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::SliceKind::*;
229 use self::Usefulness::*;
230 use self::WitnessPreference::*;
232 use rustc_data_structures::fx::FxHashMap;
233 use rustc_index::vec::Idx;
235 use super::{compare_const_vals, PatternFoldable, PatternFolder};
236 use super::{FieldPat, Pat, PatKind, PatRange};
238 use rustc::hir::def_id::DefId;
239 use rustc::hir::{HirId, RangeEnd};
240 use rustc::ty::layout::{Integer, IntegerExt, Size, VariantIdx};
241 use rustc::ty::{self, Const, Ty, TyCtxt, TypeFoldable};
244 use rustc::mir::interpret::{truncate, AllocId, ConstValue, Pointer, Scalar};
245 use rustc::mir::Field;
246 use rustc::util::captures::Captures;
247 use rustc::util::common::ErrorReported;
249 use syntax::attr::{SignedInt, UnsignedInt};
250 use syntax_pos::{Span, DUMMY_SP};
252 use arena::TypedArena;
254 use smallvec::{smallvec, SmallVec};
255 use std::cmp::{self, max, min, Ordering};
256 use std::convert::TryInto;
258 use std::iter::{FromIterator, IntoIterator};
259 use std::ops::RangeInclusive;
262 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pat<'tcx>) -> Pat<'tcx> {
263 LiteralExpander { tcx: cx.tcx }.fold_pattern(&pat)
266 struct LiteralExpander<'tcx> {
270 impl LiteralExpander<'tcx> {
271 /// Derefs `val` and potentially unsizes the value if `crty` is an array and `rty` a slice.
273 /// `crty` and `rty` can differ because you can use array constants in the presence of slice
274 /// patterns. So the pattern may end up being a slice, but the constant is an array. We convert
275 /// the array to a slice in that case.
276 fn fold_const_value_deref(
278 val: ConstValue<'tcx>,
279 // the pattern's pointee type
281 // the constant's pointee type
283 ) -> ConstValue<'tcx> {
284 debug!("fold_const_value_deref {:?} {:?} {:?}", val, rty, crty);
285 match (val, &crty.kind, &rty.kind) {
286 // the easy case, deref a reference
287 (ConstValue::Scalar(Scalar::Ptr(p)), x, y) if x == y => {
288 let alloc = self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id);
289 ConstValue::ByRef { alloc, offset: p.offset }
291 // unsize array to slice if pattern is array but match value or other patterns are slice
292 (ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
295 data: self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id),
296 start: p.offset.bytes().try_into().unwrap(),
297 end: n.eval_usize(self.tcx, ty::ParamEnv::empty()).try_into().unwrap(),
300 // fat pointers stay the same
301 (ConstValue::Slice { .. }, _, _)
302 | (_, ty::Slice(_), ty::Slice(_))
303 | (_, ty::Str, ty::Str) => val,
304 // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
305 _ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
310 impl PatternFolder<'tcx> for LiteralExpander<'tcx> {
311 fn fold_pattern(&mut self, pat: &Pat<'tcx>) -> Pat<'tcx> {
312 debug!("fold_pattern {:?} {:?} {:?}", pat, pat.ty.kind, pat.kind);
313 match (&pat.ty.kind, &*pat.kind) {
319 val: ty::ConstKind::Value(val),
320 ty: ty::TyS { kind: ty::Ref(_, crty, _), .. },
326 kind: box PatKind::Deref {
330 kind: box PatKind::Constant {
331 value: self.tcx.mk_const(Const {
332 val: ty::ConstKind::Value(
333 self.fold_const_value_deref(*val, rty, crty),
345 value: Const { val, ty: ty::TyS { kind: ty::Ref(_, crty, _), .. } },
347 ) => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
349 (_, &PatKind::Binding { subpattern: Some(ref s), .. }) => s.fold_with(self),
350 (_, &PatKind::AscribeUserType { subpattern: ref s, .. }) => s.fold_with(self),
351 _ => pat.super_fold_with(self),
356 impl<'tcx> Pat<'tcx> {
357 fn is_wildcard(&self) -> bool {
359 PatKind::Binding { subpattern: None, .. } | PatKind::Wild => true,
365 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
367 #[derive(Debug, Clone)]
368 pub struct PatStack<'p, 'tcx>(SmallVec<[&'p Pat<'tcx>; 2]>);
370 impl<'p, 'tcx> PatStack<'p, 'tcx> {
371 pub fn from_pattern(pat: &'p Pat<'tcx>) -> Self {
372 PatStack(smallvec![pat])
375 fn from_vec(vec: SmallVec<[&'p Pat<'tcx>; 2]>) -> Self {
379 fn from_slice(s: &[&'p Pat<'tcx>]) -> Self {
380 PatStack(SmallVec::from_slice(s))
383 fn is_empty(&self) -> bool {
387 fn len(&self) -> usize {
391 fn head(&self) -> &'p Pat<'tcx> {
395 fn to_tail(&self) -> Self {
396 PatStack::from_slice(&self.0[1..])
399 fn iter(&self) -> impl Iterator<Item = &Pat<'tcx>> {
400 self.0.iter().map(|p| *p)
403 // If the first pattern is an or-pattern, expand this pattern. Otherwise, return `None`.
404 fn expand_or_pat(&self) -> Option<Vec<Self>> {
407 } else if let PatKind::Or { pats } = &*self.head().kind {
411 let mut new_patstack = PatStack::from_pattern(pat);
412 new_patstack.0.extend_from_slice(&self.0[1..]);
422 /// This computes `D(self)`. See top of the file for explanations.
423 fn specialize_wildcard(&self) -> Option<Self> {
424 if self.head().is_wildcard() { Some(self.to_tail()) } else { None }
427 /// This computes `S(constructor, self)`. See top of the file for explanations.
428 fn specialize_constructor(
430 cx: &mut MatchCheckCtxt<'p, 'tcx>,
431 constructor: &Constructor<'tcx>,
432 ctor_wild_subpatterns: &'p [Pat<'tcx>],
433 ) -> Option<PatStack<'p, 'tcx>> {
434 let new_heads = specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns);
435 new_heads.map(|mut new_head| {
436 new_head.0.extend_from_slice(&self.0[1..]);
442 impl<'p, 'tcx> Default for PatStack<'p, 'tcx> {
443 fn default() -> Self {
444 PatStack(smallvec![])
448 impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
449 fn from_iter<T>(iter: T) -> Self
451 T: IntoIterator<Item = &'p Pat<'tcx>>,
453 PatStack(iter.into_iter().collect())
458 pub struct Matrix<'p, 'tcx>(Vec<PatStack<'p, 'tcx>>);
460 impl<'p, 'tcx> Matrix<'p, 'tcx> {
461 pub fn empty() -> Self {
465 /// Pushes a new row to the matrix. If the row starts with an or-pattern, this expands it.
466 pub fn push(&mut self, row: PatStack<'p, 'tcx>) {
467 if let Some(rows) = row.expand_or_pat() {
474 /// Iterate over the first component of each row
475 fn heads<'a>(&'a self) -> impl Iterator<Item = &'a Pat<'tcx>> + Captures<'p> {
476 self.0.iter().map(|r| r.head())
479 /// This computes `D(self)`. See top of the file for explanations.
480 fn specialize_wildcard(&self) -> Self {
481 self.0.iter().filter_map(|r| r.specialize_wildcard()).collect()
484 /// This computes `S(constructor, self)`. See top of the file for explanations.
485 fn specialize_constructor(
487 cx: &mut MatchCheckCtxt<'p, 'tcx>,
488 constructor: &Constructor<'tcx>,
489 ctor_wild_subpatterns: &'p [Pat<'tcx>],
490 ) -> Matrix<'p, 'tcx> {
493 .filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
498 /// Pretty-printer for matrices of patterns, example:
499 /// +++++++++++++++++++++++++++++
501 /// +++++++++++++++++++++++++++++
502 /// + true + [First] +
503 /// +++++++++++++++++++++++++++++
504 /// + true + [Second(true)] +
505 /// +++++++++++++++++++++++++++++
507 /// +++++++++++++++++++++++++++++
508 /// + _ + [_, _, tail @ ..] +
509 /// +++++++++++++++++++++++++++++
510 impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
511 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
514 let &Matrix(ref m) = self;
515 let pretty_printed_matrix: Vec<Vec<String>> =
516 m.iter().map(|row| row.iter().map(|pat| format!("{:?}", pat)).collect()).collect();
518 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
519 assert!(m.iter().all(|row| row.len() == column_count));
520 let column_widths: Vec<usize> = (0..column_count)
521 .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0))
524 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
525 let br = "+".repeat(total_width);
526 write!(f, "{}\n", br)?;
527 for row in pretty_printed_matrix {
529 for (column, pat_str) in row.into_iter().enumerate() {
531 write!(f, "{:1$}", pat_str, column_widths[column])?;
535 write!(f, "{}\n", br)?;
541 impl<'p, 'tcx> FromIterator<PatStack<'p, 'tcx>> for Matrix<'p, 'tcx> {
542 fn from_iter<T>(iter: T) -> Self
544 T: IntoIterator<Item = PatStack<'p, 'tcx>>,
546 let mut matrix = Matrix::empty();
548 // Using `push` ensures we correctly expand or-patterns.
