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::captures::Captures;
233 use rustc_index::vec::Idx;
235 use super::{compare_const_vals, PatternFoldable, PatternFolder};
236 use super::{FieldPat, Pat, PatKind, PatRange};
238 use rustc_attr::{SignedInt, UnsignedInt};
239 use rustc_errors::ErrorReported;
240 use rustc_hir::def_id::DefId;
241 use rustc_hir::{HirId, RangeEnd};
242 use rustc_middle::mir::interpret::{truncate, AllocId, ConstValue, Pointer, Scalar};
243 use rustc_middle::mir::Field;
244 use rustc_middle::ty::layout::IntegerExt;
245 use rustc_middle::ty::{self, Const, Ty, TyCtxt, TypeFoldable};
246 use rustc_session::lint;
247 use rustc_span::{Span, DUMMY_SP};
248 use rustc_target::abi::{Integer, Size, VariantIdx};
250 use arena::TypedArena;
252 use smallvec::{smallvec, SmallVec};
253 use std::borrow::Cow;
254 use std::cmp::{self, max, min, Ordering};
255 use std::convert::TryInto;
257 use std::iter::{FromIterator, IntoIterator};
258 use std::ops::RangeInclusive;
260 crate fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pat<'tcx>) -> Pat<'tcx> {
261 LiteralExpander { tcx: cx.tcx, param_env: cx.param_env }.fold_pattern(&pat)
264 struct LiteralExpander<'tcx> {
266 param_env: ty::ParamEnv<'tcx>,
269 impl<'tcx> 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(p), x, y) if x == y => {
289 let alloc = self.tcx.global_alloc(p.alloc_id).unwrap_memory();
290 ConstValue::ByRef { alloc, offset: p.offset }
292 Scalar::Raw { .. } => {
293 let layout = self.tcx.layout_of(self.param_env.and(rty)).unwrap();
295 // Deref of a reference to a ZST is a nop.
296 ConstValue::Scalar(Scalar::zst())
298 // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;`
299 bug!("cannot deref {:#?}, {} -> {}", val, crty, rty);
304 // unsize array to slice if pattern is array but match value or other patterns are slice
305 (ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
308 data: self.tcx.global_alloc(p.alloc_id).unwrap_memory(),
309 start: p.offset.bytes().try_into().unwrap(),
310 end: n.eval_usize(self.tcx, ty::ParamEnv::empty()).try_into().unwrap(),
313 // fat pointers stay the same
314 (ConstValue::Slice { .. }, _, _)
315 | (_, ty::Slice(_), ty::Slice(_))
316 | (_, ty::Str, ty::Str) => val,
317 // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
318 _ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
323 impl<'tcx> PatternFolder<'tcx> for LiteralExpander<'tcx> {
324 fn fold_pattern(&mut self, pat: &Pat<'tcx>) -> Pat<'tcx> {
325 debug!("fold_pattern {:?} {:?} {:?}", pat, pat.ty.kind, pat.kind);
326 match (&pat.ty.kind, &*pat.kind) {
332 val: ty::ConstKind::Value(val),
333 ty: ty::TyS { kind: ty::Ref(_, crty, _), .. },
339 kind: box PatKind::Deref {
343 kind: box PatKind::Constant {
344 value: Const::from_value(
346 self.fold_const_value_deref(*val, rty, crty),
357 value: Const { val, ty: ty::TyS { kind: ty::Ref(_, crty, _), .. } },
359 ) => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
361 (_, &PatKind::Binding { subpattern: Some(ref s), .. }) => s.fold_with(self),
362 (_, &PatKind::AscribeUserType { subpattern: ref s, .. }) => s.fold_with(self),
363 _ => pat.super_fold_with(self),
368 impl<'tcx> Pat<'tcx> {
369 pub(super) fn is_wildcard(&self) -> bool {
371 PatKind::Binding { subpattern: None, .. } | PatKind::Wild => true,
377 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
379 #[derive(Debug, Clone)]
380 crate struct PatStack<'p, 'tcx>(SmallVec<[&'p Pat<'tcx>; 2]>);
382 impl<'p, 'tcx> PatStack<'p, 'tcx> {
383 crate fn from_pattern(pat: &'p Pat<'tcx>) -> Self {
384 PatStack(smallvec![pat])
387 fn from_vec(vec: SmallVec<[&'p Pat<'tcx>; 2]>) -> Self {
391 fn from_slice(s: &[&'p Pat<'tcx>]) -> Self {
392 PatStack(SmallVec::from_slice(s))
395 fn is_empty(&self) -> bool {
399 fn len(&self) -> usize {
403 fn head(&self) -> &'p Pat<'tcx> {
407 fn to_tail(&self) -> Self {
408 PatStack::from_slice(&self.0[1..])
411 fn iter(&self) -> impl Iterator<Item = &Pat<'tcx>> {
412 self.0.iter().copied()
415 // If the first pattern is an or-pattern, expand this pattern. Otherwise, return `None`.
416 fn expand_or_pat(&self) -> Option<Vec<Self>> {
419 } else if let PatKind::Or { pats } = &*self.head().kind {
423 let mut new_patstack = PatStack::from_pattern(pat);
424 new_patstack.0.extend_from_slice(&self.0[1..]);
434 /// This computes `D(self)`. See top of the file for explanations.
435 fn specialize_wildcard(&self) -> Option<Self> {
436 if self.head().is_wildcard() { Some(self.to_tail()) } else { None }
439 /// This computes `S(constructor, self)`. See top of the file for explanations.
440 fn specialize_constructor(
442 cx: &mut MatchCheckCtxt<'p, 'tcx>,
443 constructor: &Constructor<'tcx>,
444 ctor_wild_subpatterns: &Fields<'p, 'tcx>,
445 ) -> Option<PatStack<'p, 'tcx>> {
447 specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns)?;
448 Some(new_fields.push_on_patstack(&self.0[1..]))
452 impl<'p, 'tcx> Default for PatStack<'p, 'tcx> {
453 fn default() -> Self {
454 PatStack(smallvec![])
458 impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
459 fn from_iter<T>(iter: T) -> Self
461 T: IntoIterator<Item = &'p Pat<'tcx>>,
463 PatStack(iter.into_iter().collect())
469 crate struct Matrix<'p, 'tcx>(Vec<PatStack<'p, 'tcx>>);
471 impl<'p, 'tcx> Matrix<'p, 'tcx> {
472 crate fn empty() -> Self {
476 /// Pushes a new row to the matrix. If the row starts with an or-pattern, this expands it.
477 crate fn push(&mut self, row: PatStack<'p, 'tcx>) {
478 if let Some(rows) = row.expand_or_pat() {
480 // We recursively expand the or-patterns of the new rows.
481 // This is necessary as we might have `0 | (1 | 2)` or e.g., `x @ 0 | x @ (1 | 2)`.
489 /// Iterate over the first component of each row
490 fn heads<'a>(&'a self) -> impl Iterator<Item = &'a Pat<'tcx>> + Captures<'p> {
491 self.0.iter().map(|r| r.head())
494 /// This computes `D(self)`. See top of the file for explanations.
495 fn specialize_wildcard(&self) -> Self {
496 self.0.iter().filter_map(|r| r.specialize_wildcard()).collect()
499 /// This computes `S(constructor, self)`. See top of the file for explanations.
500 fn specialize_constructor(
502 cx: &mut MatchCheckCtxt<'p, 'tcx>,
503 constructor: &Constructor<'tcx>,
504 ctor_wild_subpatterns: &Fields<'p, 'tcx>,
505 ) -> Matrix<'p, 'tcx> {
508 .filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
513 /// Pretty-printer for matrices of patterns, example:
516 /// +++++++++++++++++++++++++++++
518 /// +++++++++++++++++++++++++++++
519 /// + true + [First] +
520 /// +++++++++++++++++++++++++++++
521 /// + true + [Second(true)] +
522 /// +++++++++++++++++++++++++++++
524 /// +++++++++++++++++++++++++++++
525 /// + _ + [_, _, tail @ ..] +
526 /// +++++++++++++++++++++++++++++
527 impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
528 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
531 let &Matrix(ref m) = self;
532 let pretty_printed_matrix: Vec<Vec<String>> =
533 m.iter().map(|row| row.iter().map(|pat| format!("{:?}", pat)).collect()).collect();
535 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
536 assert!(m.iter().all(|row| row.len() == column_count));
537 let column_widths: Vec<usize> = (0..column_count)
538 .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0))
541 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
542 let br = "+".repeat(total_width);
543 write!(f, "{}\n", br)?;
544 for row in pretty_printed_matrix {
546 for (column, pat_str) in row.into_iter().enumerate() {
548 write!(f, "{:1$}", pat_str, column_widths[column])?;
552 write!(f, "{}\n", br)?;
558 impl<'p, 'tcx> FromIterator<PatStack<'p, 'tcx>> for Matrix<'p, 'tcx> {
559 fn from_iter<T>(iter: T) -> Self
561 T: IntoIterator<Item = PatStack<'p, 'tcx>>,
563 let mut matrix = Matrix::empty();
565 // Using `push` ensures we correctly expand or-patterns.
