
        self.state.write_str(s);
    }

    #[inline]
    fn write_length_prefix(&mut self, len: usize) {
        // Our impl for `usize` will extend it if needed.
        self.write_usize(len);
    }

    #[inline]
    fn write_u8(&mut self, i: u8) {
        self.state.write_u8(i);
    }

    #[inline]
    fn write_u16(&mut self, i: u16) {
        self.state.short_write(i.to_le_bytes());
    }

    #[inline]
    fn write_u32(&mut self, i: u32) {
        self.state.short_write(i.to_le_bytes());
    }

    #[inline]
    fn write_u64(&mut self, i: u64) {
        self.state.short_write(i.to_le_bytes());
    }

    #[inline]
    fn write_u128(&mut self, i: u128) {
        self.write_u64(i as u64);
        self.write_u64((i >> 64) as u64);
    }

    #[inline]
    fn write_usize(&mut self, i: usize) {
        // Always treat usize as u64 so we get the same results on 32 and 64 bit
        // platforms. This is important for symbol hashes when cross compiling,
        // for example.
        self.state.short_write((i as u64).to_le_bytes());
    }

    #[inline]
    fn write_i8(&mut self, i: i8) {
        self.state.write_i8(i);
    }

    #[inline]
    fn write_i16(&mut self, i: i16) {
        self.state.short_write((i as u16).to_le_bytes());
    }

    #[inline]
    fn write_i32(&mut self, i: i32) {
        self.state.short_write((i as u32).to_le_bytes());
    }

    #[inline]
    fn write_i64(&mut self, i: i64) {
        self.state.short_write((i as u64).to_le_bytes());
    }

    #[inline]
    fn write_i128(&mut self, i: i128) {
        self.state.write(&(i as u128).to_le_bytes());
    }

    #[inline]
    fn write_isize(&mut self, i: isize) {
        // Always treat isize as a 64-bit number so we get the same results on 32 and 64 bit
        // platforms. This is important for symbol hashes when cross compiling,
        // for example. Sign extending here is preferable as it means that the
        // same negative number hashes the same on both 32 and 64 bit platforms.
        let value = i as u64;

        // Cold path
        #[cold]
        #[inline(never)]
        fn hash_value(state: &mut SipHasher128, value: u64) {
            state.write_u8(0xFF);
            state.short_write(value.to_le_bytes());
        }

        // `isize` values often seem to have a small (positive) numeric value in practice.
        // To exploit this, if the value is small, we will hash a smaller amount of bytes.
        // However, we cannot just skip the leading zero bytes, as that would produce the same hash
        // e.g. if you hash two values that have the same bit pattern when they are swapped.
        // See /~https://github.com/rust-lang/rust/pull/93014 for context.
        //
        // Therefore, we employ the following strategy:
        // 1) When we encounter a value that fits within a single byte (the most common case), we
        // hash just that byte. This is the most common case that is being optimized. However, we do
        // not do this for the value 0xFF, as that is a reserved prefix (a bit like in UTF-8).
        // 2) When we encounter a larger value, we hash a "marker" 0xFF and then the corresponding
        // 8 bytes. Since this prefix cannot occur when we hash a single byte, when we hash two
        // `isize`s that fit within a different amount of bytes, they should always produce a different
        // byte stream for the hasher.
        if value < 0xFF {
            self.state.write_u8(value as u8);
        } else {
            hash_value(&mut self.state, value);
        }
    }
}