555 pub struct MatchCheckCtxt<'a, 'tcx> {
556 pub tcx: TyCtxt<'tcx>,
557 /// The module in which the match occurs. This is necessary for
558 /// checking inhabited-ness of types because whether a type is (visibly)
559 /// inhabited can depend on whether it was defined in the current module or
560 /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty
561 /// outside it's module and should not be matchable with an empty match
564 param_env: ty::ParamEnv<'tcx>,
565 pub pattern_arena: &'a TypedArena<Pat<'tcx>>,
566 pub byte_array_map: FxHashMap<*const Pat<'tcx>, Vec<&'a Pat<'tcx>>>,
569 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
570 pub fn create_and_enter<F, R>(
572 param_env: ty::ParamEnv<'tcx>,
577 F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R,
579 let pattern_arena = TypedArena::default();
585 pattern_arena: &pattern_arena,
586 byte_array_map: FxHashMap::default(),
590 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
591 if self.tcx.features().exhaustive_patterns {
592 self.tcx.is_ty_uninhabited_from(self.module, ty)
598 fn is_local(&self, ty: Ty<'tcx>) -> bool {
600 ty::Adt(adt_def, ..) => adt_def.did.is_local(),
606 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
608 /// Patterns of length `n` (`[x, y]`).
610 /// Patterns using the `..` notation (`[x, .., y]`). Captures any array constructor of `length
611 /// >= i + j`. In the case where `array_len` is `Some(_)`, this indicates that we only care
612 /// about the first `i` and the last `j` values of the array, and everything in between is a
618 fn arity(self) -> u64 {
620 FixedLen(length) => length,
621 VarLen(prefix, suffix) => prefix + suffix,
625 /// Whether this pattern includes patterns of length `other_len`.
626 fn covers_length(self, other_len: u64) -> bool {
628 FixedLen(len) => len == other_len,
629 VarLen(prefix, suffix) => prefix + suffix <= other_len,
633 /// Returns a collection of slices that spans the values covered by `self`, subtracted by the
634 /// values covered by `other`: i.e., `self \ other` (in set notation).
635 fn subtract(self, other: Self) -> SmallVec<[Self; 1]> {
636 // Remember, `VarLen(i, j)` covers the union of `FixedLen` from `i + j` to infinity.
637 // Naming: we remove the "neg" constructors from the "pos" ones.
639 FixedLen(pos_len) => {
640 if other.covers_length(pos_len) {
646 VarLen(pos_prefix, pos_suffix) => {
647 let pos_len = pos_prefix + pos_suffix;
649 FixedLen(neg_len) => {
650 if neg_len < pos_len {
655 // We know that `neg_len + 1 >= pos_len >= pos_suffix`.
656 .chain(Some(VarLen(neg_len + 1 - pos_suffix, pos_suffix)))
660 VarLen(neg_prefix, neg_suffix) => {
661 let neg_len = neg_prefix + neg_suffix;
662 if neg_len <= pos_len {
665 (pos_len..neg_len).map(FixedLen).collect()
674 /// A constructor for array and slice patterns.
675 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
677 /// `None` if the matched value is a slice, `Some(n)` if it is an array of size `n`.
678 array_len: Option<u64>,
679 /// The kind of pattern it is: fixed-length `[x, y]` or variable length `[x, .., y]`.
684 /// Returns what patterns this constructor covers: either fixed-length patterns or
685 /// variable-length patterns.
686 fn pattern_kind(self) -> SliceKind {
688 Slice { array_len: Some(len), kind: VarLen(prefix, suffix) }
689 if prefix + suffix == len =>
697 /// Returns what values this constructor covers: either values of only one given length, or
698 /// values of length above a given length.
699 /// This is different from `pattern_kind()` because in some cases the pattern only takes into
700 /// account a subset of the entries of the array, but still only captures values of a given
702 fn value_kind(self) -> SliceKind {
704 Slice { array_len: Some(len), kind: VarLen(_, _) } => FixedLen(len),
709 fn arity(self) -> u64 {
710 self.pattern_kind().arity()
714 #[derive(Clone, Debug, PartialEq)]
715 enum Constructor<'tcx> {
716 /// The constructor of all patterns that don't vary by constructor,
717 /// e.g., struct patterns and fixed-length arrays.
722 ConstantValue(&'tcx ty::Const<'tcx>),
723 /// Ranges of integer literal values (`2`, `2..=5` or `2..5`).
724 IntRange(IntRange<'tcx>),
725 /// Ranges of floating-point literal values (`2.0..=5.2`).
726 FloatRange(&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>, RangeEnd),
727 /// Array and slice patterns.
729 /// Fake extra constructor for enums that aren't allowed to be matched exhaustively.
733 impl<'tcx> Constructor<'tcx> {
734 fn is_slice(&self) -> bool {
741 fn variant_index_for_adt<'a>(
743 cx: &MatchCheckCtxt<'a, 'tcx>,
744 adt: &'tcx ty::AdtDef,
747 Variant(id) => adt.variant_index_with_id(*id),
749 assert!(!adt.is_enum());
752 ConstantValue(c) => crate::const_eval::const_variant_index(cx.tcx, cx.param_env, c),
753 _ => bug!("bad constructor {:?} for adt {:?}", self, adt),
757 // Returns the set of constructors covered by `self` but not by
758 // anything in `other_ctors`.
759 fn subtract_ctors(&self, other_ctors: &Vec<Constructor<'tcx>>) -> Vec<Constructor<'tcx>> {
761 // Those constructors can only match themselves.
762 Single | Variant(_) | ConstantValue(..) | FloatRange(..) => {
763 if other_ctors.iter().any(|c| c == self) { vec![] } else { vec![self.clone()] }
766 let mut other_slices = other_ctors
768 .filter_map(|c: &Constructor<'_>| match c {
769 Slice(slice) => Some(*slice),
770 // FIXME(oli-obk): implement `deref` for `ConstValue`
771 ConstantValue(..) => None,
772 _ => bug!("bad slice pattern constructor {:?}", c),
774 .map(Slice::value_kind);
776 match slice.value_kind() {
777 FixedLen(self_len) => {
778 if other_slices.any(|other_slice| other_slice.covers_length(self_len)) {
784 kind @ VarLen(..) => {
785 let mut remaining_slices = vec![kind];
787 // For each used slice, subtract from the current set of slices.
788 for other_slice in other_slices {
789 remaining_slices = remaining_slices
791 .flat_map(|remaining_slice| remaining_slice.subtract(other_slice))
794 // If the constructors that have been considered so far already cover
795 // the entire range of `self`, no need to look at more constructors.
796 if remaining_slices.is_empty() {
803 .map(|kind| Slice { array_len: slice.array_len, kind })
809 IntRange(self_range) => {
810 let mut remaining_ranges = vec![self_range.clone()];
811 for other_ctor in other_ctors {
812 if let IntRange(other_range) = other_ctor {
813 if other_range == self_range {
814 // If the `self` range appears directly in a `match` arm, we can
815 // eliminate it straight away.
816 remaining_ranges = vec![];
818 // Otherwise explicitely compute the remaining ranges.
819 remaining_ranges = other_range.subtract_from(remaining_ranges);
822 // If the ranges that have been considered so far already cover the entire
823 // range of values, we can return early.
824 if remaining_ranges.is_empty() {
830 // Convert the ranges back into constructors.
831 remaining_ranges.into_iter().map(IntRange).collect()
833 // This constructor is never covered by anything else
834 NonExhaustive => vec![NonExhaustive],
838 /// This returns one wildcard pattern for each argument to this constructor.
840 /// This must be consistent with `apply`, `specialize_one_pattern`, and `arity`.
