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implement the skeleton of the updated trait solver

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This commit is contained in:
lcnr 2022-12-04 03:19:10 +00:00
parent 4653c93e44
commit a213bb36c9
21 changed files with 1569 additions and 87 deletions
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6
compiler/rustc_infer/src/infer/canonical/query_response.rs
View file

@ -151,7 +151,11 @@ fn make_query_response<T>(
})
}
fn take_opaque_types_for_query_response(&self) -> Vec<(Ty<'tcx>, Ty<'tcx>)> {
/// FIXME: This method should only be used for canonical queries and therefore be private.
///
/// As the new solver does canonicalization slightly differently, this is also used there
/// for now. This should hopefully change fairly soon.
pub fn take_opaque_types_for_query_response(&self) -> Vec<(Ty<'tcx>, Ty<'tcx>)> {
self.inner
.borrow_mut()
.opaque_type_storage

10
compiler/rustc_middle/src/infer/canonical.rs
View file

@ -300,6 +300,16 @@ pub fn unchecked_map<W>(self, map_op: impl FnOnce(V) -> W) -> Canonical<'tcx, W>
let Canonical { max_universe, variables, value } = self;
Canonical { max_universe, variables, value: map_op(value) }
}
/// Allows you to map the `value` of a canonical while keeping the same set of
/// bound variables.
///
/// **WARNING:** This function is very easy to mis-use, hence the name! See
/// the comment of [Canonical::unchecked_map] for more details.
pub fn unchecked_rebind<W>(self, value: W) -> Canonical<'tcx, W> {
let Canonical { max_universe, variables, value: _ } = self;
Canonical { max_universe, variables, value }
}
}
pub type QueryOutlivesConstraint<'tcx> = (

2
compiler/rustc_middle/src/traits/query.rs
View file

@ -96,7 +96,7 @@ pub fn new(value: T) -> Self {
pub type CanonicalTypeOpNormalizeGoal<'tcx, T> =
Canonical<'tcx, ty::ParamEnvAnd<'tcx, type_op::Normalize<T>>>;
#[derive(Copy, Clone, Debug, HashStable)]
#[derive(Copy, Clone, Debug, HashStable, PartialEq, Eq)]
pub struct NoSolution;
pub type Fallible<T> = Result<T, NoSolution>;

1
compiler/rustc_middle/src/traits/specialization_graph.rs
View file

@ -180,6 +180,7 @@ fn next(&mut self) -> Option<Node> {
}
/// Information about the most specialized definition of an associated item.
#[derive(Debug)]
pub struct LeafDef {
/// The associated item described by this `LeafDef`.
pub item: ty::AssocItem,

41
compiler/rustc_middle/src/ty/mod.rs
View file

@ -535,6 +535,17 @@ pub fn without_const(mut self, tcx: TyCtxt<'tcx>) -> Self {
self
}
#[instrument(level = "debug", skip(tcx), ret)]
pub fn is_coinductive(self, tcx: TyCtxt<'tcx>) -> bool {
match self.kind().skip_binder() {
ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
tcx.trait_is_coinductive(data.def_id())
}
ty::PredicateKind::WellFormed(_) => true,
_ => false,
}
}
/// Whether this projection can be soundly normalized.
///
/// Wf predicates must not be normalized, as normalization
@ -1018,6 +1029,24 @@ pub struct ProjectionPredicate<'tcx> {
pub term: Term<'tcx>,
}
impl<'tcx> ProjectionPredicate<'tcx> {
pub fn self_ty(self) -> Ty<'tcx> {
self.projection_ty.self_ty()
}
pub fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> ProjectionPredicate<'tcx> {
Self { projection_ty: self.projection_ty.with_self_ty(tcx, self_ty), ..self }
}
pub fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
self.projection_ty.trait_def_id(tcx)
}
pub fn def_id(self) -> DefId {
self.projection_ty.def_id
}
}
pub type PolyProjectionPredicate<'tcx> = Binder<'tcx, ProjectionPredicate<'tcx>>;
impl<'tcx> PolyProjectionPredicate<'tcx> {
@ -1054,18 +1083,6 @@ pub fn projection_def_id(&self) -> DefId {
}
}
impl<'tcx> ProjectionPredicate<'tcx> {
pub fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
Self {
projection_ty: tcx.mk_alias_ty(
self.projection_ty.def_id,
[self_ty.into()].into_iter().chain(self.projection_ty.substs.iter().skip(1)),
),
..self
}
}
}
pub trait ToPolyTraitRef<'tcx> {
fn to_poly_trait_ref(&self) -> PolyTraitRef<'tcx>;
}

12
compiler/rustc_middle/src/ty/sty.rs
View file

@ -1169,7 +1169,7 @@ pub struct AliasTy<'tcx> {
}
impl<'tcx> AliasTy<'tcx> {
pub fn trait_def_id(&self, tcx: TyCtxt<'tcx>) -> DefId {
pub fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId {
match tcx.def_kind(self.def_id) {
DefKind::AssocTy | DefKind::AssocConst => tcx.parent(self.def_id),
DefKind::ImplTraitPlaceholder => {
@ -1183,7 +1183,7 @@ pub fn trait_def_id(&self, tcx: TyCtxt<'tcx>) -> DefId {
/// For example, if this is a projection of `<T as StreamingIterator>::Item<'a>`,
/// then this function would return a `T: Iterator` trait reference and `['a]` as the own substs
pub fn trait_ref_and_own_substs(
&self,
self,
tcx: TyCtxt<'tcx>,
) -> (ty::TraitRef<'tcx>, &'tcx [ty::GenericArg<'tcx>]) {
debug_assert!(matches!(tcx.def_kind(self.def_id), DefKind::AssocTy | DefKind::AssocConst));
@ -1202,14 +1202,18 @@ pub fn trait_ref_and_own_substs(
/// WARNING: This will drop the substs for generic associated types
/// consider calling [Self::trait_ref_and_own_substs] to get those
/// as well.
pub fn trait_ref(&self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
pub fn trait_ref(self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx> {
let def_id = self.trait_def_id(tcx);
tcx.mk_trait_ref(def_id, self.substs.truncate_to(tcx, tcx.generics_of(def_id)))
}
pub fn self_ty(&self) -> Ty<'tcx> {
pub fn self_ty(self) -> Ty<'tcx> {
self.substs.type_at(0)
}
pub fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
tcx.mk_alias_ty(self.def_id, [self_ty.into()].into_iter().chain(self.substs.iter().skip(1)))
}
}
#[derive(Copy, Clone, Debug, TypeFoldable, TypeVisitable, Lift)]

4
compiler/rustc_middle/src/ty/subst.rs
View file

@ -573,6 +573,10 @@ pub fn try_map_bound<F, U, E>(self, f: F) -> Result<EarlyBinder<U>, E>
pub fn rebind<U>(&self, value: U) -> EarlyBinder<U> {
EarlyBinder(value)
}
pub fn skip_binder(self) -> T {
self.0
}
}
impl<T> EarlyBinder<Option<T>> {

2
compiler/rustc_trait_selection/src/lib.rs
View file

@ -19,6 +19,7 @@
#![feature(let_chains)]
#![feature(if_let_guard)]
#![feature(never_type)]
#![feature(result_option_inspect)]
#![feature(type_alias_impl_trait)]
#![recursion_limit = "512"] // For rustdoc
@ -37,4 +38,5 @@
pub mod autoderef;
pub mod errors;
pub mod infer;
pub mod solve;
pub mod traits;

