1 /// This file includes the logic for exhaustiveness and usefulness checking for
2 /// pattern-matching. Specifically, given a list of patterns for a type, we can
4 /// (a) the patterns cover every possible constructor for the type [exhaustiveness]
5 /// (b) each pattern is necessary [usefulness]
7 /// The algorithm implemented here is a modified version of the one described in:
8 /// http://moscova.inria.fr/~maranget/papers/warn/index.html
9 /// However, to save future implementors from reading the original paper, we
10 /// summarise the algorithm here to hopefully save time and be a little clearer
11 /// (without being so rigorous).
13 /// The core of the algorithm revolves about a "usefulness" check. In particular, we
14 /// are trying to compute a predicate `U(P, p_{m + 1})` where `P` is a list of patterns
15 /// of length `m` for a compound (product) type with `n` components (we refer to this as
16 /// a matrix). `U(P, p_{m + 1})` represents whether, given an existing list of patterns
17 /// `p_1 ..= p_m`, adding a new pattern will be "useful" (that is, cover previously-
18 /// uncovered values of the type).
20 /// If we have this predicate, then we can easily compute both exhaustiveness of an
21 /// entire set of patterns and the individual usefulness of each one.
22 /// (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
23 /// match doesn't increase the number of values we're matching)
24 /// (b) a pattern `p_i` is not useful if `U(P[0..=(i-1), p_i)` is false (i.e., adding a
25 /// pattern to those that have come before it doesn't increase the number of values
28 /// For example, say we have the following:
30 /// // x: (Option<bool>, Result<()>)
32 /// (Some(true), _) => {}
33 /// (None, Err(())) => {}
34 /// (None, Err(_)) => {}
37 /// Here, the matrix `P` is 3 x 2 (rows x columns).
43 /// We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
44 /// `[Some(false), _]`, for instance). In addition, row 3 is not useful, because
45 /// all the values it covers are already covered by row 2.
47 /// To compute `U`, we must have two other concepts.
48 /// 1. `S(c, P)` is a "specialized matrix", where `c` is a constructor (like `Some` or
49 /// `None`). You can think of it as filtering `P` to just the rows whose *first* pattern
50 /// can cover `c` (and expanding OR-patterns into distinct patterns), and then expanding
51 /// the constructor into all of its components.
52 /// The specialization of a row vector is computed by `specialize`.
54 /// It is computed as follows. For each row `p_i` of P, we have four cases:
55 /// 1.1. `p_(i,1) = c(r_1, .., r_a)`. Then `S(c, P)` has a corresponding row:
56 /// r_1, .., r_a, p_(i,2), .., p_(i,n)
57 /// 1.2. `p_(i,1) = c'(r_1, .., r_a')` where `c ≠ c'`. Then `S(c, P)` has no
58 /// corresponding row.
59 /// 1.3. `p_(i,1) = _`. Then `S(c, P)` has a corresponding row:
60 /// _, .., _, p_(i,2), .., p_(i,n)
61 /// 1.4. `p_(i,1) = r_1 | r_2`. Then `S(c, P)` has corresponding rows inlined from:
62 /// S(c, (r_1, p_(i,2), .., p_(i,n)))
63 /// S(c, (r_2, p_(i,2), .., p_(i,n)))
65 /// 2. `D(P)` is a "default matrix". This is used when we know there are missing
66 /// constructor cases, but there might be existing wildcard patterns, so to check the
67 /// usefulness of the matrix, we have to check all its *other* components.
68 /// The default matrix is computed inline in `is_useful`.
70 /// It is computed as follows. For each row `p_i` of P, we have three cases:
71 /// 1.1. `p_(i,1) = c(r_1, .., r_a)`. Then `D(P)` has no corresponding row.
72 /// 1.2. `p_(i,1) = _`. Then `D(P)` has a corresponding row:
73 /// p_(i,2), .., p_(i,n)
74 /// 1.3. `p_(i,1) = r_1 | r_2`. Then `D(P)` has corresponding rows inlined from:
75 /// D((r_1, p_(i,2), .., p_(i,n)))
76 /// D((r_2, p_(i,2), .., p_(i,n)))
78 /// Note that the OR-patterns are not always used directly in Rust, but are used to derive
79 /// the exhaustive integer matching rules, so they're written here for posterity.
81 /// The algorithm for computing `U`
82 /// -------------------------------
83 /// The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
84 /// That means we're going to check the components from left-to-right, so the algorithm
85 /// operates principally on the first component of the matrix and new pattern `p_{m + 1}`.
86 /// This algorithm is realised in the `is_useful` function.
88 /// Base case. (`n = 0`, i.e., an empty tuple pattern)
89 /// - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`),
90 /// then `U(P, p_{m + 1})` is false.
91 /// - Otherwise, `P` must be empty, so `U(P, p_{m + 1})` is true.
93 /// Inductive step. (`n > 0`, i.e., whether there's at least one column
94 /// [which may then be expanded into further columns later])
95 /// We're going to match on the new pattern, `p_{m + 1}`.
96 /// - If `p_{m + 1} == c(r_1, .., r_a)`, then we have a constructor pattern.
97 /// Thus, the usefulness of `p_{m + 1}` can be reduced to whether it is useful when
98 /// we ignore all the patterns in `P` that involve other constructors. This is where
99 /// `S(c, P)` comes in:
100 /// `U(P, p_{m + 1}) := U(S(c, P), S(c, p_{m + 1}))`
101 /// This special case is handled in `is_useful_specialized`.
102 /// - If `p_{m + 1} == _`, then we have two more cases:
103 /// + All the constructors of the first component of the type exist within
104 /// all the rows (after having expanded OR-patterns). In this case:
105 /// `U(P, p_{m + 1}) := ∨(k ϵ constructors) U(S(k, P), S(k, p_{m + 1}))`
106 /// I.e., the pattern `p_{m + 1}` is only useful when all the constructors are
107 /// present *if* its later components are useful for the respective constructors
108 /// covered by `p_{m + 1}` (usually a single constructor, but all in the case of `_`).
109 /// + Some constructors are not present in the existing rows (after having expanded
110 /// OR-patterns). However, there might be wildcard patterns (`_`) present. Thus, we
111 /// are only really concerned with the other patterns leading with wildcards. This is
112 /// where `D` comes in:
113 /// `U(P, p_{m + 1}) := U(D(P), p_({m + 1},2), .., p_({m + 1},n))`
114 /// - If `p_{m + 1} == r_1 | r_2`, then the usefulness depends on each separately:
115 /// `U(P, p_{m + 1}) := U(P, (r_1, p_({m + 1},2), .., p_({m + 1},n)))
116 /// || U(P, (r_2, p_({m + 1},2), .., p_({m + 1},n)))`
118 /// Modifications to the algorithm
119 /// ------------------------------
120 /// The algorithm in the paper doesn't cover some of the special cases that arise in Rust, for
121 /// example uninhabited types and variable-length slice patterns. These are drawn attention to
122 /// throughout the code below. I'll make a quick note here about how exhaustive integer matching
123 /// is accounted for, though.
125 /// Exhaustive integer matching
126 /// ---------------------------
127 /// An integer type can be thought of as a (huge) sum type: 1 | 2 | 3 | ...
128 /// So to support exhaustive integer matching, we can make use of the logic in the paper for
129 /// OR-patterns. However, we obviously can't just treat ranges x..=y as individual sums, because
130 /// they are likely gigantic. So we instead treat ranges as constructors of the integers. This means
131 /// that we have a constructor *of* constructors (the integers themselves). We then need to work
132 /// through all the inductive step rules above, deriving how the ranges would be treated as
133 /// OR-patterns, and making sure that they're treated in the same way even when they're ranges.
134 /// There are really only four special cases here:
135 /// - When we match on a constructor that's actually a range, we have to treat it as if we would
137 /// + It turns out that we can simply extend the case for single-value patterns in
138 /// `specialize` to either be *equal* to a value constructor, or *contained within* a range
140 /// + When the pattern itself is a range, you just want to tell whether any of the values in
141 /// the pattern range coincide with values in the constructor range, which is precisely
143 /// Since when encountering a range pattern for a value constructor, we also use inclusion, it
144 /// means that whenever the constructor is a value/range and the pattern is also a value/range,
145 /// we can simply use intersection to test usefulness.
