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) {
317 val: ty::ConstKind::Value(val),
318 ty: ty::TyS { kind: ty::Ref(_, crty, _), .. }
324 kind: box PatKind::Deref {
328 kind: box PatKind::Constant {
329 value: self.tcx.mk_const(Const {
330 val: ty::ConstKind::Value(
331 self.fold_const_value_deref(*val, rty, crty)
343 value: Const { val, ty: ty::TyS { kind: ty::Ref(_, crty, _), .. } },
345 ) => bug!("cannot deref {:#?}, {} -> {}", val, crty, rty),
347 (_, &PatKind::Binding { subpattern: Some(ref s), .. }) => s.fold_with(self),
348 _ => pat.super_fold_with(self),
353 impl<'tcx> Pat<'tcx> {
354 fn is_wildcard(&self) -> bool {
356 PatKind::Binding { subpattern: None, .. } | PatKind::Wild => true,
362 /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]`
364 #[derive(Debug, Clone)]
365 pub struct PatStack<'p, 'tcx>(SmallVec<[&'p Pat<'tcx>; 2]>);
367 impl<'p, 'tcx> PatStack<'p, 'tcx> {
368 pub fn from_pattern(pat: &'p Pat<'tcx>) -> Self {
369 PatStack(smallvec![pat])
372 fn from_vec(vec: SmallVec<[&'p Pat<'tcx>; 2]>) -> Self {
376 fn from_slice(s: &[&'p Pat<'tcx>]) -> Self {
377 PatStack(SmallVec::from_slice(s))
380 fn is_empty(&self) -> bool {
384 fn len(&self) -> usize {
388 fn head(&self) -> &'p Pat<'tcx> {
392 fn to_tail(&self) -> Self {
393 PatStack::from_slice(&self.0[1..])
396 fn iter(&self) -> impl Iterator<Item = &Pat<'tcx>> {
397 self.0.iter().map(|p| *p)
400 /// This computes `D(self)`. See top of the file for explanations.
401 fn specialize_wildcard(&self) -> Option<Self> {
402 if self.head().is_wildcard() { Some(self.to_tail()) } else { None }
405 /// This computes `S(constructor, self)`. See top of the file for explanations.
406 fn specialize_constructor<'a, 'q>(
408 cx: &mut MatchCheckCtxt<'a, 'tcx>,
409 constructor: &Constructor<'tcx>,
410 ctor_wild_subpatterns: &[&'q Pat<'tcx>],
411 ) -> Option<PatStack<'q, 'tcx>>
416 let new_heads = specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns);
417 new_heads.map(|mut new_head| {
418 new_head.0.extend_from_slice(&self.0[1..]);
424 impl<'p, 'tcx> Default for PatStack<'p, 'tcx> {
425 fn default() -> Self {
426 PatStack(smallvec![])
430 impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
431 fn from_iter<T>(iter: T) -> Self
433 T: IntoIterator<Item = &'p Pat<'tcx>>,
435 PatStack(iter.into_iter().collect())
440 pub struct Matrix<'p, 'tcx>(Vec<PatStack<'p, 'tcx>>);
442 impl<'p, 'tcx> Matrix<'p, 'tcx> {
443 pub fn empty() -> Self {
447 pub fn push(&mut self, row: PatStack<'p, 'tcx>) {
451 /// Iterate over the first component of each row
452 fn heads<'a>(&'a self) -> impl Iterator<Item = &'a Pat<'tcx>> + Captures<'p> {
453 self.0.iter().map(|r| r.head())
456 /// This computes `D(self)`. See top of the file for explanations.
457 fn specialize_wildcard(&self) -> Self {
458 self.0.iter().filter_map(|r| r.specialize_wildcard()).collect()
461 /// This computes `S(constructor, self)`. See top of the file for explanations.
462 fn specialize_constructor<'a, 'q>(
464 cx: &mut MatchCheckCtxt<'a, 'tcx>,
465 constructor: &Constructor<'tcx>,
466 ctor_wild_subpatterns: &[&'q Pat<'tcx>],
467 ) -> Matrix<'q, 'tcx>
475 .filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
481 /// Pretty-printer for matrices of patterns, example:
482 /// +++++++++++++++++++++++++++++
484 /// +++++++++++++++++++++++++++++
485 /// + true + [First] +
486 /// +++++++++++++++++++++++++++++
487 /// + true + [Second(true)] +
488 /// +++++++++++++++++++++++++++++
490 /// +++++++++++++++++++++++++++++
491 /// + _ + [_, _, tail @ ..] +
492 /// +++++++++++++++++++++++++++++
493 impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> {
494 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
497 let &Matrix(ref m) = self;
498 let pretty_printed_matrix: Vec<Vec<String>> =
499 m.iter().map(|row| row.iter().map(|pat| format!("{:?}", pat)).collect()).collect();
501 let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
502 assert!(m.iter().all(|row| row.len() == column_count));
503 let column_widths: Vec<usize> = (0..column_count)
504 .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0))
507 let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
508 let br = "+".repeat(total_width);
509 write!(f, "{}\n", br)?;
510 for row in pretty_printed_matrix {
512 for (column, pat_str) in row.into_iter().enumerate() {
514 write!(f, "{:1$}", pat_str, column_widths[column])?;
518 write!(f, "{}\n", br)?;
524 impl<'p, 'tcx> FromIterator<PatStack<'p, 'tcx>> for Matrix<'p, 'tcx> {
525 fn from_iter<T>(iter: T) -> Self
527 T: IntoIterator<Item = PatStack<'p, 'tcx>>,
529 Matrix(iter.into_iter().collect())
533 pub struct MatchCheckCtxt<'a, 'tcx> {
534 pub tcx: TyCtxt<'tcx>,
535 /// The module in which the match occurs. This is necessary for
536 /// checking inhabited-ness of types because whether a type is (visibly)
537 /// inhabited can depend on whether it was defined in the current module or
538 /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty
539 /// outside it's module and should not be matchable with an empty match
542 param_env: ty::ParamEnv<'tcx>,
543 pub pattern_arena: &'a TypedArena<Pat<'tcx>>,
544 pub byte_array_map: FxHashMap<*const Pat<'tcx>, Vec<&'a Pat<'tcx>>>,
547 impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
548 pub fn create_and_enter<F, R>(
550 param_env: ty::ParamEnv<'tcx>,
555 F: for<'b> FnOnce(MatchCheckCtxt<'b, 'tcx>) -> R,
557 let pattern_arena = TypedArena::default();
563 pattern_arena: &pattern_arena,
564 byte_array_map: FxHashMap::default(),
568 fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool {
569 if self.tcx.features().exhaustive_patterns {
570 self.tcx.is_ty_uninhabited_from(self.module, ty)
576 fn is_local(&self, ty: Ty<'tcx>) -> bool {
578 ty::Adt(adt_def, ..) => adt_def.did.is_local(),
584 #[derive(Clone, Debug)]
585 enum Constructor<'tcx> {
586 /// The constructor of all patterns that don't vary by constructor,
587 /// e.g., struct patterns and fixed-length arrays.
592 ConstantValue(&'tcx ty::Const<'tcx>, Span),
593 /// Ranges of literal values (`2..=5` and `2..5`).
594 ConstantRange(u128, u128, Ty<'tcx>, RangeEnd, Span),
595 /// Array patterns of length `n`.
597 /// Slice patterns. Captures any array constructor of `length >= i + j`.
598 VarLenSlice(u64, u64),
599 /// Fake extra constructor for enums that aren't allowed to be matched exhaustively.
603 // Ignore spans when comparing, they don't carry semantic information as they are only for lints.
604 impl<'tcx> std::cmp::PartialEq for Constructor<'tcx> {
605 fn eq(&self, other: &Self) -> bool {
606 match (self, other) {
607 (Constructor::Single, Constructor::Single) => true,
608 (Constructor::NonExhaustive, Constructor::NonExhaustive) => true,
609 (Constructor::Variant(a), Constructor::Variant(b)) => a == b,
610 (Constructor::ConstantValue(a, _), Constructor::ConstantValue(b, _)) => a == b,
612 Constructor::ConstantRange(a_start, a_end, a_ty, a_range_end, _),
613 Constructor::ConstantRange(b_start, b_end, b_ty, b_range_end, _),
614 ) => a_start == b_start && a_end == b_end && a_ty == b_ty && a_range_end == b_range_end,
615 (Constructor::FixedLenSlice(a), Constructor::FixedLenSlice(b)) => a == b,
617 Constructor::VarLenSlice(a_prefix, a_suffix),
618 Constructor::VarLenSlice(b_prefix, b_suffix),
619 ) => a_prefix == b_prefix && a_suffix == b_suffix,
625 impl<'tcx> Constructor<'tcx> {
626 fn is_slice(&self) -> bool {
628 FixedLenSlice { .. } | VarLenSlice { .. } => true,
633 // Whether to evaluate a constructor using exhaustive integer matching. This is true if the
634 // constructor is a range or constant with an integer type.
