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improve readability of winnowing #108937

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Mar 11, 2023
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improve readability of winnowing #108937

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3097453
improve readability of winnowing
lcnr Mar 9, 2023
14818e2
updating comment
lcnr Mar 10, 2023
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105 changes: 66 additions & 39 deletions compiler/rustc_trait_selection/src/traits/select/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -465,14 +465,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
if candidates.len() > 1 {
let mut i = 0;
while i < candidates.len() {
let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
let should_drop_i = (0..candidates.len()).filter(|&j| i != j).any(|j| {
self.candidate_should_be_dropped_in_favor_of(
&candidates[i],
&candidates[j],
needs_infer,
)
) == DropVictim::Yes
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Maybe .is_drop_victim() (or other check method) would be nicer here, idk.

});
if is_dup {
if should_drop_i {
debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
candidates.swap_remove(i);
} else {
Expand Down Expand Up @@ -1842,15 +1842,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
ProjectionMatchesProjection::No
}
}
}

///////////////////////////////////////////////////////////////////////////
// WINNOW
//
// Winnowing is the process of attempting to resolve ambiguity by
// probing further. During the winnowing process, we unify all
// type variables and then we also attempt to evaluate recursive
// bounds to see if they are satisfied.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
enum DropVictim {
Yes,
No,
}

/// ## Winnowing
///
/// Winnowing is the process of attempting to resolve ambiguity by
/// probing further. During the winnowing process, we unify all
/// type variables and then we also attempt to evaluate recursive
/// bounds to see if they are satisfied.
impl<'tcx> SelectionContext<'_, 'tcx> {
/// Returns `true` if `victim` should be dropped in favor of
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/// `other`. Generally speaking we will drop duplicate
/// candidates and prefer where-clause candidates.
Expand All @@ -1861,9 +1867,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
victim: &EvaluatedCandidate<'tcx>,
other: &EvaluatedCandidate<'tcx>,
needs_infer: bool,
) -> bool {
) -> DropVictim {
if victim.candidate == other.candidate {
return true;
return DropVictim::Yes;
}

// Check if a bound would previously have been removed when normalizing
Expand All @@ -1887,11 +1893,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
}

// FIXME(@jswrenn): this should probably be more sophisticated
(TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => false,
(TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => DropVictim::No,

// (*)
(BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => true,
(_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => false,
(BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => {
DropVictim::Yes
}
(_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => {
DropVictim::No
}

(ParamCandidate(other), ParamCandidate(victim)) => {
let same_except_bound_vars = other.skip_binder().trait_ref
Expand All @@ -1905,28 +1915,27 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
// or the current one if tied (they should both evaluate to the same answer). This is
// probably best characterized as a "hack", since we might prefer to just do our
// best to *not* create essentially duplicate candidates in the first place.
other.bound_vars().len() <= victim.bound_vars().len()
if other.bound_vars().len() <= victim.bound_vars().len() {
DropVictim::Yes
} else {
DropVictim::No
}
} else if other.skip_binder().trait_ref == victim.skip_binder().trait_ref
&& victim.skip_binder().constness == ty::BoundConstness::NotConst
&& other.skip_binder().polarity == victim.skip_binder().polarity
{
// Drop otherwise equivalent non-const candidates in favor of const candidates.
true
DropVictim::Yes
} else {
false
DropVictim::No
}
}

// Drop otherwise equivalent non-const fn pointer candidates
(FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => true,
(FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => DropVictim::Yes,

// Global bounds from the where clause should be ignored
// here (see issue #50825). Otherwise, we have a where
// clause so don't go around looking for impls.
// Arbitrarily give param candidates priority
// over projection and object candidates.
(
ParamCandidate(ref cand),
ParamCandidate(ref other_cand),
ImplCandidate(..)
| ClosureCandidate { .. }
| GeneratorCandidate
Expand All @@ -1939,11 +1948,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
| TraitAliasCandidate
| ObjectCandidate(_)
| ProjectionCandidate(..),
) => !is_global(cand),
(ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref cand)) => {
) => {
if is_global(other_cand) {
DropVictim::No
} else {
// We have a where clause so don't go around looking
// for impls. Arbitrarily give param candidates priority
// over projection and object candidates.
//
// Global bounds from the where clause should be ignored
// here (see issue #50825).
DropVictim::Yes
}
}
(ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref victim_cand)) => {
// Prefer these to a global where-clause bound
// (see issue #50825).
is_global(cand)
if is_global(victim_cand) { DropVictim::Yes } else { DropVictim::No }
}
(
ImplCandidate(_)
Expand All @@ -1956,18 +1977,22 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { has_nested: true }
| TraitAliasCandidate,
ParamCandidate(ref cand),
ParamCandidate(ref victim_cand),
) => {
// Prefer these to a global where-clause bound
// (see issue #50825).
is_global(cand) && other.evaluation.must_apply_modulo_regions()
if is_global(victim_cand) && other.evaluation.must_apply_modulo_regions() {
DropVictim::Yes
} else {
DropVictim::No
}
}

(ProjectionCandidate(i, _), ProjectionCandidate(j, _))
| (ObjectCandidate(i), ObjectCandidate(j)) => {
// Arbitrarily pick the lower numbered candidate for backwards
// compatibility reasons. Don't let this affect inference.
i < j && !needs_infer
if i < j && !needs_infer { DropVictim::Yes } else { DropVictim::No }
}
(ObjectCandidate(_), ProjectionCandidate(..))
| (ProjectionCandidate(..), ObjectCandidate(_)) => {
Expand All @@ -1987,7 +2012,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { .. }
| TraitAliasCandidate,
) => true,
) => DropVictim::Yes,

(
ImplCandidate(..)
Expand All @@ -2001,7 +2026,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
| BuiltinCandidate { .. }
| TraitAliasCandidate,
ObjectCandidate(_) | ProjectionCandidate(..),
) => false,
) => DropVictim::No,

(&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
// See if we can toss out `victim` based on specialization.
Expand All @@ -2014,7 +2039,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
let tcx = self.tcx();
if other.evaluation.must_apply_modulo_regions() {
if tcx.specializes((other_def, victim_def)) {
return true;
return DropVictim::Yes;
}
}

Expand Down Expand Up @@ -2060,13 +2085,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
// will then correctly report an inference error, since the
// existence of multiple marker trait impls tells us nothing
// about which one should actually apply.
!needs_infer
if needs_infer { DropVictim::No } else { DropVictim::Yes }
}
Some(_) => true,
None => false,
Some(_) => DropVictim::Yes,
None => DropVictim::No,
}
} else {
false
DropVictim::No
}
}

Expand All @@ -2092,10 +2117,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
| TraitUpcastingUnsizeCandidate(_)
| BuiltinCandidate { has_nested: true }
| TraitAliasCandidate,
) => false,
) => DropVictim::No,
}
}
}

impl<'tcx> SelectionContext<'_, 'tcx> {
fn sized_conditions(
&mut self,
obligation: &TraitObligation<'tcx>,
Expand Down