146 /// - When we're testing for usefulness of a pattern and the pattern's first component is a
148 /// + If all the constructors appear in the matrix, we have a slight complication. By default,
149 /// the behaviour (i.e., a disjunction over specialised matrices for each constructor) is
150 /// invalid, because we want a disjunction over every *integer* in each range, not just a
151 /// disjunction over every range. This is a bit more tricky to deal with: essentially we need
152 /// to form equivalence classes of subranges of the constructor range for which the behaviour
153 /// of the matrix `P` and new pattern `p_{m + 1}` are the same. This is described in more
154 /// detail in `split_grouped_constructors`.
155 /// + If some constructors are missing from the matrix, it turns out we don't need to do
156 /// anything special (because we know none of the integers are actually wildcards: i.e., we
157 /// can't span wildcards using ranges).
159 use self::Constructor::*;
160 use self::Usefulness::*;
161 use self::WitnessPreference::*;
163 use rustc_data_structures::fx::FxHashMap;
164 use rustc_index::vec::Idx;
166 use super::{FieldPat, Pat, PatKind, PatRange};
167 use super::{PatternFoldable, PatternFolder, compare_const_vals};
169 use rustc::hir::def_id::DefId;
170 use rustc::hir::{RangeEnd, HirId};
171 use rustc::ty::{self, Ty, TyCtxt, TypeFoldable, Const};
172 use rustc::ty::layout::{Integer, IntegerExt, VariantIdx, Size};
174 use rustc::mir::Field;
175 use rustc::mir::interpret::{ConstValue, Scalar, truncate, AllocId, Pointer};
176 use rustc::util::common::ErrorReported;
179 use syntax::attr::{SignedInt, UnsignedInt};
180 use syntax_pos::{Span, DUMMY_SP};
182 use arena::TypedArena;
184 use smallvec::{SmallVec, smallvec};
185 use std::cmp::{self, Ordering, min, max};
187 use std::iter::{FromIterator, IntoIterator};
188 use std::ops::RangeInclusive;
190 use std::convert::TryInto;
192 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pat<'tcx>) -> Pat<'tcx> {
193 LiteralExpander { tcx: cx.tcx }.fold_pattern(&pat)
196 struct LiteralExpander<'tcx> {
200 impl LiteralExpander<'tcx> {
201 /// Derefs `val` and potentially unsizes the value if `crty` is an array and `rty` a slice.
203 /// `crty` and `rty` can differ because you can use array constants in the presence of slice
204 /// patterns. So the pattern may end up being a slice, but the constant is an array. We convert
205 /// the array to a slice in that case.
206 fn fold_const_value_deref(
208 val: ConstValue<'tcx>,
209 // the pattern's pointee type
211 // the constant's pointee type
213 ) -> ConstValue<'tcx> {
214 debug!("fold_const_value_deref {:?} {:?} {:?}", val, rty, crty);
215 match (val, &crty.kind, &rty.kind) {
216 // the easy case, deref a reference
217 (ConstValue::Scalar(Scalar::Ptr(p)), x, y) if x == y => {
218 let alloc = self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id);
224 // unsize array to slice if pattern is array but match value or other patterns are slice
225 (ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
228 data: self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id),
229 start: p.offset.bytes().try_into().unwrap(),
230 end: n.eval_usize(self.tcx, ty::ParamEnv::empty()).try_into().unwrap(),
233 // fat pointers stay the same
234 | (ConstValue::Slice { .. }, _, _)
235 | (_, ty::Slice(_), ty::Slice(_))
236 | (_, ty::Str, ty::Str)
238 // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
239 _ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
244 impl PatternFolder<'tcx> for LiteralExpander<'tcx> {
245 fn fold_pattern(&mut self, pat: &Pat<'tcx>) -> Pat<'tcx> {
246 debug!("fold_pattern {:?} {:?} {:?}", pat, pat.ty.kind, pat.kind);
247 match (&pat.ty.kind, &*pat.kind) {
250 &PatKind::Constant { value: Const {
252 ty: ty::TyS { kind: ty::Ref(_, crty, _), .. },
258 kind: box PatKind::Deref {
262 kind: box PatKind::Constant { value: self.tcx.mk_const(Const {
263 val: self.fold_const_value_deref(*val, rty, crty),
270 (_, &PatKind::Binding { subpattern: Some(ref s), .. }) => {
273 _ => pat.super_fold_with(self)
278 impl<'tcx> Pat<'tcx> {
279 fn is_wildcard(&self) -> bool {
281 PatKind::Binding { subpattern: None, .. } | PatKind::Wild =>
288 /// A 2D matrix. Nx1 matrices are very common, which is why `SmallVec[_; 2]`
289 /// works well for each row.
290 pub struct Matrix<'p, 'tcx>(Vec<SmallVec<[&'p Pat<'tcx>; 2]>>);
292 impl<'p, 'tcx> Matrix<'p, 'tcx> {
293 pub fn empty() -> Self {
297 pub fn push(&mut self, row: SmallVec<[&'p Pat<'tcx>; 2]>) {
302 /// Pretty-printer for matrices of patterns, example:
303 /// ++++++++++++++++++++++++++
305 /// ++++++++++++++++++++++++++
306 /// + true + [First] +
307 /// ++++++++++++++++++++++++++
308 /// + true + [Second(true)] +
309 /// ++++++++++++++++++++++++++
311 /// ++++++++++++++++++++++++++
312 /// + _ + [_, _, ..tail] +
313 /// ++++++++++++++++++++++++++
314 impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
315 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
318 let &Matrix(ref m) = self;
319 let pretty_printed_matrix: Vec<Vec<String>> = m.iter().map(|row| {
320 row.iter().map(|pat| format!("{:?}", pat)).collect()
323 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
324 assert!(m.iter().all(|row| row.len() == column_count));
325 let column_widths: Vec<usize> = (0..column_count).map(|col| {
326 pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)
329 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
330 let br = "+".repeat(total_width);
331 write!(f, "{}\n", br)?;
332 for row in pretty_printed_matrix {
334 for (column, pat_str) in row.into_iter().enumerate() {
336 write!(f, "{:1$}", pat_str, column_widths[column])?;
340 write!(f, "{}\n", br)?;
346 impl<'p, 'tcx> FromIterator<SmallVec<[&'p Pat<'tcx>; 2]>> for Matrix<'p, 'tcx> {
347 fn from_iter<T>(iter: T) -> Self
348 where T: IntoIterator<Item=SmallVec<[&'p Pat<'tcx>; 2]>>
350 Matrix(iter.into_iter().collect())
354 pub struct MatchCheckCtxt<'a, 'tcx> {
355 pub tcx: TyCtxt<'tcx>,
356 /// The module in which the match occurs. This is necessary for
357 /// checking inhabited-ness of types because whether a type is (visibly)
358 /// inhabited can depend on whether it was defined in the current module or
359 /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty
360 /// outside it's module and should not be matchable with an empty match
363 param_env: ty::ParamEnv<'tcx>,
364 pub pattern_arena: &'a TypedArena<Pat<'tcx>>,
365 pub byte_array_map: FxHashMap<*const Pat<'tcx>, Vec<&'a Pat<'tcx>>>,
368 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
369 pub fn create_and_enter<F, R>(
371 param_env: ty::ParamEnv<'tcx>,
376 F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R,
378 let pattern_arena = TypedArena::default();
384 pattern_arena: &pattern_arena,
385 byte_array_map: FxHashMap::default(),
389 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
390 if self.tcx.features().exhaustive_patterns {
391 self.tcx.is_ty_uninhabited_from(self.module, ty)
397 fn is_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
399 ty::Adt(adt_def, ..) => adt_def.is_variant_list_non_exhaustive(),
404 fn is_local(&self, ty: Ty<'tcx>) -> bool {
406 ty::Adt(adt_def, ..) => adt_def.did.is_local(),
412 #[derive(Clone, Debug)]
413 enum Constructor<'tcx> {
414 /// The constructor of all patterns that don't vary by constructor,
415 /// e.g., struct patterns and fixed-length arrays.
420 ConstantValue(&'tcx ty::Const<'tcx>, Span),
421 /// Ranges of literal values (`2..=5` and `2..5`).
422 ConstantRange(u128, u128, Ty<'tcx>, RangeEnd, Span),
423 /// Array patterns of length n.
427 // Ignore spans when comparing, they don't carry semantic information as they are only for lints.