635 fn is_range_and_should_match_exhaustively(&self, tcx: TyCtxt<'tcx>) -> bool {
636 let ty = match self {
637 ConstantValue(value, _) => value.ty,
638 ConstantRange(_, _, ty, _, _) => ty,
641 IntRange::should_treat_range_exhaustively(tcx, ty)
644 fn variant_index_for_adt<'a>(
646 cx: &MatchCheckCtxt<'a, 'tcx>,
647 adt: &'tcx ty::AdtDef,
650 Variant(id) => adt.variant_index_with_id(*id),
652 assert!(!adt.is_enum());
655 ConstantValue(c, _) => crate::const_eval::const_variant_index(cx.tcx, cx.param_env, c),
656 _ => bug!("bad constructor {:?} for adt {:?}", self, adt),
660 fn display(&self, tcx: TyCtxt<'tcx>) -> String {
662 Constructor::ConstantValue(val, _) => format!("{}", val),
663 Constructor::ConstantRange(lo, hi, ty, range_end, _) => {
664 // Get the right sign on the output:
665 let ty = ty::ParamEnv::empty().and(*ty);
668 ty::Const::from_bits(tcx, *lo, ty),
670 ty::Const::from_bits(tcx, *hi, ty),
673 Constructor::FixedLenSlice(val) => format!("[{}]", val),
674 Constructor::VarLenSlice(prefix, suffix) => format!("[{}, .., {}]", prefix, suffix),
675 _ => bug!("bad constructor being displayed: `{:?}", self),
679 // Returns the set of constructors covered by `self` but not by
680 // anything in `other_ctors`.
684 param_env: ty::ParamEnv<'tcx>,
685 other_ctors: &Vec<Constructor<'tcx>>,
686 ) -> Vec<Constructor<'tcx>> {
688 // Those constructors can only match themselves.
689 Single | Variant(_) => {
690 if other_ctors.iter().any(|c| c == self) {
696 FixedLenSlice(self_len) => {
697 let overlaps = |c: &Constructor<'_>| match *c {
698 FixedLenSlice(other_len) => other_len == self_len,
699 VarLenSlice(prefix, suffix) => prefix + suffix <= self_len,
702 if other_ctors.iter().any(overlaps) { vec![] } else { vec![self.clone()] }
705 let mut remaining_ctors = vec![self.clone()];
707 // For each used ctor, subtract from the current set of constructors.
708 // Naming: we remove the "neg" constructors from the "pos" ones.
709 // Remember, `VarLenSlice(i, j)` covers the union of `FixedLenSlice` from
710 // `i + j` to infinity.
711 for neg_ctor in other_ctors {
712 remaining_ctors = remaining_ctors
714 .flat_map(|pos_ctor| -> SmallVec<[Constructor<'tcx>; 1]> {
715 // Compute `pos_ctor \ neg_ctor`.
716 match (&pos_ctor, neg_ctor) {
717 (&FixedLenSlice(pos_len), &VarLenSlice(neg_prefix, neg_suffix)) => {
718 let neg_len = neg_prefix + neg_suffix;
719 if neg_len <= pos_len {
726 &VarLenSlice(pos_prefix, pos_suffix),
727 &VarLenSlice(neg_prefix, neg_suffix),
729 let neg_len = neg_prefix + neg_suffix;
730 let pos_len = pos_prefix + pos_suffix;
731 if neg_len <= pos_len {
734 (pos_len..neg_len).map(FixedLenSlice).collect()
737 (&VarLenSlice(pos_prefix, pos_suffix), &FixedLenSlice(neg_len)) => {
738 let pos_len = pos_prefix + pos_suffix;
739 if neg_len < pos_len {
744 // We know that `neg_len + 1 >= pos_len >= pos_suffix`.
745 .chain(Some(VarLenSlice(
746 neg_len + 1 - pos_suffix,
752 _ if pos_ctor == *neg_ctor => smallvec![],
753 _ => smallvec![pos_ctor],
758 // If the constructors that have been considered so far already cover
759 // the entire range of `self`, no need to look at more constructors.
760 if remaining_ctors.is_empty() {
767 ConstantRange(..) | ConstantValue(..) => {
768 let mut remaining_ctors = vec![self.clone()];
769 for other_ctor in other_ctors {
770 if other_ctor == self {
771 // If a constructor appears in a `match` arm, we can
772 // eliminate it straight away.
773 remaining_ctors = vec![]
774 } else if let Some(interval) = IntRange::from_ctor(tcx, param_env, other_ctor) {
775 // Refine the required constructors for the type by subtracting
776 // the range defined by the current constructor pattern.
777 remaining_ctors = interval.subtract_from(tcx, param_env, remaining_ctors);
780 // If the constructor patterns that have been considered so far
781 // already cover the entire range of values, then we know the
782 // constructor is not missing, and we can move on to the next one.
783 if remaining_ctors.is_empty() {
788 // If a constructor has not been matched, then it is missing.
789 // We add `remaining_ctors` instead of `self`, because then we can
790 // provide more detailed error information about precisely which
791 // ranges have been omitted.
794 // This constructor is never covered by anything else
795 NonExhaustive => vec![NonExhaustive],
799 /// This returns one wildcard pattern for each argument to this constructor.
801 /// This must be consistent with `apply`, `specialize_one_pattern` and `arity`.
802 fn wildcard_subpatterns<'a>(
804 cx: &MatchCheckCtxt<'a, 'tcx>,
806 ) -> Vec<Pat<'tcx>> {
807 debug!("wildcard_subpatterns({:#?}, {:?})", self, ty);
810 Single | Variant(_) => match ty.kind {
811 ty::Tuple(ref fs) => {
812 fs.into_iter().map(|t| t.expect_ty()).map(Pat::wildcard_from_ty).collect()
814 ty::Ref(_, rty, _) => vec![Pat::wildcard_from_ty(rty)],
815 ty::Adt(adt, substs) => {
817 // Use T as the sub pattern type of Box<T>.
818 vec![Pat::wildcard_from_ty(substs.type_at(0))]
820 let variant = &adt.variants[self.variant_index_for_adt(cx, adt)];
821 let is_non_exhaustive =
822 variant.is_field_list_non_exhaustive() && !cx.is_local(ty);
827 let is_visible = adt.is_enum()
828 || field.vis.is_accessible_from(cx.module, cx.tcx);
829 let is_uninhabited = cx.is_uninhabited(field.ty(cx.tcx, substs));
830 match (is_visible, is_non_exhaustive, is_uninhabited) {
831 // Treat all uninhabited types in non-exhaustive variants as
833 (_, true, true) => cx.tcx.types.err,
834 // Treat all non-visible fields as `TyErr`. They can't appear
835 // in any other pattern from this match (because they are
836 // private), so their type does not matter - but we don't want
837 // to know they are uninhabited.
838 (false, ..) => cx.tcx.types.err,
840 let ty = field.ty(cx.tcx, substs);
842 // If the field type returned is an array of an unknown
843 // size return an TyErr.
846 .try_eval_usize(cx.tcx, cx.param_env)
856 .map(Pat::wildcard_from_ty)
862 FixedLenSlice(_) | VarLenSlice(..) => match ty.kind {
863 ty::Slice(ty) | ty::Array(ty, _) => {
864 let arity = self.arity(cx, ty);
865 (0..arity).map(|_| Pat::wildcard_from_ty(ty)).collect()
867 _ => bug!("bad slice pattern {:?} {:?}", self, ty),
869 ConstantValue(..) | ConstantRange(..) | NonExhaustive => vec![],
873 /// This computes the arity of a constructor. The arity of a constructor
874 /// is how many subpattern patterns of that constructor should be expanded to.
876 /// For instance, a tuple pattern `(_, 42, Some([]))` has the arity of 3.
877 /// A struct pattern's arity is the number of fields it contains, etc.
879 /// This must be consistent with `wildcard_subpatterns`, `specialize_one_pattern` and `apply`.
880 fn arity<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> u64 {
881 debug!("Constructor::arity({:#?}, {:?})", self, ty);
883 Single | Variant(_) => match ty.kind {
884 ty::Tuple(ref fs) => fs.len() as u64,
885 ty::Slice(..) | ty::Array(..) => bug!("bad slice pattern {:?} {:?}", self, ty),
888 adt.variants[self.variant_index_for_adt(cx, adt)].fields.len() as u64
892 FixedLenSlice(length) => *length,
893 VarLenSlice(prefix, suffix) => prefix + suffix,
894 ConstantValue(..) | ConstantRange(..) | NonExhaustive => 0,
898 /// Apply a constructor to a list of patterns, yielding a new pattern. `pats`
899 /// must have as many elements as this constructor's arity.