841 fn wildcard_subpatterns<'a>(
843 cx: &MatchCheckCtxt<'a, 'tcx>,
845 ) -> Vec<Pat<'tcx>> {
846 debug!("wildcard_subpatterns({:#?}, {:?})", self, ty);
849 Single | Variant(_) => match ty.kind {
850 ty::Tuple(ref fs) => {
851 fs.into_iter().map(|t| t.expect_ty()).map(Pat::wildcard_from_ty).collect()
853 ty::Ref(_, rty, _) => vec![Pat::wildcard_from_ty(rty)],
854 ty::Adt(adt, substs) => {
856 // Use T as the sub pattern type of Box<T>.
857 vec![Pat::wildcard_from_ty(substs.type_at(0))]
859 let variant = &adt.variants[self.variant_index_for_adt(cx, adt)];
860 let is_non_exhaustive =
861 variant.is_field_list_non_exhaustive() && !cx.is_local(ty);
866 let is_visible = adt.is_enum()
867 || field.vis.is_accessible_from(cx.module, cx.tcx);
868 let is_uninhabited = cx.is_uninhabited(field.ty(cx.tcx, substs));
869 match (is_visible, is_non_exhaustive, is_uninhabited) {
870 // Treat all uninhabited types in non-exhaustive variants as
872 (_, true, true) => cx.tcx.types.err,
873 // Treat all non-visible fields as `TyErr`. They can't appear
874 // in any other pattern from this match (because they are
875 // private), so their type does not matter - but we don't want
876 // to know they are uninhabited.
877 (false, ..) => cx.tcx.types.err,
879 let ty = field.ty(cx.tcx, substs);
881 // If the field type returned is an array of an unknown
882 // size return an TyErr.
885 .try_eval_usize(cx.tcx, cx.param_env)
895 .map(Pat::wildcard_from_ty)
901 Slice(_) => match ty.kind {
902 ty::Slice(ty) | ty::Array(ty, _) => {
903 let arity = self.arity(cx, ty);
904 (0..arity).map(|_| Pat::wildcard_from_ty(ty)).collect()
906 _ => bug!("bad slice pattern {:?} {:?}", self, ty),
908 ConstantValue(..) | FloatRange(..) | IntRange(..) | NonExhaustive => vec![],
912 /// This computes the arity of a constructor. The arity of a constructor
913 /// is how many subpattern patterns of that constructor should be expanded to.
915 /// For instance, a tuple pattern `(_, 42, Some([]))` has the arity of 3.
916 /// A struct pattern's arity is the number of fields it contains, etc.
918 /// This must be consistent with `wildcard_subpatterns`, `specialize_one_pattern`, and `apply`.
919 fn arity<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> u64 {
920 debug!("Constructor::arity({:#?}, {:?})", self, ty);
922 Single | Variant(_) => match ty.kind {
923 ty::Tuple(ref fs) => fs.len() as u64,
924 ty::Slice(..) | ty::Array(..) => bug!("bad slice pattern {:?} {:?}", self, ty),
927 adt.variants[self.variant_index_for_adt(cx, adt)].fields.len() as u64
931 Slice(slice) => slice.arity(),
932 ConstantValue(..) | FloatRange(..) | IntRange(..) | NonExhaustive => 0,
936 /// Apply a constructor to a list of patterns, yielding a new pattern. `pats`
937 /// must have as many elements as this constructor's arity.
939 /// This must be consistent with `wildcard_subpatterns`, `specialize_one_pattern`, and `arity`.
942 /// `self`: `Constructor::Single`
943 /// `ty`: `(u32, u32, u32)`
944 /// `pats`: `[10, 20, _]`
945 /// returns `(10, 20, _)`
947 /// `self`: `Constructor::Variant(Option::Some)`
948 /// `ty`: `Option<bool>`
949 /// `pats`: `[false]`
950 /// returns `Some(false)`
953 cx: &MatchCheckCtxt<'a, 'tcx>,
955 pats: impl IntoIterator<Item = Pat<'tcx>>,
957 let mut subpatterns = pats.into_iter();
959 let pat = match self {
960 Single | Variant(_) => match ty.kind {
961 ty::Adt(..) | ty::Tuple(..) => {
962 let subpatterns = subpatterns
964 .map(|(i, p)| FieldPat { field: Field::new(i), pattern: p })
967 if let ty::Adt(adt, substs) = ty.kind {
972 variant_index: self.variant_index_for_adt(cx, adt),
976 PatKind::Leaf { subpatterns }
979 PatKind::Leaf { subpatterns }
982 ty::Ref(..) => PatKind::Deref { subpattern: subpatterns.nth(0).unwrap() },
983 ty::Slice(_) | ty::Array(..) => bug!("bad slice pattern {:?} {:?}", self, ty),
986 Slice(slice) => match slice.pattern_kind() {
988 PatKind::Slice { prefix: subpatterns.collect(), slice: None, suffix: vec![] }
990 VarLen(prefix, _) => {
991 let mut prefix: Vec<_> = subpatterns.by_ref().take(prefix as usize).collect();
992 if slice.array_len.is_some() {
993 // Improves diagnostics a bit: if the type is a known-size array, instead
994 // of reporting `[x, _, .., _, y]`, we prefer to report `[x, .., y]`.
995 // This is incorrect if the size is not known, since `[_, ..]` captures
996 // arrays of lengths `>= 1` whereas `[..]` captures any length.
997 while !prefix.is_empty() && prefix.last().unwrap().is_wildcard() {
1001 let suffix: Vec<_> = if slice.array_len.is_some() {
1003 subpatterns.skip_while(Pat::is_wildcard).collect()
1005 subpatterns.collect()
1007 let wild = Pat::wildcard_from_ty(ty);
1008 PatKind::Slice { prefix, slice: Some(wild), suffix }
1011 &ConstantValue(value) => PatKind::Constant { value },
1012 &FloatRange(lo, hi, end) => PatKind::Range(PatRange { lo, hi, end }),
1013 IntRange(range) => return range.to_pat(cx.tcx),
1014 NonExhaustive => PatKind::Wild,
1017 Pat { ty, span: DUMMY_SP, kind: Box::new(pat) }
1020 /// Like `apply`, but where all the subpatterns are wildcards `_`.
1021 fn apply_wildcards<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {
1022 let subpatterns = self.wildcard_subpatterns(cx, ty).into_iter().rev();
1023 self.apply(cx, ty, subpatterns)
1027 #[derive(Clone, Debug)]
1028 pub enum Usefulness<'tcx> {
1030 UsefulWithWitness(Vec<Witness<'tcx>>),
1034 impl<'tcx> Usefulness<'tcx> {
1035 fn new_useful(preference: WitnessPreference) -> Self {
1037 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
1038 LeaveOutWitness => Useful,
1042 fn is_useful(&self) -> bool {
1049 fn apply_constructor(
1051 cx: &MatchCheckCtxt<'_, 'tcx>,
1052 ctor: &Constructor<'tcx>,
1056 UsefulWithWitness(witnesses) => UsefulWithWitness(
1059 .map(|witness| witness.apply_constructor(cx, &ctor, ty))
1066 fn apply_wildcard(self, ty: Ty<'tcx>) -> Self {
1068 UsefulWithWitness(witnesses) => {
1069 let wild = Pat::wildcard_from_ty(ty);
1073 .map(|mut witness| {
1074 witness.0.push(wild.clone());
1084 fn apply_missing_ctors(
1086 cx: &MatchCheckCtxt<'_, 'tcx>,
1088 missing_ctors: &MissingConstructors<'tcx>,
1091 UsefulWithWitness(witnesses) => {
1092 let new_patterns: Vec<_> =
1093 missing_ctors.iter().map(|ctor| ctor.apply_wildcards(cx, ty)).collect();
1094 // Add the new patterns to each witness
1098 .flat_map(|witness| {
1099 new_patterns.iter().map(move |pat| {
1100 let mut witness = witness.clone();
1101 witness.0.push(pat.clone());
1113 #[derive(Copy, Clone, Debug)]
1114 pub enum WitnessPreference {
1119 #[derive(Copy, Clone, Debug)]
1120 struct PatCtxt<'tcx> {
1125 /// A witness of non-exhaustiveness for error reporting, represented
1126 /// as a list of patterns (in reverse order of construction) with
1127 /// wildcards inside to represent elements that can take any inhabitant
1128 /// of the type as a value.
1130 /// A witness against a list of patterns should have the same types
1131 /// and length as the pattern matched against. Because Rust `match`
1132 /// is always against a single pattern, at the end the witness will
1133 /// have length 1, but in the middle of the algorithm, it can contain
1134 /// multiple patterns.
