1 /// Note: most tests relevant to this file can be found (at the time of writing)
2 /// in src/tests/ui/pattern/usefulness.
4 /// This file includes the logic for exhaustiveness and usefulness checking for
5 /// pattern-matching. Specifically, given a list of patterns for a type, we can
7 /// (a) the patterns cover every possible constructor for the type [exhaustiveness]
8 /// (b) each pattern is necessary [usefulness]
10 /// The algorithm implemented here is a modified version of the one described in:
11 /// http://moscova.inria.fr/~maranget/papers/warn/index.html
12 /// However, to save future implementors from reading the original paper, we
13 /// summarise the algorithm here to hopefully save time and be a little clearer
14 /// (without being so rigorous).
16 /// The core of the algorithm revolves about a "usefulness" check. In particular, we
17 /// are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
18 /// a matrix). `U(P, p)` represents whether, given an existing list of patterns
19 /// `P_1 ..= P_m`, adding a new pattern `p` will be "useful" (that is, cover previously-
20 /// uncovered values of the type).
22 /// If we have this predicate, then we can easily compute both exhaustiveness of an
23 /// entire set of patterns and the individual usefulness of each one.
24 /// (a) the set of patterns is exhaustive iff `U(P, _)` is false (i.e., adding a wildcard
25 /// match doesn't increase the number of values we're matching)
26 /// (b) a pattern `P_i` is not useful if `U(P[0..=(i-1), P_i)` is false (i.e., adding a
27 /// pattern to those that have come before it doesn't increase the number of values
30 /// During the course of the algorithm, the rows of the matrix won't just be individual patterns,
31 /// but rather partially-deconstructed patterns in the form of a list of patterns. The paper
32 /// calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
35 /// For example, say we have the following:
37 /// // x: (Option<bool>, Result<()>)
39 /// (Some(true), _) => {}
40 /// (None, Err(())) => {}
41 /// (None, Err(_)) => {}
44 /// Here, the matrix `P` starts as:
46 /// [(Some(true), _)],
47 /// [(None, Err(()))],
50 /// We can tell it's not exhaustive, because `U(P, _)` is true (we're not covering
51 /// `[(Some(false), _)]`, for instance). In addition, row 3 is not useful, because
52 /// all the values it covers are already covered by row 2.
54 /// A list of patterns can be thought of as a stack, because we are mainly interested in the top of
55 /// the stack at any given point, and we can pop or apply constructors to get new pattern-stacks.
56 /// To match the paper, the top of the stack is at the beginning / on the left.
58 /// There are two important operations on pattern-stacks necessary to understand the algorithm:
59 /// 1. We can pop a given constructor off the top of a stack. This operation is called
60 /// `specialize`, and is denoted `S(c, p)` where `c` is a constructor (like `Some` or
61 /// `None`) and `p` a pattern-stack.
62 /// If the pattern on top of the stack can cover `c`, this removes the constructor and
63 /// pushes its arguments onto the stack. It also expands OR-patterns into distinct patterns.
64 /// Otherwise the pattern-stack is discarded.
65 /// This essentially filters those pattern-stacks whose top covers the constructor `c` and
66 /// discards the others.
68 /// For example, the first pattern above initially gives a stack `[(Some(true), _)]`. If we
69 /// pop the tuple constructor, we are left with `[Some(true), _]`, and if we then pop the
70 /// `Some` constructor we get `[true, _]`. If we had popped `None` instead, we would get
73 /// This returns zero or more new pattern-stacks, as follows. We look at the pattern `p_1`
74 /// on top of the stack, and we have four cases:
75 /// 1.1. `p_1 = c(r_1, .., r_a)`, i.e. the top of the stack has constructor `c`. We
76 /// push onto the stack the arguments of this constructor, and return the result:
77 /// r_1, .., r_a, p_2, .., p_n
78 /// 1.2. `p_1 = c'(r_1, .., r_a')` where `c ≠ c'`. We discard the current stack and
80 /// 1.3. `p_1 = _`. We push onto the stack as many wildcards as the constructor `c` has
81 /// arguments (its arity), and return the resulting stack:
82 /// _, .., _, p_2, .., p_n
83 /// 1.4. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
85 /// S(c, (r_1, p_2, .., p_n))
86 /// S(c, (r_2, p_2, .., p_n))
88 /// 2. We can pop a wildcard off the top of the stack. This is called `D(p)`, where `p` is
90 /// This is used when we know there are missing constructor cases, but there might be
91 /// existing wildcard patterns, so to check the usefulness of the matrix, we have to check
92 /// all its *other* components.
94 /// It is computed as follows. We look at the pattern `p_1` on top of the stack,
95 /// and we have three cases:
96 /// 1.1. `p_1 = c(r_1, .., r_a)`. We discard the current stack and return nothing.
97 /// 1.2. `p_1 = _`. We return the rest of the stack:
99 /// 1.3. `p_1 = r_1 | r_2`. We expand the OR-pattern and then recurse on each resulting
101 /// D((r_1, p_2, .., p_n))
102 /// D((r_2, p_2, .., p_n))
104 /// Note that the OR-patterns are not always used directly in Rust, but are used to derive the
105 /// exhaustive integer matching rules, so they're written here for posterity.
107 /// Both those operations extend straightforwardly to a list or pattern-stacks, i.e. a matrix, by
108 /// working row-by-row. Popping a constructor ends up keeping only the matrix rows that start with
109 /// the given constructor, and popping a wildcard keeps those rows that start with a wildcard.
112 /// The algorithm for computing `U`
113 /// -------------------------------
114 /// The algorithm is inductive (on the number of columns: i.e., components of tuple patterns).
115 /// That means we're going to check the components from left-to-right, so the algorithm
116 /// operates principally on the first component of the matrix and new pattern-stack `p`.
117 /// This algorithm is realised in the `is_useful` function.
119 /// Base case. (`n = 0`, i.e., an empty tuple pattern)
120 /// - If `P` already contains an empty pattern (i.e., if the number of patterns `m > 0`),
121 /// then `U(P, p)` is false.
122 /// - Otherwise, `P` must be empty, so `U(P, p)` is true.
124 /// Inductive step. (`n > 0`, i.e., whether there's at least one column
125 /// [which may then be expanded into further columns later])
126 /// We're going to match on the top of the new pattern-stack, `p_1`.
127 /// - If `p_1 == c(r_1, .., r_a)`, i.e. we have a constructor pattern.
128 /// Then, the usefulness of `p_1` can be reduced to whether it is useful when
129 /// we ignore all the patterns in the first column of `P` that involve other constructors.
130 /// This is where `S(c, P)` comes in:
131 /// `U(P, p) := U(S(c, P), S(c, p))`
132 /// This special case is handled in `is_useful_specialized`.
134 /// For example, if `P` is:
139 /// and `p` is [Some(false), 0], then we don't care about row 2 since we know `p` only
140 /// matches values that row 2 doesn't. For row 1 however, we need to dig into the
141 /// arguments of `Some` to know whether some new value is covered. So we compute
142 /// `U([[true, _]], [false, 0])`.
144 /// - If `p_1 == _`, then we look at the list of constructors that appear in the first
145 /// component of the rows of `P`:
146 /// + If there are some constructors that aren't present, then we might think that the
147 /// wildcard `_` is useful, since it covers those constructors that weren't covered
149 /// That's almost correct, but only works if there were no wildcards in those first
150 /// components. So we need to check that `p` is useful with respect to the rows that
151 /// start with a wildcard, if there are any. This is where `D` comes in:
152 /// `U(P, p) := U(D(P), D(p))`
154 /// For example, if `P` is:
157 /// [None, false, 1],
159 /// and `p` is [_, false, _], the `Some` constructor doesn't appear in `P`. So if we
160 /// only had row 2, we'd know that `p` is useful. However row 1 starts with a
161 /// wildcard, so we need to check whether `U([[true, _]], [false, 1])`.
163 /// + Otherwise, all possible constructors (for the relevant type) are present. In this
164 /// case we must check whether the wildcard pattern covers any unmatched value. For
165 /// that, we can think of the `_` pattern as a big OR-pattern that covers all
166 /// possible constructors. For `Option`, that would mean `_ = None | Some(_)` for
167 /// example. The wildcard pattern is useful in this case if it is useful when
168 /// specialized to one of the possible constructors. So we compute:
169 /// `U(P, p) := ∃(k ϵ constructors) U(S(k, P), S(k, p))`
171 /// For example, if `P` is:
176 /// and `p` is [_, false], both `None` and `Some` constructors appear in the first
177 /// components of `P`. We will therefore try popping both constructors in turn: we
178 /// compute U([[true, _]], [_, false]) for the `Some` constructor, and U([[false]],
179 /// [false]) for the `None` constructor. The first case returns true, so we know that
180 /// `p` is useful for `P`. Indeed, it matches `[Some(false), _]` that wasn't matched
183 /// - If `p_1 == r_1 | r_2`, then the usefulness depends on each `r_i` separately:
184 /// `U(P, p) := U(P, (r_1, p_2, .., p_n))
185 /// || U(P, (r_2, p_2, .., p_n))`
187 /// Modifications to the algorithm
188 /// ------------------------------
189 /// The algorithm in the paper doesn't cover some of the special cases that arise in Rust, for
190 /// example uninhabited types and variable-length slice patterns. These are drawn attention to
191 /// throughout the code below. I'll make a quick note here about how exhaustive integer matching is
192 /// accounted for, though.
194 /// Exhaustive integer matching
195 /// ---------------------------
196 /// An integer type can be thought of as a (huge) sum type: 1 | 2 | 3 | ...
