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Diffstat (limited to 'compiler/rustc_pattern_analysis/src/usefulness.rs')
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diff --git a/compiler/rustc_pattern_analysis/src/usefulness.rs b/compiler/rustc_pattern_analysis/src/usefulness.rs new file mode 100644 index 00000000000..9710c9e1303 --- /dev/null +++ b/compiler/rustc_pattern_analysis/src/usefulness.rs @@ -0,0 +1,1883 @@ +//! # Match exhaustiveness and redundancy algorithm +//! +//! This file contains the logic for exhaustiveness and usefulness checking for pattern-matching. +//! Specifically, given a list of patterns in a match, we can tell whether: +//! (a) a given pattern is redundant +//! (b) the patterns cover every possible value for the type (exhaustiveness) +//! +//! The algorithm implemented here is inspired from the one described in [this +//! paper](http://moscova.inria.fr/~maranget/papers/warn/index.html). We have however changed it in +//! various ways to accommodate the variety of patterns that Rust supports. We thus explain our +//! version here, without being as precise. +//! +//! Fun fact: computing exhaustiveness is NP-complete, because we can encode a SAT problem as an +//! exhaustiveness problem. See [here](https://niedzejkob.p4.team/rust-np) for the fun details. +//! +//! +//! # Summary +//! +//! The algorithm is given as input a list of patterns, one for each arm of a match, and computes +//! the following: +//! - a set of values that match none of the patterns (if any), +//! - for each subpattern (taking into account or-patterns), whether removing it would change +//! anything about how the match executes, i.e. whether it is useful/not redundant. +//! +//! To a first approximation, the algorithm works by exploring all possible values for the type +//! being matched on, and determining which arm(s) catch which value. To make this tractable we +//! cleverly group together values, as we'll see below. +//! +//! The entrypoint of this file is the [`compute_match_usefulness`] function, which computes +//! usefulness for each subpattern and exhaustiveness for the whole match. +//! +//! In this page we explain the necessary concepts to understand how the algorithm works. +//! +//! +//! # Usefulness +//! +//! The central concept of this file is the notion of "usefulness". Given some patterns `p_1 .. +//! p_n`, a pattern `q` is said to be *useful* if there is a value that is matched by `q` and by +//! none of the `p_i`. We write `usefulness(p_1 .. p_n, q)` for a function that returns a list of +//! such values. The aim of this file is to compute it efficiently. +//! +//! This is enough to compute usefulness: a pattern in a `match` expression is redundant iff it is +//! not useful w.r.t. the patterns above it: +//! ```compile_fail,E0004 +//! # fn foo() { +//! match Some(0u32) { +//! Some(0..100) => {}, +//! Some(90..190) => {}, // useful: `Some(150)` is matched by this but not the branch above +//! Some(50..150) => {}, // redundant: all the values this matches are already matched by +//! // the branches above +//! None => {}, // useful: `None` is matched by this but not the branches above +//! } +//! # } +//! ``` +//! +//! This is also enough to compute exhaustiveness: a match is exhaustive iff the wildcard `_` +//! pattern is _not_ useful w.r.t. the patterns in the match. The values returned by `usefulness` +//! are used to tell the user which values are missing. +//! ```compile_fail,E0004 +//! # fn foo(x: Option<u32>) { +//! match x { +//! None => {}, +//! Some(0) => {}, +//! // not exhaustive: `_` is useful because it matches `Some(1)` +//! } +//! # } +//! ``` +//! +//! +//! # Constructors and fields +//! +//! In the value `Pair(Some(0), true)`, `Pair` is called the constructor of the value, and `Some(0)` +//! and `true` are its fields. Every matcheable value can be decomposed in this way. Examples of +//! constructors are: `Some`, `None`, `(,)` (the 2-tuple constructor), `Foo {..}` (the constructor +//! for a struct `Foo`), and `2` (the constructor for the number `2`). +//! +//! Each constructor takes a fixed number of fields; this is called its arity. `Pair` and `(,)` have +//! arity 2, `Some` has arity 1, `None` and `42` have arity 0. Each type has a known set of +//! constructors. Some types have many constructors (like `u64`) or even an infinitely many (like +//! `&str` and `&[T]`). +//! +//! Patterns are similar: `Pair(Some(_), _)` has constructor `Pair` and two fields. The difference +//! is that we get some extra pattern-only constructors, namely: the wildcard `_`, variable +//! bindings, integer ranges like `0..