257
compiler/rustc_trait_selection/src/solve/cache.rs Normal file
View file

@ -0,0 +1,257 @@
//! This module both handles the global cache which stores "finished" goals,
//! and the provisional cache which contains partially computed goals.
//!
//! The provisional cache is necessary when dealing with coinductive cycles.
//!
//! For more information about the provisional cache and coinduction in general,
//! check out the relevant section of the rustc-dev-guide.
//!
//! FIXME(@lcnr): Write that section, feel free to ping me if you need help here
//! before then or if I still haven't done that before January 2023.
use super::overflow::OverflowData;
use super::CanonicalGoal;
use super::{EvalCtxt, QueryResult};
use rustc_data_structures::fx::FxHashMap;
use rustc_middle::ty::TyCtxt;
use std::{cmp::Ordering, collections::hash_map::Entry};
#[derive(Debug, Clone)]
struct ProvisionalEntry<'tcx> {
// In case we have a coinductive cycle, this is the
// the currently least restrictive result of this goal.
response: QueryResult<'tcx>,
// The lowest element on the stack on which this result
// relies on. Starts out as just being the depth at which
// we've proven this obligation, but gets lowered to the
// depth of another goal if we rely on it in a cycle.
depth: usize,
}
struct StackElem<'tcx> {
goal: CanonicalGoal<'tcx>,
has_been_used: bool,
}
/// The cache used for goals which are currently in progress or which depend
/// on in progress results.
///
/// Once we're done with a goal we can store it in the global trait solver
/// cache of the `TyCtxt`. For goals which we're currently proving, or which
/// have only been proven via a coinductive cycle using a goal still on our stack
/// we have to use this separate data structure.
///
/// The current data structure is not perfect, so there may still be room for
/// improvement here. We have the following requirements:
///
/// ## Is there is a provisional entry for the given goal:
///
/// ```ignore (for syntax highlighting)
/// self.entries.get(goal)
/// ```
///
/// ## Get all goals on the stack involved in a cycle:
///
/// ```ignore (for syntax highlighting)
/// let entry = self.entries.get(goal).unwrap();
/// let involved_goals = self.stack.iter().skip(entry.depth);
/// ```
///
/// ## Capping the depth of all entries
///
/// Needed whenever we encounter a cycle. The current implementation always
/// iterates over all entries instead of only the ones with a larger depth.
/// Changing this may result in notable performance improvements.
///
/// ```ignore (for syntax highlighting)
/// let cycle_depth = self.entries.get(goal).unwrap().depth;
/// for e in &mut self.entries {
/// e.depth = e.depth.min(cycle_depth);
/// }
/// ```
///
/// ## Checking whether we have to rerun the current goal
///
/// A goal has to be rerun if its provisional result was used in a cycle
/// and that result is different from its final result. We update
/// [StackElem::has_been_used] for the deepest stack element involved in a cycle.
///
/// ## Moving all finished goals into the global cache
///
/// If `stack_elem.has_been_used` is true, iterate over all entries, moving the ones
/// with equal depth. If not, simply move this single entry.
pub(super) struct ProvisionalCache<'tcx> {
stack: Vec<StackElem<'tcx>>,
entries: FxHashMap<CanonicalGoal<'tcx>, ProvisionalEntry<'tcx>>,
}
impl<'tcx> ProvisionalCache<'tcx> {
pub(super) fn empty() -> ProvisionalCache<'tcx> {
ProvisionalCache { stack: Vec::new(), entries: Default::default() }
}
pub(super) fn current_depth(&self) -> usize {
self.stack.len()
}
}
impl<'tcx> EvalCtxt<'tcx> {
/// Tries putting the new goal on the stack, returning an error if it is already cached.
///
/// This correctly updates the provisional cache if there is a cycle.
pub(super) fn try_push_stack(
&mut self,
goal: CanonicalGoal<'tcx>,
) -> Result<(), QueryResult<'tcx>> {
// FIXME: start by checking the global cache
// Look at the provisional cache to check for cycles.
let cache = &mut self.provisional_cache;
match cache.entries.entry(goal) {
// No entry, simply push this goal on the stack after dealing with overflow.
Entry::Vacant(v) => {
if self.overflow_data.has_overflow(cache.stack.len()) {
return Err(self.deal_with_overflow());
}
v.insert(ProvisionalEntry {
response: fixme_response_yes_no_constraints(),
depth: cache.stack.len(),
});
cache.stack.push(StackElem { goal, has_been_used: false });
Ok(())
}
// We have a nested goal which relies on a goal `root` deeper in the stack.
//
// We first store that we may have to rerun `evaluate_goal` for `root` in case the
// provisional response is not equal to the final response. We also update the depth
// of all goals which recursively depend on our current goal to depend on `root`
// instead.
//
// Finally we can return either the provisional response for that goal if we have a
// coinductive cycle or an ambiguous result if the cycle is inductive.
Entry::Occupied(entry) => {
// FIXME: `ProvisionalEntry` should be `Copy`.
let entry = entry.get().clone();
cache.stack[entry.depth].has_been_used = true;
for provisional_entry in cache.entries.values_mut() {
provisional_entry.depth = provisional_entry.depth.min(entry.depth);
}
// NOTE: The goals on the stack aren't the only goals involved in this cycle.
// We can also depend on goals which aren't part of the stack but coinductively
// depend on the stack themselves. We already checked whether all the goals
// between these goals and their root on the stack. This means that as long as
// each goal in a cycle is checked for coinductivity by itself simply checking
// the stack is enough.
if cache.stack[entry.depth..]
.iter()
.all(|g| g.goal.value.predicate.is_coinductive(self.tcx))
{
Err(entry.response)
} else {
Err(fixme_response_maybe_no_constraints())
}
}
}
}
/// We cannot simply store the result of [EvalCtxt::compute_goal] as we have to deal with
/// coinductive cycles.
///
/// When we encounter a coinductive cycle, we have to prove the final result of that cycle
/// while we are still computing that result. Because of this we continously recompute the
/// cycle until the result of the previous iteration is equal to the final result, at which
/// point we are done.
///
/// This function returns `true` if we were able to finalize the goal and `false` if it has
/// updated the provisional cache and we have to recompute the current goal.
///
/// FIXME: Refer to the rustc-dev-guide entry once it exists.
pub(super) fn try_finalize_goal(
&mut self,
actual_goal: CanonicalGoal<'tcx>,
response: QueryResult<'tcx>,
) -> bool {
let cache = &mut self.provisional_cache;
let StackElem { goal, has_been_used } = cache.stack.pop().unwrap();
assert_eq!(goal, actual_goal);
let provisional_entry = cache.entries.get_mut(&goal).unwrap();
// Check whether the current stack entry is the root of a cycle.
//
// If so, we either move all participants of that cycle to the global cache
// or, in case the provisional response used in the cycle is not equal to the
// final response, have to recompute the goal after updating the provisional
// response to the final response of this iteration.
if has_been_used {
if provisional_entry.response == response {
// We simply drop all entries according to an immutable condition, so
// query instability is not a concern here.
#[allow(rustc::potential_query_instability)]
cache.entries.retain(|goal, entry| match entry.depth.cmp(&cache.stack.len()) {
Ordering::Less => true,
Ordering::Equal => {
Self::try_move_finished_goal_to_global_cache(
self.tcx,
&mut self.overflow_data,
&cache.stack,
// FIXME: these should be `Copy` :(
goal.clone(),
entry.response.clone(),
);
false
}
Ordering::Greater => bug!("entry with greater depth than the current leaf"),
});
true
} else {
provisional_entry.response = response;
cache.stack.push(StackElem { goal, has_been_used: false });
false
}
} else {
Self::try_move_finished_goal_to_global_cache(
self.tcx,
&mut self.overflow_data,
&cache.stack,
goal,
response,
);
cache.entries.remove(&goal);
true
}
}
fn try_move_finished_goal_to_global_cache(
tcx: TyCtxt<'tcx>,
overflow_data: &mut OverflowData,
stack: &[StackElem<'tcx>],
goal: CanonicalGoal<'tcx>,
response: QueryResult<'tcx>,
) {
// We move goals to the global cache if we either did not hit an overflow or if it's
// the root goal as that will now always hit the same overflow limit.
//
// NOTE: We cannot move any non-root goals to the global cache even if their final result
// isn't impacted by the overflow as that goal still has unstable query dependencies
// because it didn't go its full depth.
//
// FIXME(@lcnr): We could still cache subtrees which are not impacted by overflow though.
// Tracking that info correctly isn't trivial, so I haven't implemented it for now.
let should_cache_globally = !overflow_data.did_overflow() || stack.is_empty();
if should_cache_globally {
// FIXME: move the provisional entry to the global cache.
let _ = (tcx, goal, response);
}
}
}
fn fixme_response_yes_no_constraints<'tcx>() -> QueryResult<'tcx> {
unimplemented!()
}
fn fixme_response_maybe_no_constraints<'tcx>() -> QueryResult<'tcx> {
unimplemented!()
}