1136 /// For example, if we are constructing a witness for the match against
1138 /// struct Pair(Option<(u32, u32)>, bool);
1140 /// match (p: Pair) {
1141 /// Pair(None, _) => {}
1142 /// Pair(_, false) => {}
1146 /// We'll perform the following steps:
1147 /// 1. Start with an empty witness
1148 /// `Witness(vec![])`
1149 /// 2. Push a witness `Some(_)` against the `None`
1150 /// `Witness(vec![Some(_)])`
1151 /// 3. Push a witness `true` against the `false`
1152 /// `Witness(vec![Some(_), true])`
1153 /// 4. Apply the `Pair` constructor to the witnesses
1154 /// `Witness(vec![Pair(Some(_), true)])`
1156 /// The final `Pair(Some(_), true)` is then the resulting witness.
1157 #[derive(Clone, Debug)]
1158 pub struct Witness<'tcx>(Vec<Pat<'tcx>>);
1160 impl<'tcx> Witness<'tcx> {
1161 pub fn single_pattern(self) -> Pat<'tcx> {
1162 assert_eq!(self.0.len(), 1);
1163 self.0.into_iter().next().unwrap()
1166 /// Constructs a partial witness for a pattern given a list of
1167 /// patterns expanded by the specialization step.
1169 /// When a pattern P is discovered to be useful, this function is used bottom-up
1170 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
1171 /// of values, V, where each value in that set is not covered by any previously
1172 /// used patterns and is covered by the pattern P'. Examples:
1174 /// left_ty: tuple of 3 elements
1175 /// pats: [10, 20, _] => (10, 20, _)
1177 /// left_ty: struct X { a: (bool, &'static str), b: usize}
1178 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
1179 fn apply_constructor<'a>(
1181 cx: &MatchCheckCtxt<'a, 'tcx>,
1182 ctor: &Constructor<'tcx>,
1185 let arity = ctor.arity(cx, ty);
1187 let len = self.0.len() as u64;
1188 let pats = self.0.drain((len - arity) as usize..).rev();
1189 ctor.apply(cx, ty, pats)
1198 /// This determines the set of all possible constructors of a pattern matching
1199 /// values of type `left_ty`. For vectors, this would normally be an infinite set
1200 /// but is instead bounded by the maximum fixed length of slice patterns in
1201 /// the column of patterns being analyzed.
1203 /// We make sure to omit constructors that are statically impossible. E.g., for
1204 /// `Option<!>`, we do not include `Some(_)` in the returned list of constructors.
1205 fn all_constructors<'a, 'tcx>(
1206 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1208 ) -> Vec<Constructor<'tcx>> {
1209 debug!("all_constructors({:?})", pcx.ty);
1210 let make_range = |start, end| {
1212 // `unwrap()` is ok because we know the type is an integer.
1213 IntRange::from_range(cx.tcx, start, end, pcx.ty, &RangeEnd::Included, pcx.span)
1219 [true, false].iter().map(|&b| ConstantValue(ty::Const::from_bool(cx.tcx, b))).collect()
1221 ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx, cx.param_env).is_some() => {
1222 let len = len.eval_usize(cx.tcx, cx.param_env);
1223 if len != 0 && cx.is_uninhabited(sub_ty) {
1226 vec![Slice(Slice { array_len: Some(len), kind: VarLen(0, 0) })]
1229 // Treat arrays of a constant but unknown length like slices.
1230 ty::Array(ref sub_ty, _) | ty::Slice(ref sub_ty) => {
1231 let kind = if cx.is_uninhabited(sub_ty) { FixedLen(0) } else { VarLen(0, 0) };
1232 vec![Slice(Slice { array_len: None, kind })]
1234 ty::Adt(def, substs) if def.is_enum() => {
1235 let ctors: Vec<_> = def
1239 !cx.tcx.features().exhaustive_patterns
1241 .uninhabited_from(cx.tcx, substs, def.adt_kind())
1242 .contains(cx.tcx, cx.module)
1244 .map(|v| Variant(v.def_id))
1247 // If our scrutinee is *privately* an empty enum, we must treat it as though it had an
1248 // "unknown" constructor (in that case, all other patterns obviously can't be variants)
1249 // to avoid exposing its emptyness. See the `match_privately_empty` test for details.
1250 // FIXME: currently the only way I know of something can be a privately-empty enum is
1251 // when the exhaustive_patterns feature flag is not present, so this is only needed for
1253 let is_privately_empty = ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1254 // If the enum is declared as `#[non_exhaustive]`, we treat it as if it had an
1255 // additionnal "unknown" constructor.
1256 let is_declared_nonexhaustive =
1257 def.is_variant_list_non_exhaustive() && !cx.is_local(pcx.ty);
1259 if is_privately_empty || is_declared_nonexhaustive {
1260 // There is no point in enumerating all possible variants, because the user can't
1261 // actually match against them themselves. So we return only the fictitious
1263 // E.g., in an example like:
1265 // let err: io::ErrorKind = ...;
1267 // io::ErrorKind::NotFound => {},
1270 // we don't want to show every possible IO error, but instead have only `_` as the
1279 // The valid Unicode Scalar Value ranges.
1280 make_range('\u{0000}' as u128, '\u{D7FF}' as u128),
1281 make_range('\u{E000}' as u128, '\u{10FFFF}' as u128),
1284 ty::Int(_) | ty::Uint(_)
1285 if pcx.ty.is_ptr_sized_integral()
1286 && !cx.tcx.features().precise_pointer_size_matching =>
1288 // `usize`/`isize` are not allowed to be matched exhaustively unless the
1289 // `precise_pointer_size_matching` feature is enabled. So we treat those types like
1290 // `#[non_exhaustive]` enums by returning a special unmatcheable constructor.
1294 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
1295 let min = 1u128 << (bits - 1);
1297 vec![make_range(min, max)]
1300 let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
1301 let max = truncate(u128::max_value(), size);
1302 vec![make_range(0, max)]
1305 if cx.is_uninhabited(pcx.ty) {
1314 /// An inclusive interval, used for precise integer exhaustiveness checking.
1315 /// `IntRange`s always store a contiguous range. This means that values are
1316 /// encoded such that `0` encodes the minimum value for the integer,
1317 /// regardless of the signedness.
1318 /// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
1319 /// This makes comparisons and arithmetic on interval endpoints much more
1320 /// straightforward. See `signed_bias` for details.
1322 /// `IntRange` is never used to encode an empty range or a "range" that wraps
1323 /// around the (offset) space: i.e., `range.lo <= range.hi`.
1324 #[derive(Clone, Debug)]
1325 struct IntRange<'tcx> {
1326 pub range: RangeInclusive<u128>,
1331 impl<'tcx> IntRange<'tcx> {
1333 fn is_integral(ty: Ty<'_>) -> bool {
1335 ty::Char | ty::Int(_) | ty::Uint(_) => true,
1340 fn is_singleton(&self) -> bool {
1341 self.range.start() == self.range.end()
1344 fn boundaries(&self) -> (u128, u128) {
1345 (*self.range.start(), *self.range.end())
1348 /// Don't treat `usize`/`isize` exhaustively unless the `precise_pointer_size_matching` feature
1350 fn treat_exhaustively(&self, tcx: TyCtxt<'tcx>) -> bool {
1351 !self.ty.is_ptr_sized_integral() || tcx.features().precise_pointer_size_matching
1355 fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
1357 ty::Char => Some((Size::from_bytes(4), 0)),
1359 let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
1360 Some((size, 1u128 << (size.bits() as u128 - 1)))
1362 ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
1370 param_env: ty::ParamEnv<'tcx>,
1371 value: &Const<'tcx>,
1373 ) -> Option<IntRange<'tcx>> {
1374 if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
1376 let val = if let ty::ConstKind::Value(ConstValue::Scalar(Scalar::Raw { data, size })) =
1379 // For this specific pattern we can skip a lot of effort and go
1380 // straight to the result, after doing a bit of checking. (We
1381 // could remove this branch and just use the next branch, which
1382 // is more general but much slower.)
1383 Scalar::<()>::check_raw(data, size, target_size);
1385 } else if let Some(val) = value.try_eval_bits(tcx, param_env, ty) {
1386 // This is a more general form of the previous branch.
1391 let val = val ^ bias;
1392 Some(IntRange { range: val..=val, ty, span })
1406 ) -> Option<IntRange<'tcx>> {
1407 if Self::is_integral(ty) {
1408 // Perform a shift if the underlying types are signed,
1409 // which makes the interval arithmetic simpler.
1410 let bias = IntRange::signed_bias(tcx, ty);
1411 let (lo, hi) = (lo ^ bias, hi ^ bias);
1412 let offset = (*end == RangeEnd::Excluded) as u128;
1413 if lo > hi || (lo == hi && *end == RangeEnd::Excluded) {
1414 // This should have been caught earlier by E0030.
