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 "specialized 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 specialization 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, VariantIdx};
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;
199 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pattern<'tcx>)
202 cx.pattern_arena.alloc(LiteralExpander.fold_pattern(&pat))
205 struct LiteralExpander;
206 impl<'tcx> PatternFolder<'tcx> for LiteralExpander {
207 fn fold_pattern(&mut self, pat: &Pattern<'tcx>) -> Pattern<'tcx> {
208 match (&pat.ty.sty, &*pat.kind) {
209 (&ty::Ref(_, rty, _), &PatternKind::Constant { ref value }) => {
213 kind: box PatternKind::Deref {
214 subpattern: Pattern {
217 kind: box PatternKind::Constant { value: value.clone() },
222 (_, &PatternKind::Binding { subpattern: Some(ref s), .. }) => {
225 _ => pat.super_fold_with(self)
230 impl<'tcx> Pattern<'tcx> {
231 fn is_wildcard(&self) -> bool {
233 PatternKind::Binding { subpattern: None, .. } | PatternKind::Wild =>
240 pub struct Matrix<'a, 'tcx: 'a>(Vec<Vec<&'a Pattern<'tcx>>>);
242 impl<'a, 'tcx> Matrix<'a, 'tcx> {
243 pub fn empty() -> Self {
247 pub fn push(&mut self, row: Vec<&'a Pattern<'tcx>>) {
252 /// Pretty-printer for matrices of patterns, example:
253 /// ++++++++++++++++++++++++++
255 /// ++++++++++++++++++++++++++
256 /// + true + [First] +
257 /// ++++++++++++++++++++++++++
258 /// + true + [Second(true)] +
259 /// ++++++++++++++++++++++++++
261 /// ++++++++++++++++++++++++++
262 /// + _ + [_, _, ..tail] +
263 /// ++++++++++++++++++++++++++
264 impl<'a, 'tcx> fmt::Debug for Matrix<'a, 'tcx> {
265 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
268 let &Matrix(ref m) = self;
269 let pretty_printed_matrix: Vec<Vec<String>> = m.iter().map(|row| {
270 row.iter().map(|pat| format!("{:?}", pat)).collect()
273 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
274 assert!(m.iter().all(|row| row.len() == column_count));
275 let column_widths: Vec<usize> = (0..column_count).map(|col| {
276 pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)
279 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
280 let br = "+".repeat(total_width);
281 write!(f, "{}\n", br)?;
282 for row in pretty_printed_matrix {
284 for (column, pat_str) in row.into_iter().enumerate() {
286 write!(f, "{:1$}", pat_str, column_widths[column])?;
290 write!(f, "{}\n", br)?;
296 impl<'a, 'tcx> FromIterator<Vec<&'a Pattern<'tcx>>> for Matrix<'a, 'tcx> {
297 fn from_iter<T: IntoIterator<Item=Vec<&'a Pattern<'tcx>>>>(iter: T) -> Self
299 Matrix(iter.into_iter().collect())
303 pub struct MatchCheckCtxt<'a, 'tcx: 'a> {
304 pub tcx: TyCtxt<'a, 'tcx, 'tcx>,
305 /// The module in which the match occurs. This is necessary for
306 /// checking inhabited-ness of types because whether a type is (visibly)
307 /// inhabited can depend on whether it was defined in the current module or
308 /// not. eg. `struct Foo { _private: ! }` cannot be seen to be empty
309 /// outside it's module and should not be matchable with an empty match
312 pub pattern_arena: &'a TypedArena<Pattern<'tcx>>,
313 pub byte_array_map: FxHashMap<*const Pattern<'tcx>, Vec<&'a Pattern<'tcx>>>,
316 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
317 pub fn create_and_enter<F, R>(
318 tcx: TyCtxt<'a, 'tcx, 'tcx>,
321 where F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R
323 let pattern_arena = TypedArena::default();
328 pattern_arena: &pattern_arena,
329 byte_array_map: FxHashMap::default(),
333 // convert a byte-string pattern to a list of u8 patterns.
334 fn lower_byte_str_pattern<'p>(&mut self, pat: &'p Pattern<'tcx>) -> Vec<&'p Pattern<'tcx>>
337 let pattern_arena = &*self.pattern_arena;
339 self.byte_array_map.entry(pat).or_insert_with(|| {
341 box PatternKind::Constant {
344 if let Some(ptr) = const_val.to_ptr() {
345 let is_array_ptr = const_val.ty
347 .and_then(|t| t.ty.builtin_index())
348 .map_or(false, |t| t == tcx.types.u8);
349 assert!(is_array_ptr);
350 let alloc = tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
351 assert_eq!(ptr.offset.bytes(), 0);
352 // FIXME: check length
353 alloc.bytes.iter().map(|b| {
354 &*pattern_arena.alloc(Pattern {
357 kind: box PatternKind::Constant {
358 value: ty::Const::from_bits(
361 ty::ParamEnv::empty().and(tcx.types.u8))
366 bug!("not a byte str: {:?}", const_val)
369 _ => span_bug!(pat.span, "unexpected byte array pattern {:?}", pat)
374 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
375 if self.tcx.features().exhaustive_patterns {
376 self.tcx.is_ty_uninhabited_from(self.module, ty)
382 fn is_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
384 ty::Adt(adt_def, ..) => adt_def.is_variant_list_non_exhaustive(),
389 fn is_local(&self, ty: Ty<'tcx>) -> bool {
391 ty::Adt(adt_def, ..) => adt_def.did.is_local(),
396 fn is_variant_uninhabited(&self,
397 variant: &'tcx ty::VariantDef,
398 substs: &'tcx ty::subst::Substs<'tcx>)
401 if self.tcx.features().exhaustive_patterns {
402 self.tcx.is_enum_variant_uninhabited_from(self.module, variant, substs)
409 #[derive(Clone, Debug, PartialEq)]
410 pub enum Constructor<'tcx> {
411 /// The constructor of all patterns that don't vary by constructor,
412 /// e.g. struct patterns and fixed-length arrays.
417 ConstantValue(&'tcx ty::Const<'tcx>),
418 /// Ranges of literal values (`2...5` and `2..5`).
419 ConstantRange(u128, u128, Ty<'tcx>, RangeEnd),
420 /// Array patterns of length n.