572 crate struct MatchCheckCtxt<'a, 'tcx> {
573 crate tcx: TyCtxt<'tcx>,
574 /// The module in which the match occurs. This is necessary for
575 /// checking inhabited-ness of types because whether a type is (visibly)
576 /// inhabited can depend on whether it was defined in the current module or
577 /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty
578 /// outside it's module and should not be matchable with an empty match
581 crate param_env: ty::ParamEnv<'tcx>,
582 crate pattern_arena: &'a TypedArena<Pat<'tcx>>,
585 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
586 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
587 if self.tcx.features().exhaustive_patterns {
588 self.tcx.is_ty_uninhabited_from(self.module, ty, self.param_env)
594 /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
595 crate fn is_foreign_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
597 ty::Adt(def, ..) => {
598 def.is_enum() && def.is_variant_list_non_exhaustive() && !def.did.is_local()
605 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
607 /// Patterns of length `n` (`[x, y]`).
609 /// Patterns using the `..` notation (`[x, .., y]`).
610 /// Captures any array constructor of `length >= i + j`.
611 /// In the case where `array_len` is `Some(_)`,
612 /// this indicates that we only care about the first `i` and the last `j` values of the array,
613 /// and everything in between is a wildcard `_`.
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 /// A value can be decomposed into a constructor applied to some fields. This struct represents
715 /// the constructor. See also `Fields`.
717 /// `pat_constructor` retrieves the constructor corresponding to a pattern.
718 /// `specialize_one_pattern` returns the list of fields corresponding to a pattern, given a
719 /// constructor. `Constructor::apply` reconstructs the pattern from a pair of `Constructor` and
721 #[derive(Clone, Debug, PartialEq)]
722 enum Constructor<'tcx> {
723 /// The constructor for patterns that have a single constructor, like tuples, struct patterns
724 /// and fixed-length arrays.
729 ConstantValue(&'tcx ty::Const<'tcx>),
730 /// Ranges of integer literal values (`2`, `2..=5` or `2..5`).
731 IntRange(IntRange<'tcx>),
732 /// Ranges of floating-point literal values (`2.0..=5.2`).
733 FloatRange(&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>, RangeEnd),
734 /// Array and slice patterns.
736 /// Fake extra constructor for enums that aren't allowed to be matched exhaustively.
740 impl<'tcx> Constructor<'tcx> {
741 fn is_slice(&self) -> bool {
748 fn variant_index_for_adt<'a>(
750 cx: &MatchCheckCtxt<'a, 'tcx>,
751 adt: &'tcx ty::AdtDef,
754 Variant(id) => adt.variant_index_with_id(id),
756 assert!(!adt.is_enum());
759 ConstantValue(c) => cx.tcx.destructure_const(cx.param_env.and(c)).variant,
760 _ => bug!("bad constructor {:?} for adt {:?}", self, adt),
764 // Returns the set of constructors covered by `self` but not by
765 // anything in `other_ctors`.
766 fn subtract_ctors(&self, other_ctors: &Vec<Constructor<'tcx>>) -> Vec<Constructor<'tcx>> {
767 if other_ctors.is_empty() {
768 return vec![self.clone()];
772 // Those constructors can only match themselves.
773 Single | Variant(_) | ConstantValue(..) | FloatRange(..) => {
774 if other_ctors.iter().any(|c| c == self) { vec![] } else { vec![self.clone()] }
777 let mut other_slices = other_ctors
779 .filter_map(|c: &Constructor<'_>| match c {
780 Slice(slice) => Some(*slice),
781 // FIXME(oli-obk): implement `deref` for `ConstValue`
782 ConstantValue(..) => None,
783 _ => bug!("bad slice pattern constructor {:?}", c),
785 .map(Slice::value_kind);
787 match slice.value_kind() {
788 FixedLen(self_len) => {
789 if other_slices.any(|other_slice| other_slice.covers_length(self_len)) {
795 kind @ VarLen(..) => {
796 let mut remaining_slices = vec![kind];
798 // For each used slice, subtract from the current set of slices.
799 for other_slice in other_slices {
800 remaining_slices = remaining_slices
802 .flat_map(|remaining_slice| remaining_slice.subtract(other_slice))
805 // If the constructors that have been considered so far already cover
806 // the entire range of `self`, no need to look at more constructors.
807 if remaining_slices.is_empty() {
814 .map(|kind| Slice { array_len: slice.array_len, kind })
820 IntRange(self_range) => {
821 let mut remaining_ranges = vec![self_range.clone()];
822 for other_ctor in other_ctors {
823 if let IntRange(other_range) = other_ctor {
824 if other_range == self_range {
825 // If the `self` range appears directly in a `match` arm, we can
826 // eliminate it straight away.
827 remaining_ranges = vec![];
829 // Otherwise explicitly compute the remaining ranges.
830 remaining_ranges = other_range.subtract_from(remaining_ranges);
833 // If the ranges that have been considered so far already cover the entire
834 // range of values, we can return early.
835 if remaining_ranges.is_empty() {
841 // Convert the ranges back into constructors.
842 remaining_ranges.into_iter().map(IntRange).collect()
844 // This constructor is never covered by anything else
845 NonExhaustive => vec![NonExhaustive],
849 /// Apply a constructor to a list of patterns, yielding a new pattern. `pats`
850 /// must have as many elements as this constructor's arity.
852 /// This is roughly the inverse of `specialize_one_pattern`.
855 /// `self`: `Constructor::Single`
856 /// `ty`: `(u32, u32, u32)`
857 /// `pats`: `[10, 20, _]`
858 /// returns `(10, 20, _)`
860 /// `self`: `Constructor::Variant(Option::Some)`
861 /// `ty`: `Option<bool>`
862 /// `pats`: `[false]`
863 /// returns `Some(false)`
866 cx: &MatchCheckCtxt<'p, 'tcx>,
868 fields: Fields<'p, 'tcx>,
870 let mut subpatterns = fields.all_patterns();
872 let pat = match self {
873 Single | Variant(_) => match ty.kind {
874 ty::Adt(..) | ty::Tuple(..) => {
875 let subpatterns = subpatterns
877 .map(|(i, p)| FieldPat { field: Field::new(i), pattern: p })
880 if let ty::Adt(adt, substs) = ty.kind {
885 variant_index: self.variant_index_for_adt(cx, adt),
889 PatKind::Leaf { subpatterns }
892 PatKind::Leaf { subpatterns }
895 ty::Ref(..) => PatKind::Deref { subpattern: subpatterns.next().unwrap() },
896 ty::Slice(_) | ty::Array(..) => bug!("bad slice pattern {:?} {:?}", self, ty),
899 Slice(slice) => match slice.pattern_kind() {
901 PatKind::Slice { prefix: subpatterns.collect(), slice: None, suffix: vec![] }
903 VarLen(prefix, _) => {
904 let mut prefix: Vec<_> = subpatterns.by_ref().take(prefix as usize).collect();
905 if slice.array_len.is_some() {
906 // Improves diagnostics a bit: if the type is a known-size array, instead
907 // of reporting `[x, _, .., _, y]`, we prefer to report `[x, .., y]`.
908 // This is incorrect if the size is not known, since `[_, ..]` captures
909 // arrays of lengths `>= 1` whereas `[..]` captures any length.
910 while !prefix.is_empty() && prefix.last().unwrap().is_wildcard() {
914 let suffix: Vec<_> = if slice.array_len.is_some() {
916 subpatterns.skip_while(Pat::is_wildcard).collect()
918 subpatterns.collect()
920 let wild = Pat::wildcard_from_ty(ty);
921 PatKind::Slice { prefix, slice: Some(wild), suffix }
924 &ConstantValue(value) => PatKind::Constant { value },
925 &FloatRange(lo, hi, end) => PatKind::Range(PatRange { lo, hi, end }),
926 IntRange(range) => return range.to_pat(cx.tcx),
927 NonExhaustive => PatKind::Wild,
930 Pat { ty, span: DUMMY_SP, kind: Box::new(pat) }
933 /// Like `apply`, but where all the subpatterns are wildcards `_`.
934 fn apply_wildcards<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {
935 self.apply(cx, ty, Fields::wildcards(cx, self, ty))
939 #[derive(Debug, Copy, Clone)]
940 enum FilteredField<'p, 'tcx> {
945 impl<'p, 'tcx> FilteredField<'p, 'tcx> {
946 fn kept(self) -> Option<&'p Pat<'tcx>> {
948 FilteredField::Kept(p) => Some(p),
949 FilteredField::Hidden(_) => None,
953 fn to_pattern(self) -> Pat<'tcx> {
955 FilteredField::Kept(p) => p.clone(),
956 FilteredField::Hidden(ty) => Pat::wildcard_from_ty(ty),
961 /// A value can be decomposed into a constructor applied to some fields. This struct represents
962 /// those fields, generalized to allow patterns in each field. See also `Constructor`.
964 /// If a private or `non_exhaustive` field is uninhabited, the code mustn't observe that it is
965 /// uninhabited. For that, we filter these fields out of the matrix. This is subtle because we
966 /// still need to have those fields back when going to/from a `Pat`. Mot of this is handled
967 /// automatically in `Fields`, but when constructing or deconstructing fields you need to use the
968 /// correct method. As a rule, when going to/from the matrix, use the filtered field list; when
969 /// going to/from `Pat`, use the full field list.
