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, Const};
182 use rustc::ty::layout::{Integer, IntegerExt, VariantIdx, Size};
184 use rustc::mir::Field;
185 use rustc::mir::interpret::{ConstValue, Pointer, Scalar};
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 smallvec::{SmallVec, smallvec};
194 use std::cmp::{self, Ordering, min, max};
196 use std::iter::{FromIterator, IntoIterator};
197 use std::ops::RangeInclusive;
200 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pattern<'tcx>)
203 cx.pattern_arena.alloc(LiteralExpander { tcx: cx.tcx }.fold_pattern(&pat))
206 struct LiteralExpander<'a, 'tcx> {
207 tcx: TyCtxt<'a, 'tcx, 'tcx>
210 impl<'a, 'tcx> LiteralExpander<'a, 'tcx> {
211 /// Derefs `val` and potentially unsizes the value if `crty` is an array and `rty` a slice.
213 /// `crty` and `rty` can differ because you can use array constants in the presence of slice
214 /// patterns. So the pattern may end up being a slice, but the constant is an array. We convert
215 /// the array to a slice in that case.
216 fn fold_const_value_deref(
218 val: ConstValue<'tcx>,
219 // the pattern's pointee type
221 // the constant's pointee type
223 ) -> ConstValue<'tcx> {
224 match (val, &crty.sty, &rty.sty) {
225 // the easy case, deref a reference
226 (ConstValue::Scalar(Scalar::Ptr(p)), x, y) if x == y => ConstValue::ByRef(
228 self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id),
231 // unsize array to slice if pattern is array but match value or other patterns are slice
232 (ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
234 ConstValue::ScalarPair(
236 n.val.try_to_scalar().unwrap(),
239 // fat pointers stay the same
240 (ConstValue::ScalarPair(..), _, _) => val,
241 // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
242 _ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
247 impl<'a, 'tcx> PatternFolder<'tcx> for LiteralExpander<'a, 'tcx> {
248 fn fold_pattern(&mut self, pat: &Pattern<'tcx>) -> Pattern<'tcx> {
249 match (&pat.ty.sty, &*pat.kind) {
252 &PatternKind::Constant { value: Const {
254 ty: ty::TyS { sty: ty::Ref(_, crty, _), .. },
260 kind: box PatternKind::Deref {
261 subpattern: Pattern {
264 kind: box PatternKind::Constant { value: Const::from_const_value(
266 self.fold_const_value_deref(*val, rty, crty),
273 (_, &PatternKind::Binding { subpattern: Some(ref s), .. }) => {
276 _ => pat.super_fold_with(self)
281 impl<'tcx> Pattern<'tcx> {
282 fn is_wildcard(&self) -> bool {
284 PatternKind::Binding { subpattern: None, .. } | PatternKind::Wild =>
291 /// A 2D matrix. Nx1 matrices are very common, which is why `SmallVec[_; 2]`
292 /// works well for each row.
293 pub struct Matrix<'p, 'tcx: 'p>(Vec<SmallVec<[&'p Pattern<'tcx>; 2]>>);
295 impl<'p, 'tcx> Matrix<'p, 'tcx> {
296 pub fn empty() -> Self {
300 pub fn push(&mut self, row: SmallVec<[&'p Pattern<'tcx>; 2]>) {
305 /// Pretty-printer for matrices of patterns, example:
306 /// ++++++++++++++++++++++++++
308 /// ++++++++++++++++++++++++++
309 /// + true + [First] +
310 /// ++++++++++++++++++++++++++
311 /// + true + [Second(true)] +
312 /// ++++++++++++++++++++++++++
314 /// ++++++++++++++++++++++++++
315 /// + _ + [_, _, ..tail] +
316 /// ++++++++++++++++++++++++++
317 impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
318 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
321 let &Matrix(ref m) = self;
322 let pretty_printed_matrix: Vec<Vec<String>> = m.iter().map(|row| {
323 row.iter().map(|pat| format!("{:?}", pat)).collect()
326 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
327 assert!(m.iter().all(|row| row.len() == column_count));
328 let column_widths: Vec<usize> = (0..column_count).map(|col| {
329 pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)
332 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
333 let br = "+".repeat(total_width);
334 write!(f, "{}\n", br)?;
335 for row in pretty_printed_matrix {
337 for (column, pat_str) in row.into_iter().enumerate() {
339 write!(f, "{:1$}", pat_str, column_widths[column])?;
343 write!(f, "{}\n", br)?;
349 impl<'p, 'tcx> FromIterator<SmallVec<[&'p Pattern<'tcx>; 2]>> for Matrix<'p, 'tcx> {
350 fn from_iter<T>(iter: T) -> Self
351 where T: IntoIterator<Item=SmallVec<[&'p Pattern<'tcx>; 2]>>
353 Matrix(iter.into_iter().collect())
357 pub struct MatchCheckCtxt<'a, 'tcx: 'a> {
358 pub tcx: TyCtxt<'a, 'tcx, 'tcx>,
359 /// The module in which the match occurs. This is necessary for
360 /// checking inhabited-ness of types because whether a type is (visibly)
361 /// inhabited can depend on whether it was defined in the current module or
362 /// not. eg. `struct Foo { _private: ! }` cannot be seen to be empty
363 /// outside it's module and should not be matchable with an empty match
366 param_env: ty::ParamEnv<'tcx>,
367 pub pattern_arena: &'a TypedArena<Pattern<'tcx>>,
368 pub byte_array_map: FxHashMap<*const Pattern<'tcx>, Vec<&'a Pattern<'tcx>>>,
371 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
372 pub fn create_and_enter<F, R>(
373 tcx: TyCtxt<'a, 'tcx, 'tcx>,
374 param_env: ty::ParamEnv<'tcx>,
377 where F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R
379 let pattern_arena = TypedArena::default();
385 pattern_arena: &pattern_arena,
386 byte_array_map: FxHashMap::default(),
390 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
391 if self.tcx.features().exhaustive_patterns {
392 self.tcx.is_ty_uninhabited_from(self.module, ty)
398 fn is_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
400 ty::Adt(adt_def, ..) => adt_def.is_variant_list_non_exhaustive(),
405 fn is_local(&self, ty: Ty<'tcx>) -> bool {
407 ty::Adt(adt_def, ..) => adt_def.did.is_local(),
412 fn is_variant_uninhabited(&self,
413 variant: &'tcx ty::VariantDef,
414 substs: &'tcx ty::subst::Substs<'tcx>)
417 if self.tcx.features().exhaustive_patterns {
418 self.tcx.is_enum_variant_uninhabited_from(self.module, variant, substs)
425 #[derive(Clone, Debug, PartialEq)]
426 pub enum Constructor<'tcx> {
427 /// The constructor of all patterns that don't vary by constructor,
428 /// e.g., struct patterns and fixed-length arrays.