197 /// So to support exhaustive integer matching, we can make use of the logic in the paper for
198 /// OR-patterns. However, we obviously can't just treat ranges x..=y as individual sums, because
199 /// they are likely gigantic. So we instead treat ranges as constructors of the integers. This means
200 /// that we have a constructor *of* constructors (the integers themselves). We then need to work
201 /// through all the inductive step rules above, deriving how the ranges would be treated as
202 /// OR-patterns, and making sure that they're treated in the same way even when they're ranges.
203 /// There are really only four special cases here:
204 /// - When we match on a constructor that's actually a range, we have to treat it as if we would
206 /// + It turns out that we can simply extend the case for single-value patterns in
207 /// `specialize` to either be *equal* to a value constructor, or *contained within* a range
209 /// + When the pattern itself is a range, you just want to tell whether any of the values in
210 /// the pattern range coincide with values in the constructor range, which is precisely
212 /// Since when encountering a range pattern for a value constructor, we also use inclusion, it
213 /// means that whenever the constructor is a value/range and the pattern is also a value/range,
214 /// we can simply use intersection to test usefulness.
215 /// - When we're testing for usefulness of a pattern and the pattern's first component is a
217 /// + If all the constructors appear in the matrix, we have a slight complication. By default,
218 /// the behaviour (i.e., a disjunction over specialised matrices for each constructor) is
219 /// invalid, because we want a disjunction over every *integer* in each range, not just a
220 /// disjunction over every range. This is a bit more tricky to deal with: essentially we need
221 /// to form equivalence classes of subranges of the constructor range for which the behaviour
222 /// of the matrix `P` and new pattern `p` are the same. This is described in more
223 /// detail in `split_grouped_constructors`.
224 /// + If some constructors are missing from the matrix, it turns out we don't need to do
225 /// anything special (because we know none of the integers are actually wildcards: i.e., we
226 /// can't span wildcards using ranges).
227 use self::Constructor::*;
228 use self::Usefulness::*;
229 use self::WitnessPreference::*;
231 use rustc_data_structures::fx::FxHashMap;
232 use rustc_index::vec::Idx;
234 use super::{compare_const_vals, PatternFoldable, PatternFolder};
235 use super::{FieldPat, Pat, PatKind, PatRange};
237 use rustc::hir::def_id::DefId;
238 use rustc::hir::{HirId, RangeEnd};
239 use rustc::ty::layout::{Integer, IntegerExt, Size, VariantIdx};
240 use rustc::ty::{self, Const, Ty, TyCtxt, TypeFoldable};
243 use rustc::mir::interpret::{truncate, AllocId, ConstValue, Pointer, Scalar};
244 use rustc::mir::Field;
245 use rustc::util::captures::Captures;
246 use rustc::util::common::ErrorReported;
248 use syntax::attr::{SignedInt, UnsignedInt};
249 use syntax_pos::{Span, DUMMY_SP};
251 use arena::TypedArena;
253 use smallvec::{smallvec, SmallVec};
254 use std::cmp::{self, max, min, Ordering};
255 use std::convert::TryInto;
257 use std::iter::{FromIterator, IntoIterator};
258 use std::ops::RangeInclusive;
261 pub fn expand_pattern<'a, 'tcx>(cx: &MatchCheckCtxt<'a, 'tcx>, pat: Pat<'tcx>) -> Pat<'tcx> {
262 LiteralExpander { tcx: cx.tcx }.fold_pattern(&pat)
265 struct LiteralExpander<'tcx> {
269 impl LiteralExpander<'tcx> {
270 /// Derefs `val` and potentially unsizes the value if `crty` is an array and `rty` a slice.
272 /// `crty` and `rty` can differ because you can use array constants in the presence of slice
273 /// patterns. So the pattern may end up being a slice, but the constant is an array. We convert
274 /// the array to a slice in that case.
275 fn fold_const_value_deref(
277 val: ConstValue<'tcx>,
278 // the pattern's pointee type
280 // the constant's pointee type
282 ) -> ConstValue<'tcx> {
283 debug!("fold_const_value_deref {:?} {:?} {:?}", val, rty, crty);
284 match (val, &crty.kind, &rty.kind) {
285 // the easy case, deref a reference
286 (ConstValue::Scalar(Scalar::Ptr(p)), x, y) if x == y => {
287 let alloc = self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id);
288 ConstValue::ByRef { alloc, offset: p.offset }
290 // unsize array to slice if pattern is array but match value or other patterns are slice
291 (ConstValue::Scalar(Scalar::Ptr(p)), ty::Array(t, n), ty::Slice(u)) => {
294 data: self.tcx.alloc_map.lock().unwrap_memory(p.alloc_id),
295 start: p.offset.bytes().try_into().unwrap(),
296 end: n.eval_usize(self.tcx, ty::ParamEnv::empty()).try_into().unwrap(),
299 // fat pointers stay the same
300 (ConstValue::Slice { .. }, _, _)
301 | (_, ty::Slice(_), ty::Slice(_))
302 | (_, ty::Str, ty::Str) => val,
303 // FIXME(oli-obk): this is reachable for `const FOO: &&&u32 = &&&42;` being used
304 _ => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
309 impl PatternFolder<'tcx> for LiteralExpander<'tcx> {
310 fn fold_pattern(&mut self, pat: &Pat<'tcx>) -> Pat<'tcx> {
311 debug!("fold_pattern {:?} {:?} {:?}", pat, pat.ty.kind, pat.kind);
312 match (&pat.ty.kind, &*pat.kind) {
316 value: Const { val, ty: ty::TyS { kind: ty::Ref(_, crty, _), .. } },
321 kind: box PatKind::Deref {
325 kind: box PatKind::Constant {
326 value: self.tcx.mk_const(Const {
327 val: self.fold_const_value_deref(*val, rty, crty),
334 (_, &PatKind::Binding { subpattern: Some(ref s), .. }) => s.fold_with(self),
335 _ => pat.super_fold_with(self),
340 impl<'tcx> Pat<'tcx> {
341 fn is_wildcard(&self) -> bool {
343 PatKind::Binding { subpattern: None, .. } | PatKind::Wild => true,
349 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
351 #[derive(Debug, Clone)]
352 pub struct PatStack<'p, 'tcx>(SmallVec<[&'p Pat<'tcx>; 2]>);
354 impl<'p, 'tcx> PatStack<'p, 'tcx> {
355 pub fn from_pattern(pat: &'p Pat<'tcx>) -> Self {
356 PatStack(smallvec![pat])
359 fn from_vec(vec: SmallVec<[&'p Pat<'tcx>; 2]>) -> Self {
363 fn from_slice(s: &[&'p Pat<'tcx>]) -> Self {
364 PatStack(SmallVec::from_slice(s))
367 fn is_empty(&self) -> bool {
371 fn len(&self) -> usize {
375 fn head(&self) -> &'p Pat<'tcx> {
379 fn to_tail(&self) -> Self {
380 PatStack::from_slice(&self.0[1..])
383 fn iter(&self) -> impl Iterator<Item = &Pat<'tcx>> {
384 self.0.iter().map(|p| *p)
387 /// This computes `D(self)`. See top of the file for explanations.
388 fn specialize_wildcard(&self) -> Option<Self> {
389 if self.head().is_wildcard() { Some(self.to_tail()) } else { None }
392 /// This computes `S(constructor, self)`. See top of the file for explanations.
393 fn specialize_constructor<'a, 'q>(
395 cx: &mut MatchCheckCtxt<'a, 'tcx>,
396 constructor: &Constructor<'tcx>,
397 ctor_wild_subpatterns: &[&'q Pat<'tcx>],
398 ) -> Option<PatStack<'q, 'tcx>>
403 let new_heads = specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns);
404 new_heads.map(|mut new_head| {
405 new_head.0.extend_from_slice(&self.0[1..]);
411 impl<'p, 'tcx> Default for PatStack<'p, 'tcx> {
412 fn default() -> Self {
413 PatStack(smallvec![])
417 impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
418 fn from_iter<T>(iter: T) -> Self
420 T: IntoIterator<Item = &'p Pat<'tcx>>,
422 PatStack(iter.into_iter().collect())
427 pub struct Matrix<'p, 'tcx>(Vec<PatStack<'p, 'tcx>>);
429 impl<'p, 'tcx> Matrix<'p, 'tcx> {
430 pub fn empty() -> Self {
434 pub fn push(&mut self, row: PatStack<'p, 'tcx>) {
438 /// Iterate over the first component of each row
439 fn heads<'a>(&'a self) -> impl Iterator<Item = &'a Pat<'tcx>> + Captures<'p> {
440 self.0.iter().map(|r| r.head())
443 /// This computes `D(self)`. See top of the file for explanations.
444 fn specialize_wildcard(&self) -> Self {
445 self.0.iter().filter_map(|r| r.specialize_wildcard()).collect()
448 /// This computes `S(constructor, self)`. See top of the file for explanations.