970 /// This filtering is uncommon in practice, because uninhabited fields are rarely used.
971 #[derive(Debug, Clone)]
972 enum Fields<'p, 'tcx> {
973 /// Lists of patterns that don't contain any filtered fields.
974 Slice(&'p [Pat<'tcx>]),
975 Vec(SmallVec<[&'p Pat<'tcx>; 2]>),
976 /// Patterns where some of the fields need to be hidden.
977 Filtered(SmallVec<[FilteredField<'p, 'tcx>; 2]>),
980 impl<'p, 'tcx> Fields<'p, 'tcx> {
985 /// Construct a new `Fields` from the given pattern. Must not be used if the pattern is a field
986 /// of a struct/tuple/variant.
987 fn from_single_pattern(pat: &'p Pat<'tcx>) -> Self {
988 Fields::Slice(std::slice::from_ref(pat))
991 /// Construct a new `Fields` from the given patterns. You must be sure those patterns can't
992 /// contain fields that need to be filtered out. When in doubt, prefer `replace_fields`.
993 fn from_slice_unfiltered(pats: &'p [Pat<'tcx>]) -> Self {
997 /// Convenience; internal use.
998 fn wildcards_from_tys(
999 cx: &MatchCheckCtxt<'p, 'tcx>,
1000 tys: impl IntoIterator<Item = Ty<'tcx>>,
1002 let wilds = tys.into_iter().map(Pat::wildcard_from_ty);
1003 let pats = cx.pattern_arena.alloc_from_iter(wilds);
1007 /// Creates a new list of wildcard fields for a given constructor.
1009 cx: &MatchCheckCtxt<'p, 'tcx>,
1010 constructor: &Constructor<'tcx>,
1013 debug!("Fields::wildcards({:#?}, {:?})", constructor, ty);
1014 let wildcard_from_ty = |ty| &*cx.pattern_arena.alloc(Pat::wildcard_from_ty(ty));
1017 Single | Variant(_) => match ty.kind {
1018 ty::Tuple(ref fs) => {
1019 Fields::wildcards_from_tys(cx, fs.into_iter().map(|ty| ty.expect_ty()))
1021 ty::Ref(_, rty, _) => Fields::from_single_pattern(wildcard_from_ty(rty)),
1022 ty::Adt(adt, substs) => {
1024 // Use T as the sub pattern type of Box<T>.
1025 Fields::from_single_pattern(wildcard_from_ty(substs.type_at(0)))
1027 let variant = &adt.variants[constructor.variant_index_for_adt(cx, adt)];
1028 // Whether we must not match the fields of this variant exhaustively.
1029 let is_non_exhaustive =
1030 variant.is_field_list_non_exhaustive() && !adt.did.is_local();
1031 let field_tys = variant.fields.iter().map(|field| field.ty(cx.tcx, substs));
1032 // In the following cases, we don't need to filter out any fields. This is
1033 // the vast majority of real cases, since uninhabited fields are uncommon.
1034 let has_no_hidden_fields = (adt.is_enum() && !is_non_exhaustive)
1035 || !field_tys.clone().any(|ty| cx.is_uninhabited(ty));
1037 if has_no_hidden_fields {
1038 Fields::wildcards_from_tys(cx, field_tys)
1040 let fields = variant
1044 let ty = field.ty(cx.tcx, substs);
1045 let is_visible = adt.is_enum()
1046 || field.vis.is_accessible_from(cx.module, cx.tcx);
1047 let is_uninhabited = cx.is_uninhabited(ty);
1049 // In the cases of either a `#[non_exhaustive]` field list
1050 // or a non-public field, we hide uninhabited fields in
1051 // order not to reveal the uninhabitedness of the whole
1053 if is_uninhabited && (!is_visible || is_non_exhaustive) {
1054 FilteredField::Hidden(ty)
1056 FilteredField::Kept(wildcard_from_ty(ty))
1060 Fields::Filtered(fields)
1064 _ => Fields::empty(),
1066 Slice(slice) => match ty.kind {
1067 ty::Slice(ty) | ty::Array(ty, _) => {
1068 let arity = slice.arity();
1069 Fields::wildcards_from_tys(cx, (0..arity).map(|_| ty))
1071 _ => bug!("bad slice pattern {:?} {:?}", constructor, ty),
1073 ConstantValue(..) | FloatRange(..) | IntRange(..) | NonExhaustive => Fields::empty(),
1077 fn len(&self) -> usize {
1079 Fields::Slice(pats) => pats.len(),
1080 Fields::Vec(pats) => pats.len(),
1081 Fields::Filtered(fields) => fields.iter().filter(|p| p.kept().is_some()).count(),
1085 /// Returns the complete list of patterns, including hidden fields.
1086 fn all_patterns(self) -> impl Iterator<Item = Pat<'tcx>> {
1087 let pats: SmallVec<[_; 2]> = match self {
1088 Fields::Slice(pats) => pats.iter().cloned().collect(),
1089 Fields::Vec(pats) => pats.into_iter().cloned().collect(),
1090 Fields::Filtered(fields) => {
1091 // We don't skip any fields here.
1092 fields.into_iter().map(|p| p.to_pattern()).collect()
1098 /// Overrides some of the fields with the provided patterns.
1099 fn replace_with_fieldpats(
1101 new_pats: impl IntoIterator<Item = &'p FieldPat<'tcx>>,
1103 self.replace_fields_indexed(
1104 new_pats.into_iter().map(|pat| (pat.field.index(), &pat.pattern)),
1108 /// Overrides some of the fields with the provided patterns.
1109 fn replace_fields_indexed(
1111 new_pats: impl IntoIterator<Item = (usize, &'p Pat<'tcx>)>,
1113 let mut fields = self.clone();
1114 if let Fields::Slice(pats) = fields {
1115 fields = Fields::Vec(pats.iter().collect());
1119 Fields::Vec(pats) => {
1120 for (i, pat) in new_pats {
1124 Fields::Filtered(fields) => {
1125 for (i, pat) in new_pats {
1126 if let FilteredField::Kept(p) = &mut fields[i] {
1131 Fields::Slice(_) => unreachable!(),
1136 /// Replaces contained fields with the given filtered list of patterns, e.g. taken from the
1137 /// matrix. There must be `len()` patterns in `pats`.
1140 cx: &MatchCheckCtxt<'p, 'tcx>,
1141 pats: impl IntoIterator<Item = Pat<'tcx>>,
1143 let pats: &[_] = cx.pattern_arena.alloc_from_iter(pats);
1146 Fields::Filtered(fields) => {
1147 let mut pats = pats.iter();
1148 let mut fields = fields.clone();
1149 for f in &mut fields {
1150 if let FilteredField::Kept(p) = f {
1151 // We take one input pattern for each `Kept` field, in order.
1152 *p = pats.next().unwrap();
1155 Fields::Filtered(fields)
1157 _ => Fields::Slice(pats),
1161 fn push_on_patstack(self, stack: &[&'p Pat<'tcx>]) -> PatStack<'p, 'tcx> {
1162 let pats: SmallVec<_> = match self {
1163 Fields::Slice(pats) => pats.iter().chain(stack.iter().copied()).collect(),
1164 Fields::Vec(mut pats) => {
1165 pats.extend_from_slice(stack);
1168 Fields::Filtered(fields) => {
1169 // We skip hidden fields here
1170 fields.into_iter().filter_map(|p| p.kept()).chain(stack.iter().copied()).collect()
1173 PatStack::from_vec(pats)
1177 #[derive(Clone, Debug)]
1178 crate enum Usefulness<'tcx, 'p> {
1179 /// Carries a list of unreachable subpatterns. Used only in the presence of or-patterns.
1180 Useful(Vec<&'p Pat<'tcx>>),
1181 /// Carries a list of witnesses of non-exhaustiveness.
1182 UsefulWithWitness(Vec<Witness<'tcx>>),
1186 impl<'tcx, 'p> Usefulness<'tcx, 'p> {
1187 fn new_useful(preference: WitnessPreference) -> Self {
1189 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
1190 LeaveOutWitness => Useful(vec![]),
1194 fn is_useful(&self) -> bool {
1201 fn apply_constructor(
1203 cx: &MatchCheckCtxt<'p, 'tcx>,
1204 ctor: &Constructor<'tcx>,
1206 ctor_wild_subpatterns: &Fields<'p, 'tcx>,
1209 UsefulWithWitness(witnesses) => UsefulWithWitness(
1212 .map(|witness| witness.apply_constructor(cx, &ctor, ty, ctor_wild_subpatterns))
1219 fn apply_wildcard(self, ty: Ty<'tcx>) -> Self {
1221 UsefulWithWitness(witnesses) => {
1222 let wild = Pat::wildcard_from_ty(ty);
1226 .map(|mut witness| {
1227 witness.0.push(wild.clone());
1237 fn apply_missing_ctors(
1239 cx: &MatchCheckCtxt<'_, 'tcx>,
1241 missing_ctors: &MissingConstructors<'tcx>,
1244 UsefulWithWitness(witnesses) => {
1245 let new_patterns: Vec<_> =
1246 missing_ctors.iter().map(|ctor| ctor.apply_wildcards(cx, ty)).collect();
1247 // Add the new patterns to each witness
1251 .flat_map(|witness| {
1252 new_patterns.iter().map(move |pat| {
1253 let mut witness = witness.clone();
1254 witness.0.push(pat.clone());
1266 #[derive(Copy, Clone, Debug)]
1267 crate enum WitnessPreference {
1272 #[derive(Copy, Clone, Debug)]
1273 struct PatCtxt<'tcx> {
1278 /// A witness of non-exhaustiveness for error reporting, represented
1279 /// as a list of patterns (in reverse order of construction) with
1280 /// wildcards inside to represent elements that can take any inhabitant
1281 /// of the type as a value.
