1 /// This file includes the logic for exhaustiveness and usefulness checking for
2 /// pattern-matching. Specifically, given a list of patterns for a type, we can
4 /// (a) the patterns cover every possible constructor for the type [exhaustiveness]
5 /// (b) each pattern is necessary [usefulness]
7 /// The algorithm implemented here is a modified version of the one described in:
8 /// http://moscova.inria.fr/~maranget/papers/warn/index.html
9 /// However, to save future implementors from reading the original paper, we
10 /// summarise the algorithm here to hopefully save time and be a little clearer
11 /// (without being so rigorous).
13 /// The core of the algorithm revolves about a "usefulness" check. In particular, we
14 /// are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
15 /// a matrix). `U(P, p)` represents whether, given an existing list of patterns
16 /// `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously-
17 /// uncovered values of the type).
19 /// If we have this predicate, then we can easily compute both exhaustiveness of an
20 /// entire set of patterns and the individual usefulness of each one.
21 /// (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
22 /// match doesn't increase the number of values we're matching)
23 /// (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a
24 /// pattern to those that have come before it doesn't increase the number of values
27 /// During the course of the algorithm, the rows of the matrix won't just be individual patterns,
28 /// but rather partially-deconstructed patterns in the form of a list of patterns. The paper
29 /// calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
32 /// For example, say we have the following:
34 /// // x: (Option<bool>, Result<()>)
36 /// (Some(true), _) => {}
37 /// (None, Err(())) => {}
38 /// (None, Err(_)) => {}
41 /// Here, the matrix `P` starts as:
43 /// [(Some(true), _)],
44 /// [(None, Err(()))],
47 /// We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
48 /// `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because
49 /// all the values it covers are already covered by row 2.
51 /// A list of patterns can be thought of as a stack, because we are mainly interested in the top of
52 /// the stack at any given point, and we can pop or apply constructors to get new pattern-stacks.
53 /// To match the paper, the top of the stack is at the beginning / on the left.
55 /// There are two important operations on pattern-stacks necessary to understand the algorithm:
56 /// 1. We can pop a given constructor off the top of a stack. This operation is called
57 /// `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or
58 /// `None`) and `p` a pattern-stack.
59 /// If the pattern on top of the stack can cover `c`, this removes the constructor and
60 /// pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns.
61 /// Otherwise the pattern-stack is discarded.
62 /// This essentially filters those pattern-stacks whose top covers the constructor `c` and
63 /// discards the others.
65 /// For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we
66 /// pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the
67 /// `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get
70 /// This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1`
71 /// on top of the stack, and we have four cases:
72 /// 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We
73 /// push onto the stack the arguments of this constructor, and return the result:
74 /// r_1, .., r_a, p_2, .., p_n
75 /// 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and
77 /// 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has
78 /// arguments (its arity), and return the resulting stack:
79 /// _, .., _, p_2, .., p_n
80 /// 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
82 /// S(c, (r_1, p_2, .., p_n))
83 /// S(c, (r_2, p_2, .., p_n))
85 /// 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is
87 /// This is used when we know there are missing constructor cases, but there might be
88 /// existing wildcard patterns, so to check the usefulness of the matrix, we have to check
89 /// all its *other* components.
91 /// It is computed as follows. We look at the pattern `p_1` on top of the stack,
92 /// and we have three cases:
93 /// 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing.
94 /// 1.2. `p_1 = _`. We return the rest of the stack:
96 /// 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
98 /// D((r_1, p_2, .., p_n))
99 /// D((r_2, p_2, .., p_n))
101 /// Note that the OR-patterns are not always used directly in Rust, but are used to derive the
102 /// exhaustive integer matching rules, so they're written here for posterity.
104 /// Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by
105 /// working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with
106 /// the given constructor, and popping a wildcard keeps those rows that start with a wildcard.
109 /// The algorithm for computing `U`
110 /// -------------------------------
111 /// The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
112 /// That means we're going to check the components from left-to-right, so the algorithm
113 /// operates principally on the first component of the matrix and new pattern-stack `p`.
114 /// This algorithm is realised in the `is_useful` function.
116 /// Base case. (`n = 0`, i.e., an empty tuple pattern)
117 /// - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`),
118 /// then `U(P, p)` is false.
119 /// - Otherwise, `P` must be empty, so `U(P, p)` is true.
121 /// Inductive step. (`n > 0`, i.e., whether there's at least one column
122 /// [which may then be expanded into further columns later])
123 /// We're going to match on the top of the new pattern-stack, `p_1`.
124 /// - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern.
125 /// Then, the usefulness of `p_1` can be reduced to whether it is useful when
126 /// we ignore all the patterns in the first column of `P` that involve other constructors.
127 /// This is where `S(c, P)` comes in:
128 /// `U(P, p) := U(S(c, P), S(c, p))`
129 /// This special case is handled in `is_useful_specialized`.
131 /// For example, if `P` is:
136 /// and `p` is [Some(false), 0], then we don't care about row 2 since we know `p` only
137 /// matches values that row 2 doesn't. For row 1 however, we need to dig into the
138 /// arguments of `Some` to know whether some new value is covered. So we compute
139 /// `U([[true, _]], [false, 0])`.
141 /// - If `p_1 == _`, then we look at the list of constructors that appear in the first
142 /// component of the rows of `P`:
143 /// + If there are some constructors that aren't present, then we might think that the
144 /// wildcard `_` is useful, since it covers those constructors that weren't covered
146 /// That's almost correct, but only works if there were no wildcards in those first
147 /// components. So we need to check that `p` is useful with respect to the rows that
148 /// start with a wildcard, if there are any. This is where `D` comes in:
149 /// `U(P, p) := U(D(P), D(p))`
151 /// For example, if `P` is:
154 /// [None, false, 1],
156 /// and `p` is [_, false, _], the `Some` constructor doesn't appear in `P`. So if we
157 /// only had row 2, we'd know that `p` is useful. However row 1 starts with a
158 /// wildcard, so we need to check whether `U([[true, _]], [false, 1])`.
160 /// + Otherwise, all possible constructors (for the relevant type) are present. In this
161 /// case we must check whether the wildcard pattern covers any unmatched value. For
162 /// that, we can think of the `_` pattern as a big OR-pattern that covers all
163 /// possible constructors. For `Option`, that would mean `_ = None | Some(_)` for
164 /// example. The wildcard pattern is useful in this case if it is useful when
165 /// specialized to one of the possible constructors. So we compute:
166 /// `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))`
168 /// For example, if `P` is:
173 /// and `p` is [_, false], both `None` and `Some` constructors appear in the first
174 /// components of `P`. We will therefore try popping both constructors in turn: we
175 /// compute U([[true, _]], [_, false]) for the `Some` constructor, and U([[false]],
176 /// [false]) for the `None` constructor. The first case returns true, so we know that
177 /// `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched
180 /// - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately:
181 /// `U(P, p) := U(P, (r_1, p_2, .., p_n))
182 /// || U(P, (r_2, p_2, .., p_n))`
184 /// Modifications to the algorithm
185 /// ------------------------------
186 /// The algorithm in the paper doesn't cover some of the special cases that arise in Rust, for
187 /// example uninhabited types and variable-length slice patterns. These are drawn attention to
188 /// throughout the code below. I'll make a quick note here about how exhaustive integer matching is
189 /// accounted for, though.
191 /// Exhaustive integer matching
192 /// ---------------------------
193 /// An integer type can be thought of as a (huge) sum type: 1 | 2 | 3 | ...
194 /// So to support exhaustive integer matching, we can make use of the logic in the paper for
195 /// OR-patterns. However, we obviously can't just treat ranges x..=y as individual sums, because
196 /// they are likely gigantic. So we instead treat ranges as constructors of the integers. This means
197 /// that we have a constructor *of* constructors (the integers themselves). We then need to work
198 /// through all the inductive step rules above, deriving how the ranges would be treated as
199 /// OR-patterns, and making sure that they're treated in the same way even when they're ranges.
200 /// There are really only four special cases here:
201 /// - When we match on a constructor that's actually a range, we have to treat it as if we would
203 /// + It turns out that we can simply extend the case for single-value patterns in
204 /// `specialize` to either be *equal* to a value constructor, or *contained within* a range
206 /// + When the pattern itself is a range, you just want to tell whether any of the values in
207 /// the pattern range coincide with values in the constructor range, which is precisely
209 /// Since when encountering a range pattern for a value constructor, we also use inclusion, it
210 /// means that whenever the constructor is a value/range and the pattern is also a value/range,
211 /// we can simply use intersection to test usefulness.