901 /// This must be consistent with `wildcard_subpatterns`, `specialize_one_pattern` and `arity`.
904 /// `self`: `Constructor::Single`
905 /// `ty`: `(u32, u32, u32)`
906 /// `pats`: `[10, 20, _]`
907 /// returns `(10, 20, _)`
909 /// `self`: `Constructor::Variant(Option::Some)`
910 /// `ty`: `Option<bool>`
911 /// `pats`: `[false]`
912 /// returns `Some(false)`
915 cx: &MatchCheckCtxt<'a, 'tcx>,
917 pats: impl IntoIterator<Item = Pat<'tcx>>,
919 let mut subpatterns = pats.into_iter();
921 let pat = match self {
922 Single | Variant(_) => match ty.kind {
923 ty::Adt(..) | ty::Tuple(..) => {
924 let subpatterns = subpatterns
926 .map(|(i, p)| FieldPat { field: Field::new(i), pattern: p })
929 if let ty::Adt(adt, substs) = ty.kind {
934 variant_index: self.variant_index_for_adt(cx, adt),
938 PatKind::Leaf { subpatterns }
941 PatKind::Leaf { subpatterns }
944 ty::Ref(..) => PatKind::Deref { subpattern: subpatterns.nth(0).unwrap() },
945 ty::Slice(_) | ty::Array(..) => bug!("bad slice pattern {:?} {:?}", self, ty),
948 FixedLenSlice(_) => {
949 PatKind::Slice { prefix: subpatterns.collect(), slice: None, suffix: vec![] }
951 &VarLenSlice(prefix_len, _) => {
952 let prefix = subpatterns.by_ref().take(prefix_len as usize).collect();
953 let suffix = subpatterns.collect();
954 let wild = Pat::wildcard_from_ty(ty);
955 PatKind::Slice { prefix, slice: Some(wild), suffix }
957 &ConstantValue(value, _) => PatKind::Constant { value },
958 &ConstantRange(lo, hi, ty, end, _) => PatKind::Range(PatRange {
959 lo: ty::Const::from_bits(cx.tcx, lo, ty::ParamEnv::empty().and(ty)),
960 hi: ty::Const::from_bits(cx.tcx, hi, ty::ParamEnv::empty().and(ty)),
963 NonExhaustive => PatKind::Wild,
966 Pat { ty, span: DUMMY_SP, kind: Box::new(pat) }
969 /// Like `apply`, but where all the subpatterns are wildcards `_`.
970 fn apply_wildcards<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {
971 let subpatterns = self.wildcard_subpatterns(cx, ty).into_iter().rev();
972 self.apply(cx, ty, subpatterns)
976 #[derive(Clone, Debug)]
977 pub enum Usefulness<'tcx> {
979 UsefulWithWitness(Vec<Witness<'tcx>>),
983 impl<'tcx> Usefulness<'tcx> {
984 fn new_useful(preference: WitnessPreference) -> Self {
986 ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
987 LeaveOutWitness => Useful,
991 fn is_useful(&self) -> bool {
998 fn apply_constructor(
1000 cx: &MatchCheckCtxt<'_, 'tcx>,
1001 ctor: &Constructor<'tcx>,
1005 UsefulWithWitness(witnesses) => UsefulWithWitness(
1008 .map(|witness| witness.apply_constructor(cx, &ctor, ty))
1015 fn apply_wildcard(self, ty: Ty<'tcx>) -> Self {
1017 UsefulWithWitness(witnesses) => {
1018 let wild = Pat::wildcard_from_ty(ty);
1022 .map(|mut witness| {
1023 witness.0.push(wild.clone());
1033 fn apply_missing_ctors(
1035 cx: &MatchCheckCtxt<'_, 'tcx>,
1037 missing_ctors: &MissingConstructors<'tcx>,
1040 UsefulWithWitness(witnesses) => {
1041 let new_patterns: Vec<_> =
1042 missing_ctors.iter().map(|ctor| ctor.apply_wildcards(cx, ty)).collect();
1043 // Add the new patterns to each witness
1047 .flat_map(|witness| {
1048 new_patterns.iter().map(move |pat| {
1049 let mut witness = witness.clone();
1050 witness.0.push(pat.clone());
1062 #[derive(Copy, Clone, Debug)]
1063 pub enum WitnessPreference {
1068 #[derive(Copy, Clone, Debug)]
1069 struct PatCtxt<'tcx> {
1074 /// A witness of non-exhaustiveness for error reporting, represented
1075 /// as a list of patterns (in reverse order of construction) with
1076 /// wildcards inside to represent elements that can take any inhabitant
1077 /// of the type as a value.
1079 /// A witness against a list of patterns should have the same types
1080 /// and length as the pattern matched against. Because Rust `match`
1081 /// is always against a single pattern, at the end the witness will
1082 /// have length 1, but in the middle of the algorithm, it can contain
1083 /// multiple patterns.
1085 /// For example, if we are constructing a witness for the match against
1087 /// struct Pair(Option<(u32, u32)>, bool);
1089 /// match (p: Pair) {
1090 /// Pair(None, _) => {}
1091 /// Pair(_, false) => {}
1095 /// We'll perform the following steps:
1096 /// 1. Start with an empty witness
1097 /// `Witness(vec![])`
1098 /// 2. Push a witness `Some(_)` against the `None`
1099 /// `Witness(vec![Some(_)])`
1100 /// 3. Push a witness `true` against the `false`
1101 /// `Witness(vec![Some(_), true])`
1102 /// 4. Apply the `Pair` constructor to the witnesses
1103 /// `Witness(vec![Pair(Some(_), true)])`
1105 /// The final `Pair(Some(_), true)` is then the resulting witness.
1106 #[derive(Clone, Debug)]
1107 pub struct Witness<'tcx>(Vec<Pat<'tcx>>);
1109 impl<'tcx> Witness<'tcx> {
1110 pub fn single_pattern(self) -> Pat<'tcx> {
1111 assert_eq!(self.0.len(), 1);
1112 self.0.into_iter().next().unwrap()
1115 /// Constructs a partial witness for a pattern given a list of
1116 /// patterns expanded by the specialization step.
1118 /// When a pattern P is discovered to be useful, this function is used bottom-up
1119 /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset
1120 /// of values, V, where each value in that set is not covered by any previously
1121 /// used patterns and is covered by the pattern P'. Examples:
1123 /// left_ty: tuple of 3 elements
1124 /// pats: [10, 20, _] => (10, 20, _)
1126 /// left_ty: struct X { a: (bool, &'static str), b: usize}
1127 /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
1128 fn apply_constructor<'a>(
1130 cx: &MatchCheckCtxt<'a, 'tcx>,
1131 ctor: &Constructor<'tcx>,
1134 let arity = ctor.arity(cx, ty);
1136 let len = self.0.len() as u64;
1137 let pats = self.0.drain((len - arity) as usize..).rev();
1138 ctor.apply(cx, ty, pats)
1147 /// This determines the set of all possible constructors of a pattern matching
1148 /// values of type `left_ty`. For vectors, this would normally be an infinite set
1149 /// but is instead bounded by the maximum fixed length of slice patterns in
1150 /// the column of patterns being analyzed.
1152 /// We make sure to omit constructors that are statically impossible. E.g., for
1153 /// `Option<!>`, we do not include `Some(_)` in the returned list of constructors.
1154 fn all_constructors<'a, 'tcx>(
1155 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1157 ) -> Vec<Constructor<'tcx>> {
1158 debug!("all_constructors({:?})", pcx.ty);
1159 let make_range = |start, end| ConstantRange(start, end, pcx.ty, RangeEnd::Included, pcx.span);
1161 ty::Bool => [true, false]
1163 .map(|&b| ConstantValue(ty::Const::from_bool(cx.tcx, b), pcx.span))
1165 ty::Array(ref sub_ty, len) if len.try_eval_usize(cx.tcx, cx.param_env).is_some() => {
1166 let len = len.eval_usize(cx.tcx, cx.param_env);
1167 if len != 0 && cx.is_uninhabited(sub_ty) { vec![] } else { vec![FixedLenSlice(len)] }
1169 // Treat arrays of a constant but unknown length like slices.
