1 // Copyright 2012-2016 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 /// This file includes the logic for exhaustiveness and usefulness checking for
12 /// pattern-matching. Specifically, given a list of patterns for a type, we can
14 /// (a) the patterns cover every possible constructor for the type [exhaustiveness]
15 /// (b) each pattern is necessary [usefulness]
17 /// The algorithm implemented here is a modified version of the one described in:
18 /// http://moscova.inria.fr/~maranget/papers/warn/index.html
19 /// However, to save future implementors from reading the original paper, I'm going
20 /// to summarise the algorithm here to hopefully save time and be a little clearer
21 /// (without being so rigorous).
23 /// The core of the algorithm revolves about a "usefulness" check. In particular, we
24 /// are trying to compute a predicate `U(P, p_{m + 1})` where `P` is a list of patterns
25 /// of length `m` for a compound (product) type with `n` components (we refer to this as
26 /// a matrix). `U(P, p_{m + 1})` represents whether, given an existing list of patterns
27 /// `p_1 ..= p_m`, adding a new pattern will be "useful" (that is, cover previously-
28 /// uncovered values of the type).
30 /// If we have this predicate, then we can easily compute both exhaustiveness of an
31 /// entire set of patterns and the individual usefulness of each one.
32 /// (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e. adding a wildcard
33 /// match doesn't increase the number of values we're matching)
34 /// (b) a pattern `p_i` is not useful if `U(P[0..=(i-1), p_i)` is false (i.e. adding a
35 /// pattern to those that have come before it doesn't increase the number of values
38 /// For example, say we have the following:
40 /// // x: (Option<bool>, Result<()>)
42 /// (Some(true), _) => {}
43 /// (None, Err(())) => {}
44 /// (None, Err(_)) => {}
47 /// Here, the matrix `P` is 3 x 2 (rows x columns).
53 /// We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
54 /// `[Some(false), _]`, for instance). In addition, row 3 is not useful, because
55 /// all the values it covers are already covered by row 2.
57 /// To compute `U`, we must have two other concepts.
58 /// 1. `S(c, P)` is a "specialised matrix", where `c` is a constructor (like `Some` or
59 /// `None`). You can think of it as filtering `P` to just the rows whose *first* pattern
60 /// can cover `c` (and expanding OR-patterns into distinct patterns), and then expanding
61 /// the constructor into all of its components.
62 /// The specialisation of a row vector is computed by `specialize`.
64 /// It is computed as follows. For each row `p_i` of P, we have four cases:
65 /// 1.1. `p_(i,1) = c(r_1, .., r_a)`. Then `S(c, P)` has a corresponding row:
66 /// r_1, .., r_a, p_(i,2), .., p_(i,n)
67 /// 1.2. `p_(i,1) = c'(r_1, .., r_a')` where `c ≠ c'`. Then `S(c, P)` has no
68 /// corresponding row.
69 /// 1.3. `p_(i,1) = _`. Then `S(c, P)` has a corresponding row:
70 /// _, .., _, p_(i,2), .., p_(i,n)
71 /// 1.4. `p_(i,1) = r_1 | r_2`. Then `S(c, P)` has corresponding rows inlined from:
72 /// S(c, (r_1, p_(i,2), .., p_(i,n)))
73 /// S(c, (r_2, p_(i,2), .., p_(i,n)))
75 /// 2. `D(P)` is a "default matrix". This is used when we know there are missing
76 /// constructor cases, but there might be existing wildcard patterns, so to check the
77 /// usefulness of the matrix, we have to check all its *other* components.
78 /// The default matrix is computed inline in `is_useful`.
80 /// It is computed as follows. For each row `p_i` of P, we have three cases:
81 /// 1.1. `p_(i,1) = c(r_1, .., r_a)`. Then `D(P)` has no corresponding row.
82 /// 1.2. `p_(i,1) = _`. Then `D(P)` has a corresponding row:
83 /// p_(i,2), .., p_(i,n)
84 /// 1.3. `p_(i,1) = r_1 | r_2`. Then `D(P)` has corresponding rows inlined from:
85 /// D((r_1, p_(i,2), .., p_(i,n)))
86 /// D((r_2, p_(i,2), .., p_(i,n)))
88 /// Note that the OR-patterns are not always used directly in Rust, but are used to derive
89 /// the exhaustive integer matching rules, so they're written here for posterity.
91 /// The algorithm for computing `U`
92 /// -------------------------------
93 /// The algorithm is inductive (on the number of columns: i.e. components of tuple patterns).
94 /// That means we're going to check the components from left-to-right, so the algorithm
95 /// operates principally on the first component of the matrix and new pattern `p_{m + 1}`.
96 /// This algorithm is realised in the `is_useful` function.
98 /// Base case. (`n = 0`, i.e. an empty tuple pattern)
99 /// - If `P` already contains an empty pattern (i.e. if the number of patterns `m > 0`),
100 /// then `U(P, p_{m + 1})` is false.
101 /// - Otherwise, `P` must be empty, so `U(P, p_{m + 1})` is true.
103 /// Inductive step. (`n > 0`, i.e. whether there's at least one column
104 /// [which may then be expanded into further columns later])
105 /// We're going to match on the new pattern, `p_{m + 1}`.
106 /// - If `p_{m + 1} == c(r_1, .., r_a)`, then we have a constructor pattern.
107 /// Thus, the usefulness of `p_{m + 1}` can be reduced to whether it is useful when
108 /// we ignore all the patterns in `P` that involve other constructors. This is where
109 /// `S(c, P)` comes in:
110 /// `U(P, p_{m + 1}) := U(S(c, P), S(c, p_{m + 1}))`
111 /// This special case is handled in `is_useful_specialized`.
112 /// - If `p_{m + 1} == _`, then we have two more cases:
113 /// + All the constructors of the first component of the type exist within
114 /// all the rows (after having expanded OR-patterns). In this case:
115 /// `U(P, p_{m + 1}) := ∨(k ϵ constructors) U(S(k, P), S(k, p_{m + 1}))`
116 /// I.e. the pattern `p_{m + 1}` is only useful when all the constructors are
117 /// present *if* its later components are useful for the respective constructors
118 /// covered by `p_{m + 1}` (usually a single constructor, but all in the case of `_`).
119 /// + Some constructors are not present in the existing rows (after having expanded
120 /// OR-patterns). However, there might be wildcard patterns (`_`) present. Thus, we
121 /// are only really concerned with the other patterns leading with wildcards. This is
122 /// where `D` comes in:
123 /// `U(P, p_{m + 1}) := U(D(P), p_({m + 1},2), .., p_({m + 1},n))`
124 /// - If `p_{m + 1} == r_1 | r_2`, then the usefulness depends on each separately:
125 /// `U(P, p_{m + 1}) := U(P, (r_1, p_({m + 1},2), .., p_({m + 1},n)))
126 /// || U(P, (r_2, p_({m + 1},2), .., p_({m + 1},n)))`
128 /// Modifications to the algorithm
129 /// ------------------------------
130 /// The algorithm in the paper doesn't cover some of the special cases that arise in Rust, for
131 /// example uninhabited types and variable-length slice patterns. These are drawn attention to
132 /// throughout the code below. I'll make a quick note here about how exhaustive integer matching
133 /// is accounted for, though.
135 /// Exhaustive integer matching
136 /// ---------------------------
137 /// An integer type can be thought of as a (huge) sum type: 1 | 2 | 3 | ...
138 /// So to support exhaustive integer matching, we can make use of the logic in the paper for
139 /// OR-patterns. However, we obviously can't just treat ranges x..=y as individual sums, because
140 /// they are likely gigantic. So we instead treat ranges as constructors of the integers. This means
141 /// that we have a constructor *of* constructors (the integers themselves). We then need to work
142 /// through all the inductive step rules above, deriving how the ranges would be treated as
143 /// OR-patterns, and making sure that they're treated in the same way even when they're ranges.