449 fn specialize_constructor<'a, 'q>(
451 cx: &mut MatchCheckCtxt<'a, 'tcx>,
452 constructor: &Constructor<'tcx>,
453 ctor_wild_subpatterns: &[&'q Pat<'tcx>],
454 ) -> Matrix<'q, 'tcx>
462 .filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
468 /// Pretty-printer for matrices of patterns, example:
469 /// +++++++++++++++++++++++++++++
471 /// +++++++++++++++++++++++++++++
472 /// + true + [First] +
473 /// +++++++++++++++++++++++++++++
474 /// + true + [Second(true)] +
475 /// +++++++++++++++++++++++++++++
477 /// +++++++++++++++++++++++++++++
478 /// + _ + [_, _, tail @ ..] +
479 /// +++++++++++++++++++++++++++++
480 impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
481 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
484 let &Matrix(ref m) = self;
485 let pretty_printed_matrix: Vec<Vec<String>> =
486 m.iter().map(|row| row.iter().map(|pat| format!("{:?}", pat)).collect()).collect();
488 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
489 assert!(m.iter().all(|row| row.len() == column_count));
490 let column_widths: Vec<usize> = (0..column_count)
491 .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0))
494 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
495 let br = "+".repeat(total_width);
496 write!(f, "{}\n", br)?;
497 for row in pretty_printed_matrix {
499 for (column, pat_str) in row.into_iter().enumerate() {
501 write!(f, "{:1$}", pat_str, column_widths[column])?;
505 write!(f, "{}\n", br)?;
511 impl<'p, 'tcx> FromIterator<PatStack<'p, 'tcx>> for Matrix<'p, 'tcx> {
512 fn from_iter<T>(iter: T) -> Self
514 T: IntoIterator<Item = PatStack<'p, 'tcx>>,
516 Matrix(iter.into_iter().collect())
520 pub struct MatchCheckCtxt<'a, 'tcx> {
521 pub tcx: TyCtxt<'tcx>,
522 /// The module in which the match occurs. This is necessary for
523 /// checking inhabited-ness of types because whether a type is (visibly)
524 /// inhabited can depend on whether it was defined in the current module or
525 /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty
526 /// outside it's module and should not be matchable with an empty match
529 param_env: ty::ParamEnv<'tcx>,
530 pub pattern_arena: &'a TypedArena<Pat<'tcx>>,
531 pub byte_array_map: FxHashMap<*const Pat<'tcx>, Vec<&'a Pat<'tcx>>>,
534 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
535 pub fn create_and_enter<F, R>(
537 param_env: ty::ParamEnv<'tcx>,
542 F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R,
544 let pattern_arena = TypedArena::default();
550 pattern_arena: &pattern_arena,
551 byte_array_map: FxHashMap::default(),
555 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
556 if self.tcx.features().exhaustive_patterns {
557 self.tcx.is_ty_uninhabited_from(self.module, ty)
563 fn is_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
565 ty::Adt(adt_def, ..) => adt_def.is_variant_list_non_exhaustive(),
570 fn is_local(&self, ty: Ty<'tcx>) -> bool {
572 ty::Adt(adt_def, ..) => adt_def.did.is_local(),
578 #[derive(Clone, Debug)]
579 enum Constructor<'tcx> {
580 /// The constructor of all patterns that don't vary by constructor,
581 /// e.g., struct patterns and fixed-length arrays.
586 ConstantValue(&'tcx ty::Const<'tcx>, Span),
587 /// Ranges of literal values (`2..=5` and `2..5`).
588 ConstantRange(u128, u128, Ty<'tcx>, RangeEnd, Span),
589 /// Array patterns of length `n`.
591 /// Slice patterns. Captures any array constructor of `length >= i + j`.
592 VarLenSlice(u64, u64),
595 // Ignore spans when comparing, they don't carry semantic information as they are only for lints.
596 impl<'tcx> std::cmp::PartialEq for Constructor<'tcx> {
597 fn eq(&self, other: &Self) -> bool {
598 match (self, other) {
599 (Constructor::Single, Constructor::Single) => true,
600 (Constructor::Variant(a), Constructor::Variant(b)) => a == b,
601 (Constructor::ConstantValue(a, _), Constructor::ConstantValue(b, _)) => a == b,
603 Constructor::ConstantRange(a_start, a_end, a_ty, a_range_end, _),
604 Constructor::ConstantRange(b_start, b_end, b_ty, b_range_end, _),
605 ) => a_start == b_start && a_end == b_end && a_ty == b_ty && a_range_end == b_range_end,
606 (Constructor::FixedLenSlice(a), Constructor::FixedLenSlice(b)) => a == b,
608 Constructor::VarLenSlice(a_prefix, a_suffix),
609 Constructor::VarLenSlice(b_prefix, b_suffix),
610 ) => a_prefix == b_prefix && a_suffix == b_suffix,
616 impl<'tcx> Constructor<'tcx> {
617 fn is_slice(&self) -> bool {
619 FixedLenSlice { .. } | VarLenSlice { .. } => true,
624 fn variant_index_for_adt<'a>(
626 cx: &MatchCheckCtxt<'a, 'tcx>,
627 adt: &'tcx ty::AdtDef,
630 Variant(id) => adt.variant_index_with_id(*id),
632 assert!(!adt.is_enum());
635 ConstantValue(c, _) => crate::const_eval::const_variant_index(cx.tcx, cx.param_env, c),
636 _ => bug!("bad constructor {:?} for adt {:?}", self, adt),
640 fn display(&self, tcx: TyCtxt<'tcx>) -> String {
642 Constructor::ConstantValue(val, _) => format!("{}", val),
643 Constructor::ConstantRange(lo, hi, ty, range_end, _) => {
644 // Get the right sign on the output:
645 let ty = ty::ParamEnv::empty().and(*ty);
648 ty::Const::from_bits(tcx, *lo, ty),
650 ty::Const::from_bits(tcx, *hi, ty),
653 Constructor::FixedLenSlice(val) => format!("[{}]", val),
654 Constructor::VarLenSlice(prefix, suffix) => format!("[{}, .., {}]", prefix, suffix),
655 _ => bug!("bad constructor being displayed: `{:?}", self),
659 // Returns the set of constructors covered by `self` but not by
660 // anything in `other_ctors`.
664 param_env: ty::ParamEnv<'tcx>,
665 other_ctors: &Vec<Constructor<'tcx>>,
666 ) -> Vec<Constructor<'tcx>> {
668 // Those constructors can only match themselves.
669 Single | Variant(_) => {
670 if other_ctors.iter().any(|c| c == self) {
676 FixedLenSlice(self_len) => {
677 let overlaps = |c: &Constructor<'_>| match *c {
678 FixedLenSlice(other_len) => other_len == self_len,
679 VarLenSlice(prefix, suffix) => prefix + suffix <= self_len,
682 if other_ctors.iter().any(overlaps) { vec![] } else { vec![self.clone()] }
685 let mut remaining_ctors = vec![self.clone()];
687 // For each used ctor, subtract from the current set of constructors.
688 // Naming: we remove the "neg" constructors from the "pos" ones.
689 // Remember, `VarLenSlice(i, j)` covers the union of `FixedLenSlice` from
690 // `i + j` to infinity.
691 for neg_ctor in other_ctors {
692 remaining_ctors = remaining_ctors
694 .flat_map(|pos_ctor| -> SmallVec<[Constructor<'tcx>; 1]> {
695 // Compute `pos_ctor \ neg_ctor`.
696 match (&pos_ctor, neg_ctor) {
697 (&FixedLenSlice(pos_len), &VarLenSlice(neg_prefix, neg_suffix)) => {
698 let neg_len = neg_prefix + neg_suffix;
699 if neg_len <= pos_len {
706 &VarLenSlice(pos_prefix, pos_suffix),
707 &VarLenSlice(neg_prefix, neg_suffix),
709 let neg_len = neg_prefix + neg_suffix;
710 let pos_len = pos_prefix + pos_suffix;
711 if neg_len <= pos_len {
714 (pos_len..neg_len).map(FixedLenSlice).collect()
717 (&VarLenSlice(pos_prefix, pos_suffix), &FixedLenSlice(neg_len)) => {
718 let pos_len = pos_prefix + pos_suffix;
719 if neg_len < pos_len {
724 // We know that `neg_len + 1 >= pos_len >= pos_suffix`.
725 .chain(Some(VarLenSlice(
726 neg_len + 1 - pos_suffix,
732 _ if pos_ctor == *neg_ctor => smallvec![],
733 _ => smallvec![pos_ctor],
738 // If the constructors that have been considered so far already cover
739 // the entire range of `self`, no need to look at more constructors.
740 if remaining_ctors.is_empty() {
747 ConstantRange(..) | ConstantValue(..) => {
748 let mut remaining_ctors = vec![self.clone()];
749 for other_ctor in other_ctors {
750 if other_ctor == self {
751 // If a constructor appears in a `match` arm, we can
752 // eliminate it straight away.
753 remaining_ctors = vec![]
754 } else if let Some(interval) = IntRange::from_ctor(tcx, param_env, other_ctor) {
755 // Refine the required constructors for the type by subtracting
756 // the range defined by the current constructor pattern.
757 remaining_ctors = interval.subtract_from(tcx, param_env, remaining_ctors);
760 // If the constructor patterns that have been considered so far
761 // already cover the entire range of values, then we know the
762 // constructor is not missing, and we can move on to the next one.
763 if remaining_ctors.is_empty() {
768 // If a constructor has not been matched, then it is missing.
769 // We add `remaining_ctors` instead of `self`, because then we can
770 // provide more detailed error information about precisely which
771 // ranges have been omitted.
777 /// This returns one wildcard pattern for each argument to this constructor.
778 fn wildcard_subpatterns<'a>(
780 cx: &MatchCheckCtxt<'a, 'tcx>,
782 ) -> Vec<Pat<'tcx>> {
783 debug!("wildcard_subpatterns({:#?}, {:?})", self, ty);
785 ty::Tuple(ref fs) => {
786 fs.into_iter().map(|t| t.expect_ty()).map(Pat::wildcard_from_ty).collect()
788 ty::Slice(ty) | ty::Array(ty, _) => match *self {
789 FixedLenSlice(length) => (0..length).map(|_| Pat::wildcard_from_ty(ty)).collect(),
790 VarLenSlice(prefix, suffix) => {
791 (0..prefix + suffix).map(|_| Pat::wildcard_from_ty(ty)).collect()
793 ConstantValue(..) => vec![],
794 _ => bug!("bad slice pattern {:?} {:?}", self, ty),
796 ty::Ref(_, rty, _) => vec![Pat::wildcard_from_ty(rty)],
797 ty::Adt(adt, substs) => {
799 // Use T as the sub pattern type of Box<T>.