1170 ty::Array(ref sub_ty, _) | ty::Slice(ref sub_ty) => {
1171 if cx.is_uninhabited(sub_ty) {
1172 vec![FixedLenSlice(0)]
1174 vec![VarLenSlice(0, 0)]
1177 ty::Adt(def, substs) if def.is_enum() => {
1178 let ctors: Vec<_> = def
1182 !cx.tcx.features().exhaustive_patterns
1184 .uninhabited_from(cx.tcx, substs, def.adt_kind())
1185 .contains(cx.tcx, cx.module)
1187 .map(|v| Variant(v.def_id))
1190 // If our scrutinee is *privately* an empty enum, we must treat it as though it had an
1191 // "unknown" constructor (in that case, all other patterns obviously can't be variants)
1192 // to avoid exposing its emptyness. See the `match_privately_empty` test for details.
1193 // FIXME: currently the only way I know of something can be a privately-empty enum is
1194 // when the exhaustive_patterns feature flag is not present, so this is only needed for
1196 let is_privately_empty = ctors.is_empty() && !cx.is_uninhabited(pcx.ty);
1197 // If the enum is declared as `#[non_exhaustive]`, we treat it as if it had an
1198 // additionnal "unknown" constructor.
1199 let is_declared_nonexhaustive =
1200 def.is_variant_list_non_exhaustive() && !cx.is_local(pcx.ty);
1202 if is_privately_empty || is_declared_nonexhaustive {
1203 // There is no point in enumerating all possible variants, because the user can't
1204 // actually match against them themselves. So we return only the fictitious
1206 // E.g., in an example like:
1208 // let err: io::ErrorKind = ...;
1210 // io::ErrorKind::NotFound => {},
1213 // we don't want to show every possible IO error, but instead have only `_` as the
1222 // The valid Unicode Scalar Value ranges.
1223 make_range('\u{0000}' as u128, '\u{D7FF}' as u128),
1224 make_range('\u{E000}' as u128, '\u{10FFFF}' as u128),
1227 ty::Int(_) | ty::Uint(_)
1228 if pcx.ty.is_ptr_sized_integral()
1229 && !cx.tcx.features().precise_pointer_size_matching =>
1231 // `usize`/`isize` are not allowed to be matched exhaustively unless the
1232 // `precise_pointer_size_matching` feature is enabled. So we treat those types like
1233 // `#[non_exhaustive]` enums by returning a special unmatcheable constructor.
1237 let bits = Integer::from_attr(&cx.tcx, SignedInt(ity)).size().bits() as u128;
1238 let min = 1u128 << (bits - 1);
1240 vec![make_range(min, max)]
1243 let size = Integer::from_attr(&cx.tcx, UnsignedInt(uty)).size();
1244 let max = truncate(u128::max_value(), size);
1245 vec![make_range(0, max)]
1248 if cx.is_uninhabited(pcx.ty) {
1257 /// An inclusive interval, used for precise integer exhaustiveness checking.
1258 /// `IntRange`s always store a contiguous range. This means that values are
1259 /// encoded such that `0` encodes the minimum value for the integer,
1260 /// regardless of the signedness.
1261 /// For example, the pattern `-128..=127i8` is encoded as `0..=255`.
1262 /// This makes comparisons and arithmetic on interval endpoints much more
1263 /// straightforward. See `signed_bias` for details.
1265 /// `IntRange` is never used to encode an empty range or a "range" that wraps
1266 /// around the (offset) space: i.e., `range.lo <= range.hi`.
1267 #[derive(Clone, Debug)]
1268 struct IntRange<'tcx> {
1269 pub range: RangeInclusive<u128>,
1274 impl<'tcx> IntRange<'tcx> {
1276 fn is_integral(ty: Ty<'_>) -> bool {
1278 ty::Char | ty::Int(_) | ty::Uint(_) => true,
1283 fn should_treat_range_exhaustively(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> bool {
1284 // Don't treat `usize`/`isize` exhaustively unless the `precise_pointer_size_matching`
1285 // feature is enabled.
1286 IntRange::is_integral(ty)
1287 && (!ty.is_ptr_sized_integral() || tcx.features().precise_pointer_size_matching)
1291 fn integral_size_and_signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'_>) -> Option<(Size, u128)> {
1293 ty::Char => Some((Size::from_bytes(4), 0)),
1295 let size = Integer::from_attr(&tcx, SignedInt(ity)).size();
1296 Some((size, 1u128 << (size.bits() as u128 - 1)))
1298 ty::Uint(uty) => Some((Integer::from_attr(&tcx, UnsignedInt(uty)).size(), 0)),
1306 param_env: ty::ParamEnv<'tcx>,
1307 value: &Const<'tcx>,
1309 ) -> Option<IntRange<'tcx>> {
1310 if let Some((target_size, bias)) = Self::integral_size_and_signed_bias(tcx, value.ty) {
1312 let val = if let ty::ConstKind::Value(ConstValue::Scalar(
1313 Scalar::Raw { data, size }
1315 // For this specific pattern we can skip a lot of effort and go
1316 // straight to the result, after doing a bit of checking. (We
1317 // could remove this branch and just use the next branch, which
1318 // is more general but much slower.)
1319 Scalar::<()>::check_raw(data, size, target_size);
1321 } else if let Some(val) = value.try_eval_bits(tcx, param_env, ty) {
1322 // This is a more general form of the previous branch.
1327 let val = val ^ bias;
1328 Some(IntRange { range: val..=val, ty, span })
1342 ) -> Option<IntRange<'tcx>> {
1343 if Self::is_integral(ty) {
1344 // Perform a shift if the underlying types are signed,
1345 // which makes the interval arithmetic simpler.
1346 let bias = IntRange::signed_bias(tcx, ty);
1347 let (lo, hi) = (lo ^ bias, hi ^ bias);
1348 // Make sure the interval is well-formed.
1349 if lo > hi || lo == hi && *end == RangeEnd::Excluded {
1352 let offset = (*end == RangeEnd::Excluded) as u128;
1353 Some(IntRange { range: lo..=(hi - offset), ty, span })
1362 param_env: ty::ParamEnv<'tcx>,
1363 ctor: &Constructor<'tcx>,
1364 ) -> Option<IntRange<'tcx>> {
1365 // Floating-point ranges are permitted and we don't want
1366 // to consider them when constructing integer ranges.
1368 ConstantRange(lo, hi, ty, end, span) => Self::from_range(tcx, *lo, *hi, ty, end, *span),
1369 ConstantValue(val, span) => Self::from_const(tcx, param_env, val, *span),
1376 param_env: ty::ParamEnv<'tcx>,
1377 mut pat: &Pat<'tcx>,
1378 ) -> Option<IntRange<'tcx>> {
1381 box PatKind::Constant { value } => {
1382 return Self::from_const(tcx, param_env, value, pat.span);
1384 box PatKind::Range(PatRange { lo, hi, end }) => {
1385 return Self::from_range(
1387 lo.eval_bits(tcx, param_env, lo.ty),
1388 hi.eval_bits(tcx, param_env, hi.ty),
1394 box PatKind::AscribeUserType { ref subpattern, .. } => {
1402 // The return value of `signed_bias` should be XORed with an endpoint to encode/decode it.
1403 fn signed_bias(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> u128 {
1406 let bits = Integer::from_attr(&tcx, SignedInt(ity)).size().bits() as u128;
1413 /// Converts a `RangeInclusive` to a `ConstantValue` or inclusive `ConstantRange`.
1417 r: RangeInclusive<u128>,
1419 ) -> Constructor<'tcx> {
1420 let bias = IntRange::signed_bias(tcx, ty);
1421 let (lo, hi) = r.into_inner();
1423 let ty = ty::ParamEnv::empty().and(ty);
1424 ConstantValue(ty::Const::from_bits(tcx, lo ^ bias, ty), span)
1426 ConstantRange(lo ^ bias, hi ^ bias, ty, RangeEnd::Included, span)
1430 /// Returns a collection of ranges that spans the values covered by `ranges`, subtracted
1431 /// by the values covered by `self`: i.e., `ranges \ self` (in set notation).
1435 param_env: ty::ParamEnv<'tcx>,
1436 ranges: Vec<Constructor<'tcx>>,
1437 ) -> Vec<Constructor<'tcx>> {
1440 .filter_map(|r| IntRange::from_ctor(tcx, param_env, &r).map(|i| i.range));
1441 let mut remaining_ranges = vec![];
1443 let (lo, hi) = self.range.into_inner();
1444 for subrange in ranges {
1445 let (subrange_lo, subrange_hi) = subrange.into_inner();
1446 if lo > subrange_hi || subrange_lo > hi {
1447 // The pattern doesn't intersect with the subrange at all,
1448 // so the subrange remains untouched.
1449 remaining_ranges.push(Self::range_to_ctor(
1452 subrange_lo..=subrange_hi,
1456 if lo > subrange_lo {
1457 // The pattern intersects an upper section of the
1458 // subrange, so a lower section will remain.