433 ConstantValue(&'tcx ty::Const<'tcx>),
434 /// Ranges of literal values (`2...5` and `2..5`).
435 ConstantRange(u128, u128, Ty<'tcx>, RangeEnd),
436 /// Array patterns of length n.
440 impl<'tcx> Constructor<'tcx> {
441 fn variant_index_for_adt(&self, adt: &'tcx ty::AdtDef) -> VariantIdx {
443 &Variant(vid) => adt.variant_index_with_id(vid),
445 assert!(!adt.is_enum());
448 _ => bug!("bad constructor {:?} for adt {:?}", self, adt)
453 #[derive(Clone, Debug)]
454 pub enum Usefulness<'tcx> {
456 UsefulWithWitness(Vec<Witness<'tcx>>),
460 impl<'tcx> Usefulness<'tcx> {
461 fn is_useful(&self) -> bool {
469 #[derive(Copy, Clone, Debug)]
470 pub enum WitnessPreference {
475 #[derive(Copy, Clone, Debug)]
476 struct PatternContext<'tcx> {
478 max_slice_length: u64,
481 /// A witness of non-exhaustiveness for error reporting, represented
482 /// as a list of patterns (in reverse order of construction) with
483 /// wildcards inside to represent elements that can take any inhabitant
484 /// of the type as a value.
486 /// A witness against a list of patterns should have the same types
487 /// and length as the pattern matched against. Because Rust `match`
488 /// is always against a single pattern, at the end the witness will
489 /// have length 1, but in the middle of the algorithm, it can contain
490 /// multiple patterns.
492 /// For example, if we are constructing a witness for the match against
494 /// struct Pair(Option<(u32, u32)>, bool);
496 /// match (p: Pair) {
497 /// Pair(None, _) => {}
498 /// Pair(_, false) => {}
502 /// We'll perform the following steps:
503 /// 1. Start with an empty witness
504 /// `Witness(vec![])`
505 /// 2. Push a witness `Some(_)` against the `None`
506 /// `Witness(vec![Some(_)])`
507 /// 3. Push a witness `true` against the `false`
508 /// `Witness(vec![Some(_), true])`
509 /// 4. Apply the `Pair` constructor to the witnesses
510 /// `Witness(vec![Pair(Some(_), true)])`
512 /// The final `Pair(Some(_), true)` is then the resulting witness.
513 #[derive(Clone, Debug)]
514 pub struct Witness<'tcx>(Vec<Pattern<'tcx>>);
516 impl<'tcx> Witness<'tcx> {
517 pub fn single_pattern(&self) -> &Pattern<'tcx> {
518 assert_eq!(self.0.len(), 1);
522 fn push_wild_constructor<'a>(
524 cx: &MatchCheckCtxt<'a, 'tcx>,
525 ctor: &Constructor<'tcx>,
529 let sub_pattern_tys = constructor_sub_pattern_tys(cx, ctor, ty);
530 self.0.extend(sub_pattern_tys.into_iter().map(|ty| {
534 kind: box PatternKind::Wild,
537 self.apply_constructor(cx, ctor, ty)
541 /// Constructs a partial witness for a pattern given a list of
542 /// patterns expanded by the specialization step.
544 /// When a pattern P is discovered to be useful, this function is used bottom-up
545 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
546 /// of values, V, where each value in that set is not covered by any previously
547 /// used patterns and is covered by the pattern P'. Examples:
549 /// left_ty: tuple of 3 elements
550 /// pats: [10, 20, _] => (10, 20, _)
552 /// left_ty: struct X { a: (bool, &'static str), b: usize}
553 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
554 fn apply_constructor<'a>(
556 cx: &MatchCheckCtxt<'a,'tcx>,
557 ctor: &Constructor<'tcx>,
561 let arity = constructor_arity(cx, ctor, ty);
563 let len = self.0.len() as u64;
564 let mut pats = self.0.drain((len - arity) as usize..).rev();
569 let pats = pats.enumerate().map(|(i, p)| {
571 field: Field::new(i),
576 if let ty::Adt(adt, substs) = ty.sty {
578 PatternKind::Variant {
581 variant_index: ctor.variant_index_for_adt(adt),
585 PatternKind::Leaf { subpatterns: pats }
588 PatternKind::Leaf { subpatterns: pats }
593 PatternKind::Deref { subpattern: pats.nth(0).unwrap() }
596 ty::Slice(_) | ty::Array(..) => {
598 prefix: pats.collect(),
606 ConstantValue(value) => PatternKind::Constant { value },
607 ConstantRange(lo, hi, ty, end) => PatternKind::Range {
608 lo: ty::Const::from_bits(cx.tcx, lo, ty::ParamEnv::empty().and(ty)),
609 hi: ty::Const::from_bits(cx.tcx, hi, ty::ParamEnv::empty().and(ty)),
613 _ => PatternKind::Wild,
619 self.0.push(Pattern {
629 /// This determines the set of all possible constructors of a pattern matching
630 /// values of type `left_ty`. For vectors, this would normally be an infinite set
631 /// but is instead bounded by the maximum fixed length of slice patterns in
632 /// the column of patterns being analyzed.
634 /// We make sure to omit constructors that are statically impossible. eg for
635 /// Option<!> we do not include Some(_) in the returned list of constructors.
636 fn all_constructors<'a, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
637 pcx: PatternContext<'tcx>)
638 -> Vec<Constructor<'tcx>>
640 debug!("all_constructors({:?})", pcx.ty);
641 let ctors = match pcx.ty.sty {
643 [true, false].iter().map(|&b| {
644 ConstantValue(ty::Const::from_bool(cx.tcx, b))
647 ty::Array(ref sub_ty, len) if len.assert_usize(cx.tcx).is_some() => {
648 let len = len.unwrap_usize(cx.tcx);
649 if len != 0 && cx.is_uninhabited(sub_ty) {
655 // Treat arrays of a constant but unknown length like slices.
656 ty::Array(ref sub_ty, _) |
657 ty::Slice(ref sub_ty) => {
658 if cx.is_uninhabited(sub_ty) {
661 (0..pcx.max_slice_length+1).map(|length| Slice(length)).collect()
664 ty::Adt(def, substs) if def.is_enum() => {
666 .filter(|v| !cx.is_variant_uninhabited(v, substs))
667 .map(|v| Variant(v.did))
672 // The valid Unicode Scalar Value ranges.
673 ConstantRange('\u{0000}' as u128,
678 ConstantRange('\u{E000}' as u128,
679 '\u{10FFFF}' as u128,
686 // FIXME(49937): refactor these bit manipulations into interpret.
687 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
688 let min = 1u128 << (bits - 1);
689 let max = (1u128 << (bits - 1)) - 1;
690 vec![ConstantRange(min, max, pcx.ty, RangeEnd::Included)]
693 // FIXME(49937): refactor these bit manipulations into interpret.
