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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 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.
//!
//! Subtle: when relating an opaque to another type, we emit a
//! `NormalizesTo(opaque, ?fresh_var)` goal when trying to normalize the opaque.
//! This nested goal starts out as ambiguous and does not actually define the opaque.
//! However, if `?fresh_var` ends up geteting equated to another type, we retry the
//! `NormalizesTo` goal, at which point the opaque is actually defined.
use super::{EvalCtxt, GoalSource};
use rustc_infer::traits::query::NoSolution;
use rustc_middle::traits::solve::{Certainty, Goal, QueryResult};
use rustc_middle::ty::{self, Ty};
impl<'tcx> EvalCtxt<'_, 'tcx> {
#[instrument(level = "debug", skip(self), ret)]
pub(super) fn compute_alias_relate_goal(
&mut self,
goal: Goal<'tcx, (ty::Term<'tcx>, ty::Term<'tcx>, ty::AliasRelationDirection)>,
) -> QueryResult<'tcx> {
let tcx = self.tcx();
let Goal { param_env, predicate: (lhs, rhs, direction) } = goal;
let Some(lhs) = self.try_normalize_term(param_env, lhs)? else {
return self
.evaluate_added_goals_and_make_canonical_response(Certainty::overflow(true));
};
let Some(rhs) = self.try_normalize_term(param_env, rhs)? else {
return self
.evaluate_added_goals_and_make_canonical_response(Certainty::overflow(true));
};
let variance = match direction {
ty::AliasRelationDirection::Equate => ty::Variance::Invariant,
ty::AliasRelationDirection::Subtype => ty::Variance::Covariant,
};
match (lhs.to_alias_ty(tcx), rhs.to_alias_ty(tcx)) {
(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)
}
(None, Some(alias)) => self.relate_rigid_alias_non_alias(
param_env,
alias,
variance.xform(ty::Variance::Contravariant),
lhs,
),
(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)
}
}
}
/// Relate a rigid alias with another type. This is the same as
/// an ordinary relate except that we treat the outer most alias
/// constructor as rigid.
#[instrument(level = "debug", skip(self, param_env), ret)]
fn relate_rigid_alias_non_alias(
&mut self,
param_env: ty::ParamEnv<'tcx>,
alias: ty::AliasTy<'tcx>,
variance: ty::Variance,
term: ty::Term<'tcx>,
) -> QueryResult<'tcx> {
// NOTE: this check is purely an optimization, the structural eq would
// always fail if the term is not an inference variable.
if term.is_infer() {
let tcx = self.tcx();
// We need to relate `alias` to `term` treating only the outermost
// constructor as rigid, relating any contained generic arguments as
// normal. We do this by first structurally equating the `term`
// with the alias constructor instantiated with unconstrained infer vars,
// and then relate this with the whole `alias`.
//
// Alternatively we could modify `Equate` for this case by adding another
// variant to `StructurallyRelateAliases`.
let identity_args = self.fresh_args_for_item(alias.def_id);
let rigid_ctor = ty::AliasTy::new(tcx, alias.def_id, identity_args);
self.eq_structurally_relating_aliases(param_env, term, rigid_ctor.to_ty(tcx).into())?;
self.eq(param_env, alias, rigid_ctor)?;
self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
} else {
Err(NoSolution)
}
}
// FIXME: This needs a name that reflects that it's okay to bottom-out with an inference var.
/// Normalize the `term` to equate it later.
#[instrument(level = "debug", skip(self, param_env), ret)]
fn try_normalize_term(
&mut self,
param_env: ty::ParamEnv<'tcx>,
term: ty::Term<'tcx>,
) -> Result<Option<ty::Term<'tcx>>, NoSolution> {
match term.unpack() {
ty::TermKind::Ty(ty) => {
Ok(self.try_normalize_ty_recur(param_env, 0, ty).map(Into::into))
}
ty::TermKind::Const(_) => {
if let Some(alias) = term.to_alias_ty(self.tcx()) {
let term = self.next_term_infer_of_kind(term);
self.add_goal(
GoalSource::Misc,
Goal::new(self.tcx(), param_env, ty::NormalizesTo { alias, term }),
);
self.try_evaluate_added_goals()?;
Ok(Some(self.resolve_vars_if_possible(term)))
} else {
Ok(Some(term))
}
}
}
}
#[instrument(level = "debug", skip(self, param_env), ret)]
fn try_normalize_ty_recur(
&mut self,
param_env: ty::ParamEnv<'tcx>,
depth: usize,
ty: Ty<'tcx>,
) -> Option<Ty<'tcx>> {
if !self.tcx().recursion_limit().value_within_limit(depth) {
return None;
}
let ty::Alias(_, alias) = *ty.kind() else {
return Some(ty);
};
match self.commit_if_ok(|this| {
let normalized_ty = this.next_ty_infer();
let normalizes_to_goal = Goal::new(
this.tcx(),
param_env,
ty::NormalizesTo { alias, term: normalized_ty.into() },
);
this.add_goal(GoalSource::Misc, normalizes_to_goal);
this.try_evaluate_added_goals()?;
Ok(this.resolve_vars_if_possible(normalized_ty))
}) {
Ok(ty) => self.try_normalize_ty_recur(param_env, depth + 1, ty),
Err(NoSolution) => Some(ty),
}
}
}