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fc512014 XL |
1 | //! Note: tests specific to this file can be found in: |
2 | //! | |
3 | //! - `ui/pattern/usefulness` | |
4 | //! - `ui/or-patterns` | |
5 | //! - `ui/consts/const_in_pattern` | |
6 | //! - `ui/rfc-2008-non-exhaustive` | |
7 | //! - `ui/half-open-range-patterns` | |
8 | //! - probably many others | |
9 | //! | |
10 | //! I (Nadrieril) prefer to put new tests in `ui/pattern/usefulness` unless there's a specific | |
11 | //! reason not to, for example if they depend on a particular feature like `or_patterns`. | |
12 | //! | |
13 | //! ----- | |
14 | //! | |
15 | //! This file includes the logic for exhaustiveness and reachability checking for pattern-matching. | |
16 | //! Specifically, given a list of patterns for a type, we can tell whether: | |
17 | //! (a) each pattern is reachable (reachability) | |
18 | //! (b) the patterns cover every possible value for the type (exhaustiveness) | |
19 | //! | |
20 | //! The algorithm implemented here is a modified version of the one described in [this | |
21 | //! paper](http://moscova.inria.fr/~maranget/papers/warn/index.html). We have however generalized | |
22 | //! it to accommodate the variety of patterns that Rust supports. We thus explain our version here, | |
23 | //! without being as rigorous. | |
24 | //! | |
25 | //! | |
26 | //! # Summary | |
27 | //! | |
28 | //! The core of the algorithm is the notion of "usefulness". A pattern `q` is said to be *useful* | |
29 | //! relative to another pattern `p` of the same type if there is a value that is matched by `q` and | |
30 | //! not matched by `p`. This generalizes to many `p`s: `q` is useful w.r.t. a list of patterns | |
31 | //! `p_1 .. p_n` if there is a value that is matched by `q` and by none of the `p_i`. We write | |
32 | //! `usefulness(p_1 .. p_n, q)` for a function that returns a list of such values. The aim of this | |
33 | //! file is to compute it efficiently. | |
34 | //! | |
35 | //! This is enough to compute reachability: a pattern in a `match` expression is reachable iff it | |
36 | //! is useful w.r.t. the patterns above it: | |
37 | //! ```rust | |
04454e1e | 38 | //! # fn foo(x: Option<i32>) { |
fc512014 | 39 | //! match x { |
04454e1e FG |
40 | //! Some(_) => {}, |
41 | //! None => {}, // reachable: `None` is matched by this but not the branch above | |
42 | //! Some(0) => {}, // unreachable: all the values this matches are already matched by | |
43 | //! // `Some(_)` above | |
fc512014 | 44 | //! } |
04454e1e | 45 | //! # } |
fc512014 XL |
46 | //! ``` |
47 | //! | |
48 | //! This is also enough to compute exhaustiveness: a match is exhaustive iff the wildcard `_` | |
49 | //! pattern is _not_ useful w.r.t. the patterns in the match. The values returned by `usefulness` | |
50 | //! are used to tell the user which values are missing. | |
04454e1e FG |
51 | //! ```compile_fail,E0004 |
52 | //! # fn foo(x: Option<i32>) { | |
fc512014 | 53 | //! match x { |
04454e1e FG |
54 | //! Some(0) => {}, |
55 | //! None => {}, | |
fc512014 XL |
56 | //! // not exhaustive: `_` is useful because it matches `Some(1)` |
57 | //! } | |
04454e1e | 58 | //! # } |
fc512014 XL |
59 | //! ``` |
60 | //! | |
61 | //! The entrypoint of this file is the [`compute_match_usefulness`] function, which computes | |
62 | //! reachability for each match branch and exhaustiveness for the whole match. | |
63 | //! | |
64 | //! | |
65 | //! # Constructors and fields | |
66 | //! | |
67 | //! Note: we will often abbreviate "constructor" as "ctor". | |
68 | //! | |
5e7ed085 | 69 | //! The idea that powers everything that is done in this file is the following: a (matchable) |
fc512014 XL |
70 | //! value is made from a constructor applied to a number of subvalues. Examples of constructors are |
71 | //! `Some`, `None`, `(,)` (the 2-tuple constructor), `Foo {..}` (the constructor for a struct | |
72 | //! `Foo`), and `2` (the constructor for the number `2`). This is natural when we think of | |
73 | //! pattern-matching, and this is the basis for what follows. | |
74 | //! | |
75 | //! Some of the ctors listed above might feel weird: `None` and `2` don't take any arguments. | |
76 | //! That's ok: those are ctors that take a list of 0 arguments; they are the simplest case of | |
77 | //! ctors. We treat `2` as a ctor because `u64` and other number types behave exactly like a huge | |
5e7ed085 | 78 | //! `enum`, with one variant for each number. This allows us to see any matchable value as made up |
fc512014 XL |
79 | //! from a tree of ctors, each having a set number of children. For example: `Foo { bar: None, |
80 | //! baz: Ok(0) }` is made from 4 different ctors, namely `Foo{..}`, `None`, `Ok` and `0`. | |
81 | //! | |
82 | //! This idea can be extended to patterns: they are also made from constructors applied to fields. | |
83 | //! A pattern for a given type is allowed to use all the ctors for values of that type (which we | |
84 | //! call "value constructors"), but there are also pattern-only ctors. The most important one is | |
85 | //! the wildcard (`_`), and the others are integer ranges (`0..=10`), variable-length slices (`[x, | |
86 | //! ..]`), and or-patterns (`Ok(0) | Err(_)`). Examples of valid patterns are `42`, `Some(_)`, `Foo | |
87 | //! { bar: Some(0) | None, baz: _ }`. Note that a binder in a pattern (e.g. `Some(x)`) matches the | |
88 | //! same values as a wildcard (e.g. `Some(_)`), so we treat both as wildcards. | |
89 | //! | |
90 | //! From this deconstruction we can compute whether a given value matches a given pattern; we | |
91 | //! simply look at ctors one at a time. Given a pattern `p` and a value `v`, we want to compute | |
92 | //! `matches!(v, p)`. It's mostly straightforward: we compare the head ctors and when they match | |
93 | //! we compare their fields recursively. A few representative examples: | |
94 | //! | |
95 | //! - `matches!(v, _) := true` | |
96 | //! - `matches!((v0, v1), (p0, p1)) := matches!(v0, p0) && matches!(v1, p1)` | |
97 | //! - `matches!(Foo { bar: v0, baz: v1 }, Foo { bar: p0, baz: p1 }) := matches!(v0, p0) && matches!(v1, p1)` | |
98 | //! - `matches!(Ok(v0), Ok(p0)) := matches!(v0, p0)` | |
99 | //! - `matches!(Ok(v0), Err(p0)) := false` (incompatible variants) | |
100 | //! - `matches!(v, 1..=100) := matches!(v, 1) || ... || matches!(v, 100)` | |