694 let bits = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size().bits() as u128;
695 let max = !0u128 >> (128 - bits);
696 vec![ConstantRange(0, max, pcx.ty, RangeEnd::Included)]
699 if cx.is_uninhabited(pcx.ty) {
709 fn max_slice_length<'p, 'a: 'p, 'tcx: 'a, I>(
710 cx: &mut MatchCheckCtxt<'a, 'tcx>,
712 where I: Iterator<Item=&'p Pattern<'tcx>>
714 // The exhaustiveness-checking paper does not include any details on
715 // checking variable-length slice patterns. However, they are matched
716 // by an infinite collection of fixed-length array patterns.
718 // Checking the infinite set directly would take an infinite amount
719 // of time. However, it turns out that for each finite set of
720 // patterns `P`, all sufficiently large array lengths are equivalent:
722 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
723 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
724 // `sₘ` for each sufficiently-large length `m` that applies to exactly
725 // the same subset of `P`.
727 // Because of that, each witness for reachability-checking from one
728 // of the sufficiently-large lengths can be transformed to an
729 // equally-valid witness from any other length, so we only have
730 // to check slice lengths from the "minimal sufficiently-large length"
733 // Note that the fact that there is a *single* `sₘ` for each `m`
734 // not depending on the specific pattern in `P` is important: if
735 // you look at the pair of patterns
738 // Then any slice of length ≥1 that matches one of these two
739 // patterns can be trivially turned to a slice of any
740 // other length ≥1 that matches them and vice-versa - for
741 // but the slice from length 2 `[false, true]` that matches neither
742 // of these patterns can't be turned to a slice from length 1 that
743 // matches neither of these patterns, so we have to consider
744 // slices from length 2 there.
746 // Now, to see that that length exists and find it, observe that slice
747 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
748 // "variable-length" patterns (`[_, .., _]`).
750 // For fixed-length patterns, all slices with lengths *longer* than
751 // the pattern's length have the same outcome (of not matching), so
752 // as long as `L` is greater than the pattern's length we can pick
753 // any `sₘ` from that length and get the same result.
755 // For variable-length patterns, the situation is more complicated,
756 // because as seen above the precise value of `sₘ` matters.
758 // However, for each variable-length pattern `p` with a prefix of length
759 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
760 // `slₚ` elements are examined.
762 // Therefore, as long as `L` is positive (to avoid concerns about empty
763 // types), all elements after the maximum prefix length and before
764 // the maximum suffix length are not examined by any variable-length
765 // pattern, and therefore can be added/removed without affecting
766 // them - creating equivalent patterns from any sufficiently-large
769 // Of course, if fixed-length patterns exist, we must be sure
770 // that our length is large enough to miss them all, so
771 // we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
773 // for example, with the above pair of patterns, all elements
774 // but the first and last can be added/removed, so any
775 // witness of length ≥2 (say, `[false, false, true]`) can be
776 // turned to a witness from any other length ≥2.
778 let mut max_prefix_len = 0;
779 let mut max_suffix_len = 0;
780 let mut max_fixed_len = 0;
782 for row in patterns {
784 PatternKind::Constant { value } => {
785 // extract the length of an array/slice from a constant
786 match (value.val, &value.ty.sty) {
787 (_, ty::Array(_, n)) => max_fixed_len = cmp::max(
789 n.unwrap_usize(cx.tcx),
791 (ConstValue::ScalarPair(_, n), ty::Slice(_)) => max_fixed_len = cmp::max(
793 n.to_usize(&cx.tcx).unwrap(),
798 PatternKind::Slice { ref prefix, slice: None, ref suffix } => {
799 let fixed_len = prefix.len() as u64 + suffix.len() as u64;
800 max_fixed_len = cmp::max(max_fixed_len, fixed_len);
802 PatternKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
803 max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
804 max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
810 cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len)
813 /// An inclusive interval, used for precise integer exhaustiveness checking.
814 /// `IntRange`s always store a contiguous range. This means that values are
815 /// encoded such that `0` encodes the minimum value for the integer,
816 /// regardless of the signedness.
817 /// For example, the pattern `-128...127i8` is encoded as `0..=255`.
818 /// This makes comparisons and arithmetic on interval endpoints much more
819 /// straightforward. See `signed_bias` for details.
821 /// `IntRange` is never used to encode an empty range or a "range" that wraps
822 /// around the (offset) space: i.e., `range.lo <= range.hi`.
824 struct IntRange<'tcx> {
825 pub range: RangeInclusive<u128>,
829 impl<'tcx> IntRange<'tcx> {
830 fn from_ctor(tcx: TyCtxt<'_, 'tcx, 'tcx>,
831 ctor: &Constructor<'tcx>)
832 -> Option<IntRange<'tcx>> {
833 // Floating-point ranges are permitted and we don't want
834 // to consider them when constructing integer ranges.
835 fn is_integral<'tcx>(ty: Ty<'tcx>) -> bool {
837 ty::Char | ty::Int(_) | ty::Uint(_) => true,
843 ConstantRange(lo, hi, ty, end) if is_integral(ty) => {
844 // Perform a shift if the underlying types are signed,
845 // which makes the interval arithmetic simpler.
846 let bias = IntRange::signed_bias(tcx, ty);
847 let (lo, hi) = (lo ^ bias, hi ^ bias);
848 // Make sure the interval is well-formed.
849 if lo > hi || lo == hi && *end == RangeEnd::Excluded {
852 let offset = (*end == RangeEnd::Excluded) as u128;
853 Some(IntRange { range: lo..=(hi - offset), ty })
856 ConstantValue(val) if is_integral(val.ty) => {
858 if let Some(val) = val.assert_bits(tcx, ty::ParamEnv::empty().and(ty)) {
859 let bias = IntRange::signed_bias(tcx, ty);
860 let val = val ^ bias;
861 Some(IntRange { range: val..=val, ty })
870 fn from_pat(tcx: TyCtxt<'_, 'tcx, 'tcx>,
872 -> Option<IntRange<'tcx>> {
873 Self::from_ctor(tcx, &match pat.kind {
874 box PatternKind::Constant { value } => ConstantValue(value),
875 box PatternKind::Range { lo, hi, ty, end } => ConstantRange(
876 lo.to_bits(tcx, ty::ParamEnv::empty().and(ty)).unwrap(),
877 hi.to_bits(tcx, ty::ParamEnv::empty().and(ty)).unwrap(),
885 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
886 fn signed_bias(tcx: TyCtxt<'_, 'tcx, 'tcx>, ty: Ty<'tcx>) -> u128 {
889 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
896 /// Convert a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
898 tcx: TyCtxt<'_, 'tcx, 'tcx>,
900 r: RangeInclusive<u128>,
901 ) -> Constructor<'tcx> {
902 let bias = IntRange::signed_bias(tcx, ty);
903 let (lo, hi) = r.into_inner();
905 let ty = ty::ParamEnv::empty().and(ty);
906 ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty))
908 ConstantRange(lo ^ bias, hi ^ bias, ty, RangeEnd::Included)
912 /// Return a collection of ranges that spans the values covered by `ranges`, subtracted
913 /// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
914 fn subtract_from(self,
915 tcx: TyCtxt<'_, 'tcx, 'tcx>,
916 ranges: Vec<Constructor<'tcx>>)
917 -> Vec<Constructor<'tcx>> {
918 let ranges = ranges.into_iter().filter_map(|r| {
919 IntRange::from_ctor(tcx, &r).map(|i| i.range)
921 let mut remaining_ranges = vec![];
923 let (lo, hi) = self.range.into_inner();
924 for subrange in ranges {
925 let (subrange_lo, subrange_hi) = subrange.into_inner();
926 if lo > subrange_hi || subrange_lo > hi {
927 // The pattern doesn't intersect with the subrange at all,
928 // so the subrange remains untouched.