1459 remaining_ranges.push(Self::range_to_ctor(
1462 subrange_lo..=(lo - 1),
1466 if hi < subrange_hi {
1467 // The pattern intersects a lower section of the
1468 // subrange, so an upper section will remain.
1469 remaining_ranges.push(Self::range_to_ctor(
1472 (hi + 1)..=subrange_hi,
1481 fn intersection(&self, other: &Self) -> Option<Self> {
1483 let (lo, hi) = (*self.range.start(), *self.range.end());
1484 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1485 if lo <= other_hi && other_lo <= hi {
1486 let span = other.span;
1487 Some(IntRange { range: max(lo, other_lo)..=min(hi, other_hi), ty, span })
1493 fn suspicious_intersection(&self, other: &Self) -> bool {
1494 // `false` in the following cases:
1495 // 1 ---- // 1 ---------- // 1 ---- // 1 ----
1496 // 2 ---------- // 2 ---- // 2 ---- // 2 ----
1498 // The following are currently `false`, but could be `true` in the future (#64007):
1499 // 1 --------- // 1 ---------
1500 // 2 ---------- // 2 ----------
1502 // `true` in the following cases:
1503 // 1 ------- // 1 -------
1504 // 2 -------- // 2 -------
1505 let (lo, hi) = (*self.range.start(), *self.range.end());
1506 let (other_lo, other_hi) = (*other.range.start(), *other.range.end());
1507 (lo == other_hi || hi == other_lo)
1511 // A struct to compute a set of constructors equivalent to `all_ctors \ used_ctors`.
1512 struct MissingConstructors<'tcx> {
1514 param_env: ty::ParamEnv<'tcx>,
1515 all_ctors: Vec<Constructor<'tcx>>,
1516 used_ctors: Vec<Constructor<'tcx>>,
1519 impl<'tcx> MissingConstructors<'tcx> {
1522 param_env: ty::ParamEnv<'tcx>,
1523 all_ctors: Vec<Constructor<'tcx>>,
1524 used_ctors: Vec<Constructor<'tcx>>,
1526 MissingConstructors { tcx, param_env, all_ctors, used_ctors }
1529 fn into_inner(self) -> (Vec<Constructor<'tcx>>, Vec<Constructor<'tcx>>) {
1530 (self.all_ctors, self.used_ctors)
1533 fn is_empty(&self) -> bool {
1534 self.iter().next().is_none()
1536 /// Whether this contains all the constructors for the given type or only a
1538 fn all_ctors_are_missing(&self) -> bool {
1539 self.used_ctors.is_empty()
1542 /// Iterate over all_ctors \ used_ctors
1543 fn iter<'a>(&'a self) -> impl Iterator<Item = Constructor<'tcx>> + Captures<'a> {
1544 self.all_ctors.iter().flat_map(move |req_ctor| {
1545 req_ctor.subtract_ctors(self.tcx, self.param_env, &self.used_ctors)
1550 impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
1551 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1552 let ctors: Vec<_> = self.iter().collect();
1553 write!(f, "{:?}", ctors)
1557 /// Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html.
1558 /// The algorithm from the paper has been modified to correctly handle empty
1559 /// types. The changes are:
1560 /// (0) We don't exit early if the pattern matrix has zero rows. We just
1561 /// continue to recurse over columns.
1562 /// (1) all_constructors will only return constructors that are statically
1563 /// possible. E.g., it will only return `Ok` for `Result<T, !>`.
1565 /// This finds whether a (row) vector `v` of patterns is 'useful' in relation
1566 /// to a set of such vectors `m` - this is defined as there being a set of
1567 /// inputs that will match `v` but not any of the sets in `m`.
1569 /// All the patterns at each column of the `matrix ++ v` matrix must
1570 /// have the same type, except that wildcard (PatKind::Wild) patterns
1571 /// with type `TyErr` are also allowed, even if the "type of the column"
1572 /// is not `TyErr`. That is used to represent private fields, as using their
1573 /// real type would assert that they are inhabited.
1575 /// This is used both for reachability checking (if a pattern isn't useful in
1576 /// relation to preceding patterns, it is not reachable) and exhaustiveness
1577 /// checking (if a wildcard pattern is useful in relation to a matrix, the
1578 /// matrix isn't exhaustive).
1579 pub fn is_useful<'p, 'a, 'tcx>(
1580 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1581 matrix: &Matrix<'p, 'tcx>,
1582 v: &PatStack<'_, 'tcx>,
1583 witness_preference: WitnessPreference,
1585 ) -> Usefulness<'tcx> {
1586 let &Matrix(ref rows) = matrix;
1587 debug!("is_useful({:#?}, {:#?})", matrix, v);
1589 // The base case. We are pattern-matching on () and the return value is
1590 // based on whether our matrix has a row or not.
1591 // NOTE: This could potentially be optimized by checking rows.is_empty()
1592 // first and then, if v is non-empty, the return value is based on whether
1593 // the type of the tuple we're checking is inhabited or not.
1595 return if rows.is_empty() {
1596 Usefulness::new_useful(witness_preference)
1602 assert!(rows.iter().all(|r| r.len() == v.len()));
1604 let (ty, span) = matrix
1606 .map(|r| (r.ty, r.span))
1607 .find(|(ty, _)| !ty.references_error())
1608 .unwrap_or((v.head().ty, v.head().span));
1610 // TyErr is used to represent the type of wildcard patterns matching
1611 // against inaccessible (private) fields of structs, so that we won't
1612 // be able to observe whether the types of the struct's fields are
1615 // If the field is truly inaccessible, then all the patterns
1616 // matching against it must be wildcard patterns, so its type
1619 // However, if we are matching against non-wildcard patterns, we
1620 // need to know the real type of the field so we can specialize
1621 // against it. This primarily occurs through constants - they
1622 // can include contents for fields that are inaccessible at the
1623 // location of the match. In that case, the field's type is
1624 // inhabited - by the constant - so we can just use it.
1626 // FIXME: this might lead to "unstable" behavior with macro hygiene
1627 // introducing uninhabited patterns for inaccessible fields. We
1628 // need to figure out how to model that.
1633 debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
1635 if let Some(constructor) = pat_constructor(cx, v.head(), pcx) {
1636 debug!("is_useful - expanding constructor: {:#?}", constructor);
1637 split_grouped_constructors(
1647 .map(|c| is_useful_specialized(cx, matrix, v, c, pcx.ty, witness_preference, hir_id))
1648 .find(|result| result.is_useful())
1649 .unwrap_or(NotUseful)
1651 debug!("is_useful - expanding wildcard");
1653 let used_ctors: Vec<Constructor<'_>> =
1654 matrix.heads().filter_map(|p| pat_constructor(cx, p, pcx)).collect();
1655 debug!("used_ctors = {:#?}", used_ctors);
1656 // `all_ctors` are all the constructors for the given type, which
1657 // should all be represented (or caught with the wild pattern `_`).
1658 let all_ctors = all_constructors(cx, pcx);
1659 debug!("all_ctors = {:#?}", all_ctors);
1661 // `missing_ctors` is the set of constructors from the same type as the
1662 // first column of `matrix` that are matched only by wildcard patterns
1663 // from the first column.
1665 // Therefore, if there is some pattern that is unmatched by `matrix`,
1666 // it will still be unmatched if the first constructor is replaced by
1667 // any of the constructors in `missing_ctors`
1669 // Missing constructors are those that are not matched by any non-wildcard patterns in the
1670 // current column. We only fully construct them on-demand, because they're rarely used and
1672 let missing_ctors = MissingConstructors::new(cx.tcx, cx.param_env, all_ctors, used_ctors);
1674 debug!("missing_ctors.empty()={:#?}", missing_ctors.is_empty(),);
1676 if missing_ctors.is_empty() {
1677 let (all_ctors, _) = missing_ctors.into_inner();
1678 split_grouped_constructors(cx.tcx, cx.param_env, pcx, all_ctors, matrix, DUMMY_SP, None)
1681 is_useful_specialized(cx, matrix, v, c, pcx.ty, witness_preference, hir_id)
1683 .find(|result| result.is_useful())
1684 .unwrap_or(NotUseful)
1686 let matrix = matrix.specialize_wildcard();
1687 let v = v.to_tail();
1688 let usefulness = is_useful(cx, &matrix, &v, witness_preference, hir_id);
1690 // In this case, there's at least one "free"
1691 // constructor that is only matched against by
1692 // wildcard patterns.
1694 // There are 2 ways we can report a witness here.