1415 bug!("malformed range pattern: {}..={}", lo, (hi - offset));
1417 Some(IntRange { range: lo..=(hi - offset), ty, span })
1425 param_env: ty::ParamEnv<'tcx>,
1427 ) -> Option<IntRange<'tcx>> {
1428 match pat_constructor(tcx, param_env, pat)? {
1429 IntRange(range) => Some(range),
1434 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
1435 fn signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
1438 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
1445 /// Returns a collection of ranges that spans the values covered by `ranges`, subtracted
1446 /// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
1447 fn subtract_from(&self, ranges: Vec<IntRange<'tcx>>) -> Vec<IntRange<'tcx>> {
1448 let mut remaining_ranges = vec![];
1450 let span = self.span;
1451 let (lo, hi) = self.boundaries();
1452 for subrange in ranges {
1453 let (subrange_lo, subrange_hi) = subrange.range.into_inner();
1454 if lo > subrange_hi || subrange_lo > hi {
1455 // The pattern doesn't intersect with the subrange at all,
1456 // so the subrange remains untouched.
1457 remaining_ranges.push(IntRange { range: subrange_lo..=subrange_hi, ty, span });
1459 if lo > subrange_lo {
1460 // The pattern intersects an upper section of the
1461 // subrange, so a lower section will remain.
1462 remaining_ranges.push(IntRange { range: subrange_lo..=(lo - 1), ty, span });
1464 if hi < subrange_hi {
1465 // The pattern intersects a lower section of the
1466 // subrange, so an upper section will remain.
1467 remaining_ranges.push(IntRange { range: (hi + 1)..=subrange_hi, ty, span });
1474 fn is_subrange(&self, other: &Self) -> bool {
1475 other.range.start() <= self.range.start() && self.range.end() <= other.range.end()
1478 fn intersection(&self, tcx: TyCtxt<'tcx>, other: &Self) -> Option<Self> {
1480 let (lo, hi) = self.boundaries();
1481 let (other_lo, other_hi) = other.boundaries();
1482 if self.treat_exhaustively(tcx) {
1483 if lo <= other_hi && other_lo <= hi {
1484 let span = other.span;
1485 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty, span })
1490 // If the range should not be treated exhaustively, fallback to checking for inclusion.
1491 if self.is_subrange(other) { Some(self.clone()) } else { None }
1495 fn suspicious_intersection(&self, other: &Self) -> bool {
1496 // `false` in the following cases:
1497 // 1 ---- // 1 ---------- // 1 ---- // 1 ----
1498 // 2 ---------- // 2 ---- // 2 ---- // 2 ----
1500 // The following are currently `false`, but could be `true` in the future (#64007):
1501 // 1 --------- // 1 ---------
1502 // 2 ---------- // 2 ----------
1504 // `true` in the following cases:
1505 // 1 ------- // 1 -------
1506 // 2 -------- // 2 -------
1507 let (lo, hi) = self.boundaries();
1508 let (other_lo, other_hi) = other.boundaries();
1509 (lo == other_hi || hi == other_lo)
1512 fn to_pat(&self, tcx: TyCtxt<'tcx>) -> Pat<'tcx> {
1513 let (lo, hi) = self.boundaries();
1515 let bias = IntRange::signed_bias(tcx, self.ty);
1516 let (lo, hi) = (lo ^ bias, hi ^ bias);
1518 let ty = ty::ParamEnv::empty().and(self.ty);
1519 let lo_const = ty::Const::from_bits(tcx, lo, ty);
1520 let hi_const = ty::Const::from_bits(tcx, hi, ty);
1522 let kind = if lo == hi {
1523 PatKind::Constant { value: lo_const }
1525 PatKind::Range(PatRange { lo: lo_const, hi: hi_const, end: RangeEnd::Included })
1528 // This is a brand new pattern, so we don't reuse `self.span`.
1529 Pat { ty: self.ty, span: DUMMY_SP, kind: Box::new(kind) }
1533 /// Ignore spans when comparing, they don't carry semantic information as they are only for lints.
1534 impl<'tcx> std::cmp::PartialEq for IntRange<'tcx> {
1535 fn eq(&self, other: &Self) -> bool {
1536 self.range == other.range && self.ty == other.ty
1540 // A struct to compute a set of constructors equivalent to `all_ctors \ used_ctors`.
1541 struct MissingConstructors<'tcx> {
1542 all_ctors: Vec<Constructor<'tcx>>,
1543 used_ctors: Vec<Constructor<'tcx>>,
1546 impl<'tcx> MissingConstructors<'tcx> {
1547 fn new(all_ctors: Vec<Constructor<'tcx>>, used_ctors: Vec<Constructor<'tcx>>) -> Self {
1548 MissingConstructors { all_ctors, used_ctors }
1551 fn into_inner(self) -> (Vec<Constructor<'tcx>>, Vec<Constructor<'tcx>>) {
1552 (self.all_ctors, self.used_ctors)
1555 fn is_empty(&self) -> bool {
1556 self.iter().next().is_none()
1558 /// Whether this contains all the constructors for the given type or only a
1560 fn all_ctors_are_missing(&self) -> bool {
1561 self.used_ctors.is_empty()
1564 /// Iterate over all_ctors \ used_ctors
1565 fn iter<'a>(&'a self) -> impl Iterator<Item = Constructor<'tcx>> + Captures<'a> {
1566 self.all_ctors.iter().flat_map(move |req_ctor| req_ctor.subtract_ctors(&self.used_ctors))
1570 impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
1571 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1572 let ctors: Vec<_> = self.iter().collect();
1573 write!(f, "{:?}", ctors)
1577 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html.
1578 /// The algorithm from the paper has been modified to correctly handle empty
1579 /// types. The changes are:
1580 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1581 /// continue to recurse over columns.
1582 /// (1) all_constructors will only return constructors that are statically
1583 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
1585 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1586 /// to a set of such vectors `m` - this is defined as there being a set of
1587 /// inputs that will match `v` but not any of the sets in `m`.
1589 /// All the patterns at each column of the `matrix ++ v` matrix must
1590 /// have the same type, except that wildcard (PatKind::Wild) patterns
1591 /// with type `TyErr` are also allowed, even if the "type of the column"
1592 /// is not `TyErr`. That is used to represent private fields, as using their
1593 /// real type would assert that they are inhabited.
1595 /// This is used both for reachability checking (if a pattern isn't useful in
1596 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1597 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1598 /// matrix isn't exhaustive).
1599 pub fn is_useful<'p, 'tcx>(
1600 cx: &mut MatchCheckCtxt<'p, 'tcx>,
1601 matrix: &Matrix<'p, 'tcx>,
1602 v: &PatStack<'p, 'tcx>,
1603 witness_preference: WitnessPreference,
1605 ) -> Usefulness<'tcx> {
1606 let &Matrix(ref rows) = matrix;
1607 debug!("is_useful({:#?}, {:#?})", matrix, v);
1609 // The base case. We are pattern-matching on () and the return value is
1610 // based on whether our matrix has a row or not.
1611 // NOTE: This could potentially be optimized by checking rows.is_empty()
1612 // first and then, if v is non-empty, the return value is based on whether
1613 // the type of the tuple we're checking is inhabited or not.
1615 return if rows.is_empty() {
1616 Usefulness::new_useful(witness_preference)
1622 assert!(rows.iter().all(|r| r.len() == v.len()));
1624 // If the first pattern is an or-pattern, expand it.
1625 if let Some(vs) = v.expand_or_pat() {
1628 .map(|v| is_useful(cx, matrix, &v, witness_preference, hir_id))
1629 .find(|result| result.is_useful())
1630 .unwrap_or(NotUseful);
1633 let (ty, span) = matrix
1635 .map(|r| (r.ty, r.span))
1636 .find(|(ty, _)| !ty.references_error())
1637 .unwrap_or((v.head().ty, v.head().span));
1639 // TyErr is used to represent the type of wildcard patterns matching
1640 // against inaccessible (private) fields of structs, so that we won't
1641 // be able to observe whether the types of the struct's fields are
1644 // If the field is truly inaccessible, then all the patterns
1645 // matching against it must be wildcard patterns, so its type
1648 // However, if we are matching against non-wildcard patterns, we
1649 // need to know the real type of the field so we can specialize
1650 // against it. This primarily occurs through constants - they
1651 // can include contents for fields that are inaccessible at the
1652 // location of the match. In that case, the field's type is
1653 // inhabited - by the constant - so we can just use it.