424 impl<'tcx> Constructor<'tcx> {
425 fn variant_index_for_adt(&self, adt: &'tcx ty::AdtDef) -> VariantIdx {
427 &Variant(vid) => adt.variant_index_with_id(vid),
429 assert!(!adt.is_enum());
432 _ => bug!("bad constructor {:?} for adt {:?}", self, adt)
437 #[derive(Clone, Debug)]
438 pub enum Usefulness<'tcx> {
440 UsefulWithWitness(Vec<Witness<'tcx>>),
444 impl<'tcx> Usefulness<'tcx> {
445 fn is_useful(&self) -> bool {
453 #[derive(Copy, Clone, Debug)]
454 pub enum WitnessPreference {
459 #[derive(Copy, Clone, Debug)]
460 struct PatternContext<'tcx> {
462 max_slice_length: u64,
465 /// A witness of non-exhaustiveness for error reporting, represented
466 /// as a list of patterns (in reverse order of construction) with
467 /// wildcards inside to represent elements that can take any inhabitant
468 /// of the type as a value.
470 /// A witness against a list of patterns should have the same types
471 /// and length as the pattern matched against. Because Rust `match`
472 /// is always against a single pattern, at the end the witness will
473 /// have length 1, but in the middle of the algorithm, it can contain
474 /// multiple patterns.
476 /// For example, if we are constructing a witness for the match against
478 /// struct Pair(Option<(u32, u32)>, bool);
480 /// match (p: Pair) {
481 /// Pair(None, _) => {}
482 /// Pair(_, false) => {}
486 /// We'll perform the following steps:
487 /// 1. Start with an empty witness
488 /// `Witness(vec![])`
489 /// 2. Push a witness `Some(_)` against the `None`
490 /// `Witness(vec![Some(_)])`
491 /// 3. Push a witness `true` against the `false`
492 /// `Witness(vec![Some(_), true])`
493 /// 4. Apply the `Pair` constructor to the witnesses
494 /// `Witness(vec![Pair(Some(_), true)])`
496 /// The final `Pair(Some(_), true)` is then the resulting witness.
497 #[derive(Clone, Debug)]
498 pub struct Witness<'tcx>(Vec<Pattern<'tcx>>);
500 impl<'tcx> Witness<'tcx> {
501 pub fn single_pattern(&self) -> &Pattern<'tcx> {
502 assert_eq!(self.0.len(), 1);
506 fn push_wild_constructor<'a>(
508 cx: &MatchCheckCtxt<'a, 'tcx>,
509 ctor: &Constructor<'tcx>,
513 let sub_pattern_tys = constructor_sub_pattern_tys(cx, ctor, ty);
514 self.0.extend(sub_pattern_tys.into_iter().map(|ty| {
518 kind: box PatternKind::Wild,
521 self.apply_constructor(cx, ctor, ty)
525 /// Constructs a partial witness for a pattern given a list of
526 /// patterns expanded by the specialization step.
528 /// When a pattern P is discovered to be useful, this function is used bottom-up
529 /// to reconstruct a complete witness, e.g. a pattern P' that covers a subset
530 /// of values, V, where each value in that set is not covered by any previously
531 /// used patterns and is covered by the pattern P'. Examples:
533 /// left_ty: tuple of 3 elements
534 /// pats: [10, 20, _] => (10, 20, _)
536 /// left_ty: struct X { a: (bool, &'static str), b: usize}
537 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
538 fn apply_constructor<'a>(
540 cx: &MatchCheckCtxt<'a,'tcx>,
541 ctor: &Constructor<'tcx>,
545 let arity = constructor_arity(cx, ctor, ty);
547 let len = self.0.len() as u64;
548 let mut pats = self.0.drain((len - arity) as usize..).rev();
553 let pats = pats.enumerate().map(|(i, p)| {
555 field: Field::new(i),
560 if let ty::Adt(adt, substs) = ty.sty {
562 PatternKind::Variant {
565 variant_index: ctor.variant_index_for_adt(adt),
569 PatternKind::Leaf { subpatterns: pats }
572 PatternKind::Leaf { subpatterns: pats }
577 PatternKind::Deref { subpattern: pats.nth(0).unwrap() }
580 ty::Slice(_) | ty::Array(..) => {
582 prefix: pats.collect(),
590 ConstantValue(value) => PatternKind::Constant { value },
591 ConstantRange(lo, hi, ty, end) => PatternKind::Range {
592 lo: ty::Const::from_bits(cx.tcx, lo, ty::ParamEnv::empty().and(ty)),
593 hi: ty::Const::from_bits(cx.tcx, hi, ty::ParamEnv::empty().and(ty)),
597 _ => PatternKind::Wild,
603 self.0.push(Pattern {
613 /// This determines the set of all possible constructors of a pattern matching
614 /// values of type `left_ty`. For vectors, this would normally be an infinite set
615 /// but is instead bounded by the maximum fixed length of slice patterns in
616 /// the column of patterns being analyzed.
618 /// We make sure to omit constructors that are statically impossible. eg for
619 /// Option<!> we do not include Some(_) in the returned list of constructors.
620 fn all_constructors<'a, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
621 pcx: PatternContext<'tcx>)
622 -> Vec<Constructor<'tcx>>
624 debug!("all_constructors({:?})", pcx.ty);
625 let exhaustive_integer_patterns = cx.tcx.features().exhaustive_integer_patterns;
626 let ctors = match pcx.ty.sty {
628 [true, false].iter().map(|&b| {
629 ConstantValue(ty::Const::from_bool(cx.tcx, b))
632 ty::Array(ref sub_ty, len) if len.assert_usize(cx.tcx).is_some() => {
633 let len = len.unwrap_usize(cx.tcx);
634 if len != 0 && cx.is_uninhabited(sub_ty) {
640 // Treat arrays of a constant but unknown length like slices.
641 ty::Array(ref sub_ty, _) |
642 ty::Slice(ref sub_ty) => {
643 if cx.is_uninhabited(sub_ty) {
646 (0..pcx.max_slice_length+1).map(|length| Slice(length)).collect()
649 ty::Adt(def, substs) if def.is_enum() => {
651 .filter(|v| !cx.is_variant_uninhabited(v, substs))
652 .map(|v| Variant(v.did))
655 ty::Char if exhaustive_integer_patterns => {
657 // The valid Unicode Scalar Value ranges.