1695 // Commonly, we can report all the "free"
1696 // constructors as witnesses, e.g., if we have:
1699 // enum Direction { N, S, E, W }
1700 // let Direction::N = ...;
1703 // we can report 3 witnesses: `S`, `E`, and `W`.
1705 // However, there is a case where we don't want
1706 // to do this and instead report a single `_` witness:
1707 // if the user didn't actually specify a constructor
1708 // in this arm, e.g., in
1710 // let x: (Direction, Direction, bool) = ...;
1711 // let (_, _, false) = x;
1713 // we don't want to show all 16 possible witnesses
1714 // `(<direction-1>, <direction-2>, true)` - we are
1715 // satisfied with `(_, _, true)`. In this case,
1716 // `used_ctors` is empty.
1717 if missing_ctors.all_ctors_are_missing() {
1718 // All constructors are unused. Add a wild pattern
1719 // rather than each individual constructor.
1720 usefulness.apply_wildcard(pcx.ty)
1722 // Construct for each missing constructor a "wild" version of this
1723 // constructor, that matches everything that can be built with
1724 // it. For example, if `ctor` is a `Constructor::Variant` for
1725 // `Option::Some`, we get the pattern `Some(_)`.
1726 usefulness.apply_missing_ctors(cx, pcx.ty, &missing_ctors)
1732 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
1733 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
1734 fn is_useful_specialized<'p, 'a, 'tcx>(
1735 cx: &mut MatchCheckCtxt<'a, 'tcx>,
1736 matrix: &Matrix<'p, 'tcx>,
1737 v: &PatStack<'_, 'tcx>,
1738 ctor: Constructor<'tcx>,
1740 witness_preference: WitnessPreference,
1742 ) -> Usefulness<'tcx> {
1743 debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
1745 let ctor_wild_subpatterns_owned: Vec<_> = ctor.wildcard_subpatterns(cx, lty);
1746 let ctor_wild_subpatterns: Vec<_> = ctor_wild_subpatterns_owned.iter().collect();
1747 let matrix = matrix.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns);
1748 v.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns)
1749 .map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id))
1750 .map(|u| u.apply_constructor(cx, &ctor, lty))
1751 .unwrap_or(NotUseful)
1754 /// Determines the constructor that the given pattern can be specialized to.
1755 /// Returns `None` in case of a catch-all, which can't be specialized.
1756 fn pat_constructor<'tcx>(
1757 cx: &mut MatchCheckCtxt<'_, 'tcx>,
1760 ) -> Option<Constructor<'tcx>> {
1762 PatKind::AscribeUserType { ref subpattern, .. } => pat_constructor(cx, subpattern, pcx),
1763 PatKind::Binding { .. } | PatKind::Wild => None,
1764 PatKind::Leaf { .. } | PatKind::Deref { .. } => Some(Single),
1765 PatKind::Variant { adt_def, variant_index, .. } => {
1766 Some(Variant(adt_def.variants[variant_index].def_id))
1768 PatKind::Constant { value } => Some(ConstantValue(value, pat.span)),
1769 PatKind::Range(PatRange { lo, hi, end }) => Some(ConstantRange(
1770 lo.eval_bits(cx.tcx, cx.param_env, lo.ty),
1771 hi.eval_bits(cx.tcx, cx.param_env, hi.ty),
1776 PatKind::Array { .. } => match pcx.ty.kind {
1777 ty::Array(_, length) => Some(FixedLenSlice(length.eval_usize(cx.tcx, cx.param_env))),
1778 _ => span_bug!(pat.span, "bad ty {:?} for array pattern", pcx.ty),
1780 PatKind::Slice { ref prefix, ref slice, ref suffix } => {
1781 let prefix = prefix.len() as u64;
1782 let suffix = suffix.len() as u64;
1783 if slice.is_some() {
1784 Some(VarLenSlice(prefix, suffix))
1786 Some(FixedLenSlice(prefix + suffix))
1789 PatKind::Or { .. } => {
1790 bug!("support for or-patterns has not been fully implemented yet.");
1795 // checks whether a constant is equal to a user-written slice pattern. Only supports byte slices,
1796 // meaning all other types will compare unequal and thus equal patterns often do not cause the
1797 // second pattern to lint about unreachable match arms.
1798 fn slice_pat_covered_by_const<'tcx>(
1801 const_val: &'tcx ty::Const<'tcx>,
1802 prefix: &[Pat<'tcx>],
1803 slice: &Option<Pat<'tcx>>,
1804 suffix: &[Pat<'tcx>],
1805 param_env: ty::ParamEnv<'tcx>,
1806 ) -> Result<bool, ErrorReported> {
1807 let const_val_val = if let ty::ConstKind::Value(val) = const_val.val {
1811 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1819 let data: &[u8] = match (const_val_val, &const_val.ty.kind) {
1820 (ConstValue::ByRef { offset, alloc, .. }, ty::Array(t, n)) => {
1821 assert_eq!(*t, tcx.types.u8);
1822 let n = n.eval_usize(tcx, param_env);
1823 let ptr = Pointer::new(AllocId(0), offset);
1824 alloc.get_bytes(&tcx, ptr, Size::from_bytes(n)).unwrap()
1826 (ConstValue::Slice { data, start, end }, ty::Slice(t)) => {
1827 assert_eq!(*t, tcx.types.u8);
1828 let ptr = Pointer::new(AllocId(0), Size::from_bytes(start as u64));
1829 data.get_bytes(&tcx, ptr, Size::from_bytes((end - start) as u64)).unwrap()
1831 // FIXME(oli-obk): create a way to extract fat pointers from ByRef
1832 (_, ty::Slice(_)) => return Ok(false),
1834 "slice_pat_covered_by_const: {:#?}, {:#?}, {:#?}, {:#?}",
1842 let pat_len = prefix.len() + suffix.len();
1843 if data.len() < pat_len || (slice.is_none() && data.len() > pat_len) {
1847 for (ch, pat) in data[..prefix.len()]
1850 .chain(data[data.len() - suffix.len()..].iter().zip(suffix))
1853 box PatKind::Constant { value } => {
1854 let b = value.eval_bits(tcx, param_env, pat.ty);
1855 assert_eq!(b as u8 as u128, b);
1867 /// For exhaustive integer matching, some constructors are grouped within other constructors
1868 /// (namely integer typed values are grouped within ranges). However, when specialising these
1869 /// constructors, we want to be specialising for the underlying constructors (the integers), not
1870 /// the groups (the ranges). Thus we need to split the groups up. Splitting them up naïvely would
1871 /// mean creating a separate constructor for every single value in the range, which is clearly
1872 /// impractical. However, observe that for some ranges of integers, the specialisation will be
1873 /// identical across all values in that range (i.e., there are equivalence classes of ranges of
1874 /// constructors based on their `is_useful_specialized` outcome). These classes are grouped by
1875 /// the patterns that apply to them (in the matrix `P`). We can split the range whenever the
1876 /// patterns that apply to that range (specifically: the patterns that *intersect* with that range)
1878 /// Our solution, therefore, is to split the range constructor into subranges at every single point
1879 /// the group of intersecting patterns changes (using the method described below).
1880 /// And voilà! We're testing precisely those ranges that we need to, without any exhaustive matching
1881 /// on actual integers. The nice thing about this is that the number of subranges is linear in the
1882 /// number of rows in the matrix (i.e., the number of cases in the `match` statement), so we don't
1883 /// need to be worried about matching over gargantuan ranges.
1885 /// Essentially, given the first column of a matrix representing ranges, looking like the following:
1887 /// |------| |----------| |-------| ||
1888 /// |-------| |-------| |----| ||
1891 /// We split the ranges up into equivalence classes so the ranges are no longer overlapping:
1893 /// |--|--|||-||||--||---|||-------| |-|||| ||
1895 /// The logic for determining how to split the ranges is fairly straightforward: we calculate
1896 /// boundaries for each interval range, sort them, then create constructors for each new interval
1897 /// between every pair of boundary points. (This essentially sums up to performing the intuitive
1898 /// merging operation depicted above.)
1900 /// `hir_id` is `None` when we're evaluating the wildcard pattern, do not lint for overlapping in
1901 /// ranges that case.
1903 /// This also splits variable-length slices into fixed-length slices.
1904 fn split_grouped_constructors<'p, 'tcx>(
1906 param_env: ty::ParamEnv<'tcx>,
1908 ctors: Vec<Constructor<'tcx>>,
1909 matrix: &Matrix<'p, 'tcx>,
1911 hir_id: Option<HirId>,
1912 ) -> Vec<Constructor<'tcx>> {
1914 let mut split_ctors = Vec::with_capacity(ctors.len());
1916 for ctor in ctors.into_iter() {
1918 ConstantRange(..) if ctor.is_range_and_should_match_exhaustively(tcx) => {
1919 // We only care about finding all the subranges within the range of the constructor
1920 // range. Anything else is irrelevant, because it is guaranteed to result in
1921 // `NotUseful`, which is the default case anyway, and can be ignored.