1283 /// A witness against a list of patterns should have the same types
1284 /// and length as the pattern matched against. Because Rust `match`
1285 /// is always against a single pattern, at the end the witness will
1286 /// have length 1, but in the middle of the algorithm, it can contain
1287 /// multiple patterns.
1289 /// For example, if we are constructing a witness for the match against
1291 /// struct Pair(Option<(u32, u32)>, bool);
1293 /// match (p: Pair) {
1294 /// Pair(None, _) => {}
1295 /// Pair(_, false) => {}
1299 /// We'll perform the following steps:
1300 /// 1. Start with an empty witness
1301 /// `Witness(vec![])`
1302 /// 2. Push a witness `Some(_)` against the `None`
1303 /// `Witness(vec![Some(_)])`
1304 /// 3. Push a witness `true` against the `false`
1305 /// `Witness(vec![Some(_), true])`
1306 /// 4. Apply the `Pair` constructor to the witnesses
1307 /// `Witness(vec![Pair(Some(_), true)])`
1309 /// The final `Pair(Some(_), true)` is then the resulting witness.
1310 #[derive(Clone, Debug)]
1311 crate struct Witness<'tcx>(Vec<Pat<'tcx>>);
1313 impl<'tcx> Witness<'tcx> {
1314 crate fn single_pattern(self) -> Pat<'tcx> {
1315 assert_eq!(self.0.len(), 1);
1316 self.0.into_iter().next().unwrap()
1319 /// Constructs a partial witness for a pattern given a list of
1320 /// patterns expanded by the specialization step.
1322 /// When a pattern P is discovered to be useful, this function is used bottom-up
1323 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
1324 /// of values, V, where each value in that set is not covered by any previously
1325 /// used patterns and is covered by the pattern P'. Examples:
1327 /// left_ty: tuple of 3 elements
1328 /// pats: [10, 20, _] => (10, 20, _)
1330 /// left_ty: struct X { a: (bool, &'static str), b: usize}
1331 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
1332 fn apply_constructor<'p>(
1334 cx: &MatchCheckCtxt<'p, 'tcx>,
1335 ctor: &Constructor<'tcx>,
1337 ctor_wild_subpatterns: &Fields<'p, 'tcx>,
1340 let len = self.0.len();
1341 let arity = ctor_wild_subpatterns.len();
1342 let pats = self.0.drain((len - arity)..).rev();
1343 let fields = ctor_wild_subpatterns.replace_fields(cx, pats);
1344 ctor.apply(cx, ty, fields)
1353 /// This determines the set of all possible constructors of a pattern matching
1354 /// values of type `left_ty`. For vectors, this would normally be an infinite set
1355 /// but is instead bounded by the maximum fixed length of slice patterns in
1356 /// the column of patterns being analyzed.
1358 /// We make sure to omit constructors that are statically impossible. E.g., for
1359 /// `Option<!>`, we do not include `Some(_)` in the returned list of constructors.
1360 /// Invariant: this returns an empty `Vec` if and only if the type is uninhabited (as determined by
1361 /// `cx.is_uninhabited()`).
1362 fn all_constructors<'a, 'tcx>(
1363 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1365 ) -> Vec<Constructor<'tcx>> {
1366 debug!("all_constructors({:?})", pcx.ty);
1367 let make_range = |start, end| {
1369 // `unwrap()` is ok because we know the type is an integer.
1370 IntRange::from_range(cx.tcx, start, end, pcx.ty, &RangeEnd::Included, pcx.span)
1376 [true, false].iter().map(|&b| ConstantValue(ty::Const::from_bool(cx.tcx, b))).collect()
1378 ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx, cx.param_env).is_some() => {
1379 let len = len.eval_usize(cx.tcx, cx.param_env);
1380 if len != 0 && cx.is_uninhabited(sub_ty) {
1383 vec![Slice(Slice { array_len: Some(len), kind: VarLen(0, 0) })]
1386 // Treat arrays of a constant but unknown length like slices.
1387 ty::Array(ref sub_ty, _) | ty::Slice(ref sub_ty) => {
1388 let kind = if cx.is_uninhabited(sub_ty) { FixedLen(0) } else { VarLen(0, 0) };
1389 vec![Slice(Slice { array_len: None, kind })]
1391 ty::Adt(def, substs) if def.is_enum() => {
1392 let ctors: Vec<_> = if cx.tcx.features().exhaustive_patterns {
1393 // If `exhaustive_patterns` is enabled, we exclude variants known to be
1398 !v.uninhabited_from(cx.tcx, substs, def.adt_kind(), cx.param_env)
1399 .contains(cx.tcx, cx.module)
1401 .map(|v| Variant(v.def_id))
1404 def.variants.iter().map(|v| Variant(v.def_id)).collect()
1407 // If the enum is declared as `#[non_exhaustive]`, we treat it as if it had an
1408 // additional "unknown" constructor.
1409 // There is no point in enumerating all possible variants, because the user can't
1410 // actually match against them all themselves. So we always return only the fictitious
1412 // E.g., in an example like:
1414 // let err: io::ErrorKind = ...;
1416 // io::ErrorKind::NotFound => {},
1419 // we don't want to show every possible IO error, but instead have only `_` as the
1421 let is_declared_nonexhaustive = cx.is_foreign_non_exhaustive_enum(pcx.ty);
1423 // If `exhaustive_patterns` is disabled and our scrutinee is an empty enum, we treat it
1424 // as though it had an "unknown" constructor to avoid exposing its emptyness. Note that
1425 // an empty match will still be considered exhaustive because that case is handled
1426 // separately in `check_match`.
1427 let is_secretly_empty =
1428 def.variants.is_empty() && !cx.tcx.features().exhaustive_patterns;
1430 if is_secretly_empty || is_declared_nonexhaustive { vec![NonExhaustive] } else { ctors }
1434 // The valid Unicode Scalar Value ranges.
1435 make_range('\u{0000}' as u128, '\u{D7FF}' as u128),
1436 make_range('\u{E000}' as u128, '\u{10FFFF}' as u128),
1439 ty::Int(_) | ty::Uint(_)
1440 if pcx.ty.is_ptr_sized_integral()
1441 && !cx.tcx.features().precise_pointer_size_matching =>
1443 // `usize`/`isize` are not allowed to be matched exhaustively unless the
1444 // `precise_pointer_size_matching` feature is enabled. So we treat those types like
1445 // `#[non_exhaustive]` enums by returning a special unmatcheable constructor.
1449 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
1450 let min = 1u128 << (bits - 1);
1452 vec![make_range(min, max)]
1455 let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
1456 let max = truncate(u128::max_value(), size);
1457 vec![make_range(0, max)]
1460 if cx.is_uninhabited(pcx.ty) {
1469 /// An inclusive interval, used for precise integer exhaustiveness checking.
1470 /// `IntRange`s always store a contiguous range. This means that values are
1471 /// encoded such that `0` encodes the minimum value for the integer,
1472 /// regardless of the signedness.
1473 /// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
1474 /// This makes comparisons and arithmetic on interval endpoints much more
1475 /// straightforward. See `signed_bias` for details.
1477 /// `IntRange` is never used to encode an empty range or a "range" that wraps
1478 /// around the (offset) space: i.e., `range.lo <= range.hi`.
1479 #[derive(Clone, Debug)]
1480 struct IntRange<'tcx> {
1481 range: RangeInclusive<u128>,
1486 impl<'tcx> IntRange<'tcx> {
1488 fn is_integral(ty: Ty<'_>) -> bool {
1490 ty::Char | ty::Int(_) | ty::Uint(_) => true,
1495 fn is_singleton(&self) -> bool {
1496 self.range.start() == self.range.end()
1499 fn boundaries(&self) -> (u128, u128) {
1500 (*self.range.start(), *self.range.end())
1503 /// Don't treat `usize`/`isize` exhaustively unless the `precise_pointer_size_matching` feature
1505 fn treat_exhaustively(&self, tcx: TyCtxt<'tcx>) -> bool {
1506 !self.ty.is_ptr_sized_integral() || tcx.features().precise_pointer_size_matching
1510 fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
1512 ty::Char => Some((Size::from_bytes(4), 0)),
1514 let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
1515 Some((size, 1u128 << (size.bits() as u128 - 1)))
1517 ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
1525 param_env: ty::ParamEnv<'tcx>,
1526 value: &Const<'tcx>,
1528 ) -> Option<IntRange<'tcx>> {
1529 if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
1532 if let ty::ConstKind::Value(ConstValue::Scalar(scalar)) = value.val {
1533 // For this specific pattern we can skip a lot of effort and go
1534 // straight to the result, after doing a bit of checking. (We
1535 // could remove this branch and just fall through, which
1536 // is more general but much slower.)