658 ConstantRange('\u{0000}' as u128,
663 ConstantRange('\u{E000}' as u128,
664 '\u{10FFFF}' as u128,
670 ty::Int(ity) if exhaustive_integer_patterns => {
671 // FIXME(49937): refactor these bit manipulations into interpret.
672 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
673 let min = 1u128 << (bits - 1);
674 let max = (1u128 << (bits - 1)) - 1;
675 vec![ConstantRange(min, max, pcx.ty, RangeEnd::Included)]
677 ty::Uint(uty) if exhaustive_integer_patterns => {
678 // FIXME(49937): refactor these bit manipulations into interpret.
679 let bits = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size().bits() as u128;
680 let max = !0u128 >> (128 - bits);
681 vec![ConstantRange(0, max, pcx.ty, RangeEnd::Included)]
684 if cx.is_uninhabited(pcx.ty) {
694 fn max_slice_length<'p, 'a: 'p, 'tcx: 'a, I>(
695 cx: &mut MatchCheckCtxt<'a, 'tcx>,
697 where I: Iterator<Item=&'p Pattern<'tcx>>
699 // The exhaustiveness-checking paper does not include any details on
700 // checking variable-length slice patterns. However, they are matched
701 // by an infinite collection of fixed-length array patterns.
703 // Checking the infinite set directly would take an infinite amount
704 // of time. However, it turns out that for each finite set of
705 // patterns `P`, all sufficiently large array lengths are equivalent:
707 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
708 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
709 // `sₘ` for each sufficiently-large length `m` that applies to exactly
710 // the same subset of `P`.
712 // Because of that, each witness for reachability-checking from one
713 // of the sufficiently-large lengths can be transformed to an
714 // equally-valid witness from any other length, so we only have
715 // to check slice lengths from the "minimal sufficiently-large length"
718 // Note that the fact that there is a *single* `sₘ` for each `m`
719 // not depending on the specific pattern in `P` is important: if
720 // you look at the pair of patterns
723 // Then any slice of length ≥1 that matches one of these two
724 // patterns can be trivially turned to a slice of any
725 // other length ≥1 that matches them and vice-versa - for
726 // but the slice from length 2 `[false, true]` that matches neither
727 // of these patterns can't be turned to a slice from length 1 that
728 // matches neither of these patterns, so we have to consider
729 // slices from length 2 there.
731 // Now, to see that that length exists and find it, observe that slice
732 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
733 // "variable-length" patterns (`[_, .., _]`).
735 // For fixed-length patterns, all slices with lengths *longer* than
736 // the pattern's length have the same outcome (of not matching), so
737 // as long as `L` is greater than the pattern's length we can pick
738 // any `sₘ` from that length and get the same result.
740 // For variable-length patterns, the situation is more complicated,
741 // because as seen above the precise value of `sₘ` matters.
743 // However, for each variable-length pattern `p` with a prefix of length
744 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
745 // `slₚ` elements are examined.
747 // Therefore, as long as `L` is positive (to avoid concerns about empty
748 // types), all elements after the maximum prefix length and before
749 // the maximum suffix length are not examined by any variable-length
750 // pattern, and therefore can be added/removed without affecting
751 // them - creating equivalent patterns from any sufficiently-large
754 // Of course, if fixed-length patterns exist, we must be sure
755 // that our length is large enough to miss them all, so
756 // we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
758 // for example, with the above pair of patterns, all elements
759 // but the first and last can be added/removed, so any
760 // witness of length ≥2 (say, `[false, false, true]`) can be
761 // turned to a witness from any other length ≥2.
763 let mut max_prefix_len = 0;
764 let mut max_suffix_len = 0;
765 let mut max_fixed_len = 0;
767 for row in patterns {
769 PatternKind::Constant { value } => {
770 if let Some(ptr) = value.to_ptr() {
771 let is_array_ptr = value.ty
773 .and_then(|t| t.ty.builtin_index())
774 .map_or(false, |t| t == cx.tcx.types.u8);
776 let alloc = cx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id);
777 max_fixed_len = cmp::max(max_fixed_len, alloc.bytes.len() as u64);
781 PatternKind::Slice { ref prefix, slice: None, ref suffix } => {
782 let fixed_len = prefix.len() as u64 + suffix.len() as u64;
783 max_fixed_len = cmp::max(max_fixed_len, fixed_len);
785 PatternKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
786 max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
787 max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
793 cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len)
796 /// An inclusive interval, used for precise integer exhaustiveness checking.
797 /// `IntRange`s always store a contiguous range. This means that values are
798 /// encoded such that `0` encodes the minimum value for the integer,
799 /// regardless of the signedness.
800 /// For example, the pattern `-128...127i8` is encoded as `0..=255`.
801 /// This makes comparisons and arithmetic on interval endpoints much more
802 /// straightforward. See `signed_bias` for details.
804 /// `IntRange` is never used to encode an empty range or a "range" that wraps
805 /// around the (offset) space: i.e. `range.lo <= range.hi`.