428 impl<'tcx> std::cmp::PartialEq for Constructor<'tcx> {
429 fn eq(&self, other: &Self) -> bool {
430 match (self, other) {
431 (Constructor::Single, Constructor::Single) => true,
432 (Constructor::Variant(a), Constructor::Variant(b)) => a == b,
433 (Constructor::ConstantValue(a, _), Constructor::ConstantValue(b, _)) => a == b,
435 Constructor::ConstantRange(a_start, a_end, a_ty, a_range_end, _),
436 Constructor::ConstantRange(b_start, b_end, b_ty, b_range_end, _),
437 ) => a_start == b_start && a_end == b_end && a_ty == b_ty && a_range_end == b_range_end,
438 (Constructor::Slice(a), Constructor::Slice(b)) => a == b,
444 impl<'tcx> Constructor<'tcx> {
445 fn is_slice(&self) -> bool {
447 Slice { .. } => true,
452 fn variant_index_for_adt<'a>(
454 cx: &MatchCheckCtxt<'a, 'tcx>,
455 adt: &'tcx ty::AdtDef,
458 Variant(id) => adt.variant_index_with_id(*id),
460 assert!(!adt.is_enum());
463 ConstantValue(c, _) => crate::const_eval::const_variant_index(cx.tcx, cx.param_env, c),
464 _ => bug!("bad constructor {:?} for adt {:?}", self, adt)
468 fn display(&self, tcx: TyCtxt<'tcx>) -> String {
470 Constructor::ConstantValue(val, _) => format!("{}", val),
471 Constructor::ConstantRange(lo, hi, ty, range_end, _) => {
472 // Get the right sign on the output:
473 let ty = ty::ParamEnv::empty().and(*ty);
476 ty::Const::from_bits(tcx, *lo, ty),
478 ty::Const::from_bits(tcx, *hi, ty),
481 Constructor::Slice(val) => format!("[{}]", val),
482 _ => bug!("bad constructor being displayed: `{:?}", self),
487 #[derive(Clone, Debug)]
488 pub enum Usefulness<'tcx> {
490 UsefulWithWitness(Vec<Witness<'tcx>>),
494 impl<'tcx> Usefulness<'tcx> {
495 fn is_useful(&self) -> bool {
503 #[derive(Copy, Clone, Debug)]
504 pub enum WitnessPreference {
509 #[derive(Copy, Clone, Debug)]
510 struct PatCtxt<'tcx> {
512 max_slice_length: u64,
516 /// A witness of non-exhaustiveness for error reporting, represented
517 /// as a list of patterns (in reverse order of construction) with
518 /// wildcards inside to represent elements that can take any inhabitant
519 /// of the type as a value.
521 /// A witness against a list of patterns should have the same types
522 /// and length as the pattern matched against. Because Rust `match`
523 /// is always against a single pattern, at the end the witness will
524 /// have length 1, but in the middle of the algorithm, it can contain
525 /// multiple patterns.
527 /// For example, if we are constructing a witness for the match against
529 /// struct Pair(Option<(u32, u32)>, bool);
531 /// match (p: Pair) {
532 /// Pair(None, _) => {}
533 /// Pair(_, false) => {}
537 /// We'll perform the following steps:
538 /// 1. Start with an empty witness
539 /// `Witness(vec![])`
540 /// 2. Push a witness `Some(_)` against the `None`
541 /// `Witness(vec![Some(_)])`
542 /// 3. Push a witness `true` against the `false`
543 /// `Witness(vec![Some(_), true])`
544 /// 4. Apply the `Pair` constructor to the witnesses
545 /// `Witness(vec![Pair(Some(_), true)])`
547 /// The final `Pair(Some(_), true)` is then the resulting witness.
548 #[derive(Clone, Debug)]
549 pub struct Witness<'tcx>(Vec<Pat<'tcx>>);
551 impl<'tcx> Witness<'tcx> {
552 pub fn single_pattern(self) -> Pat<'tcx> {
553 assert_eq!(self.0.len(), 1);
554 self.0.into_iter().next().unwrap()
557 fn push_wild_constructor<'a>(
559 cx: &MatchCheckCtxt<'a, 'tcx>,
560 ctor: &Constructor<'tcx>,
564 let sub_pattern_tys = constructor_sub_pattern_tys(cx, ctor, ty);
565 self.0.extend(sub_pattern_tys.into_iter().map(|ty| {
569 kind: box PatKind::Wild,
572 self.apply_constructor(cx, ctor, ty)
575 /// Constructs a partial witness for a pattern given a list of
576 /// patterns expanded by the specialization step.
578 /// When a pattern P is discovered to be useful, this function is used bottom-up
579 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
580 /// of values, V, where each value in that set is not covered by any previously
581 /// used patterns and is covered by the pattern P'. Examples:
583 /// left_ty: tuple of 3 elements
584 /// pats: [10, 20, _] => (10, 20, _)
586 /// left_ty: struct X { a: (bool, &'static str), b: usize}
587 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
588 fn apply_constructor<'a>(
590 cx: &MatchCheckCtxt<'a,'tcx>,
591 ctor: &Constructor<'tcx>,
595 let arity = constructor_arity(cx, ctor, ty);
597 let len = self.0.len() as u64;
598 let mut pats = self.0.drain((len - arity) as usize..).rev();
603 let pats = pats.enumerate().map(|(i, p)| {
605 field: Field::new(i),
610 if let ty::Adt(adt, substs) = ty.kind {
615 variant_index: ctor.variant_index_for_adt(cx, adt),
619 PatKind::Leaf { subpatterns: pats }
622 PatKind::Leaf { subpatterns: pats }
627 PatKind::Deref { subpattern: pats.nth(0).unwrap() }
630 ty::Slice(_) | ty::Array(..) => {
632 prefix: pats.collect(),
640 ConstantValue(value, _) => PatKind::Constant { value },
641 ConstantRange(lo, hi, ty, end, _) => PatKind::Range(PatRange {
642 lo: ty::Const::from_bits(cx.tcx, lo, ty::ParamEnv::empty().and(ty)),
643 hi: ty::Const::from_bits(cx.tcx, hi, ty::ParamEnv::empty().and(ty)),
662 /// This determines the set of all possible constructors of a pattern matching
663 /// values of type `left_ty`. For vectors, this would normally be an infinite set
664 /// but is instead bounded by the maximum fixed length of slice patterns in
665 /// the column of patterns being analyzed.
667 /// We make sure to omit constructors that are statically impossible. E.g., for
668 /// `Option<!>`, we do not include `Some(_)` in the returned list of constructors.
669 fn all_constructors<'a, 'tcx>(
670 cx: &mut MatchCheckCtxt<'a, 'tcx>,
672 ) -> Vec<Constructor<'tcx>> {
673 debug!("all_constructors({:?})", pcx.ty);
674 let ctors = match pcx.ty.kind {
676 [true, false].iter().map(|&b| {
677 ConstantValue(ty::Const::from_bool(cx.tcx, b), pcx.span)
680 ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx, cx.param_env).is_some() => {
681 let len = len.eval_usize(cx.tcx, cx.param_env);
682 if len != 0 && cx.is_uninhabited(sub_ty) {
688 // Treat arrays of a constant but unknown length like slices.
689 ty::Array(ref sub_ty, _) |
690 ty::Slice(ref sub_ty) => {
691 if cx.is_uninhabited(sub_ty) {
694 (0..pcx.max_slice_length+1).map(|length| Slice(length)).collect()
697 ty::Adt(def, substs) if def.is_enum() => {
700 !cx.tcx.features().exhaustive_patterns ||
701 !v.uninhabited_from(cx.tcx, substs, def.adt_kind()).contains(cx.tcx, cx.module)
703 .map(|v| Variant(v.def_id))
708 // The valid Unicode Scalar Value ranges.
718 '\u{10FFFF}' as u128,
726 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
727 let min = 1u128 << (bits - 1);
729 vec![ConstantRange(min, max, pcx.ty, RangeEnd::Included, pcx.span)]
732 let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
733 let max = truncate(u128::max_value(), size);
734 vec![ConstantRange(0, max, pcx.ty, RangeEnd::Included, pcx.span)]
737 if cx.is_uninhabited(pcx.ty) {
747 fn max_slice_length<'p, 'a, 'tcx, I>(cx: &mut MatchCheckCtxt<'a, 'tcx>, patterns: I) -> u64
749 I: Iterator<Item = &'p Pat<'tcx>>,
752 // The exhaustiveness-checking paper does not include any details on
753 // checking variable-length slice patterns. However, they are matched
754 // by an infinite collection of fixed-length array patterns.
756 // Checking the infinite set directly would take an infinite amount
757 // of time. However, it turns out that for each finite set of
758 // patterns `P`, all sufficiently large array lengths are equivalent:
760 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
761 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
762 // `sₘ` for each sufficiently-large length `m` that applies to exactly
763 // the same subset of `P`.