1655 // FIXME: this might lead to "unstable" behavior with macro hygiene
1656 // introducing uninhabited patterns for inaccessible fields. We
1657 // need to figure out how to model that.
1662 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
1664 if let Some(constructor) = pat_constructor(cx.tcx, cx.param_env, v.head()) {
1665 debug!("is_useful - expanding constructor: {:#?}", constructor);
1666 split_grouped_constructors(
1676 .map(|c| is_useful_specialized(cx, matrix, v, c, pcx.ty, witness_preference, hir_id))
1677 .find(|result| result.is_useful())
1678 .unwrap_or(NotUseful)
1680 debug!("is_useful - expanding wildcard");
1682 let used_ctors: Vec<Constructor<'_>> =
1683 matrix.heads().filter_map(|p| pat_constructor(cx.tcx, cx.param_env, p)).collect();
1684 debug!("used_ctors = {:#?}", used_ctors);
1685 // `all_ctors` are all the constructors for the given type, which
1686 // should all be represented (or caught with the wild pattern `_`).
1687 let all_ctors = all_constructors(cx, pcx);
1688 debug!("all_ctors = {:#?}", all_ctors);
1690 // `missing_ctors` is the set of constructors from the same type as the
1691 // first column of `matrix` that are matched only by wildcard patterns
1692 // from the first column.
1694 // Therefore, if there is some pattern that is unmatched by `matrix`,
1695 // it will still be unmatched if the first constructor is replaced by
1696 // any of the constructors in `missing_ctors`
1698 // Missing constructors are those that are not matched by any non-wildcard patterns in the
1699 // current column. We only fully construct them on-demand, because they're rarely used and
1701 let missing_ctors = MissingConstructors::new(all_ctors, used_ctors);
1703 debug!("missing_ctors.empty()={:#?}", missing_ctors.is_empty(),);
1705 if missing_ctors.is_empty() {
1706 let (all_ctors, _) = missing_ctors.into_inner();
1707 split_grouped_constructors(cx.tcx, cx.param_env, pcx, all_ctors, matrix, DUMMY_SP, None)
1710 is_useful_specialized(cx, matrix, v, c, pcx.ty, witness_preference, hir_id)
1712 .find(|result| result.is_useful())
1713 .unwrap_or(NotUseful)
1715 let matrix = matrix.specialize_wildcard();
1716 let v = v.to_tail();
1717 let usefulness = is_useful(cx, &matrix, &v, witness_preference, hir_id);
1719 // In this case, there's at least one "free"
1720 // constructor that is only matched against by
1721 // wildcard patterns.
1723 // There are 2 ways we can report a witness here.
1724 // Commonly, we can report all the "free"
1725 // constructors as witnesses, e.g., if we have:
1728 // enum Direction { N, S, E, W }
1729 // let Direction::N = ...;
1732 // we can report 3 witnesses: `S`, `E`, and `W`.
1734 // However, there is a case where we don't want
1735 // to do this and instead report a single `_` witness:
1736 // if the user didn't actually specify a constructor
1737 // in this arm, e.g., in
1739 // let x: (Direction, Direction, bool) = ...;
1740 // let (_, _, false) = x;
1742 // we don't want to show all 16 possible witnesses
1743 // `(<direction-1>, <direction-2>, true)` - we are
1744 // satisfied with `(_, _, true)`. In this case,
1745 // `used_ctors` is empty.
1746 if missing_ctors.all_ctors_are_missing() {
1747 // All constructors are unused. Add a wild pattern
1748 // rather than each individual constructor.
1749 usefulness.apply_wildcard(pcx.ty)
1751 // Construct for each missing constructor a "wild" version of this
1752 // constructor, that matches everything that can be built with
1753 // it. For example, if `ctor` is a `Constructor::Variant` for
1754 // `Option::Some`, we get the pattern `Some(_)`.
1755 usefulness.apply_missing_ctors(cx, pcx.ty, &missing_ctors)
1761 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
1762 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1763 fn is_useful_specialized<'p, 'tcx>(
1764 cx: &mut MatchCheckCtxt<'p, 'tcx>,
1765 matrix: &Matrix<'p, 'tcx>,
1766 v: &PatStack<'p, 'tcx>,
1767 ctor: Constructor<'tcx>,
1769 witness_preference: WitnessPreference,
1771 ) -> Usefulness<'tcx> {
1772 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1774 let ctor_wild_subpatterns =
1775 cx.pattern_arena.alloc_from_iter(ctor.wildcard_subpatterns(cx, lty));
1776 let matrix = matrix.specialize_constructor(cx, &ctor, ctor_wild_subpatterns);
1777 v.specialize_constructor(cx, &ctor, ctor_wild_subpatterns)
1778 .map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id))
1779 .map(|u| u.apply_constructor(cx, &ctor, lty))
1780 .unwrap_or(NotUseful)
1783 /// Determines the constructor that the given pattern can be specialized to.
1784 /// Returns `None` in case of a catch-all, which can't be specialized.
1785 fn pat_constructor<'tcx>(
1787 param_env: ty::ParamEnv<'tcx>,
1789 ) -> Option<Constructor<'tcx>> {
1791 PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
1792 PatKind::Binding { .. } | PatKind::Wild => None,
1793 PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(Single),
1794 PatKind::Variant { adt_def, variant_index, .. } => {
1795 Some(Variant(adt_def.variants[variant_index].def_id))
1797 PatKind::Constant { value } => {
1798 if let Some(int_range) = IntRange::from_const(tcx, param_env, value, pat.span) {
1799 Some(IntRange(int_range))
1801 match (value.val, &value.ty.kind) {
1802 (_, ty::Array(_, n)) => {
1803 let len = n.eval_usize(tcx, param_env);
1804 Some(Slice(Slice { array_len: Some(len), kind: FixedLen(len) }))
1806 (ty::ConstKind::Value(ConstValue::Slice { start, end, .. }), ty::Slice(_)) => {
1807 let len = (end - start) as u64;
1808 Some(Slice(Slice { array_len: None, kind: FixedLen(len) }))
1810 // FIXME(oli-obk): implement `deref` for `ConstValue`
1811 // (ty::ConstKind::Value(ConstValue::ByRef { .. }), ty::Slice(_)) => { ... }
1812 _ => Some(ConstantValue(value)),
1816 PatKind::Range(PatRange { lo, hi, end }) => {
1818 if let Some(int_range) = IntRange::from_range(
1820 lo.eval_bits(tcx, param_env, lo.ty),
1821 hi.eval_bits(tcx, param_env, hi.ty),
1826 Some(IntRange(int_range))
1828 Some(FloatRange(lo, hi, end))
1831 PatKind::Array { ref prefix, ref slice, ref suffix }
1832 | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
1833 let array_len = match pat.ty.kind {
1834 ty::Array(_, length) => Some(length.eval_usize(tcx, param_env)),
1835 ty::Slice(_) => None,
1836 _ => span_bug!(pat.span, "bad ty {:?} for slice pattern", pat.ty),
1838 let prefix = prefix.len() as u64;
1839 let suffix = suffix.len() as u64;
1841 if slice.is_some() { VarLen(prefix, suffix) } else { FixedLen(prefix + suffix) };
1842 Some(Slice(Slice { array_len, kind }))
1844 PatKind::Or { .. } => bug!("Or-pattern should have been expanded earlier on."),
1848 // checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
1849 // meaning all other types will compare unequal and thus equal patterns often do not cause the
1850 // second pattern to lint about unreachable match arms.
1851 fn slice_pat_covered_by_const<'tcx>(
1854 const_val: &'tcx ty::Const<'tcx>,
1855 prefix: &[Pat<'tcx>],
1856 slice: &Option<Pat<'tcx>>,
1857 suffix: &[Pat<'tcx>],
1858 param_env: ty::ParamEnv<'tcx>,
1859 ) -> Result<bool, ErrorReported> {
1860 let const_val_val = if let ty::ConstKind::Value(val) = const_val.val {
1864 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1872 let data: &[u8] = match (const_val_val, &const_val.ty.kind) {
1873 (ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
1874 assert_eq!(*t, tcx.types.u8);
1875 let n = n.eval_usize(tcx, param_env);
1876 let ptr = Pointer::new(AllocId(0), offset);
1877 alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
1879 (ConstValue::Slice { data, start, end }, ty::Slice(t)) => {
1880 assert_eq!(*t, tcx.types.u8);
1881 let ptr = Pointer::new(AllocId(0), Size::from_bytes(start as u64));
1882 data.get_bytes(&tcx, ptr, Size::from_bytes((end - start) as u64)).unwrap()
1884 // FIXME(oli-obk): create a way to extract fat pointers from ByRef
1885 (_, ty::Slice(_)) => return Ok(false),
1887 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1895 let pat_len = prefix.len() + suffix.len();
1896 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1900 for (ch, pat) in data[..prefix.len()]
1903 .chain(data[data.len() - suffix.len()..].iter().zip(suffix))
1906 box PatKind::Constant { value } => {
1907 let b = value.eval_bits(tcx, param_env, pat.ty);
1908 assert_eq!(b as u8 as u128, b);
1920 /// For exhaustive integer matching, some constructors are grouped within other constructors
1921 /// (namely integer typed values are grouped within ranges). However, when specialising these
1922 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1923 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1924 /// mean creating a separate constructor for every single value in the range, which is clearly
1925 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1926 /// identical across all values in that range (i.e., there are equivalence classes of ranges of
1927 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
1928 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
1929 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
1931 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1932 /// the group of intersecting patterns changes (using the method described below).
