1 /// Note: most tests relevant to this file can be found (at the time of writing)
2 /// in src/tests/ui/pattern/usefulness.
4 /// This file includes the logic for exhaustiveness and usefulness checking for
5 /// pattern-matching. Specifically, given a list of patterns for a type, we can
7 /// (a) the patterns cover every possible constructor for the type [exhaustiveness]
8 /// (b) each pattern is necessary [usefulness]
10 /// The algorithm implemented here is a modified version of the one described in:
11 /// http://moscova.inria.fr/~maranget/papers/warn/index.html
12 /// However, to save future implementors from reading the original paper, we
13 /// summarise the algorithm here to hopefully save time and be a little clearer
14 /// (without being so rigorous).
16 /// The core of the algorithm revolves about a "usefulness" check. In particular, we
17 /// are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
18 /// a matrix). `U(P, p)` represents whether, given an existing list of patterns
19 /// `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously-
20 /// uncovered values of the type).
22 /// If we have this predicate, then we can easily compute both exhaustiveness of an
23 /// entire set of patterns and the individual usefulness of each one.
24 /// (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
25 /// match doesn't increase the number of values we're matching)
26 /// (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a
27 /// pattern to those that have come before it doesn't increase the number of values
30 /// During the course of the algorithm, the rows of the matrix won't just be individual patterns,
31 /// but rather partially-deconstructed patterns in the form of a list of patterns. The paper
32 /// calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
35 /// For example, say we have the following:
37 /// // x: (Option<bool>, Result<()>)
39 /// (Some(true), _) => {}
40 /// (None, Err(())) => {}
41 /// (None, Err(_)) => {}
44 /// Here, the matrix `P` starts as:
46 /// [(Some(true), _)],
47 /// [(None, Err(()))],
50 /// We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
51 /// `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because
52 /// all the values it covers are already covered by row 2.
54 /// A list of patterns can be thought of as a stack, because we are mainly interested in the top of
55 /// the stack at any given point, and we can pop or apply constructors to get new pattern-stacks.
56 /// To match the paper, the top of the stack is at the beginning / on the left.
58 /// There are two important operations on pattern-stacks necessary to understand the algorithm:
59 /// 1. We can pop a given constructor off the top of a stack. This operation is called
60 /// `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or
61 /// `None`) and `p` a pattern-stack.
62 /// If the pattern on top of the stack can cover `c`, this removes the constructor and
63 /// pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns.
64 /// Otherwise the pattern-stack is discarded.
65 /// This essentially filters those pattern-stacks whose top covers the constructor `c` and
66 /// discards the others.
68 /// For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we
69 /// pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the
70 /// `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get
73 /// This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1`
74 /// on top of the stack, and we have four cases:
75 /// 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We
76 /// push onto the stack the arguments of this constructor, and return the result:
77 /// r_1, .., r_a, p_2, .., p_n
78 /// 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and
80 /// 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has
81 /// arguments (its arity), and return the resulting stack:
82 /// _, .., _, p_2, .., p_n
83 /// 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
85 /// S(c, (r_1, p_2, .., p_n))
86 /// S(c, (r_2, p_2, .., p_n))
88 /// 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is
90 /// This is used when we know there are missing constructor cases, but there might be
91 /// existing wildcard patterns, so to check the usefulness of the matrix, we have to check
92 /// all its *other* components.
94 /// It is computed as follows. We look at the pattern `p_1` on top of the stack,
95 /// and we have three cases:
96 /// 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing.
97 /// 1.2. `p_1 = _`. We return the rest of the stack:
99 /// 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
101 /// D((r_1, p_2, .., p_n))
102 /// D((r_2, p_2, .., p_n))
104 /// Note that the OR-patterns are not always used directly in Rust, but are used to derive the
105 /// exhaustive integer matching rules, so they're written here for posterity.
107 /// Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by
108 /// working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with
109 /// the given constructor, and popping a wildcard keeps those rows that start with a wildcard.
112 /// The algorithm for computing `U`
113 /// -------------------------------
114 /// The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
115 /// That means we're going to check the components from left-to-right, so the algorithm
116 /// operates principally on the first component of the matrix and new pattern-stack `p`.
117 /// This algorithm is realised in the `is_useful` function.
119 /// Base case. (`n = 0`, i.e., an empty tuple pattern)
120 /// - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`),
121 /// then `U(P, p)` is false.
122 /// - Otherwise, `P` must be empty, so `U(P, p)` is true.
124 /// Inductive step. (`n > 0`, i.e., whether there's at least one column
125 /// [which may then be expanded into further columns later])
126 /// We're going to match on the top of the new pattern-stack, `p_1`.
127 /// - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern.
128 /// Then, the usefulness of `p_1` can be reduced to whether it is useful when
129 /// we ignore all the patterns in the first column of `P` that involve other constructors.
130 /// This is where `S(c, P)` comes in:
131 /// `U(P, p) := U(S(c, P), S(c, p))`
132 /// This special case is handled in `is_useful_specialized`.
134 /// For example, if `P` is:
139 /// and `p` is [Some(false), 0], then we don't care about row 2 since we know `p` only
140 /// matches values that row 2 doesn't. For row 1 however, we need to dig into the
141 /// arguments of `Some` to know whether some new value is covered. So we compute
142 /// `U([[true, _]], [false, 0])`.
144 /// - If `p_1 == _`, then we look at the list of constructors that appear in the first
145 /// component of the rows of `P`:
146 /// + If there are some constructors that aren't present, then we might think that the
147 /// wildcard `_` is useful, since it covers those constructors that weren't covered
149 /// That's almost correct, but only works if there were no wildcards in those first
150 /// components. So we need to check that `p` is useful with respect to the rows that
151 /// start with a wildcard, if there are any. This is where `D` comes in:
152 /// `U(P, p) := U(D(P), D(p))`
154 /// For example, if `P` is:
157 /// [None, false, 1],
159 /// and `p` is [_, false, _], the `Some` constructor doesn't appear in `P`. So if we
160 /// only had row 2, we'd know that `p` is useful. However row 1 starts with a
161 /// wildcard, so we need to check whether `U([[true, _]], [false, 1])`.
163 /// + Otherwise, all possible constructors (for the relevant type) are present. In this
164 /// case we must check whether the wildcard pattern covers any unmatched value. For
165 /// that, we can think of the `_` pattern as a big OR-pattern that covers all
166 /// possible constructors. For `Option`, that would mean `_ = None | Some(_)` for
167 /// example. The wildcard pattern is useful in this case if it is useful when
168 /// specialized to one of the possible constructors. So we compute:
169 /// `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))`
171 /// For example, if `P` is:
176 /// and `p` is [_, false], both `None` and `Some` constructors appear in the first
177 /// components of `P`. We will therefore try popping both constructors in turn: we
178 /// compute U([[true, _]], [_, false]) for the `Some` constructor, and U([[false]],
179 /// [false]) for the `None` constructor. The first case returns true, so we know that
180 /// `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched
183 /// - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately:
184 /// `U(P, p) := U(P, (r_1, p_2, .., p_n))
185 /// || U(P, (r_2, p_2, .., p_n))`
187 /// Modifications to the algorithm
188 /// ------------------------------
189 /// The algorithm in the paper doesn't cover some of the special cases that arise in Rust, for
190 /// example uninhabited types and variable-length slice patterns. These are drawn attention to
191 /// throughout the code below. I'll make a quick note here about how exhaustive integer matching is
192 /// accounted for, though.
194 /// Exhaustive integer matching
195 /// ---------------------------
196 /// An integer type can be thought of as a (huge) sum type: 1 | 2 | 3 | ...
197 /// So to support exhaustive integer matching, we can make use of the logic in the paper for
198 /// OR-patterns. However, we obviously can't just treat ranges x..=y as individual sums, because
199 /// they are likely gigantic. So we instead treat ranges as constructors of the integers. This means
200 /// that we have a constructor *of* constructors (the integers themselves). We then need to work
201 /// through all the inductive step rules above, deriving how the ranges would be treated as
202 /// OR-patterns, and making sure that they're treated in the same way even when they're ranges.
203 /// There are really only four special cases here:
204 /// - When we match on a constructor that's actually a range, we have to treat it as if we would
206 /// + It turns out that we can simply extend the case for single-value patterns in
207 /// `specialize` to either be *equal* to a value constructor, or *contained within* a range
209 /// + When the pattern itself is a range, you just want to tell whether any of the values in
210 /// the pattern range coincide with values in the constructor range, which is precisely
212 /// Since when encountering a range pattern for a value constructor, we also use inclusion, it
213 /// means that whenever the constructor is a value/range and the pattern is also a value/range,
214 /// we can simply use intersection to test usefulness.
215 /// - When we're testing for usefulness of a pattern and the pattern's first component is a
217 /// + If all the constructors appear in the matrix, we have a slight complication. By default,
218 /// the behaviour (i.e., a disjunction over specialised matrices for each constructor) is
219 /// invalid, because we want a disjunction over every *integer* in each range, not just a
220 /// disjunction over every range. This is a bit more tricky to deal with: essentially we need
221 /// to form equivalence classes of subranges of the constructor range for which the behaviour
222 /// of the matrix `P` and new pattern `p` are the same. This is described in more
223 /// detail in `split_grouped_constructors`.