1922 let ctor_range = IntRange::from_ctor(tcx, param_env, &ctor).unwrap();
1924 /// Represents a border between 2 integers. Because the intervals spanning borders
1925 /// must be able to cover every integer, we need to be able to represent
1926 /// 2^128 + 1 such borders.
1927 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
1933 // A function for extracting the borders of an integer interval.
1934 fn range_borders(r: IntRange<'_>) -> impl Iterator<Item = Border> {
1935 let (lo, hi) = r.range.into_inner();
1936 let from = Border::JustBefore(lo);
1937 let to = match hi.checked_add(1) {
1938 Some(m) => Border::JustBefore(m),
1939 None => Border::AfterMax,
1941 vec![from, to].into_iter()
1944 // Collect the span and range of all the intersecting ranges to lint on likely
1945 // incorrect range patterns. (#63987)
1946 let mut overlaps = vec![];
1947 // `borders` is the set of borders between equivalence classes: each equivalence
1948 // class lies between 2 borders.
1949 let row_borders = matrix
1953 IntRange::from_pat(tcx, param_env, row.head()).map(|r| (r, row.len()))
1955 .flat_map(|(range, row_len)| {
1956 let intersection = ctor_range.intersection(&range);
1957 let should_lint = ctor_range.suspicious_intersection(&range);
1958 if let (Some(range), 1, true) = (&intersection, row_len, should_lint) {
1959 // FIXME: for now, only check for overlapping ranges on simple range
1960 // patterns. Otherwise with the current logic the following is detected
1962 // match (10u8, true) {
1963 // (0 ..= 125, false) => {}
1964 // (126 ..= 255, false) => {}
1965 // (0 ..= 255, true) => {}
1967 overlaps.push(range.clone());
1971 .flat_map(|range| range_borders(range));
1972 let ctor_borders = range_borders(ctor_range.clone());
1973 let mut borders: Vec<_> = row_borders.chain(ctor_borders).collect();
1974 borders.sort_unstable();
1976 lint_overlapping_patterns(tcx, hir_id, ctor_range, ty, overlaps);
1978 // We're going to iterate through every adjacent pair of borders, making sure that
1979 // each represents an interval of nonnegative length, and convert each such
1980 // interval into a constructor.
1981 for IntRange { range, .. } in
1982 borders.windows(2).filter_map(|window| match (window[0], window[1]) {
1983 (Border::JustBefore(n), Border::JustBefore(m)) => {
1985 Some(IntRange { range: n..=(m - 1), ty, span })
1990 (Border::JustBefore(n), Border::AfterMax) => {
1991 Some(IntRange { range: n..=u128::MAX, ty, span })
1993 (Border::AfterMax, _) => None,
1996 split_ctors.push(IntRange::range_to_ctor(tcx, ty, range, span));
1999 VarLenSlice(self_prefix, self_suffix) => {
2000 // The exhaustiveness-checking paper does not include any details on
2001 // checking variable-length slice patterns. However, they are matched
2002 // by an infinite collection of fixed-length array patterns.
2004 // Checking the infinite set directly would take an infinite amount
2005 // of time. However, it turns out that for each finite set of
2006 // patterns `P`, all sufficiently large array lengths are equivalent:
2008 // Each slice `s` with a "sufficiently-large" length `l ≥ L` that applies
2009 // to exactly the subset `Pₜ` of `P` can be transformed to a slice
2010 // `sₘ` for each sufficiently-large length `m` that applies to exactly
2011 // the same subset of `P`.
2013 // Because of that, each witness for reachability-checking from one
2014 // of the sufficiently-large lengths can be transformed to an
2015 // equally-valid witness from any other length, so we only have
2016 // to check slice lengths from the "minimal sufficiently-large length"
2019 // Note that the fact that there is a *single* `sₘ` for each `m`
2020 // not depending on the specific pattern in `P` is important: if
2021 // you look at the pair of patterns
2024 // Then any slice of length ≥1 that matches one of these two
2025 // patterns can be trivially turned to a slice of any
2026 // other length ≥1 that matches them and vice-versa - for
2027 // but the slice from length 2 `[false, true]` that matches neither
2028 // of these patterns can't be turned to a slice from length 1 that
2029 // matches neither of these patterns, so we have to consider
2030 // slices from length 2 there.
2032 // Now, to see that that length exists and find it, observe that slice
2033 // patterns are either "fixed-length" patterns (`[_, _, _]`) or
2034 // "variable-length" patterns (`[_, .., _]`).
2036 // For fixed-length patterns, all slices with lengths *longer* than
2037 // the pattern's length have the same outcome (of not matching), so
2038 // as long as `L` is greater than the pattern's length we can pick
2039 // any `sₘ` from that length and get the same result.
2041 // For variable-length patterns, the situation is more complicated,
2042 // because as seen above the precise value of `sₘ` matters.
2044 // However, for each variable-length pattern `p` with a prefix of length
2045 // `plₚ` and suffix of length `slₚ`, only the first `plₚ` and the last
2046 // `slₚ` elements are examined.
2048 // Therefore, as long as `L` is positive (to avoid concerns about empty
2049 // types), all elements after the maximum prefix length and before
2050 // the maximum suffix length are not examined by any variable-length
2051 // pattern, and therefore can be added/removed without affecting
2052 // them - creating equivalent patterns from any sufficiently-large
2055 // Of course, if fixed-length patterns exist, we must be sure
2056 // that our length is large enough to miss them all, so
2057 // we can pick `L = max(max(FIXED_LEN)+1, max(PREFIX_LEN) + max(SUFFIX_LEN))`
2059 // for example, with the above pair of patterns, all elements
2060 // but the first and last can be added/removed, so any
2061 // witness of length ≥2 (say, `[false, false, true]`) can be
2062 // turned to a witness from any other length ≥2.
2064 let mut max_prefix_len = self_prefix;
2065 let mut max_suffix_len = self_suffix;
2066 let mut max_fixed_len = 0;
2068 for row in matrix.heads() {
2070 PatKind::Constant { value } => {
2071 // extract the length of an array/slice from a constant
2072 match (value.val, &value.ty.kind) {
2073 (_, ty::Array(_, n)) => {
2075 cmp::max(max_fixed_len, n.eval_usize(tcx, param_env))
2077 (ty::ConstKind::Value(ConstValue::Slice { start, end, .. }),
2079 max_fixed_len = cmp::max(max_fixed_len, (end - start) as u64)
2084 PatKind::Slice { ref prefix, slice: None, ref suffix } => {
2085 let fixed_len = prefix.len() as u64 + suffix.len() as u64;
2086 max_fixed_len = cmp::max(max_fixed_len, fixed_len);
2088 PatKind::Slice { ref prefix, slice: Some(_), ref suffix } => {
2089 max_prefix_len = cmp::max(max_prefix_len, prefix.len() as u64);
2090 max_suffix_len = cmp::max(max_suffix_len, suffix.len() as u64);
2096 // For diagnostics, we keep the prefix and suffix lengths separate, so in the case
2097 // where `max_fixed_len + 1` is the largest, we adapt `max_prefix_len` accordingly,
2098 // so that `L = max_prefix_len + max_suffix_len`.
2099 if max_fixed_len + 1 >= max_prefix_len + max_suffix_len {
2100 // The subtraction can't overflow thanks to the above check.
2101 // The new `max_prefix_len` is also guaranteed to be larger than its previous
2103 max_prefix_len = max_fixed_len + 1 - max_suffix_len;
2106 // `ctor` originally covered the range `(self_prefix + self_suffix..infinity)`. We
2107 // now split it into two: lengths smaller than `max_prefix_len + max_suffix_len`
2108 // are treated independently as fixed-lengths slices, and lengths above are
2109 // captured by a final VarLenSlice constructor.
2111 (self_prefix + self_suffix..max_prefix_len + max_suffix_len).map(FixedLenSlice),
2113 split_ctors.push(VarLenSlice(max_prefix_len, max_suffix_len));
2115 // Any other constructor can be used unchanged.