1537 if let Ok(bits) = scalar.to_bits_or_ptr(target_size, &tcx) {
1541 // This is a more general form of the previous case.
1542 value.try_eval_bits(tcx, param_env, ty)
1544 let val = val ^ bias;
1545 Some(IntRange { range: val..=val, ty, span })
1559 ) -> Option<IntRange<'tcx>> {
1560 if Self::is_integral(ty) {
1561 // Perform a shift if the underlying types are signed,
1562 // which makes the interval arithmetic simpler.
1563 let bias = IntRange::signed_bias(tcx, ty);
1564 let (lo, hi) = (lo ^ bias, hi ^ bias);
1565 let offset = (*end == RangeEnd::Excluded) as u128;
1566 if lo > hi || (lo == hi && *end == RangeEnd::Excluded) {
1567 // This should have been caught earlier by E0030.
1568 bug!("malformed range pattern: {}..={}", lo, (hi - offset));
1570 Some(IntRange { range: lo..=(hi - offset), ty, span })
1578 param_env: ty::ParamEnv<'tcx>,
1580 ) -> Option<IntRange<'tcx>> {
1581 match pat_constructor(tcx, param_env, pat)? {
1582 IntRange(range) => Some(range),
1587 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
1588 fn signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
1591 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
1598 /// Returns a collection of ranges that spans the values covered by `ranges`, subtracted
1599 /// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
1600 fn subtract_from(&self, ranges: Vec<IntRange<'tcx>>) -> Vec<IntRange<'tcx>> {
1601 let mut remaining_ranges = vec![];
1603 let span = self.span;
1604 let (lo, hi) = self.boundaries();
1605 for subrange in ranges {
1606 let (subrange_lo, subrange_hi) = subrange.range.into_inner();
1607 if lo > subrange_hi || subrange_lo > hi {
1608 // The pattern doesn't intersect with the subrange at all,
1609 // so the subrange remains untouched.
1610 remaining_ranges.push(IntRange { range: subrange_lo..=subrange_hi, ty, span });
1612 if lo > subrange_lo {
1613 // The pattern intersects an upper section of the
1614 // subrange, so a lower section will remain.
1615 remaining_ranges.push(IntRange { range: subrange_lo..=(lo - 1), ty, span });
1617 if hi < subrange_hi {
1618 // The pattern intersects a lower section of the
1619 // subrange, so an upper section will remain.
1620 remaining_ranges.push(IntRange { range: (hi + 1)..=subrange_hi, ty, span });
1627 fn is_subrange(&self, other: &Self) -> bool {
1628 other.range.start() <= self.range.start() && self.range.end() <= other.range.end()
1631 fn intersection(&self, tcx: TyCtxt<'tcx>, other: &Self) -> Option<Self> {
1633 let (lo, hi) = self.boundaries();
1634 let (other_lo, other_hi) = other.boundaries();
1635 if self.treat_exhaustively(tcx) {
1636 if lo <= other_hi && other_lo <= hi {
1637 let span = other.span;
1638 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty, span })
1643 // If the range should not be treated exhaustively, fallback to checking for inclusion.
1644 if self.is_subrange(other) { Some(self.clone()) } else { None }
1648 fn suspicious_intersection(&self, other: &Self) -> bool {
1649 // `false` in the following cases:
1650 // 1 ---- // 1 ---------- // 1 ---- // 1 ----
1651 // 2 ---------- // 2 ---- // 2 ---- // 2 ----
1653 // The following are currently `false`, but could be `true` in the future (#64007):
1654 // 1 --------- // 1 ---------
1655 // 2 ---------- // 2 ----------
1657 // `true` in the following cases:
1658 // 1 ------- // 1 -------
1659 // 2 -------- // 2 -------
1660 let (lo, hi) = self.boundaries();
1661 let (other_lo, other_hi) = other.boundaries();
1662 lo == other_hi || hi == other_lo
1665 fn to_pat(&self, tcx: TyCtxt<'tcx>) -> Pat<'tcx> {
1666 let (lo, hi) = self.boundaries();
1668 let bias = IntRange::signed_bias(tcx, self.ty);
1669 let (lo, hi) = (lo ^ bias, hi ^ bias);
1671 let ty = ty::ParamEnv::empty().and(self.ty);
1672 let lo_const = ty::Const::from_bits(tcx, lo, ty);
1673 let hi_const = ty::Const::from_bits(tcx, hi, ty);
1675 let kind = if lo == hi {
1676 PatKind::Constant { value: lo_const }
1678 PatKind::Range(PatRange { lo: lo_const, hi: hi_const, end: RangeEnd::Included })
1681 // This is a brand new pattern, so we don't reuse `self.span`.
1682 Pat { ty: self.ty, span: DUMMY_SP, kind: Box::new(kind) }
1686 /// Ignore spans when comparing, they don't carry semantic information as they are only for lints.
1687 impl<'tcx> std::cmp::PartialEq for IntRange<'tcx> {
1688 fn eq(&self, other: &Self) -> bool {
1689 self.range == other.range && self.ty == other.ty
1693 // A struct to compute a set of constructors equivalent to `all_ctors \ used_ctors`.
1694 struct MissingConstructors<'tcx> {
1695 all_ctors: Vec<Constructor<'tcx>>,
1696 used_ctors: Vec<Constructor<'tcx>>,
1699 impl<'tcx> MissingConstructors<'tcx> {
1700 fn new(all_ctors: Vec<Constructor<'tcx>>, used_ctors: Vec<Constructor<'tcx>>) -> Self {
1701 MissingConstructors { all_ctors, used_ctors }
1704 fn into_inner(self) -> (Vec<Constructor<'tcx>>, Vec<Constructor<'tcx>>) {
1705 (self.all_ctors, self.used_ctors)
1708 fn is_empty(&self) -> bool {
1709 self.iter().next().is_none()
1711 /// Whether this contains all the constructors for the given type or only a
1713 fn all_ctors_are_missing(&self) -> bool {
1714 self.used_ctors.is_empty()
1717 /// Iterate over all_ctors \ used_ctors
1718 fn iter<'a>(&'a self) -> impl Iterator<Item = Constructor<'tcx>> + Captures<'a> {
1719 self.all_ctors.iter().flat_map(move |req_ctor| req_ctor.subtract_ctors(&self.used_ctors))
1723 impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
1724 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1725 let ctors: Vec<_> = self.iter().collect();
1726 write!(f, "{:?}", ctors)
1730 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html.
1731 /// The algorithm from the paper has been modified to correctly handle empty
1732 /// types. The changes are:
1733 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1734 /// continue to recurse over columns.
1735 /// (1) all_constructors will only return constructors that are statically
1736 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
1738 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1739 /// to a set of such vectors `m` - this is defined as there being a set of
1740 /// inputs that will match `v` but not any of the sets in `m`.
1742 /// All the patterns at each column of the `matrix ++ v` matrix must
1743 /// have the same type, except that wildcard (PatKind::Wild) patterns
1744 /// with type `TyErr` are also allowed, even if the "type of the column"
1745 /// is not `TyErr`. That is used to represent private fields, as using their
1746 /// real type would assert that they are inhabited.
1748 /// This is used both for reachability checking (if a pattern isn't useful in
1749 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1750 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1751 /// matrix isn't exhaustive).
1753 /// `is_under_guard` is used to inform if the pattern has a guard. If it
1754 /// has one it must not be inserted into the matrix. This shouldn't be
1755 /// relied on for soundness.
1756 crate fn is_useful<'p, 'tcx>(
1757 cx: &mut MatchCheckCtxt<'p, 'tcx>,
1758 matrix: &Matrix<'p, 'tcx>,
1759 v: &PatStack<'p, 'tcx>,
1760 witness_preference: WitnessPreference,
1762 is_under_guard: bool,
1764 ) -> Usefulness<'tcx, 'p> {
1765 let &Matrix(ref rows) = matrix;
1766 debug!("is_useful({:#?}, {:#?})", matrix, v);
1768 // The base case. We are pattern-matching on () and the return value is
1769 // based on whether our matrix has a row or not.
1770 // NOTE: This could potentially be optimized by checking rows.is_empty()
1771 // first and then, if v is non-empty, the return value is based on whether
1772 // the type of the tuple we're checking is inhabited or not.
1774 return if rows.is_empty() {
1775 Usefulness::new_useful(witness_preference)
1781 assert!(rows.iter().all(|r| r.len() == v.len()));
1783 // If the first pattern is an or-pattern, expand it.
1784 if let Some(vs) = v.expand_or_pat() {
1785 // We need to push the already-seen patterns into the matrix in order to detect redundant
1786 // branches like `Some(_) | Some(0)`. We also keep track of the unreachable subpatterns.
