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Reimplement Usefulness::merge in terms of a binop

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Nadrieril 2020-12-21 04:51:46 +00:00
parent 5547105f6b
commit f4f20c0663
1 changed files with 24 additions and 39 deletions
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63
compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
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@ -696,9 +696,9 @@ impl<'tcx> Usefulness<'tcx> {
}
}
/// When trying several branches and each returns a `Usefulness`, we need to combine the
/// results together.
fn merge(usefulnesses: impl Iterator<Item = Self>) -> Self {
/// Combine usefulnesses from two branches. This is an associative operation and `NotUseful` is
/// a unit.
fn extend(&mut self, other: Self) {
// If we have detected some unreachable sub-branches, we only want to keep them when they
// were unreachable in _all_ branches. Eg. in the following, the last `true` is unreachable
// in the second branch of the first or-pattern, but not otherwise. Therefore we don't want
@ -709,54 +709,39 @@ impl<'tcx> Usefulness<'tcx> {
// (false | true, false | true) => {}
// }
// ```
// Here however we _do_ want to lint that the last `false` is unreachable. So we don't want
// to intersect the spans that come directly from the or-pattern, since each branch of the
// or-pattern brings a new disjoint pattern.
// Here however we _do_ want to lint that the last `false` is unreachable. In order to
// handle that correctly, each branch of an or-pattern marks the other branches as
// unreachable (see `unsplit_or_pat`). That way, intersecting the results will correctly
// identify unreachable sub-patterns.
// ```
// match None {
// Some(false) => {}
// None | Some(true | false) => {}
// }
// ```
match (&mut *self, other) {
(Useful(s), Useful(o)) => s.intersection_mut(&o),
(UsefulWithWitness(s), UsefulWithWitness(o)) => s.extend(o),
(_, NotUseful) => {}
(NotUseful, other) => *self = other,
(UsefulWithWitness(_), Useful(_)) | (Useful(_), UsefulWithWitness(_)) => unreachable!(),
}
}
// Is `None` when no branch was useful. Will often be `Some(Spanset::new())` because the
// sets are only non-empty in the presence of or-patterns.
let mut unreachables: Option<SpanSet> = None;
// Witnesses of usefulness, if any.
let mut witnesses = Vec::new();
/// When trying several branches and each returns a `Usefulness`, we need to combine the
/// results together.
fn merge(usefulnesses: impl Iterator<Item = Self>) -> Self {
let mut ret = NotUseful;
for u in usefulnesses {
match u {
Useful(spans) if spans.is_empty() => {
ret.extend(u);
if let Useful(spans) = &ret {
if spans.is_empty() {
// Once we reach the empty set, more intersections won't change the result.
return Useful(SpanSet::new());
}
Useful(spans) => {
if let Some(unreachables) = &mut unreachables {
if !unreachables.is_empty() {
unreachables.intersection_mut(&spans);
}
if unreachables.is_empty() {
return Useful(SpanSet::new());
}
} else {
unreachables = Some(spans);
}
}
NotUseful => {}
UsefulWithWitness(wits) => {
witnesses.extend(wits);
return ret;
}
}
}
if !witnesses.is_empty() {
UsefulWithWitness(witnesses)
} else if let Some(unreachables) = unreachables {
Useful(unreachables)
} else {
NotUseful
}
ret
}
/// After calculating the usefulness for a branch of an or-pattern, call this to make this