101 | //! - `matches!([v0], [p0, .., p1]) := false` (incompatible lengths) | |
102 | //! - `matches!([v0, v1, v2], [p0, .., p1]) := matches!(v0, p0) && matches!(v2, p1)` | |
103 | //! - `matches!(v, p0 | p1) := matches!(v, p0) || matches!(v, p1)` | |
104 | //! | |
105 | //! Constructors, fields and relevant operations are defined in the [`super::deconstruct_pat`] module. | |
106 | //! | |
107 | //! Note: this constructors/fields distinction may not straightforwardly apply to every Rust type. | |
108 | //! For example a value of type `Rc<u64>` can't be deconstructed that way, and `&str` has an | |
109 | //! infinitude of constructors. There are also subtleties with visibility of fields and | |
110 | //! uninhabitedness and various other things. The constructors idea can be extended to handle most | |
111 | //! of these subtleties though; caveats are documented where relevant throughout the code. | |
112 | //! | |
113 | //! Whether constructors cover each other is computed by [`Constructor::is_covered_by`]. | |
114 | //! | |
115 | //! | |
116 | //! # Specialization | |
117 | //! | |
118 | //! Recall that we wish to compute `usefulness(p_1 .. p_n, q)`: given a list of patterns `p_1 .. | |
119 | //! p_n` and a pattern `q`, all of the same type, we want to find a list of values (called | |
120 | //! "witnesses") that are matched by `q` and by none of the `p_i`. We obviously don't just | |
121 | //! enumerate all possible values. From the discussion above we see that we can proceed | |
122 | //! ctor-by-ctor: for each value ctor of the given type, we ask "is there a value that starts with | |
123 | //! this constructor and matches `q` and none of the `p_i`?". As we saw above, there's a lot we can | |
124 | //! say from knowing only the first constructor of our candidate value. | |
125 | //! | |
126 | //! Let's take the following example: | |
04454e1e FG |
127 | //! ```compile_fail,E0004 |
128 | //! # enum Enum { Variant1(()), Variant2(Option<bool>, u32)} | |
129 | //! # fn foo(x: Enum) { | |
fc512014 XL |
130 | //! match x { |
131 | //! Enum::Variant1(_) => {} // `p1` | |
132 | //! Enum::Variant2(None, 0) => {} // `p2` | |
133 | //! Enum::Variant2(Some(_), 0) => {} // `q` | |
134 | //! } | |
04454e1e | 135 | //! # } |
fc512014 XL |
136 | //! ``` |
137 | //! | |
138 | //! We can easily see that if our candidate value `v` starts with `Variant1` it will not match `q`. | |
139 | //! If `v = Variant2(v0, v1)` however, whether or not it matches `p2` and `q` will depend on `v0` | |
140 | //! and `v1`. In fact, such a `v` will be a witness of usefulness of `q` exactly when the tuple | |
141 | //! `(v0, v1)` is a witness of usefulness of `q'` in the following reduced match: | |
142 | //! | |
04454e1e FG |
143 | //! ```compile_fail,E0004 |
144 | //! # fn foo(x: (Option<bool>, u32)) { | |
fc512014 XL |
145 | //! match x { |
146 | //! (None, 0) => {} // `p2'` | |
147 | //! (Some(_), 0) => {} // `q'` | |
148 | //! } | |
04454e1e | 149 | //! # } |
fc512014 XL |
150 | //! ``` |
151 | //! | |
152 | //! This motivates a new step in computing usefulness, that we call _specialization_. | |
153 | //! Specialization consist of filtering a list of patterns for those that match a constructor, and | |
154 | //! then looking into the constructor's fields. This enables usefulness to be computed recursively. | |
155 | //! | |
156 | //! Instead of acting on a single pattern in each row, we will consider a list of patterns for each | |
157 | //! row, and we call such a list a _pattern-stack_. The idea is that we will specialize the | |
158 | //! leftmost pattern, which amounts to popping the constructor and pushing its fields, which feels | |
159 | //! like a stack. We note a pattern-stack simply with `[p_1 ... p_n]`. | |
160 | //! Here's a sequence of specializations of a list of pattern-stacks, to illustrate what's | |
161 | //! happening: | |
04454e1e | 162 | //! ```ignore (illustrative) |
fc512014 XL |
163 | //! [Enum::Variant1(_)] |
164 | //! [Enum::Variant2(None, 0)] | |
165 | //! [Enum::Variant2(Some(_), 0)] | |
166 | //! //==>> specialize with `Variant2` | |
167 | //! [None, 0] | |
168 | //! [Some(_), 0] | |
169 | //! //==>> specialize with `Some` | |
170 | //! [_, 0] | |
171 | //! //==>> specialize with `true` (say the type was `bool`) | |
172 | //! [0] | |
173 | //! //==>> specialize with `0` | |
174 | //! [] | |
175 | //! ``` | |
176 | //! | |
177 | //! The function `specialize(c, p)` takes a value constructor `c` and a pattern `p`, and returns 0 | |
178 | //! or more pattern-stacks. If `c` does not match the head constructor of `p`, it returns nothing; | |
179 | //! otherwise if returns the fields of the constructor. This only returns more than one | |
180 | //! pattern-stack if `p` has a pattern-only constructor. | |
181 | //! | |
182 | //! - Specializing for the wrong constructor returns nothing | |
183 | //! | |
184 | //! `specialize(None, Some(p0)) := []` | |
185 | //! | |
186 | //! - Specializing for the correct constructor returns a single row with the fields | |
187 | //! | |
188 | //! `specialize(Variant1, Variant1(p0, p1, p2)) := [[p0, p1, p2]]` | |
189 | //! | |
190 | //! `specialize(Foo{..}, Foo { bar: p0, baz: p1 }) := [[p0, p1]]` | |
191 | //! | |
192 | //! - For or-patterns, we specialize each branch and concatenate the results | |
193 | //! | |
194 | //! `specialize(c, p0 | p1) := specialize(c, p0) ++ specialize(c, p1)` | |
195 | //! | |
196 | //! - We treat the other pattern constructors as if they were a large or-pattern of all the | |
197 | //! possibilities: | |
198 | //! | |
199 | //! `specialize(c, _) := specialize(c, Variant1(_) | Variant2(_, _) | ...)` | |
200 | //! | |
201 | //! `specialize(c, 1..=100) := specialize(c, 1 | ... | 100)` | |
202 | //! | |
203 | //! `specialize(c, [p0, .., p1]) := specialize(c, [p0, p1] | [p0, _, p1] | [p0, _, _, p1] | ...)` | |
204 | //! | |
205 | //! - If `c` is a pattern-only constructor, `specialize` is defined on a case-by-case basis. See | |
206 | //! the discussion about constructor splitting in [`super::deconstruct_pat`]. | |
207 | //! | |
208 | //! | |
209 | //! We then extend this function to work with pattern-stacks as input, by acting on the first | |
210 | //! column and keeping the other columns untouched. | |
211 | //! | |