929 remaining_ranges.push(Self::range_to_ctor(tcx, ty, subrange_lo..=subrange_hi));
931 if lo > subrange_lo {
932 // The pattern intersects an upper section of the
933 // subrange, so a lower section will remain.
934 remaining_ranges.push(Self::range_to_ctor(tcx, ty, subrange_lo..=(lo - 1)));
936 if hi < subrange_hi {
937 // The pattern intersects a lower section of the
938 // subrange, so an upper section will remain.
939 remaining_ranges.push(Self::range_to_ctor(tcx, ty, (hi + 1)..=subrange_hi));
946 fn intersection(&self, other: &Self) -> Option<Self> {
948 let (lo, hi) = (*self.range.start(), *self.range.end());
949 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
950 if lo <= other_hi && other_lo <= hi {
951 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty })
958 // A request for missing constructor data in terms of either:
959 // - whether or not there any missing constructors; or
960 // - the actual set of missing constructors.
962 enum MissingCtorsInfo {
967 // Used by `compute_missing_ctors`.
968 #[derive(Debug, PartialEq)]
969 enum MissingCtors<'tcx> {
973 // Note that the Vec can be empty.
974 Ctors(Vec<Constructor<'tcx>>),
977 // When `info` is `MissingCtorsInfo::Ctors`, compute a set of constructors
978 // equivalent to `all_ctors \ used_ctors`. When `info` is
979 // `MissingCtorsInfo::Emptiness`, just determines if that set is empty or not.
980 // (The split logic gives a performance win, because we always need to know if
981 // the set is empty, but we rarely need the full set, and it can be expensive
982 // to compute the full set.)
983 fn compute_missing_ctors<'a, 'tcx: 'a>(
984 info: MissingCtorsInfo,
985 tcx: TyCtxt<'a, 'tcx, 'tcx>,
986 all_ctors: &Vec<Constructor<'tcx>>,
987 used_ctors: &Vec<Constructor<'tcx>>,
988 ) -> MissingCtors<'tcx> {
989 let mut missing_ctors = vec![];
991 for req_ctor in all_ctors {
992 let mut refined_ctors = vec![req_ctor.clone()];
993 for used_ctor in used_ctors {
994 if used_ctor == req_ctor {
995 // If a constructor appears in a `match` arm, we can
996 // eliminate it straight away.
997 refined_ctors = vec![]
998 } else if let Some(interval) = IntRange::from_ctor(tcx, used_ctor) {
999 // Refine the required constructors for the type by subtracting
1000 // the range defined by the current constructor pattern.
1001 refined_ctors = interval.subtract_from(tcx, refined_ctors);
1004 // If the constructor patterns that have been considered so far
1005 // already cover the entire range of values, then we the
1006 // constructor is not missing, and we can move on to the next one.
1007 if refined_ctors.is_empty() {
1011 // If a constructor has not been matched, then it is missing.
1012 // We add `refined_ctors` instead of `req_ctor`, because then we can
1013 // provide more detailed error information about precisely which
1014 // ranges have been omitted.
1015 if info == MissingCtorsInfo::Emptiness {
1016 if !refined_ctors.is_empty() {
1017 // The set is non-empty; return early.
1018 return MissingCtors::NonEmpty;
1021 missing_ctors.extend(refined_ctors);
1025 if info == MissingCtorsInfo::Emptiness {
1026 // If we reached here, the set is empty.
1029 MissingCtors::Ctors(missing_ctors)
1033 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html
1034 /// The algorithm from the paper has been modified to correctly handle empty
1035 /// types. The changes are:
1036 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1037 /// continue to recurse over columns.
1038 /// (1) all_constructors will only return constructors that are statically
1039 /// possible. eg. it will only return Ok for Result<T, !>
1041 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1042 /// to a set of such vectors `m` - this is defined as there being a set of
1043 /// inputs that will match `v` but not any of the sets in `m`.
1045 /// All the patterns at each column of the `matrix ++ v` matrix must
1046 /// have the same type, except that wildcard (PatternKind::Wild) patterns
1047 /// with type TyErr are also allowed, even if the "type of the column"
1048 /// is not TyErr. That is used to represent private fields, as using their
1049 /// real type would assert that they are inhabited.
1051 /// This is used both for reachability checking (if a pattern isn't useful in
1052 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1053 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1054 /// matrix isn't exhaustive).
1055 pub fn is_useful<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
1056 matrix: &Matrix<'p, 'tcx>,
1057 v: &[&Pattern<'tcx>],
1058 witness: WitnessPreference)
1059 -> Usefulness<'tcx> {
1060 let &Matrix(ref rows) = matrix;
1061 debug!("is_useful({:#?}, {:#?})", matrix, v);
1063 // The base case. We are pattern-matching on () and the return value is
1064 // based on whether our matrix has a row or not.
1065 // NOTE: This could potentially be optimized by checking rows.is_empty()
1066 // first and then, if v is non-empty, the return value is based on whether
1067 // the type of the tuple we're checking is inhabited or not.
1069 return if rows.is_empty() {
1071 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
1072 LeaveOutWitness => Useful,
1079 assert!(rows.iter().all(|r| r.len() == v.len()));
1081 let pcx = PatternContext {
1082 // TyErr is used to represent the type of wildcard patterns matching
1083 // against inaccessible (private) fields of structs, so that we won't
1084 // be able to observe whether the types of the struct's fields are
1087 // If the field is truly inaccessible, then all the patterns
1088 // matching against it must be wildcard patterns, so its type
1091 // However, if we are matching against non-wildcard patterns, we
1092 // need to know the real type of the field so we can specialize
1093 // against it. This primarily occurs through constants - they
1094 // can include contents for fields that are inaccessible at the
1095 // location of the match. In that case, the field's type is
1096 // inhabited - by the constant - so we can just use it.