1787 let mut matrix = matrix.clone();
1788 let mut unreachable_pats = Vec::new();
1789 let mut any_is_useful = false;
1791 let res = is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false);
1794 any_is_useful = true;
1795 unreachable_pats.extend(pats);
1797 NotUseful => unreachable_pats.push(v.head()),
1798 UsefulWithWitness(_) => {
1799 bug!("Encountered or-pat in `v` during exhaustiveness checking")
1802 // If pattern has a guard don't add it to the matrix
1803 if !is_under_guard {
1807 return if any_is_useful { Useful(unreachable_pats) } else { NotUseful };
1810 let (ty, span) = matrix
1812 .map(|r| (r.ty, r.span))
1813 .find(|(ty, _)| !ty.references_error())
1814 .unwrap_or((v.head().ty, v.head().span));
1816 // TyErr is used to represent the type of wildcard patterns matching
1817 // against inaccessible (private) fields of structs, so that we won't
1818 // be able to observe whether the types of the struct's fields are
1821 // If the field is truly inaccessible, then all the patterns
1822 // matching against it must be wildcard patterns, so its type
1825 // However, if we are matching against non-wildcard patterns, we
1826 // need to know the real type of the field so we can specialize
1827 // against it. This primarily occurs through constants - they
1828 // can include contents for fields that are inaccessible at the
1829 // location of the match. In that case, the field's type is
1830 // inhabited - by the constant - so we can just use it.
1832 // FIXME: this might lead to "unstable" behavior with macro hygiene
1833 // introducing uninhabited patterns for inaccessible fields. We
1834 // need to figure out how to model that.
1839 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
1841 if let Some(constructor) = pat_constructor(cx.tcx, cx.param_env, v.head()) {
1842 debug!("is_useful - expanding constructor: {:#?}", constructor);
1843 split_grouped_constructors(
1854 is_useful_specialized(
1865 .find(|result| result.is_useful())
1866 .unwrap_or(NotUseful)
1868 debug!("is_useful - expanding wildcard");
1870 let used_ctors: Vec<Constructor<'_>> =
1871 matrix.heads().filter_map(|p| pat_constructor(cx.tcx, cx.param_env, p)).collect();
1872 debug!("used_ctors = {:#?}", used_ctors);
1873 // `all_ctors` are all the constructors for the given type, which
1874 // should all be represented (or caught with the wild pattern `_`).
1875 let all_ctors = all_constructors(cx, pcx);
1876 debug!("all_ctors = {:#?}", all_ctors);
1878 // `missing_ctors` is the set of constructors from the same type as the
1879 // first column of `matrix` that are matched only by wildcard patterns
1880 // from the first column.
1882 // Therefore, if there is some pattern that is unmatched by `matrix`,
1883 // it will still be unmatched if the first constructor is replaced by
1884 // any of the constructors in `missing_ctors`
1886 // Missing constructors are those that are not matched by any non-wildcard patterns in the
1887 // current column. We only fully construct them on-demand, because they're rarely used and
1889 let missing_ctors = MissingConstructors::new(all_ctors, used_ctors);
1891 debug!("missing_ctors.empty()={:#?}", missing_ctors.is_empty(),);
1893 if missing_ctors.is_empty() {
1894 let (all_ctors, _) = missing_ctors.into_inner();
1895 split_grouped_constructors(cx.tcx, cx.param_env, pcx, all_ctors, matrix, DUMMY_SP, None)
1898 is_useful_specialized(
1909 .find(|result| result.is_useful())
1910 .unwrap_or(NotUseful)
1912 let matrix = matrix.specialize_wildcard();
1913 let v = v.to_tail();
1915 is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false);
1917 // In this case, there's at least one "free"
1918 // constructor that is only matched against by
1919 // wildcard patterns.
1921 // There are 2 ways we can report a witness here.
1922 // Commonly, we can report all the "free"
1923 // constructors as witnesses, e.g., if we have:
1926 // enum Direction { N, S, E, W }
1927 // let Direction::N = ...;
1930 // we can report 3 witnesses: `S`, `E`, and `W`.
1932 // However, there is a case where we don't want
1933 // to do this and instead report a single `_` witness:
1934 // if the user didn't actually specify a constructor
1935 // in this arm, e.g., in
1937 // let x: (Direction, Direction, bool) = ...;
1938 // let (_, _, false) = x;
1940 // we don't want to show all 16 possible witnesses
1941 // `(<direction-1>, <direction-2>, true)` - we are
1942 // satisfied with `(_, _, true)`. In this case,
1943 // `used_ctors` is empty.
1944 // The exception is: if we are at the top-level, for example in an empty match, we
1945 // sometimes prefer reporting the list of constructors instead of just `_`.
1946 let report_ctors_rather_than_wildcard = is_top_level && !IntRange::is_integral(pcx.ty);
1947 if missing_ctors.all_ctors_are_missing() && !report_ctors_rather_than_wildcard {
1948 // All constructors are unused. Add a wild pattern
1949 // rather than each individual constructor.
1950 usefulness.apply_wildcard(pcx.ty)
1952 // Construct for each missing constructor a "wild" version of this
1953 // constructor, that matches everything that can be built with
1954 // it. For example, if `ctor` is a `Constructor::Variant` for
1955 // `Option::Some`, we get the pattern `Some(_)`.
1956 usefulness.apply_missing_ctors(cx, pcx.ty, &missing_ctors)
1962 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
1963 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1964 fn is_useful_specialized<'p, 'tcx>(
1965 cx: &mut MatchCheckCtxt<'p, 'tcx>,
1966 matrix: &Matrix<'p, 'tcx>,
1967 v: &PatStack<'p, 'tcx>,
1968 ctor: Constructor<'tcx>,
1970 witness_preference: WitnessPreference,
1972 is_under_guard: bool,
1973 ) -> Usefulness<'tcx, 'p> {
1974 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, ty);
1976 // We cache the result of `Fields::wildcards` because it is used a lot.
1977 let ctor_wild_subpatterns = Fields::wildcards(cx, &ctor, ty);
1978 let matrix = matrix.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns);
1979 v.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns)
1980 .map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false))
1981 .map(|u| u.apply_constructor(cx, &ctor, ty, &ctor_wild_subpatterns))
1982 .unwrap_or(NotUseful)
1985 /// Determines the constructor that the given pattern can be specialized to.
1986 /// Returns `None` in case of a catch-all, which can't be specialized.
1987 fn pat_constructor<'tcx>(
1989 param_env: ty::ParamEnv<'tcx>,
1991 ) -> Option<Constructor<'tcx>> {
1993 PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
1994 PatKind::Binding { .. } | PatKind::Wild => None,
1995 PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(Single),
1996 PatKind::Variant { adt_def, variant_index, .. } => {
1997 Some(Variant(adt_def.variants[variant_index].def_id))
1999 PatKind::Constant { value } => {
2000 if let Some(int_range) = IntRange::from_const(tcx, param_env, value, pat.span) {
2001 Some(IntRange(int_range))
2003 match (value.val, &value.ty.kind) {
2004 (_, ty::Array(_, n)) => {
2005 let len = n.eval_usize(tcx, param_env);
2006 Some(Slice(Slice { array_len: Some(len), kind: FixedLen(len) }))
2008 (ty::ConstKind::Value(ConstValue::Slice { start, end, .. }), ty::Slice(_)) => {
2009 let len = (end - start) as u64;
2010 Some(Slice(Slice { array_len: None, kind: FixedLen(len) }))
2012 // FIXME(oli-obk): implement `deref` for `ConstValue`
2013 // (ty::ConstKind::Value(ConstValue::ByRef { .. }), ty::Slice(_)) => { ... }
2014 _ => Some(ConstantValue(value)),
2018 PatKind::Range(PatRange { lo, hi, end }) => {
2020 if let Some(int_range) = IntRange::from_range(
2022 lo.eval_bits(tcx, param_env, lo.ty),
2023 hi.eval_bits(tcx, param_env, hi.ty),
2028 Some(IntRange(int_range))
2030 Some(FloatRange(lo, hi, end))
2033 PatKind::Array { ref prefix, ref slice, ref suffix }
2034 | PatKind::Slice { ref prefix, ref slice, ref suffix } => {
2035 let array_len = match pat.ty.kind {
2036 ty::Array(_, length) => Some(length.eval_usize(tcx, param_env)),
2037 ty::Slice(_) => None,
2038 _ => span_bug!(pat.span, "bad ty {:?} for slice pattern", pat.ty),
2040 let prefix = prefix.len() as u64;
2041 let suffix = suffix.len() as u64;
2043 if slice.is_some() { VarLen(prefix, suffix) } else { FixedLen(prefix + suffix) };
2044 Some(Slice(Slice { array_len, kind }))
2046 PatKind::Or { .. } => bug!("Or-pattern should have been expanded earlier on."),
2050 // checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
2051 // meaning all other types will compare unequal and thus equal patterns often do not cause the
2052 // second pattern to lint about unreachable match arms.