1933 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1934 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1935 /// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
1936 /// need to be worried about matching over gargantuan ranges.
1938 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
1940 /// |------| |----------| |-------| ||
1941 /// |-------| |-------| |----| ||
1944 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
1946 /// |--|--|||-||||--||---|||-------| |-|||| ||
1948 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
1949 /// boundaries for each interval range, sort them, then create constructors for each new interval
1950 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
1951 /// merging operation depicted above.)
1953 /// `hir_id` is `None` when we're evaluating the wildcard pattern, do not lint for overlapping in
1954 /// ranges that case.
1956 /// This also splits variable-length slices into fixed-length slices.
1957 fn split_grouped_constructors<'p, 'tcx>(
1959 param_env: ty::ParamEnv<'tcx>,
1961 ctors: Vec<Constructor<'tcx>>,
1962 matrix: &Matrix<'p, 'tcx>,
1964 hir_id: Option<HirId>,
1965 ) -> Vec<Constructor<'tcx>> {
1967 let mut split_ctors = Vec::with_capacity(ctors.len());
1968 debug!("split_grouped_constructors({:#?}, {:#?})", matrix, ctors);
1970 for ctor in ctors.into_iter() {
1972 IntRange(ctor_range) if ctor_range.treat_exhaustively(tcx) => {
1973 // Fast-track if the range is trivial. In particular, don't do the overlapping
1975 if ctor_range.is_singleton() {
1976 split_ctors.push(IntRange(ctor_range));
1980 /// Represents a border between 2 integers. Because the intervals spanning borders
1981 /// must be able to cover every integer, we need to be able to represent
1982 /// 2^128 + 1 such borders.
1983 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
1989 // A function for extracting the borders of an integer interval.
1990 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
1991 let (lo, hi) = r.range.into_inner();
1992 let from = Border::JustBefore(lo);
1993 let to = match hi.checked_add(1) {
1994 Some(m) => Border::JustBefore(m),
1995 None => Border::AfterMax,
1997 vec![from, to].into_iter()
2000 // Collect the span and range of all the intersecting ranges to lint on likely
2001 // incorrect range patterns. (#63987)
2002 let mut overlaps = vec![];
2003 // `borders` is the set of borders between equivalence classes: each equivalence
2004 // class lies between 2 borders.
2005 let row_borders = matrix
2009 IntRange::from_pat(tcx, param_env, row.head()).map(|r| (r, row.len()))
2011 .flat_map(|(range, row_len)| {
2012 let intersection = ctor_range.intersection(tcx, &range);
2013 let should_lint = ctor_range.suspicious_intersection(&range);
2014 if let (Some(range), 1, true) = (&intersection, row_len, should_lint) {
2015 // FIXME: for now, only check for overlapping ranges on simple range
2016 // patterns. Otherwise with the current logic the following is detected
2018 // match (10u8, true) {
2019 // (0 ..= 125, false) => {}
2020 // (126 ..= 255, false) => {}
2021 // (0 ..= 255, true) => {}
2023 overlaps.push(range.clone());
2027 .flat_map(|range| range_borders(range));
2028 let ctor_borders = range_borders(ctor_range.clone());
2029 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
2030 borders.sort_unstable();
2032 lint_overlapping_patterns(tcx, hir_id, ctor_range, ty, overlaps);
2034 // We're going to iterate through every adjacent pair of borders, making sure that
2035 // each represents an interval of nonnegative length, and convert each such
2036 // interval into a constructor.
2040 .filter_map(|window| match (window[0], window[1]) {
2041 (Border::JustBefore(n), Border::JustBefore(m)) => {
2043 Some(IntRange { range: n..=(m - 1), ty, span })
2048 (Border::JustBefore(n), Border::AfterMax) => {
2049 Some(IntRange { range: n..=u128::MAX, ty, span })
2051 (Border::AfterMax, _) => None,
2056 Slice(Slice { array_len, kind: VarLen(self_prefix, self_suffix) }) => {
2057 // The exhaustiveness-checking paper does not include any details on
2058 // checking variable-length slice patterns. However, they are matched
2059 // by an infinite collection of fixed-length array patterns.
2061 // Checking the infinite set directly would take an infinite amount
2062 // of time. However, it turns out that for each finite set of
2063 // patterns `P`, all sufficiently large array lengths are equivalent:
2065 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
2066 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
2067 // `sₘ` for each sufficiently-large length `m` that applies to exactly
2068 // the same subset of `P`.
2070 // Because of that, each witness for reachability-checking from one
2071 // of the sufficiently-large lengths can be transformed to an
2072 // equally-valid witness from any other length, so we only have
2073 // to check slice lengths from the "minimal sufficiently-large length"
2076 // Note that the fact that there is a *single* `sₘ` for each `m`
2077 // not depending on the specific pattern in `P` is important: if
2078 // you look at the pair of patterns
2081 // Then any slice of length ≥1 that matches one of these two
2082 // patterns can be trivially turned to a slice of any
2083 // other length ≥1 that matches them and vice-versa - for
2084 // but the slice from length 2 `[false, true]` that matches neither
2085 // of these patterns can't be turned to a slice from length 1 that
2086 // matches neither of these patterns, so we have to consider
2087 // slices from length 2 there.
2089 // Now, to see that that length exists and find it, observe that slice
2090 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
2091 // "variable-length" patterns (`[_, .., _]`).
2093 // For fixed-length patterns, all slices with lengths *longer* than
2094 // the pattern's length have the same outcome (of not matching), so
2095 // as long as `L` is greater than the pattern's length we can pick
2096 // any `sₘ` from that length and get the same result.
2098 // For variable-length patterns, the situation is more complicated,
2099 // because as seen above the precise value of `sₘ` matters.
2101 // However, for each variable-length pattern `p` with a prefix of length
2102 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
2103 // `slₚ` elements are examined.
2105 // Therefore, as long as `L` is positive (to avoid concerns about empty
2106 // types), all elements after the maximum prefix length and before
2107 // the maximum suffix length are not examined by any variable-length
2108 // pattern, and therefore can be added/removed without affecting
2109 // them - creating equivalent patterns from any sufficiently-large
2112 // Of course, if fixed-length patterns exist, we must be sure
2113 // that our length is large enough to miss them all, so
2114 // we can pick `L = max(max(FIXED_LEN)+1, max(PREFIX_LEN) + max(SUFFIX_LEN))`
2116 // for example, with the above pair of patterns, all elements
2117 // but the first and last can be added/removed, so any
2118 // witness of length ≥2 (say, `[false, false, true]`) can be
2119 // turned to a witness from any other length ≥2.
2121 let mut max_prefix_len = self_prefix;
2122 let mut max_suffix_len = self_suffix;
2123 let mut max_fixed_len = 0;
2126 matrix.heads().filter_map(|pat| pat_constructor(tcx, param_env, pat));
2127 for ctor in head_ctors {
2129 Slice(slice) => match slice.pattern_kind() {
2131 max_fixed_len = cmp::max(max_fixed_len, len);
2133 VarLen(prefix, suffix) => {
2134 max_prefix_len = cmp::max(max_prefix_len, prefix);
2135 max_suffix_len = cmp::max(max_suffix_len, suffix);
2142 // For diagnostics, we keep the prefix and suffix lengths separate, so in the case
2143 // where `max_fixed_len + 1` is the largest, we adapt `max_prefix_len` accordingly,
2144 // so that `L = max_prefix_len + max_suffix_len`.
2145 if max_fixed_len + 1 >= max_prefix_len + max_suffix_len {
2146 // The subtraction can't overflow thanks to the above check.