765 // Because of that, each witness for reachability-checking from one
766 // of the sufficiently-large lengths can be transformed to an
767 // equally-valid witness from any other length, so we only have
768 // to check slice lengths from the "minimal sufficiently-large length"
771 // Note that the fact that there is a *single* `sₘ` for each `m`
772 // not depending on the specific pattern in `P` is important: if
773 // you look at the pair of patterns
776 // Then any slice of length ≥1 that matches one of these two
777 // patterns can be trivially turned to a slice of any
778 // other length ≥1 that matches them and vice-versa - for
779 // but the slice from length 2 `[false, true]` that matches neither
780 // of these patterns can't be turned to a slice from length 1 that
781 // matches neither of these patterns, so we have to consider
782 // slices from length 2 there.
784 // Now, to see that that length exists and find it, observe that slice
785 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
786 // "variable-length" patterns (`[_, .., _]`).
788 // For fixed-length patterns, all slices with lengths *longer* than
789 // the pattern's length have the same outcome (of not matching), so
790 // as long as `L` is greater than the pattern's length we can pick
791 // any `sₘ` from that length and get the same result.
793 // For variable-length patterns, the situation is more complicated,
794 // because as seen above the precise value of `sₘ` matters.
796 // However, for each variable-length pattern `p` with a prefix of length
797 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
798 // `slₚ` elements are examined.
800 // Therefore, as long as `L` is positive (to avoid concerns about empty
801 // types), all elements after the maximum prefix length and before
802 // the maximum suffix length are not examined by any variable-length
803 // pattern, and therefore can be added/removed without affecting
804 // them - creating equivalent patterns from any sufficiently-large
807 // Of course, if fixed-length patterns exist, we must be sure
808 // that our length is large enough to miss them all, so
809 // we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
811 // for example, with the above pair of patterns, all elements
812 // but the first and last can be added/removed, so any
813 // witness of length ≥2 (say, `[false, false, true]`) can be
814 // turned to a witness from any other length ≥2.
816 let mut max_prefix_len = 0;
817 let mut max_suffix_len = 0;
818 let mut max_fixed_len = 0;
820 for row in patterns {
822 PatKind::Constant { value } => {
823 // extract the length of an array/slice from a constant
824 match (value.val, &value.ty.kind) {
825 (_, ty::Array(_, n)) => max_fixed_len = cmp::max(
827 n.eval_usize(cx.tcx, cx.param_env),
829 (ConstValue::Slice{ start, end, .. }, ty::Slice(_)) => max_fixed_len = cmp::max(
831 (end - start) as u64,
836 PatKind::Slice { ref prefix, slice: None, ref suffix } => {
837 let fixed_len = prefix.len() as u64 + suffix.len() as u64;
838 max_fixed_len = cmp::max(max_fixed_len, fixed_len);
840 PatKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
841 max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
842 max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
848 cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len)
851 /// An inclusive interval, used for precise integer exhaustiveness checking.
852 /// `IntRange`s always store a contiguous range. This means that values are
853 /// encoded such that `0` encodes the minimum value for the integer,
854 /// regardless of the signedness.
855 /// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
856 /// This makes comparisons and arithmetic on interval endpoints much more
857 /// straightforward. See `signed_bias` for details.
859 /// `IntRange` is never used to encode an empty range or a "range" that wraps
860 /// around the (offset) space: i.e., `range.lo <= range.hi`.
861 #[derive(Clone, Debug)]
862 struct IntRange<'tcx> {
863 pub range: RangeInclusive<u128>,
868 impl<'tcx> IntRange<'tcx> {
870 fn is_integral(ty: Ty<'_>) -> bool {
872 ty::Char | ty::Int(_) | ty::Uint(_) => true,
878 fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
880 ty::Char => Some((Size::from_bytes(4), 0)),
882 let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
883 Some((size, 1u128 << (size.bits() as u128 - 1)))
885 ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
893 param_env: ty::ParamEnv<'tcx>,
896 ) -> Option<IntRange<'tcx>> {
897 if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
899 let val = if let ConstValue::Scalar(Scalar::Raw { data, size }) = value.val {
900 // For this specific pattern we can skip a lot of effort and go
901 // straight to the result, after doing a bit of checking. (We
902 // could remove this branch and just use the next branch, which
903 // is more general but much slower.)
904 Scalar::<()>::check_raw(data, size, target_size);
906 } else if let Some(val) = value.try_eval_bits(tcx, param_env, ty) {
907 // This is a more general form of the previous branch.
912 let val = val ^ bias;
913 Some(IntRange { range: val..=val, ty, span })
927 ) -> Option<IntRange<'tcx>> {
928 if Self::is_integral(ty) {
929 // Perform a shift if the underlying types are signed,
930 // which makes the interval arithmetic simpler.
931 let bias = IntRange::signed_bias(tcx, ty);
932 let (lo, hi) = (lo ^ bias, hi ^ bias);
933 // Make sure the interval is well-formed.
934 if lo > hi || lo == hi && *end == RangeEnd::Excluded {
937 let offset = (*end == RangeEnd::Excluded) as u128;
938 Some(IntRange { range: lo..=(hi - offset), ty, span })
947 param_env: ty::ParamEnv<'tcx>,
948 ctor: &Constructor<'tcx>,
949 ) -> Option<IntRange<'tcx>> {
950 // Floating-point ranges are permitted and we don't want
951 // to consider them when constructing integer ranges.
953 ConstantRange(lo, hi, ty, end, span) => Self::from_range(tcx, *lo, *hi, ty, end, *span),
954 ConstantValue(val, span) => Self::from_const(tcx, param_env, val, *span),
961 param_env: ty::ParamEnv<'tcx>,
963 ) -> Option<IntRange<'tcx>> {
966 box PatKind::Constant { value } => {
967 return Self::from_const(tcx, param_env, value, pat.span);
969 box PatKind::Range(PatRange { lo, hi, end }) => {
970 return Self::from_range(
972 lo.eval_bits(tcx, param_env, lo.ty),
973 hi.eval_bits(tcx, param_env, hi.ty),
979 box PatKind::AscribeUserType { ref subpattern, .. } => {
987 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
988 fn signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
991 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
998 /// Converts a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
1002 r: RangeInclusive<u128>,
1004 ) -> Constructor<'tcx> {
1005 let bias = IntRange::signed_bias(tcx, ty);
1006 let (lo, hi) = r.into_inner();
1008 let ty = ty::ParamEnv::empty().and(ty);
1009 ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty), span)
1011 ConstantRange(lo ^ bias, hi ^ bias, ty, RangeEnd::Included, span)
1015 /// Returns a collection of ranges that spans the values covered by `ranges`, subtracted
1016 /// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
1020 param_env: ty::ParamEnv<'tcx>,
1021 ranges: Vec<Constructor<'tcx>>,
1022 ) -> Vec<Constructor<'tcx>> {
1023 let ranges = ranges.into_iter().filter_map(|r| {
1024 IntRange::from_ctor(tcx, param_env, &r).map(|i| i.range)
1026 let mut remaining_ranges = vec![];
1028 let (lo, hi) = self.range.into_inner();
1029 for subrange in ranges {
1030 let (subrange_lo, subrange_hi) = subrange.into_inner();
1031 if lo > subrange_hi || subrange_lo > hi {
1032 // The pattern doesn't intersect with the subrange at all,
1033 // so the subrange remains untouched.
1034 remaining_ranges.push(
1035 Self::range_to_ctor(tcx, ty, subrange_lo..=subrange_hi, self.span),
1038 if lo > subrange_lo {
1039 // The pattern intersects an upper section of the
1040 // subrange, so a lower section will remain.
1041 remaining_ranges.push(
1042 Self::range_to_ctor(tcx, ty, subrange_lo..=(lo - 1), self.span),
1045 if hi < subrange_hi {
1046 // The pattern intersects a lower section of the
1047 // subrange, so an upper section will remain.