144 /// There are really only four special cases here:
145 /// - When we match on a constructor that's actually a range, we have to treat it as if we would
147 /// + It turns out that we can simply extend the case for single-value patterns in
148 /// `specialize` to either be *equal* to a value constructor, or *contained within* a range
150 /// + When the pattern itself is a range, you just want to tell whether any of the values in
151 /// the pattern range coincide with values in the constructor range, which is precisely
153 /// Since when encountering a range pattern for a value constructor, we also use inclusion, it
154 /// means that whenever the constructor is a value/range and the pattern is also a value/range,
155 /// we can simply use intersection to test usefulness.
156 /// - When we're testing for usefulness of a pattern and the pattern's first component is a
158 /// + If all the constructors appear in the matrix, we have a slight complication. By default,
159 /// the behaviour (i.e. a disjunction over specialised matrices for each constructor) is
160 /// invalid, because we want a disjunction over every *integer* in each range, not just a
161 /// disjunction over every range. This is a bit more tricky to deal with: essentially we need
162 /// to form equivalence classes of subranges of the constructor range for which the behaviour
163 /// of the matrix `P` and new pattern `p_{m + 1}` are the same. This is described in more
164 /// detail in `split_grouped_constructors`.
165 /// + If some constructors are missing from the matrix, it turns out we don't need to do
166 /// anything special (because we know none of the integers are actually wildcards: i.e. we
167 /// can't span wildcards using ranges).
169 use self::Constructor::*;
170 use self::Usefulness::*;
171 use self::WitnessPreference::*;
173 use rustc_data_structures::fx::FxHashMap;
174 use rustc_data_structures::indexed_vec::Idx;
176 use super::{FieldPattern, Pattern, PatternKind};
177 use super::{PatternFoldable, PatternFolder, compare_const_vals};
179 use rustc::hir::def_id::DefId;
180 use rustc::hir::RangeEnd;
181 use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
182 use rustc::ty::layout::{Integer, IntegerExt};
184 use rustc::mir::Field;
185 use rustc::mir::interpret::ConstValue;
186 use rustc::util::common::ErrorReported;
188 use syntax::attr::{SignedInt, UnsignedInt};
189 use syntax_pos::{Span, DUMMY_SP};
191 use arena::TypedArena;
193 use std::cmp::{self, Ordering, min, max};
195 use std::iter::{FromIterator, IntoIterator};
196 use std::ops::RangeInclusive;
198 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pattern<'tcx>)
201 cx.pattern_arena.alloc(LiteralExpander.fold_pattern(&pat))
204 struct LiteralExpander;
205 impl<'tcx> PatternFolder<'tcx> for LiteralExpander {
206 fn fold_pattern(&mut self, pat: &Pattern<'tcx>) -> Pattern<'tcx> {
207 match (&pat.ty.sty, &*pat.kind) {
208 (&ty::TyRef(_, rty, _), &PatternKind::Constant { ref value }) => {
212 kind: box PatternKind::Deref {
213 subpattern: Pattern {
216 kind: box PatternKind::Constant { value: value.clone() },
221 (_, &PatternKind::Binding { subpattern: Some(ref s), .. }) => {
224 _ => pat.super_fold_with(self)
229 impl<'tcx> Pattern<'tcx> {
230 fn is_wildcard(&self) -> bool {
232 PatternKind::Binding { subpattern: None, .. } | PatternKind::Wild =>
239 pub struct Matrix<'a, 'tcx: 'a>(Vec<Vec<&'a Pattern<'tcx>>>);
241 impl<'a, 'tcx> Matrix<'a, 'tcx> {
242 pub fn empty() -> Self {
246 pub fn push(&mut self, row: Vec<&'a Pattern<'tcx>>) {
251 /// Pretty-printer for matrices of patterns, example:
252 /// ++++++++++++++++++++++++++
254 /// ++++++++++++++++++++++++++
255 /// + true + [First] +
256 /// ++++++++++++++++++++++++++
257 /// + true + [Second(true)] +
258 /// ++++++++++++++++++++++++++
260 /// ++++++++++++++++++++++++++
261 /// + _ + [_, _, ..tail] +
262 /// ++++++++++++++++++++++++++
263 impl<'a, 'tcx> fmt::Debug for Matrix<'a, 'tcx> {
264 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
267 let &Matrix(ref m) = self;
268 let pretty_printed_matrix: Vec<Vec<String>> = m.iter().map(|row| {
269 row.iter().map(|pat| format!("{:?}", pat)).collect()
272 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
273 assert!(m.iter().all(|row| row.len() == column_count));
274 let column_widths: Vec<usize> = (0..column_count).map(|col| {
275 pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)
278 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
279 let br = "+".repeat(total_width);
280 write!(f, "{}\n", br)?;
281 for row in pretty_printed_matrix {
283 for (column, pat_str) in row.into_iter().enumerate() {
285 write!(f, "{:1$}", pat_str, column_widths[column])?;
289 write!(f, "{}\n", br)?;
295 impl<'a, 'tcx> FromIterator<Vec<&'a Pattern<'tcx>>> for Matrix<'a, 'tcx> {
296 fn from_iter<T: IntoIterator<Item=Vec<&'a Pattern<'tcx>>>>(iter: T) -> Self
298 Matrix(iter.into_iter().collect())
302 pub struct MatchCheckCtxt<'a, 'tcx: 'a> {
303 pub tcx: TyCtxt<'a, 'tcx, 'tcx>,
304 /// The module in which the match occurs. This is necessary for
305 /// checking inhabited-ness of types because whether a type is (visibly)
306 /// inhabited can depend on whether it was defined in the current module or
307 /// not. eg. `struct Foo { _private: ! }` cannot be seen to be empty
308 /// outside it's module and should not be matchable with an empty match
311 pub pattern_arena: &'a TypedArena<Pattern<'tcx>>,
312 pub byte_array_map: FxHashMap<*const Pattern<'tcx>, Vec<&'a Pattern<'tcx>>>,
315 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
316 pub fn create_and_enter<F, R>(
317 tcx: TyCtxt<'a, 'tcx, 'tcx>,
320 where F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R
322 let pattern_arena = TypedArena::new();
327 pattern_arena: &pattern_arena,
328 byte_array_map: FxHashMap::default(),
332 // convert a byte-string pattern to a list of u8 patterns.
333 fn lower_byte_str_pattern<'p>(&mut self, pat: &'p Pattern<'tcx>) -> Vec<&'p Pattern<'tcx>>
336 let pattern_arena = &*self.pattern_arena;
338 self.byte_array_map.entry(pat).or_insert_with(|| {
340 box PatternKind::Constant {
343 if let Some(ptr) = const_val.to_ptr() {
344 let is_array_ptr = const_val.ty
346 .and_then(|t| t.ty.builtin_index())
347 .map_or(false, |t| t == tcx.types.u8);
348 assert!(is_array_ptr);
349 let alloc = tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
350 assert_eq!(ptr.offset.bytes(), 0);
351 // FIXME: check length
352 alloc.bytes.iter().map(|b| {
353 &*pattern_arena.alloc(Pattern {
356 kind: box PatternKind::Constant {
357 value: ty::Const::from_bits(
360 ty::ParamEnv::empty().and(tcx.types.u8))
365 bug!("not a byte str: {:?}", const_val)
368 _ => span_bug!(pat.span, "unexpected byte array pattern {:?}", pat)
373 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
374 if self.tcx.features().exhaustive_patterns {
375 self.tcx.is_ty_uninhabited_from(self.module, ty)
381 fn is_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
383 ty::TyAdt(adt_def, ..) => adt_def.is_enum() && adt_def.is_non_exhaustive(),
388 fn is_local(&self, ty: Ty<'tcx>) -> bool {
390 ty::TyAdt(adt_def, ..) => adt_def.did.is_local(),
395 fn is_variant_uninhabited(&self,
396 variant: &'tcx ty::VariantDef,
397 substs: &'tcx ty::subst::Substs<'tcx>)
400 if self.tcx.features().exhaustive_patterns {
401 self.tcx.is_enum_variant_uninhabited_from(self.module, variant, substs)
408 #[derive(Clone, Debug, PartialEq)]
409 pub enum Constructor<'tcx> {
410 /// The constructor of all patterns that don't vary by constructor,
411 /// e.g. struct patterns and fixed-length arrays.