1098 // FIXME: this might lead to "unstable" behavior with macro hygiene
1099 // introducing uninhabited patterns for inaccessible fields. We
1100 // need to figure out how to model that.
1101 ty: rows.iter().map(|r| r[0].ty).find(|ty| !ty.references_error()).unwrap_or(v[0].ty),
1102 max_slice_length: max_slice_length(cx, rows.iter().map(|r| r[0]).chain(Some(v[0])))
1105 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v[0]);
1107 if let Some(constructors) = pat_constructors(cx, v[0], pcx) {
1108 debug!("is_useful - expanding constructors: {:#?}", constructors);
1109 split_grouped_constructors(cx.tcx, constructors, matrix, pcx.ty).into_iter().map(|c|
1110 is_useful_specialized(cx, matrix, v, c, pcx.ty, witness)
1111 ).find(|result| result.is_useful()).unwrap_or(NotUseful)
1113 debug!("is_useful - expanding wildcard");
1115 let used_ctors: Vec<Constructor> = rows.iter().flat_map(|row| {
1116 pat_constructors(cx, row[0], pcx).unwrap_or(vec![])
1118 debug!("used_ctors = {:#?}", used_ctors);
1119 // `all_ctors` are all the constructors for the given type, which
1120 // should all be represented (or caught with the wild pattern `_`).
1121 let all_ctors = all_constructors(cx, pcx);
1122 debug!("all_ctors = {:#?}", all_ctors);
1124 // `missing_ctors` is the set of constructors from the same type as the
1125 // first column of `matrix` that are matched only by wildcard patterns
1126 // from the first column.
1128 // Therefore, if there is some pattern that is unmatched by `matrix`,
1129 // it will still be unmatched if the first constructor is replaced by
1130 // any of the constructors in `missing_ctors`
1132 // However, if our scrutinee is *privately* an empty enum, we
1133 // must treat it as though it had an "unknown" constructor (in
1134 // that case, all other patterns obviously can't be variants)
1135 // to avoid exposing its emptyness. See the `match_privately_empty`
1136 // test for details.
1138 // FIXME: currently the only way I know of something can
1139 // be a privately-empty enum is when the exhaustive_patterns
1140 // feature flag is not present, so this is only
1141 // needed for that case.
1143 // Missing constructors are those that are not matched by any
1144 // non-wildcard patterns in the current column. We always determine if
1145 // the set is empty, but we only fully construct them on-demand,
1146 // because they're rarely used and can be big.
1147 let cheap_missing_ctors =
1148 compute_missing_ctors(MissingCtorsInfo::Emptiness, cx.tcx, &all_ctors, &used_ctors);
1150 let is_privately_empty = all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1151 let is_declared_nonexhaustive = cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
1152 debug!("cheap_missing_ctors={:#?} is_privately_empty={:#?} is_declared_nonexhaustive={:#?}",
1153 cheap_missing_ctors, is_privately_empty, is_declared_nonexhaustive);
1155 // For privately empty and non-exhaustive enums, we work as if there were an "extra"
1156 // `_` constructor for the type, so we can never match over all constructors.
1157 let is_non_exhaustive = is_privately_empty || is_declared_nonexhaustive ||
1158 (pcx.ty.is_pointer_sized() && !cx.tcx.features().precise_pointer_size_matching);
1160 if cheap_missing_ctors == MissingCtors::Empty && !is_non_exhaustive {
1161 split_grouped_constructors(cx.tcx, all_ctors, matrix, pcx.ty).into_iter().map(|c| {
1162 is_useful_specialized(cx, matrix, v, c, pcx.ty, witness)
1163 }).find(|result| result.is_useful()).unwrap_or(NotUseful)
1165 let matrix = rows.iter().filter_map(|r| {
1166 if r[0].is_wildcard() {
1167 Some(SmallVec::from_slice(&r[1..]))
1172 match is_useful(cx, &matrix, &v[1..], witness) {
1173 UsefulWithWitness(pats) => {
1175 // In this case, there's at least one "free"
1176 // constructor that is only matched against by
1177 // wildcard patterns.
1179 // There are 2 ways we can report a witness here.
1180 // Commonly, we can report all the "free"
1181 // constructors as witnesses, e.g., if we have:
1184 // enum Direction { N, S, E, W }
1185 // let Direction::N = ...;
1188 // we can report 3 witnesses: `S`, `E`, and `W`.
1190 // However, there are 2 cases where we don't want
1191 // to do this and instead report a single `_` witness:
1193 // 1) If the user is matching against a non-exhaustive
1194 // enum, there is no point in enumerating all possible
1195 // variants, because the user can't actually match
1196 // against them himself, e.g., in an example like:
1198 // let err: io::ErrorKind = ...;
1200 // io::ErrorKind::NotFound => {},
1203 // we don't want to show every possible IO error,
1204 // but instead have `_` as the witness (this is
1205 // actually *required* if the user specified *all*
1206 // IO errors, but is probably what we want in every
1209 // 2) If the user didn't actually specify a constructor
1210 // in this arm, e.g., in
1212 // let x: (Direction, Direction, bool) = ...;
1213 // let (_, _, false) = x;
1215 // we don't want to show all 16 possible witnesses
1216 // `(<direction-1>, <direction-2>, true)` - we are
1217 // satisfied with `(_, _, true)`. In this case,
1218 // `used_ctors` is empty.
1219 let new_witnesses = if is_non_exhaustive || used_ctors.is_empty() {
1220 // All constructors are unused. Add wild patterns
1221 // rather than each individual constructor.
1222 pats.into_iter().map(|mut witness| {
1223 witness.0.push(Pattern {
1226 kind: box PatternKind::Wild,
1231 let expensive_missing_ctors =
1232 compute_missing_ctors(MissingCtorsInfo::Ctors, cx.tcx, &all_ctors,
1234 if let MissingCtors::Ctors(missing_ctors) = expensive_missing_ctors {
1235 pats.into_iter().flat_map(|witness| {
1236 missing_ctors.iter().map(move |ctor| {
1237 // Extends the witness with a "wild" version of this
1238 // constructor, that matches everything that can be built with
1239 // it. For example, if `ctor` is a `Constructor::Variant` for
1240 // `Option::Some`, this pushes the witness for `Some(_)`.