212 | //! Specialization for the whole matrix is done in [`Matrix::specialize_constructor`]. Note that | |
213 | //! or-patterns in the first column are expanded before being stored in the matrix. Specialization | |
214 | //! for a single patstack is done from a combination of [`Constructor::is_covered_by`] and | |
215 | //! [`PatStack::pop_head_constructor`]. The internals of how it's done mostly live in the | |
216 | //! [`Fields`] struct. | |
217 | //! | |
218 | //! | |
219 | //! # Computing usefulness | |
220 | //! | |
221 | //! We now have all we need to compute usefulness. The inputs to usefulness are a list of | |
222 | //! pattern-stacks `p_1 ... p_n` (one per row), and a new pattern_stack `q`. The paper and this | |
223 | //! file calls the list of patstacks a _matrix_. They must all have the same number of columns and | |
224 | //! the patterns in a given column must all have the same type. `usefulness` returns a (possibly | |
225 | //! empty) list of witnesses of usefulness. These witnesses will also be pattern-stacks. | |
226 | //! | |
227 | //! - base case: `n_columns == 0`. | |
228 | //! Since a pattern-stack functions like a tuple of patterns, an empty one functions like the | |
229 | //! unit type. Thus `q` is useful iff there are no rows above it, i.e. if `n == 0`. | |
230 | //! | |
231 | //! - inductive case: `n_columns > 0`. | |
232 | //! We need a way to list the constructors we want to try. We will be more clever in the next | |
233 | //! section but for now assume we list all value constructors for the type of the first column. | |
234 | //! | |
235 | //! - for each such ctor `c`: | |
236 | //! | |
237 | //! - for each `q'` returned by `specialize(c, q)`: | |
238 | //! | |
239 | //! - we compute `usefulness(specialize(c, p_1) ... specialize(c, p_n), q')` | |
240 | //! | |
241 | //! - for each witness found, we revert specialization by pushing the constructor `c` on top. | |
242 | //! | |
243 | //! - We return the concatenation of all the witnesses found, if any. | |
244 | //! | |
245 | //! Example: | |
04454e1e | 246 | //! ```ignore (illustrative) |
fc512014 XL |
247 | //! [Some(true)] // p_1 |
248 | //! [None] // p_2 | |
249 | //! [Some(_)] // q | |
250 | //! //==>> try `None`: `specialize(None, q)` returns nothing | |
251 | //! //==>> try `Some`: `specialize(Some, q)` returns a single row | |
252 | //! [true] // p_1' | |
253 | //! [_] // q' | |
254 | //! //==>> try `true`: `specialize(true, q')` returns a single row | |
255 | //! [] // p_1'' | |
256 | //! [] // q'' | |
257 | //! //==>> base case; `n != 0` so `q''` is not useful. | |
258 | //! //==>> go back up a step | |
259 | //! [true] // p_1' | |
260 | //! [_] // q' | |
261 | //! //==>> try `false`: `specialize(false, q')` returns a single row | |
262 | //! [] // q'' | |
263 | //! //==>> base case; `n == 0` so `q''` is useful. We return the single witness `[]` | |
264 | //! witnesses: | |
265 | //! [] | |
266 | //! //==>> undo the specialization with `false` | |
267 | //! witnesses: | |
268 | //! [false] | |
269 | //! //==>> undo the specialization with `Some` | |
270 | //! witnesses: | |
271 | //! [Some(false)] | |
272 | //! //==>> we have tried all the constructors. The output is the single witness `[Some(false)]`. | |
273 | //! ``` | |
274 | //! | |
275 | //! This computation is done in [`is_useful`]. In practice we don't care about the list of | |
276 | //! witnesses when computing reachability; we only need to know whether any exist. We do keep the | |
277 | //! witnesses when computing exhaustiveness to report them to the user. | |
278 | //! | |
279 | //! | |
280 | //! # Making usefulness tractable: constructor splitting | |
281 | //! | |
282 | //! We're missing one last detail: which constructors do we list? Naively listing all value | |
283 | //! constructors cannot work for types like `u64` or `&str`, so we need to be more clever. The | |
284 | //! first obvious insight is that we only want to list constructors that are covered by the head | |
285 | //! constructor of `q`. If it's a value constructor, we only try that one. If it's a pattern-only | |
286 | //! constructor, we use the final clever idea for this algorithm: _constructor splitting_, where we | |
287 | //! group together constructors that behave the same. | |
288 | //! | |
289 | //! The details are not necessary to understand this file, so we explain them in | |
290 | //! [`super::deconstruct_pat`]. Splitting is done by the [`Constructor::split`] function. | |
49aad941 FG |
291 | //! |
292 | //! # Constants in patterns | |
293 | //! | |
294 | //! There are two kinds of constants in patterns: | |
295 | //! | |
296 | //! * literals (`1`, `true`, `"foo"`) | |
297 | //! * named or inline consts (`FOO`, `const { 5 + 6 }`) | |
298 | //! | |
299 | //! The latter are converted into other patterns with literals at the leaves. For example | |
300 | //! `const_to_pat(const { [1, 2, 3] })` becomes an `Array(vec![Const(1), Const(2), Const(3)])` | |
301 | //! pattern. This gets problematic when comparing the constant via `==` would behave differently | |
302 | //! from matching on the constant converted to a pattern. Situations like that can occur, when | |
303 | //! the user implements `PartialEq` manually, and thus could make `==` behave arbitrarily different. | |
304 | //! In order to honor the `==` implementation, constants of types that implement `PartialEq` manually | |
305 | //! stay as a full constant and become an `Opaque` pattern. These `Opaque` patterns do not participate | |
306 | //! in exhaustiveness, specialization or overlap checking. | |
fc512014 | 307 | |
c295e0f8 | 308 | use self::ArmType::*; |
fc512014 | 309 | use self::Usefulness::*; |
c295e0f8 | 310 | use super::deconstruct_pat::{Constructor, DeconstructedPat, Fields, SplitWildcard}; |
9c376795 | 311 | use crate::errors::{NonExhaustiveOmittedPattern, Uncovered}; |
fc512014 XL |
312 | |
313 | use rustc_data_structures::captures::Captures; | |
fc512014 XL |
314 | |
315 | use rustc_arena::TypedArena; | |
3c0e092e | 316 | use rustc_data_structures::stack::ensure_sufficient_stack; |
fc512014 XL |
317 | use rustc_hir::def_id::DefId; |
318 | use rustc_hir::HirId; | |
319 | use rustc_middle::ty::{self, Ty, TyCtxt}; | |
c295e0f8 XL |
320 | use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS; |
321 | use rustc_span::{Span, DUMMY_SP}; | |
fc512014 XL |
322 | |