224 /// + If some constructors are missing from the matrix, it turns out we don't need to do
225 /// anything special (because we know none of the integers are actually wildcards: i.e., we
226 /// can't span wildcards using ranges).
227 use self::Constructor::*;
228 use self::Usefulness::*;
229 use self::WitnessPreference::*;
231 use rustc_data_structures::fx::FxHashMap;
232 use rustc_index::vec::Idx;
234 use super::{compare_const_vals, PatternFoldable, PatternFolder};
235 use super::{FieldPat, Pat, PatKind, PatRange};
237 use rustc::hir::def_id::DefId;
238 use rustc::hir::{HirId, RangeEnd};
239 use rustc::ty::layout::{Integer, IntegerExt, Size, VariantIdx};
240 use rustc::ty::{self, Const, Ty, TyCtxt, TypeFoldable};
243 use rustc::mir::interpret::{truncate, AllocId, ConstValue, Pointer, Scalar};
244 use rustc::mir::Field;
245 use rustc::util::captures::Captures;
246 use rustc::util::common::ErrorReported;
248 use syntax::attr::{SignedInt, UnsignedInt};
249 use syntax_pos::{Span, DUMMY_SP};
251 use arena::TypedArena;
253 use smallvec::{smallvec, SmallVec};
254 use std::cmp::{self, max, min, Ordering};
255 use std::convert::TryInto;
257 use std::iter::{FromIterator, IntoIterator};
258 use std::ops::RangeInclusive;
261 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pat<'tcx>) -> Pat<'tcx> {
262 LiteralExpander { tcx: cx.tcx }.fold_pattern(&pat)
265 struct LiteralExpander<'tcx> {
269 impl LiteralExpander<'tcx> {
270 /// Derefs `val` and potentially unsizes the value if `crty` is an array and `rty` a slice.
272 /// `crty` and `rty` can differ because you can use array constants in the presence of slice
273 /// patterns. So the pattern may end up being a slice, but the constant is an array. We convert
274 /// the array to a slice in that case.
275 fn fold_const_value_deref(
277 val: ConstValue<'tcx>,
278 // the pattern's pointee type
280 // the constant's pointee type
282 ) -> ConstValue<'tcx> {
283 debug!("fold_const_value_deref {:?} {:?} {:?}", val, rty, crty);
284 match (val, &crty.kind, &rty.kind) {
285 // the easy case, deref a reference
286 (ConstValue::Scalar(Scalar::Ptr(p)), x, y) if x == y => {
287 let alloc = self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id);
288 ConstValue::ByRef { alloc, offset: p.offset }
290 // unsize array to slice if pattern is array but match value or other patterns are slice
291 (ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
294 data: self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id),
295 start: p.offset.bytes().try_into().unwrap(),
296 end: n.eval_usize(self.tcx, ty::ParamEnv::empty()).try_into().unwrap(),
299 // fat pointers stay the same
300 (ConstValue::Slice { .. }, _, _)
301 | (_, ty::Slice(_), ty::Slice(_))
302 | (_, ty::Str, ty::Str) => val,
303 // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
304 _ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
309 impl PatternFolder<'tcx> for LiteralExpander<'tcx> {
310 fn fold_pattern(&mut self, pat: &Pat<'tcx>) -> Pat<'tcx> {
311 debug!("fold_pattern {:?} {:?} {:?}", pat, pat.ty.kind, pat.kind);
312 match (&pat.ty.kind, &*pat.kind) {
316 value: Const { val, ty: ty::TyS { kind: ty::Ref(_, crty, _), .. } },
321 kind: box PatKind::Deref {
325 kind: box PatKind::Constant {
326 value: self.tcx.mk_const(Const {
327 val: self.fold_const_value_deref(*val, rty, crty),
334 (_, &PatKind::Binding { subpattern: Some(ref s), .. }) => s.fold_with(self),
335 _ => pat.super_fold_with(self),
340 impl<'tcx> Pat<'tcx> {
341 fn is_wildcard(&self) -> bool {
343 PatKind::Binding { subpattern: None, .. } | PatKind::Wild => true,
349 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
351 #[derive(Debug, Clone)]
352 pub struct PatStack<'p, 'tcx>(SmallVec<[&'p Pat<'tcx>; 2]>);
354 impl<'p, 'tcx> PatStack<'p, 'tcx> {
355 pub fn from_pattern(pat: &'p Pat<'tcx>) -> Self {
356 PatStack(smallvec![pat])
359 fn from_vec(vec: SmallVec<[&'p Pat<'tcx>; 2]>) -> Self {
363 fn from_slice(s: &[&'p Pat<'tcx>]) -> Self {
364 PatStack(SmallVec::from_slice(s))
367 fn is_empty(&self) -> bool {
371 fn len(&self) -> usize {
375 fn head(&self) -> &'p Pat<'tcx> {
379 fn to_tail(&self) -> Self {
380 PatStack::from_slice(&self.0[1..])
383 fn iter(&self) -> impl Iterator<Item = &Pat<'tcx>> {
384 self.0.iter().map(|p| *p)
387 /// This computes `D(self)`. See top of the file for explanations.
388 fn specialize_wildcard(&self) -> Option<Self> {
389 if self.head().is_wildcard() { Some(self.to_tail()) } else { None }
392 /// This computes `S(constructor, self)`. See top of the file for explanations.
393 fn specialize_constructor<'a, 'q>(
395 cx: &mut MatchCheckCtxt<'a, 'tcx>,
396 constructor: &Constructor<'tcx>,
397 ctor_wild_subpatterns: &[&'q Pat<'tcx>],
398 ) -> Option<PatStack<'q, 'tcx>>
403 let new_heads = specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns);
404 new_heads.map(|mut new_head| {
405 new_head.0.extend_from_slice(&self.0[1..]);
411 impl<'p, 'tcx> Default for PatStack<'p, 'tcx> {
412 fn default() -> Self {
413 PatStack(smallvec![])
417 impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
418 fn from_iter<T>(iter: T) -> Self
420 T: IntoIterator<Item = &'p Pat<'tcx>>,
422 PatStack(iter.into_iter().collect())
427 pub struct Matrix<'p, 'tcx>(Vec<PatStack<'p, 'tcx>>);
429 impl<'p, 'tcx> Matrix<'p, 'tcx> {
430 pub fn empty() -> Self {
434 pub fn push(&mut self, row: PatStack<'p, 'tcx>) {
438 /// Iterate over the first component of each row
439 fn heads<'a>(&'a self) -> impl Iterator<Item = &'a Pat<'tcx>> + Captures<'p> {
440 self.0.iter().map(|r| r.head())
443 /// This computes `D(self)`. See top of the file for explanations.
444 fn specialize_wildcard(&self) -> Self {
445 self.0.iter().filter_map(|r| r.specialize_wildcard()).collect()
448 /// This computes `S(constructor, self)`. See top of the file for explanations.
449 fn specialize_constructor<'a, 'q>(
451 cx: &mut MatchCheckCtxt<'a, 'tcx>,
452 constructor: &Constructor<'tcx>,
453 ctor_wild_subpatterns: &[&'q Pat<'tcx>],
454 ) -> Matrix<'q, 'tcx>
462 .filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
468 /// Pretty-printer for matrices of patterns, example:
469 /// +++++++++++++++++++++++++++++
471 /// +++++++++++++++++++++++++++++
472 /// + true + [First] +
473 /// +++++++++++++++++++++++++++++
474 /// + true + [Second(true)] +
475 /// +++++++++++++++++++++++++++++
477 /// +++++++++++++++++++++++++++++
478 /// + _ + [_, _, tail @ ..] +
479 /// +++++++++++++++++++++++++++++
480 impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
481 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
484 let &Matrix(ref m) = self;
485 let pretty_printed_matrix: Vec<Vec<String>> =
486 m.iter().map(|row| row.iter().map(|pat| format!("{:?}", pat)).collect()).collect();
488 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
489 assert!(m.iter().all(|row| row.len() == column_count));
490 let column_widths: Vec<usize> = (0..column_count)
491 .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0))
494 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
495 let br = "+".repeat(total_width);
496 write!(f, "{}\n", br)?;
497 for row in pretty_printed_matrix {
499 for (column, pat_str) in row.into_iter().enumerate() {
501 write!(f, "{:1$}", pat_str, column_widths[column])?;
505 write!(f, "{}\n", br)?;
511 impl<'p, 'tcx> FromIterator<PatStack<'p, 'tcx>> for Matrix<'p, 'tcx> {
512 fn from_iter<T>(iter: T) -> Self
514 T: IntoIterator<Item = PatStack<'p, 'tcx>>,
516 Matrix(iter.into_iter().collect())
520 pub struct MatchCheckCtxt<'a, 'tcx> {
521 pub tcx: TyCtxt<'tcx>,
522 /// The module in which the match occurs. This is necessary for
523 /// checking inhabited-ness of types because whether a type is (visibly)
524 /// inhabited can depend on whether it was defined in the current module or
525 /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty
526 /// outside it's module and should not be matchable with an empty match
529 param_env: ty::ParamEnv<'tcx>,
530 pub pattern_arena: &'a TypedArena<Pat<'tcx>>,
531 pub byte_array_map: FxHashMap<*const Pat<'tcx>, Vec<&'a Pat<'tcx>>>,
534 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
535 pub fn create_and_enter<F, R>(
537 param_env: ty::ParamEnv<'tcx>,
542 F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R,
544 let pattern_arena = TypedArena::default();
550 pattern_arena: &pattern_arena,
551 byte_array_map: FxHashMap::default(),
555 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
556 if self.tcx.features().exhaustive_patterns {
557 self.tcx.is_ty_uninhabited_from(self.module, ty)
563 fn is_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
565 ty::Adt(adt_def, ..) => adt_def.is_variant_list_non_exhaustive(),
570 fn is_local(&self, ty: Ty<'tcx>) -> bool {
572 ty::Adt(adt_def, ..) => adt_def.did.is_local(),
578 #[derive(Clone, Debug)]
579 enum Constructor<'tcx> {
580 /// The constructor of all patterns that don't vary by constructor,
581 /// e.g., struct patterns and fixed-length arrays.