/// Something that implements `HashStable<CTX>` can be hashed in a way that is
/// stable across multiple compilation sessions.
///
/// Note that `HashStable` imposes rather more strict requirements than usual
/// hash functions:
///
/// - Stable hashes are sometimes used as identifiers. Therefore they must
///   conform to the corresponding `PartialEq` implementations:
///
///     - `x == y` implies `hash_stable(x) == hash_stable(y)`, and
///     - `x != y` implies `hash_stable(x) != hash_stable(y)`.
///
///   That second condition is usually not required for hash functions
///   (e.g. `Hash`). In practice this means that `hash_stable` must feed any
///   information into the hasher that a `PartialEq` comparison takes into
///   account. See [#49300](/~https://github.com/rust-lang/rust/issues/49300)
///   for an example where violating this invariant has caused trouble in the
///   past.
///
/// - `hash_stable()` must be independent of the current
///    compilation session. E.g. they must not hash memory addresses or other
///    things that are "randomly" assigned per compilation session.
///
/// - `hash_stable()` must be independent of the host architecture. The
///   `StableHasher` takes care of endianness and `isize`/`usize` platform
///   differences.
pub trait HashStable<CTX> {
    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher);
}

/// Implement this for types that can be turned into stable keys like, for
/// example, for DefId that can be converted to a DefPathHash. This is used for
/// bringing maps into a predictable order before hashing them.
pub trait ToStableHashKey<HCX> {
    type KeyType: Ord + Sized + HashStable<HCX>;
    fn to_stable_hash_key(&self, hcx: &HCX) -> Self::KeyType;
}

/// Trait for marking a type as having a sort order that is
/// stable across compilation session boundaries. More formally:
///
/// ```txt
/// Ord::cmp(a1, b1) == Ord::cmp(a2, b2)
///    where a2 = decode(encode(a1, context1), context2)
///          b2 = decode(encode(b1, context1), context2)
/// ```
///
/// i.e. the result of `Ord::cmp` is not influenced by encoding
/// the values in one session and then decoding them in another
/// session.
///
/// This is trivially true for types where encoding and decoding
/// don't change the bytes of the values that are used during
/// comparison and comparison only depends on these bytes (as
/// opposed to some non-local state). Examples are u32, String,
/// Path, etc.
///
/// But it is not true for:
///  - `*const T` and `*mut T` because the values of these pointers
///    will change between sessions.
///  - `DefIndex`, `CrateNum`, `LocalDefId`, because their concrete
///    values depend on state that might be different between
///    compilation sessions.
///
/// The associated constant `CAN_USE_UNSTABLE_SORT` denotes whether
/// unstable sorting can be used for this type. Set to true if and
/// only if `a == b` implies `a` and `b` are fully indistinguishable.
pub unsafe trait StableOrd: Ord {
    const CAN_USE_UNSTABLE_SORT: bool;
}

unsafe impl<T: StableOrd> StableOrd for &T {
    const CAN_USE_UNSTABLE_SORT: bool = T::CAN_USE_UNSTABLE_SORT;
}

/// Implement HashStable by just calling `Hash::hash()`. Also implement `StableOrd` for the type since
/// that has the same requirements.
///
/// **WARNING** This is only valid for types that *really* don't need any context for fingerprinting.
/// But it is easy to misuse this macro (see [#96013](/~https://github.com/rust-lang/rust/issues/96013)
/// for examples). Therefore this macro is not exported and should only be used in the limited cases
/// here in this module.
///
/// Use `#[derive(HashStable_Generic)]` instead.
macro_rules! impl_stable_traits_for_trivial_type {
    ($t:ty) => {
        impl<CTX> $crate::stable_hasher::HashStable<CTX> for $t {
            #[inline]
            fn hash_stable(&self, _: &mut CTX, hasher: &mut $crate::stable_hasher::StableHasher) {
                ::std::hash::Hash::hash(self, hasher);
            }
        }

        unsafe impl $crate::stable_hasher::StableOrd for $t {
            const CAN_USE_UNSTABLE_SORT: bool = true;
        }
    };
}

impl_stable_traits_for_trivial_type!(i8);
impl_stable_traits_for_trivial_type!(i16);
impl_stable_traits_for_trivial_type!(i32);
impl_stable_traits_for_trivial_type!(i64);
impl_stable_traits_for_trivial_type!(isize);