212 /// - When we're testing for usefulness of a pattern and the pattern's first component is a
214 /// + If all the constructors appear in the matrix, we have a slight complication. By default,
215 /// the behaviour (i.e., a disjunction over specialised matrices for each constructor) is
216 /// invalid, because we want a disjunction over every *integer* in each range, not just a
217 /// disjunction over every range. This is a bit more tricky to deal with: essentially we need
218 /// to form equivalence classes of subranges of the constructor range for which the behaviour
219 /// of the matrix `P` and new pattern `p` are the same. This is described in more
220 /// detail in `split_grouped_constructors`.
221 /// + If some constructors are missing from the matrix, it turns out we don't need to do
222 /// anything special (because we know none of the integers are actually wildcards: i.e., we
223 /// can't span wildcards using ranges).
224 use self::Constructor::*;
225 use self::Usefulness::*;
226 use self::WitnessPreference::*;
228 use rustc_data_structures::fx::FxHashMap;
229 use rustc_index::vec::Idx;
231 use super::{compare_const_vals, PatternFoldable, PatternFolder};
232 use super::{FieldPat, Pat, PatKind, PatRange};
234 use rustc::hir::def_id::DefId;
235 use rustc::hir::{HirId, RangeEnd};
236 use rustc::ty::layout::{Integer, IntegerExt, Size, VariantIdx};
237 use rustc::ty::{self, Const, Ty, TyCtxt, TypeFoldable};
240 use rustc::mir::interpret::{truncate, AllocId, ConstValue, Pointer, Scalar};
241 use rustc::mir::Field;
242 use rustc::util::captures::Captures;
243 use rustc::util::common::ErrorReported;
245 use syntax::attr::{SignedInt, UnsignedInt};
246 use syntax_pos::{Span, DUMMY_SP};
248 use arena::TypedArena;
250 use smallvec::{smallvec, SmallVec};
251 use std::cmp::{self, max, min, Ordering};
252 use std::convert::TryInto;
254 use std::iter::{FromIterator, IntoIterator};
255 use std::ops::RangeInclusive;
258 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pat<'tcx>) -> Pat<'tcx> {
259 LiteralExpander { tcx: cx.tcx }.fold_pattern(&pat)
262 struct LiteralExpander<'tcx> {
266 impl LiteralExpander<'tcx> {
267 /// Derefs `val` and potentially unsizes the value if `crty` is an array and `rty` a slice.
269 /// `crty` and `rty` can differ because you can use array constants in the presence of slice
270 /// patterns. So the pattern may end up being a slice, but the constant is an array. We convert
271 /// the array to a slice in that case.
272 fn fold_const_value_deref(
274 val: ConstValue<'tcx>,
275 // the pattern's pointee type
277 // the constant's pointee type
279 ) -> ConstValue<'tcx> {
280 debug!("fold_const_value_deref {:?} {:?} {:?}", val, rty, crty);
281 match (val, &crty.kind, &rty.kind) {
282 // the easy case, deref a reference
283 (ConstValue::Scalar(Scalar::Ptr(p)), x, y) if x == y => {
284 let alloc = self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id);
285 ConstValue::ByRef { alloc, offset: p.offset }
287 // unsize array to slice if pattern is array but match value or other patterns are slice
288 (ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
291 data: self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id),
292 start: p.offset.bytes().try_into().unwrap(),
293 end: n.eval_usize(self.tcx, ty::ParamEnv::empty()).try_into().unwrap(),
296 // fat pointers stay the same
297 (ConstValue::Slice { .. }, _, _)
298 | (_, ty::Slice(_), ty::Slice(_))
299 | (_, ty::Str, ty::Str) => val,
300 // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
301 _ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
306 impl PatternFolder<'tcx> for LiteralExpander<'tcx> {
307 fn fold_pattern(&mut self, pat: &Pat<'tcx>) -> Pat<'tcx> {
308 debug!("fold_pattern {:?} {:?} {:?}", pat, pat.ty.kind, pat.kind);
309 match (&pat.ty.kind, &*pat.kind) {
313 value: Const { val, ty: ty::TyS { kind: ty::Ref(_, crty, _), .. } },
318 kind: box PatKind::Deref {
322 kind: box PatKind::Constant {
323 value: self.tcx.mk_const(Const {
324 val: self.fold_const_value_deref(*val, rty, crty),
331 (_, &PatKind::Binding { subpattern: Some(ref s), .. }) => s.fold_with(self),
332 _ => pat.super_fold_with(self),
337 impl<'tcx> Pat<'tcx> {
338 fn is_wildcard(&self) -> bool {
340 PatKind::Binding { subpattern: None, .. } | PatKind::Wild => true,
346 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
348 #[derive(Debug, Clone)]
349 pub struct PatStack<'p, 'tcx>(SmallVec<[&'p Pat<'tcx>; 2]>);
351 impl<'p, 'tcx> PatStack<'p, 'tcx> {
352 pub fn from_pattern(pat: &'p Pat<'tcx>) -> Self {
353 PatStack(smallvec![pat])
356 fn from_vec(vec: SmallVec<[&'p Pat<'tcx>; 2]>) -> Self {
360 fn from_slice(s: &[&'p Pat<'tcx>]) -> Self {
361 PatStack(SmallVec::from_slice(s))
364 fn is_empty(&self) -> bool {
368 fn len(&self) -> usize {
372 fn head(&self) -> &'p Pat<'tcx> {
376 fn to_tail(&self) -> Self {
377 PatStack::from_slice(&self.0[1..])
380 fn iter(&self) -> impl Iterator<Item = &Pat<'tcx>> {
381 self.0.iter().map(|p| *p)
384 /// This computes `D(self)`. See top of the file for explanations.
385 fn specialize_wildcard(&self) -> Option<Self> {
386 if self.head().is_wildcard() { Some(self.to_tail()) } else { None }
389 /// This computes `S(constructor, self)`. See top of the file for explanations.
390 fn specialize_constructor<'a, 'q>(
392 cx: &mut MatchCheckCtxt<'a, 'tcx>,
393 constructor: &Constructor<'tcx>,
394 wild_patterns: &[&'q Pat<'tcx>],
395 ) -> Option<PatStack<'q, 'tcx>>
400 specialize(cx, self, constructor, wild_patterns)
404 impl<'p, 'tcx> Default for PatStack<'p, 'tcx> {
405 fn default() -> Self {
406 PatStack(smallvec![])
410 impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
411 fn from_iter<T>(iter: T) -> Self
413 T: IntoIterator<Item = &'p Pat<'tcx>>,
415 PatStack(iter.into_iter().collect())
420 pub struct Matrix<'p, 'tcx>(Vec<PatStack<'p, 'tcx>>);
422 impl<'p, 'tcx> Matrix<'p, 'tcx> {
423 pub fn empty() -> Self {
427 pub fn push(&mut self, row: PatStack<'p, 'tcx>) {
431 /// Iterate over the first component of each row
432 fn heads<'a>(&'a self) -> impl Iterator<Item = &'a Pat<'tcx>> + Captures<'p> {
433 self.0.iter().map(|r| r.head())
436 /// This computes `D(self)`. See top of the file for explanations.
437 fn specialize_wildcard(&self) -> Self {
438 self.0.iter().filter_map(|r| r.specialize_wildcard()).collect()
441 /// This computes `S(constructor, self)`. See top of the file for explanations.
442 fn specialize_constructor<'a, 'q>(
444 cx: &mut MatchCheckCtxt<'a, 'tcx>,
445 constructor: &Constructor<'tcx>,
446 wild_patterns: &[&'q Pat<'tcx>],
447 ) -> Matrix<'q, 'tcx>
455 .filter_map(|r| r.specialize_constructor(cx, constructor, wild_patterns))
461 /// Pretty-printer for matrices of patterns, example:
462 /// +++++++++++++++++++++++++++++
464 /// +++++++++++++++++++++++++++++
465 /// + true + [First] +
466 /// +++++++++++++++++++++++++++++
467 /// + true + [Second(true)] +
468 /// +++++++++++++++++++++++++++++
470 /// +++++++++++++++++++++++++++++
471 /// + _ + [_, _, tail @ ..] +
472 /// +++++++++++++++++++++++++++++
473 impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
474 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
477 let &Matrix(ref m) = self;
478 let pretty_printed_matrix: Vec<Vec<String>> =
479 m.iter().map(|row| row.iter().map(|pat| format!("{:?}", pat)).collect()).collect();
481 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
482 assert!(m.iter().all(|row| row.len() == column_count));
483 let column_widths: Vec<usize> = (0..column_count)
484 .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0))
487 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
488 let br = "+".repeat(total_width);
489 write!(f, "{}\n", br)?;
490 for row in pretty_printed_matrix {
492 for (column, pat_str) in row.into_iter().enumerate() {
494 write!(f, "{:1$}", pat_str, column_widths[column])?;
498 write!(f, "{}\n", br)?;
504 impl<'p, 'tcx> FromIterator<PatStack<'p, 'tcx>> for Matrix<'p, 'tcx> {
505 fn from_iter<T>(iter: T) -> Self
507 T: IntoIterator<Item = PatStack<'p, 'tcx>>,
509 Matrix(iter.into_iter().collect())
513 pub struct MatchCheckCtxt<'a, 'tcx> {
514 pub tcx: TyCtxt<'tcx>,
515 /// The module in which the match occurs. This is necessary for
516 /// checking inhabited-ness of types because whether a type is (visibly)
517 /// inhabited can depend on whether it was defined in the current module or
518 /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty
519 /// outside it's module and should not be matchable with an empty match
522 param_env: ty::ParamEnv<'tcx>,
523 pub pattern_arena: &'a TypedArena<Pat<'tcx>>,
524 pub byte_array_map: FxHashMap<*const Pat<'tcx>, Vec<&'a Pat<'tcx>>>,
527 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
528 pub fn create_and_enter<F, R>(
530 param_env: ty::ParamEnv<'tcx>,
535 F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R,
537 let pattern_arena = TypedArena::default();
543 pattern_arena: &pattern_arena,
544 byte_array_map: FxHashMap::default(),
548 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
549 if self.tcx.features().exhaustive_patterns {
550 self.tcx.is_ty_uninhabited_from(self.module, ty)
556 fn is_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
558 ty::Adt(adt_def, ..) => adt_def.is_variant_list_non_exhaustive(),
563 fn is_local(&self, ty: Ty<'tcx>) -> bool {
565 ty::Adt(adt_def, ..) => adt_def.did.is_local(),
571 #[derive(Clone, Debug)]
572 enum Constructor<'tcx> {
573 /// The constructor of all patterns that don't vary by constructor,
574 /// e.g., struct patterns and fixed-length arrays.