800 vec![Pat::wildcard_from_ty(substs.type_at(0))]
802 let variant = &adt.variants[self.variant_index_for_adt(cx, adt)];
803 let is_non_exhaustive =
804 variant.is_field_list_non_exhaustive() && !cx.is_local(ty);
810 adt.is_enum() || field.vis.is_accessible_from(cx.module, cx.tcx);
811 let is_uninhabited = cx.is_uninhabited(field.ty(cx.tcx, substs));
812 match (is_visible, is_non_exhaustive, is_uninhabited) {
813 // Treat all uninhabited types in non-exhaustive variants as `TyErr`.
814 (_, true, true) => cx.tcx.types.err,
815 // Treat all non-visible fields as `TyErr`. They can't appear in any
816 // other pattern from this match (because they are private), so their
817 // type does not matter - but we don't want to know they are
819 (false, ..) => cx.tcx.types.err,
821 let ty = field.ty(cx.tcx, substs);
823 // If the field type returned is an array of an unknown size
827 .try_eval_usize(cx.tcx, cx.param_env)
837 .map(Pat::wildcard_from_ty)
845 /// This computes the arity of a constructor. The arity of a constructor
846 /// is how many subpattern patterns of that constructor should be expanded to.
848 /// For instance, a tuple pattern `(_, 42, Some([]))` has the arity of 3.
849 /// A struct pattern's arity is the number of fields it contains, etc.
850 fn arity<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> u64 {
851 debug!("Constructor::arity({:#?}, {:?})", self, ty);
853 ty::Tuple(ref fs) => fs.len() as u64,
854 ty::Slice(..) | ty::Array(..) => match *self {
855 FixedLenSlice(length) => length,
856 VarLenSlice(prefix, suffix) => prefix + suffix,
857 ConstantValue(..) => 0,
858 _ => bug!("bad slice pattern {:?} {:?}", self, ty),
862 adt.variants[self.variant_index_for_adt(cx, adt)].fields.len() as u64
868 /// Apply a constructor to a list of patterns, yielding a new pattern. `pats`
869 /// must have as many elements as this constructor's arity.
872 /// `self`: `Constructor::Single`
873 /// `ty`: `(u32, u32, u32)`
874 /// `pats`: `[10, 20, _]`
875 /// returns `(10, 20, _)`
877 /// `self`: `Constructor::Variant(Option::Some)`
878 /// `ty`: `Option<bool>`
879 /// `pats`: `[false]`
880 /// returns `Some(false)`
883 cx: &MatchCheckCtxt<'a, 'tcx>,
885 pats: impl IntoIterator<Item = Pat<'tcx>>,
887 let mut subpatterns = pats.into_iter();
888 let pat = match ty.kind {
889 ty::Adt(..) | ty::Tuple(..) => {
890 let subpatterns = subpatterns
892 .map(|(i, p)| FieldPat { field: Field::new(i), pattern: p })
895 if let ty::Adt(adt, substs) = ty.kind {
900 variant_index: self.variant_index_for_adt(cx, adt),
904 PatKind::Leaf { subpatterns }
907 PatKind::Leaf { subpatterns }
911 ty::Ref(..) => PatKind::Deref { subpattern: subpatterns.nth(0).unwrap() },
913 ty::Slice(_) | ty::Array(..) => match self {
914 FixedLenSlice(_) => {
915 PatKind::Slice { prefix: subpatterns.collect(), slice: None, suffix: vec![] }
917 VarLenSlice(prefix_len, _suffix_len) => {
918 let prefix = subpatterns.by_ref().take(*prefix_len as usize).collect();
919 let suffix = subpatterns.collect();
920 let wild = Pat::wildcard_from_ty(ty);
921 PatKind::Slice { prefix, slice: Some(wild), suffix }
923 _ => bug!("bad slice pattern {:?} {:?}", self, ty),
927 ConstantValue(value, _) => PatKind::Constant { value },
928 ConstantRange(lo, hi, ty, end, _) => PatKind::Range(PatRange {
929 lo: ty::Const::from_bits(cx.tcx, lo, ty::ParamEnv::empty().and(ty)),
930 hi: ty::Const::from_bits(cx.tcx, hi, ty::ParamEnv::empty().and(ty)),
937 Pat { ty, span: DUMMY_SP, kind: Box::new(pat) }
940 /// Like `apply`, but where all the subpatterns are wildcards `_`.
941 fn apply_wildcards<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {
942 let subpatterns = self.wildcard_subpatterns(cx, ty).into_iter().rev();
943 self.apply(cx, ty, subpatterns)
947 #[derive(Clone, Debug)]
948 pub enum Usefulness<'tcx> {
950 UsefulWithWitness(Vec<Witness<'tcx>>),
954 impl<'tcx> Usefulness<'tcx> {
955 fn new_useful(preference: WitnessPreference) -> Self {
957 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
958 LeaveOutWitness => Useful,
962 fn is_useful(&self) -> bool {
969 fn apply_constructor(
971 cx: &MatchCheckCtxt<'_, 'tcx>,
972 ctor: &Constructor<'tcx>,
976 UsefulWithWitness(witnesses) => UsefulWithWitness(
979 .map(|witness| witness.apply_constructor(cx, &ctor, ty))
986 fn apply_wildcard(self, ty: Ty<'tcx>) -> Self {
988 UsefulWithWitness(witnesses) => {
989 let wild = Pat::wildcard_from_ty(ty);
994 witness.0.push(wild.clone());
1004 fn apply_missing_ctors(
1006 cx: &MatchCheckCtxt<'_, 'tcx>,
1008 missing_ctors: &MissingConstructors<'tcx>,
1011 UsefulWithWitness(witnesses) => {
1012 let new_patterns: Vec<_> =
1013 missing_ctors.iter().map(|ctor| ctor.apply_wildcards(cx, ty)).collect();
1014 // Add the new patterns to each witness
1018 .flat_map(|witness| {
1019 new_patterns.iter().map(move |pat| {
1020 let mut witness = witness.clone();
1021 witness.0.push(pat.clone());
1033 #[derive(Copy, Clone, Debug)]
1034 pub enum WitnessPreference {
1039 #[derive(Copy, Clone, Debug)]
1040 struct PatCtxt<'tcx> {
1045 /// A witness of non-exhaustiveness for error reporting, represented
1046 /// as a list of patterns (in reverse order of construction) with
1047 /// wildcards inside to represent elements that can take any inhabitant
1048 /// of the type as a value.
1050 /// A witness against a list of patterns should have the same types
1051 /// and length as the pattern matched against. Because Rust `match`
1052 /// is always against a single pattern, at the end the witness will
1053 /// have length 1, but in the middle of the algorithm, it can contain
1054 /// multiple patterns.
1056 /// For example, if we are constructing a witness for the match against
1058 /// struct Pair(Option<(u32, u32)>, bool);
1060 /// match (p: Pair) {
1061 /// Pair(None, _) => {}
1062 /// Pair(_, false) => {}
1066 /// We'll perform the following steps:
1067 /// 1. Start with an empty witness
1068 /// `Witness(vec![])`
1069 /// 2. Push a witness `Some(_)` against the `None`
1070 /// `Witness(vec![Some(_)])`
1071 /// 3. Push a witness `true` against the `false`
1072 /// `Witness(vec![Some(_), true])`
1073 /// 4. Apply the `Pair` constructor to the witnesses
1074 /// `Witness(vec![Pair(Some(_), true)])`
1076 /// The final `Pair(Some(_), true)` is then the resulting witness.
1077 #[derive(Clone, Debug)]
1078 pub struct Witness<'tcx>(Vec<Pat<'tcx>>);
1080 impl<'tcx> Witness<'tcx> {
1081 pub fn single_pattern(self) -> Pat<'tcx> {
1082 assert_eq!(self.0.len(), 1);
1083 self.0.into_iter().next().unwrap()
1086 /// Constructs a partial witness for a pattern given a list of
1087 /// patterns expanded by the specialization step.
1089 /// When a pattern P is discovered to be useful, this function is used bottom-up
1090 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
1091 /// of values, V, where each value in that set is not covered by any previously
1092 /// used patterns and is covered by the pattern P'. Examples:
1094 /// left_ty: tuple of 3 elements
1095 /// pats: [10, 20, _] => (10, 20, _)
1097 /// left_ty: struct X { a: (bool, &'static str), b: usize}
1098 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
1099 fn apply_constructor<'a>(
1101 cx: &MatchCheckCtxt<'a, 'tcx>,
1102 ctor: &Constructor<'tcx>,
1105 let arity = ctor.arity(cx, ty);
1107 let len = self.0.len() as u64;
1108 let pats = self.0.drain((len - arity) as usize..).rev();
1109 ctor.apply(cx, ty, pats)
1118 /// This determines the set of all possible constructors of a pattern matching
1119 /// values of type `left_ty`. For vectors, this would normally be an infinite set
1120 /// but is instead bounded by the maximum fixed length of slice patterns in
1121 /// the column of patterns being analyzed.
1123 /// We make sure to omit constructors that are statically impossible. E.g., for
1124 /// `Option<!>`, we do not include `Some(_)` in the returned list of constructors.
1125 fn all_constructors<'a, 'tcx>(
1126 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1128 ) -> Vec<Constructor<'tcx>> {
1129 debug!("all_constructors({:?})", pcx.ty);
1130 let ctors = match pcx.ty.kind {
1131 ty::Bool => [true, false]
1133 .map(|&b| ConstantValue(ty::Const::from_bool(cx.tcx, b), pcx.span))
1135 ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx, cx.param_env).is_some() => {
1136 let len = len.eval_usize(cx.tcx, cx.param_env);
1137 if len != 0 && cx.is_uninhabited(sub_ty) { vec![] } else { vec![FixedLenSlice(len)] }
1139 // Treat arrays of a constant but unknown length like slices.