impl_stable_traits_for_trivial_type!(u8);
impl_stable_traits_for_trivial_type!(u16);
impl_stable_traits_for_trivial_type!(u32);
impl_stable_traits_for_trivial_type!(u64);
impl_stable_traits_for_trivial_type!(usize);

impl_stable_traits_for_trivial_type!(u128);
impl_stable_traits_for_trivial_type!(i128);

impl_stable_traits_for_trivial_type!(char);
impl_stable_traits_for_trivial_type!(());

impl_stable_traits_for_trivial_type!(Hash64);
impl_stable_traits_for_trivial_type!(Hash128);

impl<CTX> HashStable<CTX> for ! {
    fn hash_stable(&self, _ctx: &mut CTX, _hasher: &mut StableHasher) {
        unreachable!()
    }
}

impl<CTX, T> HashStable<CTX> for PhantomData<T> {
    fn hash_stable(&self, _ctx: &mut CTX, _hasher: &mut StableHasher) {}
}

impl<CTX> HashStable<CTX> for ::std::num::NonZeroU32 {
    #[inline]
    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        self.get().hash_stable(ctx, hasher)
    }
}

impl<CTX> HashStable<CTX> for ::std::num::NonZeroUsize {
    #[inline]
    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(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        let val: u32 = self.to_bits();
        val.hash_stable(ctx, hasher);
    }
}

impl<CTX> HashStable<CTX> for f64 {
    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        let val: u64 = self.to_bits();
        val.hash_stable(ctx, hasher);
    }
}

impl<CTX> HashStable<CTX> for ::std::cmp::Ordering {
    #[inline]
    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,) {
    #[inline]
    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(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        let (ref _0, ref _1) = *self;
        _0.hash_stable(ctx, hasher);
        _1.hash_stable(ctx, hasher);
    }
}

unsafe impl<T1: StableOrd, T2: StableOrd> StableOrd for (T1, T2) {
    const CAN_USE_UNSTABLE_SORT: bool = T1::CAN_USE_UNSTABLE_SORT && T2::CAN_USE_UNSTABLE_SORT;
}

impl<T1, T2, T3, CTX> HashStable<CTX> for (T1, T2, T3)
where
    T1: HashStable<CTX>,
    T2: HashStable<CTX>,
    T3: HashStable<CTX>,
{
    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);
        _2.hash_stable(ctx, hasher);
    }
}

unsafe impl<T1: StableOrd, T2: StableOrd, T3: StableOrd> StableOrd for (T1, T2, T3) {
    const CAN_USE_UNSTABLE_SORT: bool =
        T1::CAN_USE_UNSTABLE_SORT && T2::CAN_USE_UNSTABLE_SORT && T3::CAN_USE_UNSTABLE_SORT;
}

impl<T1, T2, T3, T4, CTX> HashStable<CTX> for (T1, T2, T3, T4)
where
    T1: HashStable<CTX>,
    T2: HashStable<CTX>,
    T3: HashStable<CTX>,
    T4: HashStable<CTX>,
{
    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);
        _2.hash_stable(ctx, hasher);
        _3.hash_stable(ctx, hasher);
    }
}

unsafe impl<T1: StableOrd, T2: StableOrd, T3: StableOrd, T4: StableOrd> StableOrd
    for (T1, T2, T3, T4)
{
    const CAN_USE_UNSTABLE_SORT: bool = T1::CAN_USE_UNSTABLE_SORT
        && T2::CAN_USE_UNSTABLE_SORT
        && T3::CAN_USE_UNSTABLE_SORT
        && T4::CAN_USE_UNSTABLE_SORT;
}

impl<T: HashStable<CTX>, CTX> HashStable<CTX> for [T] {
    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);
        }
    }
}

impl<CTX> HashStable<CTX> for [u8] {
    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        self.len().hash_stable(ctx, hasher);
        hasher.write(self);
    }
}

impl<T: HashStable<CTX>, CTX> HashStable<CTX> for Vec<T> {
    #[inline]
    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        self[..].hash_stable(ctx, hasher);
    }
}