579 ConstantValue(&'tcx ty::Const<'tcx>, Span),
580 /// Ranges of literal values (`2..=5` and `2..5`).
581 ConstantRange(u128, u128, Ty<'tcx>, RangeEnd, Span),
582 /// Array patterns of length n.
586 // Ignore spans when comparing, they don't carry semantic information as they are only for lints.
587 impl<'tcx> std::cmp::PartialEq for Constructor<'tcx> {
588 fn eq(&self, other: &Self) -> bool {
589 match (self, other) {
590 (Constructor::Single, Constructor::Single) => true,
591 (Constructor::Variant(a), Constructor::Variant(b)) => a == b,
592 (Constructor::ConstantValue(a, _), Constructor::ConstantValue(b, _)) => a == b,
594 Constructor::ConstantRange(a_start, a_end, a_ty, a_range_end, _),
595 Constructor::ConstantRange(b_start, b_end, b_ty, b_range_end, _),
596 ) => a_start == b_start && a_end == b_end && a_ty == b_ty && a_range_end == b_range_end,
597 (Constructor::Slice(a), Constructor::Slice(b)) => a == b,
603 impl<'tcx> Constructor<'tcx> {
604 fn is_slice(&self) -> bool {
606 Slice { .. } => true,
611 fn variant_index_for_adt<'a>(
613 cx: &MatchCheckCtxt<'a, 'tcx>,
614 adt: &'tcx ty::AdtDef,
617 Variant(id) => adt.variant_index_with_id(*id),
619 assert!(!adt.is_enum());
622 ConstantValue(c, _) => crate::const_eval::const_variant_index(cx.tcx, cx.param_env, c),
623 _ => bug!("bad constructor {:?} for adt {:?}", self, adt),
627 fn display(&self, tcx: TyCtxt<'tcx>) -> String {
629 Constructor::ConstantValue(val, _) => format!("{}", val),
630 Constructor::ConstantRange(lo, hi, ty, range_end, _) => {
631 // Get the right sign on the output:
632 let ty = ty::ParamEnv::empty().and(*ty);
635 ty::Const::from_bits(tcx, *lo, ty),
637 ty::Const::from_bits(tcx, *hi, ty),
640 Constructor::Slice(val) => format!("[{}]", val),
641 _ => bug!("bad constructor being displayed: `{:?}", self),
645 fn wildcard_subpatterns<'a>(
647 cx: &MatchCheckCtxt<'a, 'tcx>,
649 ) -> Vec<Pat<'tcx>> {
650 constructor_sub_pattern_tys(cx, self, ty)
652 .map(|ty| Pat { ty, span: DUMMY_SP, kind: box PatKind::Wild })
656 /// This computes the arity of a constructor. The arity of a constructor
657 /// is how many subpattern patterns of that constructor should be expanded to.
659 /// For instance, a tuple pattern `(_, 42, Some([]))` has the arity of 3.
660 /// A struct pattern's arity is the number of fields it contains, etc.
661 fn arity<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> u64 {
662 debug!("Constructor::arity({:#?}, {:?})", self, ty);
664 ty::Tuple(ref fs) => fs.len() as u64,
665 ty::Slice(..) | ty::Array(..) => match *self {
666 Slice(length) => length,
667 ConstantValue(..) => 0,
668 _ => bug!("bad slice pattern {:?} {:?}", self, ty),
672 adt.variants[self.variant_index_for_adt(cx, adt)].fields.len() as u64
679 #[derive(Clone, Debug)]
680 pub enum Usefulness<'tcx> {
682 UsefulWithWitness(Vec<Witness<'tcx>>),
686 impl<'tcx> Usefulness<'tcx> {
687 fn is_useful(&self) -> bool {
695 #[derive(Copy, Clone, Debug)]
696 pub enum WitnessPreference {
701 #[derive(Copy, Clone, Debug)]
702 struct PatCtxt<'tcx> {
704 max_slice_length: u64,
708 /// A witness of non-exhaustiveness for error reporting, represented
709 /// as a list of patterns (in reverse order of construction) with
710 /// wildcards inside to represent elements that can take any inhabitant
711 /// of the type as a value.
713 /// A witness against a list of patterns should have the same types
714 /// and length as the pattern matched against. Because Rust `match`
715 /// is always against a single pattern, at the end the witness will
716 /// have length 1, but in the middle of the algorithm, it can contain
717 /// multiple patterns.
719 /// For example, if we are constructing a witness for the match against
721 /// struct Pair(Option<(u32, u32)>, bool);
723 /// match (p: Pair) {
724 /// Pair(None, _) => {}
725 /// Pair(_, false) => {}
729 /// We'll perform the following steps:
730 /// 1. Start with an empty witness
731 /// `Witness(vec![])`
732 /// 2. Push a witness `Some(_)` against the `None`
733 /// `Witness(vec![Some(_)])`
734 /// 3. Push a witness `true` against the `false`
735 /// `Witness(vec![Some(_), true])`
736 /// 4. Apply the `Pair` constructor to the witnesses
737 /// `Witness(vec![Pair(Some(_), true)])`
739 /// The final `Pair(Some(_), true)` is then the resulting witness.
740 #[derive(Clone, Debug)]
741 pub struct Witness<'tcx>(Vec<Pat<'tcx>>);
743 impl<'tcx> Witness<'tcx> {
744 pub fn single_pattern(self) -> Pat<'tcx> {
745 assert_eq!(self.0.len(), 1);
746 self.0.into_iter().next().unwrap()
749 fn push_wild_constructor<'a>(
751 cx: &MatchCheckCtxt<'a, 'tcx>,
752 ctor: &Constructor<'tcx>,
755 self.0.extend(ctor.wildcard_subpatterns(cx, ty));
756 self.apply_constructor(cx, ctor, ty)
759 /// Constructs a partial witness for a pattern given a list of
760 /// patterns expanded by the specialization step.
762 /// When a pattern P is discovered to be useful, this function is used bottom-up
763 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
764 /// of values, V, where each value in that set is not covered by any previously
765 /// used patterns and is covered by the pattern P'. Examples:
767 /// left_ty: tuple of 3 elements
768 /// pats: [10, 20, _] => (10, 20, _)
770 /// left_ty: struct X { a: (bool, &'static str), b: usize}
771 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
772 fn apply_constructor<'a>(
774 cx: &MatchCheckCtxt<'a, 'tcx>,
775 ctor: &Constructor<'tcx>,
778 let arity = ctor.arity(cx, ty);
780 let len = self.0.len() as u64;
781 let mut pats = self.0.drain((len - arity) as usize..).rev();
784 ty::Adt(..) | ty::Tuple(..) => {
787 .map(|(i, p)| FieldPat { field: Field::new(i), pattern: p })
790 if let ty::Adt(adt, substs) = ty.kind {
795 variant_index: ctor.variant_index_for_adt(cx, adt),
799 PatKind::Leaf { subpatterns: pats }
802 PatKind::Leaf { subpatterns: pats }
806 ty::Ref(..) => PatKind::Deref { subpattern: pats.nth(0).unwrap() },
808 ty::Slice(_) | ty::Array(..) => {
809 PatKind::Slice { prefix: pats.collect(), slice: None, suffix: vec![] }
813 ConstantValue(value, _) => PatKind::Constant { value },
814 ConstantRange(lo, hi, ty, end, _) => PatKind::Range(PatRange {
815 lo: ty::Const::from_bits(cx.tcx, lo, ty::ParamEnv::empty().and(ty)),
816 hi: ty::Const::from_bits(cx.tcx, hi, ty::ParamEnv::empty().and(ty)),
824 self.0.push(Pat { ty, span: DUMMY_SP, kind: Box::new(pat) });
830 /// This determines the set of all possible constructors of a pattern matching
831 /// values of type `left_ty`. For vectors, this would normally be an infinite set
832 /// but is instead bounded by the maximum fixed length of slice patterns in
833 /// the column of patterns being analyzed.