1048 remaining_ranges.push(
1049 Self::range_to_ctor(tcx, ty, (hi + 1)..=subrange_hi, self.span),
1057 fn intersection(&self, other: &Self) -> Option<Self> {
1059 let (lo, hi) = (*self.range.start(), *self.range.end());
1060 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1061 if lo <= other_hi && other_lo <= hi {
1062 let span = other.span;
1063 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty, span })
1069 fn suspicious_intersection(&self, other: &Self) -> bool {
1070 // `false` in the following cases:
1071 // 1 ---- // 1 ---------- // 1 ---- // 1 ----
1072 // 2 ---------- // 2 ---- // 2 ---- // 2 ----
1074 // The following are currently `false`, but could be `true` in the future (#64007):
1075 // 1 --------- // 1 ---------
1076 // 2 ---------- // 2 ----------
1078 // `true` in the following cases:
1079 // 1 ------- // 1 -------
1080 // 2 -------- // 2 -------
1081 let (lo, hi) = (*self.range.start(), *self.range.end());
1082 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1083 (lo == other_hi || hi == other_lo)
1087 // A request for missing constructor data in terms of either:
1088 // - whether or not there any missing constructors; or
1089 // - the actual set of missing constructors.
1090 #[derive(PartialEq)]
1091 enum MissingCtorsInfo {
1096 // Used by `compute_missing_ctors`.
1097 #[derive(Debug, PartialEq)]
1098 enum MissingCtors<'tcx> {
1102 // Note that the Vec can be empty.
1103 Ctors(Vec<Constructor<'tcx>>),
1106 // When `info` is `MissingCtorsInfo::Ctors`, compute a set of constructors
1107 // equivalent to `all_ctors \ used_ctors`. When `info` is
1108 // `MissingCtorsInfo::Emptiness`, just determines if that set is empty or not.
1109 // (The split logic gives a performance win, because we always need to know if
1110 // the set is empty, but we rarely need the full set, and it can be expensive
1111 // to compute the full set.)
1112 fn compute_missing_ctors<'tcx>(
1113 info: MissingCtorsInfo,
1115 param_env: ty::ParamEnv<'tcx>,
1116 all_ctors: &Vec<Constructor<'tcx>>,
1117 used_ctors: &Vec<Constructor<'tcx>>,
1118 ) -> MissingCtors<'tcx> {
1119 let mut missing_ctors = vec![];
1121 for req_ctor in all_ctors {
1122 let mut refined_ctors = vec![req_ctor.clone()];
1123 for used_ctor in used_ctors {
1124 if used_ctor == req_ctor {
1125 // If a constructor appears in a `match` arm, we can
1126 // eliminate it straight away.
1127 refined_ctors = vec![]
1128 } else if let Some(interval) = IntRange::from_ctor(tcx, param_env, used_ctor) {
1129 // Refine the required constructors for the type by subtracting
1130 // the range defined by the current constructor pattern.
1131 refined_ctors = interval.subtract_from(tcx, param_env, refined_ctors);
1134 // If the constructor patterns that have been considered so far
1135 // already cover the entire range of values, then we the
1136 // constructor is not missing, and we can move on to the next one.
1137 if refined_ctors.is_empty() {
1141 // If a constructor has not been matched, then it is missing.
1142 // We add `refined_ctors` instead of `req_ctor`, because then we can
1143 // provide more detailed error information about precisely which
1144 // ranges have been omitted.
1145 if info == MissingCtorsInfo::Emptiness {
1146 if !refined_ctors.is_empty() {
1147 // The set is non-empty; return early.
1148 return MissingCtors::NonEmpty;
1151 missing_ctors.extend(refined_ctors);
1155 if info == MissingCtorsInfo::Emptiness {
1156 // If we reached here, the set is empty.
1159 MissingCtors::Ctors(missing_ctors)
1163 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html.
1164 /// The algorithm from the paper has been modified to correctly handle empty
1165 /// types. The changes are:
1166 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1167 /// continue to recurse over columns.
1168 /// (1) all_constructors will only return constructors that are statically
1169 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
1171 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1172 /// to a set of such vectors `m` - this is defined as there being a set of
1173 /// inputs that will match `v` but not any of the sets in `m`.
1175 /// All the patterns at each column of the `matrix ++ v` matrix must
1176 /// have the same type, except that wildcard (PatKind::Wild) patterns
1177 /// with type `TyErr` are also allowed, even if the "type of the column"
1178 /// is not `TyErr`. That is used to represent private fields, as using their
1179 /// real type would assert that they are inhabited.
1181 /// This is used both for reachability checking (if a pattern isn't useful in
1182 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1183 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1184 /// matrix isn't exhaustive).
1185 pub fn is_useful<'p, 'a, 'tcx>(
1186 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1187 matrix: &Matrix<'p, 'tcx>,
1189 witness: WitnessPreference,
1191 ) -> Usefulness<'tcx> {
1192 let &Matrix(ref rows) = matrix;
1193 debug!("is_useful({:#?}, {:#?})", matrix, v);
1195 // The base case. We are pattern-matching on () and the return value is
1196 // based on whether our matrix has a row or not.
1197 // NOTE: This could potentially be optimized by checking rows.is_empty()
1198 // first and then, if v is non-empty, the return value is based on whether
1199 // the type of the tuple we're checking is inhabited or not.
1201 return if rows.is_empty() {
1203 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
1204 LeaveOutWitness => Useful,
1211 assert!(rows.iter().all(|r| r.len() == v.len()));
1213 let (ty, span) = rows.iter()
1214 .map(|r| (r[0].ty, r[0].span))
1215 .find(|(ty, _)| !ty.references_error())
1216 .unwrap_or((v[0].ty, v[0].span));
1218 // TyErr is used to represent the type of wildcard patterns matching
1219 // against inaccessible (private) fields of structs, so that we won't
1220 // be able to observe whether the types of the struct's fields are
1223 // If the field is truly inaccessible, then all the patterns
1224 // matching against it must be wildcard patterns, so its type
1227 // However, if we are matching against non-wildcard patterns, we
1228 // need to know the real type of the field so we can specialize
1229 // against it. This primarily occurs through constants - they
1230 // can include contents for fields that are inaccessible at the
1231 // location of the match. In that case, the field's type is
1232 // inhabited - by the constant - so we can just use it.
1234 // FIXME: this might lead to "unstable" behavior with macro hygiene
1235 // introducing uninhabited patterns for inaccessible fields. We
1236 // need to figure out how to model that.
1238 max_slice_length: max_slice_length(cx, rows.iter().map(|r| r[0]).chain(Some(v[0]))),
1242 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v[0]);
1244 if let Some(constructors) = pat_constructors(cx, v[0], pcx) {
1245 debug!("is_useful - expanding constructors: {:#?}", constructors);
1246 split_grouped_constructors(
1247 cx.tcx, cx.param_env, constructors, matrix, pcx.ty, pcx.span, Some(hir_id),
1248 ).into_iter().map(|c|
1249 is_useful_specialized(cx, matrix, v, c, pcx.ty, witness, hir_id)
1250 ).find(|result| result.is_useful()).unwrap_or(NotUseful)
1252 debug!("is_useful - expanding wildcard");
1254 let used_ctors: Vec<Constructor<'_>> = rows.iter().flat_map(|row| {
1255 pat_constructors(cx, row[0], pcx).unwrap_or(vec![])
1257 debug!("used_ctors = {:#?}", used_ctors);
1258 // `all_ctors` are all the constructors for the given type, which
1259 // should all be represented (or caught with the wild pattern `_`).
1260 let all_ctors = all_constructors(cx, pcx);
1261 debug!("all_ctors = {:#?}", all_ctors);
1263 // `missing_ctors` is the set of constructors from the same type as the
1264 // first column of `matrix` that are matched only by wildcard patterns
1265 // from the first column.
1267 // Therefore, if there is some pattern that is unmatched by `matrix`,
1268 // it will still be unmatched if the first constructor is replaced by
1269 // any of the constructors in `missing_ctors`
1271 // However, if our scrutinee is *privately* an empty enum, we
1272 // must treat it as though it had an "unknown" constructor (in
1273 // that case, all other patterns obviously can't be variants)
1274 // to avoid exposing its emptyness. See the `match_privately_empty`
1275 // test for details.
1277 // FIXME: currently the only way I know of something can
1278 // be a privately-empty enum is when the exhaustive_patterns
1279 // feature flag is not present, so this is only
1280 // needed for that case.
1282 // Missing constructors are those that are not matched by any
1283 // non-wildcard patterns in the current column. We always determine if
1284 // the set is empty, but we only fully construct them on-demand,
1285 // because they're rarely used and can be big.
1286 let cheap_missing_ctors = compute_missing_ctors(
1287 MissingCtorsInfo::Emptiness, cx.tcx, cx.param_env, &all_ctors, &used_ctors,
1290 let is_privately_empty = all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1291 let is_declared_nonexhaustive = cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
1292 debug!("cheap_missing_ctors={:#?} is_privately_empty={:#?} is_declared_nonexhaustive={:#?}",
1293 cheap_missing_ctors, is_privately_empty, is_declared_nonexhaustive);
1295 // For privately empty and non-exhaustive enums, we work as if there were an "extra"
1296 // `_` constructor for the type, so we can never match over all constructors.