586 ConstantValue(&'tcx ty::Const<'tcx>, Span),
587 /// Ranges of literal values (`2..=5` and `2..5`).
588 ConstantRange(u128, u128, Ty<'tcx>, RangeEnd, Span),
589 /// Array patterns of length n.
593 // Ignore spans when comparing, they don't carry semantic information as they are only for lints.
594 impl<'tcx> std::cmp::PartialEq for Constructor<'tcx> {
595 fn eq(&self, other: &Self) -> bool {
596 match (self, other) {
597 (Constructor::Single, Constructor::Single) => true,
598 (Constructor::Variant(a), Constructor::Variant(b)) => a == b,
599 (Constructor::ConstantValue(a, _), Constructor::ConstantValue(b, _)) => a == b,
601 Constructor::ConstantRange(a_start, a_end, a_ty, a_range_end, _),
602 Constructor::ConstantRange(b_start, b_end, b_ty, b_range_end, _),
603 ) => a_start == b_start && a_end == b_end && a_ty == b_ty && a_range_end == b_range_end,
604 (Constructor::FixedLenSlice(a), Constructor::FixedLenSlice(b)) => a == b,
610 impl<'tcx> Constructor<'tcx> {
611 fn is_slice(&self) -> bool {
613 FixedLenSlice { .. } => true,
618 fn variant_index_for_adt<'a>(
620 cx: &MatchCheckCtxt<'a, 'tcx>,
621 adt: &'tcx ty::AdtDef,
624 Variant(id) => adt.variant_index_with_id(*id),
626 assert!(!adt.is_enum());
629 ConstantValue(c, _) => crate::const_eval::const_variant_index(cx.tcx, cx.param_env, c),
630 _ => bug!("bad constructor {:?} for adt {:?}", self, adt),
634 fn display(&self, tcx: TyCtxt<'tcx>) -> String {
636 Constructor::ConstantValue(val, _) => format!("{}", val),
637 Constructor::ConstantRange(lo, hi, ty, range_end, _) => {
638 // Get the right sign on the output:
639 let ty = ty::ParamEnv::empty().and(*ty);
642 ty::Const::from_bits(tcx, *lo, ty),
644 ty::Const::from_bits(tcx, *hi, ty),
647 Constructor::FixedLenSlice(val) => format!("[{}]", val),
648 _ => bug!("bad constructor being displayed: `{:?}", self),
652 // Returns the set of constructors covered by `self` but not by
653 // anything in `other_ctors`.
657 param_env: ty::ParamEnv<'tcx>,
658 other_ctors: &Vec<Constructor<'tcx>>,
659 ) -> Vec<Constructor<'tcx>> {
660 let mut refined_ctors = vec![self.clone()];
661 for other_ctor in other_ctors {
662 if other_ctor == self {
663 // If a constructor appears in a `match` arm, we can
664 // eliminate it straight away.
665 refined_ctors = vec![]
666 } else if let Some(interval) = IntRange::from_ctor(tcx, param_env, other_ctor) {
667 // Refine the required constructors for the type by subtracting
668 // the range defined by the current constructor pattern.
669 refined_ctors = interval.subtract_from(tcx, param_env, refined_ctors);
672 // If the constructor patterns that have been considered so far
673 // already cover the entire range of values, then we know the
674 // constructor is not missing, and we can move on to the next one.
675 if refined_ctors.is_empty() {
680 // If a constructor has not been matched, then it is missing.
681 // We add `refined_ctors` instead of `self`, because then we can
682 // provide more detailed error information about precisely which
683 // ranges have been omitted.
687 /// This returns one wildcard pattern for each argument to this constructor.
688 fn wildcard_subpatterns<'a>(
690 cx: &MatchCheckCtxt<'a, 'tcx>,
692 ) -> impl Iterator<Item = Pat<'tcx>> + DoubleEndedIterator {
693 constructor_sub_pattern_tys(cx, self, ty).into_iter().map(|ty| Pat {
696 kind: box PatKind::Wild,
700 /// This computes the arity of a constructor. The arity of a constructor
701 /// is how many subpattern patterns of that constructor should be expanded to.
703 /// For instance, a tuple pattern `(_, 42, Some([]))` has the arity of 3.
704 /// A struct pattern's arity is the number of fields it contains, etc.
705 fn arity<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> u64 {
706 debug!("Constructor::arity({:#?}, {:?})", self, ty);
708 ty::Tuple(ref fs) => fs.len() as u64,
709 ty::Slice(..) | ty::Array(..) => match *self {
710 FixedLenSlice(length) => length,
711 ConstantValue(..) => 0,
712 _ => bug!("bad slice pattern {:?} {:?}", self, ty),
716 adt.variants[self.variant_index_for_adt(cx, adt)].fields.len() as u64
722 /// Apply a constructor to a list of patterns, yielding a new pattern. `pats`
723 /// must have as many elements as this constructor's arity.
726 /// `self`: `Constructor::Single`
727 /// `ty`: `(u32, u32, u32)`
728 /// `pats`: `[10, 20, _]`
729 /// returns `(10, 20, _)`
731 /// `self`: `Constructor::Variant(Option::Some)`
732 /// `ty`: `Option<bool>`
733 /// `pats`: `[false]`
734 /// returns `Some(false)`
737 cx: &MatchCheckCtxt<'a, 'tcx>,
739 pats: impl IntoIterator<Item = Pat<'tcx>>,
741 let mut subpatterns = pats.into_iter();
742 let pat = match ty.kind {
743 ty::Adt(..) | ty::Tuple(..) => {
744 let subpatterns = subpatterns
746 .map(|(i, p)| FieldPat { field: Field::new(i), pattern: p })
749 if let ty::Adt(adt, substs) = ty.kind {
754 variant_index: self.variant_index_for_adt(cx, adt),
758 PatKind::Leaf { subpatterns }
761 PatKind::Leaf { subpatterns }
765 ty::Ref(..) => PatKind::Deref { subpattern: subpatterns.nth(0).unwrap() },
767 ty::Slice(_) | ty::Array(..) => {
768 PatKind::Slice { prefix: subpatterns.collect(), slice: None, suffix: vec![] }
772 ConstantValue(value, _) => PatKind::Constant { value },
773 ConstantRange(lo, hi, ty, end, _) => PatKind::Range(PatRange {
774 lo: ty::Const::from_bits(cx.tcx, lo, ty::ParamEnv::empty().and(ty)),
775 hi: ty::Const::from_bits(cx.tcx, hi, ty::ParamEnv::empty().and(ty)),
782 Pat { ty, span: DUMMY_SP, kind: Box::new(pat) }
785 /// Like `apply`, but where all the subpatterns are wildcards `_`.