416 ConstantValue(&'tcx ty::Const<'tcx>),
417 /// Ranges of literal values (`2...5` and `2..5`).
418 ConstantRange(&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>, RangeEnd),
419 /// Array patterns of length n.
423 impl<'tcx> Constructor<'tcx> {
424 fn variant_index_for_adt(&self, adt: &'tcx ty::AdtDef) -> usize {
426 &Variant(vid) => adt.variant_index_with_id(vid),
428 assert!(!adt.is_enum());
431 _ => bug!("bad constructor {:?} for adt {:?}", self, adt)
436 #[derive(Clone, Debug)]
437 pub enum Usefulness<'tcx> {
439 UsefulWithWitness(Vec<Witness<'tcx>>),
443 impl<'tcx> Usefulness<'tcx> {
444 fn is_useful(&self) -> bool {
452 #[derive(Copy, Clone, Debug)]
453 pub enum WitnessPreference {
458 #[derive(Copy, Clone, Debug)]
459 struct PatternContext<'tcx> {
461 max_slice_length: u64,
464 /// A witness of non-exhaustiveness for error reporting, represented
465 /// as a list of patterns (in reverse order of construction) with
466 /// wildcards inside to represent elements that can take any inhabitant
467 /// of the type as a value.
469 /// A witness against a list of patterns should have the same types
470 /// and length as the pattern matched against. Because Rust `match`
471 /// is always against a single pattern, at the end the witness will
472 /// have length 1, but in the middle of the algorithm, it can contain
473 /// multiple patterns.
475 /// For example, if we are constructing a witness for the match against
477 /// struct Pair(Option<(u32, u32)>, bool);
479 /// match (p: Pair) {
480 /// Pair(None, _) => {}
481 /// Pair(_, false) => {}
485 /// We'll perform the following steps:
486 /// 1. Start with an empty witness
487 /// `Witness(vec![])`
488 /// 2. Push a witness `Some(_)` against the `None`
489 /// `Witness(vec![Some(_)])`
490 /// 3. Push a witness `true` against the `false`
491 /// `Witness(vec![Some(_), true])`
492 /// 4. Apply the `Pair` constructor to the witnesses
493 /// `Witness(vec![Pair(Some(_), true)])`
495 /// The final `Pair(Some(_), true)` is then the resulting witness.
496 #[derive(Clone, Debug)]
497 pub struct Witness<'tcx>(Vec<Pattern<'tcx>>);
499 impl<'tcx> Witness<'tcx> {
500 pub fn single_pattern(&self) -> &Pattern<'tcx> {
501 assert_eq!(self.0.len(), 1);
505 fn push_wild_constructor<'a>(
507 cx: &MatchCheckCtxt<'a, 'tcx>,
508 ctor: &Constructor<'tcx>,
512 let sub_pattern_tys = constructor_sub_pattern_tys(cx, ctor, ty);
513 self.0.extend(sub_pattern_tys.into_iter().map(|ty| {
517 kind: box PatternKind::Wild,
520 self.apply_constructor(cx, ctor, ty)
524 /// Constructs a partial witness for a pattern given a list of
525 /// patterns expanded by the specialization step.
527 /// When a pattern P is discovered to be useful, this function is used bottom-up
528 /// to reconstruct a complete witness, e.g. a pattern P' that covers a subset
529 /// of values, V, where each value in that set is not covered by any previously
530 /// used patterns and is covered by the pattern P'. Examples:
532 /// left_ty: tuple of 3 elements
533 /// pats: [10, 20, _] => (10, 20, _)
535 /// left_ty: struct X { a: (bool, &'static str), b: usize}
536 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
537 fn apply_constructor<'a>(
539 cx: &MatchCheckCtxt<'a,'tcx>,
540 ctor: &Constructor<'tcx>,
544 let arity = constructor_arity(cx, ctor, ty);
546 let len = self.0.len() as u64;
547 let mut pats = self.0.drain((len - arity) as usize..).rev();
552 let pats = pats.enumerate().map(|(i, p)| {
554 field: Field::new(i),
559 if let ty::TyAdt(adt, substs) = ty.sty {
561 PatternKind::Variant {
564 variant_index: ctor.variant_index_for_adt(adt),
568 PatternKind::Leaf { subpatterns: pats }
571 PatternKind::Leaf { subpatterns: pats }
576 PatternKind::Deref { subpattern: pats.nth(0).unwrap() }
579 ty::TySlice(_) | ty::TyArray(..) => {
581 prefix: pats.collect(),
589 ConstantValue(value) => PatternKind::Constant { value },
590 ConstantRange(lo, hi, end) => PatternKind::Range { lo, hi, end },
591 _ => PatternKind::Wild,
597 self.0.push(Pattern {
607 /// This determines the set of all possible constructors of a pattern matching
608 /// values of type `left_ty`. For vectors, this would normally be an infinite set
609 /// but is instead bounded by the maximum fixed length of slice patterns in
610 /// the column of patterns being analyzed.
612 /// We make sure to omit constructors that are statically impossible. eg for
613 /// Option<!> we do not include Some(_) in the returned list of constructors.
614 fn all_constructors<'a, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
615 pcx: PatternContext<'tcx>)
616 -> Vec<Constructor<'tcx>>
618 debug!("all_constructors({:?})", pcx.ty);
619 let exhaustive_integer_patterns = cx.tcx.features().exhaustive_integer_patterns;
620 let ctors = match pcx.ty.sty {
622 [true, false].iter().map(|&b| {
623 ConstantValue(ty::Const::from_bool(cx.tcx, b))
626 ty::TyArray(ref sub_ty, len) if len.assert_usize(cx.tcx).is_some() => {
627 let len = len.unwrap_usize(cx.tcx);
628 if len != 0 && cx.is_uninhabited(sub_ty) {
634 // Treat arrays of a constant but unknown length like slices.
635 ty::TyArray(ref sub_ty, _) |
636 ty::TySlice(ref sub_ty) => {
637 if cx.is_uninhabited(sub_ty) {
640 (0..pcx.max_slice_length+1).map(|length| Slice(length)).collect()
643 ty::TyAdt(def, substs) if def.is_enum() => {
645 .filter(|v| !cx.is_variant_uninhabited(v, substs))
646 .map(|v| Variant(v.did))
649 ty::TyChar if exhaustive_integer_patterns => {
650 let endpoint = |c: char| {
651 let ty = ty::ParamEnv::empty().and(cx.tcx.types.char);
652 ty::Const::from_bits(cx.tcx, c as u128, ty)
655 // The valid Unicode Scalar Value ranges.
656 ConstantRange(endpoint('\u{0000}'), endpoint('\u{D7FF}'), RangeEnd::Included),
657 ConstantRange(endpoint('\u{E000}'), endpoint('\u{10FFFF}'), RangeEnd::Included),
660 ty::TyInt(ity) if exhaustive_integer_patterns => {
661 // FIXME(49937): refactor these bit manipulations into interpret.