1297 let is_non_exhaustive = is_privately_empty || is_declared_nonexhaustive ||
1298 (pcx.ty.is_ptr_sized_integral() && !cx.tcx.features().precise_pointer_size_matching);
1300 if cheap_missing_ctors == MissingCtors::Empty && !is_non_exhaustive {
1301 split_grouped_constructors(
1302 cx.tcx, cx.param_env, all_ctors, matrix, pcx.ty, DUMMY_SP, None,
1305 .map(|c| is_useful_specialized(cx, matrix, v, c, pcx.ty, witness, hir_id))
1306 .find(|result| result.is_useful())
1307 .unwrap_or(NotUseful)
1309 let matrix = rows.iter().filter_map(|r| {
1310 if r[0].is_wildcard() {
1311 Some(SmallVec::from_slice(&r[1..]))
1316 match is_useful(cx, &matrix, &v[1..], witness, hir_id) {
1317 UsefulWithWitness(pats) => {
1319 // In this case, there's at least one "free"
1320 // constructor that is only matched against by
1321 // wildcard patterns.
1323 // There are 2 ways we can report a witness here.
1324 // Commonly, we can report all the "free"
1325 // constructors as witnesses, e.g., if we have:
1328 // enum Direction { N, S, E, W }
1329 // let Direction::N = ...;
1332 // we can report 3 witnesses: `S`, `E`, and `W`.
1334 // However, there are 2 cases where we don't want
1335 // to do this and instead report a single `_` witness:
1337 // 1) If the user is matching against a non-exhaustive
1338 // enum, there is no point in enumerating all possible
1339 // variants, because the user can't actually match
1340 // against them himself, e.g., in an example like:
1342 // let err: io::ErrorKind = ...;
1344 // io::ErrorKind::NotFound => {},
1347 // we don't want to show every possible IO error,
1348 // but instead have `_` as the witness (this is
1349 // actually *required* if the user specified *all*
1350 // IO errors, but is probably what we want in every
1353 // 2) If the user didn't actually specify a constructor
1354 // in this arm, e.g., in
1356 // let x: (Direction, Direction, bool) = ...;
1357 // let (_, _, false) = x;
1359 // we don't want to show all 16 possible witnesses
1360 // `(<direction-1>, <direction-2>, true)` - we are
1361 // satisfied with `(_, _, true)`. In this case,
1362 // `used_ctors` is empty.
1363 let new_witnesses = if is_non_exhaustive || used_ctors.is_empty() {
1364 // All constructors are unused. Add wild patterns
1365 // rather than each individual constructor.
1366 pats.into_iter().map(|mut witness| {
1367 witness.0.push(Pat {
1370 kind: box PatKind::Wild,
1375 let expensive_missing_ctors = compute_missing_ctors(
1376 MissingCtorsInfo::Ctors, cx.tcx, cx.param_env, &all_ctors, &used_ctors,
1378 if let MissingCtors::Ctors(missing_ctors) = expensive_missing_ctors {
1379 pats.into_iter().flat_map(|witness| {
1380 missing_ctors.iter().map(move |ctor| {
1381 // Extends the witness with a "wild" version of this
1382 // constructor, that matches everything that can be built with
1383 // it. For example, if `ctor` is a `Constructor::Variant` for
1384 // `Option::Some`, this pushes the witness for `Some(_)`.
1385 witness.clone().push_wild_constructor(cx, ctor, pcx.ty)
1389 bug!("cheap missing ctors")
1392 UsefulWithWitness(new_witnesses)
1400 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
1401 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1402 fn is_useful_specialized<'p, 'a, 'tcx>(
1403 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1404 &Matrix(ref m): &Matrix<'p, 'tcx>,
1406 ctor: Constructor<'tcx>,
1408 witness: WitnessPreference,
1410 ) -> Usefulness<'tcx> {
1411 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1412 let sub_pat_tys = constructor_sub_pattern_tys(cx, &ctor, lty);
1413 let wild_patterns_owned: Vec<_> = sub_pat_tys.iter().map(|ty| {
1417 kind: box PatKind::Wild,
1420 let wild_patterns: Vec<_> = wild_patterns_owned.iter().collect();
1421 let matrix = Matrix(
1423 .filter_map(|r| specialize(cx, &r, &ctor, &wild_patterns))
1426 match specialize(cx, v, &ctor, &wild_patterns) {
1427 Some(v) => match is_useful(cx, &matrix, &v, witness, hir_id) {
1428 UsefulWithWitness(witnesses) => UsefulWithWitness(
1429 witnesses.into_iter()
1430 .map(|witness| witness.apply_constructor(cx, &ctor, lty))
1439 /// Determines the constructors that the given pattern can be specialized to.
1441 /// In most cases, there's only one constructor that a specific pattern
1442 /// represents, such as a specific enum variant or a specific literal value.
1443 /// Slice patterns, however, can match slices of different lengths. For instance,
1444 /// `[a, b, ..tail]` can match a slice of length 2, 3, 4 and so on.
1446 /// Returns `None` in case of a catch-all, which can't be specialized.
1447 fn pat_constructors<'tcx>(
1448 cx: &mut MatchCheckCtxt<'_, 'tcx>,
1451 ) -> Option<Vec<Constructor<'tcx>>> {
1453 PatKind::AscribeUserType { ref subpattern, .. } =>
1454 pat_constructors(cx, subpattern, pcx),
1455 PatKind::Binding { .. } | PatKind::Wild => None,
1456 PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(vec![Single]),
1457 PatKind::Variant { adt_def, variant_index, .. } => {
1458 Some(vec![Variant(adt_def.variants[variant_index].def_id)])
1460 PatKind::Constant { value } => Some(vec![ConstantValue(value, pat.span)]),
1461 PatKind::Range(PatRange { lo, hi, end }) =>
1462 Some(vec![ConstantRange(
1463 lo.eval_bits(cx.tcx, cx.param_env, lo.ty),
1464 hi.eval_bits(cx.tcx, cx.param_env, hi.ty),
1469 PatKind::Array { .. } => match pcx.ty.kind {
1470 ty::Array(_, length) => Some(vec![
1471 Slice(length.eval_usize(cx.tcx, cx.param_env))
1473 _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty)
1475 PatKind::Slice { ref prefix, ref slice, ref suffix } => {
1476 let pat_len = prefix.len() as u64 + suffix.len() as u64;
1477 if slice.is_some() {
1478 Some((pat_len..pcx.max_slice_length+1).map(Slice).collect())
1480 Some(vec![Slice(pat_len)])
1483 PatKind::Or { .. } => {
1484 bug!("support for or-patterns has not been fully implemented yet.");
1489 /// This computes the arity of a constructor. The arity of a constructor
1490 /// is how many subpattern patterns of that constructor should be expanded to.
1492 /// For instance, a tuple pattern `(_, 42, Some([]))` has the arity of 3.
1493 /// A struct pattern's arity is the number of fields it contains, etc.
1494 fn constructor_arity(cx: &MatchCheckCtxt<'a, 'tcx>, ctor: &Constructor<'tcx>, ty: Ty<'tcx>) -> u64 {
1495 debug!("constructor_arity({:#?}, {:?})", ctor, ty);
1497 ty::Tuple(ref fs) => fs.len() as u64,
1498 ty::Slice(..) | ty::Array(..) => match *ctor {
1499 Slice(length) => length,
1500 ConstantValue(..) => 0,
1501 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty)
1504 ty::Adt(adt, _) => {
1505 adt.variants[ctor.variant_index_for_adt(cx, adt)].fields.len() as u64
1511 /// This computes the types of the sub patterns that a constructor should be
1514 /// For instance, a tuple pattern (43u32, 'a') has sub pattern types [u32, char].
1515 fn constructor_sub_pattern_tys<'a, 'tcx>(
1516 cx: &MatchCheckCtxt<'a, 'tcx>,
1517 ctor: &Constructor<'tcx>,
1519 ) -> Vec<Ty<'tcx>> {
1520 debug!("constructor_sub_pattern_tys({:#?}, {:?})", ctor, ty);
1522 ty::Tuple(ref fs) => fs.into_iter().map(|t| t.expect_ty()).collect(),
1523 ty::Slice(ty) | ty::Array(ty, _) => match *ctor {
1524 Slice(length) => (0..length).map(|_| ty).collect(),
1525 ConstantValue(..) => vec![],
1526 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty)
1528 ty::Ref(_, rty, _) => vec![rty],
1529 ty::Adt(adt, substs) => {
1531 // Use T as the sub pattern type of Box<T>.