=10`, and variable-length slices like `[_, .., _]`. We treat +//! or-patterns separately, see the dedicated section below. +//! +//! Now to check if a value `v` matches a pattern `p`, we check if `v`'s constructor matches `p`'s +//! constructor, then recursively compare their fields if necessary. A few representative examples: +//! +//! - `matches!(v, _) := true` +//! - `matches!((v0, v1), (p0, p1)) := matches!(v0, p0) && matches!(v1, p1)` +//! - `matches!(Foo { bar: v0, baz: v1 }, Foo { bar: p0, baz: p1 }) := matches!(v0, p0) && matches!(v1, p1)` +//! - `matches!(Ok(v0), Ok(p0)) := matches!(v0, p0)` +//! - `matches!(Ok(v0), Err(p0)) := false` (incompatible variants) +//! - `matches!(v, 1..=100) := matches!(v, 1) || ... || matches!(v, 100)` +//! - `matches!([v0], [p0, .., p1]) := false` (incompatible lengths) +//! - `matches!([v0, v1, v2], [p0, .., p1]) := matches!(v0, p0) && matches!(v2, p1)` +//! +//! Constructors and relevant operations are defined in the [`crate::constructor`] module. A +//! representation of patterns that uses constructors is available in [`crate::pat`]. The question +//! of whether a constructor is matched by another one is answered by +//! [`Constructor::is_covered_by`]. +//! +//! Note 1: variable bindings (like the `x` in `Some(x)`) match anything, so we treat them as wildcards. +//! Note 2: this only applies to matcheable values. For example a value of type `Rc<u64>` can't be +//! deconstructed that way. +//! +//! +//! +//! # Specialization +//! +//! The examples in the previous section motivate the operation at the heart of the algorithm: +//! "specialization". It captures this idea of "removing one layer of constructor". +//! +//! `specialize(c, p)` takes a value-only constructor `c` and a pattern `p`, and returns a +//! pattern-tuple or nothing. It works as follows: +//! +//! - Specializing for the wrong constructor returns nothing +//! +//! - `specialize(None, Some(p0)) := <nothing>` +//! - `specialize([,,,], [p0]) := <nothing>` +//! +//! - Specializing for the correct constructor returns a tuple of the fields +//! +//! - `specialize(Variant1, Variant1(p0, p1, p2)) := (p0, p1, p2)` +//! - `specialize(Foo{ bar, baz, quz }, Foo { bar: p0, baz: p1, .. }) := (p0, p1, _)` +//! - `specialize([,,,], [p0, .., p1]) := (p0, _, _, p1)` +//! +//! We get the following property: for any values `v_1, .., v_n` of appropriate types, we have: +//! ```text +//! matches!(c(v_1, .., v_n), p) +//! <=> specialize(c, p) returns something +//! && matches!((v_1, .., v_n), specialize(c, p)) +//! ``` +//! +//! We also extend specialization to pattern-tuples by applying it to the first pattern: +//! `specialize(c, (p_0, .., p_n)) := specialize(c, p_0) ++ (p_1, .., p_m)` +//! where `++` is concatenation of tuples. +//! +//! +//! The previous property extends to pattern-tuples: +//! ```text +//! matches!((c(v_1, .., v_n), w_1, .., w_m), (p_0, p_1, .., p_m)) +//! <=> specialize(c, p_0) does not error +//! && matches!((v_1, .., v_n, w_1, .., w_m), specialize(c, (p_0, p_1, .., p_m))) +//! ``` +//! +//! Whether specialization returns something or not is given by [`Constructor::is_covered_by`]. +//! Specialization of a pattern is computed in [`DeconstructedPat::specialize`]. Specialization for +//! a pattern-tuple is computed in [`PatStack::pop_head_constructor`]. Finally, specialization for a +//! set of pattern-tuples is computed in [`Matrix::specialize_constructor`]. +//! +//! +//! +//! # Undoing specialization +//! +//! To construct witnesses we will need an inverse of specialization. If `c` is a constructor of +//! arity `n`, we define `unspecialize` as: +//! `unspecialize(c, (p_1, .., p_n, q_1, .., q_m)) := (c(p_1, .., p_n), q_1, .., q_m)`. +//! +//! This is done for a single witness-tuple in [`WitnessStack::apply_constructor`], and for a set of +//! witness-tuples in [`WitnessMatrix::apply_constructor`]. +//! +//! +//! +//! # Computing usefulness +//! +//! We now present a naive version of the algorithm for computing usefulness. From now on we operate +//! on pattern-tuples. +//! +//! Let `pt_1, .., pt_n` and `qt` be length-m tuples of patterns for the same type `(T_1, .., T_m)`. +//! We compute `usefulness(tp_1, .., tp_n, tq)` as follows: +//! +//! - Base case: `m == 0`. +//! The pattern-tuples are all empty, i.e. they're all `()`. Thus `tq` is useful iff there are +//! no rows above it, i.e. if `n == 