323 | use smallvec::{smallvec, SmallVec}; | |
324 | use std::fmt; | |
c295e0f8 | 325 | use std::iter::once; |
fc512014 | 326 | |
923072b8 FG |
327 | pub(crate) struct MatchCheckCtxt<'p, 'tcx> { |
328 | pub(crate) tcx: TyCtxt<'tcx>, | |
fc512014 XL |
329 | /// The module in which the match occurs. This is necessary for |
330 | /// checking inhabited-ness of types because whether a type is (visibly) | |
331 | /// inhabited can depend on whether it was defined in the current module or | |
332 | /// not. E.g., `struct Foo { _private: ! }` cannot be seen to be empty | |
333 | /// outside its module and should not be matchable with an empty match statement. | |
923072b8 FG |
334 | pub(crate) module: DefId, |
335 | pub(crate) param_env: ty::ParamEnv<'tcx>, | |
336 | pub(crate) pattern_arena: &'p TypedArena<DeconstructedPat<'p, 'tcx>>, | |
353b0b11 FG |
337 | /// Only produce `NON_EXHAUSTIVE_OMITTED_PATTERNS` lint on refutable patterns. |
338 | pub(crate) refutable: bool, | |
fc512014 XL |
339 | } |
340 | ||
341 | impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> { | |
342 | pub(super) fn is_uninhabited(&self, ty: Ty<'tcx>) -> bool { | |
343 | if self.tcx.features().exhaustive_patterns { | |
487cf647 | 344 | !ty.is_inhabited_from(self.tcx, self.module, self.param_env) |
fc512014 XL |
345 | } else { |
346 | false | |
347 | } | |
348 | } | |
349 | ||
350 | /// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`. | |
351 | pub(super) fn is_foreign_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool { | |
352 | match ty.kind() { | |
353 | ty::Adt(def, ..) => { | |
5e7ed085 | 354 | def.is_enum() && def.is_variant_list_non_exhaustive() && !def.did().is_local() |
fc512014 XL |
355 | } |
356 | _ => false, | |
357 | } | |
358 | } | |
359 | } | |
360 | ||
361 | #[derive(Copy, Clone)] | |
362 | pub(super) struct PatCtxt<'a, 'p, 'tcx> { | |
363 | pub(super) cx: &'a MatchCheckCtxt<'p, 'tcx>, | |
364 | /// Type of the current column under investigation. | |
365 | pub(super) ty: Ty<'tcx>, | |
366 | /// Span of the current pattern under investigation. | |
367 | pub(super) span: Span, | |
368 | /// Whether the current pattern is the whole pattern as found in a match arm, or if it's a | |
369 | /// subpattern. | |
370 | pub(super) is_top_level: bool, | |
5e7ed085 | 371 | /// Whether the current pattern is from a `non_exhaustive` enum. |
c295e0f8 | 372 | pub(super) is_non_exhaustive: bool, |
fc512014 XL |
373 | } |
374 | ||
6a06907d XL |
375 | impl<'a, 'p, 'tcx> fmt::Debug for PatCtxt<'a, 'p, 'tcx> { |
376 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | |
377 | f.debug_struct("PatCtxt").field("ty", &self.ty).finish() | |
378 | } | |
379 | } | |
380 | ||
fc512014 XL |
381 | /// A row of a matrix. Rows of len 1 are very common, which is why `SmallVec[_; 2]` |
382 | /// works well. | |
6a06907d | 383 | #[derive(Clone)] |
f2b60f7d FG |
384 | pub(crate) struct PatStack<'p, 'tcx> { |
385 | pub(crate) pats: SmallVec<[&'p DeconstructedPat<'p, 'tcx>; 2]>, | |
fc512014 XL |
386 | } |
387 | ||
388 | impl<'p, 'tcx> PatStack<'p, 'tcx> { | |
c295e0f8 | 389 | fn from_pattern(pat: &'p DeconstructedPat<'p, 'tcx>) -> Self { |
fc512014 XL |
390 | Self::from_vec(smallvec![pat]) |
391 | } | |
392 | ||
c295e0f8 XL |
393 | fn from_vec(vec: SmallVec<[&'p DeconstructedPat<'p, 'tcx>; 2]>) -> Self { |
394 | PatStack { pats: vec } | |
fc512014 XL |
395 | } |
396 | ||
397 | fn is_empty(&self) -> bool { | |
398 | self.pats.is_empty() | |
399 | } | |
400 | ||
401 | fn len(&self) -> usize { | |
402 | self.pats.len() | |
403 | } | |
404 | ||
c295e0f8 | 405 | fn head(&self) -> &'p DeconstructedPat<'p, 'tcx> { |
fc512014 XL |
406 | self.pats[0] |
407 | } | |
408 | ||
c295e0f8 | 409 | fn iter(&self) -> impl Iterator<Item = &DeconstructedPat<'p, 'tcx>> { |
fc512014 XL |
410 | self.pats.iter().copied() |
411 | } | |
412 | ||
6a06907d XL |
413 | // Recursively expand the first pattern into its subpatterns. Only useful if the pattern is an |
414 | // or-pattern. Panics if `self` is empty. | |
415 | fn expand_or_pat<'a>(&'a self) -> impl Iterator<Item = PatStack<'p, 'tcx>> + Captures<'a> { | |
c295e0f8 | 416 | self.head().iter_fields().map(move |pat| { |
6a06907d XL |
417 | let mut new_patstack = PatStack::from_pattern(pat); |
418 | new_patstack.pats.extend_from_slice(&self.pats[1..]); | |
419 | new_patstack | |
420 | }) | |
fc512014 XL |
421 | } |
422 | ||
f2b60f7d FG |
423 | // Recursively expand all patterns into their subpatterns and push each `PatStack` to matrix. |
424 | fn expand_and_extend<'a>(&'a self, matrix: &mut Matrix<'p, 'tcx>) { | |
425 | if !self.is_empty() && self.head().is_or_pat() { | |
426 | for pat in self.head().iter_fields() { | |
427 | let mut new_patstack = PatStack::from_pattern(pat); | |
428 | new_patstack.pats.extend_from_slice(&self.pats[1..]); | |
429 | if !new_patstack.is_empty() && new_patstack.head().is_or_pat() { | |
430 | new_patstack.expand_and_extend(matrix); | |
431 | } else if !new_patstack.is_empty() { | |
432 | matrix.push(new_patstack); | |
433 | } | |
434 | } | |
435 | } | |
436 | } | |
437 | ||
c295e0f8 | 438 | /// This computes `S(self.head().ctor(), self)`. See top of the file for explanations. |
fc512014 XL |
439 | /// |
440 | /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing | |
441 | /// fields filled with wild patterns. | |
442 | /// | |
443 | /// This is roughly the inverse of `Constructor::apply`. | |
c295e0f8 XL |
444 | fn pop_head_constructor( |
445 | &self, | |
064997fb | 446 | pcx: &PatCtxt<'_, 'p, 'tcx>, |
c295e0f8 XL |
447 | ctor: &Constructor<'tcx>, |
448 | ) -> PatStack<'p, 'tcx> { | |
fc512014 XL |
449 | // We pop the head pattern and push the new fields extracted from the arguments of |
450 | // `self.head()`. | |
064997fb | 451 | let mut new_fields: SmallVec<[_; 2]> = self.head().specialize(pcx, ctor); |
fc512014 XL |
452 | new_fields.extend_from_slice(&self.pats[1..]); |
453 | PatStack::from_vec(new_fields) | |
454 | } | |
455 | } | |
456 | ||
6a06907d XL |
457 | /// Pretty-printing for matrix row. |
458 | impl<'p, 'tcx> fmt::Debug for PatStack<'p, 'tcx> { | |
459 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | |
460 | write!(f, "+")?; | |
461 | for pat in self.iter() { | |
add651ee | 462 | write!(f, " {pat:?} +")?; |
6a06907d XL |
463 | } |
464 | Ok(()) | |
465 | } | |
466 | } | |
467 | ||