2147 // The new `max_prefix_len` is also guaranteed to be larger than its previous
2149 max_prefix_len = max_fixed_len + 1 - max_suffix_len;
2154 let kind = if max_prefix_len + max_suffix_len < len {
2155 VarLen(max_prefix_len, max_suffix_len)
2159 split_ctors.push(Slice(Slice { array_len, kind }));
2162 // `ctor` originally covered the range `(self_prefix +
2163 // self_suffix..infinity)`. We now split it into two: lengths smaller than
2164 // `max_prefix_len + max_suffix_len` are treated independently as
2165 // fixed-lengths slices, and lengths above are captured by a final VarLen
2168 (self_prefix + self_suffix..max_prefix_len + max_suffix_len)
2169 .map(|len| Slice(Slice { array_len, kind: FixedLen(len) })),
2171 split_ctors.push(Slice(Slice {
2173 kind: VarLen(max_prefix_len, max_suffix_len),
2178 // Any other constructor can be used unchanged.
2179 _ => split_ctors.push(ctor),
2183 debug!("split_grouped_constructors(..)={:#?}", split_ctors);
2187 fn lint_overlapping_patterns(
2189 hir_id: Option<HirId>,
2190 ctor_range: IntRange<'tcx>,
2192 overlaps: Vec<IntRange<'tcx>>,
2194 if let (true, Some(hir_id)) = (!overlaps.is_empty(), hir_id) {
2195 let mut err = tcx.struct_span_lint_hir(
2196 lint::builtin::OVERLAPPING_PATTERNS,
2199 "multiple patterns covering the same range",
2201 err.span_label(ctor_range.span, "overlapping patterns");
2202 for int_range in overlaps {
2203 // Use the real type for user display of the ranges:
2207 "this range overlaps on `{}`",
2208 IntRange { range: int_range.range, ty, span: DUMMY_SP }.to_pat(tcx),
2216 fn constructor_covered_by_range<'tcx>(
2218 param_env: ty::ParamEnv<'tcx>,
2219 ctor: &Constructor<'tcx>,
2222 if let Single = ctor {
2226 let (pat_from, pat_to, pat_end, ty) = match *pat.kind {
2227 PatKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
2228 PatKind::Range(PatRange { lo, hi, end }) => (lo, hi, end, lo.ty),
2229 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
2231 let (ctor_from, ctor_to, ctor_end) = match *ctor {
2232 ConstantValue(value) => (value, value, RangeEnd::Included),
2233 FloatRange(from, to, ctor_end) => (from, to, ctor_end),
2234 _ => bug!("`constructor_covered_by_range` called with {:?}", ctor),
2236 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, pat_from, pat_to, ty);
2238 let to = compare_const_vals(tcx, ctor_to, pat_to, param_env, ty)?;
2239 let from = compare_const_vals(tcx, ctor_from, pat_from, param_env, ty)?;
2240 let intersects = (from == Ordering::Greater || from == Ordering::Equal)
2241 && (to == Ordering::Less || (pat_end == ctor_end && to == Ordering::Equal));
2242 if intersects { Some(()) } else { None }
2245 fn patterns_for_variant<'p, 'tcx>(
2246 cx: &mut MatchCheckCtxt<'p, 'tcx>,
2247 subpatterns: &'p [FieldPat<'tcx>],
2248 ctor_wild_subpatterns: &'p [Pat<'tcx>],
2249 is_non_exhaustive: bool,
2250 ) -> PatStack<'p, 'tcx> {
2251 let mut result: SmallVec<_> = ctor_wild_subpatterns.iter().collect();
2253 for subpat in subpatterns {
2254 if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
2255 result[subpat.field.index()] = &subpat.pattern;
2260 "patterns_for_variant({:#?}, {:#?}) = {:#?}",
2261 subpatterns, ctor_wild_subpatterns, result
2263 PatStack::from_vec(result)
2266 /// This is the main specialization step. It expands the pattern
2267 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
2268 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
2269 /// Returns `None` if the pattern does not have the given constructor.
2271 /// OTOH, slice patterns with a subslice pattern (tail @ ..) can be expanded into multiple
2272 /// different patterns.
2273 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
2274 /// fields filled with wild patterns.
2275 fn specialize_one_pattern<'p, 'tcx>(
2276 cx: &mut MatchCheckCtxt<'p, 'tcx>,
2278 constructor: &Constructor<'tcx>,
2279 ctor_wild_subpatterns: &'p [Pat<'tcx>],
2280 ) -> Option<PatStack<'p, 'tcx>> {
2281 if let NonExhaustive = constructor {
2282 // Only a wildcard pattern can match the special extra constructor
2283 return if pat.is_wildcard() { Some(PatStack::default()) } else { None };
2286 let result = match *pat.kind {
2287 PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
2289 PatKind::Binding { .. } | PatKind::Wild => Some(ctor_wild_subpatterns.iter().collect()),
2291 PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
2292 let ref variant = adt_def.variants[variant_index];
2293 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(pat.ty);
2294 Some(Variant(variant.def_id))
2295 .filter(|variant_constructor| variant_constructor == constructor)
2297 patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, is_non_exhaustive)
2301 PatKind::Leaf { ref subpatterns } => {
2302 Some(patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, false))
2305 PatKind::Deref { ref subpattern } => Some(PatStack::from_pattern(subpattern)),
2307 PatKind::Constant { value } if constructor.is_slice() => {
2308 // We extract an `Option` for the pointer because slices of zero
2309 // elements don't necessarily point to memory, they are usually
2310 // just integers. The only time they should be pointing to memory
2311 // is when they are subslices of nonzero slices.
2312 let (alloc, offset, n, ty) = match value.ty.kind {
2313 ty::Array(t, n) => match value.val {
2314 ty::ConstKind::Value(ConstValue::ByRef { offset, alloc, .. }) => {
2315 (alloc, offset, n.eval_usize(cx.tcx, cx.param_env), t)
2317 _ => span_bug!(pat.span, "array pattern is {:?}", value,),
2321 ty::ConstKind::Value(ConstValue::Slice { data, start, end }) => {
2322 (data, Size::from_bytes(start as u64), (end - start) as u64, t)
2324 ty::ConstKind::Value(ConstValue::ByRef { .. }) => {
2325 // FIXME(oli-obk): implement `deref` for `ConstValue`
2330 "slice pattern constant must be scalar pair but is {:?}",
2337 "unexpected const-val {:?} with ctor {:?}",
2342 if ctor_wild_subpatterns.len() as u64 == n {
2343 // convert a constant slice/array pattern to a list of patterns.
2344 let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
2345 let ptr = Pointer::new(AllocId(0), offset);
2348 let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
2349 let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
2350 let scalar = scalar.not_undef().ok()?;
2351 let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
2353 Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
2354 Some(&*cx.pattern_arena.alloc(pattern))
2362 PatKind::Constant { .. } | PatKind::Range { .. } => {
2363 // If the constructor is a:
2364 // - Single value: add a row if the pattern contains the constructor.
2365 // - Range: add a row if the constructor intersects the pattern.
2366 if let IntRange(ctor) = constructor {
2367 match IntRange::from_pat(cx.tcx, cx.param_env, pat) {
2368 Some(pat) => ctor.intersection(cx.tcx, &pat).map(|_| {
2369 // Constructor splitting should ensure that all intersections we encounter
2370 // are actually inclusions.
2371 assert!(ctor.is_subrange(&pat));
2377 // Fallback for non-ranges and ranges that involve
2378 // floating-point numbers, which are not conveniently handled
2379 // by `IntRange`. For these cases, the constructor may not be a
2380 // range so intersection actually devolves into being covered
2382 constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat)
2383 .map(|()| PatStack::default())
2387 PatKind::Array { ref prefix, ref slice, ref suffix }
2388 | PatKind::Slice { ref prefix, ref slice, ref suffix } => match *constructor {
2390 let pat_len = prefix.len() + suffix.len();
2391 if let Some(slice_count) = ctor_wild_subpatterns.len().checked_sub(pat_len) {
2392 if slice_count == 0 || slice.is_some() {
2397 ctor_wild_subpatterns
2401 .chain(suffix.iter()),
2412 ConstantValue(cv) => {
2413 match slice_pat_covered_by_const(
2422 Ok(true) => Some(PatStack::default()),
2424 Err(ErrorReported) => None,
2427 _ => span_bug!(pat.span, "unexpected ctor {:?} for slice pat", constructor),
2430 PatKind::Or { .. } => bug!("Or-pattern should have been expanded earlier on."),
2432 debug!("specialize({:#?}, {:#?}) = {:#?}", pat, ctor_wild_subpatterns, result);