0`. In that case we return `()` as a witness-tuple of +//! usefulness of `tq`. +//! +//! - Inductive case: `m > 0`. +//! In this naive version, we list all the possible constructors for values of type `T1` (we +//! will be more clever in the next section). +//! +//! - For each such constructor `c` for which `specialize(c, tq)` is not nothing: +//! - We recursively compute `usefulness(specialize(c, tp_1) ... specialize(c, tp_n), specialize(c, tq))`, +//! where we discard any `specialize(c, p_i)` that returns nothing. +//! - For each witness-tuple `w` found, we apply `unspecialize(c, w)` to it. +//! +//! - We return the all the witnesses found, if any. +//! +//! +//! Let's take the following example: +//! ```compile_fail,E0004 +//! # enum Enum { Variant1(()), Variant2(Option<bool>, u32)} +//! # use Enum::*; +//! # fn foo(x: Enum) { +//! match x { +//! Variant1(_) => {} // `p1` +//! Variant2(None, 0) => {} // `p2` +//! Variant2(Some(_), 0) => {} // `q` +//! } +//! # } +//! ``` +//! +//! To compute the usefulness of `q`, we would proceed as follows: +//! ```text +//! Start: +//! `tp1 = [Variant1(_)]` +//! `tp2 = [Variant2(None, 0)]` +//! `tq = [Variant2(Some(true), 0)]` +//! +//! Constructors are `Variant1` and `Variant2`. Only `Variant2` can specialize `tq`. +//! Specialize with `Variant2`: +//! `tp2 = [None, 0]` +//! `tq = [Some(true), 0]` +//! +//! Constructors are `None` and `Some`. Only `Some` can specialize `tq`. +//! Specialize with `Some`: +//! `tq = [true, 0]` +//! +//! Constructors are `false` and `true`. Only `true` can specialize `tq`. +//! Specialize with `true`: +//! `tq = [0]` +//! +//! Constructors are `0`, `1`, .. up to infinity. Only `0` can specialize `tq`. +//! Specialize with `0`: +//! `tq = []` +//! +//! m == 0 and n == 0, so `tq` is useful with witness `[]`. +//! `witness = []` +//! +//! Unspecialize with `0`: +//! `witness = [0]` +//! Unspecialize with `true`: +//! `witness = [true, 0]` +//! Unspecialize with `Some`: +//! `witness = [Some(true), 0]` +//! Unspecialize with `Variant2`: +//! `witness = [Variant2(Some(true), 0)]` +//! ``` +//! +//! Therefore `usefulness(tp_1, tp_2, tq)` returns the single witness-tuple `[Variant2(Some(true), 0)]`. +//! +//! +//! Computing the set of constructors for a type is done in [`PatCx::ctors_for_ty`]. See +//! the following sections for more accurate versions of the algorithm and corresponding links. +//! +//! +//! +//! # Computing usefulness and exhaustiveness in one go +//! +//! The algorithm we have described so far computes usefulness of each pattern in turn, and ends by +//! checking if `_` is useful to determine exhaustiveness of the whole match. In practice, instead +//! of doing "for each pattern { for each constructor { ... } }", we do "for each constructor { for +//! each pattern { ... } }". This allows us to compute everything in one go. +//! +//! [`Matrix`] stores the set of pattern-tuples under consideration. We track usefulness of each +//! row mutably in the matrix as we go along. We ignore witnesses of usefulness of the match rows. +//! We gather witnesses of the usefulness of `_` in [`WitnessMatrix`]. The algorithm that computes +//! all this is in [`compute_exhaustiveness_and_usefulness`]. +//! +//! See the full example at the bottom of this documentation. +//! +//! +//! +//! # Making usefulness tractable: constructor splitting +//! +//! We're missing one last detail: which constructors do we list? Naively listing all value +//! constructors cannot work for types like `u64` or `&str`, so we need to be more clever. The final +//! clever idea for this algorithm is that we can group together constructors that behave the same. +//! +//! Examples: +//! ```compile_fail,E0004 +//! match (0, false) { +//! (0 ..=100, true) => {} +//! (50..=150, false) => {} +//! (0 ..=200, _) => {} +//! } +//! ``` +//! +//! In this example, trying any of `0`, `1`, .., `49` will give the same specialized matrix, and +//! thus the same usefulness/exhaustiveness results. We can thus accelerate the algorithm by +//! trying them all at once. Here in fact, the only cases we need to consider are: `0..50`, +//! `50..=100`, `101..=150`,`151..=200` and `201..