1241 witness.clone().push_wild_constructor(cx, ctor, pcx.ty)
1245 bug!("cheap missing ctors")
1248 UsefulWithWitness(new_witnesses)
1256 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
1257 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1258 fn is_useful_specialized<'p, 'a: 'p, 'tcx: 'a>(
1259 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1260 &Matrix(ref m): &Matrix<'p, 'tcx>,
1261 v: &[&Pattern<'tcx>],
1262 ctor: Constructor<'tcx>,
1264 witness: WitnessPreference,
1265 ) -> Usefulness<'tcx> {
1266 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1267 let sub_pat_tys = constructor_sub_pattern_tys(cx, &ctor, lty);
1268 let wild_patterns_owned: Vec<_> = sub_pat_tys.iter().map(|ty| {
1272 kind: box PatternKind::Wild,
1275 let wild_patterns: Vec<_> = wild_patterns_owned.iter().collect();
1276 let matrix = Matrix(m.iter().flat_map(|r| {
1277 specialize(cx, &r, &ctor, &wild_patterns)
1279 match specialize(cx, v, &ctor, &wild_patterns) {
1280 Some(v) => match is_useful(cx, &matrix, &v, witness) {
1281 UsefulWithWitness(witnesses) => UsefulWithWitness(
1282 witnesses.into_iter()
1283 .map(|witness| witness.apply_constructor(cx, &ctor, lty))
1292 /// Determines the constructors that the given pattern can be specialized to.
1294 /// In most cases, there's only one constructor that a specific pattern
1295 /// represents, such as a specific enum variant or a specific literal value.
1296 /// Slice patterns, however, can match slices of different lengths. For instance,
1297 /// `[a, b, ..tail]` can match a slice of length 2, 3, 4 and so on.
1299 /// Returns None in case of a catch-all, which can't be specialized.
1300 fn pat_constructors<'tcx>(cx: &mut MatchCheckCtxt<'_, 'tcx>,
1301 pat: &Pattern<'tcx>,
1302 pcx: PatternContext)
1303 -> Option<Vec<Constructor<'tcx>>>
1306 PatternKind::AscribeUserType { ref subpattern, .. } =>
1307 pat_constructors(cx, subpattern, pcx),
1308 PatternKind::Binding { .. } | PatternKind::Wild => None,
1309 PatternKind::Leaf { .. } | PatternKind::Deref { .. } => Some(vec![Single]),
1310 PatternKind::Variant { adt_def, variant_index, .. } => {
1311 Some(vec![Variant(adt_def.variants[variant_index].did)])
1313 PatternKind::Constant { value } => Some(vec![ConstantValue(value)]),
1314 PatternKind::Range { lo, hi, ty, end } =>
1315 Some(vec![ConstantRange(
1316 lo.to_bits(cx.tcx, ty::ParamEnv::empty().and(ty)).unwrap(),
1317 hi.to_bits(cx.tcx, ty::ParamEnv::empty().and(ty)).unwrap(),
1321 PatternKind::Array { .. } => match pcx.ty.sty {
1322 ty::Array(_, length) => Some(vec![
1323 Slice(length.unwrap_usize(cx.tcx))
1325 _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty)
1327 PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
1328 let pat_len = prefix.len() as u64 + suffix.len() as u64;
1329 if slice.is_some() {
1330 Some((pat_len..pcx.max_slice_length+1).map(Slice).collect())
1332 Some(vec![Slice(pat_len)])
1338 /// This computes the arity of a constructor. The arity of a constructor
1339 /// is how many subpattern patterns of that constructor should be expanded to.
1341 /// For instance, a tuple pattern (_, 42, Some([])) has the arity of 3.
1342 /// A struct pattern's arity is the number of fields it contains, etc.
1343 fn constructor_arity(_cx: &MatchCheckCtxt, ctor: &Constructor, ty: Ty) -> u64 {
1344 debug!("constructor_arity({:#?}, {:?})", ctor, ty);
1346 ty::Tuple(ref fs) => fs.len() as u64,
1347 ty::Slice(..) | ty::Array(..) => match *ctor {
1348 Slice(length) => length,
1349 ConstantValue(_) => 0,
1350 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty)
1353 ty::Adt(adt, _) => {
1354 adt.variants[ctor.variant_index_for_adt(adt)].fields.len() as u64
1360 /// This computes the types of the sub patterns that a constructor should be
1363 /// For instance, a tuple pattern (43u32, 'a') has sub pattern types [u32, char].
1364 fn constructor_sub_pattern_tys<'a, 'tcx: 'a>(cx: &MatchCheckCtxt<'a, 'tcx>,
1366 ty: Ty<'tcx>) -> Vec<Ty<'tcx>>
1368 debug!("constructor_sub_pattern_tys({:#?}, {:?})", ctor, ty);
1370 ty::Tuple(ref fs) => fs.into_iter().map(|t| *t).collect(),
1371 ty::Slice(ty) | ty::Array(ty, _) => match *ctor {
1372 Slice(length) => (0..length).map(|_| ty).collect(),
1373 ConstantValue(_) => vec![],
1374 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty)
1376 ty::Ref(_, rty, _) => vec![rty],
1377 ty::Adt(adt, substs) => {
1379 // Use T as the sub pattern type of Box<T>.
1380 vec![substs.type_at(0)]
1382 adt.variants[ctor.variant_index_for_adt(adt)].fields.iter().map(|field| {
1383 let is_visible = adt.is_enum()
1384 || field.vis.is_accessible_from(cx.module, cx.tcx);
1386 field.ty(cx.tcx, substs)
1388 // Treat all non-visible fields as TyErr. They
1389 // can't appear in any other pattern from
1390 // this match (because they are private),
1391 // so their type does not matter - but
1392 // we don't want to know they are
1403 // checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
1404 // meaning all other types will compare unequal and thus equal patterns often do not cause the
1405 // second pattern to lint about unreachable match arms.
1406 fn slice_pat_covered_by_const<'tcx>(
1407 tcx: TyCtxt<'_, 'tcx, '_>,
1409 const_val: &ty::Const<'tcx>,
1410 prefix: &[Pattern<'tcx>],
1411 slice: &Option<Pattern<'tcx>>,
1412 suffix: &[Pattern<'tcx>]
1413 ) -> Result<bool, ErrorReported> {
1414 let data: &[u8] = match (const_val.val, &const_val.ty.sty) {
1415 (ConstValue::ByRef(id, alloc, offset), ty::Array(t, n)) => {
1416 if *t != tcx.types.u8 {
1417 // FIXME(oli-obk): can't mix const patterns with slice patterns and get
1418 // any sort of exhaustiveness/unreachable check yet
1421 let ptr = Pointer::new(id, offset);
1422 let n = n.assert_usize(tcx).unwrap();
1423 alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
1425 // a slice fat pointer to a zero length slice
1426 (ConstValue::ScalarPair(Scalar::Bits { .. }, n), ty::Slice(t)) => {
1427 if *t != tcx.types.u8 {
1428 // FIXME(oli-obk): can't mix const patterns with slice patterns and get
1429 // any sort of exhaustiveness/unreachable check yet
1432 assert_eq!(n.to_usize(&tcx).unwrap(), 0);
1436 (ConstValue::ScalarPair(Scalar::Ptr(ptr), n), ty::Slice(t)) => {
1437 if *t != tcx.types.u8 {
1438 // FIXME(oli-obk): can't mix const patterns with slice patterns and get
1439 // any sort of exhaustiveness/unreachable check yet
1442 let n = n.to_usize(&tcx).unwrap();
1445 .unwrap_memory(ptr.alloc_id)
1446 .get_bytes(&tcx, ptr, Size::from_bytes(n))
1450 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1451 const_val, prefix, slice, suffix,
1455 let pat_len = prefix.len() + suffix.len();
1456 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1461 data[..prefix.len()].iter().zip(prefix).chain(
1462 data[data.len()-suffix.len()..].iter().zip(suffix))
1465 box PatternKind::Constant { value } => {
1466 let b = value.unwrap_bits(tcx, ty::ParamEnv::empty().and(pat.ty));
1467 assert_eq!(b as u8 as u128, b);
1479 // Whether to evaluate a constructor using exhaustive integer matching. This is true if the
1480 // constructor is a range or constant with an integer type.
