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//! Code for projecting associated types out of trait references.
use super::PredicateObligation;
use crate::infer::InferCtxtUndoLogs;
use rustc_data_structures::{
snapshot_map::{self, SnapshotMapRef, SnapshotMapStorage},
undo_log::Rollback,
};
use rustc_middle::ty::{self, Ty};
pub use rustc_middle::traits::{EvaluationResult, Reveal};
pub(crate) type UndoLog<'tcx> =
snapshot_map::UndoLog<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>;
#[derive(Clone)]
pub struct MismatchedProjectionTypes<'tcx> {
pub err: ty::error::TypeError<'tcx>,
}
#[derive(Clone, TypeFoldable, TypeVisitable)]
pub struct Normalized<'tcx, T> {
pub value: T,
pub obligations: Vec<PredicateObligation<'tcx>>,
}
pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
impl<'tcx, T> Normalized<'tcx, T> {
pub fn with<U>(self, value: U) -> Normalized<'tcx, U> {
Normalized { value, obligations: self.obligations }
}
}
// # Cache
/// The projection cache. Unlike the standard caches, this can include
/// infcx-dependent type variables, therefore we have to roll the
/// cache back each time we roll a snapshot back, to avoid assumptions
/// on yet-unresolved inference variables. Types with placeholder
/// regions also have to be removed when the respective snapshot ends.
///
/// Because of that, projection cache entries can be "stranded" and left
/// inaccessible when type variables inside the key are resolved. We make no
/// attempt to recover or remove "stranded" entries, but rather let them be
/// (for the lifetime of the infcx).
///
/// Entries in the projection cache might contain inference variables
/// that will be resolved by obligations on the projection cache entry (e.g.,
/// when a type parameter in the associated type is constrained through
/// an "RFC 447" projection on the impl).
///
/// When working with a fulfillment context, the derived obligations of each
/// projection cache entry will be registered on the fulfillcx, so any users
/// that can wait for a fulfillcx fixed point need not care about this. However,
/// users that don't wait for a fixed point (e.g., trait evaluation) have to
/// resolve the obligations themselves to make sure the projected result is
/// ok and avoid issues like #43132.
///
/// If that is done, after evaluation the obligations, it is a good idea to
/// call `ProjectionCache::complete` to make sure the obligations won't be
/// re-evaluated and avoid an exponential worst-case.
//
// FIXME: we probably also want some sort of cross-infcx cache here to
// reduce the amount of duplication. Let's see what we get with the Chalk reforms.
pub struct ProjectionCache<'a, 'tcx> {
map: &'a mut SnapshotMapStorage<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
}
#[derive(Clone, Default)]
pub struct ProjectionCacheStorage<'tcx> {
map: SnapshotMapStorage<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
}
#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
pub struct ProjectionCacheKey<'tcx> {
ty: ty::ProjectionTy<'tcx>,
}
impl<'tcx> ProjectionCacheKey<'tcx> {
pub fn new(ty: ty::ProjectionTy<'tcx>) -> Self {
Self { ty }
}
}
#[derive(Clone, Debug)]
pub enum ProjectionCacheEntry<'tcx> {
InProgress,
Ambiguous,
Recur,
Error,
NormalizedTy {
ty: Normalized<'tcx, ty::Term<'tcx>>,
/// If we were able to successfully evaluate the
/// corresponding cache entry key during predicate
/// evaluation, then this field stores the final
/// result obtained from evaluating all of the projection
/// sub-obligations. During evaluation, we will skip
/// evaluating the cached sub-obligations in `ty`
/// if this field is set. Evaluation only
/// cares about the final result, so we don't
/// care about any region constraint side-effects
/// produced by evaluating the sub-boligations.
///
/// Additionally, we will clear out the sub-obligations
/// entirely if we ever evaluate the cache entry (along
/// with all its sub obligations) to `EvaluatedToOk`.
/// This affects all users of the cache, not just evaluation.
/// Since a result of `EvaluatedToOk` means that there were
/// no region obligations that need to be tracked, it's
/// fine to forget about the sub-obligations - they
/// don't provide any additional information. However,
/// we do *not* discard any obligations when we see
/// `EvaluatedToOkModuloRegions` - we don't know
/// which sub-obligations may introduce region constraints,
/// so we keep them all to be safe.