92
compiler/rustc_trait_selection/src/solve/fulfill.rs Normal file
View file

@ -0,0 +1,92 @@
use std::mem;
use rustc_data_structures::fx::FxHashMap;
use rustc_infer::{
infer::InferCtxt,
traits::{query::NoSolution, FulfillmentError, PredicateObligation, TraitEngine},
};
use rustc_middle::ty;
use super::{Certainty, EvalCtxt};
/// A trait engine using the new trait solver.
///
/// This is mostly identical to how `evaluate_all` works inside of the
/// solver, except that the requirements are slightly different.
///
/// Unlike `evaluate_all` it is possible to add new obligations later on
/// and we also have to track diagnostics information by using `Obligation`
/// instead of `Goal`.
///
/// It is also likely that we want to use slightly different datastructures
/// here as this will have to deal with far more root goals than `evaluate_all`.
pub struct FulfillmentCtxt<'tcx> {
obligations: Vec<PredicateObligation<'tcx>>,
}
impl<'tcx> FulfillmentCtxt<'tcx> {
pub fn new() -> FulfillmentCtxt<'tcx> {
FulfillmentCtxt { obligations: Vec::new() }
}
}
impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
fn register_predicate_obligation(
&mut self,
_infcx: &InferCtxt<'tcx>,
obligation: PredicateObligation<'tcx>,
) {
self.obligations.push(obligation);
}
fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
let errors = self.select_where_possible(infcx);
if !errors.is_empty() {
return errors;
}
if self.obligations.is_empty() {
Vec::new()
} else {
unimplemented!("ambiguous obligations")
}
}
fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
let errors = Vec::new();
for i in 0.. {
if !infcx.tcx.recursion_limit().value_within_limit(i) {
unimplemented!("overflow")
}
let mut has_changed = false;
for o in mem::take(&mut self.obligations) {
let mut cx = EvalCtxt::new(infcx.tcx);
let (changed, certainty) = match cx.evaluate_goal(infcx, o.clone().into()) {
Ok(result) => result,
Err(NoSolution) => unimplemented!("error"),
};
has_changed |= changed;
match certainty {
Certainty::Yes => {}
Certainty::Maybe(_) => self.obligations.push(o),
}
}
if !has_changed {
break;
}
}
errors
}
fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
self.obligations.clone()
}
fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
unimplemented!("Should be moved out of `TraitEngine`")
}
}

55
compiler/rustc_trait_selection/src/solve/infcx_ext.rs Normal file
View file

@ -0,0 +1,55 @@
use rustc_infer::infer::canonical::CanonicalVarValues;
use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use rustc_infer::infer::InferCtxt;
use rustc_infer::traits::query::NoSolution;
use rustc_middle::ty::Ty;
use rustc_span::DUMMY_SP;
use crate::solve::ExternalConstraints;
use super::{Certainty, QueryResult, Response};
/// Methods used inside of the canonical queries of the solver.
pub(super) trait InferCtxtExt<'tcx> {
fn next_ty_infer(&self) -> Ty<'tcx>;
fn make_canonical_response(
&self,
var_values: CanonicalVarValues<'tcx>,
certainty: Certainty,
) -> QueryResult<'tcx>;
}
impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
fn next_ty_infer(&self) -> Ty<'tcx> {
self.next_ty_var(TypeVariableOrigin {
kind: TypeVariableOriginKind::MiscVariable,
span: DUMMY_SP,
})
}
fn make_canonical_response(
&self,
var_values: CanonicalVarValues<'tcx>,
certainty: Certainty,
) -> QueryResult<'tcx> {
let external_constraints = take_external_constraints(self)?;
Ok(self.canonicalize_response(Response { var_values, external_constraints, certainty }))
}
}
#[instrument(level = "debug", skip(infcx), ret)]
fn take_external_constraints<'tcx>(
infcx: &InferCtxt<'tcx>,
) -> Result<ExternalConstraints<'tcx>, NoSolution> {
let region_obligations = infcx.take_registered_region_obligations();
let opaque_types = infcx.take_opaque_types_for_query_response();
Ok(ExternalConstraints {
// FIXME: Now that's definitely wrong :)
//
// Should also do the leak check here I think
regions: drop(region_obligations),
opaque_types,
})
}