fc512014 | 468 | /// A 2D matrix. |
c295e0f8 | 469 | #[derive(Clone)] |
fc512014 | 470 | pub(super) struct Matrix<'p, 'tcx> { |
f2b60f7d | 471 | pub patterns: Vec<PatStack<'p, 'tcx>>, |
fc512014 XL |
472 | } |
473 | ||
474 | impl<'p, 'tcx> Matrix<'p, 'tcx> { | |
475 | fn empty() -> Self { | |
476 | Matrix { patterns: vec![] } | |
477 | } | |
478 | ||
479 | /// Number of columns of this matrix. `None` is the matrix is empty. | |
480 | pub(super) fn column_count(&self) -> Option<usize> { | |
481 | self.patterns.get(0).map(|r| r.len()) | |
482 | } | |
483 | ||
6a06907d XL |
484 | /// Pushes a new row to the matrix. If the row starts with an or-pattern, this recursively |
485 | /// expands it. | |
fc512014 | 486 | fn push(&mut self, row: PatStack<'p, 'tcx>) { |
c295e0f8 | 487 | if !row.is_empty() && row.head().is_or_pat() { |
f2b60f7d | 488 | row.expand_and_extend(self); |
fc512014 XL |
489 | } else { |
490 | self.patterns.push(row); | |
491 | } | |
492 | } | |
493 | ||
494 | /// Iterate over the first component of each row | |
c295e0f8 | 495 | fn heads<'a>( |
fc512014 | 496 | &'a self, |
c295e0f8 XL |
497 | ) -> impl Iterator<Item = &'p DeconstructedPat<'p, 'tcx>> + Clone + Captures<'a> { |
498 | self.patterns.iter().map(|r| r.head()) | |
fc512014 XL |
499 | } |
500 | ||
501 | /// This computes `S(constructor, self)`. See top of the file for explanations. | |
502 | fn specialize_constructor( | |
503 | &self, | |
064997fb | 504 | pcx: &PatCtxt<'_, 'p, 'tcx>, |
fc512014 | 505 | ctor: &Constructor<'tcx>, |
fc512014 | 506 | ) -> Matrix<'p, 'tcx> { |
c295e0f8 XL |
507 | let mut matrix = Matrix::empty(); |
508 | for row in &self.patterns { | |
509 | if ctor.is_covered_by(pcx, row.head().ctor()) { | |
064997fb | 510 | let new_row = row.pop_head_constructor(pcx, ctor); |
c295e0f8 XL |
511 | matrix.push(new_row); |
512 | } | |
513 | } | |
514 | matrix | |
fc512014 XL |
515 | } |
516 | } | |
517 | ||
518 | /// Pretty-printer for matrices of patterns, example: | |
519 | /// | |
520 | /// ```text | |
fc512014 | 521 | /// + _ + [] + |
fc512014 | 522 | /// + true + [First] + |
fc512014 | 523 | /// + true + [Second(true)] + |
fc512014 | 524 | /// + false + [_] + |
fc512014 | 525 | /// + _ + [_, _, tail @ ..] + |
fc512014 XL |
526 | /// ``` |
527 | impl<'p, 'tcx> fmt::Debug for Matrix<'p, 'tcx> { | |
528 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | |
529 | write!(f, "\n")?; | |
530 | ||
531 | let Matrix { patterns: m, .. } = self; | |
532 | let pretty_printed_matrix: Vec<Vec<String>> = | |
add651ee | 533 | m.iter().map(|row| row.iter().map(|pat| format!("{pat:?}")).collect()).collect(); |
fc512014 | 534 | |
6a06907d | 535 | let column_count = m.iter().map(|row| row.len()).next().unwrap_or(0); |
fc512014 XL |
536 | assert!(m.iter().all(|row| row.len() == column_count)); |
537 | let column_widths: Vec<usize> = (0..column_count) | |
538 | .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)) | |
539 | .collect(); | |
540 | ||
fc512014 XL |
541 | for row in pretty_printed_matrix { |
542 | write!(f, "+")?; | |
543 | for (column, pat_str) in row.into_iter().enumerate() { | |
544 | write!(f, " ")?; | |
545 | write!(f, "{:1$}", pat_str, column_widths[column])?; | |
546 | write!(f, " +")?; | |
547 | } | |
548 | write!(f, "\n")?; | |
fc512014 XL |
549 | } |
550 | Ok(()) | |
551 | } | |
552 | } | |
553 | ||
6a06907d XL |
554 | /// This carries the results of computing usefulness, as described at the top of the file. When |
555 | /// checking usefulness of a match branch, we use the `NoWitnesses` variant, which also keeps track | |
556 | /// of potential unreachable sub-patterns (in the presence of or-patterns). When checking | |
557 | /// exhaustiveness of a whole match, we use the `WithWitnesses` variant, which carries a list of | |
558 | /// witnesses of non-exhaustiveness when there are any. | |
c295e0f8 XL |
559 | /// Which variant to use is dictated by `ArmType`. |
560 | #[derive(Debug)] | |
6a06907d | 561 | enum Usefulness<'p, 'tcx> { |
c295e0f8 XL |
562 | /// If we don't care about witnesses, simply remember if the pattern was useful. |
563 | NoWitnesses { useful: bool }, | |
6a06907d XL |
564 | /// Carries a list of witnesses of non-exhaustiveness. If empty, indicates that the whole |
565 | /// pattern is unreachable. | |
c295e0f8 | 566 | WithWitnesses(Vec<Witness<'p, 'tcx>>), |
fc512014 XL |
567 | } |
568 | ||
6a06907d | 569 | impl<'p, 'tcx> Usefulness<'p, 'tcx> { |
c295e0f8 | 570 | fn new_useful(preference: ArmType) -> Self { |
fc512014 | 571 | match preference { |
c295e0f8 XL |
572 | // A single (empty) witness of reachability. |
573 | FakeExtraWildcard => WithWitnesses(vec![Witness(vec![])]), | |
574 | RealArm => NoWitnesses { useful: true }, | |
6a06907d XL |
575 | } |
576 | } | |
c295e0f8 XL |
577 | |
578 | fn new_not_useful(preference: ArmType) -> Self { | |
6a06907d | 579 | match preference { |
c295e0f8 XL |
580 | FakeExtraWildcard => WithWitnesses(vec![]), |
581 | RealArm => NoWitnesses { useful: false }, | |
582 | } | |
583 | } | |
584 | ||
585 | fn is_useful(&self) -> bool { | |
586 | match self { | |
587 | Usefulness::NoWitnesses { useful } => *useful, | |
588 | Usefulness::WithWitnesses(witnesses) => !witnesses.is_empty(), | |
6a06907d XL |
589 | } |
590 | } | |
591 | ||
592 | /// Combine usefulnesses from two branches. This is an associative operation. | |
593 | fn extend(&mut self, other: Self) { | |
594 | match (&mut *self, other) { | |
595 | (WithWitnesses(_), WithWitnesses(o)) if o.is_empty() => {} | |
596 | (WithWitnesses(s), WithWitnesses(o)) if s.is_empty() => *self = WithWitnesses(o), | |
597 | (WithWitnesses(s), WithWitnesses(o)) => s.extend(o), | |
c295e0f8 XL |
598 | (NoWitnesses { useful: s_useful }, NoWitnesses { useful: o_useful }) => { |
599 | *s_useful = *s_useful || o_useful | |
fc512014 | 600 | } |
c295e0f8 | 601 | _ => unreachable!(), |
fc512014 XL |
602 | } |
603 | } | |
604 | ||
c295e0f8 | 605 | /// After calculating usefulness after a specialization, call this to reconstruct a usefulness |
fc512014 XL |
606 | /// that makes sense for the matrix pre-specialization. This new usefulness can then be merged |
607 | /// with the results of specializing with the other constructors. | |
6a06907d | 608 | fn apply_constructor( |
fc512014 | 609 | self, |
064997fb | 610 | pcx: &PatCtxt<'_, 'p, 'tcx>, |
fc512014 XL |