impl<K, V, R, CTX> HashStable<CTX> for indexmap::IndexMap<K, V, R>
where
    K: HashStable<CTX> + Eq + Hash,
    V: HashStable<CTX>,
    R: BuildHasher,
{
    #[inline]
    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);
        }
    }
}

impl<K, R, CTX> HashStable<CTX> for indexmap::IndexSet<K, R>
where
    K: HashStable<CTX> + Eq + Hash,
    R: BuildHasher,
{
    #[inline]
    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);
        }
    }
}

impl<A, const N: usize, CTX> HashStable<CTX> for SmallVec<[A; N]>
where
    A: HashStable<CTX>,
{
    #[inline]
    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(&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(&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(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        (**self).hash_stable(ctx, hasher);
    }
}

impl<CTX> HashStable<CTX> for str {
    #[inline]
    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        self.as_bytes().hash_stable(ctx, hasher);
    }
}

unsafe impl StableOrd for &str {
    const CAN_USE_UNSTABLE_SORT: bool = true;
}

impl<CTX> HashStable<CTX> for String {
    #[inline]
    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
        self[..].hash_stable(hcx, hasher);
    }
}

// Safety: String comparison only depends on their contents and the
// contents are not changed by (de-)serialization.
unsafe impl StableOrd for String {
    const CAN_USE_UNSTABLE_SORT: bool = true;
}

impl<HCX> ToStableHashKey<HCX> for String {
    type KeyType = String;
    #[inline]
    fn to_stable_hash_key(&self, _: &HCX) -> Self::KeyType {
        self.clone()
    }
}

impl<HCX, T1: ToStableHashKey<HCX>, T2: ToStableHashKey<HCX>> ToStableHashKey<HCX> for (T1, T2) {
    type KeyType = (T1::KeyType, T2::KeyType);
    #[inline]
    fn to_stable_hash_key(&self, hcx: &HCX) -> Self::KeyType {
        (self.0.to_stable_hash_key(hcx), self.1.to_stable_hash_key(hcx))
    }
}

impl<CTX> HashStable<CTX> for bool {
    #[inline]
    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        (if *self { 1u8 } else { 0u8 }).hash_stable(ctx, hasher);
    }
}

// Safety: sort order of bools is not changed by (de-)serialization.
unsafe impl StableOrd for bool {
    const CAN_USE_UNSTABLE_SORT: bool = true;
}

impl<T, CTX> HashStable<CTX> for Option<T>
where
    T: HashStable<CTX>,
{
    #[inline]
    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);
        } else {
            0u8.hash_stable(ctx, hasher);
        }
    }
}

// Safety: the Option wrapper does not add instability to comparison.
unsafe impl<T: StableOrd> StableOrd for Option<T> {
    const CAN_USE_UNSTABLE_SORT: bool = T::CAN_USE_UNSTABLE_SORT;
}

impl<T1, T2, CTX> HashStable<CTX> for Result<T1, T2>
where
    T1: HashStable<CTX>,
    T2: HashStable<CTX>,
{
    #[inline]
    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),
            Err(ref x) => x.hash_stable(ctx, hasher),
        }
    }
}

impl<'a, T, CTX> HashStable<CTX> for &'a T
where
    T: HashStable<CTX> + ?Sized,
{
    #[inline]
    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(&self, _: &mut CTX, hasher: &mut StableHasher) {
        ::std::hash::Hash::hash(self, hasher);
    }
}

impl<T, CTX> HashStable<CTX> for ::std::ops::RangeInclusive<T>
where
    T: HashStable<CTX>,
{
    #[inline]
    fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
        self.start().hash_stable(ctx, hasher);
        self.end().hash_stable(ctx, hasher);
    }
}

impl<I: Idx, T, CTX> HashStable<CTX> for IndexSlice<I, T>
where
    T: HashStable<CTX>,
{
    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);
        }
    }
}