807 struct IntRange<'tcx> {
808 pub range: RangeInclusive<u128>,
812 impl<'tcx> IntRange<'tcx> {
813 fn from_ctor(tcx: TyCtxt<'_, 'tcx, 'tcx>,
814 ctor: &Constructor<'tcx>)
815 -> Option<IntRange<'tcx>> {
817 ConstantRange(lo, hi, ty, end) => {
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 if lo > hi || lo == hi && *end == RangeEnd::Excluded {
826 let offset = (*end == RangeEnd::Excluded) as u128;
827 Some(IntRange { range: lo..=(hi - offset), ty })
830 ConstantValue(val) => {
832 if let Some(val) = val.assert_bits(tcx, ty::ParamEnv::empty().and(ty)) {
833 let bias = IntRange::signed_bias(tcx, ty);
834 let val = val ^ bias;
835 Some(IntRange { range: val..=val, ty })
840 Single | Variant(_) | Slice(_) => {
846 fn from_pat(tcx: TyCtxt<'_, 'tcx, 'tcx>,
848 -> Option<IntRange<'tcx>> {
849 Self::from_ctor(tcx, &match pat.kind {
850 box PatternKind::Constant { value } => ConstantValue(value),
851 box PatternKind::Range { lo, hi, ty, end } => ConstantRange(
852 lo.to_bits(tcx, ty::ParamEnv::empty().and(ty)).unwrap(),
853 hi.to_bits(tcx, ty::ParamEnv::empty().and(ty)).unwrap(),
861 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
862 fn signed_bias(tcx: TyCtxt<'_, 'tcx, 'tcx>, ty: Ty<'tcx>) -> u128 {
865 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
872 /// Convert a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
874 tcx: TyCtxt<'_, 'tcx, 'tcx>,
876 r: RangeInclusive<u128>,
877 ) -> Constructor<'tcx> {
878 let bias = IntRange::signed_bias(tcx, ty);
879 let (lo, hi) = r.into_inner();
881 let ty = ty::ParamEnv::empty().and(ty);
882 ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty))
884 ConstantRange(lo ^ bias, hi ^ bias, ty, RangeEnd::Included)
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 // A request for missing constructor data in terms of either:
935 // - whether or not there any missing constructors; or
936 // - the actual set of missing constructors.
938 enum MissingCtorsInfo {
943 // Used by `compute_missing_ctors`.
944 #[derive(Debug, PartialEq)]
945 enum MissingCtors<'tcx> {
949 // Note that the Vec can be empty.
950 Ctors(Vec<Constructor<'tcx>>),
953 // When `info` is `MissingCtorsInfo::Ctors`, compute a set of constructors
954 // equivalent to `all_ctors \ used_ctors`. When `info` is
955 // `MissingCtorsInfo::Emptiness`, just determines if that set is empty or not.
956 // (The split logic gives a performance win, because we always need to know if
957 // the set is empty, but we rarely need the full set, and it can be expensive
958 // to compute the full set.)
959 fn compute_missing_ctors<'a, 'tcx: 'a>(
960 info: MissingCtorsInfo,
961 tcx: TyCtxt<'a, 'tcx, 'tcx>,
962 all_ctors: &Vec<Constructor<'tcx>>,
963 used_ctors: &Vec<Constructor<'tcx>>,
964 ) -> MissingCtors<'tcx> {
965 let mut missing_ctors = vec![];
967 for req_ctor in all_ctors {
968 let mut refined_ctors = vec![req_ctor.clone()];
969 for used_ctor in used_ctors {
970 if used_ctor == req_ctor {
971 // If a constructor appears in a `match` arm, we can
972 // eliminate it straight away.
973 refined_ctors = vec![]
974 } else if tcx.features().exhaustive_integer_patterns {
975 if let Some(interval) = IntRange::from_ctor(tcx, used_ctor) {
976 // Refine the required constructors for the type by subtracting
977 // the range defined by the current constructor pattern.
978 refined_ctors = interval.subtract_from(tcx, refined_ctors);
982 // If the constructor patterns that have been considered so far
983 // already cover the entire range of values, then we the
984 // constructor is not missing, and we can move on to the next one.
985 if refined_ctors.is_empty() {
989 // If a constructor has not been matched, then it is missing.
990 // We add `refined_ctors` instead of `req_ctor`, because then we can
991 // provide more detailed error information about precisely which
992 // ranges have been omitted.
993 if info == MissingCtorsInfo::Emptiness {
994 if !refined_ctors.is_empty() {
995 // The set is non-empty; return early.
996 return MissingCtors::NonEmpty;
999 missing_ctors.extend(refined_ctors);
1003 if info == MissingCtorsInfo::Emptiness {
1004 // If we reached here, the set is empty.
1007 MissingCtors::Ctors(missing_ctors)
1011 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html
1012 /// The algorithm from the paper has been modified to correctly handle empty
1013 /// types. The changes are:
1014 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1015 /// continue to recurse over columns.
1016 /// (1) all_constructors will only return constructors that are statically
1017 /// possible. eg. it will only return Ok for Result<T, !>
1019 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1020 /// to a set of such vectors `m` - this is defined as there being a set of
1021 /// inputs that will match `v` but not any of the sets in `m`.
1023 /// All the patterns at each column of the `matrix ++ v` matrix must
1024 /// have the same type, except that wildcard (PatternKind::Wild) patterns
1025 /// with type TyErr are also allowed, even if the "type of the column"
1026 /// is not TyErr. That is used to represent private fields, as using their
1027 /// real type would assert that they are inhabited.
1029 /// This is used both for reachability checking (if a pattern isn't useful in
1030 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1031 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1032 /// matrix isn't exhaustive).
1033 pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
1034 matrix: &Matrix<'p, 'tcx>,
1035 v: &[&'p Pattern<'tcx>],
1036 witness: WitnessPreference)
1037 -> Usefulness<'tcx> {
1038 let &Matrix(ref rows) = matrix;
1039 debug!("is_useful({:#?}, {:#?})", matrix, v);
1041 // The base case. We are pattern-matching on () and the return value is
1042 // based on whether our matrix has a row or not.
1043 // NOTE: This could potentially be optimized by checking rows.is_empty()
1044 // first and then, if v is non-empty, the return value is based on whether
1045 // the type of the tuple we're checking is inhabited or not.
1047 return if rows.is_empty() {
1049 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
1050 LeaveOutWitness => Useful,
1057 assert!(rows.iter().all(|r| r.len() == v.len()));
1059 let pcx = PatternContext {
1060 // TyErr is used to represent the type of wildcard patterns matching
1061 // against inaccessible (private) fields of structs, so that we won't
1062 // be able to observe whether the types of the struct's fields are
1065 // If the field is truly inaccessible, then all the patterns
1066 // matching against it must be wildcard patterns, so its type
1069 // However, if we are matching against non-wildcard patterns, we
1070 // need to know the real type of the field so we can specialize
1071 // against it. This primarily occurs through constants - they
1072 // can include contents for fields that are inaccessible at the
1073 // location of the match. In that case, the field's type is
1074 // inhabited - by the constant - so we can just use it.
1076 // FIXME: this might lead to "unstable" behavior with macro hygiene
1077 // introducing uninhabited patterns for inaccessible fields. We
1078 // need to figure out how to model that.
