pat_constructors returns at most one constructor
I tracked the origins of `pat_constructors` returning a `Vec` to commit 9b3f9d94441340f0cdf6ec59aab739baef0f1ac0. It was indeed specifically for variable-length slice patterns.
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@ -1615,13 +1615,13 @@ pub fn is_useful<'p, 'a, 'tcx>(
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debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
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if let Some(constructors) = pat_constructors(cx, v.head(), pcx) {
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debug!("is_useful - expanding constructors: {:#?}", constructors);
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if let Some(constructor) = pat_constructor(cx, v.head(), pcx) {
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debug!("is_useful - expanding constructor: {:#?}", constructor);
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split_grouped_constructors(
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cx.tcx,
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cx.param_env,
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pcx,
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constructors,
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vec![constructor],
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matrix,
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pcx.span,
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Some(hir_id),
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@ -1634,7 +1634,7 @@ pub fn is_useful<'p, 'a, 'tcx>(
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debug!("is_useful - expanding wildcard");
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let used_ctors: Vec<Constructor<'_>> =
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matrix.heads().flat_map(|p| pat_constructors(cx, p, pcx).unwrap_or(vec![])).collect();
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matrix.heads().filter_map(|p| pat_constructor(cx, p, pcx)).collect();
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debug!("used_ctors = {:#?}", used_ctors);
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// `all_ctors` are all the constructors for the given type, which
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// should all be represented (or caught with the wild pattern `_`).
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@ -1777,47 +1777,39 @@ fn is_useful_specialized<'p, 'a, 'tcx>(
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.unwrap_or(NotUseful)
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}
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/// Determines the constructors that the given pattern can be specialized to.
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///
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/// In most cases, there's only one constructor that a specific pattern
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/// represents, such as a specific enum variant or a specific literal value.
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/// Slice patterns, however, can match slices of different lengths. For instance,
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/// `[a, b, tail @ ..]` can match a slice of length 2, 3, 4 and so on.
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///
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/// Determines the constructor that the given pattern can be specialized to.
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/// Returns `None` in case of a catch-all, which can't be specialized.
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fn pat_constructors<'tcx>(
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fn pat_constructor<'tcx>(
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cx: &mut MatchCheckCtxt<'_, 'tcx>,
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pat: &Pat<'tcx>,
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pcx: PatCtxt<'tcx>,
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) -> Option<Vec<Constructor<'tcx>>> {
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) -> Option<Constructor<'tcx>> {
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match *pat.kind {
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PatKind::AscribeUserType { ref subpattern, .. } => pat_constructors(cx, subpattern, pcx),
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PatKind::AscribeUserType { ref subpattern, .. } => pat_constructor(cx, subpattern, pcx),
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PatKind::Binding { .. } | PatKind::Wild => None,
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PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(vec![Single]),
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PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(Single),
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PatKind::Variant { adt_def, variant_index, .. } => {
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Some(vec![Variant(adt_def.variants[variant_index].def_id)])
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Some(Variant(adt_def.variants[variant_index].def_id))
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}
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PatKind::Constant { value } => Some(vec![ConstantValue(value, pat.span)]),
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PatKind::Range(PatRange { lo, hi, end }) => Some(vec![ConstantRange(
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PatKind::Constant { value } => Some(ConstantValue(value, pat.span)),
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PatKind::Range(PatRange { lo, hi, end }) => Some(ConstantRange(
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lo.eval_bits(cx.tcx, cx.param_env, lo.ty),
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hi.eval_bits(cx.tcx, cx.param_env, hi.ty),
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lo.ty,
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end,
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pat.span,
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)]),
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)),
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PatKind::Array { .. } => match pcx.ty.kind {
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ty::Array(_, length) => {
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Some(vec![FixedLenSlice(length.eval_usize(cx.tcx, cx.param_env))])
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}
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ty::Array(_, length) => Some(FixedLenSlice(length.eval_usize(cx.tcx, cx.param_env))),
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_ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty),
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},
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PatKind::Slice { ref prefix, ref slice, ref suffix } => {
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let prefix = prefix.len() as u64;
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let suffix = suffix.len() as u64;
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if slice.is_some() {
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Some(vec![VarLenSlice(prefix, suffix)])
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Some(VarLenSlice(prefix, suffix))
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} else {
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Some(vec![FixedLenSlice(prefix + suffix)])
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Some(FixedLenSlice(prefix + suffix))
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}
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}
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PatKind::Or { .. } => {
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