835 /// We make sure to omit constructors that are statically impossible. E.g., for
836 /// `Option<!>`, we do not include `Some(_)` in the returned list of constructors.
837 fn all_constructors<'a, 'tcx>(
838 cx: &mut MatchCheckCtxt<'a, 'tcx>,
840 ) -> Vec<Constructor<'tcx>> {
841 debug!("all_constructors({:?})", pcx.ty);
842 let ctors = match pcx.ty.kind {
843 ty::Bool => [true, false]
845 .map(|&b| ConstantValue(ty::Const::from_bool(cx.tcx, b), pcx.span))
847 ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx, cx.param_env).is_some() => {
848 let len = len.eval_usize(cx.tcx, cx.param_env);
849 if len != 0 && cx.is_uninhabited(sub_ty) { vec![] } else { vec![Slice(len)] }
851 // Treat arrays of a constant but unknown length like slices.
852 ty::Array(ref sub_ty, _) | ty::Slice(ref sub_ty) => {
853 if cx.is_uninhabited(sub_ty) {
856 (0..pcx.max_slice_length + 1).map(|length| Slice(length)).collect()
859 ty::Adt(def, substs) if def.is_enum() => def
863 !cx.tcx.features().exhaustive_patterns
865 .uninhabited_from(cx.tcx, substs, def.adt_kind())
866 .contains(cx.tcx, cx.module)
868 .map(|v| Variant(v.def_id))
872 // The valid Unicode Scalar Value ranges.
882 '\u{10FFFF}' as u128,
890 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
891 let min = 1u128 << (bits - 1);
893 vec![ConstantRange(min, max, pcx.ty, RangeEnd::Included, pcx.span)]
896 let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
897 let max = truncate(u128::max_value(), size);
898 vec![ConstantRange(0, max, pcx.ty, RangeEnd::Included, pcx.span)]
901 if cx.is_uninhabited(pcx.ty) {
911 fn max_slice_length<'p, 'a, 'tcx, I>(cx: &mut MatchCheckCtxt<'a, 'tcx>, patterns: I) -> u64
913 I: Iterator<Item = &'p Pat<'tcx>>,
916 // The exhaustiveness-checking paper does not include any details on
917 // checking variable-length slice patterns. However, they are matched
918 // by an infinite collection of fixed-length array patterns.
920 // Checking the infinite set directly would take an infinite amount
921 // of time. However, it turns out that for each finite set of
922 // patterns `P`, all sufficiently large array lengths are equivalent:
924 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
925 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
926 // `sₘ` for each sufficiently-large length `m` that applies to exactly
927 // the same subset of `P`.
929 // Because of that, each witness for reachability-checking from one
930 // of the sufficiently-large lengths can be transformed to an
931 // equally-valid witness from any other length, so we only have
932 // to check slice lengths from the "minimal sufficiently-large length"
935 // Note that the fact that there is a *single* `sₘ` for each `m`
936 // not depending on the specific pattern in `P` is important: if
937 // you look at the pair of patterns
940 // Then any slice of length ≥1 that matches one of these two
941 // patterns can be trivially turned to a slice of any
942 // other length ≥1 that matches them and vice-versa - for
943 // but the slice from length 2 `[false, true]` that matches neither
944 // of these patterns can't be turned to a slice from length 1 that
945 // matches neither of these patterns, so we have to consider
946 // slices from length 2 there.
948 // Now, to see that that length exists and find it, observe that slice
949 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
950 // "variable-length" patterns (`[_, .., _]`).
952 // For fixed-length patterns, all slices with lengths *longer* than
953 // the pattern's length have the same outcome (of not matching), so
954 // as long as `L` is greater than the pattern's length we can pick
955 // any `sₘ` from that length and get the same result.
957 // For variable-length patterns, the situation is more complicated,
958 // because as seen above the precise value of `sₘ` matters.
960 // However, for each variable-length pattern `p` with a prefix of length
961 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
962 // `slₚ` elements are examined.
964 // Therefore, as long as `L` is positive (to avoid concerns about empty
965 // types), all elements after the maximum prefix length and before
966 // the maximum suffix length are not examined by any variable-length
967 // pattern, and therefore can be added/removed without affecting
968 // them - creating equivalent patterns from any sufficiently-large
971 // Of course, if fixed-length patterns exist, we must be sure
972 // that our length is large enough to miss them all, so
973 // we can pick `L = max(FIXED_LEN+1 ∪ {max(PREFIX_LEN) + max(SUFFIX_LEN)})`
975 // for example, with the above pair of patterns, all elements
976 // but the first and last can be added/removed, so any
977 // witness of length ≥2 (say, `[false, false, true]`) can be
978 // turned to a witness from any other length ≥2.
980 let mut max_prefix_len = 0;
981 let mut max_suffix_len = 0;
982 let mut max_fixed_len = 0;
984 for row in patterns {
986 PatKind::Constant { value } => {
987 // extract the length of an array/slice from a constant
988 match (value.val, &value.ty.kind) {
989 (_, ty::Array(_, n)) => {
990 max_fixed_len = cmp::max(max_fixed_len, n.eval_usize(cx.tcx, cx.param_env))
992 (ConstValue::Slice { start, end, .. }, ty::Slice(_)) => {
993 max_fixed_len = cmp::max(max_fixed_len, (end - start) as u64)
998 PatKind::Slice { ref prefix, slice: None, ref suffix } => {
999 let fixed_len = prefix.len() as u64 + suffix.len() as u64;
1000 max_fixed_len = cmp::max(max_fixed_len, fixed_len);
1002 PatKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
1003 max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
1004 max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
1010 cmp::max(max_fixed_len + 1, max_prefix_len + max_suffix_len)
1013 /// An inclusive interval, used for precise integer exhaustiveness checking.
1014 /// `IntRange`s always store a contiguous range. This means that values are
1015 /// encoded such that `0` encodes the minimum value for the integer,
1016 /// regardless of the signedness.
1017 /// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
1018 /// This makes comparisons and arithmetic on interval endpoints much more
1019 /// straightforward. See `signed_bias` for details.
1021 /// `IntRange` is never used to encode an empty range or a "range" that wraps
1022 /// around the (offset) space: i.e., `range.lo <= range.hi`.
1023 #[derive(Clone, Debug)]
1024 struct IntRange<'tcx> {
1025 pub range: RangeInclusive<u128>,
1030 impl<'tcx> IntRange<'tcx> {
1032 fn is_integral(ty: Ty<'_>) -> bool {
1034 ty::Char | ty::Int(_) | ty::Uint(_) => true,
1040 fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
1042 ty::Char => Some((Size::from_bytes(4), 0)),
1044 let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
1045 Some((size, 1u128 << (size.bits() as u128 - 1)))
1047 ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
1055 param_env: ty::ParamEnv<'tcx>,
1056 value: &Const<'tcx>,
1058 ) -> Option<IntRange<'tcx>> {
1059 if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
1061 let val = if let ConstValue::Scalar(Scalar::Raw { data, size }) = value.val {
1062 // For this specific pattern we can skip a lot of effort and go
1063 // straight to the result, after doing a bit of checking. (We
1064 // could remove this branch and just use the next branch, which
1065 // is more general but much slower.)
1066 Scalar::<()>::check_raw(data, size, target_size);
1068 } else if let Some(val) = value.try_eval_bits(tcx, param_env, ty) {
1069 // This is a more general form of the previous branch.
1074 let val = val ^ bias;
1075 Some(IntRange { range: val..=val, ty, span })
1089 ) -> Option<IntRange<'tcx>> {
1090 if Self::is_integral(ty) {
1091 // Perform a shift if the underlying types are signed,
1092 // which makes the interval arithmetic simpler.
1093 let bias = IntRange::signed_bias(tcx, ty);
1094 let (lo, hi) = (lo ^ bias, hi ^ bias);
1095 // Make sure the interval is well-formed.