786 fn apply_wildcards<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {
787 let subpatterns = self.wildcard_subpatterns(cx, ty).rev();
788 self.apply(cx, ty, subpatterns)
792 #[derive(Clone, Debug)]
793 pub enum Usefulness<'tcx> {
795 UsefulWithWitness(Vec<Witness<'tcx>>),
799 impl<'tcx> Usefulness<'tcx> {
800 fn new_useful(preference: WitnessPreference) -> Self {
802 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
803 LeaveOutWitness => Useful,
807 fn is_useful(&self) -> bool {
814 fn apply_constructor(
816 cx: &MatchCheckCtxt<'_, 'tcx>,
817 ctor: &Constructor<'tcx>,
821 UsefulWithWitness(witnesses) => UsefulWithWitness(
824 .map(|witness| witness.apply_constructor(cx, &ctor, ty))
831 fn apply_wildcard(self, ty: Ty<'tcx>) -> Self {
833 UsefulWithWitness(witnesses) => {
834 let wild = Pat { ty, span: DUMMY_SP, kind: box PatKind::Wild };
839 witness.0.push(wild.clone());
849 fn apply_missing_ctors(
851 cx: &MatchCheckCtxt<'_, 'tcx>,
853 missing_ctors: &MissingConstructors<'tcx>,
856 UsefulWithWitness(witnesses) => {
857 let new_patterns: Vec<_> =
858 missing_ctors.iter().map(|ctor| ctor.apply_wildcards(cx, ty)).collect();
859 // Add the new patterns to each witness
863 .flat_map(|witness| {
864 new_patterns.iter().map(move |pat| {
865 let mut witness = witness.clone();
866 witness.0.push(pat.clone());
878 #[derive(Copy, Clone, Debug)]
879 pub enum WitnessPreference {
884 #[derive(Copy, Clone, Debug)]
885 struct PatCtxt<'tcx> {
887 max_slice_length: u64,
891 /// A witness of non-exhaustiveness for error reporting, represented
892 /// as a list of patterns (in reverse order of construction) with
893 /// wildcards inside to represent elements that can take any inhabitant
894 /// of the type as a value.
896 /// A witness against a list of patterns should have the same types
897 /// and length as the pattern matched against. Because Rust `match`
898 /// is always against a single pattern, at the end the witness will
899 /// have length 1, but in the middle of the algorithm, it can contain
900 /// multiple patterns.
902 /// For example, if we are constructing a witness for the match against
904 /// struct Pair(Option<(u32, u32)>, bool);
906 /// match (p: Pair) {
907 /// Pair(None, _) => {}
908 /// Pair(_, false) => {}
912 /// We'll perform the following steps:
913 /// 1. Start with an empty witness
914 /// `Witness(vec![])`
915 /// 2. Push a witness `Some(_)` against the `None`
916 /// `Witness(vec![Some(_)])`
917 /// 3. Push a witness `true` against the `false`
918 /// `Witness(vec![Some(_), true])`
919 /// 4. Apply the `Pair` constructor to the witnesses
920 /// `Witness(vec![Pair(Some(_), true)])`
922 /// The final `Pair(Some(_), true)` is then the resulting witness.
923 #[derive(Clone, Debug)]
924 pub struct Witness<'tcx>(Vec<Pat<'tcx>>);
926 impl<'tcx> Witness<'tcx> {
927 pub fn single_pattern(self) -> Pat<'tcx> {
928 assert_eq!(self.0.len(), 1);
929 self.0.into_iter().next().unwrap()
932 /// Constructs a partial witness for a pattern given a list of
933 /// patterns expanded by the specialization step.
935 /// When a pattern P is discovered to be useful, this function is used bottom-up
936 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
937 /// of values, V, where each value in that set is not covered by any previously
938 /// used patterns and is covered by the pattern P'. Examples:
940 /// left_ty: tuple of 3 elements
941 /// pats: [10, 20, _] => (10, 20, _)
943 /// left_ty: struct X { a: (bool, &'static str), b: usize}
944 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
945 fn apply_constructor<'a>(
947 cx: &MatchCheckCtxt<'a, 'tcx>,
948 ctor: &Constructor<'tcx>,
951 let arity = ctor.arity(cx, ty);
953 let len = self.0.len() as u64;
954 let pats = self.0.drain((len - arity) as usize..).rev();
955 ctor.apply(cx, ty, pats)
964 /// This determines the set of all possible constructors of a pattern matching
965 /// values of type `left_ty`. For vectors, this would normally be an infinite set
966 /// but is instead bounded by the maximum fixed length of slice patterns in
967 /// the column of patterns being analyzed.
969 /// We make sure to omit constructors that are statically impossible. E.g., for
970 /// `Option<!>`, we do not include `Some(_)` in the returned list of constructors.
971 fn all_constructors<'a, 'tcx>(
972 cx: &mut MatchCheckCtxt<'a, 'tcx>,
974 ) -> Vec<Constructor<'tcx>> {
975 debug!("all_constructors({:?})", pcx.ty);
976 let ctors = match pcx.ty.kind {
977 ty::Bool => [true, false]
979 .map(|&b| ConstantValue(ty::Const::from_bool(cx.tcx, b), pcx.span))
981 ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx, cx.param_env).is_some() => {
982 let len = len.eval_usize(cx.tcx, cx.param_env);
983 if len != 0 && cx.is_uninhabited(sub_ty) { vec![] } else { vec![FixedLenSlice(len)] }
985 // Treat arrays of a constant but unknown length like slices.
986 ty::Array(ref sub_ty, _) | ty::Slice(ref sub_ty) => {
987 if cx.is_uninhabited(sub_ty) {
988 vec![FixedLenSlice(0)]
990 (0..pcx.max_slice_length + 1).map(|length| FixedLenSlice(length)).collect()
993 ty::Adt(def, substs) if def.is_enum() => def
997 !cx.tcx.features().exhaustive_patterns
999 .uninhabited_from(cx.tcx, substs, def.adt_kind())
1000 .contains(cx.tcx, cx.module)
1002 .map(|v| Variant(v.def_id))
1006 // The valid Unicode Scalar Value ranges.
1016 '\u{10FFFF}' as u128,
1024 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
1025 let min = 1u128 << (bits - 1);
1027 vec![ConstantRange(min, max, pcx.ty, RangeEnd::Included, pcx.span)]
1030 let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
1031 let max = truncate(u128::max_value(), size);
1032 vec![ConstantRange(0, max, pcx.ty, RangeEnd::Included, pcx.span)]
1035 if cx.is_uninhabited(pcx.ty) {
1045 fn max_slice_length<'p, 'a, 'tcx, I>(cx: &mut MatchCheckCtxt<'a, 'tcx>, patterns: I) -> u64
1047 I: Iterator<Item = &'p Pat<'tcx>>,
1050 // The exhaustiveness-checking paper does not include any details on
1051 // checking variable-length slice patterns. However, they are matched
1052 // by an infinite collection of fixed-length array patterns.
1054 // Checking the infinite set directly would take an infinite amount
1055 // of time. However, it turns out that for each finite set of
1056 // patterns `P`, all sufficiently large array lengths are equivalent:
1058 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
1059 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
1060 // `sₘ` for each sufficiently-large length `m` that applies to exactly
1061 // the same subset of `P`.
1063 // Because of that, each witness for reachability-checking from one
1064 // of the sufficiently-large lengths can be transformed to an
1065 // equally-valid witness from any other length, so we only have
1066 // to check slice lengths from the "minimal sufficiently-large length"
1069 // Note that the fact that there is a *single* `sₘ` for each `m`
1070 // not depending on the specific pattern in `P` is important: if
1071 // you look at the pair of patterns
1074 // Then any slice of length ≥1 that matches one of these two
1075 // patterns can be trivially turned to a slice of any
1076 // other length ≥1 that matches them and vice-versa - for
1077 // but the slice from length 2 `[false, true]` that matches neither
1078 // of these patterns can't be turned to a slice from length 1 that
1079 // matches neither of these patterns, so we have to consider
1080 // slices from length 2 there.
1082 // Now, to see that that length exists and find it, observe that slice
1083 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
1084 // "variable-length" patterns (`[_, .., _]`).
1086 // For fixed-length patterns, all slices with lengths *longer* than
1087 // the pattern's length have the same outcome (of not matching), so
1088 // as long as `L` is greater than the pattern's length we can pick
1089 // any `sₘ` from that length and get the same result.
1091 // For variable-length patterns, the situation is more complicated,
1092 // because as seen above the precise value of `sₘ` matters.
1094 // However, for each variable-length pattern `p` with a prefix of length
1095 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
1096 // `slₚ` elements are examined.
1098 // Therefore, as long as `L` is positive (to avoid concerns about empty
1099 // types), all elements after the maximum prefix length and before
1100 // the maximum suffix length are not examined by any variable-length
1101 // pattern, and therefore can be added/removed without affecting
1102 // them - creating equivalent patterns from any sufficiently-large
1105 // Of course, if fixed-length patterns exist, we must be sure
1106 // that our length is large enough to miss them all, so
1107 // we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
1109 // for example, with the above pair of patterns, all elements
1110 // but the first and last can be added/removed, so any
1111 // witness of length ≥2 (say, `[false, false, true]`) can be
1112 // turned to a witness from any other length ≥2.
1114 let mut max_prefix_len = 0;
1115 let mut max_suffix_len = 0;
1116 let mut max_fixed_len = 0;
1118 for row in patterns {
1120 PatKind::Constant { value } => {
1121 // extract the length of an array/slice from a constant
1122 match (value.val, &value.ty.kind) {
1123 (_, ty::Array(_, n)) => {
1124 max_fixed_len = cmp::max(max_fixed_len, n.eval_usize(cx.tcx, cx.param_env))
1126 (ConstValue::Slice { start, end, .. }, ty::Slice(_)) => {
1127 max_fixed_len = cmp::max(max_fixed_len, (end - start) as u64)
1132 PatKind::Slice { ref prefix, slice: None, ref suffix } => {
1133 let fixed_len = prefix.len() as u64 + suffix.len() as u64;
1134 max_fixed_len = cmp::max(max_fixed_len, fixed_len);
1136 PatKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
1137 max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
1138 max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
1144 cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len)
1147 /// An inclusive interval, used for precise integer exhaustiveness checking.