1481 fn should_treat_range_exhaustively(tcx: TyCtxt<'_, 'tcx, 'tcx>, ctor: &Constructor<'tcx>) -> bool {
1482 let ty = match ctor {
1483 ConstantValue(value) => value.ty,
1484 ConstantRange(_, _, ty, _) => ty,
1487 if let ty::Char | ty::Int(_) | ty::Uint(_) = ty.sty {
1488 !ty.is_pointer_sized() || tcx.features().precise_pointer_size_matching
1494 /// For exhaustive integer matching, some constructors are grouped within other constructors
1495 /// (namely integer typed values are grouped within ranges). However, when specialising these
1496 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1497 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1498 /// mean creating a separate constructor for every single value in the range, which is clearly
1499 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1500 /// identical across all values in that range (i.e., there are equivalence classes of ranges of
1501 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
1502 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
1503 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
1505 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1506 /// the group of intersecting patterns changes (using the method described below).
1507 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1508 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1509 /// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
1510 /// need to be worried about matching over gargantuan ranges.
1512 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
1514 /// |------| |----------| |-------| ||
1515 /// |-------| |-------| |----| ||
1518 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
1520 /// |--|--|||-||||--||---|||-------| |-|||| ||
1522 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
1523 /// boundaries for each interval range, sort them, then create constructors for each new interval
1524 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
1525 /// merging operation depicted above.)
1526 fn split_grouped_constructors<'p, 'a: 'p, 'tcx: 'a>(
1527 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1528 ctors: Vec<Constructor<'tcx>>,
1529 &Matrix(ref m): &Matrix<'p, 'tcx>,
1531 ) -> Vec<Constructor<'tcx>> {
1532 let mut split_ctors = Vec::with_capacity(ctors.len());
1534 for ctor in ctors.into_iter() {
1536 // For now, only ranges may denote groups of "subconstructors", so we only need to
1537 // special-case constant ranges.
1538 ConstantRange(..) if should_treat_range_exhaustively(tcx, &ctor) => {
1539 // We only care about finding all the subranges within the range of the constructor
1540 // range. Anything else is irrelevant, because it is guaranteed to result in
1541 // `NotUseful`, which is the default case anyway, and can be ignored.
1542 let ctor_range = IntRange::from_ctor(tcx, &ctor).unwrap();
1544 /// Represents a border between 2 integers. Because the intervals spanning borders
1545 /// must be able to cover every integer, we need to be able to represent
1546 /// 2^128 + 1 such borders.
1547 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
1553 // A function for extracting the borders of an integer interval.
1554 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
1555 let (lo, hi) = r.range.into_inner();
1556 let from = Border::JustBefore(lo);
1557 let to = match hi.checked_add(1) {
1558 Some(m) => Border::JustBefore(m),
1559 None => Border::AfterMax,
1561 vec![from, to].into_iter()
1564 // `borders` is the set of borders between equivalence classes: each equivalence
1565 // class lies between 2 borders.
1566 let row_borders = m.iter()
1567 .flat_map(|row| IntRange::from_pat(tcx, row[0]))
1568 .flat_map(|range| ctor_range.intersection(&range))
1569 .flat_map(|range| range_borders(range));
1570 let ctor_borders = range_borders(ctor_range.clone());
1571 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
1572 borders.sort_unstable();
1574 // We're going to iterate through every pair of borders, making sure that each
1575 // represents an interval of nonnegative length, and convert each such interval
1576 // into a constructor.
1577 for IntRange { range, .. } in borders.windows(2).filter_map(|window| {
1578 match (window[0], window[1]) {
1579 (Border::JustBefore(n), Border::JustBefore(m)) => {
1581 Some(IntRange { range: n..=(m - 1), ty })
1586 (Border::JustBefore(n), Border::AfterMax) => {
1587 Some(IntRange { range: n..=u128::MAX, ty })
1589 (Border::AfterMax, _) => None,
1592 split_ctors.push(IntRange::range_to_ctor(tcx, ty, range));
1595 // Any other constructor can be used unchanged.
1596 _ => split_ctors.push(ctor),
1603 /// Check whether there exists any shared value in either `ctor` or `pat` by intersecting them.
1604 fn constructor_intersects_pattern<'p, 'a: 'p, 'tcx: 'a>(
1605 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1606 ctor: &Constructor<'tcx>,
1607 pat: &'p Pattern<'tcx>,
1608 ) -> Option<SmallVec<[&'p Pattern<'tcx>; 2]>> {
1609 if should_treat_range_exhaustively(tcx, ctor) {
1610 match (IntRange::from_ctor(tcx, ctor), IntRange::from_pat(tcx, pat)) {
1611 (Some(ctor), Some(pat)) => {
1612 ctor.intersection(&pat).map(|_| {
1613 let (pat_lo, pat_hi) = pat.range.into_inner();
1614 let (ctor_lo, ctor_hi) = ctor.range.into_inner();
1615 assert!(pat_lo <= ctor_lo && ctor_hi <= pat_hi);
1622 // Fallback for non-ranges and ranges that involve floating-point numbers, which are not
1623 // conveniently handled by `IntRange`. For these cases, the constructor may not be a range
1624 // so intersection actually devolves into being covered by the pattern.
