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Inline max_slice_length
Note that where we previously ran `max_slice_len` with input having not only matrix.heads() but also v.head(). Now we run it on matrix.heads() only, but also take into account the currently processed constructor. This preserves behavior since `pat_constructors` returns only one constructor in the case that is of interest for us.
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‎src/librustc_mir/hair/pattern/_match.rs‎

Lines changed: 99 additions & 106 deletions
Original file line numberDiff line numberDiff line change
@@ -985,7 +985,6 @@ pub enum WitnessPreference {
985985
#[derive(Copy, Clone, Debug)]
986986
struct PatCtxt<'tcx> {
987987
ty: Ty<'tcx>,
988-
max_slice_length: u64,
989988
span: Span,
990989
}
991990

@@ -1143,108 +1142,6 @@ fn all_constructors<'a, 'tcx>(
11431142
ctors
11441143
}
11451144

1146-
fn max_slice_length<'p, 'a, 'tcx, I>(cx: &mut MatchCheckCtxt<'a, 'tcx>, patterns: I) -> u64
1147-
where
1148-
I: Iterator<Item = &'p Pat<'tcx>>,
1149-
'tcx: 'p,
1150-
{
1151-
// The exhaustiveness-checking paper does not include any details on
1152-
// checking variable-length slice patterns. However, they are matched
1153-
// by an infinite collection of fixed-length array patterns.
1154-
//
1155-
// Checking the infinite set directly would take an infinite amount
1156-
// of time. However, it turns out that for each finite set of
1157-
// patterns `P`, all sufficiently large array lengths are equivalent:
1158-
//
1159-
// Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
1160-
// to exactly the subset `Pₜ` of `P` can be transformed to a slice
1161-
// `sₘ` for each sufficiently-large length `m` that applies to exactly
1162-
// the same subset of `P`.
1163-
//
1164-
// Because of that, each witness for reachability-checking from one
1165-
// of the sufficiently-large lengths can be transformed to an
1166-
// equally-valid witness from any other length, so we only have
1167-
// to check slice lengths from the "minimal sufficiently-large length"
1168-
// and below.
1169-
//
1170-
// Note that the fact that there is a *single* `sₘ` for each `m`
1171-
// not depending on the specific pattern in `P` is important: if
1172-
// you look at the pair of patterns
1173-
// `[true, ..]`
1174-
// `[.., false]`
1175-
// Then any slice of length ≥1 that matches one of these two
1176-
// patterns can be trivially turned to a slice of any
1177-
// other length ≥1 that matches them and vice-versa - for
1178-
// but the slice from length 2 `[false, true]` that matches neither
1179-
// of these patterns can't be turned to a slice from length 1 that
1180-
// matches neither of these patterns, so we have to consider
1181-
// slices from length 2 there.
1182-
//
1183-
// Now, to see that that length exists and find it, observe that slice
1184-
// patterns are either "fixed-length" patterns (`[_, _, _]`) or
1185-
// "variable-length" patterns (`[_, .., _]`).
1186-
//
1187-
// For fixed-length patterns, all slices with lengths *longer* than
1188-
// the pattern's length have the same outcome (of not matching), so
1189-
// as long as `L` is greater than the pattern's length we can pick
1190-
// any `sₘ` from that length and get the same result.
1191-
//
1192-
// For variable-length patterns, the situation is more complicated,
1193-
// because as seen above the precise value of `sₘ` matters.
1194-
//
1195-
// However, for each variable-length pattern `p` with a prefix of length
1196-
// `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
1197-
// `slₚ` elements are examined.
1198-
//
1199-
// Therefore, as long as `L` is positive (to avoid concerns about empty
1200-
// types), all elements after the maximum prefix length and before
1201-
// the maximum suffix length are not examined by any variable-length
1202-
// pattern, and therefore can be added/removed without affecting
1203-
// them - creating equivalent patterns from any sufficiently-large
1204-
// length.
1205-
//
1206-
// Of course, if fixed-length patterns exist, we must be sure
1207-
// that our length is large enough to miss them all, so
1208-
// we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
1209-
//
1210-
// for example, with the above pair of patterns, all elements
1211-
// but the first and last can be added/removed, so any
1212-
// witness of length ≥2 (say, `[false, false, true]`) can be
1213-
// turned to a witness from any other length ≥2.
1214-
1215-
let mut max_prefix_len = 0;
1216-
let mut max_suffix_len = 0;
1217-
let mut max_fixed_len = 0;
1218-
1219-
for row in patterns {
1220-
match *row.kind {
1221-
PatKind::Constant { value } => {
1222-
// extract the length of an array/slice from a constant
1223-
match (value.val, &value.ty.kind) {
1224-
(_, ty::Array(_, n)) => {
1225-
max_fixed_len = cmp::max(max_fixed_len, n.eval_usize(cx.tcx, cx.param_env))
1226-
}
1227-
(ConstValue::Slice { start, end, .. }, ty::Slice(_)) => {
1228-
max_fixed_len = cmp::max(max_fixed_len, (end - start) as u64)
1229-
}
1230-
_ => {}
1231-
}
1232-
}
1233-
PatKind::Slice { ref prefix, slice: None, ref suffix } => {
1234-
let fixed_len = prefix.len() as u64 + suffix.len() as u64;
1235-
max_fixed_len = cmp::max(max_fixed_len, fixed_len);
1236-
}
1237-
PatKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
1238-
max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
1239-
max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
1240-
}
1241-
_ => {}
1242-
}
1243-
}
1244-
1245-
cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len)
1246-
}
1247-
12481145
/// An inclusive interval, used for precise integer exhaustiveness checking.
12491146
/// `IntRange`s always store a contiguous range. This means that values are
12501147
/// encoded such that `0` encodes the minimum value for the integer,
@@ -1609,7 +1506,6 @@ pub fn is_useful<'p, 'a, 'tcx>(
16091506
// introducing uninhabited patterns for inaccessible fields. We
16101507
// need to figure out how to model that.
16111508
ty,
1612-
max_slice_length: max_slice_length(cx, matrix.heads().chain(Some(v.head()))),
16131509
span,
16141510
};
16151511