1096 if lo > hi || lo == hi && *end == RangeEnd::Excluded {
1099 let offset = (*end == RangeEnd::Excluded) as u128;
1100 Some(IntRange { range: lo..=(hi - offset), ty, span })
1109 param_env: ty::ParamEnv<'tcx>,
1110 ctor: &Constructor<'tcx>,
1111 ) -> Option<IntRange<'tcx>> {
1112 // Floating-point ranges are permitted and we don't want
1113 // to consider them when constructing integer ranges.
1115 ConstantRange(lo, hi, ty, end, span) => Self::from_range(tcx, *lo, *hi, ty, end, *span),
1116 ConstantValue(val, span) => Self::from_const(tcx, param_env, val, *span),
1123 param_env: ty::ParamEnv<'tcx>,
1124 mut pat: &Pat<'tcx>,
1125 ) -> Option<IntRange<'tcx>> {
1128 box PatKind::Constant { value } => {
1129 return Self::from_const(tcx, param_env, value, pat.span);
1131 box PatKind::Range(PatRange { lo, hi, end }) => {
1132 return Self::from_range(
1134 lo.eval_bits(tcx, param_env, lo.ty),
1135 hi.eval_bits(tcx, param_env, hi.ty),
1141 box PatKind::AscribeUserType { ref subpattern, .. } => {
1149 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
1150 fn signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
1153 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
1160 /// Converts a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
1164 r: RangeInclusive<u128>,
1166 ) -> Constructor<'tcx> {
1167 let bias = IntRange::signed_bias(tcx, ty);
1168 let (lo, hi) = r.into_inner();
1170 let ty = ty::ParamEnv::empty().and(ty);
1171 ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty), span)
1173 ConstantRange(lo ^ bias, hi ^ bias, ty, RangeEnd::Included, span)
1177 /// Returns a collection of ranges that spans the values covered by `ranges`, subtracted
1178 /// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
1182 param_env: ty::ParamEnv<'tcx>,
1183 ranges: Vec<Constructor<'tcx>>,
1184 ) -> Vec<Constructor<'tcx>> {
1187 .filter_map(|r| IntRange::from_ctor(tcx, param_env, &r).map(|i| i.range));
1188 let mut remaining_ranges = vec![];
1190 let (lo, hi) = self.range.into_inner();
1191 for subrange in ranges {
1192 let (subrange_lo, subrange_hi) = subrange.into_inner();
1193 if lo > subrange_hi || subrange_lo > hi {
1194 // The pattern doesn't intersect with the subrange at all,
1195 // so the subrange remains untouched.
1196 remaining_ranges.push(Self::range_to_ctor(
1199 subrange_lo..=subrange_hi,
1203 if lo > subrange_lo {
1204 // The pattern intersects an upper section of the
1205 // subrange, so a lower section will remain.
1206 remaining_ranges.push(Self::range_to_ctor(
1209 subrange_lo..=(lo - 1),
1213 if hi < subrange_hi {
1214 // The pattern intersects a lower section of the
1215 // subrange, so an upper section will remain.
1216 remaining_ranges.push(Self::range_to_ctor(
1219 (hi + 1)..=subrange_hi,
1228 fn intersection(&self, other: &Self) -> Option<Self> {
1230 let (lo, hi) = (*self.range.start(), *self.range.end());
1231 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1232 if lo <= other_hi && other_lo <= hi {
1233 let span = other.span;
1234 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty, span })
1240 fn suspicious_intersection(&self, other: &Self) -> bool {
1241 // `false` in the following cases:
1242 // 1 ---- // 1 ---------- // 1 ---- // 1 ----
1243 // 2 ---------- // 2 ---- // 2 ---- // 2 ----
1245 // The following are currently `false`, but could be `true` in the future (#64007):
1246 // 1 --------- // 1 ---------
1247 // 2 ---------- // 2 ----------
1249 // `true` in the following cases:
1250 // 1 ------- // 1 -------
1251 // 2 -------- // 2 -------
1252 let (lo, hi) = (*self.range.start(), *self.range.end());
1253 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1254 (lo == other_hi || hi == other_lo)
1258 // A request for missing constructor data in terms of either:
1259 // - whether or not there any missing constructors; or
1260 // - the actual set of missing constructors.
1261 #[derive(PartialEq)]
1262 enum MissingCtorsInfo {
1267 // Used by `compute_missing_ctors`.
1268 #[derive(Debug, PartialEq)]
1269 enum MissingCtors<'tcx> {
1273 // Note that the Vec can be empty.
1274 Ctors(Vec<Constructor<'tcx>>),
1277 // When `info` is `MissingCtorsInfo::Ctors`, compute a set of constructors
1278 // equivalent to `all_ctors \ used_ctors`. When `info` is
1279 // `MissingCtorsInfo::Emptiness`, just determines if that set is empty or not.
1280 // (The split logic gives a performance win, because we always need to know if
1281 // the set is empty, but we rarely need the full set, and it can be expensive
1282 // to compute the full set.)
1283 fn compute_missing_ctors<'tcx>(
1284 info: MissingCtorsInfo,
1286 param_env: ty::ParamEnv<'tcx>,
1287 all_ctors: &Vec<Constructor<'tcx>>,
1288 used_ctors: &Vec<Constructor<'tcx>>,
1289 ) -> MissingCtors<'tcx> {
1290 let mut missing_ctors = vec![];
1292 for req_ctor in all_ctors {
1293 let mut refined_ctors = vec![req_ctor.clone()];
1294 for used_ctor in used_ctors {
1295 if used_ctor == req_ctor {
1296 // If a constructor appears in a `match` arm, we can
1297 // eliminate it straight away.
1298 refined_ctors = vec![]
1299 } else if let Some(interval) = IntRange::from_ctor(tcx, param_env, used_ctor) {
1300 // Refine the required constructors for the type by subtracting
1301 // the range defined by the current constructor pattern.
1302 refined_ctors = interval.subtract_from(tcx, param_env, refined_ctors);
1305 // If the constructor patterns that have been considered so far
1306 // already cover the entire range of values, then we the
1307 // constructor is not missing, and we can move on to the next one.
1308 if refined_ctors.is_empty() {
1312 // If a constructor has not been matched, then it is missing.
1313 // We add `refined_ctors` instead of `req_ctor`, because then we can
1314 // provide more detailed error information about precisely which
1315 // ranges have been omitted.
1316 if info == MissingCtorsInfo::Emptiness {
1317 if !refined_ctors.is_empty() {
1318 // The set is non-empty; return early.
1319 return MissingCtors::NonEmpty;
1322 missing_ctors.extend(refined_ctors);
1326 if info == MissingCtorsInfo::Emptiness {
1327 // If we reached here, the set is empty.
1330 MissingCtors::Ctors(missing_ctors)
1334 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html.
1335 /// The algorithm from the paper has been modified to correctly handle empty
1336 /// types. The changes are:
1337 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1338 /// continue to recurse over columns.
1339 /// (1) all_constructors will only return constructors that are statically
1340 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
1342 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1343 /// to a set of such vectors `m` - this is defined as there being a set of
1344 /// inputs that will match `v` but not any of the sets in `m`.
1346 /// All the patterns at each column of the `matrix ++ v` matrix must
1347 /// have the same type, except that wildcard (PatKind::Wild) patterns
1348 /// with type `TyErr` are also allowed, even if the "type of the column"
1349 /// is not `TyErr`. That is used to represent private fields, as using their
1350 /// real type would assert that they are inhabited.
1352 /// This is used both for reachability checking (if a pattern isn't useful in
1353 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1354 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1355 /// matrix isn't exhaustive).
1356 pub fn is_useful<'p, 'a, 'tcx>(
1357 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1358 matrix: &Matrix<'p, 'tcx>,
1359 v: &PatStack<'_, 'tcx>,
1360 witness: WitnessPreference,
1362 ) -> Usefulness<'tcx> {
1363 let &Matrix(ref rows) = matrix;
1364 debug!("is_useful({:#?}, {:#?})", matrix, v);
1366 // The base case. We are pattern-matching on () and the return value is
1367 // based on whether our matrix has a row or not.
1368 // NOTE: This could potentially be optimized by checking rows.is_empty()
1369 // first and then, if v is non-empty, the return value is based on whether
1370 // the type of the tuple we're checking is inhabited or not.
1372 return if rows.is_empty() {
1374 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
1375 LeaveOutWitness => Useful,
1382 assert!(rows.iter().all(|r| r.len() == v.len()));
1384 let (ty, span) = matrix
1386 .map(|r| (r.ty, r.span))
1387 .find(|(ty, _)| !ty.references_error())
1388 .unwrap_or((v.head().ty, v.head().span));
1390 // TyErr is used to represent the type of wildcard patterns matching
1391 // against inaccessible (private) fields of structs, so that we won't
1392 // be able to observe whether the types of the struct's fields are
1395 // If the field is truly inaccessible, then all the patterns
1396 // matching against it must be wildcard patterns, so its type
1399 // However, if we are matching against non-wildcard patterns, we
1400 // need to know the real type of the field so we can specialize
1401 // against it. This primarily occurs through constants - they
1402 // can include contents for fields that are inaccessible at the
1403 // location of the match. In that case, the field's type is
1404 // inhabited - by the constant - so we can just use it.