662 let bits = Integer::from_attr(cx.tcx, SignedInt(ity)).size().bits() as u128;
663 let min = 1u128 << (bits - 1);
664 let max = (1u128 << (bits - 1)) - 1;
665 let ty = ty::ParamEnv::empty().and(pcx.ty);
666 vec![ConstantRange(ty::Const::from_bits(cx.tcx, min as u128, ty),
667 ty::Const::from_bits(cx.tcx, max as u128, ty),
670 ty::TyUint(uty) if exhaustive_integer_patterns => {
671 // FIXME(49937): refactor these bit manipulations into interpret.
672 let bits = Integer::from_attr(cx.tcx, UnsignedInt(uty)).size().bits() as u128;
673 let max = !0u128 >> (128 - bits);
674 let ty = ty::ParamEnv::empty().and(pcx.ty);
675 vec![ConstantRange(ty::Const::from_bits(cx.tcx, 0, ty),
676 ty::Const::from_bits(cx.tcx, max, ty),
680 if cx.is_uninhabited(pcx.ty) {
690 fn max_slice_length<'p, 'a: 'p, 'tcx: 'a, I>(
691 cx: &mut MatchCheckCtxt<'a, 'tcx>,
693 where I: Iterator<Item=&'p Pattern<'tcx>>
695 // The exhaustiveness-checking paper does not include any details on
696 // checking variable-length slice patterns. However, they are matched
697 // by an infinite collection of fixed-length array patterns.
699 // Checking the infinite set directly would take an infinite amount
700 // of time. However, it turns out that for each finite set of
701 // patterns `P`, all sufficiently large array lengths are equivalent:
703 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
704 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
705 // `sₘ` for each sufficiently-large length `m` that applies to exactly
706 // the same subset of `P`.
708 // Because of that, each witness for reachability-checking from one
709 // of the sufficiently-large lengths can be transformed to an
710 // equally-valid witness from any other length, so we only have
711 // to check slice lengths from the "minimal sufficiently-large length"
714 // Note that the fact that there is a *single* `sₘ` for each `m`
715 // not depending on the specific pattern in `P` is important: if
716 // you look at the pair of patterns
719 // Then any slice of length ≥1 that matches one of these two
720 // patterns can be trivially turned to a slice of any
721 // other length ≥1 that matches them and vice-versa - for
722 // but the slice from length 2 `[false, true]` that matches neither
723 // of these patterns can't be turned to a slice from length 1 that
724 // matches neither of these patterns, so we have to consider
725 // slices from length 2 there.
727 // Now, to see that that length exists and find it, observe that slice
728 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
729 // "variable-length" patterns (`[_, .., _]`).
731 // For fixed-length patterns, all slices with lengths *longer* than
732 // the pattern's length have the same outcome (of not matching), so
733 // as long as `L` is greater than the pattern's length we can pick
734 // any `sₘ` from that length and get the same result.
736 // For variable-length patterns, the situation is more complicated,
737 // because as seen above the precise value of `sₘ` matters.
739 // However, for each variable-length pattern `p` with a prefix of length
740 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
741 // `slₚ` elements are examined.
743 // Therefore, as long as `L` is positive (to avoid concerns about empty
744 // types), all elements after the maximum prefix length and before
745 // the maximum suffix length are not examined by any variable-length
746 // pattern, and therefore can be added/removed without affecting
747 // them - creating equivalent patterns from any sufficiently-large
750 // Of course, if fixed-length patterns exist, we must be sure
751 // that our length is large enough to miss them all, so
752 // we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
754 // for example, with the above pair of patterns, all elements
755 // but the first and last can be added/removed, so any
756 // witness of length ≥2 (say, `[false, false, true]`) can be
757 // turned to a witness from any other length ≥2.
759 let mut max_prefix_len = 0;
760 let mut max_suffix_len = 0;
761 let mut max_fixed_len = 0;
763 for row in patterns {
765 PatternKind::Constant { value } => {
766 if let Some(ptr) = value.to_ptr() {
767 let is_array_ptr = value.ty
769 .and_then(|t| t.ty.builtin_index())
770 .map_or(false, |t| t == cx.tcx.types.u8);
772 let alloc = cx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
773 max_fixed_len = cmp::max(max_fixed_len, alloc.bytes.len() as u64);
777 PatternKind::Slice { ref prefix, slice: None, ref suffix } => {
778 let fixed_len = prefix.len() as u64 + suffix.len() as u64;
779 max_fixed_len = cmp::max(max_fixed_len, fixed_len);
781 PatternKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
782 max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
783 max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
789 cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len)
792 /// An inclusive interval, used for precise integer exhaustiveness checking.
793 /// `IntRange`s always store a contiguous range. This means that values are
794 /// encoded such that `0` encodes the minimum value for the integer,
795 /// regardless of the signedness.
796 /// For example, the pattern `-128...127i8` is encoded as `0..=255`.
797 /// This makes comparisons and arithmetic on interval endpoints much more
798 /// straightforward. See `signed_bias` for details.
800 /// `IntRange` is never used to encode an empty range or a "range" that wraps
801 /// around the (offset) space: i.e. `range.lo <= range.hi`.
802 struct IntRange<'tcx> {
803 pub range: RangeInclusive<u128>,
807 impl<'tcx> IntRange<'tcx> {
808 fn from_ctor(tcx: TyCtxt<'_, 'tcx, 'tcx>,
809 ctor: &Constructor<'tcx>)
810 -> Option<IntRange<'tcx>> {
812 ConstantRange(lo, hi, end) => {
813 assert_eq!(lo.ty, hi.ty);
815 let env_ty = ty::ParamEnv::empty().and(ty);
816 if let Some(lo) = lo.assert_bits(tcx, env_ty) {
817 if let Some(hi) = hi.assert_bits(tcx, env_ty) {
818 // Perform a shift if the underlying types are signed,
819 // which makes the interval arithmetic simpler.
820 let bias = IntRange::signed_bias(tcx, ty);
821 let (lo, hi) = (lo ^ bias, hi ^ bias);
822 // Make sure the interval is well-formed.
823 return if lo > hi || lo == hi && *end == RangeEnd::Excluded {
826 let offset = (*end == RangeEnd::Excluded) as u128;
827 Some(IntRange { range: lo..=(hi - offset), ty })
833 ConstantValue(val) => {
835 if let Some(val) = val.assert_bits(tcx, ty::ParamEnv::empty().and(ty)) {
836 let bias = IntRange::signed_bias(tcx, ty);
837 let val = val ^ bias;
838 Some(IntRange { range: val..=val, ty })
843 Single | Variant(_) | Slice(_) => {
849 fn from_pat(tcx: TyCtxt<'_, 'tcx, 'tcx>,
851 -> Option<IntRange<'tcx>> {
852 Self::from_ctor(tcx, &match pat.kind {
853 box PatternKind::Constant { value } => ConstantValue(value),
854 box PatternKind::Range { lo, hi, end } => ConstantRange(lo, hi, end),
859 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
860 fn signed_bias(tcx: TyCtxt<'_, 'tcx, 'tcx>, ty: Ty<'tcx>) -> u128 {
863 let bits = Integer::from_attr(tcx, SignedInt(ity)).size().bits() as u128;
870 /// Convert a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
872 tcx: TyCtxt<'_, 'tcx, 'tcx>,
874 r: RangeInclusive<u128>,
875 ) -> Constructor<'tcx> {
876 let bias = IntRange::signed_bias(tcx, ty);
877 let ty = ty::ParamEnv::empty().and(ty);
878 let (lo, hi) = r.into_inner();
880 ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty))
882 ConstantRange(ty::Const::from_bits(tcx, lo ^ bias, ty),
883 ty::Const::from_bits(tcx, hi ^ bias, ty),
888 /// Return a collection of ranges that spans the values covered by `ranges`, subtracted
889 /// by the values covered by `self`: i.e. `ranges \ self` (in set notation).