1140 ty::Array(ref sub_ty, _) | ty::Slice(ref sub_ty) => {
1141 if cx.is_uninhabited(sub_ty) {
1142 vec![FixedLenSlice(0)]
1144 vec![VarLenSlice(0, 0)]
1147 ty::Adt(def, substs) if def.is_enum() => def
1151 !cx.tcx.features().exhaustive_patterns
1153 .uninhabited_from(cx.tcx, substs, def.adt_kind())
1154 .contains(cx.tcx, cx.module)
1156 .map(|v| Variant(v.def_id))
1160 // The valid Unicode Scalar Value ranges.
1170 '\u{10FFFF}' as u128,
1178 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
1179 let min = 1u128 << (bits - 1);
1181 vec![ConstantRange(min, max, pcx.ty, RangeEnd::Included, pcx.span)]
1184 let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
1185 let max = truncate(u128::max_value(), size);
1186 vec![ConstantRange(0, max, pcx.ty, RangeEnd::Included, pcx.span)]
1189 if cx.is_uninhabited(pcx.ty) {
1199 /// An inclusive interval, used for precise integer exhaustiveness checking.
1200 /// `IntRange`s always store a contiguous range. This means that values are
1201 /// encoded such that `0` encodes the minimum value for the integer,
1202 /// regardless of the signedness.
1203 /// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
1204 /// This makes comparisons and arithmetic on interval endpoints much more
1205 /// straightforward. See `signed_bias` for details.
1207 /// `IntRange` is never used to encode an empty range or a "range" that wraps
1208 /// around the (offset) space: i.e., `range.lo <= range.hi`.
1209 #[derive(Clone, Debug)]
1210 struct IntRange<'tcx> {
1211 pub range: RangeInclusive<u128>,
1216 impl<'tcx> IntRange<'tcx> {
1218 fn is_integral(ty: Ty<'_>) -> bool {
1220 ty::Char | ty::Int(_) | ty::Uint(_) => true,
1226 fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
1228 ty::Char => Some((Size::from_bytes(4), 0)),
1230 let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
1231 Some((size, 1u128 << (size.bits() as u128 - 1)))
1233 ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
1241 param_env: ty::ParamEnv<'tcx>,
1242 value: &Const<'tcx>,
1244 ) -> Option<IntRange<'tcx>> {
1245 if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
1247 let val = if let ConstValue::Scalar(Scalar::Raw { data, size }) = value.val {
1248 // For this specific pattern we can skip a lot of effort and go
1249 // straight to the result, after doing a bit of checking. (We
1250 // could remove this branch and just use the next branch, which
1251 // is more general but much slower.)
1252 Scalar::<()>::check_raw(data, size, target_size);
1254 } else if let Some(val) = value.try_eval_bits(tcx, param_env, ty) {
1255 // This is a more general form of the previous branch.
1260 let val = val ^ bias;
1261 Some(IntRange { range: val..=val, ty, span })
1275 ) -> Option<IntRange<'tcx>> {
1276 if Self::is_integral(ty) {
1277 // Perform a shift if the underlying types are signed,
1278 // which makes the interval arithmetic simpler.
1279 let bias = IntRange::signed_bias(tcx, ty);
1280 let (lo, hi) = (lo ^ bias, hi ^ bias);
1281 // Make sure the interval is well-formed.
1282 if lo > hi || lo == hi && *end == RangeEnd::Excluded {
1285 let offset = (*end == RangeEnd::Excluded) as u128;
1286 Some(IntRange { range: lo..=(hi - offset), ty, span })
1295 param_env: ty::ParamEnv<'tcx>,
1296 ctor: &Constructor<'tcx>,
1297 ) -> Option<IntRange<'tcx>> {
1298 // Floating-point ranges are permitted and we don't want
1299 // to consider them when constructing integer ranges.
1301 ConstantRange(lo, hi, ty, end, span) => Self::from_range(tcx, *lo, *hi, ty, end, *span),
1302 ConstantValue(val, span) => Self::from_const(tcx, param_env, val, *span),
1309 param_env: ty::ParamEnv<'tcx>,
1310 mut pat: &Pat<'tcx>,
1311 ) -> Option<IntRange<'tcx>> {
1314 box PatKind::Constant { value } => {
1315 return Self::from_const(tcx, param_env, value, pat.span);
1317 box PatKind::Range(PatRange { lo, hi, end }) => {
1318 return Self::from_range(
1320 lo.eval_bits(tcx, param_env, lo.ty),
1321 hi.eval_bits(tcx, param_env, hi.ty),
1327 box PatKind::AscribeUserType { ref subpattern, .. } => {
1335 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
1336 fn signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
1339 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
1346 /// Converts a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
1350 r: RangeInclusive<u128>,
1352 ) -> Constructor<'tcx> {
1353 let bias = IntRange::signed_bias(tcx, ty);
1354 let (lo, hi) = r.into_inner();
1356 let ty = ty::ParamEnv::empty().and(ty);
1357 ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty), span)
1359 ConstantRange(lo ^ bias, hi ^ bias, ty, RangeEnd::Included, span)
1363 /// Returns a collection of ranges that spans the values covered by `ranges`, subtracted
1364 /// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
1368 param_env: ty::ParamEnv<'tcx>,
1369 ranges: Vec<Constructor<'tcx>>,
1370 ) -> Vec<Constructor<'tcx>> {
1373 .filter_map(|r| IntRange::from_ctor(tcx, param_env, &r).map(|i| i.range));
1374 let mut remaining_ranges = vec![];
1376 let (lo, hi) = self.range.into_inner();
1377 for subrange in ranges {
1378 let (subrange_lo, subrange_hi) = subrange.into_inner();
1379 if lo > subrange_hi || subrange_lo > hi {
1380 // The pattern doesn't intersect with the subrange at all,
1381 // so the subrange remains untouched.
1382 remaining_ranges.push(Self::range_to_ctor(
1385 subrange_lo..=subrange_hi,
1389 if lo > subrange_lo {
1390 // The pattern intersects an upper section of the
1391 // subrange, so a lower section will remain.
1392 remaining_ranges.push(Self::range_to_ctor(
1395 subrange_lo..=(lo - 1),
1399 if hi < subrange_hi {
1400 // The pattern intersects a lower section of the
1401 // subrange, so an upper section will remain.
1402 remaining_ranges.push(Self::range_to_ctor(
1405 (hi + 1)..=subrange_hi,
1414 fn intersection(&self, other: &Self) -> Option<Self> {
1416 let (lo, hi) = (*self.range.start(), *self.range.end());
1417 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1418 if lo <= other_hi && other_lo <= hi {
1419 let span = other.span;
1420 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty, span })
1426 fn suspicious_intersection(&self, other: &Self) -> bool {
1427 // `false` in the following cases:
1428 // 1 ---- // 1 ---------- // 1 ---- // 1 ----
1429 // 2 ---------- // 2 ---- // 2 ---- // 2 ----
1431 // The following are currently `false`, but could be `true` in the future (#64007):
1432 // 1 --------- // 1 ---------
1433 // 2 ---------- // 2 ----------
1435 // `true` in the following cases:
1436 // 1 ------- // 1 -------
1437 // 2 -------- // 2 -------
1438 let (lo, hi) = (*self.range.start(), *self.range.end());
1439 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1440 (lo == other_hi || hi == other_lo)
1444 // A struct to compute a set of constructors equivalent to `all_ctors \ used_ctors`.
1445 struct MissingConstructors<'tcx> {
1447 param_env: ty::ParamEnv<'tcx>,
1448 all_ctors: Vec<Constructor<'tcx>>,
1449 used_ctors: Vec<Constructor<'tcx>>,
1452 impl<'tcx> MissingConstructors<'tcx> {
1455 param_env: ty::ParamEnv<'tcx>,
1456 all_ctors: Vec<Constructor<'tcx>>,
1457 used_ctors: Vec<Constructor<'tcx>>,
1459 MissingConstructors { tcx, param_env, all_ctors, used_ctors }
1462 fn into_inner(self) -> (Vec<Constructor<'tcx>>, Vec<Constructor<'tcx>>) {
1463 (self.all_ctors, self.used_ctors)
1466 fn is_empty(&self) -> bool {
1467 self.iter().next().is_none()
1469 /// Whether this contains all the constructors for the given type or only a
1471 fn all_ctors_are_missing(&self) -> bool {
1472 self.used_ctors.is_empty()
1475 /// Iterate over all_ctors \ used_ctors
1476 fn iter<'a>(&'a self) -> impl Iterator<Item = Constructor<'tcx>> + Captures<'a> {
1477 self.all_ctors.iter().flat_map(move |req_ctor| {
1478 req_ctor.subtract_ctors(self.tcx, self.param_env, &self.used_ctors)
1483 impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
1484 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1485 let ctors: Vec<_> = self.iter().collect();
1486 write!(f, "{:?}", ctors)
1490 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html.
1491 /// The algorithm from the paper has been modified to correctly handle empty
1492 /// types. The changes are:
1493 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1494 /// continue to recurse over columns.
1495 /// (1) all_constructors will only return constructors that are statically
1496 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
1498 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1499 /// to a set of such vectors `m` - this is defined as there being a set of
1500 /// inputs that will match `v` but not any of the sets in `m`.
1502 /// All the patterns at each column of the `matrix ++ v` matrix must
1503 /// have the same type, except that wildcard (PatKind::Wild) patterns
1504 /// with type `TyErr` are also allowed, even if the "type of the column"
1505 /// is not `TyErr`. That is used to represent private fields, as using their
1506 /// real type would assert that they are inhabited.
1508 /// This is used both for reachability checking (if a pattern isn't useful in
1509 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1510 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1511 /// matrix isn't exhaustive).