impl<I: Idx, T, CTX> HashStable<CTX> for IndexVec<I, T>
where
    T: HashStable<CTX>,
{
    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);
        }
    }
}

impl<I: Idx, CTX> HashStable<CTX> for BitSet<I> {
    fn hash_stable(&self, _ctx: &mut CTX, hasher: &mut StableHasher) {
        ::std::hash::Hash::hash(self, hasher);
    }
}

impl<R: Idx, C: Idx, CTX> HashStable<CTX> for bit_set::BitMatrix<R, C> {
    fn hash_stable(&self, _ctx: &mut CTX, hasher: &mut StableHasher) {
        ::std::hash::Hash::hash(self, hasher);
    }
}

impl<T, CTX> HashStable<CTX> for bit_set::FiniteBitSet<T>
where
    T: HashStable<CTX> + bit_set::FiniteBitSetTy,
{
    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
        self.0.hash_stable(hcx, hasher);
    }
}

impl_stable_traits_for_trivial_type!(::std::path::Path);
impl_stable_traits_for_trivial_type!(::std::path::PathBuf);

impl<K, V, R, HCX> HashStable<HCX> for ::std::collections::HashMap<K, V, R>
where
    K: ToStableHashKey<HCX> + Eq,
    V: HashStable<HCX>,
    R: BuildHasher,
{
    #[inline]
    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
        stable_hash_reduce(hcx, hasher, self.iter(), self.len(), |hasher, hcx, (key, value)| {
            let key = key.to_stable_hash_key(hcx);
            key.hash_stable(hcx, hasher);
            value.hash_stable(hcx, hasher);
        });
    }
}

// It is not safe to implement HashStable for HashSet or any other collection type
// with unstable but observable iteration order.
// See /~https://github.com/rust-lang/compiler-team/issues/533 for further information.
impl<V, HCX> !HashStable<HCX> for std::collections::HashSet<V> {}

impl<K, V, HCX> HashStable<HCX> for ::std::collections::BTreeMap<K, V>
where
    K: HashStable<HCX> + StableOrd,
    V: HashStable<HCX>,
{
    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
        self.len().hash_stable(hcx, hasher);
        for entry in self.iter() {
            entry.hash_stable(hcx, hasher);
        }
    }
}

impl<K, HCX> HashStable<HCX> for ::std::collections::BTreeSet<K>
where
    K: HashStable<HCX> + StableOrd,
{
    fn hash_stable(&self, hcx: &mut HCX, hasher: &mut StableHasher) {
        self.len().hash_stable(hcx, hasher);
        for entry in self.iter() {
            entry.hash_stable(hcx, hasher);
        }
    }
}

fn stable_hash_reduce<HCX, I, C, F>(
    hcx: &mut HCX,
    hasher: &mut StableHasher,
    mut collection: C,
    length: usize,
    hash_function: F,
) where
    C: Iterator<Item = I>,
    F: Fn(&mut StableHasher, &mut HCX, I),
{
    length.hash_stable(hcx, hasher);

    match length {
        1 => {
            hash_function(hasher, hcx, collection.next().unwrap());
        }
        _ => {
            let hash = collection
                .map(|value| {
                    let mut hasher = StableHasher::new();
                    hash_function(&mut hasher, hcx, value);
                    hasher.finish::<Hash128>()
                })
                .reduce(|accum, value| accum.wrapping_add(value));
            hash.hash_stable(hcx, hasher);
        }
    }
}

/// Controls what data we do or do not hash.
/// Whenever a `HashStable` implementation caches its
/// result, it needs to include `HashingControls` as part
/// of the key, to ensure that it does not produce an incorrect
/// result (for example, using a `Fingerprint` produced while
/// hashing `Span`s when a `Fingerprint` without `Span`s is
/// being requested)
#[derive(Clone, Hash, Eq, PartialEq, Debug)]
pub struct HashingControls {
    pub hash_spans: bool,
}