308
compiler/rustc_trait_selection/src/solve/mod.rs Normal file
View file

@ -0,0 +1,308 @@
//! The new trait solver, currently still WIP.
//!
//! As a user of the trait system, you can use `TyCtxt::evaluate_goal` to
//! interact with this solver.
//!
//! For a high-level overview of how this solver works, check out the relevant
//! section of the rustc-dev-guide.
//!
//! FIXME(@lcnr): Write that section. If you read this before then ask me
//! about it on zulip.
// FIXME: Instead of using `infcx.canonicalize_query` we have to add a new routine which
// preserves universes and creates a unique var (in the highest universe) for each
// appearance of a region.
// FIXME: `CanonicalVarValues` should be interned and `Copy`.
// FIXME: uses of `infcx.at` need to enable deferred projection equality once that's implemented.
use std::mem;
use rustc_infer::infer::canonical::OriginalQueryValues;
use rustc_infer::infer::{InferCtxt, TyCtxtInferExt};
use rustc_infer::traits::query::NoSolution;
use rustc_infer::traits::Obligation;
use rustc_middle::infer::canonical::{Canonical, CanonicalVarValues};
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::ty::{RegionOutlivesPredicate, ToPredicate, TypeOutlivesPredicate};
use rustc_span::DUMMY_SP;
use self::infcx_ext::InferCtxtExt;
mod cache;
mod fulfill;
mod infcx_ext;
mod overflow;
mod project_goals;
mod trait_goals;
pub use fulfill::FulfillmentCtxt;
/// A goal is a statement, i.e. `predicate`, we want to prove
/// given some assumptions, i.e. `param_env`.
///
/// Most of the time the `param_env` contains the `where`-bounds of the function
/// we're currently typechecking while the `predicate` is some trait bound.
#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
pub struct Goal<'tcx, P> {
param_env: ty::ParamEnv<'tcx>,
predicate: P,
}
impl<'tcx, P> Goal<'tcx, P> {
pub fn new(
tcx: TyCtxt<'tcx>,
param_env: ty::ParamEnv<'tcx>,
predicate: impl ToPredicate<'tcx, P>,
) -> Goal<'tcx, P> {
Goal { param_env, predicate: predicate.to_predicate(tcx) }
}
/// Updates the goal to one with a different `predicate` but the same `param_env`.
fn with<Q>(self, tcx: TyCtxt<'tcx>, predicate: impl ToPredicate<'tcx, Q>) -> Goal<'tcx, Q> {
Goal { param_env: self.param_env, predicate: predicate.to_predicate(tcx) }
}
}
impl<'tcx, P> From<Obligation<'tcx, P>> for Goal<'tcx, P> {
fn from(obligation: Obligation<'tcx, P>) -> Goal<'tcx, P> {
Goal { param_env: obligation.param_env, predicate: obligation.predicate }
}
}
#[derive(Debug, PartialEq, Eq, Clone, Hash, TypeFoldable, TypeVisitable)]
pub struct Response<'tcx> {
pub var_values: CanonicalVarValues<'tcx>,
/// Additional constraints returned by this query.
pub external_constraints: ExternalConstraints<'tcx>,
pub certainty: Certainty,
}
#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
pub enum Certainty {
Yes,
Maybe(MaybeCause),
}
impl Certainty {
/// When proving multiple goals using **AND**, e.g. nested obligations for an impl,
/// use this function to unify the certainty of these goals
pub fn unify_and(self, other: Certainty) -> Certainty {
match (self, other) {
(Certainty::Yes, Certainty::Yes) => Certainty::Yes,
(Certainty::Yes, Certainty::Maybe(_)) => other,
(Certainty::Maybe(_), Certainty::Yes) => self,
(Certainty::Maybe(MaybeCause::Overflow), Certainty::Maybe(MaybeCause::Overflow)) => {
Certainty::Maybe(MaybeCause::Overflow)
}
// If at least one of the goals is ambiguous, hide the overflow as the ambiguous goal
// may still result in failure.
(Certainty::Maybe(MaybeCause::Ambiguity), Certainty::Maybe(_))
| (Certainty::Maybe(_), Certainty::Maybe(MaybeCause::Ambiguity)) => {
Certainty::Maybe(MaybeCause::Ambiguity)
}
}
}
}
/// Why we failed to evaluate a goal.
#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash, TypeFoldable, TypeVisitable)]
pub enum MaybeCause {
/// We failed due to ambiguity. This ambiguity can either
/// be a true ambiguity, i.e. there are multiple different answers,
/// or we hit a case where we just don't bother, e.g. `?x: Trait` goals.
Ambiguity,
/// We gave up due to an overflow, most often by hitting the recursion limit.
Overflow,
}
/// Additional constraints returned on success.
#[derive(Debug, PartialEq, Eq, Clone, Hash, TypeFoldable, TypeVisitable)]
pub struct ExternalConstraints<'tcx> {
// FIXME: implement this.
regions: (),
opaque_types: Vec<(Ty<'tcx>, Ty<'tcx>)>,
}
type CanonicalGoal<'tcx, T = ty::Predicate<'tcx>> = Canonical<'tcx, Goal<'tcx, T>>;
type CanonicalResponse<'tcx> = Canonical<'tcx, Response<'tcx>>;
/// The result of evaluating a canonical query.
///
/// FIXME: We use a different type than the existing canonical queries. This is because
/// we need to add a `Certainty` for `overflow` and may want to restructure this code without
/// having to worry about changes to currently used code. Once we've made progress on this
/// solver, merge the two responses again.
pub type QueryResult<'tcx> = Result<CanonicalResponse<'tcx>, NoSolution>;
pub trait TyCtxtExt<'tcx> {
fn evaluate_goal(self, goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx>;
}
impl<'tcx> TyCtxtExt<'tcx> for TyCtxt<'tcx> {
fn evaluate_goal(self, goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx> {
let mut cx = EvalCtxt::new(self);
cx.evaluate_canonical_goal(goal)
}
}
struct EvalCtxt<'tcx> {
tcx: TyCtxt<'tcx>,
provisional_cache: cache::ProvisionalCache<'tcx>,
overflow_data: overflow::OverflowData,
}
impl<'tcx> EvalCtxt<'tcx> {
fn new(tcx: TyCtxt<'tcx>) -> EvalCtxt<'tcx> {
EvalCtxt {
tcx,
provisional_cache: cache::ProvisionalCache::empty(),
overflow_data: overflow::OverflowData::new(tcx),
}
}
/// Recursively evaluates `goal`, returning whether any inference vars have
/// been constrained and the certainty of the result.
fn evaluate_goal(
&mut self,
infcx: &InferCtxt<'tcx>,
goal: Goal<'tcx, ty::Predicate<'tcx>>,
) -> Result<(bool, Certainty), NoSolution> {
let mut orig_values = OriginalQueryValues::default();
let canonical_goal = infcx.canonicalize_query(goal, &mut orig_values);
let canonical_response = self.evaluate_canonical_goal(canonical_goal)?;
Ok((
true, // FIXME: check whether `var_values` are an identity substitution.
fixme_instantiate_canonical_query_response(infcx, &orig_values, canonical_response),
))
}
fn evaluate_canonical_goal(&mut self, goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx> {
match self.try_push_stack(goal) {
Ok(()) => {}
// Our goal is already on the stack, eager return.
Err(response) => return response,
}
// We may have to repeatedly recompute the goal in case of coinductive cycles,
// check out the `cache` module for more information.
//
// FIXME: Similar to `evaluate_all`, this has to check for overflow.
loop {
let result = self.compute_goal(goal);
// FIXME: `Response` should be `Copy`
if self.try_finalize_goal(goal, result.clone()) {
return result;
}
}
}
fn compute_goal(&mut self, canonical_goal: CanonicalGoal<'tcx>) -> QueryResult<'tcx> {
// WARNING: We're looking at a canonical value without instantiating it here.
//
// We have to be incredibly careful to not change the order of bound variables or
// remove any. As we go from `Goal<'tcx, Predicate>` to `Goal` with the variants
// of `PredicateKind` this is the case and it is and faster than instantiating and
// recanonicalizing.
let Goal { param_env, predicate } = canonical_goal.value;
if let Some(kind) = predicate.kind().no_bound_vars() {
match kind {
ty::PredicateKind::Clause(ty::Clause::Trait(predicate)) => self.compute_trait_goal(
canonical_goal.unchecked_rebind(Goal { param_env, predicate }),
),
ty::PredicateKind::Clause(ty::Clause::Projection(predicate)) => self
.compute_projection_goal(
canonical_goal.unchecked_rebind(Goal { param_env, predicate }),
),
ty::PredicateKind::Clause(ty::Clause::TypeOutlives(predicate)) => self
.compute_type_outlives_goal(
canonical_goal.unchecked_rebind(Goal { param_env, predicate }),
),
ty::PredicateKind::Clause(ty::Clause::RegionOutlives(predicate)) => self
.compute_region_outlives_goal(
canonical_goal.unchecked_rebind(Goal { param_env, predicate }),
),
// FIXME: implement these predicates :)
ty::PredicateKind::WellFormed(_)
| ty::PredicateKind::ObjectSafe(_)
| ty::PredicateKind::ClosureKind(_, _, _)
| ty::PredicateKind::Subtype(_)
| ty::PredicateKind::Coerce(_)
| ty::PredicateKind::ConstEvaluatable(_)
| ty::PredicateKind::ConstEquate(_, _)
| ty::PredicateKind::TypeWellFormedFromEnv(_)
| ty::PredicateKind::Ambiguous => unimplemented!(),
}
} else {
let (infcx, goal, var_values) =
self.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &canonical_goal);
let kind = infcx.replace_bound_vars_with_placeholders(goal.predicate.kind());
let goal = goal.with(self.tcx, ty::Binder::dummy(kind));
let (_, certainty) = self.evaluate_goal(&infcx, goal)?;
infcx.make_canonical_response(var_values, certainty)
}
}
fn compute_type_outlives_goal(
&mut self,
_goal: CanonicalGoal<'tcx, TypeOutlivesPredicate<'tcx>>,
) -> QueryResult<'tcx> {
todo!()
}
fn compute_region_outlives_goal(
&mut self,
_goal: CanonicalGoal<'tcx, RegionOutlivesPredicate<'tcx>>,
) -> QueryResult<'tcx> {
todo!()
}
}
impl<'tcx> EvalCtxt<'tcx> {
fn evaluate_all(
&mut self,
infcx: &InferCtxt<'tcx>,
mut goals: Vec<Goal<'tcx, ty::Predicate<'tcx>>>,
) -> Result<Certainty, NoSolution> {
let mut new_goals = Vec::new();
self.repeat_while_none(|this| {
let mut has_changed = Err(Certainty::Yes);
for goal in goals.drain(..) {
let (changed, certainty) = match this.evaluate_goal(infcx, goal) {
Ok(result) => result,
Err(NoSolution) => return Some(Err(NoSolution)),
};
if changed {
has_changed = Ok(());
}
match certainty {
Certainty::Yes => {}
Certainty::Maybe(_) => {
new_goals.push(goal);
has_changed = has_changed.map_err(|c| c.unify_and(certainty));
}
}
}
match has_changed {
Ok(()) => {
mem::swap(&mut new_goals, &mut goals);
None
}
Err(certainty) => Some(Ok(certainty)),
}
})
}
}
fn fixme_instantiate_canonical_query_response<'tcx>(
_: &InferCtxt<'tcx>,
_: &OriginalQueryValues<'tcx>,
_: CanonicalResponse<'tcx>,
) -> Certainty {
unimplemented!()
}