1079 ty: rows.iter().map(|r| r[0].ty).find(|ty| !ty.references_error()).unwrap_or(v[0].ty),
1080 max_slice_length: max_slice_length(cx, rows.iter().map(|r| r[0]).chain(Some(v[0])))
1083 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v[0]);
1085 if let Some(constructors) = pat_constructors(cx, v[0], pcx) {
1086 debug!("is_useful - expanding constructors: {:#?}", constructors);
1087 split_grouped_constructors(cx.tcx, constructors, matrix, pcx.ty).into_iter().map(|c|
1088 is_useful_specialized(cx, matrix, v, c, pcx.ty, witness)
1089 ).find(|result| result.is_useful()).unwrap_or(NotUseful)
1091 debug!("is_useful - expanding wildcard");
1093 let used_ctors: Vec<Constructor> = rows.iter().flat_map(|row| {
1094 pat_constructors(cx, row[0], pcx).unwrap_or(vec![])
1096 debug!("used_ctors = {:#?}", used_ctors);
1097 // `all_ctors` are all the constructors for the given type, which
1098 // should all be represented (or caught with the wild pattern `_`).
1099 let all_ctors = all_constructors(cx, pcx);
1100 debug!("all_ctors = {:#?}", all_ctors);
1102 // `missing_ctors` is the set of constructors from the same type as the
1103 // first column of `matrix` that are matched only by wildcard patterns
1104 // from the first column.
1106 // Therefore, if there is some pattern that is unmatched by `matrix`,
1107 // it will still be unmatched if the first constructor is replaced by
1108 // any of the constructors in `missing_ctors`
1110 // However, if our scrutinee is *privately* an empty enum, we
1111 // must treat it as though it had an "unknown" constructor (in
1112 // that case, all other patterns obviously can't be variants)
1113 // to avoid exposing its emptyness. See the `match_privately_empty`
1114 // test for details.
1116 // FIXME: currently the only way I know of something can
1117 // be a privately-empty enum is when the exhaustive_patterns
1118 // feature flag is not present, so this is only
1119 // needed for that case.
1121 // Missing constructors are those that are not matched by any
1122 // non-wildcard patterns in the current column. We always determine if
1123 // the set is empty, but we only fully construct them on-demand,
1124 // because they're rarely used and can be big.
1125 let cheap_missing_ctors =
1126 compute_missing_ctors(MissingCtorsInfo::Emptiness, cx.tcx, &all_ctors, &used_ctors);
1128 let is_privately_empty = all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1129 let is_declared_nonexhaustive = cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
1130 debug!("cheap_missing_ctors={:#?} is_privately_empty={:#?} is_declared_nonexhaustive={:#?}",
1131 cheap_missing_ctors, is_privately_empty, is_declared_nonexhaustive);
1133 // For privately empty and non-exhaustive enums, we work as if there were an "extra"
1134 // `_` constructor for the type, so we can never match over all constructors.
1135 let is_non_exhaustive = is_privately_empty || is_declared_nonexhaustive;
1137 if cheap_missing_ctors == MissingCtors::Empty && !is_non_exhaustive {
1138 split_grouped_constructors(cx.tcx, all_ctors, matrix, pcx.ty).into_iter().map(|c| {
1139 is_useful_specialized(cx, matrix, v, c, pcx.ty, witness)
1140 }).find(|result| result.is_useful()).unwrap_or(NotUseful)
1142 let matrix = rows.iter().filter_map(|r| {
1143 if r[0].is_wildcard() {
1144 Some(r[1..].to_vec())
1149 match is_useful(cx, &matrix, &v[1..], witness) {
1150 UsefulWithWitness(pats) => {
1152 // In this case, there's at least one "free"
1153 // constructor that is only matched against by
1154 // wildcard patterns.
1156 // There are 2 ways we can report a witness here.
1157 // Commonly, we can report all the "free"
1158 // constructors as witnesses, e.g. if we have:
1161 // enum Direction { N, S, E, W }
1162 // let Direction::N = ...;
1165 // we can report 3 witnesses: `S`, `E`, and `W`.
1167 // However, there are 2 cases where we don't want
1168 // to do this and instead report a single `_` witness:
1170 // 1) If the user is matching against a non-exhaustive
1171 // enum, there is no point in enumerating all possible
1172 // variants, because the user can't actually match
1173 // against them himself, e.g. in an example like:
1175 // let err: io::ErrorKind = ...;
1177 // io::ErrorKind::NotFound => {},
1180 // we don't want to show every possible IO error,
1181 // but instead have `_` as the witness (this is
1182 // actually *required* if the user specified *all*
1183 // IO errors, but is probably what we want in every
1186 // 2) If the user didn't actually specify a constructor
1187 // in this arm, e.g. in
1189 // let x: (Direction, Direction, bool) = ...;
1190 // let (_, _, false) = x;
1192 // we don't want to show all 16 possible witnesses
1193 // `(<direction-1>, <direction-2>, true)` - we are
1194 // satisfied with `(_, _, true)`. In this case,
1195 // `used_ctors` is empty.
1196 let new_witnesses = if is_non_exhaustive || used_ctors.is_empty() {
1197 // All constructors are unused. Add wild patterns
1198 // rather than each individual constructor.
1199 pats.into_iter().map(|mut witness| {
1200 witness.0.push(Pattern {
1203 kind: box PatternKind::Wild,
1208 let expensive_missing_ctors =
1209 compute_missing_ctors(MissingCtorsInfo::Ctors, cx.tcx, &all_ctors,
1211 if let MissingCtors::Ctors(missing_ctors) = expensive_missing_ctors {
1212 pats.into_iter().flat_map(|witness| {
1213 missing_ctors.iter().map(move |ctor| {
1214 // Extends the witness with a "wild" version of this
1215 // constructor, that matches everything that can be built with
1216 // it. For example, if `ctor` is a `Constructor::Variant` for
1217 // `Option::Some`, this pushes the witness for `Some(_)`.