1148 /// `IntRange`s always store a contiguous range. This means that values are
1149 /// encoded such that `0` encodes the minimum value for the integer,
1150 /// regardless of the signedness.
1151 /// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
1152 /// This makes comparisons and arithmetic on interval endpoints much more
1153 /// straightforward. See `signed_bias` for details.
1155 /// `IntRange` is never used to encode an empty range or a "range" that wraps
1156 /// around the (offset) space: i.e., `range.lo <= range.hi`.
1157 #[derive(Clone, Debug)]
1158 struct IntRange<'tcx> {
1159 pub range: RangeInclusive<u128>,
1164 impl<'tcx> IntRange<'tcx> {
1166 fn is_integral(ty: Ty<'_>) -> bool {
1168 ty::Char | ty::Int(_) | ty::Uint(_) => true,
1174 fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
1176 ty::Char => Some((Size::from_bytes(4), 0)),
1178 let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
1179 Some((size, 1u128 << (size.bits() as u128 - 1)))
1181 ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
1189 param_env: ty::ParamEnv<'tcx>,
1190 value: &Const<'tcx>,
1192 ) -> Option<IntRange<'tcx>> {
1193 if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
1195 let val = if let ConstValue::Scalar(Scalar::Raw { data, size }) = value.val {
1196 // For this specific pattern we can skip a lot of effort and go
1197 // straight to the result, after doing a bit of checking. (We
1198 // could remove this branch and just use the next branch, which
1199 // is more general but much slower.)
1200 Scalar::<()>::check_raw(data, size, target_size);
1202 } else if let Some(val) = value.try_eval_bits(tcx, param_env, ty) {
1203 // This is a more general form of the previous branch.
1208 let val = val ^ bias;
1209 Some(IntRange { range: val..=val, ty, span })
1223 ) -> Option<IntRange<'tcx>> {
1224 if Self::is_integral(ty) {
1225 // Perform a shift if the underlying types are signed,
1226 // which makes the interval arithmetic simpler.
1227 let bias = IntRange::signed_bias(tcx, ty);
1228 let (lo, hi) = (lo ^ bias, hi ^ bias);
1229 // Make sure the interval is well-formed.
1230 if lo > hi || lo == hi && *end == RangeEnd::Excluded {
1233 let offset = (*end == RangeEnd::Excluded) as u128;
1234 Some(IntRange { range: lo..=(hi - offset), ty, span })
1243 param_env: ty::ParamEnv<'tcx>,
1244 ctor: &Constructor<'tcx>,
1245 ) -> Option<IntRange<'tcx>> {
1246 // Floating-point ranges are permitted and we don't want
1247 // to consider them when constructing integer ranges.
1249 ConstantRange(lo, hi, ty, end, span) => Self::from_range(tcx, *lo, *hi, ty, end, *span),
1250 ConstantValue(val, span) => Self::from_const(tcx, param_env, val, *span),
1257 param_env: ty::ParamEnv<'tcx>,
1258 mut pat: &Pat<'tcx>,
1259 ) -> Option<IntRange<'tcx>> {
1262 box PatKind::Constant { value } => {
1263 return Self::from_const(tcx, param_env, value, pat.span);
1265 box PatKind::Range(PatRange { lo, hi, end }) => {
1266 return Self::from_range(
1268 lo.eval_bits(tcx, param_env, lo.ty),
1269 hi.eval_bits(tcx, param_env, hi.ty),
1275 box PatKind::AscribeUserType { ref subpattern, .. } => {
1283 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
1284 fn signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
1287 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
1294 /// Converts a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
1298 r: RangeInclusive<u128>,
1300 ) -> Constructor<'tcx> {
1301 let bias = IntRange::signed_bias(tcx, ty);
1302 let (lo, hi) = r.into_inner();
1304 let ty = ty::ParamEnv::empty().and(ty);
1305 ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty), span)
1307 ConstantRange(lo ^ bias, hi ^ bias, ty, RangeEnd::Included, span)
1311 /// Returns a collection of ranges that spans the values covered by `ranges`, subtracted
1312 /// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
1316 param_env: ty::ParamEnv<'tcx>,
1317 ranges: Vec<Constructor<'tcx>>,
1318 ) -> Vec<Constructor<'tcx>> {
1321 .filter_map(|r| IntRange::from_ctor(tcx, param_env, &r).map(|i| i.range));
1322 let mut remaining_ranges = vec![];
1324 let (lo, hi) = self.range.into_inner();
1325 for subrange in ranges {
1326 let (subrange_lo, subrange_hi) = subrange.into_inner();
1327 if lo > subrange_hi || subrange_lo > hi {
1328 // The pattern doesn't intersect with the subrange at all,
1329 // so the subrange remains untouched.
1330 remaining_ranges.push(Self::range_to_ctor(
1333 subrange_lo..=subrange_hi,
1337 if lo > subrange_lo {
1338 // The pattern intersects an upper section of the
1339 // subrange, so a lower section will remain.
1340 remaining_ranges.push(Self::range_to_ctor(
1343 subrange_lo..=(lo - 1),
1347 if hi < subrange_hi {
1348 // The pattern intersects a lower section of the
1349 // subrange, so an upper section will remain.
1350 remaining_ranges.push(Self::range_to_ctor(
1353 (hi + 1)..=subrange_hi,
1362 fn intersection(&self, other: &Self) -> Option<Self> {
1364 let (lo, hi) = (*self.range.start(), *self.range.end());
1365 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1366 if lo <= other_hi && other_lo <= hi {
1367 let span = other.span;
1368 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty, span })
1374 fn suspicious_intersection(&self, other: &Self) -> bool {
1375 // `false` in the following cases:
1376 // 1 ---- // 1 ---------- // 1 ---- // 1 ----
1377 // 2 ---------- // 2 ---- // 2 ---- // 2 ----
1379 // The following are currently `false`, but could be `true` in the future (#64007):
1380 // 1 --------- // 1 ---------
1381 // 2 ---------- // 2 ----------
1383 // `true` in the following cases:
1384 // 1 ------- // 1 -------
1385 // 2 -------- // 2 -------
1386 let (lo, hi) = (*self.range.start(), *self.range.end());
1387 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1388 (lo == other_hi || hi == other_lo)
1392 // A struct to compute a set of constructors equivalent to `all_ctors \ used_ctors`.
1393 struct MissingConstructors<'tcx> {
1395 param_env: ty::ParamEnv<'tcx>,
1396 all_ctors: Vec<Constructor<'tcx>>,
1397 used_ctors: Vec<Constructor<'tcx>>,
1400 impl<'tcx> MissingConstructors<'tcx> {
1403 param_env: ty::ParamEnv<'tcx>,
1404 all_ctors: Vec<Constructor<'tcx>>,
1405 used_ctors: Vec<Constructor<'tcx>>,
1407 MissingConstructors { tcx, param_env, all_ctors, used_ctors }
1410 fn into_inner(self) -> (Vec<Constructor<'tcx>>, Vec<Constructor<'tcx>>) {
1411 (self.all_ctors, self.used_ctors)
1414 fn is_empty(&self) -> bool {
1415 self.iter().next().is_none()
1417 /// Whether this contains all the constructors for the given type or only a
1419 fn all_ctors_are_missing(&self) -> bool {
1420 self.used_ctors.is_empty()
1423 /// Iterate over all_ctors \ used_ctors
1424 fn iter<'a>(&'a self) -> impl Iterator<Item = Constructor<'tcx>> + Captures<'a> {
1425 self.all_ctors.iter().flat_map(move |req_ctor| {
1426 req_ctor.subtract_ctors(self.tcx, self.param_env, &self.used_ctors)
1431 impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
1432 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1433 let ctors: Vec<_> = self.iter().collect();
1434 write!(f, "{:?}", ctors)
1438 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html.
1439 /// The algorithm from the paper has been modified to correctly handle empty
1440 /// types. The changes are:
1441 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1442 /// continue to recurse over columns.
1443 /// (1) all_constructors will only return constructors that are statically
1444 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
1446 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1447 /// to a set of such vectors `m` - this is defined as there being a set of
1448 /// inputs that will match `v` but not any of the sets in `m`.
1450 /// All the patterns at each column of the `matrix ++ v` matrix must
1451 /// have the same type, except that wildcard (PatKind::Wild) patterns
1452 /// with type `TyErr` are also allowed, even if the "type of the column"
1453 /// is not `TyErr`. That is used to represent private fields, as using their
1454 /// real type would assert that they are inhabited.
1456 /// This is used both for reachability checking (if a pattern isn't useful in
1457 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1458 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1459 /// matrix isn't exhaustive).