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use rustc_infer::traits::query::NoSolution;
use rustc_middle::ty::TyCtxt;
use rustc_session::Limit;
use super::{Certainty, EvalCtxt, MaybeCause, QueryResult};
/// When detecting a solver overflow, we return ambiguity. Overflow can be
/// *hidden* by either a fatal error in an **AND** or a trivial success in an **OR**.
///
/// This is in issue in case of exponential blowup, e.g. if each goal on the stack
/// has multiple nested (overflowing) candidates. To deal with this, we reduce the limit
/// used by the solver when hitting the default limit for the first time.
///
/// FIXME: Get tests where always using the `default_limit` results in a hang and refer
/// to them here. We can also improve the overflow strategy if necessary.
pub(super) struct OverflowData {
default_limit: Limit,
current_limit: Limit,
/// When proving an **AND** we have to repeatedly iterate over the yet unproven goals.
///
/// Because of this each iteration also increases the depth in addition to the stack
/// depth.
additional_depth: usize,
}
impl OverflowData {
pub(super) fn new(tcx: TyCtxt<'_>) -> OverflowData {
let default_limit = tcx.recursion_limit();
OverflowData { default_limit, current_limit: default_limit, additional_depth: 0 }
}
#[inline]
pub(super) fn did_overflow(&self) -> bool {
self.default_limit.0 != self.current_limit.0
}
#[inline]
pub(super) fn has_overflow(&self, depth: usize) -> bool {
self.current_limit.value_within_limit(depth + self.additional_depth)
}
/// Updating the current limit when hitting overflow.
fn deal_with_overflow(&mut self) {
// When first hitting overflow we reduce the overflow limit
// for all future goals to prevent hangs if there's an exponental
// blowup.
self.current_limit.0 = self.default_limit.0 / 8;
}
}
impl<'tcx> EvalCtxt<'tcx> {
pub(super) fn deal_with_overflow(&mut self) -> QueryResult<'tcx> {
self.overflow_data.deal_with_overflow();
fixme_response_overflow_no_constraints()
}
/// A `while`-loop which tracks overflow.
pub(super) fn repeat_while_none(
&mut self,
mut loop_body: impl FnMut(&mut Self) -> Option<Result<Certainty, NoSolution>>,
) -> Result<Certainty, NoSolution> {
let start_depth = self.overflow_data.additional_depth;
let depth = self.provisional_cache.current_depth();
while !self.overflow_data.has_overflow(depth) {
if let Some(result) = loop_body(self) {
self.overflow_data.additional_depth = start_depth;
return result;
}
self.overflow_data.additional_depth += 1;
}
self.overflow_data.additional_depth = start_depth;
self.overflow_data.deal_with_overflow();
Ok(Certainty::Maybe(MaybeCause::Overflow))
}
}
fn fixme_response_overflow_no_constraints<'tcx>() -> QueryResult<'tcx> {
unimplemented!()
}

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use crate::traits::{specialization_graph, translate_substs};
use super::infcx_ext::InferCtxtExt;
use super::{
fixme_instantiate_canonical_query_response, CanonicalGoal, CanonicalResponse, Certainty,
EvalCtxt, Goal, QueryResult,
};
use rustc_errors::ErrorGuaranteed;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::DefId;
use rustc_infer::infer::canonical::{CanonicalVarValues, OriginalQueryValues};
use rustc_infer::infer::{InferCtxt, InferOk, TyCtxtInferExt};
use rustc_infer::traits::query::NoSolution;
use rustc_infer::traits::specialization_graph::LeafDef;
use rustc_infer::traits::{ObligationCause, Reveal};
use rustc_middle::ty;
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
use rustc_middle::ty::ProjectionPredicate;
use rustc_middle::ty::TypeVisitable;
use rustc_span::DUMMY_SP;
use std::iter;
// FIXME: Deduplicate the candidate code between projection and trait goal.
/// Similar to [super::trait_goals::Candidate] but for `Projection` goals.
#[derive(Debug, Clone)]
struct Candidate<'tcx> {
source: CandidateSource,
result: CanonicalResponse<'tcx>,
}
#[allow(dead_code)] // FIXME: implement and use all variants.
#[derive(Debug, Clone, Copy)]
enum CandidateSource {
Impl(DefId),
ParamEnv(usize),
Builtin,
}
impl<'tcx> EvalCtxt<'tcx> {
pub(super) fn compute_projection_goal(
&mut self,
goal: CanonicalGoal<'tcx, ProjectionPredicate<'tcx>>,
) -> QueryResult<'tcx> {
let candidates = self.assemble_and_evaluate_project_candidates(goal);
self.merge_project_candidates(candidates)
}
fn assemble_and_evaluate_project_candidates(
&mut self,
goal: CanonicalGoal<'tcx, ProjectionPredicate<'tcx>>,
) -> Vec<Candidate<'tcx>> {
let (ref infcx, goal, var_values) =
self.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
let mut acx = AssemblyCtxt { cx: self, infcx, var_values, candidates: Vec::new() };
acx.assemble_candidates_after_normalizing_self_ty(goal);
acx.assemble_impl_candidates(goal);
acx.candidates
}
fn merge_project_candidates(
&mut self,
mut candidates: Vec<Candidate<'tcx>>,
) -> QueryResult<'tcx> {
match candidates.len() {
0 => return Err(NoSolution),
1 => return Ok(candidates.pop().unwrap().result),
_ => {}
}
if candidates.len() > 1 {
let mut i = 0;
'outer: while i < candidates.len() {
for j in (0..candidates.len()).filter(|&j| i != j) {
if self.project_candidate_should_be_dropped_in_favor_of(
&candidates[i],
&candidates[j],
) {
debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
candidates.swap_remove(i);
continue 'outer;
}
}
debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
// If there are *STILL* multiple candidates, give up
// and report ambiguity.
i += 1;
if i > 1 {
debug!("multiple matches, ambig");
// FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
unimplemented!();
}
}
}
Ok(candidates.pop().unwrap().result)
}
fn project_candidate_should_be_dropped_in_favor_of(
&self,
candidate: &Candidate<'tcx>,
other: &Candidate<'tcx>,
) -> bool {
// FIXME: implement this
match (candidate.source, other.source) {
(CandidateSource::Impl(_), _)
| (CandidateSource::ParamEnv(_), _)
| (CandidateSource::Builtin, _) => unimplemented!(),
}
}
}
/// Similar to [super::trait_goals::AssemblyCtxt] but for `Projection` goals.
struct AssemblyCtxt<'a, 'tcx> {
cx: &'a mut EvalCtxt<'tcx>,
infcx: &'a InferCtxt<'tcx>,
var_values: CanonicalVarValues<'tcx>,
candidates: Vec<Candidate<'tcx>>,
}
impl<'tcx> AssemblyCtxt<'_, 'tcx> {
fn try_insert_candidate(&mut self, source: CandidateSource, certainty: Certainty) {
match self.infcx.make_canonical_response(self.var_values.clone(), certainty) {