611 | matrix: &Matrix<'p, 'tcx>, // used to compute missing ctors |
612 | ctor: &Constructor<'tcx>, | |
fc512014 XL |
613 | ) -> Self { |
614 | match self { | |
c295e0f8 XL |
615 | NoWitnesses { .. } => self, |
616 | WithWitnesses(ref witnesses) if witnesses.is_empty() => self, | |
6a06907d | 617 | WithWitnesses(witnesses) => { |
c295e0f8 XL |
618 | let new_witnesses = if let Constructor::Missing { .. } = ctor { |
619 | // We got the special `Missing` constructor, so each of the missing constructors | |
620 | // gives a new pattern that is not caught by the match. We list those patterns. | |
781aab86 FG |
621 | if pcx.is_non_exhaustive { |
622 | witnesses | |
623 | .into_iter() | |
624 | // Here we don't want the user to try to list all variants, we want them to add | |
625 | // a wildcard, so we only suggest that. | |
626 | .map(|witness| { | |
627 | witness.apply_constructor(pcx, &Constructor::NonExhaustive) | |
628 | }) | |
629 | .collect() | |
c295e0f8 XL |
630 | } else { |
631 | let mut split_wildcard = SplitWildcard::new(pcx); | |
632 | split_wildcard.split(pcx, matrix.heads().map(DeconstructedPat::ctor)); | |
633 | ||
634 | // This lets us know if we skipped any variants because they are marked | |
635 | // `doc(hidden)` or they are unstable feature gate (only stdlib types). | |
636 | let mut hide_variant_show_wild = false; | |
637 | // Construct for each missing constructor a "wild" version of this | |
638 | // constructor, that matches everything that can be built with | |
639 | // it. For example, if `ctor` is a `Constructor::Variant` for | |
640 | // `Option::Some`, we get the pattern `Some(_)`. | |
781aab86 | 641 | let mut new_patterns: Vec<DeconstructedPat<'_, '_>> = split_wildcard |
c295e0f8 XL |
642 | .iter_missing(pcx) |
643 | .filter_map(|missing_ctor| { | |
644 | // Check if this variant is marked `doc(hidden)` | |
645 | if missing_ctor.is_doc_hidden_variant(pcx) | |
646 | || missing_ctor.is_unstable_variant(pcx) | |
647 | { | |
648 | hide_variant_show_wild = true; | |
649 | return None; | |
650 | } | |
651 | Some(DeconstructedPat::wild_from_ctor(pcx, missing_ctor.clone())) | |
652 | }) | |
653 | .collect(); | |
654 | ||
655 | if hide_variant_show_wild { | |
781aab86 | 656 | new_patterns.push(DeconstructedPat::wildcard(pcx.ty, pcx.span)); |
c295e0f8 XL |
657 | } |
658 | ||
781aab86 FG |
659 | witnesses |
660 | .into_iter() | |
661 | .flat_map(|witness| { | |
662 | new_patterns.iter().map(move |pat| { | |
663 | Witness( | |
664 | witness | |
665 | .0 | |
666 | .iter() | |
667 | .chain(once(pat)) | |
668 | .map(DeconstructedPat::clone_and_forget_reachability) | |
669 | .collect(), | |
670 | ) | |
671 | }) | |
fc512014 | 672 | }) |
781aab86 FG |
673 | .collect() |
674 | } | |
fc512014 XL |
675 | } else { |
676 | witnesses | |
677 | .into_iter() | |
c295e0f8 | 678 | .map(|witness| witness.apply_constructor(pcx, &ctor)) |
fc512014 XL |
679 | .collect() |
680 | }; | |
6a06907d | 681 | WithWitnesses(new_witnesses) |
fc512014 | 682 | } |
fc512014 XL |
683 | } |
684 | } | |
685 | } | |
686 | ||
687 | #[derive(Copy, Clone, Debug)] | |
c295e0f8 XL |
688 | enum ArmType { |
689 | FakeExtraWildcard, | |
690 | RealArm, | |
fc512014 XL |
691 | } |
692 | ||
693 | /// A witness of non-exhaustiveness for error reporting, represented | |
694 | /// as a list of patterns (in reverse order of construction) with | |
695 | /// wildcards inside to represent elements that can take any inhabitant | |
696 | /// of the type as a value. | |
697 | /// | |
698 | /// A witness against a list of patterns should have the same types | |
699 | /// and length as the pattern matched against. Because Rust `match` | |
700 | /// is always against a single pattern, at the end the witness will | |
701 | /// have length 1, but in the middle of the algorithm, it can contain | |
702 | /// multiple patterns. | |
703 | /// | |
704 | /// For example, if we are constructing a witness for the match against | |
705 | /// | |
04454e1e | 706 | /// ```compile_fail,E0004 |
fc512014 | 707 | /// struct Pair(Option<(u32, u32)>, bool); |
04454e1e | 708 | /// # fn foo(p: Pair) { |
49aad941 | 709 | /// match p { |
fc512014 XL |
710 | /// Pair(None, _) => {} |
711 | /// Pair(_, false) => {} | |
712 | /// } | |
04454e1e | 713 | /// # } |
fc512014 XL |
714 | /// ``` |
715 | /// | |
716 | /// We'll perform the following steps: | |
717 | /// 1. Start with an empty witness | |
718 | /// `Witness(vec![])` | |
719 | /// 2. Push a witness `true` against the `false` | |
720 | /// `Witness(vec![true])` | |
721 | /// 3. Push a witness `Some(_)` against the `None` | |
722 | /// `Witness(vec![true, Some(_)])` | |
723 | /// 4. Apply the `Pair` constructor to the witnesses | |
724 | /// `Witness(vec![Pair(Some(_), true)])` | |
725 | /// | |
726 | /// The final `Pair(Some(_), true)` is then the resulting witness. | |
c295e0f8 | 727 | #[derive(Debug)] |
923072b8 | 728 | pub(crate) struct Witness<'p, 'tcx>(Vec<DeconstructedPat<'p, 'tcx>>); |
fc512014 | 729 | |
c295e0f8 | 730 | impl<'p, 'tcx> Witness<'p, 'tcx> { |
fc512014 | 731 | /// Asserts that the witness contains a single pattern, and returns it. |
c295e0f8 | 732 | fn single_pattern(self) -> DeconstructedPat<'p, 'tcx> { |
fc512014 XL |
733 | assert_eq!(self.0.len(), 1); |
734 | self.0.into_iter().next().unwrap() | |
735 | } | |
736 | ||
737 | /// Constructs a partial witness for a pattern given a list of | |
738 | /// patterns expanded by the specialization step. | |
739 | /// | |
740 | /// When a pattern P is discovered to be useful, this function is used bottom-up | |
741 | /// to reconstruct a complete witness, e.g., a pattern P' that covers a subset | |
742 | /// of values, V, where each value in that set is not covered by any previously | |
743 | /// used patterns and is covered by the pattern P'. Examples: | |
744 | /// | |
745 | /// left_ty: tuple of 3 elements | |
746 | /// pats: [10, 20, _] => (10, 20, _) | |
747 | /// | |
748 | /// left_ty: struct X { a: (bool, &'static str), b: usize} | |
749 | /// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 } | |
064997fb | 750 | fn apply_constructor(mut self, pcx: &PatCtxt<'_, 'p, 'tcx>, ctor: &Constructor<'tcx>) -> Self { |
fc512014 XL |
751 | let pat = { |
752 | let len = self.0.len(); | |
c295e0f8 | 753 | let arity = ctor.arity(pcx); |
fc512014 | 754 | let pats = self.0.drain((len - arity)..).rev(); |