1460 pub fn is_useful<'p, 'a, 'tcx>(
1461 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1462 matrix: &Matrix<'p, 'tcx>,
1463 v: &PatStack<'_, 'tcx>,
1464 witness_preference: WitnessPreference,
1466 ) -> Usefulness<'tcx> {
1467 let &Matrix(ref rows) = matrix;
1468 debug!("is_useful({:#?}, {:#?})", matrix, v);
1470 // The base case. We are pattern-matching on () and the return value is
1471 // based on whether our matrix has a row or not.
1472 // NOTE: This could potentially be optimized by checking rows.is_empty()
1473 // first and then, if v is non-empty, the return value is based on whether
1474 // the type of the tuple we're checking is inhabited or not.
1476 return if rows.is_empty() {
1477 Usefulness::new_useful(witness_preference)
1483 assert!(rows.iter().all(|r| r.len() == v.len()));
1485 let (ty, span) = matrix
1487 .map(|r| (r.ty, r.span))
1488 .find(|(ty, _)| !ty.references_error())
1489 .unwrap_or((v.head().ty, v.head().span));
1491 // TyErr is used to represent the type of wildcard patterns matching
1492 // against inaccessible (private) fields of structs, so that we won't
1493 // be able to observe whether the types of the struct's fields are
1496 // If the field is truly inaccessible, then all the patterns
1497 // matching against it must be wildcard patterns, so its type
1500 // However, if we are matching against non-wildcard patterns, we
1501 // need to know the real type of the field so we can specialize
1502 // against it. This primarily occurs through constants - they
1503 // can include contents for fields that are inaccessible at the
1504 // location of the match. In that case, the field's type is
1505 // inhabited - by the constant - so we can just use it.
1507 // FIXME: this might lead to "unstable" behavior with macro hygiene
1508 // introducing uninhabited patterns for inaccessible fields. We
1509 // need to figure out how to model that.
1511 max_slice_length: max_slice_length(cx, matrix.heads().chain(Some(v.head()))),
1515 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
1517 if let Some(constructors) = pat_constructors(cx, v.head(), pcx) {
1518 debug!("is_useful - expanding constructors: {:#?}", constructors);
1519 split_grouped_constructors(
1529 .map(|c| is_useful_specialized(cx, matrix, v, c, pcx.ty, witness_preference, hir_id))
1530 .find(|result| result.is_useful())
1531 .unwrap_or(NotUseful)
1533 debug!("is_useful - expanding wildcard");
1535 let used_ctors: Vec<Constructor<'_>> =
1536 matrix.heads().flat_map(|p| pat_constructors(cx, p, pcx).unwrap_or(vec![])).collect();
1537 debug!("used_ctors = {:#?}", used_ctors);
1538 // `all_ctors` are all the constructors for the given type, which
1539 // should all be represented (or caught with the wild pattern `_`).
1540 let all_ctors = all_constructors(cx, pcx);
1541 debug!("all_ctors = {:#?}", all_ctors);
1543 let is_privately_empty = all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1544 let is_declared_nonexhaustive = cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
1546 // `missing_ctors` is the set of constructors from the same type as the
1547 // first column of `matrix` that are matched only by wildcard patterns
1548 // from the first column.
1550 // Therefore, if there is some pattern that is unmatched by `matrix`,
1551 // it will still be unmatched if the first constructor is replaced by
1552 // any of the constructors in `missing_ctors`
1554 // However, if our scrutinee is *privately* an empty enum, we
1555 // must treat it as though it had an "unknown" constructor (in
1556 // that case, all other patterns obviously can't be variants)
1557 // to avoid exposing its emptyness. See the `match_privately_empty`
1558 // test for details.
1560 // FIXME: currently the only way I know of something can
1561 // be a privately-empty enum is when the exhaustive_patterns
1562 // feature flag is not present, so this is only
1563 // needed for that case.
1565 // Missing constructors are those that are not matched by any
1566 // non-wildcard patterns in the current column. To determine if
1567 // the set is empty, we can check that `.peek().is_none()`, so
1568 // we only fully construct them on-demand, because they're rarely used and can be big.
1569 let missing_ctors = MissingConstructors::new(cx.tcx, cx.param_env, all_ctors, used_ctors);
1572 "missing_ctors.empty()={:#?} is_privately_empty={:#?} is_declared_nonexhaustive={:#?}",
1573 missing_ctors.is_empty(),
1575 is_declared_nonexhaustive
1578 // For privately empty and non-exhaustive enums, we work as if there were an "extra"
1579 // `_` constructor for the type, so we can never match over all constructors.
1580 let is_non_exhaustive = is_privately_empty
1581 || is_declared_nonexhaustive
1582 || (pcx.ty.is_ptr_sized_integral() && !cx.tcx.features().precise_pointer_size_matching);
1584 if missing_ctors.is_empty() && !is_non_exhaustive {
1585 let (all_ctors, _) = missing_ctors.into_inner();
1586 split_grouped_constructors(
1596 .map(|c| is_useful_specialized(cx, matrix, v, c, pcx.ty, witness_preference, hir_id))
1597 .find(|result| result.is_useful())
1598 .unwrap_or(NotUseful)
1600 let matrix = matrix.specialize_wildcard();
1601 let v = v.to_tail();
1602 let usefulness = is_useful(cx, &matrix, &v, witness_preference, hir_id);
1604 // In this case, there's at least one "free"
1605 // constructor that is only matched against by
1606 // wildcard patterns.
1608 // There are 2 ways we can report a witness here.
1609 // Commonly, we can report all the "free"
1610 // constructors as witnesses, e.g., if we have:
1613 // enum Direction { N, S, E, W }
1614 // let Direction::N = ...;
1617 // we can report 3 witnesses: `S`, `E`, and `W`.
1619 // However, there are 2 cases where we don't want
1620 // to do this and instead report a single `_` witness:
1622 // 1) If the user is matching against a non-exhaustive
1623 // enum, there is no point in enumerating all possible
1624 // variants, because the user can't actually match
1625 // against them themselves, e.g., in an example like:
1627 // let err: io::ErrorKind = ...;
1629 // io::ErrorKind::NotFound => {},
1632 // we don't want to show every possible IO error,
1633 // but instead have `_` as the witness (this is
1634 // actually *required* if the user specified *all*
1635 // IO errors, but is probably what we want in every
1638 // 2) If the user didn't actually specify a constructor
1639 // in this arm, e.g., in
1641 // let x: (Direction, Direction, bool) = ...;
1642 // let (_, _, false) = x;
1644 // we don't want to show all 16 possible witnesses
1645 // `(<direction-1>, <direction-2>, true)` - we are
1646 // satisfied with `(_, _, true)`. In this case,
1647 // `used_ctors` is empty.
1648 if is_non_exhaustive || missing_ctors.all_ctors_are_missing() {
1649 // All constructors are unused. Add a wild pattern
1650 // rather than each individual constructor.
1651 usefulness.apply_wildcard(pcx.ty)
1653 // Construct for each missing constructor a "wild" version of this
1654 // constructor, that matches everything that can be built with
1655 // it. For example, if `ctor` is a `Constructor::Variant` for
1656 // `Option::Some`, we get the pattern `Some(_)`.
1657 usefulness.apply_missing_ctors(cx, pcx.ty, &missing_ctors)
1663 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
1664 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1665 fn is_useful_specialized<'p, 'a, 'tcx>(
1666 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1667 matrix: &Matrix<'p, 'tcx>,
1668 v: &PatStack<'_, 'tcx>,
1669 ctor: Constructor<'tcx>,
1671 witness_preference: WitnessPreference,
1673 ) -> Usefulness<'tcx> {
1674 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1676 let ctor_wild_subpatterns_owned: Vec<_> = ctor.wildcard_subpatterns(cx, lty).collect();
1677 let ctor_wild_subpatterns: Vec<_> = ctor_wild_subpatterns_owned.iter().collect();
1678 let matrix = matrix.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns);
1679 v.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns)
1680 .map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id))
1681 .map(|u| u.apply_constructor(cx, &ctor, lty))
1682 .unwrap_or(NotUseful)
1685 /// Determines the constructors that the given pattern can be specialized to.
1687 /// In most cases, there's only one constructor that a specific pattern
1688 /// represents, such as a specific enum variant or a specific literal value.
1689 /// Slice patterns, however, can match slices of different lengths. For instance,
1690 /// `[a, b, tail @ ..]` can match a slice of length 2, 3, 4 and so on.
1692 /// Returns `None` in case of a catch-all, which can't be specialized.