Ok(result) => self.candidates.push(Candidate { source, result }),
Err(NoSolution) => debug!(?source, ?certainty, "failed leakcheck"),
}
}
fn assemble_candidates_after_normalizing_self_ty(
&mut self,
goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
) {
let tcx = self.cx.tcx;
let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.projection_ty.self_ty().kind() else {
return
};
self.infcx.probe(|_| {
let normalized_ty = self.infcx.next_ty_infer();
let normalizes_to_goal = goal.with(
tcx,
ty::Binder::dummy(ty::ProjectionPredicate {
projection_ty,
term: normalized_ty.into(),
}),
);
let normalization_certainty =
match self.cx.evaluate_goal(&self.infcx, normalizes_to_goal) {
Ok((_, certainty)) => certainty,
Err(NoSolution) => return,
};
// NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate.
// This doesn't work as long as we use `CandidateSource` in both winnowing and to resolve associated items.
let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
let mut orig_values = OriginalQueryValues::default();
let goal = self.infcx.canonicalize_query(goal, &mut orig_values);
let normalized_candidates = self.cx.assemble_and_evaluate_project_candidates(goal);
// Map each candidate from being canonical wrt the current inference context to being
// canonical wrt the caller.
for Candidate { source, result } in normalized_candidates {
self.infcx.probe(|_| {
let candidate_certainty = fixme_instantiate_canonical_query_response(
self.infcx,
&orig_values,
result,
);
self.try_insert_candidate(
source,
normalization_certainty.unify_and(candidate_certainty),
)
})
}
})
}
fn assemble_impl_candidates(&mut self, goal: Goal<'tcx, ProjectionPredicate<'tcx>>) {
self.cx.tcx.for_each_relevant_impl(
goal.predicate.trait_def_id(self.cx.tcx),
goal.predicate.self_ty(),
|impl_def_id| self.consider_impl_candidate(goal, impl_def_id),
);
}
fn consider_impl_candidate(
&mut self,
goal: Goal<'tcx, ProjectionPredicate<'tcx>>,
impl_def_id: DefId,
) {
let tcx = self.cx.tcx;
let goal_trait_ref = goal.predicate.projection_ty.trait_ref(tcx);
let impl_trait_ref = tcx.bound_impl_trait_ref(impl_def_id).unwrap();
let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
if iter::zip(goal_trait_ref.substs, impl_trait_ref.skip_binder().substs)
.any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
{
return;
}
self.infcx.probe(|_| {
let impl_substs = self.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
let impl_trait_ref = impl_trait_ref.subst(tcx, impl_substs);
let Ok(InferOk { obligations, .. }) = self
.infcx
.at(&ObligationCause::dummy(), goal.param_env)
.define_opaque_types(false)
.eq(goal_trait_ref, impl_trait_ref)
.map_err(|e| debug!("failed to equate trait refs: {e:?}"))
else {
return
};
let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
let Ok(trait_ref_certainty) = self.cx.evaluate_all(self.infcx, nested_goals) else { return };
let Some(assoc_def) = self.fetch_eligible_assoc_item_def(
goal.param_env,
goal_trait_ref,
goal.predicate.def_id(),
impl_def_id
) else {
return
};
if !assoc_def.item.defaultness(tcx).has_value() {
tcx.sess.delay_span_bug(
tcx.def_span(assoc_def.item.def_id),
"missing value for assoc item in impl",
);
}
// Getting the right substitutions here is complex, e.g. given:
// - a goal `<Vec<u32> as Trait<i32>>::Assoc<u64>`
// - the applicable impl `impl<T> Trait<i32> for Vec<T>`
// - and the impl which defines `Assoc` being `impl<T, U> Trait<U> for Vec<T>`
//
// We first rebase the goal substs onto the impl, going from `[Vec<u32>, i32, u64]`
// to `[u32, u64]`.
//
// And then map these substs to the substs of the defining impl of `Assoc`, going
// from `[u32, u64]` to `[u32, i32, u64]`.
let impl_substs_with_gat = goal.predicate.projection_ty.substs.rebase_onto(
tcx,
goal_trait_ref.def_id,
impl_trait_ref.substs,
);
let substs = translate_substs(
self.infcx,
goal.param_env,
impl_def_id,
impl_substs_with_gat,
assoc_def.defining_node,
);
// Finally we construct the actual value of the associated type.
let is_const = matches!(tcx.def_kind(assoc_def.item.def_id), DefKind::AssocConst);
let ty = tcx.bound_type_of(assoc_def.item.def_id);
let term: ty::EarlyBinder<ty::Term<'tcx>> = if is_const {
let identity_substs = ty::InternalSubsts::identity_for_item(tcx, assoc_def.item.def_id);
let did = ty::WithOptConstParam::unknown(assoc_def.item.def_id);
let kind =
ty::ConstKind::Unevaluated(ty::UnevaluatedConst::new(did, identity_substs));
ty.map_bound(|ty| tcx.mk_const(kind, ty).into())
} else {
ty.map_bound(|ty| ty.into())
};
let Ok(InferOk { obligations, .. }) = self
.infcx
.at(&ObligationCause::dummy(), goal.param_env)
.define_opaque_types(false)
.eq(goal.predicate.term, term.subst(tcx, substs))
.map_err(|e| debug!("failed to equate trait refs: {e:?}"))
else {
return
};
let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
let Ok(rhs_certainty) = self.cx.evaluate_all(self.infcx, nested_goals) else { return };
let certainty = trait_ref_certainty.unify_and(rhs_certainty);
self.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
})
}
/// This behavior is also implemented in `rustc_ty_utils` and in the old `project` code.
///
/// FIXME: We should merge these 3 implementations as it's likely that they otherwise
/// diverge.
#[instrument(level = "debug", skip(self, param_env), ret)]
fn fetch_eligible_assoc_item_def(
&self,
param_env: ty::ParamEnv<'tcx>,
goal_trait_ref: ty::TraitRef<'tcx>,
trait_assoc_def_id: DefId,
impl_def_id: DefId,
) -> Option<LeafDef> {
let node_item =
specialization_graph::assoc_def(self.cx.tcx, impl_def_id, trait_assoc_def_id)
.map_err(|ErrorGuaranteed { .. }| ())
.ok()?;
let eligible = if node_item.is_final() {
// Non-specializable items are always projectable.
true
} else {
// Only reveal a specializable default if we're past type-checking
// and the obligation is monomorphic, otherwise passes such as
// transmute checking and polymorphic MIR optimizations could
// get a result which isn't correct for all monomorphizations.
if param_env.reveal() == Reveal::All {
let poly_trait_ref = self.infcx.resolve_vars_if_possible(goal_trait_ref);
!poly_trait_ref.still_further_specializable()
} else {
debug!(?node_item.item.def_id, "not eligible due to default");
false
}
};
if eligible { Some(node_item) } else { None }
}
}