1512 pub fn is_useful<'p, 'a, 'tcx>(
1513 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1514 matrix: &Matrix<'p, 'tcx>,
1515 v: &PatStack<'_, 'tcx>,
1516 witness_preference: WitnessPreference,
1518 ) -> Usefulness<'tcx> {
1519 let &Matrix(ref rows) = matrix;
1520 debug!("is_useful({:#?}, {:#?})", matrix, v);
1522 // The base case. We are pattern-matching on () and the return value is
1523 // based on whether our matrix has a row or not.
1524 // NOTE: This could potentially be optimized by checking rows.is_empty()
1525 // first and then, if v is non-empty, the return value is based on whether
1526 // the type of the tuple we're checking is inhabited or not.
1528 return if rows.is_empty() {
1529 Usefulness::new_useful(witness_preference)
1535 assert!(rows.iter().all(|r| r.len() == v.len()));
1537 let (ty, span) = matrix
1539 .map(|r| (r.ty, r.span))
1540 .find(|(ty, _)| !ty.references_error())
1541 .unwrap_or((v.head().ty, v.head().span));
1543 // TyErr is used to represent the type of wildcard patterns matching
1544 // against inaccessible (private) fields of structs, so that we won't
1545 // be able to observe whether the types of the struct's fields are
1548 // If the field is truly inaccessible, then all the patterns
1549 // matching against it must be wildcard patterns, so its type
1552 // However, if we are matching against non-wildcard patterns, we
1553 // need to know the real type of the field so we can specialize
1554 // against it. This primarily occurs through constants - they
1555 // can include contents for fields that are inaccessible at the
1556 // location of the match. In that case, the field's type is
1557 // inhabited - by the constant - so we can just use it.
1559 // FIXME: this might lead to "unstable" behavior with macro hygiene
1560 // introducing uninhabited patterns for inaccessible fields. We
1561 // need to figure out how to model that.
1566 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
1568 if let Some(constructor) = pat_constructor(cx, v.head(), pcx) {
1569 debug!("is_useful - expanding constructor: {:#?}", constructor);
1570 split_grouped_constructors(
1580 .map(|c| is_useful_specialized(cx, matrix, v, c, pcx.ty, witness_preference, hir_id))
1581 .find(|result| result.is_useful())
1582 .unwrap_or(NotUseful)
1584 debug!("is_useful - expanding wildcard");
1586 let used_ctors: Vec<Constructor<'_>> =
1587 matrix.heads().filter_map(|p| pat_constructor(cx, p, pcx)).collect();
1588 debug!("used_ctors = {:#?}", used_ctors);
1589 // `all_ctors` are all the constructors for the given type, which
1590 // should all be represented (or caught with the wild pattern `_`).
1591 let all_ctors = all_constructors(cx, pcx);
1592 debug!("all_ctors = {:#?}", all_ctors);
1594 let is_privately_empty = all_ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1595 let is_declared_nonexhaustive = cx.is_non_exhaustive_enum(pcx.ty) && !cx.is_local(pcx.ty);
1597 // `missing_ctors` is the set of constructors from the same type as the
1598 // first column of `matrix` that are matched only by wildcard patterns
1599 // from the first column.
1601 // Therefore, if there is some pattern that is unmatched by `matrix`,
1602 // it will still be unmatched if the first constructor is replaced by
1603 // any of the constructors in `missing_ctors`
1605 // However, if our scrutinee is *privately* an empty enum, we
1606 // must treat it as though it had an "unknown" constructor (in
1607 // that case, all other patterns obviously can't be variants)
1608 // to avoid exposing its emptyness. See the `match_privately_empty`
1609 // test for details.
1611 // FIXME: currently the only way I know of something can
1612 // be a privately-empty enum is when the exhaustive_patterns
1613 // feature flag is not present, so this is only
1614 // needed for that case.
1616 // Missing constructors are those that are not matched by any
1617 // non-wildcard patterns in the current column. To determine if
1618 // the set is empty, we can check that `.peek().is_none()`, so
1619 // we only fully construct them on-demand, because they're rarely used and can be big.
1620 let missing_ctors = MissingConstructors::new(cx.tcx, cx.param_env, all_ctors, used_ctors);
1623 "missing_ctors.empty()={:#?} is_privately_empty={:#?} is_declared_nonexhaustive={:#?}",
1624 missing_ctors.is_empty(),
1626 is_declared_nonexhaustive
1629 // For privately empty and non-exhaustive enums, we work as if there were an "extra"
1630 // `_` constructor for the type, so we can never match over all constructors.
1631 let is_non_exhaustive = is_privately_empty
1632 || is_declared_nonexhaustive
1633 || (pcx.ty.is_ptr_sized_integral() && !cx.tcx.features().precise_pointer_size_matching);
1635 if missing_ctors.is_empty() && !is_non_exhaustive {
1636 let (all_ctors, _) = missing_ctors.into_inner();
1637 split_grouped_constructors(cx.tcx, cx.param_env, pcx, all_ctors, matrix, DUMMY_SP, None)
1640 is_useful_specialized(cx, matrix, v, c, pcx.ty, witness_preference, hir_id)
1642 .find(|result| result.is_useful())
1643 .unwrap_or(NotUseful)
1645 let matrix = matrix.specialize_wildcard();
1646 let v = v.to_tail();
1647 let usefulness = is_useful(cx, &matrix, &v, witness_preference, hir_id);
1649 // In this case, there's at least one "free"
1650 // constructor that is only matched against by
1651 // wildcard patterns.
1653 // There are 2 ways we can report a witness here.
1654 // Commonly, we can report all the "free"
1655 // constructors as witnesses, e.g., if we have:
1658 // enum Direction { N, S, E, W }
1659 // let Direction::N = ...;
1662 // we can report 3 witnesses: `S`, `E`, and `W`.
1664 // However, there are 2 cases where we don't want
1665 // to do this and instead report a single `_` witness:
1667 // 1) If the user is matching against a non-exhaustive
1668 // enum, there is no point in enumerating all possible
1669 // variants, because the user can't actually match
1670 // against them themselves, e.g., in an example like:
1672 // let err: io::ErrorKind = ...;
1674 // io::ErrorKind::NotFound => {},
1677 // we don't want to show every possible IO error,
1678 // but instead have `_` as the witness (this is
1679 // actually *required* if the user specified *all*
1680 // IO errors, but is probably what we want in every
1683 // 2) If the user didn't actually specify a constructor
1684 // in this arm, e.g., in
1686 // let x: (Direction, Direction, bool) = ...;
1687 // let (_, _, false) = x;
1689 // we don't want to show all 16 possible witnesses
1690 // `(<direction-1>, <direction-2>, true)` - we are
1691 // satisfied with `(_, _, true)`. In this case,
1692 // `used_ctors` is empty.
1693 if is_non_exhaustive || missing_ctors.all_ctors_are_missing() {
1694 // All constructors are unused. Add a wild pattern
1695 // rather than each individual constructor.
1696 usefulness.apply_wildcard(pcx.ty)
1698 // Construct for each missing constructor a "wild" version of this
1699 // constructor, that matches everything that can be built with
1700 // it. For example, if `ctor` is a `Constructor::Variant` for
1701 // `Option::Some`, we get the pattern `Some(_)`.
1702 usefulness.apply_missing_ctors(cx, pcx.ty, &missing_ctors)
1708 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
1709 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1710 fn is_useful_specialized<'p, 'a, 'tcx>(
1711 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1712 matrix: &Matrix<'p, 'tcx>,
1713 v: &PatStack<'_, 'tcx>,
1714 ctor: Constructor<'tcx>,
1716 witness_preference: WitnessPreference,
1718 ) -> Usefulness<'tcx> {
1719 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1721 let ctor_wild_subpatterns_owned: Vec<_> = ctor.wildcard_subpatterns(cx, lty);
1722 let ctor_wild_subpatterns: Vec<_> = ctor_wild_subpatterns_owned.iter().collect();
1723 let matrix = matrix.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns);
1724 v.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns)
1725 .map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id))
1726 .map(|u| u.apply_constructor(cx, &ctor, lty))
1727 .unwrap_or(NotUseful)
1730 /// Determines the constructor that the given pattern can be specialized to.
1731 /// Returns `None` in case of a catch-all, which can't be specialized.
1732 fn pat_constructor<'tcx>(
1733 cx: &mut MatchCheckCtxt<'_, 'tcx>,
1736 ) -> Option<Constructor<'tcx>> {
1738 PatKind::AscribeUserType { ref subpattern, .. } => pat_constructor(cx, subpattern, pcx),
1739 PatKind::Binding { .. } | PatKind::Wild => None,
1740 PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(Single),
1741 PatKind::Variant { adt_def, variant_index, .. } => {
1742 Some(Variant(adt_def.variants[variant_index].def_id))
1744 PatKind::Constant { value } => Some(ConstantValue(value, pat.span)),
1745 PatKind::Range(PatRange { lo, hi, end }) => Some(ConstantRange(
1746 lo.eval_bits(cx.tcx, cx.param_env, lo.ty),
1747 hi.eval_bits(cx.tcx, cx.param_env, hi.ty),
1752 PatKind::Array { .. } => match pcx.ty.kind {
1753 ty::Array(_, length) => Some(FixedLenSlice(length.eval_usize(cx.tcx, cx.param_env))),
1754 _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty),
1756 PatKind::Slice { ref prefix, ref slice, ref suffix } => {
1757 let prefix = prefix.len() as u64;
1758 let suffix = suffix.len() as u64;
1759 if slice.is_some() {
1760 Some(VarLenSlice(prefix, suffix))
1762 Some(FixedLenSlice(prefix + suffix))
1765 PatKind::Or { .. } => {
1766 bug!("support for or-patterns has not been fully implemented yet.");
1771 // checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
1772 // meaning all other types will compare unequal and thus equal patterns often do not cause the
1773 // second pattern to lint about unreachable match arms.