1218 witness.clone().push_wild_constructor(cx, ctor, pcx.ty)
1222 bug!("cheap missing ctors")
1225 UsefulWithWitness(new_witnesses)
1233 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e. `is_useful` applied
1234 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1235 fn is_useful_specialized<'p, 'a:'p, 'tcx: 'a>(
1236 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1237 &Matrix(ref m): &Matrix<'p, 'tcx>,
1238 v: &[&'p Pattern<'tcx>],
1239 ctor: Constructor<'tcx>,
1241 witness: WitnessPreference,
1242 ) -> Usefulness<'tcx> {
1243 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1244 let sub_pat_tys = constructor_sub_pattern_tys(cx, &ctor, lty);
1245 let wild_patterns_owned: Vec<_> = sub_pat_tys.iter().map(|ty| {
1249 kind: box PatternKind::Wild,
1252 let wild_patterns: Vec<_> = wild_patterns_owned.iter().collect();
1253 let matrix = Matrix(m.iter().flat_map(|r| {
1254 specialize(cx, &r, &ctor, &wild_patterns)
1256 match specialize(cx, v, &ctor, &wild_patterns) {
1257 Some(v) => match is_useful(cx, &matrix, &v, witness) {
1258 UsefulWithWitness(witnesses) => UsefulWithWitness(
1259 witnesses.into_iter()
1260 .map(|witness| witness.apply_constructor(cx, &ctor, lty))
1269 /// Determines the constructors that the given pattern can be specialized to.
1271 /// In most cases, there's only one constructor that a specific pattern
1272 /// represents, such as a specific enum variant or a specific literal value.
1273 /// Slice patterns, however, can match slices of different lengths. For instance,
1274 /// `[a, b, ..tail]` can match a slice of length 2, 3, 4 and so on.
1276 /// Returns None in case of a catch-all, which can't be specialized.
1277 fn pat_constructors<'tcx>(cx: &mut MatchCheckCtxt<'_, 'tcx>,
1278 pat: &Pattern<'tcx>,
1279 pcx: PatternContext)
1280 -> Option<Vec<Constructor<'tcx>>>
1283 PatternKind::AscribeUserType { ref subpattern, .. } =>
1284 pat_constructors(cx, subpattern, pcx),
1285 PatternKind::Binding { .. } | PatternKind::Wild => None,
1286 PatternKind::Leaf { .. } | PatternKind::Deref { .. } => Some(vec![Single]),
1287 PatternKind::Variant { adt_def, variant_index, .. } => {
1288 Some(vec![Variant(adt_def.variants[variant_index].did)])
1290 PatternKind::Constant { value } => Some(vec![ConstantValue(value)]),
1291 PatternKind::Range { lo, hi, ty, end } =>
1292 Some(vec![ConstantRange(
1293 lo.to_bits(cx.tcx, ty::ParamEnv::empty().and(ty)).unwrap(),
1294 hi.to_bits(cx.tcx, ty::ParamEnv::empty().and(ty)).unwrap(),
1298 PatternKind::Array { .. } => match pcx.ty.sty {
1299 ty::Array(_, length) => Some(vec![
1300 Slice(length.unwrap_usize(cx.tcx))
1302 _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty)
1304 PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
1305 let pat_len = prefix.len() as u64 + suffix.len() as u64;
1306 if slice.is_some() {
1307 Some((pat_len..pcx.max_slice_length+1).map(Slice).collect())
1309 Some(vec![Slice(pat_len)])
1315 /// This computes the arity of a constructor. The arity of a constructor
1316 /// is how many subpattern patterns of that constructor should be expanded to.
1318 /// For instance, a tuple pattern (_, 42, Some([])) has the arity of 3.
1319 /// A struct pattern's arity is the number of fields it contains, etc.
1320 fn constructor_arity(_cx: &MatchCheckCtxt, ctor: &Constructor, ty: Ty) -> u64 {
1321 debug!("constructor_arity({:#?}, {:?})", ctor, ty);
1323 ty::Tuple(ref fs) => fs.len() as u64,
1324 ty::Slice(..) | ty::Array(..) => match *ctor {
1325 Slice(length) => length,
1326 ConstantValue(_) => 0,
1327 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty)
1330 ty::Adt(adt, _) => {
1331 adt.variants[ctor.variant_index_for_adt(adt)].fields.len() as u64
1337 /// This computes the types of the sub patterns that a constructor should be
1340 /// For instance, a tuple pattern (43u32, 'a') has sub pattern types [u32, char].
1341 fn constructor_sub_pattern_tys<'a, 'tcx: 'a>(cx: &MatchCheckCtxt<'a, 'tcx>,
1343 ty: Ty<'tcx>) -> Vec<Ty<'tcx>>
1345 debug!("constructor_sub_pattern_tys({:#?}, {:?})", ctor, ty);
1347 ty::Tuple(ref fs) => fs.into_iter().map(|t| *t).collect(),
1348 ty::Slice(ty) | ty::Array(ty, _) => match *ctor {
1349 Slice(length) => (0..length).map(|_| ty).collect(),
1350 ConstantValue(_) => vec![],
1351 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty)
1353 ty::Ref(_, rty, _) => vec![rty],
1354 ty::Adt(adt, substs) => {
1356 // Use T as the sub pattern type of Box<T>.
1357 vec![substs.type_at(0)]
1359 adt.variants[ctor.variant_index_for_adt(adt)].fields.iter().map(|field| {
1360 let is_visible = adt.is_enum()
1361 || field.vis.is_accessible_from(cx.module, cx.tcx);
1363 field.ty(cx.tcx, substs)
1365 // Treat all non-visible fields as TyErr. They
1366 // can't appear in any other pattern from
1367 // this match (because they are private),
1368 // so their type does not matter - but
1369 // we don't want to know they are
1380 fn slice_pat_covered_by_constructor<'tcx>(
1381 tcx: TyCtxt<'_, 'tcx, '_>,
1384 prefix: &[Pattern<'tcx>],
1385 slice: &Option<Pattern<'tcx>>,
1386 suffix: &[Pattern<'tcx>]
1387 ) -> Result<bool, ErrorReported> {
1388 let data: &[u8] = match *ctor {
1389 ConstantValue(const_val) => {
1390 let val = match const_val.val {
1391 ConstValue::Unevaluated(..) |
1392 ConstValue::ByRef(..) => bug!("unexpected ConstValue: {:?}", const_val),
1393 ConstValue::Scalar(val) | ConstValue::ScalarPair(val, _) => val,
1395 if let Ok(ptr) = val.to_ptr() {
1396 let is_array_ptr = const_val.ty
1397 .builtin_deref(true)
1398 .and_then(|t| t.ty.builtin_index())
1399 .map_or(false, |t| t == tcx.types.u8);
1400 assert!(is_array_ptr);
1401 tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id).bytes.as_ref()
1403 bug!("unexpected non-ptr ConstantValue")
1409 let pat_len = prefix.len() + suffix.len();
1410 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1415 data[..prefix.len()].iter().zip(prefix).chain(
1416 data[data.len()-suffix.len()..].iter().zip(suffix))
1419 box PatternKind::Constant { value } => {
1420 let b = value.unwrap_bits(tcx, ty::ParamEnv::empty().and(pat.ty));
1421 assert_eq!(b as u8 as u128, b);
1433 // Whether to evaluate a constructor using exhaustive integer matching. This is true if the
1434 // constructor is a range or constant with an integer type.