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compiler/rustc_trait_selection/src/solve/trait_goals.rs Normal file
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//! Dealing with trait goals, i.e. `T: Trait<'a, U>`.
use std::iter;
use super::infcx_ext::InferCtxtExt;
use super::{
fixme_instantiate_canonical_query_response, CanonicalGoal, CanonicalResponse, Certainty,
EvalCtxt, Goal, QueryResult,
};
use rustc_hir::def_id::DefId;
use rustc_infer::infer::canonical::{CanonicalVarValues, OriginalQueryValues};
use rustc_infer::infer::TyCtxtInferExt;
use rustc_infer::infer::{InferCtxt, InferOk};
use rustc_infer::traits::query::NoSolution;
use rustc_infer::traits::ObligationCause;
use rustc_middle::ty;
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
use rustc_middle::ty::TraitPredicate;
use rustc_span::DUMMY_SP;
/// A candidate is a possible way to prove a goal.
///
/// It consists of both the `source`, which describes how that goal
/// would be proven, and the `result` when using the given `source`.
///
/// For the list of possible candidates, please look at the documentation
/// of [CandidateSource].
#[derive(Debug, Clone)]
pub(super) struct Candidate<'tcx> {
source: CandidateSource,
result: CanonicalResponse<'tcx>,
}
#[allow(dead_code)] // FIXME: implement and use all variants.
#[derive(Debug, Clone, Copy)]
pub(super) enum CandidateSource {
/// Some user-defined impl with the given `DefId`.
Impl(DefId),
/// The n-th caller bound in the `param_env` of our goal.
///
/// This is pretty much always a bound from the `where`-clauses of the
/// currently checked item.
ParamEnv(usize),
/// A bound on the `self_ty` in case it is a projection or an opaque type.
///
/// # Examples
///
/// ```ignore (for syntax highlighting)
/// trait Trait {
/// type Assoc: OtherTrait;
/// }
/// ```
///
/// We know that `<Whatever as Trait>::Assoc: OtherTrait` holds by looking at
/// the bounds on `Trait::Assoc`.
AliasBound(usize),
/// A builtin implementation for some specific traits, used in cases
/// where we cannot rely an ordinary library implementations.
///
/// The most notable examples are `Sized`, `Copy` and `Clone`. This is also
/// used for the `DiscriminantKind` and `Pointee` trait, both of which have
/// an associated type.
Builtin,
/// An automatic impl for an auto trait, e.g. `Send`. These impls recursively look
/// at the constituent types of the `self_ty` to check whether the auto trait
/// is implemented for those.
AutoImpl,
}
struct AssemblyCtxt<'a, 'tcx> {
cx: &'a mut EvalCtxt<'tcx>,
infcx: &'a InferCtxt<'tcx>,
var_values: CanonicalVarValues<'tcx>,
candidates: Vec<Candidate<'tcx>>,
}
impl<'tcx> EvalCtxt<'tcx> {
pub(super) fn compute_trait_goal(
&mut self,
goal: CanonicalGoal<'tcx, TraitPredicate<'tcx>>,
) -> QueryResult<'tcx> {
let candidates = self.assemble_and_evaluate_trait_candidates(goal);
self.merge_trait_candidates_discard_reservation_impls(candidates)
}
pub(super) fn assemble_and_evaluate_trait_candidates(
&mut self,
goal: CanonicalGoal<'tcx, TraitPredicate<'tcx>>,
) -> Vec<Candidate<'tcx>> {
let (ref infcx, goal, var_values) =
self.tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
let mut acx = AssemblyCtxt { cx: self, infcx, var_values, candidates: Vec::new() };
acx.assemble_candidates_after_normalizing_self_ty(goal);
acx.assemble_impl_candidates(goal);
// FIXME: Remaining candidates
acx.candidates
}
#[instrument(level = "debug", skip(self), ret)]
pub(super) fn merge_trait_candidates_discard_reservation_impls(
&mut self,
mut candidates: Vec<Candidate<'tcx>>,
) -> QueryResult<'tcx> {
match candidates.len() {
0 => return Err(NoSolution),
1 => return Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result),
_ => {}
}
if candidates.len() > 1 {
let mut i = 0;
'outer: while i < candidates.len() {
for j in (0..candidates.len()).filter(|&j| i != j) {
if self.trait_candidate_should_be_dropped_in_favor_of(
&candidates[i],
&candidates[j],
) {
debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
candidates.swap_remove(i);
continue 'outer;
}
}
debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
// If there are *STILL* multiple candidates, give up
// and report ambiguity.
i += 1;
if i > 1 {
debug!("multiple matches, ambig");
// FIXME: return overflow if all candidates overflow, otherwise return ambiguity.
unimplemented!();
}
}
}
Ok(self.discard_reservation_impl(candidates.pop().unwrap()).result)
}
fn trait_candidate_should_be_dropped_in_favor_of(
&self,
candidate: &Candidate<'tcx>,
other: &Candidate<'tcx>,
) -> bool {
// FIXME: implement this
match (candidate.source, other.source) {
(CandidateSource::Impl(_), _)
| (CandidateSource::ParamEnv(_), _)
| (CandidateSource::AliasBound(_), _)
| (CandidateSource::Builtin, _)
| (CandidateSource::AutoImpl, _) => unimplemented!(),
}
}
fn discard_reservation_impl(&self, candidate: Candidate<'tcx>) -> Candidate<'tcx> {
if let CandidateSource::Impl(def_id) = candidate.source {
if let ty::ImplPolarity::Reservation = self.tcx.impl_polarity(def_id) {
debug!("Selected reservation impl");
// FIXME: reduce candidate to ambiguous
// FIXME: replace `var_values` with identity, yeet external constraints.
unimplemented!()
}
}
candidate
}
}
impl<'tcx> AssemblyCtxt<'_, 'tcx> {
/// Adds a new candidate using the current state of the inference context.
///
/// This does require each assembly method to correctly use `probe` to not taint
/// the results of other candidates.
fn try_insert_candidate(&mut self, source: CandidateSource, certainty: Certainty) {
match self.infcx.make_canonical_response(self.var_values.clone(), certainty) {
Ok(result) => self.candidates.push(Candidate { source, result }),
Err(NoSolution) => debug!(?source, ?certainty, "failed leakcheck"),
}
}
/// If the self type of a trait goal is a projection, computing the relevant candidates is difficult.
///
/// To deal with this, we first try to normalize the self type and add the candidates for the normalized
/// self type to the list of candidates in case that succeeds. Note that we can't just eagerly return in
/// this case as projections as self types add `
fn assemble_candidates_after_normalizing_self_ty(
&mut self,
goal: Goal<'tcx, TraitPredicate<'tcx>>,
) {
let tcx = self.cx.tcx;
// FIXME: We also have to normalize opaque types, not sure where to best fit that in.
let &ty::Alias(ty::Projection, projection_ty) = goal.predicate.self_ty().kind() else {
return
};
self.infcx.probe(|_| {
let normalized_ty = self.infcx.next_ty_infer();
let normalizes_to_goal = goal.with(
tcx,
ty::Binder::dummy(ty::ProjectionPredicate {
projection_ty,
term: normalized_ty.into(),
}),
);
let normalization_certainty =
match self.cx.evaluate_goal(&self.infcx, normalizes_to_goal) {
Ok((_, certainty)) => certainty,
Err(NoSolution) => return,
};
// NOTE: Alternatively we could call `evaluate_goal` here and only have a `Normalized` candidate.
// This doesn't work as long as we use `CandidateSource` in both winnowing and to resolve associated items.
let goal = goal.with(tcx, goal.predicate.with_self_type(tcx, normalized_ty));
let mut orig_values = OriginalQueryValues::default();
let goal = self.infcx.canonicalize_query(goal, &mut orig_values);
let normalized_candidates = self.cx.assemble_and_evaluate_trait_candidates(goal);
// Map each candidate from being canonical wrt the current inference context to being
// canonical wrt the caller.
for Candidate { source, result } in normalized_candidates {
self.infcx.probe(|_| {
let candidate_certainty = fixme_instantiate_canonical_query_response(
self.infcx,
&orig_values,
result,
);
// FIXME: This is a bit scary if the `normalizes_to_goal` overflows.
//
// If we have an ambiguous candidate it hides that normalization
// caused an overflow which may cause issues.
self.try_insert_candidate(
source,
normalization_certainty.unify_and(candidate_certainty),
)
})
}
})
}
fn assemble_impl_candidates(&mut self, goal: Goal<'tcx, TraitPredicate<'tcx>>) {
self.cx.tcx.for_each_relevant_impl(
goal.predicate.def_id(),
goal.predicate.self_ty(),
|impl_def_id| self.consider_impl_candidate(goal, impl_def_id),
);
}
fn consider_impl_candidate(
&mut self,
goal: Goal<'tcx, TraitPredicate<'tcx>>,
impl_def_id: DefId,
) {
let impl_trait_ref = self.cx.tcx.bound_impl_trait_ref(impl_def_id).unwrap();
let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
if iter::zip(goal.predicate.trait_ref.substs, impl_trait_ref.skip_binder().substs)
.any(|(goal, imp)| !drcx.generic_args_may_unify(goal, imp))
{
return;
}
self.infcx.probe(|_| {
let impl_substs = self.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
let impl_trait_ref = impl_trait_ref.subst(self.cx.tcx, impl_substs);
let Ok(InferOk { obligations, .. }) = self
.infcx
.at(&ObligationCause::dummy(), goal.param_env)
.define_opaque_types(false)
.eq(goal.predicate.trait_ref, impl_trait_ref)
.map_err(|e| debug!("failed to equate trait refs: {e:?}"))
else {
return
};
let nested_goals = obligations.into_iter().map(|o| o.into()).collect();
let Ok(certainty) = self.cx.evaluate_all(self.infcx, nested_goals) else { return };
self.try_insert_candidate(CandidateSource::Impl(impl_def_id), certainty);
})
}
}