890 fn subtract_from(self,
891 tcx: TyCtxt<'_, 'tcx, 'tcx>,
892 ranges: Vec<Constructor<'tcx>>)
893 -> Vec<Constructor<'tcx>> {
894 let ranges = ranges.into_iter().filter_map(|r| {
895 IntRange::from_ctor(tcx, &r).map(|i| i.range)
897 let mut remaining_ranges = vec![];
899 let (lo, hi) = self.range.into_inner();
900 for subrange in ranges {
901 let (subrange_lo, subrange_hi) = subrange.into_inner();
902 if lo > subrange_hi || subrange_lo > hi {
903 // The pattern doesn't intersect with the subrange at all,
904 // so the subrange remains untouched.
905 remaining_ranges.push(Self::range_to_ctor(tcx, ty, subrange_lo..=subrange_hi));
907 if lo > subrange_lo {
908 // The pattern intersects an upper section of the
909 // subrange, so a lower section will remain.
910 remaining_ranges.push(Self::range_to_ctor(tcx, ty, subrange_lo..=(lo - 1)));
912 if hi < subrange_hi {
913 // The pattern intersects a lower section of the
914 // subrange, so an upper section will remain.
915 remaining_ranges.push(Self::range_to_ctor(tcx, ty, (hi + 1)..=subrange_hi));
922 fn intersection(&self, other: &Self) -> Option<Self> {
924 let (lo, hi) = (*self.range.start(), *self.range.end());
925 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
926 if lo <= other_hi && other_lo <= hi {
927 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty })
934 // Return a set of constructors equivalent to `all_ctors \ used_ctors`.
935 fn compute_missing_ctors<'a, 'tcx: 'a>(
936 tcx: TyCtxt<'a, 'tcx, 'tcx>,
937 all_ctors: &Vec<Constructor<'tcx>>,
938 used_ctors: &Vec<Constructor<'tcx>>,
939 ) -> Vec<Constructor<'tcx>> {
940 let mut missing_ctors = vec![];
942 for req_ctor in all_ctors {
943 let mut refined_ctors = vec![req_ctor.clone()];
944 for used_ctor in used_ctors {
945 if used_ctor == req_ctor {
946 // If a constructor appears in a `match` arm, we can
947 // eliminate it straight away.
948 refined_ctors = vec![]
949 } else if tcx.features().exhaustive_integer_patterns {
950 if let Some(interval) = IntRange::from_ctor(tcx, used_ctor) {
951 // Refine the required constructors for the type by subtracting
952 // the range defined by the current constructor pattern.
953 refined_ctors = interval.subtract_from(tcx, refined_ctors);
957 // If the constructor patterns that have been considered so far
958 // already cover the entire range of values, then we the
959 // constructor is not missing, and we can move on to the next one.
960 if refined_ctors.is_empty() {
964 // If a constructor has not been matched, then it is missing.
965 // We add `refined_ctors` instead of `req_ctor`, because then we can
966 // provide more detailed error information about precisely which
967 // ranges have been omitted.
968 missing_ctors.extend(refined_ctors);
974 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html
975 /// The algorithm from the paper has been modified to correctly handle empty
976 /// types. The changes are:
977 /// (0) We don't exit early if the pattern matrix has zero rows. We just
978 /// continue to recurse over columns.
979 /// (1) all_constructors will only return constructors that are statically
980 /// possible. eg. it will only return Ok for Result<T, !>
982 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
983 /// to a set of such vectors `m` - this is defined as there being a set of
984 /// inputs that will match `v` but not any of the sets in `m`.
986 /// All the patterns at each column of the `matrix ++ v` matrix must
987 /// have the same type, except that wildcard (PatternKind::Wild) patterns
988 /// with type TyErr are also allowed, even if the "type of the column"
989 /// is not TyErr. That is used to represent private fields, as using their
990 /// real type would assert that they are inhabited.
992 /// This is used both for reachability checking (if a pattern isn't useful in
993 /// relation to preceding patterns, it is not reachable) and exhaustiveness
994 /// checking (if a wildcard pattern is useful in relation to a matrix, the
995 /// matrix isn't exhaustive).
996 pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
997 matrix: &Matrix<'p, 'tcx>,
998 v: &[&'p Pattern<'tcx>],
999 witness: WitnessPreference)
1000 -> Usefulness<'tcx> {
1001 let &Matrix(ref rows) = matrix;
1002 debug!("is_useful({:#?}, {:#?})", matrix, v);
1004 // The base case. We are pattern-matching on () and the return value is
1005 // based on whether our matrix has a row or not.
1006 // NOTE: This could potentially be optimized by checking rows.is_empty()
1007 // first and then, if v is non-empty, the return value is based on whether
1008 // the type of the tuple we're checking is inhabited or not.
1010 return if rows.is_empty() {
1012 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
1013 LeaveOutWitness => Useful,
1020 assert!(rows.iter().all(|r| r.len() == v.len()));
1022 let pcx = PatternContext {
1023 // TyErr is used to represent the type of wildcard patterns matching
1024 // against inaccessible (private) fields of structs, so that we won't
1025 // be able to observe whether the types of the struct's fields are
1028 // If the field is truly inaccessible, then all the patterns
1029 // matching against it must be wildcard patterns, so its type
1032 // However, if we are matching against non-wildcard patterns, we
1033 // need to know the real type of the field so we can specialize
1034 // against it. This primarily occurs through constants - they
1035 // can include contents for fields that are inaccessible at the
1036 // location of the match. In that case, the field's type is
1037 // inhabited - by the constant - so we can just use it.
1039 // FIXME: this might lead to "unstable" behavior with macro hygiene
1040 // introducing uninhabited patterns for inaccessible fields. We
1041 // need to figure out how to model that.
1042 ty: rows.iter().map(|r| r[0].ty).find(|ty| !ty.references_error()).unwrap_or(v[0].ty),
1043 max_slice_length: max_slice_length(cx, rows.iter().map(|r| r[0]).chain(Some(v[0])))
1046 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v[0]);
1048 if let Some(constructors) = pat_constructors(cx, v[0], pcx) {
1049 debug!("is_useful - expanding constructors: {:#?}", constructors);
1050 split_grouped_constructors(cx.tcx, constructors, matrix, v, pcx.ty).into_iter().map(|c|
1051 is_useful_specialized(cx, matrix, v, c.clone(), pcx.ty, witness)
1052 ).find(|result| result.is_useful()).unwrap_or(NotUseful)
1054 debug!("is_useful - expanding wildcard");
1056 let used_ctors: Vec<Constructor> = rows.iter().flat_map(|row| {
1057 pat_constructors(cx, row[0], pcx).unwrap_or(vec![])
1059 debug!("used_ctors = {:#?}", used_ctors);
1060 // `all_ctors` are all the constructors for the given type, which
1061 // should all be represented (or caught with the wild pattern `_`).
1062 let all_ctors = all_constructors(cx, pcx);
1063 debug!("all_ctors = {:#?}", all_ctors);
1065 // `missing_ctors` is the set of constructors from the same type as the
1066 // first column of `matrix` that are matched only by wildcard patterns
1067 // from the first column.
1069 // Therefore, if there is some pattern that is unmatched by `matrix`,
1070 // it will still be unmatched if the first constructor is replaced by
1071 // any of the constructors in `missing_ctors`
1073 // However, if our scrutinee is *privately* an empty enum, we
1074 // must treat it as though it had an "unknown" constructor (in
1075 // that case, all other patterns obviously can't be variants)
1076 // to avoid exposing its emptyness. See the `match_privately_empty`
1077 // test for details.
1079 // FIXME: currently the only way I know of something can
1080 // be a privately-empty enum is when the exhaustive_patterns
1081 // feature flag is not present, so this is only
1082 // needed for that case.
