compiler/rustc_data_structures/src/intern.rs

use crate::stable_hasher::{HashStable, StableHasher};
use std::cmp::Ordering;
use std::hash::{Hash, Hasher};
use std::ops::Deref;
use std::ptr;

use crate::fingerprint::Fingerprint;

mod private {
    #[derive(Clone, Copy, Debug)]
    pub struct PrivateZst;
}

/// A reference to a value that is interned, and is known to be unique.
///
/// Note that it is possible to have a `T` and a `Interned<T>` that are (or
/// refer to) equal but different values. But if you have two different
/// `Interned<T>`s, they both refer to the same value, at a single location in
/// memory. This means that equality and hashing can be done on the value's
/// address rather than the value's contents, which can improve performance.
///
/// The `PrivateZst` field means you can pattern match with `Interned(v, _)`
/// but you can only construct a `Interned` with `new_unchecked`, and not
/// directly.
#[derive(Debug)]
#[rustc_pass_by_value]
pub struct Interned<'a, T>(pub &'a T, pub private::PrivateZst);

impl<'a, T> Interned<'a, T> {
    /// Create a new `Interned` value. The value referred to *must* be interned
    /// and thus be unique, and it *must* remain unique in the future. This
    /// function has `_unchecked` in the name but is not `unsafe`, because if
    /// the uniqueness condition is violated condition it will cause incorrect
    /// behaviour but will not affect memory safety.
    #[inline]
    pub const fn new_unchecked(t: &'a T) -> Self {
        Interned(t, private::PrivateZst)
    }
}

impl<'a, T> Clone for Interned<'a, T> {
    fn clone(&self) -> Self {
        *self
    }
}

impl<'a, T> Copy for Interned<'a, T> {}

impl<'a, T> Deref for Interned<'a, T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        self.0
    }
}

impl<'a, T> PartialEq for Interned<'a, T> {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        // Pointer equality implies equality, due to the uniqueness constraint.
        ptr::eq(self.0, other.0)
    }
}

impl<'a, T> Eq for Interned<'a, T> {}

impl<'a, T: PartialOrd> PartialOrd for Interned<'a, T> {
    fn partial_cmp(&self, other: &Interned<'a, T>) -> Option<Ordering> {
        // Pointer equality implies equality, due to the uniqueness constraint,
        // but the contents must be compared otherwise.
        if ptr::eq(self.0, other.0) {
            Some(Ordering::Equal)
        } else {
            let res = self.0.partial_cmp(&other.0);
            debug_assert_ne!(res, Some(Ordering::Equal));
            res
        }
    }
}

impl<'a, T: Ord> Ord for Interned<'a, T> {
    fn cmp(&self, other: &Interned<'a, T>) -> Ordering {
        // Pointer equality implies equality, due to the uniqueness constraint,
        // but the contents must be compared otherwise.
        if ptr::eq(self.0, other.0) {
            Ordering::Equal
        } else {
            let res = self.0.cmp(&other.0);
            debug_assert_ne!(res, Ordering::Equal);
            res
        }
    }
}

impl<'a, T> Hash for Interned<'a, T> {
    #[inline]
    fn hash<H: Hasher>(&self, s: &mut H) {
        // Pointer hashing is sufficient, due to the uniqueness constraint.
        ptr::hash(self.0, s)
    }
}

impl<T, CTX> HashStable<CTX> for Interned<'_, T>
where
    T: HashStable<CTX>,
{
    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
        self.0.hash_stable(hcx, hasher);
    }
}

/// A helper trait so that `Interned` things can cache stable hashes reproducibly.
pub trait InternedHashingContext {
    fn with_def_path_and_no_spans(&mut self, f: impl FnOnce(&mut Self));
}

/// A helper type that you can wrap round your own type in order to automatically
/// cache the stable hash on creation and not recompute it whenever the stable hash
/// of the type is computed.
/// This is only done in incremental mode. You can also opt out of caching by using
/// StableHash::ZERO for the hash, in which case the hash gets computed each time.
/// This is useful if you have values that you intern but never (can?) use for stable
/// hashing.
#[derive(Copy, Clone)]
pub struct WithStableHash<T> {
    pub internee: T,
    pub stable_hash: Fingerprint,
}

impl<T: PartialEq> PartialEq for WithStableHash<T> {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        self.internee.eq(&other.internee)
    }
}

impl<T: Eq> Eq for WithStableHash<T> {}

impl<T: Ord> PartialOrd for WithStableHash<T> {
    fn partial_cmp(&self, other: &WithStableHash<T>) -> Option<Ordering> {
        Some(self.internee.cmp(&other.internee))
    }
}

impl<T: Ord> Ord for WithStableHash<T> {
    fn cmp(&self, other: &WithStableHash<T>) -> Ordering {
        self.internee.cmp(&other.internee)
    }
}

impl<T> Deref for WithStableHash<T> {
    type Target = T;

    #[inline]
    fn deref(&self) -> &T {
        &self.internee
    }
}

impl<T: Hash> Hash for WithStableHash<T> {
    #[inline]
    fn hash<H: Hasher>(&self, s: &mut H) {
        self.internee.hash(s)
    }
}

impl<T: HashStable<CTX>, CTX: InternedHashingContext> HashStable<CTX> for WithStableHash<T> {
    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
        if self.stable_hash == Fingerprint::ZERO || cfg!(debug_assertions) {
            // No cached hash available. This can only mean that incremental is disabled.
            // We don't cache stable hashes in non-incremental mode, because they are used
            // so rarely that the performance actually suffers.

            // We need to build the hash as if we cached it and then hash that hash, as
            // otherwise the hashes will differ between cached and non-cached mode.
            let stable_hash: Fingerprint = {
                let mut hasher = StableHasher::new();
                hcx.with_def_path_and_no_spans(|hcx| self.internee.hash_stable(hcx, &mut hasher));
                hasher.finish()
            };
            if cfg!(debug_assertions) && self.stable_hash != Fingerprint::ZERO {
                assert_eq!(
                    stable_hash, self.stable_hash,
                    "cached stable hash does not match freshly computed stable hash"
                );
            }
            stable_hash.hash_stable(hcx, hasher);
        } else {
            self.stable_hash.hash_stable(hcx, hasher);
        }
    }
}

#[cfg(test)]
mod tests;