1774 fn slice_pat_covered_by_const<'tcx>(
1777 const_val: &'tcx ty::Const<'tcx>,
1778 prefix: &[Pat<'tcx>],
1779 slice: &Option<Pat<'tcx>>,
1780 suffix: &[Pat<'tcx>],
1781 param_env: ty::ParamEnv<'tcx>,
1782 ) -> Result<bool, ErrorReported> {
1783 let data: &[u8] = match (const_val.val, &const_val.ty.kind) {
1784 (ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
1785 assert_eq!(*t, tcx.types.u8);
1786 let n = n.eval_usize(tcx, param_env);
1787 let ptr = Pointer::new(AllocId(0), offset);
1788 alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
1790 (ConstValue::Slice { data, start, end }, ty::Slice(t)) => {
1791 assert_eq!(*t, tcx.types.u8);
1792 let ptr = Pointer::new(AllocId(0), Size::from_bytes(start as u64));
1793 data.get_bytes(&tcx, ptr, Size::from_bytes((end - start) as u64)).unwrap()
1795 // FIXME(oli-obk): create a way to extract fat pointers from ByRef
1796 (_, ty::Slice(_)) => return Ok(false),
1798 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1806 let pat_len = prefix.len() + suffix.len();
1807 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1811 for (ch, pat) in data[..prefix.len()]
1814 .chain(data[data.len() - suffix.len()..].iter().zip(suffix))
1817 box PatKind::Constant { value } => {
1818 let b = value.eval_bits(tcx, param_env, pat.ty);
1819 assert_eq!(b as u8 as u128, b);
1831 // Whether to evaluate a constructor using exhaustive integer matching. This is true if the
1832 // constructor is a range or constant with an integer type.
1833 fn should_treat_range_exhaustively(tcx: TyCtxt<'tcx>, ctor: &Constructor<'tcx>) -> bool {
1834 let ty = match ctor {
1835 ConstantValue(value, _) => value.ty,
1836 ConstantRange(_, _, ty, _, _) => ty,
1839 if let ty::Char | ty::Int(_) | ty::Uint(_) = ty.kind {
1840 !ty.is_ptr_sized_integral() || tcx.features().precise_pointer_size_matching
1846 /// For exhaustive integer matching, some constructors are grouped within other constructors
1847 /// (namely integer typed values are grouped within ranges). However, when specialising these
1848 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1849 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1850 /// mean creating a separate constructor for every single value in the range, which is clearly
1851 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1852 /// identical across all values in that range (i.e., there are equivalence classes of ranges of
1853 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
1854 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
1855 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
1857 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1858 /// the group of intersecting patterns changes (using the method described below).
1859 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1860 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1861 /// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
1862 /// need to be worried about matching over gargantuan ranges.
1864 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
1866 /// |------| |----------| |-------| ||
1867 /// |-------| |-------| |----| ||
1870 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
1872 /// |--|--|||-||||--||---|||-------| |-|||| ||
1874 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
1875 /// boundaries for each interval range, sort them, then create constructors for each new interval
1876 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
1877 /// merging operation depicted above.)
1879 /// `hir_id` is `None` when we're evaluating the wildcard pattern, do not lint for overlapping in
1880 /// ranges that case.
1882 /// This also splits variable-length slices into fixed-length slices.
1883 fn split_grouped_constructors<'p, 'tcx>(
1885 param_env: ty::ParamEnv<'tcx>,
1887 ctors: Vec<Constructor<'tcx>>,
1888 matrix: &Matrix<'p, 'tcx>,
1890 hir_id: Option<HirId>,
1891 ) -> Vec<Constructor<'tcx>> {
1893 let mut split_ctors = Vec::with_capacity(ctors.len());
1895 for ctor in ctors.into_iter() {
1897 ConstantRange(..) if should_treat_range_exhaustively(tcx, &ctor) => {
1898 // We only care about finding all the subranges within the range of the constructor
1899 // range. Anything else is irrelevant, because it is guaranteed to result in
1900 // `NotUseful`, which is the default case anyway, and can be ignored.
1901 let ctor_range = IntRange::from_ctor(tcx, param_env, &ctor).unwrap();
1903 /// Represents a border between 2 integers. Because the intervals spanning borders
1904 /// must be able to cover every integer, we need to be able to represent
1905 /// 2^128 + 1 such borders.
1906 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
1912 // A function for extracting the borders of an integer interval.
1913 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
1914 let (lo, hi) = r.range.into_inner();
1915 let from = Border::JustBefore(lo);
1916 let to = match hi.checked_add(1) {
1917 Some(m) => Border::JustBefore(m),
1918 None => Border::AfterMax,
1920 vec![from, to].into_iter()
1923 // Collect the span and range of all the intersecting ranges to lint on likely
1924 // incorrect range patterns. (#63987)
1925 let mut overlaps = vec![];
1926 // `borders` is the set of borders between equivalence classes: each equivalence
1927 // class lies between 2 borders.
1928 let row_borders = matrix
1932 IntRange::from_pat(tcx, param_env, row.head()).map(|r| (r, row.len()))
1934 .flat_map(|(range, row_len)| {
1935 let intersection = ctor_range.intersection(&range);
1936 let should_lint = ctor_range.suspicious_intersection(&range);
1937 if let (Some(range), 1, true) = (&intersection, row_len, should_lint) {
1938 // FIXME: for now, only check for overlapping ranges on simple range
1939 // patterns. Otherwise with the current logic the following is detected
1941 // match (10u8, true) {
1942 // (0 ..= 125, false) => {}
1943 // (126 ..= 255, false) => {}
1944 // (0 ..= 255, true) => {}
1946 overlaps.push(range.clone());
1950 .flat_map(|range| range_borders(range));
1951 let ctor_borders = range_borders(ctor_range.clone());
1952 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
1953 borders.sort_unstable();
1955 lint_overlapping_patterns(tcx, hir_id, ctor_range, ty, overlaps);
1957 // We're going to iterate through every adjacent pair of borders, making sure that
1958 // each represents an interval of nonnegative length, and convert each such
1959 // interval into a constructor.
1960 for IntRange { range, .. } in
1961 borders.windows(2).filter_map(|window| match (window[0], window[1]) {
1962 (Border::JustBefore(n), Border::JustBefore(m)) => {
1964 Some(IntRange { range: n..=(m - 1), ty, span })
1969 (Border::JustBefore(n), Border::AfterMax) => {
1970 Some(IntRange { range: n..=u128::MAX, ty, span })
1972 (Border::AfterMax, _) => None,
1975 split_ctors.push(IntRange::range_to_ctor(tcx, ty, range, span));
1978 VarLenSlice(self_prefix, self_suffix) => {
1979 // The exhaustiveness-checking paper does not include any details on
1980 // checking variable-length slice patterns. However, they are matched
1981 // by an infinite collection of fixed-length array patterns.
1983 // Checking the infinite set directly would take an infinite amount
1984 // of time. However, it turns out that for each finite set of
1985 // patterns `P`, all sufficiently large array lengths are equivalent:
1987 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
1988 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
1989 // `sₘ` for each sufficiently-large length `m` that applies to exactly
1990 // the same subset of `P`.
1992 // Because of that, each witness for reachability-checking from one
1993 // of the sufficiently-large lengths can be transformed to an
1994 // equally-valid witness from any other length, so we only have
1995 // to check slice lengths from the "minimal sufficiently-large length"
1998 // Note that the fact that there is a *single* `sₘ` for each `m`
1999 // not depending on the specific pattern in `P` is important: if
2000 // you look at the pair of patterns
2003 // Then any slice of length ≥1 that matches one of these two
2004 // patterns can be trivially turned to a slice of any
2005 // other length ≥1 that matches them and vice-versa - for
2006 // but the slice from length 2 `[false, true]` that matches neither
2007 // of these patterns can't be turned to a slice from length 1 that
2008 // matches neither of these patterns, so we have to consider
2009 // slices from length 2 there.
2011 // Now, to see that that length exists and find it, observe that slice
2012 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
2013 // "variable-length" patterns (`[_, .., _]`).
2015 // For fixed-length patterns, all slices with lengths *longer* than
2016 // the pattern's length have the same outcome (of not matching), so
2017 // as long as `L` is greater than the pattern's length we can pick
2018 // any `sₘ` from that length and get the same result.
2020 // For variable-length patterns, the situation is more complicated,
2021 // because as seen above the precise value of `sₘ` matters.
2023 // However, for each variable-length pattern `p` with a prefix of length
2024 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
2025 // `slₚ` elements are examined.
2027 // Therefore, as long as `L` is positive (to avoid concerns about empty
2028 // types), all elements after the maximum prefix length and before
2029 // the maximum suffix length are not examined by any variable-length
2030 // pattern, and therefore can be added/removed without affecting
2031 // them - creating equivalent patterns from any sufficiently-large
2034 // Of course, if fixed-length patterns exist, we must be sure
2035 // that our length is large enough to miss them all, so
2036 // we can pick `L = max(max(FIXED_LEN)+1, max(PREFIX_LEN) + max(SUFFIX_LEN))`
2038 // for example, with the above pair of patterns, all elements
2039 // but the first and last can be added/removed, so any
2040 // witness of length ≥2 (say, `[false, false, true]`) can be
2041 // turned to a witness from any other length ≥2.