1084 // Find those constructors that are not matched by any non-wildcard patterns in the
1086 let missing_ctors = compute_missing_ctors(cx.tcx, &all_ctors, &used_ctors);
1088 let is_privately_empty = all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1089 let is_declared_nonexhaustive = cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
1090 debug!("missing_ctors={:#?} is_privately_empty={:#?} is_declared_nonexhaustive={:#?}",
1091 missing_ctors, is_privately_empty, is_declared_nonexhaustive);
1093 // For privately empty and non-exhaustive enums, we work as if there were an "extra"
1094 // `_` constructor for the type, so we can never match over all constructors.
1095 let is_non_exhaustive = is_privately_empty || is_declared_nonexhaustive;
1097 if missing_ctors.is_empty() && !is_non_exhaustive {
1098 split_grouped_constructors(cx.tcx, all_ctors, matrix, v, pcx.ty).into_iter().map(|c| {
1099 is_useful_specialized(cx, matrix, v, c.clone(), pcx.ty, witness)
1100 }).find(|result| result.is_useful()).unwrap_or(NotUseful)
1102 let matrix = rows.iter().filter_map(|r| {
1103 if r[0].is_wildcard() {
1104 Some(r[1..].to_vec())
1109 match is_useful(cx, &matrix, &v[1..], witness) {
1110 UsefulWithWitness(pats) => {
1112 // In this case, there's at least one "free"
1113 // constructor that is only matched against by
1114 // wildcard patterns.
1116 // There are 2 ways we can report a witness here.
1117 // Commonly, we can report all the "free"
1118 // constructors as witnesses, e.g. if we have:
1121 // enum Direction { N, S, E, W }
1122 // let Direction::N = ...;
1125 // we can report 3 witnesses: `S`, `E`, and `W`.
1127 // However, there are 2 cases where we don't want
1128 // to do this and instead report a single `_` witness:
1130 // 1) If the user is matching against a non-exhaustive
1131 // enum, there is no point in enumerating all possible
1132 // variants, because the user can't actually match
1133 // against them himself, e.g. in an example like:
1135 // let err: io::ErrorKind = ...;
1137 // io::ErrorKind::NotFound => {},
1140 // we don't want to show every possible IO error,
1141 // but instead have `_` as the witness (this is
1142 // actually *required* if the user specified *all*
1143 // IO errors, but is probably what we want in every
1146 // 2) If the user didn't actually specify a constructor
1147 // in this arm, e.g. in
1149 // let x: (Direction, Direction, bool) = ...;
1150 // let (_, _, false) = x;
1152 // we don't want to show all 16 possible witnesses
1153 // `(<direction-1>, <direction-2>, true)` - we are
1154 // satisfied with `(_, _, true)`. In this case,
1155 // `used_ctors` is empty.
1156 let new_witnesses = if is_non_exhaustive || used_ctors.is_empty() {
1157 // All constructors are unused. Add wild patterns
1158 // rather than each individual constructor.
1159 pats.into_iter().map(|mut witness| {
1160 witness.0.push(Pattern {
1163 kind: box PatternKind::Wild,
1168 pats.into_iter().flat_map(|witness| {
1169 missing_ctors.iter().map(move |ctor| {
1170 // Extends the witness with a "wild" version of this
1171 // constructor, that matches everything that can be built with
1172 // it. For example, if `ctor` is a `Constructor::Variant` for
1173 // `Option::Some`, this pushes the witness for `Some(_)`.
1174 witness.clone().push_wild_constructor(cx, ctor, pcx.ty)
1178 UsefulWithWitness(new_witnesses)
1186 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e. `is_useful` applied
1187 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1188 fn is_useful_specialized<'p, 'a:'p, 'tcx: 'a>(
1189 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1190 &Matrix(ref m): &Matrix<'p, 'tcx>,
1191 v: &[&'p Pattern<'tcx>],
1192 ctor: Constructor<'tcx>,
1194 witness: WitnessPreference,
1195 ) -> Usefulness<'tcx> {
1196 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1197 let sub_pat_tys = constructor_sub_pattern_tys(cx, &ctor, lty);
1198 let wild_patterns_owned: Vec<_> = sub_pat_tys.iter().map(|ty| {
1202 kind: box PatternKind::Wild,
1205 let wild_patterns: Vec<_> = wild_patterns_owned.iter().collect();
1206 let matrix = Matrix(m.iter().flat_map(|r| {
1207 specialize(cx, &r, &ctor, &wild_patterns)
1209 match specialize(cx, v, &ctor, &wild_patterns) {
1210 Some(v) => match is_useful(cx, &matrix, &v, witness) {
1211 UsefulWithWitness(witnesses) => UsefulWithWitness(
1212 witnesses.into_iter()
1213 .map(|witness| witness.apply_constructor(cx, &ctor, lty))
1222 /// Determines the constructors that the given pattern can be specialized to.
1224 /// In most cases, there's only one constructor that a specific pattern
1225 /// represents, such as a specific enum variant or a specific literal value.
1226 /// Slice patterns, however, can match slices of different lengths. For instance,
1227 /// `[a, b, ..tail]` can match a slice of length 2, 3, 4 and so on.
1229 /// Returns `None` in case of a catch-all, which can't be specialized.
1230 fn pat_constructors<'tcx>(cx: &mut MatchCheckCtxt,
1231 pat: &Pattern<'tcx>,
1232 pcx: PatternContext)
1233 -> Option<Vec<Constructor<'tcx>>>
1236 PatternKind::Binding { .. } | PatternKind::Wild => None,
1237 PatternKind::Leaf { .. } | PatternKind::Deref { .. } => Some(vec![Single]),
1238 PatternKind::Variant { adt_def, variant_index, .. } => {
1239 Some(vec![Variant(adt_def.variants[variant_index].did)])
1241 PatternKind::Constant { value } => Some(vec![ConstantValue(value)]),
1242 PatternKind::Range { lo, hi, end } => Some(vec![ConstantRange(lo, hi, end)]),
1243 PatternKind::Array { .. } => match pcx.ty.sty {
1244 ty::TyArray(_, length) => Some(vec![
1245 Slice(length.unwrap_usize(cx.tcx))
1247 _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty)
1249 PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
1250 let pat_len = prefix.len() as u64 + suffix.len() as u64;
1251 if slice.is_some() {
1252 Some((pat_len..pcx.max_slice_length+1).map(Slice).collect())
1254 Some(vec![Slice(pat_len)])
1260 /// This computes the arity of a constructor. The arity of a constructor
1261 /// is how many subpattern patterns of that constructor should be expanded to.
1263 /// For instance, a tuple pattern (_, 42, Some([])) has the arity of 3.
1264 /// A struct pattern's arity is the number of fields it contains, etc.
1265 fn constructor_arity(_cx: &MatchCheckCtxt, ctor: &Constructor, ty: Ty) -> u64 {
1266 debug!("constructor_arity({:#?}, {:?})", ctor, ty);
1268 ty::TyTuple(ref fs) => fs.len() as u64,
1269 ty::TySlice(..) | ty::TyArray(..) => match *ctor {
1270 Slice(length) => length,
1271 ConstantValue(_) => 0,
1272 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty)
1275 ty::TyAdt(adt, _) => {
1276 adt.variants[ctor.variant_index_for_adt(adt)].fields.len() as u64
1282 /// This computes the types of the sub patterns that a constructor should be
1285 /// For instance, a tuple pattern (43u32, 'a') has sub pattern types [u32, char].
1286 fn constructor_sub_pattern_tys<'a, 'tcx: 'a>(cx: &MatchCheckCtxt<'a, 'tcx>,
1288 ty: Ty<'tcx>) -> Vec<Ty<'tcx>>
1290 debug!("constructor_sub_pattern_tys({:#?}, {:?})", ctor, ty);
1292 ty::TyTuple(ref fs) => fs.into_iter().map(|t| *t).collect(),
1293 ty::TySlice(ty) | ty::TyArray(ty, _) => match *ctor {
1294 Slice(length) => (0..length).map(|_| ty).collect(),
1295 ConstantValue(_) => vec![],
1296 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty)
1298 ty::TyRef(_, rty, _) => vec![rty],
1299 ty::TyAdt(adt, substs) => {
1301 // Use T as the sub pattern type of Box<T>.