2053 fn slice_pat_covered_by_const<'tcx>(
2056 const_val: &'tcx ty::Const<'tcx>,
2057 prefix: &[Pat<'tcx>],
2058 slice: &Option<Pat<'tcx>>,
2059 suffix: &[Pat<'tcx>],
2060 param_env: ty::ParamEnv<'tcx>,
2061 ) -> Result<bool, ErrorReported> {
2062 let const_val_val = if let ty::ConstKind::Value(val) = const_val.val {
2066 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
2074 let data: &[u8] = match (const_val_val, &const_val.ty.kind) {
2075 (ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
2076 assert_eq!(*t, tcx.types.u8);
2077 let n = n.eval_usize(tcx, param_env);
2078 let ptr = Pointer::new(AllocId(0), offset);
2079 alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
2081 (ConstValue::Slice { data, start, end }, ty::Slice(t)) => {
2082 assert_eq!(*t, tcx.types.u8);
2083 let ptr = Pointer::new(AllocId(0), Size::from_bytes(start));
2084 data.get_bytes(&tcx, ptr, Size::from_bytes(end - start)).unwrap()
2086 // FIXME(oli-obk): create a way to extract fat pointers from ByRef
2087 (_, ty::Slice(_)) => return Ok(false),
2089 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
2097 let pat_len = prefix.len() + suffix.len();
2098 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
2102 for (ch, pat) in data[..prefix.len()]
2105 .chain(data[data.len() - suffix.len()..].iter().zip(suffix))
2107 if let box PatKind::Constant { value } = pat.kind {
2108 let b = value.eval_bits(tcx, param_env, pat.ty);
2109 assert_eq!(b as u8 as u128, b);
2119 /// For exhaustive integer matching, some constructors are grouped within other constructors
2120 /// (namely integer typed values are grouped within ranges). However, when specialising these
2121 /// constructors, we want to be specialising for the underlying constructors (the integers), not
2122 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
2123 /// mean creating a separate constructor for every single value in the range, which is clearly
2124 /// impractical. However, observe that for some ranges of integers, the specialisation will be
2125 /// identical across all values in that range (i.e., there are equivalence classes of ranges of
2126 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
2127 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
2128 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
2130 /// Our solution, therefore, is to split the range constructor into subranges at every single point
2131 /// the group of intersecting patterns changes (using the method described below).
2132 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
2133 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
2134 /// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
2135 /// need to be worried about matching over gargantuan ranges.
2137 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
2139 /// |------| |----------| |-------| ||
2140 /// |-------| |-------| |----| ||
2143 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
2145 /// |--|--|||-||||--||---|||-------| |-|||| ||
2147 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
2148 /// boundaries for each interval range, sort them, then create constructors for each new interval
2149 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
2150 /// merging operation depicted above.)
2152 /// `hir_id` is `None` when we're evaluating the wildcard pattern, do not lint for overlapping in
2153 /// ranges that case.
2155 /// This also splits variable-length slices into fixed-length slices.
2156 fn split_grouped_constructors<'p, 'tcx>(
2158 param_env: ty::ParamEnv<'tcx>,
2160 ctors: Vec<Constructor<'tcx>>,
2161 matrix: &Matrix<'p, 'tcx>,
2163 hir_id: Option<HirId>,
2164 ) -> Vec<Constructor<'tcx>> {
2166 let mut split_ctors = Vec::with_capacity(ctors.len());
2167 debug!("split_grouped_constructors({:#?}, {:#?})", matrix, ctors);
2169 for ctor in ctors.into_iter() {
2171 IntRange(ctor_range) if ctor_range.treat_exhaustively(tcx) => {
2172 // Fast-track if the range is trivial. In particular, don't do the overlapping
2174 if ctor_range.is_singleton() {
2175 split_ctors.push(IntRange(ctor_range));
2179 /// Represents a border between 2 integers. Because the intervals spanning borders
2180 /// must be able to cover every integer, we need to be able to represent
2181 /// 2^128 + 1 such borders.
2182 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
2188 // A function for extracting the borders of an integer interval.
2189 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
2190 let (lo, hi) = r.range.into_inner();
2191 let from = Border::JustBefore(lo);
2192 let to = match hi.checked_add(1) {
2193 Some(m) => Border::JustBefore(m),
2194 None => Border::AfterMax,
2196 vec![from, to].into_iter()
2199 // Collect the span and range of all the intersecting ranges to lint on likely
2200 // incorrect range patterns. (#63987)
2201 let mut overlaps = vec![];
2202 // `borders` is the set of borders between equivalence classes: each equivalence
2203 // class lies between 2 borders.
2204 let row_borders = matrix
2208 IntRange::from_pat(tcx, param_env, row.head()).map(|r| (r, row.len()))
2210 .flat_map(|(range, row_len)| {
2211 let intersection = ctor_range.intersection(tcx, &range);
2212 let should_lint = ctor_range.suspicious_intersection(&range);
2213 if let (Some(range), 1, true) = (&intersection, row_len, should_lint) {
2214 // FIXME: for now, only check for overlapping ranges on simple range
2215 // patterns. Otherwise with the current logic the following is detected
2217 // match (10u8, true) {
2218 // (0 ..= 125, false) => {}
2219 // (126 ..= 255, false) => {}
2220 // (0 ..= 255, true) => {}
2222 overlaps.push(range.clone());
2226 .flat_map(range_borders);
2227 let ctor_borders = range_borders(ctor_range.clone());
2228 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
2229 borders.sort_unstable();
2231 lint_overlapping_patterns(tcx, hir_id, ctor_range, ty, overlaps);
2233 // We're going to iterate through every adjacent pair of borders, making sure that
2234 // each represents an interval of nonnegative length, and convert each such
2235 // interval into a constructor.
2239 .filter_map(|window| match (window[0], window[1]) {
2240 (Border::JustBefore(n), Border::JustBefore(m)) => {
2242 Some(IntRange { range: n..=(m - 1), ty, span })
2247 (Border::JustBefore(n), Border::AfterMax) => {
2248 Some(IntRange { range: n..=u128::MAX, ty, span })
2250 (Border::AfterMax, _) => None,
2255 Slice(Slice { array_len, kind: VarLen(self_prefix, self_suffix) }) => {
2256 // The exhaustiveness-checking paper does not include any details on
2257 // checking variable-length slice patterns. However, they are matched
2258 // by an infinite collection of fixed-length array patterns.
2260 // Checking the infinite set directly would take an infinite amount
2261 // of time. However, it turns out that for each finite set of
2262 // patterns `P`, all sufficiently large array lengths are equivalent:
2264 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
2265 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
2266 // `sₘ` for each sufficiently-large length `m` that applies to exactly
2267 // the same subset of `P`.
2269 // Because of that, each witness for reachability-checking from one
2270 // of the sufficiently-large lengths can be transformed to an
2271 // equally-valid witness from any other length, so we only have
2272 // to check slice lengths from the "minimal sufficiently-large length"
2275 // Note that the fact that there is a *single* `sₘ` for each `m`
2276 // not depending on the specific pattern in `P` is important: if
2277 // you look at the pair of patterns
2280 // Then any slice of length ≥1 that matches one of these two
2281 // patterns can be trivially turned to a slice of any
2282 // other length ≥1 that matches them and vice-versa - for
2283 // but the slice from length 2 `[false, true]` that matches neither
2284 // of these patterns can't be turned to a slice from length 1 that
2285 // matches neither of these patterns, so we have to consider
2286 // slices from length 2 there.
2288 // Now, to see that that length exists and find it, observe that slice
2289 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
2290 // "variable-length" patterns (`[_, .., _]`).
2292 // For fixed-length patterns, all slices with lengths *longer* than
2293 // the pattern's length have the same outcome (of not matching), so
2294 // as long as `L` is greater than the pattern's length we can pick
2295 // any `sₘ` from that length and get the same result.
2297 // For variable-length patterns, the situation is more complicated,
2298 // because as seen above the precise value of `sₘ` matters.
2300 // However, for each variable-length pattern `p` with a prefix of length
2301 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
2302 // `slₚ` elements are examined.
2304 // Therefore, as long as `L` is positive (to avoid concerns about empty
2305 // types), all elements after the maximum prefix length and before
2306 // the maximum suffix length are not examined by any variable-length
2307 // pattern, and therefore can be added/removed without affecting
2308 // them - creating equivalent patterns from any sufficiently-large
2311 // Of course, if fixed-length patterns exist, we must be sure
2312 // that our length is large enough to miss them all, so
2313 // we can pick `L = max(max(FIXED_LEN)+1, max(PREFIX_LEN) + max(SUFFIX_LEN))`
2315 // for example, with the above pair of patterns, all elements
2316 // but the first and last can be added/removed, so any
2317 // witness of length ≥2 (say, `[false, false, true]`) can be
2318 // turned to a witness from any other length ≥2.
2320 let mut max_prefix_len = self_prefix;
2321 let mut max_suffix_len = self_suffix;
2322 let mut max_fixed_len = 0;
2325 matrix.heads().filter_map(|pat| pat_constructor(tcx, param_env, pat));
2326 for ctor in head_ctors {
2327 if let Slice(slice) = ctor {
2328 match slice.pattern_kind() {
2330 max_fixed_len = cmp::max(max_fixed_len, len);
2332 VarLen(prefix, suffix) => {
2333 max_prefix_len = cmp::max(max_prefix_len, prefix);
2334 max_suffix_len = cmp::max(max_suffix_len, suffix);
2340 // For diagnostics, we keep the prefix and suffix lengths separate, so in the case
2341 // where `max_fixed_len + 1` is the largest, we adapt `max_prefix_len` accordingly,
2342 // so that `L = max_prefix_len + max_suffix_len`.