1532 vec![substs.type_at(0)]
1534 let variant = &adt.variants[ctor.variant_index_for_adt(cx, adt)];
1535 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(ty);
1536 variant.fields.iter().map(|field| {
1537 let is_visible = adt.is_enum()
1538 || field.vis.is_accessible_from(cx.module, cx.tcx);
1539 let is_uninhabited = cx.is_uninhabited(field.ty(cx.tcx, substs));
1540 match (is_visible, is_non_exhaustive, is_uninhabited) {
1541 // Treat all uninhabited types in non-exhaustive variants as `TyErr`.
1542 (_, true, true) => cx.tcx.types.err,
1543 // Treat all non-visible fields as `TyErr`. They can't appear in any
1544 // other pattern from this match (because they are private), so their
1545 // type does not matter - but we don't want to know they are uninhabited.
1546 (false, ..) => cx.tcx.types.err,
1548 let ty = field.ty(cx.tcx, substs);
1550 // If the field type returned is an array of an unknown
1551 // size return an TyErr.
1553 if len.try_eval_usize(cx.tcx, cx.param_env).is_none() =>
1566 // checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
1567 // meaning all other types will compare unequal and thus equal patterns often do not cause the
1568 // second pattern to lint about unreachable match arms.
1569 fn slice_pat_covered_by_const<'tcx>(
1572 const_val: &'tcx ty::Const<'tcx>,
1573 prefix: &[Pat<'tcx>],
1574 slice: &Option<Pat<'tcx>>,
1575 suffix: &[Pat<'tcx>],
1576 param_env: ty::ParamEnv<'tcx>,
1577 ) -> Result<bool, ErrorReported> {
1578 let data: &[u8] = match (const_val.val, &const_val.ty.kind) {
1579 (ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
1580 assert_eq!(*t, tcx.types.u8);
1581 let n = n.eval_usize(tcx, param_env);
1582 let ptr = Pointer::new(AllocId(0), offset);
1583 alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
1585 (ConstValue::Slice { data, start, end }, ty::Slice(t)) => {
1586 assert_eq!(*t, tcx.types.u8);
1587 let ptr = Pointer::new(AllocId(0), Size::from_bytes(start as u64));
1588 data.get_bytes(&tcx, ptr, Size::from_bytes((end - start) as u64)).unwrap()
1590 // FIXME(oli-obk): create a way to extract fat pointers from ByRef
1591 (_, ty::Slice(_)) => return Ok(false),
1593 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1594 const_val, prefix, slice, suffix,
1598 let pat_len = prefix.len() + suffix.len();
1599 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1604 data[..prefix.len()].iter().zip(prefix).chain(
1605 data[data.len()-suffix.len()..].iter().zip(suffix))
1608 box PatKind::Constant { value } => {
1609 let b = value.eval_bits(tcx, param_env, pat.ty);
1610 assert_eq!(b as u8 as u128, b);
1622 // Whether to evaluate a constructor using exhaustive integer matching. This is true if the
1623 // constructor is a range or constant with an integer type.
1624 fn should_treat_range_exhaustively(tcx: TyCtxt<'tcx>, ctor: &Constructor<'tcx>) -> bool {
1625 let ty = match ctor {
1626 ConstantValue(value, _) => value.ty,
1627 ConstantRange(_, _, ty, _, _) => ty,
1630 if let ty::Char | ty::Int(_) | ty::Uint(_) = ty.kind {
1631 !ty.is_ptr_sized_integral() || tcx.features().precise_pointer_size_matching
1637 /// For exhaustive integer matching, some constructors are grouped within other constructors
1638 /// (namely integer typed values are grouped within ranges). However, when specialising these
1639 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1640 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1641 /// mean creating a separate constructor for every single value in the range, which is clearly
1642 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1643 /// identical across all values in that range (i.e., there are equivalence classes of ranges of
1644 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
1645 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
1646 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
1648 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1649 /// the group of intersecting patterns changes (using the method described below).
1650 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1651 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1652 /// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
1653 /// need to be worried about matching over gargantuan ranges.
1655 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
1657 /// |------| |----------| |-------| ||
1658 /// |-------| |-------| |----| ||
1661 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
1663 /// |--|--|||-||||--||---|||-------| |-|||| ||
1665 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
1666 /// boundaries for each interval range, sort them, then create constructors for each new interval
1667 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
1668 /// merging operation depicted above.)
1670 /// `hir_id` is `None` when we're evaluating the wildcard pattern, do not lint for overlapping in
1671 /// ranges that case.
1672 fn split_grouped_constructors<'p, 'tcx>(
1674 param_env: ty::ParamEnv<'tcx>,
1675 ctors: Vec<Constructor<'tcx>>,
1676 &Matrix(ref m): &Matrix<'p, 'tcx>,
1679 hir_id: Option<HirId>,
1680 ) -> Vec<Constructor<'tcx>> {
1681 let mut split_ctors = Vec::with_capacity(ctors.len());
1683 for ctor in ctors.into_iter() {
1685 // For now, only ranges may denote groups of "subconstructors", so we only need to
1686 // special-case constant ranges.
1687 ConstantRange(..) if should_treat_range_exhaustively(tcx, &ctor) => {
1688 // We only care about finding all the subranges within the range of the constructor
1689 // range. Anything else is irrelevant, because it is guaranteed to result in
1690 // `NotUseful`, which is the default case anyway, and can be ignored.
1691 let ctor_range = IntRange::from_ctor(tcx, param_env, &ctor).unwrap();
1693 /// Represents a border between 2 integers. Because the intervals spanning borders
1694 /// must be able to cover every integer, we need to be able to represent
1695 /// 2^128 + 1 such borders.
1696 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
1702 // A function for extracting the borders of an integer interval.
1703 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
1704 let (lo, hi) = r.range.into_inner();
1705 let from = Border::JustBefore(lo);
1706 let to = match hi.checked_add(1) {
1707 Some(m) => Border::JustBefore(m),
1708 None => Border::AfterMax,
1710 vec![from, to].into_iter()
1713 // Collect the span and range of all the intersecting ranges to lint on likely
1714 // incorrect range patterns. (#63987)
1715 let mut overlaps = vec![];
1716 // `borders` is the set of borders between equivalence classes: each equivalence
1717 // class lies between 2 borders.
1718 let row_borders = m.iter()
1720 IntRange::from_pat(tcx, param_env, row[0]).map(|r| (r, row.len()))
1722 .flat_map(|(range, row_len)| {
1723 let intersection = ctor_range.intersection(&range);
1724 let should_lint = ctor_range.suspicious_intersection(&range);
1725 if let (Some(range), 1, true) = (&intersection, row_len, should_lint) {
1726 // FIXME: for now, only check for overlapping ranges on simple range
1727 // patterns. Otherwise with the current logic the following is detected
1729 // match (10u8, true) {
1730 // (0 ..= 125, false) => {}
1731 // (126 ..= 255, false) => {}
1732 // (0 ..= 255, true) => {}
1734 overlaps.push(range.clone());
1738 .flat_map(|range| range_borders(range));
1739 let ctor_borders = range_borders(ctor_range.clone());
1740 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
1741 borders.sort_unstable();
1743 lint_overlapping_patterns(tcx, hir_id, ctor_range, ty, overlaps);
1745 // We're going to iterate through every pair of borders, making sure that each
1746 // represents an interval of nonnegative length, and convert each such interval
1747 // into a constructor.
1748 for IntRange { range, .. } in borders.windows(2).filter_map(|window| {
1749 match (window[0], window[1]) {
1750 (Border::JustBefore(n), Border::JustBefore(m)) => {
1752 Some(IntRange { range: n..=(m - 1), ty, span })
1757 (Border::JustBefore(n), Border::AfterMax) => {
1758 Some(IntRange { range: n..=u128::MAX, ty, span })
1760 (Border::AfterMax, _) => None,
1763 split_ctors.push(IntRange::range_to_ctor(tcx, ty, range, span));
1766 // Any other constructor can be used unchanged.