1302 vec![substs.type_at(0)]
1304 adt.variants[ctor.variant_index_for_adt(adt)].fields.iter().map(|field| {
1305 let is_visible = adt.is_enum()
1306 || field.vis.is_accessible_from(cx.module, cx.tcx);
1308 field.ty(cx.tcx, substs)
1310 // Treat all non-visible fields as TyErr. They
1311 // can't appear in any other pattern from
1312 // this match (because they are private),
1313 // so their type does not matter - but
1314 // we don't want to know they are
1325 fn slice_pat_covered_by_constructor<'tcx>(
1326 tcx: TyCtxt<'_, 'tcx, '_>,
1329 prefix: &[Pattern<'tcx>],
1330 slice: &Option<Pattern<'tcx>>,
1331 suffix: &[Pattern<'tcx>]
1332 ) -> Result<bool, ErrorReported> {
1333 let data: &[u8] = match *ctor {
1334 ConstantValue(const_val) => {
1335 let val = match const_val.val {
1336 ConstValue::Unevaluated(..) |
1337 ConstValue::ByRef(..) => bug!("unexpected ConstValue: {:?}", const_val),
1338 ConstValue::Scalar(val) | ConstValue::ScalarPair(val, _) => val,
1340 if let Ok(ptr) = val.to_ptr() {
1341 let is_array_ptr = const_val.ty
1342 .builtin_deref(true)
1343 .and_then(|t| t.ty.builtin_index())
1344 .map_or(false, |t| t == tcx.types.u8);
1345 assert!(is_array_ptr);
1346 tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id).bytes.as_ref()
1348 bug!("unexpected non-ptr ConstantValue")
1354 let pat_len = prefix.len() + suffix.len();
1355 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1360 data[..prefix.len()].iter().zip(prefix).chain(
1361 data[data.len()-suffix.len()..].iter().zip(suffix))
1364 box PatternKind::Constant { value } => {
1365 let b = value.unwrap_bits(tcx, ty::ParamEnv::empty().and(pat.ty));
1366 assert_eq!(b as u8 as u128, b);
1378 // Whether to evaluate a constructor using exhaustive integer matching. This is true if the
1379 // constructor is a range or constant with an integer type.
1380 fn should_treat_range_exhaustively(tcx: TyCtxt<'_, 'tcx, 'tcx>, ctor: &Constructor<'tcx>) -> bool {
1381 if tcx.features().exhaustive_integer_patterns {
1382 if let ConstantValue(value) | ConstantRange(value, _, _) = ctor {
1383 if let ty::TyChar | ty::TyInt(_) | ty::TyUint(_) = value.ty.sty {
1391 /// For exhaustive integer matching, some constructors are grouped within other constructors
1392 /// (namely integer typed values are grouped within ranges). However, when specialising these
1393 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1394 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1395 /// mean creating a separate constructor for every single value in the range, which is clearly
1396 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1397 /// identical across all values in that range (i.e. there are equivalence classes of ranges of
1398 /// constructors based on their `is_useful_specialised` outcome). These classes are grouped by
1399 /// the patterns that apply to them (both in the matrix `P` and in the new row `p_{m + 1}`). We
1400 /// can split the range whenever the patterns that apply to that range (specifically: the patterns
1401 /// that *intersect* with that range) change.
1402 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1403 /// the group of intersecting patterns changes, which we can compute by converting each pattern to
1404 /// a range and recording its endpoints, then creating subranges between each consecutive pair of
1406 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1407 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1408 /// number of rows in the matrix (i.e. the number of cases in the `match` statement), so we don't
1409 /// need to be worried about matching over gargantuan ranges.
1410 fn split_grouped_constructors<'p, 'a: 'p, 'tcx: 'a>(
1411 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1412 ctors: Vec<Constructor<'tcx>>,
1413 &Matrix(ref m): &Matrix<'p, 'tcx>,
1414 p: &[&'p Pattern<'tcx>],
1416 ) -> Vec<Constructor<'tcx>> {
1419 let mut split_ctors = Vec::with_capacity(ctors.len());
1421 for ctor in ctors.into_iter() {
1423 // For now, only ranges may denote groups of "subconstructors", so we only need to
1424 // special-case constant ranges.
1425 ConstantRange(..) if should_treat_range_exhaustively(tcx, &ctor) => {
1426 // We only care about finding all the subranges within the range of the intersection
1427 // of the new pattern `p_({m + 1},1)` (here `pat`) and the constructor range.
1428 // Anything else is irrelevant, because it is guaranteed to result in `NotUseful`,
1429 // which is the default case anyway, and can be ignored.
1430 let mut ctor_range = IntRange::from_ctor(tcx, &ctor).unwrap();
1431 if let Some(pat_range) = IntRange::from_pat(tcx, pat) {
1432 if let Some(new_range) = ctor_range.intersection(&pat_range) {
1433 ctor_range = new_range;
1435 // If the intersection between `pat` and the constructor is empty, the
1436 // entire range is `NotUseful`.
1441 box PatternKind::Wild => {
1442 // A wild pattern matches the entire range of values,
1443 // so the current values are fine.
1445 // If the pattern is not a value (i.e. a degenerate range), a range or a
1446 // wildcard (which stands for the entire range), then it's guaranteed to
1451 // We're going to collect all the endpoints in the new pattern so we can create
1452 // subranges between them.
1453 // If there's a single point, we need to identify it as belonging
1454 // to a length-1 range, so it can be treated as an individual
1455 // constructor, rather than as an endpoint. To do this, we keep track of which
1456 // endpoint a point corresponds to. Whenever a point corresponds to both a start
1457 // and an end, then we create a unit range for it.
1458 #[derive(PartialEq, Clone, Copy, Debug)]
1464 let mut points = FxHashMap::default();
1465 let add_endpoint = |points: &mut FxHashMap<_, _>, x, endpoint| {
1466 points.entry(x).and_modify(|ex_x| {
1467 if *ex_x != endpoint {
1468 *ex_x = Endpoint::Both
1470 }).or_insert(endpoint);
1472 let add_endpoints = |points: &mut FxHashMap<_, _>, lo, hi| {
1473 // Insert the endpoints, taking care to keep track of to
1474 // which endpoints a point corresponds.
1475 add_endpoint(points, lo, Endpoint::Start);
1476 add_endpoint(points, hi, Endpoint::End);
1478 let (lo, hi) = (*ctor_range.range.start(), *ctor_range.range.end());
1479 add_endpoints(&mut points, lo, hi);
1480 // We're going to iterate through every row pattern, adding endpoints in.
1481 for row in m.iter() {
1482 if let Some(r) = IntRange::from_pat(tcx, row[0]) {
1483 // We're only interested in endpoints that lie (at least partially)
1484 // within the subrange domain.
1485 if let Some(r) = ctor_range.intersection(&r) {
1486 let (r_lo, r_hi) = r.range.into_inner();
1487 add_endpoints(&mut points, r_lo, r_hi);
1492 // The patterns were iterated in an arbitrary order (i.e. in the order the user
1493 // wrote them), so we need to make sure our endpoints are sorted.
1494 let mut points: Vec<(u128, Endpoint)> = points.into_iter().collect();
1495 points.sort_unstable_by_key(|(x, _)| *x);
1496 let mut points = points.into_iter();
1497 let mut a = points.next().unwrap();
1499 // Iterate through pairs of points, adding the subranges to `split_ctors`.