1406 // FIXME: this might lead to "unstable" behavior with macro hygiene
1407 // introducing uninhabited patterns for inaccessible fields. We
1408 // need to figure out how to model that.
1410 max_slice_length: max_slice_length(cx, matrix.heads().chain(Some(v.head()))),
1414 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
1416 if let Some(constructors) = pat_constructors(cx, v.head(), pcx) {
1417 debug!("is_useful - expanding constructors: {:#?}", constructors);
1418 split_grouped_constructors(
1428 .map(|c| is_useful_specialized(cx, matrix, v, c, pcx.ty, witness, hir_id))
1429 .find(|result| result.is_useful())
1430 .unwrap_or(NotUseful)
1432 debug!("is_useful - expanding wildcard");
1434 let used_ctors: Vec<Constructor<'_>> =
1435 matrix.heads().flat_map(|p| pat_constructors(cx, p, pcx).unwrap_or(vec![])).collect();
1436 debug!("used_ctors = {:#?}", used_ctors);
1437 // `all_ctors` are all the constructors for the given type, which
1438 // should all be represented (or caught with the wild pattern `_`).
1439 let all_ctors = all_constructors(cx, pcx);
1440 debug!("all_ctors = {:#?}", all_ctors);
1442 // `missing_ctors` is the set of constructors from the same type as the
1443 // first column of `matrix` that are matched only by wildcard patterns
1444 // from the first column.
1446 // Therefore, if there is some pattern that is unmatched by `matrix`,
1447 // it will still be unmatched if the first constructor is replaced by
1448 // any of the constructors in `missing_ctors`
1450 // However, if our scrutinee is *privately* an empty enum, we
1451 // must treat it as though it had an "unknown" constructor (in
1452 // that case, all other patterns obviously can't be variants)
1453 // to avoid exposing its emptyness. See the `match_privately_empty`
1454 // test for details.
1456 // FIXME: currently the only way I know of something can
1457 // be a privately-empty enum is when the exhaustive_patterns
1458 // feature flag is not present, so this is only
1459 // needed for that case.
1461 // Missing constructors are those that are not matched by any
1462 // non-wildcard patterns in the current column. We always determine if
1463 // the set is empty, but we only fully construct them on-demand,
1464 // because they're rarely used and can be big.
1465 let cheap_missing_ctors = compute_missing_ctors(
1466 MissingCtorsInfo::Emptiness,
1473 let is_privately_empty = all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1474 let is_declared_nonexhaustive = cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
1476 "cheap_missing_ctors={:#?} is_privately_empty={:#?} is_declared_nonexhaustive={:#?}",
1477 cheap_missing_ctors, is_privately_empty, is_declared_nonexhaustive
1480 // For privately empty and non-exhaustive enums, we work as if there were an "extra"
1481 // `_` constructor for the type, so we can never match over all constructors.
1482 let is_non_exhaustive = is_privately_empty
1483 || is_declared_nonexhaustive
1484 || (pcx.ty.is_ptr_sized_integral() && !cx.tcx.features().precise_pointer_size_matching);
1486 if cheap_missing_ctors == MissingCtors::Empty && !is_non_exhaustive {
1487 split_grouped_constructors(
1497 .map(|c| is_useful_specialized(cx, matrix, v, c, pcx.ty, witness, hir_id))
1498 .find(|result| result.is_useful())
1499 .unwrap_or(NotUseful)
1501 let matrix = matrix.specialize_wildcard();
1502 let v = v.to_tail();
1503 match is_useful(cx, &matrix, &v, witness, hir_id) {
1504 UsefulWithWitness(pats) => {
1506 // In this case, there's at least one "free"
1507 // constructor that is only matched against by
1508 // wildcard patterns.
1510 // There are 2 ways we can report a witness here.
1511 // Commonly, we can report all the "free"
1512 // constructors as witnesses, e.g., if we have:
1515 // enum Direction { N, S, E, W }
1516 // let Direction::N = ...;
1519 // we can report 3 witnesses: `S`, `E`, and `W`.
1521 // However, there are 2 cases where we don't want
1522 // to do this and instead report a single `_` witness:
1524 // 1) If the user is matching against a non-exhaustive
1525 // enum, there is no point in enumerating all possible
1526 // variants, because the user can't actually match
1527 // against them himself, e.g., in an example like:
1529 // let err: io::ErrorKind = ...;
1531 // io::ErrorKind::NotFound => {},
1534 // we don't want to show every possible IO error,
1535 // but instead have `_` as the witness (this is
1536 // actually *required* if the user specified *all*
1537 // IO errors, but is probably what we want in every
1540 // 2) If the user didn't actually specify a constructor
1541 // in this arm, e.g., in
1543 // let x: (Direction, Direction, bool) = ...;
1544 // let (_, _, false) = x;
1546 // we don't want to show all 16 possible witnesses
1547 // `(<direction-1>, <direction-2>, true)` - we are
1548 // satisfied with `(_, _, true)`. In this case,
1549 // `used_ctors` is empty.
1550 let new_witnesses = if is_non_exhaustive || used_ctors.is_empty() {
1551 // All constructors are unused. Add wild patterns
1552 // rather than each individual constructor.
1554 .map(|mut witness| {
1555 witness.0.push(Pat {
1558 kind: box PatKind::Wild,
1564 let expensive_missing_ctors = compute_missing_ctors(
1565 MissingCtorsInfo::Ctors,
1571 if let MissingCtors::Ctors(missing_ctors) = expensive_missing_ctors {
1573 .flat_map(|witness| {
1574 missing_ctors.iter().map(move |ctor| {
1575 // Extends the witness with a "wild" version of this
1576 // constructor, that matches everything that can be built with
1577 // it. For example, if `ctor` is a `Constructor::Variant` for
1578 // `Option::Some`, this pushes the witness for `Some(_)`.
1579 witness.clone().push_wild_constructor(cx, ctor, pcx.ty)
1584 bug!("cheap missing ctors")
1587 UsefulWithWitness(new_witnesses)
1595 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
1596 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1597 fn is_useful_specialized<'p, 'a, 'tcx>(
1598 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1599 matrix: &Matrix<'p, 'tcx>,
1600 v: &PatStack<'_, 'tcx>,
1601 ctor: Constructor<'tcx>,
1603 witness: WitnessPreference,
1605 ) -> Usefulness<'tcx> {
1606 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1608 let wild_patterns_owned = ctor.wildcard_subpatterns(cx, lty);
1609 let wild_patterns: Vec<_> = wild_patterns_owned.iter().collect();
1610 let matrix = matrix.specialize_constructor(cx, &ctor, &wild_patterns);
1611 match v.specialize_constructor(cx, &ctor, &wild_patterns) {
1612 Some(v) => match is_useful(cx, &matrix, &v, witness, hir_id) {
1613 UsefulWithWitness(witnesses) => UsefulWithWitness(
1616 .map(|witness| witness.apply_constructor(cx, &ctor, lty))
1625 /// Determines the constructors that the given pattern can be specialized to.
1627 /// In most cases, there's only one constructor that a specific pattern
1628 /// represents, such as a specific enum variant or a specific literal value.
1629 /// Slice patterns, however, can match slices of different lengths. For instance,
1630 /// `[a, b, tail @ ..]` can match a slice of length 2, 3, 4 and so on.
1632 /// Returns `None` in case of a catch-all, which can't be specialized.
1633 fn pat_constructors<'tcx>(
1634 cx: &mut MatchCheckCtxt<'_, 'tcx>,
1637 ) -> Option<Vec<Constructor<'tcx>>> {
1639 PatKind::AscribeUserType { ref subpattern, .. } => pat_constructors(cx, subpattern, pcx),
1640 PatKind::Binding { .. } | PatKind::Wild => None,
1641 PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(vec![Single]),
1642 PatKind::Variant { adt_def, variant_index, .. } => {
1643 Some(vec![Variant(adt_def.variants[variant_index].def_id)])
1645 PatKind::Constant { value } => Some(vec![ConstantValue(value, pat.span)]),
1646 PatKind::Range(PatRange { lo, hi, end }) => Some(vec![ConstantRange(
1647 lo.eval_bits(cx.tcx, cx.param_env, lo.ty),
1648 hi.eval_bits(cx.tcx, cx.param_env, hi.ty),
1653 PatKind::Array { .. } => match pcx.ty.kind {
1654 ty::Array(_, length) => Some(vec![Slice(length.eval_usize(cx.tcx, cx.param_env))]),
1655 _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty),
1657 PatKind::Slice { ref prefix, ref slice, ref suffix } => {
1658 let pat_len = prefix.len() as u64 + suffix.len() as u64;
1659 if slice.is_some() {
1660 Some((pat_len..pcx.max_slice_length + 1).map(Slice).collect())
1662 Some(vec![Slice(pat_len)])
1665 PatKind::Or { .. } => {
1666 bug!("support for or-patterns has not been fully implemented yet.");
1671 /// This computes the types of the sub patterns that a constructor should be
1674 /// For instance, a tuple pattern (43u32, 'a') has sub pattern types [u32, char].