///
/// When we are not performing evaluation
/// (e.g. in `FulfillmentContext`), we ignore this field,
/// and always re-process the cached sub-obligations
/// (which may have been cleared out - see the above
/// paragraph).
/// This ensures that we do not lose any regions
/// constraints that arise from processing the
/// sub-obligations.
complete: Option<EvaluationResult>,
},
}
impl<'tcx> ProjectionCacheStorage<'tcx> {
#[inline]
pub(crate) fn with_log<'a>(
&'a mut self,
undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
) -> ProjectionCache<'a, 'tcx> {
ProjectionCache { map: &mut self.map, undo_log }
}
}
impl<'tcx> ProjectionCache<'_, 'tcx> {
#[inline]
fn map(
&mut self,
) -> SnapshotMapRef<
'_,
ProjectionCacheKey<'tcx>,
ProjectionCacheEntry<'tcx>,
InferCtxtUndoLogs<'tcx>,
> {
self.map.with_log(self.undo_log)
}
pub fn clear(&mut self) {
self.map().clear();
}
/// Try to start normalize `key`; returns an error if
/// normalization already occurred (this error corresponds to a
/// cache hit, so it's actually a good thing).
pub fn try_start(
&mut self,
key: ProjectionCacheKey<'tcx>,
) -> Result<(), ProjectionCacheEntry<'tcx>> {
let mut map = self.map();
if let Some(entry) = map.get(&key) {
return Err(entry.clone());
}
map.insert(key, ProjectionCacheEntry::InProgress);
Ok(())
}
/// Indicates that `key` was normalized to `value`.
pub fn insert_term(
&mut self,
key: ProjectionCacheKey<'tcx>,
value: Normalized<'tcx, ty::Term<'tcx>>,
) {
debug!(
"ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
key, value
);
let mut map = self.map();
if let Some(ProjectionCacheEntry::Recur) = map.get(&key) {
debug!("Not overwriting Recur");
return;
}
let fresh_key =
map.insert(key, ProjectionCacheEntry::NormalizedTy { ty: value, complete: None });
assert!(!fresh_key, "never started projecting `{:?}`", key);
}
/// Mark the relevant projection cache key as having its derived obligations
/// complete, so they won't have to be re-computed (this is OK to do in a
/// snapshot - if the snapshot is rolled back, the obligations will be
/// marked as incomplete again).
pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>, result: EvaluationResult) {
let mut map = self.map();
match map.get(&key) {
Some(&ProjectionCacheEntry::NormalizedTy { ref ty, complete: _ }) => {
info!("ProjectionCacheEntry::complete({:?}) - completing {:?}", key, ty);
let mut ty = ty.clone();
if result.must_apply_considering_regions() {
ty.obligations = vec![];
}
map.insert(key, ProjectionCacheEntry::NormalizedTy { ty, complete: Some(result) });
}
ref value => {
// Type inference could "strand behind" old cache entries. Leave
// them alone for now.
info!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}", key, value);
}
};
}
pub fn is_complete(&mut self, key: ProjectionCacheKey<'tcx>) -> Option<EvaluationResult> {
self.map().get(&key).and_then(|res| match res {
ProjectionCacheEntry::NormalizedTy { ty: _, complete } => *complete,
_ => None,
})
}
/// Indicates that trying to normalize `key` resulted in
/// ambiguity. No point in trying it again then until we gain more
/// type information (in which case, the "fully resolved" key will
/// be different).
pub fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
let fresh = self.map().insert(key, ProjectionCacheEntry::Ambiguous);
assert!(!fresh, "never started projecting `{:?}`", key);
}
/// Indicates that while trying to normalize `key`, `key` was required to
/// be normalized again. Selection or evaluation should eventually report
/// an error here.
pub fn recur(&mut self, key: ProjectionCacheKey<'tcx>) {
let fresh = self.map().insert(key, ProjectionCacheEntry::Recur);
assert!(!fresh, "never started projecting `{:?}`", key);
}
/// Indicates that trying to normalize `key` resulted in
/// error.
pub fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
let fresh = self.map().insert(key, ProjectionCacheEntry::Error);
assert!(!fresh, "never started projecting `{:?}`", key);
}
}
impl<'tcx> Rollback<UndoLog<'tcx>> for ProjectionCacheStorage<'tcx> {
fn reverse(&mut self, undo: UndoLog<'tcx>) {
self.map.reverse(undo);
}
}
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