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//! Canonicalization is used to separate some goal from its context,
//! throwing away unnecessary information in the process.
//!
//! This is necessary to cache goals containing inference variables
//! and placeholders without restricting them to the current `InferCtxt`.
//!
//! Canonicalization is fairly involved, for more details see the relevant
//! section of the [rustc-dev-guide][c].
//!
//! [c]: https://rustc-dev-guide.rust-lang.org/solve/canonicalization.html
use std::iter;
use rustc_index::IndexVec;
use rustc_type_ir::fold::TypeFoldable;
use rustc_type_ir::inherent::*;
use rustc_type_ir::{self as ty, Canonical, CanonicalVarValues, InferCtxtLike, Interner};
use tracing::{instrument, trace};
use crate::canonicalizer::{CanonicalizeMode, Canonicalizer};
use crate::delegate::SolverDelegate;
use crate::resolve::EagerResolver;
use crate::solve::eval_ctxt::NestedGoals;
use crate::solve::{
inspect, response_no_constraints_raw, CanonicalInput, CanonicalResponse, Certainty, EvalCtxt,
ExternalConstraintsData, Goal, MaybeCause, NestedNormalizationGoals, NoSolution,
PredefinedOpaquesData, QueryInput, QueryResult, Response,
};
trait ResponseT<I: Interner> {
fn var_values(&self) -> CanonicalVarValues<I>;
}
impl<I: Interner> ResponseT<I> for Response<I> {
fn var_values(&self) -> CanonicalVarValues<I> {
self.var_values
}
}
impl<I: Interner, T> ResponseT<I> for inspect::State<I, T> {
fn var_values(&self) -> CanonicalVarValues<I> {
self.var_values
}
}
impl<D, I> EvalCtxt<'_, D>
where
D: SolverDelegate<Interner = I>,
I: Interner,
{
/// Canonicalizes the goal remembering the original values
/// for each bound variable.
pub(super) fn canonicalize_goal<T: TypeFoldable<I>>(
&self,
goal: Goal<I, T>,
) -> (Vec<I::GenericArg>, CanonicalInput<I, T>) {
let opaque_types = self.delegate.clone_opaque_types_for_query_response();
let (goal, opaque_types) =
(goal, opaque_types).fold_with(&mut EagerResolver::new(self.delegate));
let mut orig_values = Default::default();
let canonical_goal = Canonicalizer::canonicalize(
self.delegate,
CanonicalizeMode::Input,
&mut orig_values,
QueryInput {
goal,
predefined_opaques_in_body: self
.cx()
.mk_predefined_opaques_in_body(PredefinedOpaquesData { opaque_types }),
},
);
(orig_values, canonical_goal)
}
/// To return the constraints of a canonical query to the caller, we canonicalize:
///
/// - `var_values`: a map from bound variables in the canonical goal to
/// the values inferred while solving the instantiated goal.
/// - `external_constraints`: additional constraints which aren't expressible
/// using simple unification of inference variables.
#[instrument(level = "trace", skip(self), ret)]
pub(in crate::solve) fn evaluate_added_goals_and_make_canonical_response(
&mut self,
certainty: Certainty,
) -> QueryResult<I> {
self.inspect.make_canonical_response(certainty);
let goals_certainty = self.try_evaluate_added_goals()?;
assert_eq!(
self.tainted,
Ok(()),
"EvalCtxt is tainted -- nested goals may have been dropped in a \
previous call to `try_evaluate_added_goals!`"
);
// We only check for leaks from universes which were entered inside
// of the query.
self.delegate.leak_check(self.max_input_universe).map_err(|NoSolution| {
trace!("failed the leak check");
NoSolution
})?;
// When normalizing, we've replaced the expected term with an unconstrained
// inference variable. This means that we dropped information which could
// have been important. We handle this by instead returning the nested goals
// to the caller, where they are then handled.
