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pat_constructors returns at most one constructor

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I tracked the origins of `pat_constructors` returning a `Vec` to commit
9b3f9d94441340f0cdf6ec59aab739baef0f1ac0. It was indeed specifically
for variable-length slice patterns.
This commit is contained in:
Nadrieril 2019-11-05 16:16:47 +00:00
parent d7044f23e9
commit 149792b608
1 changed files with 16 additions and 24 deletions

40
src/librustc_mir/hair/pattern/_match.rs

@ -1615,13 +1615,13 @@ pub fn is_useful<'p, 'a, 'tcx>(
debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
if let Some(constructors) = pat_constructors(cx, v.head(), pcx) {
debug!("is_useful - expanding constructors: {:#?}", constructors);
if let Some(constructor) = pat_constructor(cx, v.head(), pcx) {
debug!("is_useful - expanding constructor: {:#?}", constructor);
split_grouped_constructors(
cx.tcx,
cx.param_env,
pcx,
constructors,
vec![constructor],
matrix,
pcx.span,
Some(hir_id),
@ -1634,7 +1634,7 @@ pub fn is_useful<'p, 'a, 'tcx>(
debug!("is_useful - expanding wildcard");
let used_ctors: Vec<Constructor<'_>> =
matrix.heads().flat_map(|p| pat_constructors(cx, p, pcx).unwrap_or(vec![])).collect();
matrix.heads().filter_map(|p| pat_constructor(cx, p, pcx)).collect();
debug!("used_ctors = {:#?}", used_ctors);
// `all_ctors` are all the constructors for the given type, which
// should all be represented (or caught with the wild pattern `_`).
@ -1777,47 +1777,39 @@ fn is_useful_specialized<'p, 'a, 'tcx>(
.unwrap_or(NotUseful)
}
/// Determines the constructors that the given pattern can be specialized to.
///
/// In most cases, there's only one constructor that a specific pattern
/// represents, such as a specific enum variant or a specific literal value.
/// Slice patterns, however, can match slices of different lengths. For instance,
/// `[a, b, tail @ ..]` can match a slice of length 2, 3, 4 and so on.
///
/// Determines the constructor that the given pattern can be specialized to.
/// Returns `None` in case of a catch-all, which can't be specialized.
fn pat_constructors<'tcx>(
fn pat_constructor<'tcx>(
cx: &mut MatchCheckCtxt<'_, 'tcx>,
pat: &Pat<'tcx>,
pcx: PatCtxt<'tcx>,
) -> Option<Vec<Constructor<'tcx>>> {
) -> Option<Constructor<'tcx>> {
match *pat.kind {
PatKind::AscribeUserType { ref subpattern, .. } => pat_constructors(cx, subpattern, pcx),
PatKind::AscribeUserType { ref subpattern, .. } => pat_constructor(cx, subpattern, pcx),
PatKind::Binding { .. } | PatKind::Wild => None,
PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(vec![Single]),
PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(Single),
PatKind::Variant { adt_def, variant_index, .. } => {
Some(vec![Variant(adt_def.variants[variant_index].def_id)])
Some(Variant(adt_def.variants[variant_index].def_id))
}
PatKind::Constant { value } => Some(vec![ConstantValue(value, pat.span)]),
PatKind::Range(PatRange { lo, hi, end }) => Some(vec![ConstantRange(
PatKind::Constant { value } => Some(ConstantValue(value, pat.span)),
PatKind::Range(PatRange { lo, hi, end }) => Some(ConstantRange(
lo.eval_bits(cx.tcx, cx.param_env, lo.ty),
hi.eval_bits(cx.tcx, cx.param_env, hi.ty),
lo.ty,
end,
pat.span,
)]),
)),
PatKind::Array { .. } => match pcx.ty.kind {
ty::Array(_, length) => {
Some(vec![FixedLenSlice(length.eval_usize(cx.tcx, cx.param_env))])
}
ty::Array(_, length) => Some(FixedLenSlice(length.eval_usize(cx.tcx, cx.param_env))),
_ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty),
},
PatKind::Slice { ref prefix, ref slice, ref suffix } => {
let prefix = prefix.len() as u64;
let suffix = suffix.len() as u64;
if slice.is_some() {
Some(vec![VarLenSlice(prefix, suffix)])
Some(VarLenSlice(prefix, suffix))
} else {
Some(vec![FixedLenSlice(prefix + suffix)])
Some(FixedLenSlice(prefix + suffix))
}
}
PatKind::Or { .. } => {