2116 _ => split_ctors.push(ctor),
2123 fn lint_overlapping_patterns(
2125 hir_id: Option<HirId>,
2126 ctor_range: IntRange<'tcx>,
2128 overlaps: Vec<IntRange<'tcx>>,
2130 if let (true, Some(hir_id)) = (!overlaps.is_empty(), hir_id) {
2131 let mut err = tcx.struct_span_lint_hir(
2132 lint::builtin::OVERLAPPING_PATTERNS,
2135 "multiple patterns covering the same range",
2137 err.span_label(ctor_range.span, "overlapping patterns");
2138 for int_range in overlaps {
2139 // Use the real type for user display of the ranges:
2143 "this range overlaps on `{}`",
2144 IntRange::range_to_ctor(tcx, ty, int_range.range, DUMMY_SP).display(tcx),
2152 fn constructor_covered_by_range<'tcx>(
2154 param_env: ty::ParamEnv<'tcx>,
2155 ctor: &Constructor<'tcx>,
2157 ) -> Result<bool, ErrorReported> {
2158 let (from, to, end, ty) = match pat.kind {
2159 box PatKind::Constant { value } => (value, value, RangeEnd::Included, value.ty),
2160 box PatKind::Range(PatRange { lo, hi, end }) => (lo, hi, end, lo.ty),
2161 _ => bug!("`constructor_covered_by_range` called with {:?}", pat),
2163 trace!("constructor_covered_by_range {:#?}, {:#?}, {:#?}, {}", ctor, from, to, ty);
2164 let cmp_from = |c_from| {
2165 compare_const_vals(tcx, c_from, from, param_env, ty).map(|res| res != Ordering::Less)
2167 let cmp_to = |c_to| compare_const_vals(tcx, c_to, to, param_env, ty);
2168 macro_rules! some_or_ok {
2172 None => return Ok(false), // not char or int
2177 ConstantValue(value, _) => {
2178 let to = some_or_ok!(cmp_to(value));
2180 (to == Ordering::Less) || (end == RangeEnd::Included && to == Ordering::Equal);
2181 Ok(some_or_ok!(cmp_from(value)) && end)
2183 ConstantRange(from, to, ty, RangeEnd::Included, _) => {
2185 some_or_ok!(cmp_to(ty::Const::from_bits(tcx, to, ty::ParamEnv::empty().and(ty),)));
2187 (to == Ordering::Less) || (end == RangeEnd::Included && to == Ordering::Equal);
2188 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
2191 ty::ParamEnv::empty().and(ty),
2194 ConstantRange(from, to, ty, RangeEnd::Excluded, _) => {
2196 some_or_ok!(cmp_to(ty::Const::from_bits(tcx, to, ty::ParamEnv::empty().and(ty))));
2198 (to == Ordering::Less) || (end == RangeEnd::Excluded && to == Ordering::Equal);
2199 Ok(some_or_ok!(cmp_from(ty::Const::from_bits(
2202 ty::ParamEnv::empty().and(ty)
2210 fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
2211 cx: &mut MatchCheckCtxt<'a, 'tcx>,
2212 subpatterns: &'p [FieldPat<'tcx>],
2213 ctor_wild_subpatterns: &[&'p Pat<'tcx>],
2214 is_non_exhaustive: bool,
2215 ) -> PatStack<'p, 'tcx> {
2216 let mut result = SmallVec::from_slice(ctor_wild_subpatterns);
2218 for subpat in subpatterns {
2219 if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
2220 result[subpat.field.index()] = &subpat.pattern;
2225 "patterns_for_variant({:#?}, {:#?}) = {:#?}",
2226 subpatterns, ctor_wild_subpatterns, result
2228 PatStack::from_vec(result)
2231 /// This is the main specialization step. It expands the pattern
2232 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
2233 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
2234 /// Returns `None` if the pattern does not have the given constructor.
2236 /// OTOH, slice patterns with a subslice pattern (tail @ ..) can be expanded into multiple
2237 /// different patterns.
2238 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
2239 /// fields filled with wild patterns.
2240 fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
2241 cx: &mut MatchCheckCtxt<'a, 'tcx>,
2242 mut pat: &'q Pat<'tcx>,
2243 constructor: &Constructor<'tcx>,
2244 ctor_wild_subpatterns: &[&'p Pat<'tcx>],
2245 ) -> Option<PatStack<'p, 'tcx>> {
2246 while let PatKind::AscribeUserType { ref subpattern, .. } = *pat.kind {
2250 if let NonExhaustive = constructor {
2251 // Only a wildcard pattern can match the special extra constructor
2252 return if pat.is_wildcard() { Some(PatStack::default()) } else { None };
2255 let result = match *pat.kind {
2256 PatKind::AscribeUserType { .. } => bug!(), // Handled above
2258 PatKind::Binding { .. } | PatKind::Wild => {
2259 Some(PatStack::from_slice(ctor_wild_subpatterns))
2262 PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
2263 let ref variant = adt_def.variants[variant_index];
2264 let is_non_exhaustive = variant.is_field_list_non_exhaustive() && !cx.is_local(pat.ty);
2265 Some(Variant(variant.def_id))
2266 .filter(|variant_constructor| variant_constructor == constructor)
2268 patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, is_non_exhaustive)
2272 PatKind::Leaf { ref subpatterns } => {
2273 Some(patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, false))
2276 PatKind::Deref { ref subpattern } => Some(PatStack::from_pattern(subpattern)),
2278 PatKind::Constant { value } if constructor.is_slice() => {
2279 // We extract an `Option` for the pointer because slices of zero
2280 // elements don't necessarily point to memory, they are usually
2281 // just integers. The only time they should be pointing to memory
2282 // is when they are subslices of nonzero slices.
2283 let (alloc, offset, n, ty) = match value.ty.kind {
2284 ty::Array(t, n) => match value.val {
2285 ty::ConstKind::Value(ConstValue::ByRef { offset, alloc, .. }) => {
2286 (alloc, offset, n.eval_usize(cx.tcx, cx.param_env), t)
2288 _ => span_bug!(pat.span, "array pattern is {:?}", value,),
2292 ty::ConstKind::Value(ConstValue::Slice { data, start, end }) => {
2293 (data, Size::from_bytes(start as u64), (end - start) as u64, t)
2295 ty::ConstKind::Value(ConstValue::ByRef { .. }) => {
2296 // FIXME(oli-obk): implement `deref` for `ConstValue`
2301 "slice pattern constant must be scalar pair but is {:?}",
2308 "unexpected const-val {:?} with ctor {:?}",
2313 if ctor_wild_subpatterns.len() as u64 == n {
2314 // convert a constant slice/array pattern to a list of patterns.
2315 let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
2316 let ptr = Pointer::new(AllocId(0), offset);
2319 let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
2320 let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
2321 let scalar = scalar.not_undef().ok()?;
2322 let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
2324 Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
2325 Some(&*cx.pattern_arena.alloc(pattern))
2333 PatKind::Constant { .. } | PatKind::Range { .. } => {
2334 // If the constructor is a:
2335 // - Single value: add a row if the pattern contains the constructor.
2336 // - Range: add a row if the constructor intersects the pattern.
2337 if constructor.is_range_and_should_match_exhaustively(cx.tcx) {
2339 IntRange::from_ctor(cx.tcx, cx.param_env, constructor),
2340 IntRange::from_pat(cx.tcx, cx.param_env, pat),
2342 (Some(ctor), Some(pat)) => ctor.intersection(&pat).map(|_| {
2343 let (pat_lo, pat_hi) = pat.range.into_inner();
2344 let (ctor_lo, ctor_hi) = ctor.range.into_inner();
2345 assert!(pat_lo <= ctor_lo && ctor_hi <= pat_hi);
2351 // Fallback for non-ranges and ranges that involve
2352 // floating-point numbers, which are not conveniently handled
2353 // by `IntRange`. For these cases, the constructor may not be a
2354 // range so intersection actually devolves into being covered
2356 match constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat) {
2357 Ok(true) => Some(PatStack::default()),
2358 Ok(false) | Err(ErrorReported) => None,
2363 PatKind::Array { ref prefix, ref slice, ref suffix }
2364 | PatKind::Slice { ref prefix, ref slice, ref suffix } => match *constructor {
2365 FixedLenSlice(..) | VarLenSlice(..) => {
2366 let pat_len = prefix.len() + suffix.len();
2367 if let Some(slice_count) = ctor_wild_subpatterns.len().checked_sub(pat_len) {
2368 if slice_count == 0 || slice.is_some() {
2373 ctor_wild_subpatterns
2378 .chain(suffix.iter()),
2389 ConstantValue(cv, _) => {
2390 match slice_pat_covered_by_const(
2399 Ok(true) => Some(PatStack::default()),
2401 Err(ErrorReported) => None,
2404 _ => span_bug!(pat.span, "unexpected ctor {:?} for slice pat", constructor),
2407 PatKind::Or { .. } => {
2408 bug!("support for or-patterns has not been fully implemented yet.");
2411 debug!("specialize({:#?}, {:#?}) = {:#?}", pat, ctor_wild_subpatterns, result);