1500 // We have to be careful about the orientation of the points as endpoints, to make
1501 // sure we're enumerating precisely the correct ranges. Too few and the matching is
1502 // actually incorrect. Too many and our diagnostics are poorer. This involves some
1504 while let Some(b) = points.next() {
1506 if let Endpoint::Both = a.1 {
1507 split_ctors.push(IntRange::range_to_ctor(tcx, ty, a.0..=a.0));
1510 Endpoint::Start => a.0,
1511 Endpoint::End | Endpoint::Both => a.0 + 1,
1514 Endpoint::Start | Endpoint::Both => b.0 - 1,
1515 Endpoint::End => b.0,
1517 // In some cases, we won't need an intermediate range between two ranges
1518 // lie immediately adjacent to one another.
1520 split_ctors.push(IntRange::range_to_ctor(tcx, ty, c..=d));
1526 // Any other constructor can be used unchanged.
1527 _ => split_ctors.push(ctor),
1534 /// Check whether there exists any shared value in either `ctor` or `pat` by intersecting them.
1535 fn constructor_intersects_pattern<'p, 'a: 'p, 'tcx: 'a>(
1536 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1537 ctor: &Constructor<'tcx>,
1538 pat: &'p Pattern<'tcx>,
1539 ) -> Option<Vec<&'p Pattern<'tcx>>> {
1540 if should_treat_range_exhaustively(tcx, ctor) {
1541 match (IntRange::from_ctor(tcx, ctor), IntRange::from_pat(tcx, pat)) {
1542 (Some(ctor), Some(pat)) => ctor.intersection(&pat).map(|_| vec![]),
1546 // Fallback for non-ranges and ranges that involve floating-point numbers, which are not
1547 // conveniently handled by `IntRange`. For these cases, the constructor may not be a range
1548 // so intersection actually devolves into being covered by the pattern.
1549 match constructor_covered_by_range(tcx, ctor, pat) {
1550 Ok(true) => Some(vec![]),
1551 Ok(false) | Err(ErrorReported) => None,
1556 fn constructor_covered_by_range<'a, 'tcx>(
1557 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1558 ctor: &Constructor<'tcx>,
1559 pat: &Pattern<'tcx>,
1560 ) -> Result<bool, ErrorReported> {
1561 let (from, to, end, ty) = match pat.kind {
1562 box PatternKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
1563 box PatternKind::Range { lo, hi, end } => (lo, hi, end, lo.ty),
1564 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
1566 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
1567 let cmp_from = |c_from| compare_const_vals(tcx, c_from, from, ty::ParamEnv::empty().and(ty))
1568 .map(|res| res != Ordering::Less);
1569 let cmp_to = |c_to| compare_const_vals(tcx, c_to, to, ty::ParamEnv::empty().and(ty));
1570 macro_rules! some_or_ok {
1574 None => return Ok(false), // not char or int
1579 ConstantValue(value) => {
1580 let to = some_or_ok!(cmp_to(value));
1581 let end = (to == Ordering::Less) ||
1582 (end == RangeEnd::Included && to == Ordering::Equal);
1583 Ok(some_or_ok!(cmp_from(value)) && end)
1585 ConstantRange(from, to, RangeEnd::Included) => {
1586 let to = some_or_ok!(cmp_to(to));
1587 let end = (to == Ordering::Less) ||
1588 (end == RangeEnd::Included && to == Ordering::Equal);
1589 Ok(some_or_ok!(cmp_from(from)) && end)
1591 ConstantRange(from, to, RangeEnd::Excluded) => {
1592 let to = some_or_ok!(cmp_to(to));
1593 let end = (to == Ordering::Less) ||
1594 (end == RangeEnd::Excluded && to == Ordering::Equal);
1595 Ok(some_or_ok!(cmp_from(from)) && end)
1602 fn patterns_for_variant<'p, 'a: 'p, 'tcx: 'a>(
1603 subpatterns: &'p [FieldPattern<'tcx>],
1604 wild_patterns: &[&'p Pattern<'tcx>])
1605 -> Vec<&'p Pattern<'tcx>>
1607 let mut result = wild_patterns.to_owned();
1609 for subpat in subpatterns {
1610 result[subpat.field.index()] = &subpat.pattern;
1613 debug!("patterns_for_variant({:#?}, {:#?}) = {:#?}", subpatterns, wild_patterns, result);
1617 /// This is the main specialization step. It expands the first pattern in the given row
1618 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
1619 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
1621 /// OTOH, slice patterns with a subslice pattern (..tail) can be expanded into multiple
1622 /// different patterns.
1623 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
1624 /// fields filled with wild patterns.
1625 fn specialize<'p, 'a: 'p, 'tcx: 'a>(
1626 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1627 r: &[&'p Pattern<'tcx>],
1628 constructor: &Constructor<'tcx>,
1629 wild_patterns: &[&'p Pattern<'tcx>],
1630 ) -> Option<Vec<&'p Pattern<'tcx>>> {
1633 let head: Option<Vec<&Pattern>> = match *pat.kind {
1634 PatternKind::Binding { .. } | PatternKind::Wild => {
1635 Some(wild_patterns.to_owned())
1638 PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
1639 let ref variant = adt_def.variants[variant_index];
1640 if *constructor == Variant(variant.did) {
1641 Some(patterns_for_variant(subpatterns, wild_patterns))
1647 PatternKind::Leaf { ref subpatterns } => {
1648 Some(patterns_for_variant(subpatterns, wild_patterns))
1651 PatternKind::Deref { ref subpattern } => {
1652 Some(vec![subpattern])
1655 PatternKind::Constant { value } => {
1656 match *constructor {
1658 if let Some(ptr) = value.to_ptr() {
1659 let is_array_ptr = value.ty
1660 .builtin_deref(true)
1661 .and_then(|t| t.ty.builtin_index())
1662 .map_or(false, |t| t == cx.tcx.types.u8);
1663 assert!(is_array_ptr);
1664 let data_len = cx.tcx
1667 .unwrap_memory(ptr.alloc_id)
1670 if wild_patterns.len() == data_len {
1671 Some(cx.lower_byte_str_pattern(pat))
1677 "unexpected const-val {:?} with ctor {:?}", value, constructor)
1681 // If the constructor is a single value, we add a row to the specialised matrix
1682 // if the pattern is equal to the constructor. If the constructor is a range of
1683 // values, we add a row to the specialised matrix if the pattern is contained
1684 // within the constructor. These two cases (for a single value pattern) can be
1685 // treated as intersection.
1686 constructor_intersects_pattern(cx.tcx, constructor, pat)
1691 PatternKind::Range { .. } => {
1692 // If the constructor is a single value, we add a row to the specialised matrix if the
1693 // pattern contains the constructor. If the constructor is a range of values, we add a
1694 // row to the specialised matrix if there exists any value that lies both within the
1695 // pattern and the constructor. These two cases reduce to intersection.
1696 constructor_intersects_pattern(cx.tcx, constructor, pat)
1699 PatternKind::Array { ref prefix, ref slice, ref suffix } |
1700 PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
1701 match *constructor {
1703 let pat_len = prefix.len() + suffix.len();
1704 if let Some(slice_count) = wild_patterns.len().checked_sub(pat_len) {
1705 if slice_count == 0 || slice.is_some() {
1706 Some(prefix.iter().chain(
1707 wild_patterns.iter().map(|p| *p)
1710 .chain(suffix.iter())
1719 ConstantValue(..) => {
1720 match slice_pat_covered_by_constructor(
1721 cx.tcx, pat.span, constructor, prefix, slice, suffix
1723 Ok(true) => Some(vec![]),
1725 Err(ErrorReported) => None
1728 _ => span_bug!(pat.span,
1729 "unexpected ctor {:?} for slice pat", constructor)
1733 debug!("specialize({:#?}, {:#?}) = {:#?}", r[0], wild_patterns, head);
1735 head.map(|mut head| {
1736 head.extend_from_slice(&r[1 ..]);