1435 fn should_treat_range_exhaustively(tcx: TyCtxt<'_, 'tcx, 'tcx>, ctor: &Constructor<'tcx>) -> bool {
1436 if tcx.features().exhaustive_integer_patterns {
1437 let ty = match ctor {
1438 ConstantValue(value) => value.ty,
1439 ConstantRange(_, _, ty, _) => ty,
1442 if let ty::Char | ty::Int(_) | ty::Uint(_) = ty.sty {
1449 /// For exhaustive integer matching, some constructors are grouped within other constructors
1450 /// (namely integer typed values are grouped within ranges). However, when specialising these
1451 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1452 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1453 /// mean creating a separate constructor for every single value in the range, which is clearly
1454 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1455 /// identical across all values in that range (i.e. there are equivalence classes of ranges of
1456 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
1457 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
1458 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
1460 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1461 /// the group of intersecting patterns changes (using the method described below).
1462 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1463 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1464 /// number of rows in the matrix (i.e. the number of cases in the `match` statement), so we don't
1465 /// need to be worried about matching over gargantuan ranges.
1467 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
1469 /// |------| |----------| |-------| ||
1470 /// |-------| |-------| |----| ||
1473 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
1475 /// |--|--|||-||||--||---|||-------| |-|||| ||
1477 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
1478 /// boundaries for each interval range, sort them, then create constructors for each new interval
1479 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
1480 /// merging operation depicted above.)
1481 fn split_grouped_constructors<'p, 'a: 'p, 'tcx: 'a>(
1482 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1483 ctors: Vec<Constructor<'tcx>>,
1484 &Matrix(ref m): &Matrix<'p, 'tcx>,
1486 ) -> Vec<Constructor<'tcx>> {
1487 let mut split_ctors = Vec::with_capacity(ctors.len());
1489 for ctor in ctors.into_iter() {
1491 // For now, only ranges may denote groups of "subconstructors", so we only need to
1492 // special-case constant ranges.
1493 ConstantRange(..) if should_treat_range_exhaustively(tcx, &ctor) => {
1494 // We only care about finding all the subranges within the range of the constructor
1495 // range. Anything else is irrelevant, because it is guaranteed to result in
1496 // `NotUseful`, which is the default case anyway, and can be ignored.
1497 let ctor_range = IntRange::from_ctor(tcx, &ctor).unwrap();
1499 /// Represents a border between 2 integers. Because the intervals spanning borders
1500 /// must be able to cover every integer, we need to be able to represent
1501 /// 2^128 + 1 such borders.
1502 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
1508 // A function for extracting the borders of an integer interval.
1509 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
1510 let (lo, hi) = r.range.into_inner();
1511 let from = Border::JustBefore(lo);
1512 let to = match hi.checked_add(1) {
1513 Some(m) => Border::JustBefore(m),
1514 None => Border::AfterMax,
1516 vec![from, to].into_iter()
1519 // `borders` is the set of borders between equivalence classes: each equivalence
1520 // class lies between 2 borders.
1521 let row_borders = m.iter()
1522 .flat_map(|row| IntRange::from_pat(tcx, row[0]))
1523 .flat_map(|range| ctor_range.intersection(&range))
1524 .flat_map(|range| range_borders(range));
1525 let ctor_borders = range_borders(ctor_range.clone());
1526 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
1527 borders.sort_unstable();
1529 // We're going to iterate through every pair of borders, making sure that each
1530 // represents an interval of nonnegative length, and convert each such interval
1531 // into a constructor.
1532 for IntRange { range, .. } in borders.windows(2).filter_map(|window| {
1533 match (window[0], window[1]) {
1534 (Border::JustBefore(n), Border::JustBefore(m)) => {
1536 Some(IntRange { range: n..=(m - 1), ty })
1541 (Border::JustBefore(n), Border::AfterMax) => {
1542 Some(IntRange { range: n..=u128::MAX, ty })
1544 (Border::AfterMax, _) => None,
1547 split_ctors.push(IntRange::range_to_ctor(tcx, ty, range));
1550 // Any other constructor can be used unchanged.
1551 _ => split_ctors.push(ctor),
1558 /// Check whether there exists any shared value in either `ctor` or `pat` by intersecting them.
1559 fn constructor_intersects_pattern<'p, 'a: 'p, 'tcx: 'a>(
1560 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1561 ctor: &Constructor<'tcx>,
1562 pat: &'p Pattern<'tcx>,
1563 ) -> Option<Vec<&'p Pattern<'tcx>>> {
1564 if should_treat_range_exhaustively(tcx, ctor) {
1565 match (IntRange::from_ctor(tcx, ctor), IntRange::from_pat(tcx, pat)) {
1566 (Some(ctor), Some(pat)) => {
1567 ctor.intersection(&pat).map(|_| {
1568 let (pat_lo, pat_hi) = pat.range.into_inner();
1569 let (ctor_lo, ctor_hi) = ctor.range.into_inner();
1570 assert!(pat_lo <= ctor_lo && ctor_hi <= pat_hi);
1577 // Fallback for non-ranges and ranges that involve floating-point numbers, which are not
1578 // conveniently handled by `IntRange`. For these cases, the constructor may not be a range
1579 // so intersection actually devolves into being covered by the pattern.