//
// As we return all ambiguous nested goals, we can ignore the certainty returned
// by `try_evaluate_added_goals()`.
let (certainty, normalization_nested_goals) = if self.is_normalizes_to_goal {
let NestedGoals { normalizes_to_goals, goals } = std::mem::take(&mut self.nested_goals);
if cfg!(debug_assertions) {
assert!(normalizes_to_goals.is_empty());
if goals.is_empty() {
assert!(matches!(goals_certainty, Certainty::Yes));
}
}
(certainty, NestedNormalizationGoals(goals))
} else {
let certainty = certainty.unify_with(goals_certainty);
(certainty, NestedNormalizationGoals::empty())
};
let external_constraints =
self.compute_external_query_constraints(certainty, normalization_nested_goals);
let (var_values, mut external_constraints) = (self.var_values, external_constraints)
.fold_with(&mut EagerResolver::new(self.delegate));
// Remove any trivial region constraints once we've resolved regions
external_constraints
.region_constraints
.retain(|outlives| outlives.0.as_region().map_or(true, |re| re != outlives.1));
let canonical = Canonicalizer::canonicalize(
self.delegate,
CanonicalizeMode::Response { max_input_universe: self.max_input_universe },
&mut Default::default(),
Response {
var_values,
certainty,
external_constraints: self.cx().mk_external_constraints(external_constraints),
},
);
Ok(canonical)
}
/// Constructs a totally unconstrained, ambiguous response to a goal.
///
/// Take care when using this, since often it's useful to respond with
/// ambiguity but return constrained variables to guide inference.
pub(in crate::solve) fn make_ambiguous_response_no_constraints(
&self,
maybe_cause: MaybeCause,
) -> CanonicalResponse<I> {
response_no_constraints_raw(
self.cx(),
self.max_input_universe,
self.variables,
Certainty::Maybe(maybe_cause),
)
}
/// Computes the region constraints and *new* opaque types registered when
/// proving a goal.
///
/// If an opaque was already constrained before proving this goal, then the
/// external constraints do not need to record that opaque, since if it is
/// further constrained by inference, that will be passed back in the var
/// values.
#[instrument(level = "trace", skip(self), ret)]
fn compute_external_query_constraints(
&self,
certainty: Certainty,
normalization_nested_goals: NestedNormalizationGoals<I>,
) -> ExternalConstraintsData<I> {
// We only return region constraints once the certainty is `Yes`. This
// is necessary as we may drop nested goals on ambiguity, which may result
// in unconstrained inference variables in the region constraints. It also
// prevents us from emitting duplicate region constraints, avoiding some
// unnecessary work. This slightly weakens the leak check in case it uses
// region constraints from an ambiguous nested goal. This is tested in both
// `tests/ui/higher-ranked/leak-check/leak-check-in-selection-5-ambig.rs` and
// `tests/ui/higher-ranked/leak-check/leak-check-in-selection-6-ambig-unify.rs`.
let region_constraints = if certainty == Certainty::Yes {
self.delegate.make_deduplicated_outlives_constraints()
} else {
Default::default()
};
ExternalConstraintsData {
region_constraints,
opaque_types: self
.delegate
.clone_opaque_types_for_query_response()
.into_iter()
// Only return *newly defined* opaque types.
.filter(|(a, _)| {
self.predefined_opaques_in_body.opaque_types.iter().all(|(pa, _)| pa != a)
})
.collect(),
normalization_nested_goals,
}
}
/// After calling a canonical query, we apply the constraints returned
/// by the query using this function.