1693 fn pat_constructors<'tcx>(
1694 cx: &mut MatchCheckCtxt<'_, 'tcx>,
1697 ) -> Option<Vec<Constructor<'tcx>>> {
1699 PatKind::AscribeUserType { ref subpattern, .. } => pat_constructors(cx, subpattern, pcx),
1700 PatKind::Binding { .. } | PatKind::Wild => None,
1701 PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(vec![Single]),
1702 PatKind::Variant { adt_def, variant_index, .. } => {
1703 Some(vec![Variant(adt_def.variants[variant_index].def_id)])
1705 PatKind::Constant { value } => Some(vec![ConstantValue(value, pat.span)]),
1706 PatKind::Range(PatRange { lo, hi, end }) => Some(vec![ConstantRange(
1707 lo.eval_bits(cx.tcx, cx.param_env, lo.ty),
1708 hi.eval_bits(cx.tcx, cx.param_env, hi.ty),
1713 PatKind::Array { .. } => match pcx.ty.kind {
1714 ty::Array(_, length) => {
1715 Some(vec![FixedLenSlice(length.eval_usize(cx.tcx, cx.param_env))])
1717 _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty),
1719 PatKind::Slice { ref prefix, ref slice, ref suffix } => {
1720 let pat_len = prefix.len() as u64 + suffix.len() as u64;
1721 if slice.is_some() {
1722 Some((pat_len..pcx.max_slice_length + 1).map(FixedLenSlice).collect())
1724 Some(vec![FixedLenSlice(pat_len)])
1727 PatKind::Or { .. } => {
1728 bug!("support for or-patterns has not been fully implemented yet.");
1733 /// This computes the types of the sub patterns that a constructor should be
1736 /// For instance, a tuple pattern (43u32, 'a') has sub pattern types [u32, char].
1737 fn constructor_sub_pattern_tys<'a, 'tcx>(
1738 cx: &MatchCheckCtxt<'a, 'tcx>,
1739 ctor: &Constructor<'tcx>,
1741 ) -> Vec<Ty<'tcx>> {
1742 debug!("constructor_sub_pattern_tys({:#?}, {:?})", ctor, ty);
1744 ty::Tuple(ref fs) => fs.into_iter().map(|t| t.expect_ty()).collect(),
1745 ty::Slice(ty) | ty::Array(ty, _) => match *ctor {
1746 FixedLenSlice(length) => (0..length).map(|_| ty).collect(),
1747 ConstantValue(..) => vec![],
1748 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty),
1750 ty::Ref(_, rty, _) => vec![rty],
1751 ty::Adt(adt, substs) => {
1753 // Use T as the sub pattern type of Box<T>.
1754 vec![substs.type_at(0)]
1756 let variant = &adt.variants[ctor.variant_index_for_adt(cx, adt)];
1757 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(ty);
1763 adt.is_enum() || field.vis.is_accessible_from(cx.module, cx.tcx);
1764 let is_uninhabited = cx.is_uninhabited(field.ty(cx.tcx, substs));
1765 match (is_visible, is_non_exhaustive, is_uninhabited) {
1766 // Treat all uninhabited types in non-exhaustive variants as `TyErr`.
1767 (_, true, true) => cx.tcx.types.err,
1768 // Treat all non-visible fields as `TyErr`. They can't appear in any
1769 // other pattern from this match (because they are private), so their
1770 // type does not matter - but we don't want to know they are
1772 (false, ..) => cx.tcx.types.err,
1774 let ty = field.ty(cx.tcx, substs);
1776 // If the field type returned is an array of an unknown size
1779 if len.try_eval_usize(cx.tcx, cx.param_env).is_none() =>
1795 // checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
1796 // meaning all other types will compare unequal and thus equal patterns often do not cause the
1797 // second pattern to lint about unreachable match arms.
1798 fn slice_pat_covered_by_const<'tcx>(
1801 const_val: &'tcx ty::Const<'tcx>,
1802 prefix: &[Pat<'tcx>],
1803 slice: &Option<Pat<'tcx>>,
1804 suffix: &[Pat<'tcx>],
1805 param_env: ty::ParamEnv<'tcx>,
1806 ) -> Result<bool, ErrorReported> {
1807 let data: &[u8] = match (const_val.val, &const_val.ty.kind) {
1808 (ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
1809 assert_eq!(*t, tcx.types.u8);
1810 let n = n.eval_usize(tcx, param_env);
1811 let ptr = Pointer::new(AllocId(0), offset);
1812 alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
1814 (ConstValue::Slice { data, start, end }, ty::Slice(t)) => {
1815 assert_eq!(*t, tcx.types.u8);
1816 let ptr = Pointer::new(AllocId(0), Size::from_bytes(start as u64));
1817 data.get_bytes(&tcx, ptr, Size::from_bytes((end - start) as u64)).unwrap()
1819 // FIXME(oli-obk): create a way to extract fat pointers from ByRef
1820 (_, ty::Slice(_)) => return Ok(false),
1822 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1830 let pat_len = prefix.len() + suffix.len();
1831 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1835 for (ch, pat) in data[..prefix.len()]
1838 .chain(data[data.len() - suffix.len()..].iter().zip(suffix))
1841 box PatKind::Constant { value } => {
1842 let b = value.eval_bits(tcx, param_env, pat.ty);
1843 assert_eq!(b as u8 as u128, b);
1855 // Whether to evaluate a constructor using exhaustive integer matching. This is true if the
1856 // constructor is a range or constant with an integer type.
1857 fn should_treat_range_exhaustively(tcx: TyCtxt<'tcx>, ctor: &Constructor<'tcx>) -> bool {
1858 let ty = match ctor {
1859 ConstantValue(value, _) => value.ty,
1860 ConstantRange(_, _, ty, _, _) => ty,
1863 if let ty::Char | ty::Int(_) | ty::Uint(_) = ty.kind {
1864 !ty.is_ptr_sized_integral() || tcx.features().precise_pointer_size_matching
1870 /// For exhaustive integer matching, some constructors are grouped within other constructors
1871 /// (namely integer typed values are grouped within ranges). However, when specialising these
1872 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1873 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1874 /// mean creating a separate constructor for every single value in the range, which is clearly
1875 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1876 /// identical across all values in that range (i.e., there are equivalence classes of ranges of
1877 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
1878 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
1879 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
1881 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1882 /// the group of intersecting patterns changes (using the method described below).
1883 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1884 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1885 /// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
1886 /// need to be worried about matching over gargantuan ranges.
1888 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
1890 /// |------| |----------| |-------| ||
1891 /// |-------| |-------| |----| ||
1894 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
1896 /// |--|--|||-||||--||---|||-------| |-|||| ||
1898 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
1899 /// boundaries for each interval range, sort them, then create constructors for each new interval
1900 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
1901 /// merging operation depicted above.)
1903 /// `hir_id` is `None` when we're evaluating the wildcard pattern, do not lint for overlapping in
1904 /// ranges that case.
1905 fn split_grouped_constructors<'p, 'tcx>(
1907 param_env: ty::ParamEnv<'tcx>,
1908 ctors: Vec<Constructor<'tcx>>,
1909 matrix: &Matrix<'p, 'tcx>,
1912 hir_id: Option<HirId>,
1913 ) -> Vec<Constructor<'tcx>> {
1914 let mut split_ctors = Vec::with_capacity(ctors.len());
1916 for ctor in ctors.into_iter() {
1918 // For now, only ranges may denote groups of "subconstructors", so we only need to
1919 // special-case constant ranges.
1920 ConstantRange(..) if should_treat_range_exhaustively(tcx, &ctor) => {
1921 // We only care about finding all the subranges within the range of the constructor
1922 // range. Anything else is irrelevant, because it is guaranteed to result in
1923 // `NotUseful`, which is the default case anyway, and can be ignored.
1924 let ctor_range = IntRange::from_ctor(tcx, param_env, &ctor).unwrap();
1926 /// Represents a border between 2 integers. Because the intervals spanning borders
1927 /// must be able to cover every integer, we need to be able to represent
1928 /// 2^128 + 1 such borders.
1929 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
1935 // A function for extracting the borders of an integer interval.
1936 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
1937 let (lo, hi) = r.range.into_inner();
1938 let from = Border::JustBefore(lo);
1939 let to = match hi.checked_add(1) {
1940 Some(m) => Border::JustBefore(m),
1941 None => Border::AfterMax,
1943 vec![from, to].into_iter()
1946 // Collect the span and range of all the intersecting ranges to lint on likely
1947 // incorrect range patterns. (#63987)
1948 let mut overlaps = vec![];
1949 // `borders` is the set of borders between equivalence classes: each equivalence
1950 // class lies between 2 borders.
1951 let row_borders = matrix
1955 IntRange::from_pat(tcx, param_env, row.head()).map(|r| (r, row.len()))
1957 .flat_map(|(range, row_len)| {
1958 let intersection = ctor_range.intersection(&range);
1959 let should_lint = ctor_range.suspicious_intersection(&range);
1960 if let (Some(range), 1, true) = (&intersection, row_len, should_lint) {
1961 // FIXME: for now, only check for overlapping ranges on simple range
1962 // patterns. Otherwise with the current logic the following is detected
1964 // match (10u8, true) {
1965 // (0 ..= 125, false) => {}
1966 // (126 ..= 255, false) => {}
1967 // (0 ..= 255, true) => {}
1969 overlaps.push(range.clone());
1973 .flat_map(|range| range_borders(range));
1974 let ctor_borders = range_borders(ctor_range.clone());
1975 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
1976 borders.sort_unstable();
1978 lint_overlapping_patterns(tcx, hir_id, ctor_range, ty, overlaps);
1980 // We're going to iterate through every adjacent pair of borders, making sure that
1981 // each represents an interval of nonnegative length, and convert each such
1982 // interval into a constructor.