1580 match constructor_covered_by_range(tcx, ctor, pat) {
1581 Ok(true) => Some(vec![]),
1582 Ok(false) | Err(ErrorReported) => None,
1587 fn constructor_covered_by_range<'a, 'tcx>(
1588 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1589 ctor: &Constructor<'tcx>,
1590 pat: &Pattern<'tcx>,
1591 ) -> Result<bool, ErrorReported> {
1592 let (from, to, end, ty) = match pat.kind {
1593 box PatternKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
1594 box PatternKind::Range { lo, hi, ty, end } => (lo, hi, end, ty),
1595 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
1597 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
1598 let cmp_from = |c_from| compare_const_vals(tcx, c_from, from, ty::ParamEnv::empty().and(ty))
1599 .map(|res| res != Ordering::Less);
1600 let cmp_to = |c_to| compare_const_vals(tcx, c_to, to, ty::ParamEnv::empty().and(ty));
1601 macro_rules! some_or_ok {
1605 None => return Ok(false), // not char or int
1610 ConstantValue(value) => {
1611 let to = some_or_ok!(cmp_to(value));
1612 let end = (to == Ordering::Less) ||
1613 (end == RangeEnd::Included && to == Ordering::Equal);
1614 Ok(some_or_ok!(cmp_from(value)) && end)
1616 ConstantRange(from, to, ty, RangeEnd::Included) => {
1617 let to = some_or_ok!(cmp_to(ty::Const::from_bits(
1620 ty::ParamEnv::empty().and(ty),
1622 let end = (to == Ordering::Less) ||
1623 (end == RangeEnd::Included && to == Ordering::Equal);
1624 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
1627 ty::ParamEnv::empty().and(ty),
1630 ConstantRange(from, to, ty, RangeEnd::Excluded) => {
1631 let to = some_or_ok!(cmp_to(ty::Const::from_bits(
1634 ty::ParamEnv::empty().and(ty)
1636 let end = (to == Ordering::Less) ||
1637 (end == RangeEnd::Excluded && to == Ordering::Equal);
1638 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
1641 ty::ParamEnv::empty().and(ty)))
1649 fn patterns_for_variant<'p, 'a: 'p, 'tcx: 'a>(
1650 subpatterns: &'p [FieldPattern<'tcx>],
1651 wild_patterns: &[&'p Pattern<'tcx>])
1652 -> Vec<&'p Pattern<'tcx>>
1654 let mut result = wild_patterns.to_owned();
1656 for subpat in subpatterns {
1657 result[subpat.field.index()] = &subpat.pattern;
1660 debug!("patterns_for_variant({:#?}, {:#?}) = {:#?}", subpatterns, wild_patterns, result);
1664 /// This is the main specialization step. It expands the first pattern in the given row
1665 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
1666 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
1668 /// OTOH, slice patterns with a subslice pattern (..tail) can be expanded into multiple
1669 /// different patterns.
1670 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
1671 /// fields filled with wild patterns.
1672 fn specialize<'p, 'a: 'p, 'tcx: 'a>(
1673 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1674 r: &[&'p Pattern<'tcx>],
1675 constructor: &Constructor<'tcx>,
1676 wild_patterns: &[&'p Pattern<'tcx>],
1677 ) -> Option<Vec<&'p Pattern<'tcx>>> {
1680 let head: Option<Vec<&Pattern>> = match *pat.kind {
1681 PatternKind::AscribeUserType { ref subpattern, .. } =>
1682 specialize(cx, ::std::slice::from_ref(&subpattern), constructor, wild_patterns),
1684 PatternKind::Binding { .. } | PatternKind::Wild => {
1685 Some(wild_patterns.to_owned())
1688 PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
1689 let ref variant = adt_def.variants[variant_index];
1690 if *constructor == Variant(variant.did) {
1691 Some(patterns_for_variant(subpatterns, wild_patterns))
1697 PatternKind::Leaf { ref subpatterns } => {
1698 Some(patterns_for_variant(subpatterns, wild_patterns))
1701 PatternKind::Deref { ref subpattern } => {
1702 Some(vec![subpattern])
1705 PatternKind::Constant { value } => {
1706 match *constructor {
1708 if let Some(ptr) = value.to_ptr() {
1709 let is_array_ptr = value.ty
1710 .builtin_deref(true)
1711 .and_then(|t| t.ty.builtin_index())
1712 .map_or(false, |t| t == cx.tcx.types.u8);
1713 assert!(is_array_ptr);
1714 let data_len = cx.tcx
1717 .unwrap_memory(ptr.alloc_id)
1720 if wild_patterns.len() == data_len {
1721 Some(cx.lower_byte_str_pattern(pat))
1727 "unexpected const-val {:?} with ctor {:?}", value, constructor)
1731 // If the constructor is a:
1732 // Single value: add a row if the constructor equals the pattern.
1733 // Range: add a row if the constructor contains the pattern.
1734 constructor_intersects_pattern(cx.tcx, constructor, pat)
1739 PatternKind::Range { .. } => {
1740 // If the constructor is a:
1741 // Single value: add a row if the pattern contains the constructor.
1742 // Range: add a row if the constructor intersects the pattern.
1743 constructor_intersects_pattern(cx.tcx, constructor, pat)
1746 PatternKind::Array { ref prefix, ref slice, ref suffix } |
1747 PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
1748 match *constructor {
1750 let pat_len = prefix.len() + suffix.len();
1751 if let Some(slice_count) = wild_patterns.len().checked_sub(pat_len) {
1752 if slice_count == 0 || slice.is_some() {
1753 Some(prefix.iter().chain(
1754 wild_patterns.iter().map(|p| *p)
1757 .chain(suffix.iter())
1766 ConstantValue(..) => {
1767 match slice_pat_covered_by_constructor(
1768 cx.tcx, pat.span, constructor, prefix, slice, suffix
1770 Ok(true) => Some(vec![]),
1772 Err(ErrorReported) => None
1775 _ => span_bug!(pat.span,
1776 "unexpected ctor {:?} for slice pat", constructor)
1780 debug!("specialize({:#?}, {:#?}) = {:#?}", r[0], wild_patterns, head);
1782 head.map(|mut head| {
1783 head.extend_from_slice(&r[1 ..]);