///
/// This happens in three steps:
/// - we instantiate the bound variables of the query response
/// - we unify the `var_values` of the response with the `original_values`
/// - we apply the `external_constraints` returned by the query, returning
/// the `normalization_nested_goals`
pub(super) fn instantiate_and_apply_query_response(
&mut self,
param_env: I::ParamEnv,
original_values: Vec<I::GenericArg>,
response: CanonicalResponse<I>,
) -> (NestedNormalizationGoals<I>, Certainty) {
let instantiation = Self::compute_query_response_instantiation_values(
self.delegate,
&original_values,
&response,
);
let Response { var_values, external_constraints, certainty } =
self.delegate.instantiate_canonical(response, instantiation);
Self::unify_query_var_values(self.delegate, param_env, &original_values, var_values);
let ExternalConstraintsData {
region_constraints,
opaque_types,
normalization_nested_goals,
} = &*external_constraints;
self.register_region_constraints(region_constraints);
self.register_new_opaque_types(opaque_types);
(normalization_nested_goals.clone(), certainty)
}
/// This returns the canoncial variable values to instantiate the bound variables of
/// the canonical response. This depends on the `original_values` for the
/// bound variables.
fn compute_query_response_instantiation_values<T: ResponseT<I>>(
delegate: &D,
original_values: &[I::GenericArg],
response: &Canonical<I, T>,
) -> CanonicalVarValues<I> {
// FIXME: Longterm canonical queries should deal with all placeholders
// created inside of the query directly instead of returning them to the
// caller.
let prev_universe = delegate.universe();
let universes_created_in_query = response.max_universe.index();
for _ in 0..universes_created_in_query {
delegate.create_next_universe();
}
let var_values = response.value.var_values();
assert_eq!(original_values.len(), var_values.len());
// If the query did not make progress with constraining inference variables,
// we would normally create a new inference variables for bound existential variables
// only then unify this new inference variable with the inference variable from
// the input.
//
// We therefore instantiate the existential variable in the canonical response with the
// inference variable of the input right away, which is more performant.
let mut opt_values = IndexVec::from_elem_n(None, response.variables.len());
for (original_value, result_value) in
iter::zip(original_values, var_values.var_values.iter())
{
match result_value.kind() {
ty::GenericArgKind::Type(t) => {
if let ty::Bound(debruijn, b) = t.kind() {
assert_eq!(debruijn, ty::INNERMOST);
opt_values[b.var()] = Some(*original_value);
}
}
ty::GenericArgKind::Lifetime(r) => {
if let ty::ReBound(debruijn, br) = r.kind() {
assert_eq!(debruijn, ty::INNERMOST);
opt_values[br.var()] = Some(*original_value);
}
}
ty::GenericArgKind::Const(c) => {
if let ty::ConstKind::Bound(debruijn, bv) = c.kind() {
assert_eq!(debruijn, ty::INNERMOST);
opt_values[bv.var()] = Some(*original_value);
}
}
}
}
let var_values = delegate.cx().mk_args_from_iter(
response.variables.iter().enumerate().map(|(index, info)| {
if info.universe() != ty::UniverseIndex::ROOT {
// A variable from inside a binder of the query. While ideally these shouldn't
// exist at all (see the FIXME at the start of this method), we have to deal with
// them for now.
delegate.instantiate_canonical_var_with_infer(info, |idx| {
ty::UniverseIndex::from(prev_universe.index() + idx.index())
})
} else if info.is_existential() {
// As an optimization we sometimes avoid creating a new inference variable here.
//
// All new inference variables we create start out in the current universe of the caller.
// This is conceptually wrong as these inference variables would be able to name
// more placeholders then they should be able to. However the inference variables have
// to "come from somewhere", so by equating them with the original values of the caller
// later on, we pull them down into their correct universe again.
if let Some(v) = opt_values[ty::BoundVar::from_usize(index)] {
v
} else {
delegate.instantiate_canonical_var_with_infer(info, |_| prev_universe)
}
} else {
// For placeholders which were already part of the input, we simply map this
// universal bound variable back the placeholder of the input.
original_values[info.expect_placeholder_index()]
}
}),
);
CanonicalVarValues { var_values }
}
/// Unify the `original_values` with the `var_values` returned by the canonical query..