1675 fn constructor_sub_pattern_tys<'a, 'tcx>(
1676 cx: &MatchCheckCtxt<'a, 'tcx>,
1677 ctor: &Constructor<'tcx>,
1679 ) -> Vec<Ty<'tcx>> {
1680 debug!("constructor_sub_pattern_tys({:#?}, {:?})", ctor, ty);
1682 ty::Tuple(ref fs) => fs.into_iter().map(|t| t.expect_ty()).collect(),
1683 ty::Slice(ty) | ty::Array(ty, _) => match *ctor {
1684 Slice(length) => (0..length).map(|_| ty).collect(),
1685 ConstantValue(..) => vec![],
1686 _ => bug!("bad slice pattern {:?} {:?}", ctor, ty),
1688 ty::Ref(_, rty, _) => vec![rty],
1689 ty::Adt(adt, substs) => {
1691 // Use T as the sub pattern type of Box<T>.
1692 vec![substs.type_at(0)]
1694 let variant = &adt.variants[ctor.variant_index_for_adt(cx, adt)];
1695 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(ty);
1701 adt.is_enum() || field.vis.is_accessible_from(cx.module, cx.tcx);
1702 let is_uninhabited = cx.is_uninhabited(field.ty(cx.tcx, substs));
1703 match (is_visible, is_non_exhaustive, is_uninhabited) {
1704 // Treat all uninhabited types in non-exhaustive variants as `TyErr`.
1705 (_, true, true) => cx.tcx.types.err,
1706 // Treat all non-visible fields as `TyErr`. They can't appear in any
1707 // other pattern from this match (because they are private), so their
1708 // type does not matter - but we don't want to know they are
1710 (false, ..) => cx.tcx.types.err,
1712 let ty = field.ty(cx.tcx, substs);
1714 // If the field type returned is an array of an unknown size
1717 if len.try_eval_usize(cx.tcx, cx.param_env).is_none() =>
1733 // checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
1734 // meaning all other types will compare unequal and thus equal patterns often do not cause the
1735 // second pattern to lint about unreachable match arms.
1736 fn slice_pat_covered_by_const<'tcx>(
1739 const_val: &'tcx ty::Const<'tcx>,
1740 prefix: &[Pat<'tcx>],
1741 slice: &Option<Pat<'tcx>>,
1742 suffix: &[Pat<'tcx>],
1743 param_env: ty::ParamEnv<'tcx>,
1744 ) -> Result<bool, ErrorReported> {
1745 let data: &[u8] = match (const_val.val, &const_val.ty.kind) {
1746 (ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
1747 assert_eq!(*t, tcx.types.u8);
1748 let n = n.eval_usize(tcx, param_env);
1749 let ptr = Pointer::new(AllocId(0), offset);
1750 alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
1752 (ConstValue::Slice { data, start, end }, ty::Slice(t)) => {
1753 assert_eq!(*t, tcx.types.u8);
1754 let ptr = Pointer::new(AllocId(0), Size::from_bytes(start as u64));
1755 data.get_bytes(&tcx, ptr, Size::from_bytes((end - start) as u64)).unwrap()
1757 // FIXME(oli-obk): create a way to extract fat pointers from ByRef
1758 (_, ty::Slice(_)) => return Ok(false),
1760 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1768 let pat_len = prefix.len() + suffix.len();
1769 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1773 for (ch, pat) in data[..prefix.len()]
1776 .chain(data[data.len() - suffix.len()..].iter().zip(suffix))
1779 box PatKind::Constant { value } => {
1780 let b = value.eval_bits(tcx, param_env, pat.ty);
1781 assert_eq!(b as u8 as u128, b);
1793 // Whether to evaluate a constructor using exhaustive integer matching. This is true if the
1794 // constructor is a range or constant with an integer type.
1795 fn should_treat_range_exhaustively(tcx: TyCtxt<'tcx>, ctor: &Constructor<'tcx>) -> bool {
1796 let ty = match ctor {
1797 ConstantValue(value, _) => value.ty,
1798 ConstantRange(_, _, ty, _, _) => ty,
1801 if let ty::Char | ty::Int(_) | ty::Uint(_) = ty.kind {
1802 !ty.is_ptr_sized_integral() || tcx.features().precise_pointer_size_matching
1808 /// For exhaustive integer matching, some constructors are grouped within other constructors
1809 /// (namely integer typed values are grouped within ranges). However, when specialising these
1810 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1811 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1812 /// mean creating a separate constructor for every single value in the range, which is clearly
1813 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1814 /// identical across all values in that range (i.e., there are equivalence classes of ranges of
1815 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
1816 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
1817 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
1819 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1820 /// the group of intersecting patterns changes (using the method described below).
1821 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1822 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1823 /// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
1824 /// need to be worried about matching over gargantuan ranges.
1826 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
1828 /// |------| |----------| |-------| ||
1829 /// |-------| |-------| |----| ||
1832 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
1834 /// |--|--|||-||||--||---|||-------| |-|||| ||
1836 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
1837 /// boundaries for each interval range, sort them, then create constructors for each new interval
1838 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
1839 /// merging operation depicted above.)
1841 /// `hir_id` is `None` when we're evaluating the wildcard pattern, do not lint for overlapping in
1842 /// ranges that case.
1843 fn split_grouped_constructors<'p, 'tcx>(
1845 param_env: ty::ParamEnv<'tcx>,
1846 ctors: Vec<Constructor<'tcx>>,
1847 matrix: &Matrix<'p, 'tcx>,
1850 hir_id: Option<HirId>,
1851 ) -> Vec<Constructor<'tcx>> {
1852 let mut split_ctors = Vec::with_capacity(ctors.len());
1854 for ctor in ctors.into_iter() {
1856 // For now, only ranges may denote groups of "subconstructors", so we only need to
1857 // special-case constant ranges.
1858 ConstantRange(..) if should_treat_range_exhaustively(tcx, &ctor) => {
1859 // We only care about finding all the subranges within the range of the constructor
1860 // range. Anything else is irrelevant, because it is guaranteed to result in
1861 // `NotUseful`, which is the default case anyway, and can be ignored.
1862 let ctor_range = IntRange::from_ctor(tcx, param_env, &ctor).unwrap();
1864 /// Represents a border between 2 integers. Because the intervals spanning borders
1865 /// must be able to cover every integer, we need to be able to represent
1866 /// 2^128 + 1 such borders.
1867 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
1873 // A function for extracting the borders of an integer interval.
1874 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
1875 let (lo, hi) = r.range.into_inner();
1876 let from = Border::JustBefore(lo);
1877 let to = match hi.checked_add(1) {
1878 Some(m) => Border::JustBefore(m),
1879 None => Border::AfterMax,
1881 vec![from, to].into_iter()
1884 // Collect the span and range of all the intersecting ranges to lint on likely
1885 // incorrect range patterns. (#63987)
1886 let mut overlaps = vec![];
1887 // `borders` is the set of borders between equivalence classes: each equivalence
1888 // class lies between 2 borders.
1889 let row_borders = matrix
1893 IntRange::from_pat(tcx, param_env, row.head()).map(|r| (r, row.len()))
1895 .flat_map(|(range, row_len)| {
1896 let intersection = ctor_range.intersection(&range);
1897 let should_lint = ctor_range.suspicious_intersection(&range);
1898 if let (Some(range), 1, true) = (&intersection, row_len, should_lint) {
1899 // FIXME: for now, only check for overlapping ranges on simple range
1900 // patterns. Otherwise with the current logic the following is detected
1902 // match (10u8, true) {
1903 // (0 ..= 125, false) => {}
1904 // (126 ..= 255, false) => {}
1905 // (0 ..= 255, true) => {}
1907 overlaps.push(range.clone());
1911 .flat_map(|range| range_borders(range));
1912 let ctor_borders = range_borders(ctor_range.clone());
1913 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
1914 borders.sort_unstable();
1916 lint_overlapping_patterns(tcx, hir_id, ctor_range, ty, overlaps);
1918 // We're going to iterate through every pair of borders, making sure that each
1919 // represents an interval of nonnegative length, and convert each such interval
1920 // into a constructor.