1625 match constructor_covered_by_range(tcx, ctor, pat) {
1626 Ok(true) => Some(smallvec![]),
1627 Ok(false) | Err(ErrorReported) => None,
1632 fn constructor_covered_by_range<'a, 'tcx>(
1633 tcx: TyCtxt<'a, 'tcx, 'tcx>,
1634 ctor: &Constructor<'tcx>,
1635 pat: &Pattern<'tcx>,
1636 ) -> Result<bool, ErrorReported> {
1637 let (from, to, end, ty) = match pat.kind {
1638 box PatternKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
1639 box PatternKind::Range { lo, hi, ty, end } => (lo, hi, end, ty),
1640 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
1642 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
1643 let cmp_from = |c_from| compare_const_vals(tcx, c_from, from, ty::ParamEnv::empty().and(ty))
1644 .map(|res| res != Ordering::Less);
1645 let cmp_to = |c_to| compare_const_vals(tcx, c_to, to, ty::ParamEnv::empty().and(ty));
1646 macro_rules! some_or_ok {
1650 None => return Ok(false), // not char or int
1655 ConstantValue(value) => {
1656 let to = some_or_ok!(cmp_to(value));
1657 let end = (to == Ordering::Less) ||
1658 (end == RangeEnd::Included && to == Ordering::Equal);
1659 Ok(some_or_ok!(cmp_from(value)) && end)
1661 ConstantRange(from, to, ty, RangeEnd::Included) => {
1662 let to = some_or_ok!(cmp_to(ty::Const::from_bits(
1665 ty::ParamEnv::empty().and(ty),
1667 let end = (to == Ordering::Less) ||
1668 (end == RangeEnd::Included && to == Ordering::Equal);
1669 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
1672 ty::ParamEnv::empty().and(ty),
1675 ConstantRange(from, to, ty, RangeEnd::Excluded) => {
1676 let to = some_or_ok!(cmp_to(ty::Const::from_bits(
1679 ty::ParamEnv::empty().and(ty)
1681 let end = (to == Ordering::Less) ||
1682 (end == RangeEnd::Excluded && to == Ordering::Equal);
1683 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
1686 ty::ParamEnv::empty().and(ty)))
1694 fn patterns_for_variant<'p, 'a: 'p, 'tcx: 'a>(
1695 subpatterns: &'p [FieldPattern<'tcx>],
1696 wild_patterns: &[&'p Pattern<'tcx>])
1697 -> SmallVec<[&'p Pattern<'tcx>; 2]>
1699 let mut result = SmallVec::from_slice(wild_patterns);
1701 for subpat in subpatterns {
1702 result[subpat.field.index()] = &subpat.pattern;
1705 debug!("patterns_for_variant({:#?}, {:#?}) = {:#?}", subpatterns, wild_patterns, result);
1709 /// This is the main specialization step. It expands the first pattern in the given row
1710 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
1711 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
1713 /// OTOH, slice patterns with a subslice pattern (..tail) can be expanded into multiple
1714 /// different patterns.
1715 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
1716 /// fields filled with wild patterns.
1717 fn specialize<'p, 'a: 'p, 'tcx: 'a>(
1718 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1719 r: &[&'p Pattern<'tcx>],
1720 constructor: &Constructor<'tcx>,
1721 wild_patterns: &[&'p Pattern<'tcx>],
1722 ) -> Option<SmallVec<[&'p Pattern<'tcx>; 2]>> {
1725 let head = match *pat.kind {
1726 PatternKind::AscribeUserType { ref subpattern, .. } => {
1727 specialize(cx, ::std::slice::from_ref(&subpattern), constructor, wild_patterns)
1730 PatternKind::Binding { .. } | PatternKind::Wild => {
1731 Some(SmallVec::from_slice(wild_patterns))
1734 PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
1735 let ref variant = adt_def.variants[variant_index];
1736 if *constructor == Variant(variant.did) {
1737 Some(patterns_for_variant(subpatterns, wild_patterns))
1743 PatternKind::Leaf { ref subpatterns } => {
1744 Some(patterns_for_variant(subpatterns, wild_patterns))
1747 PatternKind::Deref { ref subpattern } => {
1748 Some(smallvec![subpattern])
1751 PatternKind::Constant { value } => {
1752 match *constructor {
1754 // we extract an `Option` for the pointer because slices of zero elements don't
1755 // necessarily point to memory, they are usually just integers. The only time
1756 // they should be pointing to memory is when they are subslices of nonzero
1758 let (opt_ptr, n, ty) = match (value.val, &value.ty.sty) {
1759 (ConstValue::ByRef(id, alloc, offset), ty::TyKind::Array(t, n)) => (
1761 Pointer::new(id, offset),
1764 n.unwrap_usize(cx.tcx),
1767 (ConstValue::ScalarPair(ptr, n), ty::TyKind::Slice(t)) => (
1768 ptr.to_ptr().ok().map(|ptr| (
1770 cx.tcx.alloc_map.lock().unwrap_memory(ptr.alloc_id),
1772 n.to_bits(cx.tcx.data_layout.pointer_size).unwrap() as u64,
1777 "unexpected const-val {:?} with ctor {:?}",
1782 if wild_patterns.len() as u64 == n {
1783 // convert a constant slice/array pattern to a list of patterns.
1784 match (n, opt_ptr) {
1785 (0, _) => Some(SmallVec::new()),
1786 (_, Some((ptr, alloc))) => {
1787 let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
1789 let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
1790 let scalar = alloc.read_scalar(
1791 &cx.tcx, ptr, layout.size,
1793 let scalar = scalar.not_undef().ok()?;
1794 let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
1795 let pattern = Pattern {
1798 kind: box PatternKind::Constant { value },
1800 Some(&*cx.pattern_arena.alloc(pattern))
1803 (_, None) => span_bug!(
1805 "non zero length slice with const-val {:?}",
1814 // If the constructor is a:
1815 // Single value: add a row if the constructor equals the pattern.
1816 // Range: add a row if the constructor contains the pattern.
1817 constructor_intersects_pattern(cx.tcx, constructor, pat)
1822 PatternKind::Range { .. } => {
1823 // If the constructor is a:
1824 // Single value: add a row if the pattern contains the constructor.
1825 // Range: add a row if the constructor intersects the pattern.
1826 constructor_intersects_pattern(cx.tcx, constructor, pat)
1829 PatternKind::Array { ref prefix, ref slice, ref suffix } |
1830 PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
1831 match *constructor {
1833 let pat_len = prefix.len() + suffix.len();
1834 if let Some(slice_count) = wild_patterns.len().checked_sub(pat_len) {
1835 if slice_count == 0 || slice.is_some() {
1836 Some(prefix.iter().chain(
1837 wild_patterns.iter().map(|p| *p)
1840 .chain(suffix.iter())
1849 ConstantValue(cv) => {
1850 match slice_pat_covered_by_const(cx.tcx, pat.span, cv, prefix, slice, suffix) {
1851 Ok(true) => Some(smallvec![]),
1853 Err(ErrorReported) => None
1856 _ => span_bug!(pat.span,
1857 "unexpected ctor {:?} for slice pat", constructor)
1861 debug!("specialize({:#?}, {:#?}) = {:#?}", r[0], wild_patterns, head);
1863 head.map(|mut head| {
1864 head.extend_from_slice(&r[1 ..]);