c295e0f8 | 755 | let fields = Fields::from_iter(pcx.cx, pats); |
353b0b11 | 756 | DeconstructedPat::new(ctor.clone(), fields, pcx.ty, pcx.span) |
fc512014 XL |
757 | }; |
758 | ||
759 | self.0.push(pat); | |
760 | ||
761 | self | |
762 | } | |
763 | } | |
764 | ||
765 | /// Algorithm from <http://moscova.inria.fr/~maranget/papers/warn/index.html>. | |
766 | /// The algorithm from the paper has been modified to correctly handle empty | |
767 | /// types. The changes are: | |
768 | /// (0) We don't exit early if the pattern matrix has zero rows. We just | |
769 | /// continue to recurse over columns. | |
770 | /// (1) all_constructors will only return constructors that are statically | |
771 | /// possible. E.g., it will only return `Ok` for `Result<T, !>`. | |
772 | /// | |
773 | /// This finds whether a (row) vector `v` of patterns is 'useful' in relation | |
774 | /// to a set of such vectors `m` - this is defined as there being a set of | |
775 | /// inputs that will match `v` but not any of the sets in `m`. | |
776 | /// | |
777 | /// All the patterns at each column of the `matrix ++ v` matrix must have the same type. | |
778 | /// | |
779 | /// This is used both for reachability checking (if a pattern isn't useful in | |
780 | /// relation to preceding patterns, it is not reachable) and exhaustiveness | |
781 | /// checking (if a wildcard pattern is useful in relation to a matrix, the | |
782 | /// matrix isn't exhaustive). | |
783 | /// | |
784 | /// `is_under_guard` is used to inform if the pattern has a guard. If it | |
785 | /// has one it must not be inserted into the matrix. This shouldn't be | |
786 | /// relied on for soundness. | |
353b0b11 | 787 | #[instrument(level = "debug", skip(cx, matrix, lint_root), ret)] |
fc512014 XL |
788 | fn is_useful<'p, 'tcx>( |
789 | cx: &MatchCheckCtxt<'p, 'tcx>, | |
790 | matrix: &Matrix<'p, 'tcx>, | |
791 | v: &PatStack<'p, 'tcx>, | |
c295e0f8 | 792 | witness_preference: ArmType, |
353b0b11 | 793 | lint_root: HirId, |
fc512014 XL |
794 | is_under_guard: bool, |
795 | is_top_level: bool, | |
6a06907d | 796 | ) -> Usefulness<'p, 'tcx> { |
064997fb | 797 | debug!(?matrix, ?v); |
fc512014 | 798 | let Matrix { patterns: rows, .. } = matrix; |
fc512014 XL |
799 | |
800 | // The base case. We are pattern-matching on () and the return value is | |
801 | // based on whether our matrix has a row or not. | |
802 | // NOTE: This could potentially be optimized by checking rows.is_empty() | |
803 | // first and then, if v is non-empty, the return value is based on whether | |
804 | // the type of the tuple we're checking is inhabited or not. | |
805 | if v.is_empty() { | |
6a06907d | 806 | let ret = if rows.is_empty() { |
fc512014 XL |
807 | Usefulness::new_useful(witness_preference) |
808 | } else { | |
6a06907d | 809 | Usefulness::new_not_useful(witness_preference) |
fc512014 | 810 | }; |
6a06907d XL |
811 | debug!(?ret); |
812 | return ret; | |
813 | } | |
fc512014 | 814 | |
3c0e092e | 815 | debug_assert!(rows.iter().all(|r| r.len() == v.len())); |
fc512014 | 816 | |
fc512014 | 817 | // If the first pattern is an or-pattern, expand it. |
c295e0f8 XL |
818 | let mut ret = Usefulness::new_not_useful(witness_preference); |
819 | if v.head().is_or_pat() { | |
6a06907d | 820 | debug!("expanding or-pattern"); |
6a06907d | 821 | // We try each or-pattern branch in turn. |
fc512014 | 822 | let mut matrix = matrix.clone(); |
c295e0f8 | 823 | for v in v.expand_or_pat() { |
04454e1e | 824 | debug!(?v); |
3c0e092e | 825 | let usefulness = ensure_sufficient_stack(|| { |
353b0b11 | 826 | is_useful(cx, &matrix, &v, witness_preference, lint_root, is_under_guard, false) |
3c0e092e | 827 | }); |
04454e1e | 828 | debug!(?usefulness); |
c295e0f8 | 829 | ret.extend(usefulness); |
fc512014 XL |
830 | // If pattern has a guard don't add it to the matrix. |
831 | if !is_under_guard { | |
832 | // We push the already-seen patterns into the matrix in order to detect redundant | |
833 | // branches like `Some(_) | Some(0)`. | |
834 | matrix.push(v); | |
835 | } | |
c295e0f8 | 836 | } |
fc512014 | 837 | } else { |
2b03887a FG |
838 | let mut ty = v.head().ty(); |
839 | ||
840 | // Opaque types can't get destructured/split, but the patterns can | |
841 | // actually hint at hidden types, so we use the patterns' types instead. | |
9c376795 | 842 | if let ty::Alias(ty::Opaque, ..) = ty.kind() { |
2b03887a FG |
843 | if let Some(row) = rows.first() { |
844 | ty = row.head().ty(); | |
845 | } | |
846 | } | |
064997fb FG |
847 | let is_non_exhaustive = cx.is_foreign_non_exhaustive_enum(ty); |
848 | debug!("v.head: {:?}, v.span: {:?}", v.head(), v.head().span()); | |
849 | let pcx = &PatCtxt { cx, ty, span: v.head().span(), is_top_level, is_non_exhaustive }; | |
850 | ||
c295e0f8 | 851 | let v_ctor = v.head().ctor(); |
04454e1e | 852 | debug!(?v_ctor); |
fc512014 XL |
853 | if let Constructor::IntRange(ctor_range) = &v_ctor { |
854 | // Lint on likely incorrect range patterns (#63987) | |
855 | ctor_range.lint_overlapping_range_endpoints( | |
856 | pcx, | |
c295e0f8 | 857 | matrix.heads(), |
fc512014 | 858 | matrix.column_count().unwrap_or(0), |
353b0b11 | 859 | lint_root, |
fc512014 XL |
860 | ) |
861 | } | |
862 | // We split the head constructor of `v`. | |
c295e0f8 XL |
863 | let split_ctors = v_ctor.split(pcx, matrix.heads().map(DeconstructedPat::ctor)); |
864 | let is_non_exhaustive_and_wild = is_non_exhaustive && v_ctor.is_wildcard(); | |
fc512014 XL |
865 | // For each constructor, we compute whether there's a value that starts with it that would |
866 | // witness the usefulness of `v`. | |
867 | let start_matrix = &matrix; | |
c295e0f8 | 868 | for ctor in split_ctors { |
6a06907d | 869 | debug!("specialize({:?})", ctor); |
fc512014 | 870 | // We cache the result of `Fields::wildcards` because it is used a lot. |
c295e0f8 | 871 | let spec_matrix = start_matrix.specialize_constructor(pcx, &ctor); |
064997fb | 872 | let v = v.pop_head_constructor(pcx, &ctor); |
3c0e092e | 873 | let usefulness = ensure_sufficient_stack(|| { |
353b0b11 FG |
874 | is_useful( |
875 | cx, | |
876 | &spec_matrix, | |
877 | &v, | |
878 | witness_preference, | |
879 | lint_root, | |
880 | is_under_guard, | |
881 | false, | |
882 | ) | |
3c0e092e | 883 | }); |
c295e0f8 XL |
884 | let usefulness = usefulness.apply_constructor(pcx, start_matrix, &ctor); |
