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//! Implements the `AliasRelate` goal, which is used when unifying aliases.
//! Doing this via a separate goal is called "deferred alias relation" and part
//! of our more general approach to "lazy normalization".
//!
//! This is done by first structurally normalizing both sides of the goal, ending
//! up in either a concrete type, rigid alias, or an infer variable.
//! These are related further according to the rules below:
//!
//! (1.) If we end up with two rigid aliases, then we relate them structurally.
//!
//! (2.) If we end up with an infer var and a rigid alias, then we instantiate
//! the infer var with the constructor of the alias and then recursively relate
//! the terms.
//!
//! (3.) Otherwise, if we end with two rigid (non-projection) or infer types,
//! relate them structurally.
use rustc_type_ir::inherent::*;
use rustc_type_ir::{self as ty, Interner};
use tracing::{instrument, trace};
use crate::delegate::SolverDelegate;
use crate::solve::{Certainty, EvalCtxt, Goal, QueryResult};
impl<D, I> EvalCtxt<'_, D>
where
D: SolverDelegate<Interner = I>,
I: Interner,
{
#[instrument(level = "trace", skip(self), ret)]
pub(super) fn compute_alias_relate_goal(
&mut self,
goal: Goal<I, (I::Term, I::Term, ty::AliasRelationDirection)>,
) -> QueryResult<I> {
let cx = self.cx();
let Goal { param_env, predicate: (lhs, rhs, direction) } = goal;
debug_assert!(lhs.to_alias_term().is_some() || rhs.to_alias_term().is_some());
// Structurally normalize the lhs.
let lhs = if let Some(alias) = lhs.to_alias_term() {
let term = self.next_term_infer_of_kind(lhs);
self.add_normalizes_to_goal(goal.with(cx, ty::NormalizesTo { alias, term }));
term
} else {
lhs
};
// Structurally normalize the rhs.
let rhs = if let Some(alias) = rhs.to_alias_term() {
let term = self.next_term_infer_of_kind(rhs);
self.add_normalizes_to_goal(goal.with(cx, ty::NormalizesTo { alias, term }));
term
} else {
rhs
};
// Add a `make_canonical_response` probe step so that we treat this as
// a candidate, even if `try_evaluate_added_goals` bails due to an error.
// It's `Certainty::AMBIGUOUS` because this candidate is not "finished",
// since equating the normalized terms will lead to additional constraints.
self.inspect.make_canonical_response(Certainty::AMBIGUOUS);
// Apply the constraints.
self.try_evaluate_added_goals()?;
let lhs = self.resolve_vars_if_possible(lhs);
let rhs = self.resolve_vars_if_possible(rhs);
trace!(?lhs, ?rhs);
let variance = match direction {
ty::AliasRelationDirection::Equate => ty::Invariant,
ty::AliasRelationDirection::Subtype => ty::Covariant,
};
match (lhs.to_alias_term(), rhs.to_alias_term()) {
(None, None) => {
self.relate(param_env, lhs, variance, rhs)?;
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
(Some(alias), None) => {
self.relate_rigid_alias_non_alias(param_env, alias, variance, rhs)?;
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
(None, Some(alias)) => {
self.relate_rigid_alias_non_alias(
param_env,
alias,
variance.xform(ty::Contravariant),
lhs,
)?;
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
(Some(alias_lhs), Some(alias_rhs)) => {
self.relate(param_env, alias_lhs, variance, alias_rhs)?;
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
}
}
}
}