1983 for IntRange { range, .. } in
1984 borders.windows(2).filter_map(|window| match (window[0], window[1]) {
1985 (Border::JustBefore(n), Border::JustBefore(m)) => {
1987 Some(IntRange { range: n..=(m - 1), ty, span })
1992 (Border::JustBefore(n), Border::AfterMax) => {
1993 Some(IntRange { range: n..=u128::MAX, ty, span })
1995 (Border::AfterMax, _) => None,
1998 split_ctors.push(IntRange::range_to_ctor(tcx, ty, range, span));
2001 // Any other constructor can be used unchanged.
2002 _ => split_ctors.push(ctor),
2009 fn lint_overlapping_patterns(
2011 hir_id: Option<HirId>,
2012 ctor_range: IntRange<'tcx>,
2014 overlaps: Vec<IntRange<'tcx>>,
2016 if let (true, Some(hir_id)) = (!overlaps.is_empty(), hir_id) {
2017 let mut err = tcx.struct_span_lint_hir(
2018 lint::builtin::OVERLAPPING_PATTERNS,
2021 "multiple patterns covering the same range",
2023 err.span_label(ctor_range.span, "overlapping patterns");
2024 for int_range in overlaps {
2025 // Use the real type for user display of the ranges:
2029 "this range overlaps on `{}`",
2030 IntRange::range_to_ctor(tcx, ty, int_range.range, DUMMY_SP).display(tcx),
2038 fn constructor_covered_by_range<'tcx>(
2040 param_env: ty::ParamEnv<'tcx>,
2041 ctor: &Constructor<'tcx>,
2043 ) -> Result<bool, ErrorReported> {
2044 let (from, to, end, ty) = match pat.kind {
2045 box PatKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
2046 box PatKind::Range(PatRange { lo, hi, end }) => (lo, hi, end, lo.ty),
2047 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
2049 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
2050 let cmp_from = |c_from| {
2051 compare_const_vals(tcx, c_from, from, param_env, ty).map(|res| res != Ordering::Less)
2053 let cmp_to = |c_to| compare_const_vals(tcx, c_to, to, param_env, ty);
2054 macro_rules! some_or_ok {
2058 None => return Ok(false), // not char or int
2063 ConstantValue(value, _) => {
2064 let to = some_or_ok!(cmp_to(value));
2066 (to == Ordering::Less) || (end == RangeEnd::Included && to == Ordering::Equal);
2067 Ok(some_or_ok!(cmp_from(value)) && end)
2069 ConstantRange(from, to, ty, RangeEnd::Included, _) => {
2071 some_or_ok!(cmp_to(ty::Const::from_bits(tcx, to, ty::ParamEnv::empty().and(ty),)));
2073 (to == Ordering::Less) || (end == RangeEnd::Included && to == Ordering::Equal);
2074 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
2077 ty::ParamEnv::empty().and(ty),
2080 ConstantRange(from, to, ty, RangeEnd::Excluded, _) => {
2082 some_or_ok!(cmp_to(ty::Const::from_bits(tcx, to, ty::ParamEnv::empty().and(ty))));
2084 (to == Ordering::Less) || (end == RangeEnd::Excluded && to == Ordering::Equal);
2085 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
2088 ty::ParamEnv::empty().and(ty)
2096 fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
2097 cx: &mut MatchCheckCtxt<'a, 'tcx>,
2098 subpatterns: &'p [FieldPat<'tcx>],
2099 ctor_wild_subpatterns: &[&'p Pat<'tcx>],
2100 is_non_exhaustive: bool,
2101 ) -> PatStack<'p, 'tcx> {
2102 let mut result = SmallVec::from_slice(ctor_wild_subpatterns);
2104 for subpat in subpatterns {
2105 if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
2106 result[subpat.field.index()] = &subpat.pattern;
2111 "patterns_for_variant({:#?}, {:#?}) = {:#?}",
2112 subpatterns, ctor_wild_subpatterns, result
2114 PatStack::from_vec(result)
2117 /// This is the main specialization step. It expands the pattern
2118 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
2119 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
2120 /// Returns `None` if the pattern does not have the given constructor.
2122 /// OTOH, slice patterns with a subslice pattern (tail @ ..) can be expanded into multiple
2123 /// different patterns.
2124 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
2125 /// fields filled with wild patterns.
2126 fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
2127 cx: &mut MatchCheckCtxt<'a, 'tcx>,
2129 constructor: &Constructor<'tcx>,
2130 ctor_wild_subpatterns: &[&'p Pat<'tcx>],
2131 ) -> Option<PatStack<'p, 'tcx>> {
2132 let result = match *pat.kind {
2133 PatKind::AscribeUserType { ref subpattern, .. } => PatStack::from_pattern(subpattern)
2134 .specialize_constructor(cx, constructor, ctor_wild_subpatterns),
2136 PatKind::Binding { .. } | PatKind::Wild => {
2137 Some(PatStack::from_slice(ctor_wild_subpatterns))
2140 PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
2141 let ref variant = adt_def.variants[variant_index];
2142 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(pat.ty);
2143 Some(Variant(variant.def_id))
2144 .filter(|variant_constructor| variant_constructor == constructor)
2146 patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, is_non_exhaustive)
2150 PatKind::Leaf { ref subpatterns } => {
2151 Some(patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, false))
2154 PatKind::Deref { ref subpattern } => Some(PatStack::from_pattern(subpattern)),
2156 PatKind::Constant { value } if constructor.is_slice() => {
2157 // We extract an `Option` for the pointer because slices of zero
2158 // elements don't necessarily point to memory, they are usually
2159 // just integers. The only time they should be pointing to memory
2160 // is when they are subslices of nonzero slices.
2161 let (alloc, offset, n, ty) = match value.ty.kind {
2162 ty::Array(t, n) => match value.val {
2163 ConstValue::ByRef { offset, alloc, .. } => {
2164 (alloc, offset, n.eval_usize(cx.tcx, cx.param_env), t)
2166 _ => span_bug!(pat.span, "array pattern is {:?}", value,),
2170 ConstValue::Slice { data, start, end } => {
2171 (data, Size::from_bytes(start as u64), (end - start) as u64, t)
2173 ConstValue::ByRef { .. } => {
2174 // FIXME(oli-obk): implement `deref` for `ConstValue`
2179 "slice pattern constant must be scalar pair but is {:?}",
2186 "unexpected const-val {:?} with ctor {:?}",
2191 if ctor_wild_subpatterns.len() as u64 == n {
2192 // convert a constant slice/array pattern to a list of patterns.
2193 let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
2194 let ptr = Pointer::new(AllocId(0), offset);
2197 let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
2198 let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
2199 let scalar = scalar.not_undef().ok()?;
2200 let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
2202 Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
2203 Some(&*cx.pattern_arena.alloc(pattern))
2211 PatKind::Constant { .. } | PatKind::Range { .. } => {
2212 // If the constructor is a:
2213 // - Single value: add a row if the pattern contains the constructor.
2214 // - Range: add a row if the constructor intersects the pattern.
2215 if should_treat_range_exhaustively(cx.tcx, constructor) {
2217 IntRange::from_ctor(cx.tcx, cx.param_env, constructor),
2218 IntRange::from_pat(cx.tcx, cx.param_env, pat),
2220 (Some(ctor), Some(pat)) => ctor.intersection(&pat).map(|_| {
2221 let (pat_lo, pat_hi) = pat.range.into_inner();
2222 let (ctor_lo, ctor_hi) = ctor.range.into_inner();
2223 assert!(pat_lo <= ctor_lo && ctor_hi <= pat_hi);
2229 // Fallback for non-ranges and ranges that involve
2230 // floating-point numbers, which are not conveniently handled
2231 // by `IntRange`. For these cases, the constructor may not be a
2232 // range so intersection actually devolves into being covered
2234 match constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat) {
2235 Ok(true) => Some(PatStack::default()),
2236 Ok(false) | Err(ErrorReported) => None,
2241 PatKind::Array { ref prefix, ref slice, ref suffix }
2242 | PatKind::Slice { ref prefix, ref slice, ref suffix } => match *constructor {
2243 FixedLenSlice(..) => {
2244 let pat_len = prefix.len() + suffix.len();
2245 if let Some(slice_count) = ctor_wild_subpatterns.len().checked_sub(pat_len) {
2246 if slice_count == 0 || slice.is_some() {
2251 ctor_wild_subpatterns
2256 .chain(suffix.iter()),
2267 ConstantValue(cv, _) => {
2268 match slice_pat_covered_by_const(
2277 Ok(true) => Some(PatStack::default()),
2279 Err(ErrorReported) => None,
2282 _ => span_bug!(pat.span, "unexpected ctor {:?} for slice pat", constructor),
2285 PatKind::Or { .. } => {
2286 bug!("support for or-patterns has not been fully implemented yet.");
2289 debug!("specialize({:#?}, {:#?}) = {:#?}", pat, ctor_wild_subpatterns, result);