///
/// This assumes that this unification will always succeed. This is the case when
/// applying a query response right away. However, calling a canonical query, doing any
/// other kind of trait solving, and only then instantiating the result of the query
/// can cause the instantiation to fail. This is not supported and we ICE in this case.
///
/// We always structurally instantiate aliases. Relating aliases needs to be different
/// depending on whether the alias is *rigid* or not. We're only really able to tell
/// whether an alias is rigid by using the trait solver. When instantiating a response
/// from the solver we assume that the solver correctly handled aliases and therefore
/// always relate them structurally here.
#[instrument(level = "trace", skip(delegate))]
fn unify_query_var_values(
delegate: &D,
param_env: I::ParamEnv,
original_values: &[I::GenericArg],
var_values: CanonicalVarValues<I>,
) {
assert_eq!(original_values.len(), var_values.len());
for (&orig, response) in iter::zip(original_values, var_values.var_values.iter()) {
let goals =
delegate.eq_structurally_relating_aliases(param_env, orig, response).unwrap();
assert!(goals.is_empty());
}
}
fn register_region_constraints(
&mut self,
outlives: &[ty::OutlivesPredicate<I, I::GenericArg>],
) {
for &ty::OutlivesPredicate(lhs, rhs) in outlives {
match lhs.kind() {
ty::GenericArgKind::Lifetime(lhs) => self.register_region_outlives(lhs, rhs),
ty::GenericArgKind::Type(lhs) => self.register_ty_outlives(lhs, rhs),
ty::GenericArgKind::Const(_) => panic!("const outlives: {lhs:?}: {rhs:?}"),
}
}
}
fn register_new_opaque_types(&mut self, opaque_types: &[(ty::OpaqueTypeKey<I>, I::Ty)]) {
for &(key, ty) in opaque_types {
self.delegate.inject_new_hidden_type_unchecked(key, ty);
}
}
}
/// Used by proof trees to be able to recompute intermediate actions while
/// evaluating a goal. The `var_values` not only include the bound variables
/// of the query input, but also contain all unconstrained inference vars
/// created while evaluating this goal.
pub(in crate::solve) fn make_canonical_state<D, T, I>(
delegate: &D,
var_values: &[I::GenericArg],
max_input_universe: ty::UniverseIndex,
data: T,
) -> inspect::CanonicalState<I, T>
where
D: SolverDelegate<Interner = I>,
I: Interner,
T: TypeFoldable<I>,
{
let var_values = CanonicalVarValues { var_values: delegate.cx().mk_args(var_values) };
let state = inspect::State { var_values, data };
let state = state.fold_with(&mut EagerResolver::new(delegate));
Canonicalizer::canonicalize(
delegate,
CanonicalizeMode::Response { max_input_universe },
&mut vec![],
state,
)
}
// FIXME: needs to be pub to be accessed by downstream
// `rustc_trait_selection::solve::inspect::analyse`.
pub fn instantiate_canonical_state<D, I, T: TypeFoldable<I>>(
delegate: &D,
span: D::Span,
param_env: I::ParamEnv,
orig_values: &mut Vec<I::GenericArg>,
state: inspect::CanonicalState<I, T>,
) -> T
where
D: SolverDelegate<Interner = I>,
I: Interner,
{
// In case any fresh inference variables have been created between `state`
// and the previous instantiation, extend `orig_values` for it.
assert!(orig_values.len() <= state.value.var_values.len());
for &arg in &state.value.var_values.var_values.as_slice()
[orig_values.len()..state.value.var_values.len()]
{
// FIXME: This is so ugly.
let unconstrained = delegate.fresh_var_for_kind_with_span(arg, span);
orig_values.push(unconstrained);
}
let instantiation =
EvalCtxt::compute_query_response_instantiation_values(delegate, orig_values, &state);
let inspect::State { var_values, data } = delegate.instantiate_canonical(state, instantiation);
EvalCtxt::unify_query_var_values(delegate, param_env, orig_values, var_values);
data
}