2343 if max_fixed_len + 1 >= max_prefix_len + max_suffix_len {
2344 // The subtraction can't overflow thanks to the above check.
2345 // The new `max_prefix_len` is also guaranteed to be larger than its previous
2347 max_prefix_len = max_fixed_len + 1 - max_suffix_len;
2352 let kind = if max_prefix_len + max_suffix_len < len {
2353 VarLen(max_prefix_len, max_suffix_len)
2357 split_ctors.push(Slice(Slice { array_len, kind }));
2360 // `ctor` originally covered the range `(self_prefix +
2361 // self_suffix..infinity)`. We now split it into two: lengths smaller than
2362 // `max_prefix_len + max_suffix_len` are treated independently as
2363 // fixed-lengths slices, and lengths above are captured by a final VarLen
2366 (self_prefix + self_suffix..max_prefix_len + max_suffix_len)
2367 .map(|len| Slice(Slice { array_len, kind: FixedLen(len) })),
2369 split_ctors.push(Slice(Slice {
2371 kind: VarLen(max_prefix_len, max_suffix_len),
2376 // Any other constructor can be used unchanged.
2377 _ => split_ctors.push(ctor),
2381 debug!("split_grouped_constructors(..)={:#?}", split_ctors);
2385 fn lint_overlapping_patterns<'tcx>(
2387 hir_id: Option<HirId>,
2388 ctor_range: IntRange<'tcx>,
2390 overlaps: Vec<IntRange<'tcx>>,
2392 if let (true, Some(hir_id)) = (!overlaps.is_empty(), hir_id) {
2393 tcx.struct_span_lint_hir(
2394 lint::builtin::OVERLAPPING_PATTERNS,
2398 let mut err = lint.build("multiple patterns covering the same range");
2399 err.span_label(ctor_range.span, "overlapping patterns");
2400 for int_range in overlaps {
2401 // Use the real type for user display of the ranges:
2405 "this range overlaps on `{}`",
2406 IntRange { range: int_range.range, ty, span: DUMMY_SP }.to_pat(tcx),
2416 fn constructor_covered_by_range<'tcx>(
2418 param_env: ty::ParamEnv<'tcx>,
2419 ctor: &Constructor<'tcx>,
2422 if let Single = ctor {
2426 let (pat_from, pat_to, pat_end, ty) = match *pat.kind {
2427 PatKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
2428 PatKind::Range(PatRange { lo, hi, end }) => (lo, hi, end, lo.ty),
2429 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
2431 let (ctor_from, ctor_to, ctor_end) = match *ctor {
2432 ConstantValue(value) => (value, value, RangeEnd::Included),
2433 FloatRange(from, to, ctor_end) => (from, to, ctor_end),
2434 _ => bug!("`constructor_covered_by_range` called with {:?}", ctor),
2436 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, pat_from, pat_to, ty);
2438 let to = compare_const_vals(tcx, ctor_to, pat_to, param_env, ty)?;
2439 let from = compare_const_vals(tcx, ctor_from, pat_from, param_env, ty)?;
2440 let intersects = (from == Ordering::Greater || from == Ordering::Equal)
2441 && (to == Ordering::Less || (pat_end == ctor_end && to == Ordering::Equal));
2442 if intersects { Some(()) } else { None }
2445 /// This is the main specialization step. It expands the pattern
2446 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
2447 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
2448 /// Returns `None` if the pattern does not have the given constructor.
2450 /// OTOH, slice patterns with a subslice pattern (tail @ ..) can be expanded into multiple
2451 /// different patterns.
2452 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
2453 /// fields filled with wild patterns.
2455 /// This is roughly the inverse of `Constructor::apply`.
2456 fn specialize_one_pattern<'p, 'tcx>(
2457 cx: &mut MatchCheckCtxt<'p, 'tcx>,
2459 constructor: &Constructor<'tcx>,
2460 ctor_wild_subpatterns: &Fields<'p, 'tcx>,
2461 ) -> Option<Fields<'p, 'tcx>> {
2462 if let NonExhaustive = constructor {
2463 // Only a wildcard pattern can match the special extra constructor
2464 if !pat.is_wildcard() {
2467 return Some(Fields::empty());
2470 let result = match *pat.kind {
2471 PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
2473 PatKind::Binding { .. } | PatKind::Wild => Some(ctor_wild_subpatterns.clone()),
2475 PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
2476 let variant = &adt_def.variants[variant_index];
2477 if constructor != &Variant(variant.def_id) {
2480 Some(ctor_wild_subpatterns.replace_with_fieldpats(subpatterns))
2483 PatKind::Leaf { ref subpatterns } => {
2484 Some(ctor_wild_subpatterns.replace_with_fieldpats(subpatterns))
2487 PatKind::Deref { ref subpattern } => Some(Fields::from_single_pattern(subpattern)),
2489 PatKind::Constant { value } if constructor.is_slice() => {
2490 // We extract an `Option` for the pointer because slices of zero
2491 // elements don't necessarily point to memory, they are usually
2492 // just integers. The only time they should be pointing to memory
2493 // is when they are subslices of nonzero slices.
2494 let (alloc, offset, n, ty) = match value.ty.kind {
2495 ty::Array(t, n) => {
2496 let n = n.eval_usize(cx.tcx, cx.param_env);
2497 // Shortcut for `n == 0` where no matter what `alloc` and `offset` we produce,
2498 // the result would be exactly what we early return here.
2500 if ctor_wild_subpatterns.len() as u64 != n {
2503 return Some(Fields::empty());
2506 ty::ConstKind::Value(ConstValue::ByRef { offset, alloc, .. }) => {
2507 (Cow::Borrowed(alloc), offset, n, t)
2509 _ => span_bug!(pat.span, "array pattern is {:?}", value,),
2514 ty::ConstKind::Value(ConstValue::Slice { data, start, end }) => {
2515 let offset = Size::from_bytes(start);
2516 let n = (end - start) as u64;
2517 (Cow::Borrowed(data), offset, n, t)
2519 ty::ConstKind::Value(ConstValue::ByRef { .. }) => {
2520 // FIXME(oli-obk): implement `deref` for `ConstValue`
2525 "slice pattern constant must be scalar pair but is {:?}",
2532 "unexpected const-val {:?} with ctor {:?}",
2537 if ctor_wild_subpatterns.len() as u64 != n {
2541 // Convert a constant slice/array pattern to a list of patterns.
2542 let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
2543 let ptr = Pointer::new(AllocId(0), offset);
2544 let pats = cx.pattern_arena.alloc_from_iter((0..n).filter_map(|i| {
2545 let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
2546 let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
2547 let scalar = scalar.not_undef().ok()?;
2548 let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
2549 let pattern = Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
2552 // Ensure none of the dereferences failed.
2553 if pats.len() as u64 != n {
2556 Some(Fields::from_slice_unfiltered(pats))
2559 PatKind::Constant { .. } | PatKind::Range { .. } => {
2560 // If the constructor is a:
2561 // - Single value: add a row if the pattern contains the constructor.
2562 // - Range: add a row if the constructor intersects the pattern.
2563 if let IntRange(ctor) = constructor {
2564 let pat = IntRange::from_pat(cx.tcx, cx.param_env, pat)?;
2565 ctor.intersection(cx.tcx, &pat)?;
2566 // Constructor splitting should ensure that all intersections we encounter
2567 // are actually inclusions.
2568 assert!(ctor.is_subrange(&pat));
2570 // Fallback for non-ranges and ranges that involve
2571 // floating-point numbers, which are not conveniently handled
2572 // by `IntRange`. For these cases, the constructor may not be a
2573 // range so intersection actually devolves into being covered
2575 constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat)?;
2577 Some(Fields::empty())
2580 PatKind::Array { ref prefix, ref slice, ref suffix }
2581 | PatKind::Slice { ref prefix, ref slice, ref suffix } => match *constructor {
2583 // Number of subpatterns for this pattern
2584 let pat_len = prefix.len() + suffix.len();
2585 // Number of subpatterns for this constructor
2586 let arity = ctor_wild_subpatterns.len();
2588 if (slice.is_none() && arity != pat_len) || pat_len > arity {
2592 // Replace the prefix and the suffix with the given patterns, leaving wildcards in
2593 // the middle if there was a subslice pattern `..`.
2594 let prefix = prefix.iter().enumerate();
2595 let suffix = suffix.iter().enumerate().map(|(i, p)| (arity - suffix.len() + i, p));
2596 Some(ctor_wild_subpatterns.replace_fields_indexed(prefix.chain(suffix)))
2598 ConstantValue(cv) => {
2599 match slice_pat_covered_by_const(
2608 Ok(true) => Some(Fields::empty()),
2610 Err(ErrorReported) => None,
2613 _ => span_bug!(pat.span, "unexpected ctor {:?} for slice pat", constructor),
2616 PatKind::Or { .. } => bug!("Or-pattern should have been expanded earlier on."),
2618 debug!("specialize({:#?}, {:#?}) = {:#?}", pat, ctor_wild_subpatterns, result);