2043 let mut max_prefix_len = self_prefix;
2044 let mut max_suffix_len = self_suffix;
2045 let mut max_fixed_len = 0;
2047 for row in matrix.heads() {
2049 PatKind::Constant { value } => {
2050 // extract the length of an array/slice from a constant
2051 match (value.val, &value.ty.kind) {
2052 (_, ty::Array(_, n)) => {
2054 cmp::max(max_fixed_len, n.eval_usize(tcx, param_env))
2056 (ConstValue::Slice { start, end, .. }, ty::Slice(_)) => {
2057 max_fixed_len = cmp::max(max_fixed_len, (end - start) as u64)
2062 PatKind::Slice { ref prefix, slice: None, ref suffix } => {
2063 let fixed_len = prefix.len() as u64 + suffix.len() as u64;
2064 max_fixed_len = cmp::max(max_fixed_len, fixed_len);
2066 PatKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
2067 max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
2068 max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
2074 // For diagnostics, we keep the prefix and suffix lengths separate, so in the case
2075 // where `max_fixed_len + 1` is the largest, we adapt `max_prefix_len` accordingly,
2076 // so that `L = max_prefix_len + max_suffix_len`.
2077 if max_fixed_len + 1 >= max_prefix_len + max_suffix_len {
2078 // The subtraction can't overflow thanks to the above check.
2079 // The new `max_prefix_len` is also guaranteed to be larger than its previous
2081 max_prefix_len = max_fixed_len + 1 - max_suffix_len;
2084 // `ctor` originally covered the range `(self_prefix + self_suffix..infinity)`. We
2085 // now split it into two: lengths smaller than `max_prefix_len + max_suffix_len`
2086 // are treated independently as fixed-lengths slices, and lengths above are
2087 // captured by a final VarLenSlice constructor.
2089 (self_prefix + self_suffix..max_prefix_len + max_suffix_len).map(FixedLenSlice),
2091 split_ctors.push(VarLenSlice(max_prefix_len, max_suffix_len));
2093 // Any other constructor can be used unchanged.
2094 _ => split_ctors.push(ctor),
2101 fn lint_overlapping_patterns(
2103 hir_id: Option<HirId>,
2104 ctor_range: IntRange<'tcx>,
2106 overlaps: Vec<IntRange<'tcx>>,
2108 if let (true, Some(hir_id)) = (!overlaps.is_empty(), hir_id) {
2109 let mut err = tcx.struct_span_lint_hir(
2110 lint::builtin::OVERLAPPING_PATTERNS,
2113 "multiple patterns covering the same range",
2115 err.span_label(ctor_range.span, "overlapping patterns");
2116 for int_range in overlaps {
2117 // Use the real type for user display of the ranges:
2121 "this range overlaps on `{}`",
2122 IntRange::range_to_ctor(tcx, ty, int_range.range, DUMMY_SP).display(tcx),
2130 fn constructor_covered_by_range<'tcx>(
2132 param_env: ty::ParamEnv<'tcx>,
2133 ctor: &Constructor<'tcx>,
2135 ) -> Result<bool, ErrorReported> {
2136 let (from, to, end, ty) = match pat.kind {
2137 box PatKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
2138 box PatKind::Range(PatRange { lo, hi, end }) => (lo, hi, end, lo.ty),
2139 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
2141 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
2142 let cmp_from = |c_from| {
2143 compare_const_vals(tcx, c_from, from, param_env, ty).map(|res| res != Ordering::Less)
2145 let cmp_to = |c_to| compare_const_vals(tcx, c_to, to, param_env, ty);
2146 macro_rules! some_or_ok {
2150 None => return Ok(false), // not char or int
2155 ConstantValue(value, _) => {
2156 let to = some_or_ok!(cmp_to(value));
2158 (to == Ordering::Less) || (end == RangeEnd::Included && to == Ordering::Equal);
2159 Ok(some_or_ok!(cmp_from(value)) && end)
2161 ConstantRange(from, to, ty, RangeEnd::Included, _) => {
2163 some_or_ok!(cmp_to(ty::Const::from_bits(tcx, to, ty::ParamEnv::empty().and(ty),)));
2165 (to == Ordering::Less) || (end == RangeEnd::Included && to == Ordering::Equal);
2166 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
2169 ty::ParamEnv::empty().and(ty),
2172 ConstantRange(from, to, ty, RangeEnd::Excluded, _) => {
2174 some_or_ok!(cmp_to(ty::Const::from_bits(tcx, to, ty::ParamEnv::empty().and(ty))));
2176 (to == Ordering::Less) || (end == RangeEnd::Excluded && to == Ordering::Equal);
2177 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
2180 ty::ParamEnv::empty().and(ty)
2188 fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
2189 cx: &mut MatchCheckCtxt<'a, 'tcx>,
2190 subpatterns: &'p [FieldPat<'tcx>],
2191 ctor_wild_subpatterns: &[&'p Pat<'tcx>],
2192 is_non_exhaustive: bool,
2193 ) -> PatStack<'p, 'tcx> {
2194 let mut result = SmallVec::from_slice(ctor_wild_subpatterns);
2196 for subpat in subpatterns {
2197 if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
2198 result[subpat.field.index()] = &subpat.pattern;
2203 "patterns_for_variant({:#?}, {:#?}) = {:#?}",
2204 subpatterns, ctor_wild_subpatterns, result
2206 PatStack::from_vec(result)
2209 /// This is the main specialization step. It expands the pattern
2210 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
2211 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
2212 /// Returns `None` if the pattern does not have the given constructor.
2214 /// OTOH, slice patterns with a subslice pattern (tail @ ..) can be expanded into multiple
2215 /// different patterns.
2216 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
2217 /// fields filled with wild patterns.
2218 fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
2219 cx: &mut MatchCheckCtxt<'a, 'tcx>,
2221 constructor: &Constructor<'tcx>,
2222 ctor_wild_subpatterns: &[&'p Pat<'tcx>],
2223 ) -> Option<PatStack<'p, 'tcx>> {
2224 let result = match *pat.kind {
2225 PatKind::AscribeUserType { ref subpattern, .. } => PatStack::from_pattern(subpattern)
2226 .specialize_constructor(cx, constructor, ctor_wild_subpatterns),
2228 PatKind::Binding { .. } | PatKind::Wild => {
2229 Some(PatStack::from_slice(ctor_wild_subpatterns))
2232 PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
2233 let ref variant = adt_def.variants[variant_index];
2234 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(pat.ty);
2235 Some(Variant(variant.def_id))
2236 .filter(|variant_constructor| variant_constructor == constructor)
2238 patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, is_non_exhaustive)
2242 PatKind::Leaf { ref subpatterns } => {
2243 Some(patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, false))
2246 PatKind::Deref { ref subpattern } => Some(PatStack::from_pattern(subpattern)),
2248 PatKind::Constant { value } if constructor.is_slice() => {
2249 // We extract an `Option` for the pointer because slices of zero
2250 // elements don't necessarily point to memory, they are usually
2251 // just integers. The only time they should be pointing to memory
2252 // is when they are subslices of nonzero slices.
2253 let (alloc, offset, n, ty) = match value.ty.kind {
2254 ty::Array(t, n) => match value.val {
2255 ConstValue::ByRef { offset, alloc, .. } => {
2256 (alloc, offset, n.eval_usize(cx.tcx, cx.param_env), t)
2258 _ => span_bug!(pat.span, "array pattern is {:?}", value,),
2262 ConstValue::Slice { data, start, end } => {
2263 (data, Size::from_bytes(start as u64), (end - start) as u64, t)
2265 ConstValue::ByRef { .. } => {
2266 // FIXME(oli-obk): implement `deref` for `ConstValue`
2271 "slice pattern constant must be scalar pair but is {:?}",
2278 "unexpected const-val {:?} with ctor {:?}",
2283 if ctor_wild_subpatterns.len() as u64 == n {
2284 // convert a constant slice/array pattern to a list of patterns.
2285 let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
2286 let ptr = Pointer::new(AllocId(0), offset);
2289 let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
2290 let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
2291 let scalar = scalar.not_undef().ok()?;
2292 let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
2294 Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
2295 Some(&*cx.pattern_arena.alloc(pattern))
2303 PatKind::Constant { .. } | PatKind::Range { .. } => {
2304 // If the constructor is a:
2305 // - Single value: add a row if the pattern contains the constructor.
2306 // - Range: add a row if the constructor intersects the pattern.
2307 if should_treat_range_exhaustively(cx.tcx, constructor) {
2309 IntRange::from_ctor(cx.tcx, cx.param_env, constructor),
2310 IntRange::from_pat(cx.tcx, cx.param_env, pat),
2312 (Some(ctor), Some(pat)) => ctor.intersection(&pat).map(|_| {
2313 let (pat_lo, pat_hi) = pat.range.into_inner();
2314 let (ctor_lo, ctor_hi) = ctor.range.into_inner();
2315 assert!(pat_lo <= ctor_lo && ctor_hi <= pat_hi);
2321 // Fallback for non-ranges and ranges that involve
2322 // floating-point numbers, which are not conveniently handled
2323 // by `IntRange`. For these cases, the constructor may not be a
2324 // range so intersection actually devolves into being covered
2326 match constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat) {
2327 Ok(true) => Some(PatStack::default()),
2328 Ok(false) | Err(ErrorReported) => None,
2333 PatKind::Array { ref prefix, ref slice, ref suffix }
2334 | PatKind::Slice { ref prefix, ref slice, ref suffix } => match *constructor {
2335 FixedLenSlice(..) | VarLenSlice(..) => {
2336 let pat_len = prefix.len() + suffix.len();
2337 if let Some(slice_count) = ctor_wild_subpatterns.len().checked_sub(pat_len) {
2338 if slice_count == 0 || slice.is_some() {
2343 ctor_wild_subpatterns
2348 .chain(suffix.iter()),
2359 ConstantValue(cv, _) => {
2360 match slice_pat_covered_by_const(
2369 Ok(true) => Some(PatStack::default()),
2371 Err(ErrorReported) => None,
2374 _ => span_bug!(pat.span, "unexpected ctor {:?} for slice pat", constructor),
2377 PatKind::Or { .. } => {
2378 bug!("support for or-patterns has not been fully implemented yet.");
2381 debug!("specialize({:#?}, {:#?}) = {:#?}", pat, ctor_wild_subpatterns, result);