1921 for IntRange { range, .. } in
1922 borders.windows(2).filter_map(|window| match (window[0], window[1]) {
1923 (Border::JustBefore(n), Border::JustBefore(m)) => {
1925 Some(IntRange { range: n..=(m - 1), ty, span })
1930 (Border::JustBefore(n), Border::AfterMax) => {
1931 Some(IntRange { range: n..=u128::MAX, ty, span })
1933 (Border::AfterMax, _) => None,
1936 split_ctors.push(IntRange::range_to_ctor(tcx, ty, range, span));
1939 // Any other constructor can be used unchanged.
1940 _ => split_ctors.push(ctor),
1947 fn lint_overlapping_patterns(
1949 hir_id: Option<HirId>,
1950 ctor_range: IntRange<'tcx>,
1952 overlaps: Vec<IntRange<'tcx>>,
1954 if let (true, Some(hir_id)) = (!overlaps.is_empty(), hir_id) {
1955 let mut err = tcx.struct_span_lint_hir(
1956 lint::builtin::OVERLAPPING_PATTERNS,
1959 "multiple patterns covering the same range",
1961 err.span_label(ctor_range.span, "overlapping patterns");
1962 for int_range in overlaps {
1963 // Use the real type for user display of the ranges:
1967 "this range overlaps on `{}`",
1968 IntRange::range_to_ctor(tcx, ty, int_range.range, DUMMY_SP).display(tcx),
1976 fn constructor_covered_by_range<'tcx>(
1978 param_env: ty::ParamEnv<'tcx>,
1979 ctor: &Constructor<'tcx>,
1981 ) -> Result<bool, ErrorReported> {
1982 let (from, to, end, ty) = match pat.kind {
1983 box PatKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
1984 box PatKind::Range(PatRange { lo, hi, end }) => (lo, hi, end, lo.ty),
1985 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
1987 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
1988 let cmp_from = |c_from| {
1989 compare_const_vals(tcx, c_from, from, param_env, ty).map(|res| res != Ordering::Less)
1991 let cmp_to = |c_to| compare_const_vals(tcx, c_to, to, param_env, ty);
1992 macro_rules! some_or_ok {
1996 None => return Ok(false), // not char or int
2001 ConstantValue(value, _) => {
2002 let to = some_or_ok!(cmp_to(value));
2004 (to == Ordering::Less) || (end == RangeEnd::Included && to == Ordering::Equal);
2005 Ok(some_or_ok!(cmp_from(value)) && end)
2007 ConstantRange(from, to, ty, RangeEnd::Included, _) => {
2009 some_or_ok!(cmp_to(ty::Const::from_bits(tcx, to, ty::ParamEnv::empty().and(ty),)));
2011 (to == Ordering::Less) || (end == RangeEnd::Included && to == Ordering::Equal);
2012 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
2015 ty::ParamEnv::empty().and(ty),
2018 ConstantRange(from, to, ty, RangeEnd::Excluded, _) => {
2020 some_or_ok!(cmp_to(ty::Const::from_bits(tcx, to, ty::ParamEnv::empty().and(ty))));
2022 (to == Ordering::Less) || (end == RangeEnd::Excluded && to == Ordering::Equal);
2023 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
2026 ty::ParamEnv::empty().and(ty)
2034 fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
2035 cx: &mut MatchCheckCtxt<'a, 'tcx>,
2036 subpatterns: &'p [FieldPat<'tcx>],
2037 wild_patterns: &[&'p Pat<'tcx>],
2038 is_non_exhaustive: bool,
2039 ) -> PatStack<'p, 'tcx> {
2040 let mut result = SmallVec::from_slice(wild_patterns);
2042 for subpat in subpatterns {
2043 if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
2044 result[subpat.field.index()] = &subpat.pattern;
2048 debug!("patterns_for_variant({:#?}, {:#?}) = {:#?}", subpatterns, wild_patterns, result);
2049 PatStack::from_vec(result)
2052 /// This is the main specialization step. It expands the first pattern in the given row
2053 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
2054 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
2056 /// OTOH, slice patterns with a subslice pattern (tail @ ..) can be expanded into multiple
2057 /// different patterns.
2058 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
2059 /// fields filled with wild patterns.
2060 fn specialize<'p, 'a: 'p, 'q: 'p, 'tcx>(
2061 cx: &mut MatchCheckCtxt<'a, 'tcx>,
2062 r: &PatStack<'q, 'tcx>,
2063 constructor: &Constructor<'tcx>,
2064 wild_patterns: &[&'p Pat<'tcx>],
2065 ) -> Option<PatStack<'p, 'tcx>> {
2068 let new_head = match *pat.kind {
2069 PatKind::AscribeUserType { ref subpattern, .. } => {
2070 specialize(cx, &PatStack::from_pattern(subpattern), constructor, wild_patterns)
2073 PatKind::Binding { .. } | PatKind::Wild => Some(PatStack::from_slice(wild_patterns)),
2075 PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
2076 let ref variant = adt_def.variants[variant_index];
2077 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(pat.ty);
2078 Some(Variant(variant.def_id))
2079 .filter(|variant_constructor| variant_constructor == constructor)
2080 .map(|_| patterns_for_variant(cx, subpatterns, wild_patterns, is_non_exhaustive))
2083 PatKind::Leaf { ref subpatterns } => {
2084 Some(patterns_for_variant(cx, subpatterns, wild_patterns, false))
2087 PatKind::Deref { ref subpattern } => Some(PatStack::from_pattern(subpattern)),
2089 PatKind::Constant { value } if constructor.is_slice() => {
2090 // We extract an `Option` for the pointer because slices of zero
2091 // elements don't necessarily point to memory, they are usually
2092 // just integers. The only time they should be pointing to memory
2093 // is when they are subslices of nonzero slices.
2094 let (alloc, offset, n, ty) = match value.ty.kind {
2095 ty::Array(t, n) => match value.val {
2096 ConstValue::ByRef { offset, alloc, .. } => {
2097 (alloc, offset, n.eval_usize(cx.tcx, cx.param_env), t)
2099 _ => span_bug!(pat.span, "array pattern is {:?}", value,),
2103 ConstValue::Slice { data, start, end } => {
2104 (data, Size::from_bytes(start as u64), (end - start) as u64, t)
2106 ConstValue::ByRef { .. } => {
2107 // FIXME(oli-obk): implement `deref` for `ConstValue`
2112 "slice pattern constant must be scalar pair but is {:?}",
2119 "unexpected const-val {:?} with ctor {:?}",
2124 if wild_patterns.len() as u64 == n {
2125 // convert a constant slice/array pattern to a list of patterns.
2126 let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
2127 let ptr = Pointer::new(AllocId(0), offset);
2130 let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
2131 let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
2132 let scalar = scalar.not_undef().ok()?;
2133 let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
2135 Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
2136 Some(&*cx.pattern_arena.alloc(pattern))
2144 PatKind::Constant { .. } | PatKind::Range { .. } => {
2145 // If the constructor is a:
2146 // - Single value: add a row if the pattern contains the constructor.
2147 // - Range: add a row if the constructor intersects the pattern.
2148 if should_treat_range_exhaustively(cx.tcx, constructor) {
2150 IntRange::from_ctor(cx.tcx, cx.param_env, constructor),
2151 IntRange::from_pat(cx.tcx, cx.param_env, pat),
2153 (Some(ctor), Some(pat)) => ctor.intersection(&pat).map(|_| {
2154 let (pat_lo, pat_hi) = pat.range.into_inner();
2155 let (ctor_lo, ctor_hi) = ctor.range.into_inner();
2156 assert!(pat_lo <= ctor_lo && ctor_hi <= pat_hi);
2162 // Fallback for non-ranges and ranges that involve
2163 // floating-point numbers, which are not conveniently handled
2164 // by `IntRange`. For these cases, the constructor may not be a
2165 // range so intersection actually devolves into being covered
2167 match constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat) {
2168 Ok(true) => Some(PatStack::default()),
2169 Ok(false) | Err(ErrorReported) => None,
2174 PatKind::Array { ref prefix, ref slice, ref suffix }
2175 | PatKind::Slice { ref prefix, ref slice, ref suffix } => match *constructor {
2177 let pat_len = prefix.len() + suffix.len();
2178 if let Some(slice_count) = wild_patterns.len().checked_sub(pat_len) {
2179 if slice_count == 0 || slice.is_some() {
2189 .chain(suffix.iter()),
2200 ConstantValue(cv, _) => {
2201 match slice_pat_covered_by_const(
2210 Ok(true) => Some(PatStack::default()),
2212 Err(ErrorReported) => None,
2215 _ => span_bug!(pat.span, "unexpected ctor {:?} for slice pat", constructor),
2218 PatKind::Or { .. } => {
2219 bug!("support for or-patterns has not been fully implemented yet.");
2222 debug!("specialize({:#?}, {:#?}) = {:#?}", r.head(), wild_patterns, new_head);
2224 new_head.map(|head| {
2225 let mut head = head.0;
2226 head.extend_from_slice(&r.0[1..]);
2227 PatStack::from_vec(head)