@@ -2088,8 +1984,105 @@ fn split_grouped_constructors<'p, 'tcx>(
20881984
split_ctors.push(IntRange::range_to_ctor(tcx, ty, range, span));
20891985
}
20901986
}
2091-
VarLenSlice(prefix, suffix) => {
2092-
split_ctors.extend((prefix + suffix..pcx.max_slice_length + 1).map(FixedLenSlice))
1987+
VarLenSlice(self_prefix, self_suffix) => {
1988+
// The exhaustiveness-checking paper does not include any details on
1989+
// checking variable-length slice patterns. However, they are matched
1990+
// by an infinite collection of fixed-length array patterns.
1991+
//
1992+
// Checking the infinite set directly would take an infinite amount
1993+
// of time. However, it turns out that for each finite set of
1994+
// patterns `P`, all sufficiently large array lengths are equivalent:
1995+
//
1996+
// Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
1997+
// to exactly the subset `Pₜ` of `P` can be transformed to a slice
1998+
// `sₘ` for each sufficiently-large length `m` that applies to exactly
1999+
// the same subset of `P`.
2000+
//
2001+
// Because of that, each witness for reachability-checking from one
2002+
// of the sufficiently-large lengths can be transformed to an
2003+
// equally-valid witness from any other length, so we only have
2004+
// to check slice lengths from the "minimal sufficiently-large length"
2005+
// and below.
2006+
//
2007+
// Note that the fact that there is a *single* `sₘ` for each `m`
2008+
// not depending on the specific pattern in `P` is important: if
2009+
// you look at the pair of patterns
2010+
// `[true, ..]`
2011+
// `[.., false]`
2012+
// Then any slice of length ≥1 that matches one of these two
2013+
// patterns can be trivially turned to a slice of any
2014+
// other length ≥1 that matches them and vice-versa - for
2015+
// but the slice from length 2 `[false, true]` that matches neither
2016+
// of these patterns can't be turned to a slice from length 1 that
2017+
// matches neither of these patterns, so we have to consider
2018+
// slices from length 2 there.
2019+
//
2020+
// Now, to see that that length exists and find it, observe that slice
2021+
// patterns are either "fixed-length" patterns (`[_, _, _]`) or
2022+
// "variable-length" patterns (`[_, .., _]`).
2023+
//
2024+
// For fixed-length patterns, all slices with lengths *longer* than
2025+
// the pattern's length have the same outcome (of not matching), so
2026+
// as long as `L` is greater than the pattern's length we can pick
2027+
// any `sₘ` from that length and get the same result.
2028+
//
2029+
// For variable-length patterns, the situation is more complicated,
2030+
// because as seen above the precise value of `sₘ` matters.
2031+
//
2032+
// However, for each variable-length pattern `p` with a prefix of length
2033+
// `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
2034+
// `slₚ` elements are examined.
2035+
//
2036+
// Therefore, as long as `L` is positive (to avoid concerns about empty
2037+
// types), all elements after the maximum prefix length and before
2038+
// the maximum suffix length are not examined by any variable-length
2039+
// pattern, and therefore can be added/removed without affecting
2040+
// them - creating equivalent patterns from any sufficiently-large
2041+
// length.
2042+
//
2043+
// Of course, if fixed-length patterns exist, we must be sure
2044+
// that our length is large enough to miss them all, so
2045+
// we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
2046+
//
2047+
// for example, with the above pair of patterns, all elements
2048+
// but the first and last can be added/removed, so any
2049+
// witness of length ≥2 (say, `[false, false, true]`) can be
2050+
// turned to a witness from any other length ≥2.
2051+
2052+
let mut max_prefix_len = self_prefix;
2053+
let mut max_suffix_len = self_suffix;
2054+
let mut max_fixed_len = 0;
2055+
2056+
for row in matrix.heads() {
2057+
match *row.kind {
2058+
PatKind::Constant { value } => {
2059+
// extract the length of an array/slice from a constant
2060+
match (value.val, &value.ty.kind) {
2061+
(_, ty::Array(_, n)) => {
2062+
max_fixed_len =
2063+
cmp::max(max_fixed_len, n.eval_usize(tcx, param_env))
2064+
}
2065+
(ConstValue::Slice { start, end, .. }, ty::Slice(_)) => {
2066+
max_fixed_len = cmp::max(max_fixed_len, (end - start) as u64)
2067+
}
2068+
_ => {}
2069+
}
2070+
}
2071+
PatKind::Slice { ref prefix, slice: None, ref suffix } => {
2072+
let fixed_len = prefix.len() as u64 + suffix.len() as u64;
2073+
max_fixed_len = cmp::max(max_fixed_len, fixed_len);
2074+
}
2075+
PatKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
2076+
max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
2077+
max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
2078+
}
2079+
_ => {}
2080+
}
2081+
}
2082+
2083+
let max_slice_length = cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len);
2084+
split_ctors
2085+
.extend((self_prefix + self_suffix..=max_slice_length).map(FixedLenSlice))
20932086
}
20942087
// Any other constructor can be used unchanged.
20952088
_ => split_ctors.push(ctor),

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