1767 _ => split_ctors.push(ctor),
1774 fn lint_overlapping_patterns(
1776 hir_id: Option<HirId>,
1777 ctor_range: IntRange<'tcx>,
1779 overlaps: Vec<IntRange<'tcx>>,
1781 if let (true, Some(hir_id)) = (!overlaps.is_empty(), hir_id) {
1782 let mut err = tcx.struct_span_lint_hir(
1783 lint::builtin::OVERLAPPING_PATTERNS,
1786 "multiple patterns covering the same range",
1788 err.span_label(ctor_range.span, "overlapping patterns");
1789 for int_range in overlaps {
1790 // Use the real type for user display of the ranges:
1791 err.span_label(int_range.span, &format!(
1792 "this range overlaps on `{}`",
1793 IntRange::range_to_ctor(tcx, ty, int_range.range, DUMMY_SP).display(tcx),
1800 fn constructor_covered_by_range<'tcx>(
1802 param_env: ty::ParamEnv<'tcx>,
1803 ctor: &Constructor<'tcx>,
1805 ) -> Result<bool, ErrorReported> {
1806 let (from, to, end, ty) = match pat.kind {
1807 box PatKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
1808 box PatKind::Range(PatRange { lo, hi, end }) => (lo, hi, end, lo.ty),
1809 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
1811 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
1812 let cmp_from = |c_from| compare_const_vals(tcx, c_from, from, param_env, ty)
1813 .map(|res| res != Ordering::Less);
1814 let cmp_to = |c_to| compare_const_vals(tcx, c_to, to, param_env, ty);
1815 macro_rules! some_or_ok {
1819 None => return Ok(false), // not char or int
1824 ConstantValue(value, _) => {
1825 let to = some_or_ok!(cmp_to(value));
1826 let end = (to == Ordering::Less) ||
1827 (end == RangeEnd::Included && to == Ordering::Equal);
1828 Ok(some_or_ok!(cmp_from(value)) && end)
1830 ConstantRange(from, to, ty, RangeEnd::Included, _) => {
1831 let to = some_or_ok!(cmp_to(ty::Const::from_bits(
1834 ty::ParamEnv::empty().and(ty),
1836 let end = (to == Ordering::Less) ||
1837 (end == RangeEnd::Included && to == Ordering::Equal);
1838 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
1841 ty::ParamEnv::empty().and(ty),
1844 ConstantRange(from, to, ty, RangeEnd::Excluded, _) => {
1845 let to = some_or_ok!(cmp_to(ty::Const::from_bits(
1848 ty::ParamEnv::empty().and(ty)
1850 let end = (to == Ordering::Less) ||
1851 (end == RangeEnd::Excluded && to == Ordering::Equal);
1852 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
1855 ty::ParamEnv::empty().and(ty)))
1863 fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
1864 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1865 subpatterns: &'p [FieldPat<'tcx>],
1866 wild_patterns: &[&'p Pat<'tcx>],
1867 is_non_exhaustive: bool,
1868 ) -> SmallVec<[&'p Pat<'tcx>; 2]> {
1869 let mut result = SmallVec::from_slice(wild_patterns);
1871 for subpat in subpatterns {
1872 if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
1873 result[subpat.field.index()] = &subpat.pattern;
1877 debug!("patterns_for_variant({:#?}, {:#?}) = {:#?}", subpatterns, wild_patterns, result);
1881 /// This is the main specialization step. It expands the first pattern in the given row
1882 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
1883 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
1885 /// OTOH, slice patterns with a subslice pattern (..tail) can be expanded into multiple
1886 /// different patterns.
1887 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
1888 /// fields filled with wild patterns.
1889 fn specialize<'p, 'a: 'p, 'tcx>(
1890 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1891 r: &[&'p Pat<'tcx>],
1892 constructor: &Constructor<'tcx>,
1893 wild_patterns: &[&'p Pat<'tcx>],
1894 ) -> Option<SmallVec<[&'p Pat<'tcx>; 2]>> {
1897 let head = match *pat.kind {
1898 PatKind::AscribeUserType { ref subpattern, .. } => {
1899 specialize(cx, ::std::slice::from_ref(&subpattern), constructor, wild_patterns)
1902 PatKind::Binding { .. } | PatKind::Wild => {
1903 Some(SmallVec::from_slice(wild_patterns))
1906 PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
1907 let ref variant = adt_def.variants[variant_index];
1908 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(pat.ty);
1909 Some(Variant(variant.def_id))
1910 .filter(|variant_constructor| variant_constructor == constructor)
1911 .map(|_| patterns_for_variant(cx, subpatterns, wild_patterns, is_non_exhaustive))
1914 PatKind::Leaf { ref subpatterns } => {
1915 Some(patterns_for_variant(cx, subpatterns, wild_patterns, false))
1918 PatKind::Deref { ref subpattern } => {
1919 Some(smallvec![subpattern])
1922 PatKind::Constant { value } if constructor.is_slice() => {
1923 // We extract an `Option` for the pointer because slices of zero
1924 // elements don't necessarily point to memory, they are usually
1925 // just integers. The only time they should be pointing to memory
1926 // is when they are subslices of nonzero slices.
1927 let (alloc, offset, n, ty) = match value.ty.kind {
1928 ty::Array(t, n) => {
1930 ConstValue::ByRef { offset, alloc, .. } => (
1933 n.eval_usize(cx.tcx, cx.param_env),
1938 "array pattern is {:?}", value,
1944 ConstValue::Slice { data, start, end } => (
1946 Size::from_bytes(start as u64),
1947 (end - start) as u64,
1950 ConstValue::ByRef { .. } => {
1951 // FIXME(oli-obk): implement `deref` for `ConstValue`
1956 "slice pattern constant must be scalar pair but is {:?}",
1963 "unexpected const-val {:?} with ctor {:?}",
1968 if wild_patterns.len() as u64 == n {
1969 // convert a constant slice/array pattern to a list of patterns.
1970 let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
1971 let ptr = Pointer::new(AllocId(0), offset);
1973 let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
1974 let scalar = alloc.read_scalar(
1975 &cx.tcx, ptr, layout.size,
1977 let scalar = scalar.not_undef().ok()?;
1978 let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
1982 kind: box PatKind::Constant { value },
1984 Some(&*cx.pattern_arena.alloc(pattern))
1991 PatKind::Constant { .. } |
1992 PatKind::Range { .. } => {
1993 // If the constructor is a:
1994 // - Single value: add a row if the pattern contains the constructor.
1995 // - Range: add a row if the constructor intersects the pattern.
1996 if should_treat_range_exhaustively(cx.tcx, constructor) {
1997 match (IntRange::from_ctor(cx.tcx, cx.param_env, constructor),
1998 IntRange::from_pat(cx.tcx, cx.param_env, pat)) {
1999 (Some(ctor), Some(pat)) => {
2000 ctor.intersection(&pat).map(|_| {
2001 let (pat_lo, pat_hi) = pat.range.into_inner();
2002 let (ctor_lo, ctor_hi) = ctor.range.into_inner();
2003 assert!(pat_lo <= ctor_lo && ctor_hi <= pat_hi);
2010 // Fallback for non-ranges and ranges that involve
2011 // floating-point numbers, which are not conveniently handled
2012 // by `IntRange`. For these cases, the constructor may not be a
2013 // range so intersection actually devolves into being covered
2015 match constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat) {
2016 Ok(true) => Some(smallvec![]),
2017 Ok(false) | Err(ErrorReported) => None,
2022 PatKind::Array { ref prefix, ref slice, ref suffix } |
2023 PatKind::Slice { ref prefix, ref slice, ref suffix } => {
2024 match *constructor {
2026 let pat_len = prefix.len() + suffix.len();
2027 if let Some(slice_count) = wild_patterns.len().checked_sub(pat_len) {
2028 if slice_count == 0 || slice.is_some() {
2029 Some(prefix.iter().chain(
2030 wild_patterns.iter().map(|p| *p)
2033 .chain(suffix.iter())
2042 ConstantValue(cv, _) => {
2043 match slice_pat_covered_by_const(
2044 cx.tcx, pat.span, cv, prefix, slice, suffix, cx.param_env,
2046 Ok(true) => Some(smallvec![]),
2048 Err(ErrorReported) => None
2051 _ => span_bug!(pat.span,
2052 "unexpected ctor {:?} for slice pat", constructor)
2056 PatKind::Or { .. } => {
2057 bug!("support for or-patterns has not been fully implemented yet.");
2060 debug!("specialize({:#?}, {:#?}) = {:#?}", r[0], wild_patterns, head);
2062 head.map(|mut head| {
2063 head.extend_from_slice(&r[1 ..]);