59
compiler/rustc_trait_selection/src/traits/project.rs
View file

@ -25,7 +25,6 @@
use rustc_data_structures::stack::ensure_sufficient_stack;
use rustc_errors::ErrorGuaranteed;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::DefId;
use rustc_hir::lang_items::LangItem;
use rustc_infer::infer::at::At;
use rustc_infer::infer::resolve::OpportunisticRegionResolver;
@ -1553,7 +1552,7 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
// NOTE: This should be kept in sync with the similar code in
// `rustc_ty_utils::instance::resolve_associated_item()`.
let node_item =
assoc_def(selcx, impl_data.impl_def_id, obligation.predicate.def_id)
specialization_graph::assoc_def(selcx.tcx(), impl_data.impl_def_id, obligation.predicate.def_id)
.map_err(|ErrorGuaranteed { .. }| ())?;
if node_item.is_final() {
@ -2113,7 +2112,7 @@ fn confirm_impl_candidate<'cx, 'tcx>(
let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
let param_env = obligation.param_env;
let Ok(assoc_ty) = assoc_def(selcx, impl_def_id, assoc_item_id) else {
let Ok(assoc_ty) = specialization_graph::assoc_def(tcx, impl_def_id, assoc_item_id) else {
return Progress { term: tcx.ty_error().into(), obligations: nested };
};
@ -2210,7 +2209,7 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
let mut obligations = data.nested;
let trait_fn_def_id = tcx.impl_trait_in_trait_parent(obligation.predicate.def_id);
let Ok(leaf_def) = assoc_def(selcx, data.impl_def_id, trait_fn_def_id) else {
let Ok(leaf_def) = specialization_graph::assoc_def(tcx, data.impl_def_id, trait_fn_def_id) else {
return Progress { term: tcx.ty_error().into(), obligations };
};
if !leaf_def.item.defaultness(tcx).has_value() {
@ -2347,58 +2346,6 @@ fn assoc_ty_own_obligations<'cx, 'tcx>(
}
}
/// Locate the definition of an associated type in the specialization hierarchy,
/// starting from the given impl.
///
/// Based on the "projection mode", this lookup may in fact only examine the
/// topmost impl. See the comments for `Reveal` for more details.
fn assoc_def(
selcx: &SelectionContext<'_, '_>,
impl_def_id: DefId,
assoc_def_id: DefId,
) -> Result<specialization_graph::LeafDef, ErrorGuaranteed> {
let tcx = selcx.tcx();
let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
let trait_def = tcx.trait_def(trait_def_id);
// This function may be called while we are still building the
// specialization graph that is queried below (via TraitDef::ancestors()),
// so, in order to avoid unnecessary infinite recursion, we manually look
// for the associated item at the given impl.
// If there is no such item in that impl, this function will fail with a
// cycle error if the specialization graph is currently being built.
if let Some(&impl_item_id) = tcx.impl_item_implementor_ids(impl_def_id).get(&assoc_def_id) {
let item = tcx.associated_item(impl_item_id);
let impl_node = specialization_graph::Node::Impl(impl_def_id);
return Ok(specialization_graph::LeafDef {
item: *item,
defining_node: impl_node,
finalizing_node: if item.defaultness(tcx).is_default() {
None
} else {
Some(impl_node)
},
});
}
let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_def_id) {
Ok(assoc_item)
} else {
// This is saying that neither the trait nor
// the impl contain a definition for this
// associated type. Normally this situation
// could only arise through a compiler bug --
// if the user wrote a bad item name, it
// should have failed in astconv.
bug!(
"No associated type `{}` for {}",
tcx.item_name(assoc_def_id),
tcx.def_path_str(impl_def_id)
)
}
}
pub(crate) trait ProjectionCacheKeyExt<'cx, 'tcx>: Sized {
fn from_poly_projection_predicate(
selcx: &mut SelectionContext<'cx, 'tcx>,

14
compiler/rustc_trait_selection/src/traits/select/mod.rs
View file

@ -1171,19 +1171,7 @@ pub(crate) fn coinductive_match<I>(&mut self, mut cycle: I) -> bool
where
I: Iterator<Item = ty::Predicate<'tcx>>,
{
cycle.all(|predicate| self.coinductive_predicate(predicate))
}
fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
let result = match predicate.kind().skip_binder() {
ty::PredicateKind::Clause(ty::Clause::Trait(ref data)) => {
self.tcx().trait_is_coinductive(data.def_id())
}
ty::PredicateKind::WellFormed(_) => true,
_ => false,
};
debug!(?predicate, ?result, "coinductive_predicate");
result
cycle.all(|predicate| predicate.is_coinductive(self.tcx()))
}
/// Further evaluates `candidate` to decide whether all type parameters match and whether nested

49
compiler/rustc_trait_selection/src/traits/specialize/specialization_graph.rs
View file

@ -1,6 +1,7 @@
use super::OverlapError;
use crate::traits;
use rustc_errors::ErrorGuaranteed;
use rustc_hir::def_id::DefId;
use rustc_middle::ty::fast_reject::{self, SimplifiedType, TreatParams};
use rustc_middle::ty::{self, TyCtxt, TypeVisitable};
@ -379,3 +380,51 @@ fn record_impl_from_cstore(&mut self, tcx: TyCtxt<'tcx>, parent: DefId, child: D
self.children.entry(parent).or_default().insert_blindly(tcx, child);
}
}
/// Locate the definition of an associated type in the specialization hierarchy,
/// starting from the given impl.
pub(crate) fn assoc_def(
tcx: TyCtxt<'_>,
impl_def_id: DefId,
assoc_def_id: DefId,
) -> Result<LeafDef, ErrorGuaranteed> {
let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
let trait_def = tcx.trait_def(trait_def_id);
// This function may be called while we are still building the
// specialization graph that is queried below (via TraitDef::ancestors()),
// so, in order to avoid unnecessary infinite recursion, we manually look
// for the associated item at the given impl.
// If there is no such item in that impl, this function will fail with a
// cycle error if the specialization graph is currently being built.
if let Some(&impl_item_id) = tcx.impl_item_implementor_ids(impl_def_id).get(&assoc_def_id) {
let &item = tcx.associated_item(impl_item_id);
let impl_node = Node::Impl(impl_def_id);
return Ok(LeafDef {
item,
defining_node: impl_node,
finalizing_node: if item.defaultness(tcx).is_default() {
None
} else {
Some(impl_node)
},
});
}
let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_def_id) {
Ok(assoc_item)
} else {
// This is saying that neither the trait nor
// the impl contain a definition for this
// associated type. Normally this situation
// could only arise through a compiler bug --
// if the user wrote a bad item name, it
// should have failed in astconv.
bug!(
"No associated type `{}` for {}",
tcx.item_name(assoc_def_id),
tcx.def_path_str(impl_def_id)
)
}
}

28
src/test/ui/traits/solver-cycles/inductive-canonical-cycle.rs Normal file
View file

@ -0,0 +1,28 @@
// known-bug
// This should compile but fails with the current solver.
//
// This checks that the new solver uses `Ambiguous` when hitting the
// inductive cycle here when proving `exists<^0, ^1> (): Trait<^0, ^1>`
// which requires proving `Trait<?1, ?0>` but that has the same
// canonical representation.
trait Trait<T, U> {}
impl<T, U> Trait<T, U> for ()
where
(): Trait<U, T>,
T: OtherTrait,
{}
trait OtherTrait {}
impl OtherTrait for u32 {}
fn require_trait<T, U>()
where
(): Trait<T, U>
{}
fn main() {
require_trait::<_, _>();
//~^ ERROR overflow evaluating
}

26
src/test/ui/traits/solver-cycles/inductive-canonical-cycle.stderr Normal file
View file

@ -0,0 +1,26 @@
error[E0275]: overflow evaluating the requirement `_: Sized`
--> $DIR/inductive-canonical-cycle.rs:26:5
|
LL | require_trait::<_, _>();
| ^^^^^^^^^^^^^^^^^^^^^
|
= help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`inductive_canonical_cycle`)
note: required for `()` to implement `Trait<_, _>`
--> $DIR/inductive-canonical-cycle.rs:11:12
|
LL | impl<T, U> Trait<T, U> for ()
| ^^^^^^^^^^^ ^^
= note: 128 redundant requirements hidden
= note: required for `()` to implement `Trait<_, _>`
note: required by a bound in `require_trait`
--> $DIR/inductive-canonical-cycle.rs:22:9
|
LL | fn require_trait<T, U>()
| ------------- required by a bound in this
LL | where
LL | (): Trait<T, U>
| ^^^^^^^^^^^ required by this bound in `require_trait`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0275`.

4
triagebot.toml
View file

@ -342,6 +342,10 @@ cc = ["@BoxyUwU"]
message = "Some changes occured in `rustc_ty_utils::consts.rs`"
cc = ["@BoxyUwU"]
[mentions."compiler/rustc_trait_selection/src/solve]
message = "Some changes occurred to the core trait solver"
cc = ["@lcnr"]
[mentions."compiler/rustc_trait_selection/src/traits/engine.rs"]
message = """
Some changes occurred in engine.rs, potentially modifying the public API \