885 | ||
886 | // When all the conditions are met we have a match with a `non_exhaustive` enum | |
887 | // that has the potential to trigger the `non_exhaustive_omitted_patterns` lint. | |
888 | // To understand the workings checkout `Constructor::split` and `SplitWildcard::new/into_ctors` | |
889 | if is_non_exhaustive_and_wild | |
353b0b11 FG |
890 | // Only emit a lint on refutable patterns. |
891 | && cx.refutable | |
2b03887a | 892 | // We check that the match has a wildcard pattern and that wildcard is useful, |
c295e0f8 XL |
893 | // meaning there are variants that are covered by the wildcard. Without the check |
894 | // for `witness_preference` the lint would trigger on `if let NonExhaustiveEnum::A = foo {}` | |
895 | && usefulness.is_useful() && matches!(witness_preference, RealArm) | |
896 | && matches!( | |
897 | &ctor, | |
898 | Constructor::Missing { nonexhaustive_enum_missing_real_variants: true } | |
899 | ) | |
900 | { | |
901 | let patterns = { | |
902 | let mut split_wildcard = SplitWildcard::new(pcx); | |
903 | split_wildcard.split(pcx, matrix.heads().map(DeconstructedPat::ctor)); | |
904 | // Construct for each missing constructor a "wild" version of this | |
905 | // constructor, that matches everything that can be built with | |
906 | // it. For example, if `ctor` is a `Constructor::Variant` for | |
907 | // `Option::Some`, we get the pattern `Some(_)`. | |
908 | split_wildcard | |
909 | .iter_missing(pcx) | |
910 | // Filter out the `NonExhaustive` because we want to list only real | |
911 | // variants. Also remove any unstable feature gated variants. | |
912 | // Because of how we computed `nonexhaustive_enum_missing_real_variants`, | |
913 | // this will not return an empty `Vec`. | |
914 | .filter(|c| !(c.is_non_exhaustive() || c.is_unstable_variant(pcx))) | |
915 | .cloned() | |
916 | .map(|missing_ctor| DeconstructedPat::wild_from_ctor(pcx, missing_ctor)) | |
917 | .collect::<Vec<_>>() | |
918 | }; | |
919 | ||
9c376795 FG |
920 | // Report that a match of a `non_exhaustive` enum marked with `non_exhaustive_omitted_patterns` |
921 | // is not exhaustive enough. | |
922 | // | |
923 | // NB: The partner lint for structs lives in `compiler/rustc_hir_analysis/src/check/pat.rs`. | |
924 | cx.tcx.emit_spanned_lint( | |
925 | NON_EXHAUSTIVE_OMITTED_PATTERNS, | |
353b0b11 | 926 | lint_root, |
9c376795 FG |
927 | pcx.span, |
928 | NonExhaustiveOmittedPattern { | |
929 | scrut_ty: pcx.ty, | |
930 | uncovered: Uncovered::new(pcx.span, pcx.cx, patterns), | |
931 | }, | |
932 | ); | |
c295e0f8 XL |
933 | } |
934 | ||
935 | ret.extend(usefulness); | |
936 | } | |
937 | } | |
938 | ||
939 | if ret.is_useful() { | |
940 | v.head().set_reachable(); | |
941 | } | |
942 | ||
fc512014 XL |
943 | ret |
944 | } | |
945 | ||
946 | /// The arm of a match expression. | |
04454e1e | 947 | #[derive(Clone, Copy, Debug)] |
923072b8 | 948 | pub(crate) struct MatchArm<'p, 'tcx> { |
fc512014 | 949 | /// The pattern must have been lowered through `check_match::MatchVisitor::lower_pattern`. |
923072b8 FG |
950 | pub(crate) pat: &'p DeconstructedPat<'p, 'tcx>, |
951 | pub(crate) hir_id: HirId, | |
952 | pub(crate) has_guard: bool, | |
fc512014 XL |
953 | } |
954 | ||
6a06907d XL |
955 | /// Indicates whether or not a given arm is reachable. |
956 | #[derive(Clone, Debug)] | |
923072b8 | 957 | pub(crate) enum Reachability { |
6a06907d XL |
958 | /// The arm is reachable. This additionally carries a set of or-pattern branches that have been |
959 | /// found to be unreachable despite the overall arm being reachable. Used only in the presence | |
960 | /// of or-patterns, otherwise it stays empty. | |
961 | Reachable(Vec<Span>), | |
962 | /// The arm is unreachable. | |
963 | Unreachable, | |
964 | } | |
965 | ||
fc512014 | 966 | /// The output of checking a match for exhaustiveness and arm reachability. |
923072b8 | 967 | pub(crate) struct UsefulnessReport<'p, 'tcx> { |
fc512014 | 968 | /// For each arm of the input, whether that arm is reachable after the arms above it. |
923072b8 | 969 | pub(crate) arm_usefulness: Vec<(MatchArm<'p, 'tcx>, Reachability)>, |
fc512014 XL |
970 | /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of |
971 | /// exhaustiveness. | |
923072b8 | 972 | pub(crate) non_exhaustiveness_witnesses: Vec<DeconstructedPat<'p, 'tcx>>, |
fc512014 XL |
973 | } |
974 | ||
975 | /// The entrypoint for the usefulness algorithm. Computes whether a match is exhaustive and which | |
976 | /// of its arms are reachable. | |
977 | /// | |
978 | /// Note: the input patterns must have been lowered through | |
979 | /// `check_match::MatchVisitor::lower_pattern`. | |
04454e1e | 980 | #[instrument(skip(cx, arms), level = "debug")] |
923072b8 | 981 | pub(crate) fn compute_match_usefulness<'p, 'tcx>( |
fc512014 XL |
982 | cx: &MatchCheckCtxt<'p, 'tcx>, |
983 | arms: &[MatchArm<'p, 'tcx>], | |
353b0b11 | 984 | lint_root: HirId, |
fc512014 XL |
985 | scrut_ty: Ty<'tcx>, |
986 | ) -> UsefulnessReport<'p, 'tcx> { | |
987 | let mut matrix = Matrix::empty(); | |
988 | let arm_usefulness: Vec<_> = arms | |
989 | .iter() | |
990 | .copied() | |
991 | .map(|arm| { | |
04454e1e | 992 | debug!(?arm); |
fc512014 | 993 | let v = PatStack::from_pattern(arm.pat); |
c295e0f8 | 994 | is_useful(cx, &matrix, &v, RealArm, arm.hir_id, arm.has_guard, true); |
fc512014 XL |
995 | if !arm.has_guard { |
996 | matrix.push(v); | |
997 | } | |
c295e0f8 XL |
998 | let reachability = if arm.pat.is_reachable() { |
999 | Reachability::Reachable(arm.pat.unreachable_spans()) | |
1000 | } else { | |
1001 | Reachability::Unreachable | |
6a06907d XL |
1002 | }; |
1003 | (arm, reachability) | |
fc512014 XL |
1004 | }) |
1005 | .collect(); | |
1006 | ||
353b0b11 | 1007 | let wild_pattern = cx.pattern_arena.alloc(DeconstructedPat::wildcard(scrut_ty, DUMMY_SP)); |
fc512014 | 1008 | let v = PatStack::from_pattern(wild_pattern); |
353b0b11 | 1009 | let usefulness = is_useful(cx, &matrix, &v, FakeExtraWildcard, lint_root, false, true); |
fc512014 | 1010 | let non_exhaustiveness_witnesses = match usefulness { |
6a06907d | 1011 | WithWitnesses(pats) => pats.into_iter().map(|w| w.single_pattern()).collect(), |
c295e0f8 | 1012 | NoWitnesses { .. } => bug!(), |
fc512014 XL |
1013 | }; |
1014 | UsefulnessReport { arm_usefulness, non_exhaustiveness_witnesses } | |
1015 | } |