`. +//! +//! ``` +//! enum Direction { North, South, East, West } +//! # let wind = (Direction::North, 0u8); +//! match wind { +//! (Direction::North, 50..) => {} +//! (_, _) => {} +//! } +//! ``` +//! +//! In this example, trying any of `South`, `East`, `West` will give the same specialized matrix. By +//! the same reasoning, we only need to try two cases: `North`, and "everything else". +//! +//! We call _constructor splitting_ the operation that computes such a minimal set of cases to try. +//! This is done in [`ConstructorSet::split`] and explained in [`crate::constructor`]. +//! +//! +//! +//! # `Missing` and relevancy +//! +//! ## Relevant values +//! +//! Take the following example: +//! +//! ```compile_fail,E0004 +//! # let foo = (true, true); +//! match foo { +//! (true, _) => 1, +//! (_, true) => 2, +//! }; +//! ``` +//! +//! Consider the value `(true, true)`: +//! - Row 2 does not distinguish `(true, true)` and `(false, true)`; +//! - `false` does not show up in the first column of the match, so without knowing anything else we +//! can deduce that `(false, true)` matches the same or fewer rows than `(true, true)`. +//! +//! Using those two facts together, we deduce that `(true, true)` will not give us more usefulness +//! information about row 2 than `(false, true)` would. We say that "`(true, true)` is made +//! irrelevant for row 2 by `(false, true)`". We will use this idea to prune the search tree. +//! +//! +//! ## Computing relevancy +//! +//! We now generalize from the above example to approximate relevancy in a simple way. Note that we +//! will only compute an approximation: we can sometimes determine when a case is irrelevant, but +//! computing this precisely is at least as hard as computing usefulness. +//! +//! Our computation of relevancy relies on the `Missing` constructor. As explained in +//! [`crate::constructor`], `Missing` represents the constructors not present in a given column. For +//! example in the following: +//! +//! ```compile_fail,E0004 +//! enum Direction { North, South, East, West } +//! # let wind = (Direction::North, 0u8); +//! match wind { +//! (Direction::North, _) => 1, +//! (_, 50..) => 2, +//! }; +//! ``` +//! +//! Here `South`, `East` and `West` are missing in the first column, and `0..50` is missing in the +//! second. Both of these sets are represented by `Constructor::Missing` in their corresponding +//! column. +//! +//! We then compute relevancy as follows: during the course of the algorithm, for a row `r`: +//! - if `r` has a wildcard in the first column; +//! - and some constructors are missing in that column; +//! - then any `c != Missing` is considered irrelevant for row `r`. +//! +//! By this we mean that continuing the algorithm by specializing with `c` is guaranteed not to +//! contribute more information about the usefulness of row `r` than what we would get by +//! specializing with `Missing`. The argument is the same as in the previous subsection. +//! +//! Once we've specialized by a constructor `c` that is irrelevant for row `r`, we're guaranteed to +//! only explore values irrelevant for `r`. If we then ever reach a point where we're only exploring +//! values that are irrelevant to all of the rows (including the virtual wildcard row used for +//! exhaustiveness), we skip that case entirely. +//! +//! +//! ## Example +//! +//! Let's go through a variation on the first example: +//! +//! ```compile_fail,E0004 +//! # let foo = (true, true, true); +//! match foo { +//! (true, _, true) => 1, +//! (_, true, _) => 2, +//! }; +//! ``` +//! +//! ```text +//! ┐ Patterns: +//! │ 1. `[(true, _, true)]` +//! │ 2. `[(_, true, _)]` +//! │ 3. `[_]` // virtual extra wildcard row +//! │ +//! │ Specialize with `(,,)`: +//! ├─┐ Patterns: +//! │ │ 1. `[true, _, true]` +//! │ │ 2. `[_, true, _]` +//! │ │ 3. `[_, _, _]` +//! │ │ +//! │ │ There are missing constructors in the first column (namely `false`), hence +//! │ │ `true` is irrelevant for rows 2 and 3. +//! │ │ +//! │ │ Specialize with `true`: +//! │ ├─┐ Patterns: +//! │ │ │ 1. `[_, true]` +//! │ │ │ 2. `[true, _]` // now exploring irrelevant cases +//! │ │ │ 3. `[_, _]` // now exploring irrelevant cases +//! │ │ │ +//! │ │ │ There are missing constructors in the first column (namely `false`), hence +//! │ │ │ `true` is irrelevant for rows 1 and 3. +//! │ │ │ +//! │ │ │ Specialize with `true`: +//! │ │ ├─┐ Patterns: +//! │ │ │ │ 1. `[true]` // now exploring irrelevant cases +//! │ │ │ │ 2. `[_]` // now exploring irrelevant cases +//! │ │ │ │ 3. `[_]` // now exploring irrelevant cases +//! │ │ │ │ +//! │ │ │ │ The current case is irrelevant for all rows: we backtrack immediately. +//! │ │ ├─┘ +//! │ │ │ +//! │ │ │ Specialize with `false`: +//! │ │ ├─┐ Patterns: +//! │ │ │ │ 1. `[true]` +//! │ │ │ │ 3. `[_]` // now exploring irrelevant cases +//! │ │ │ │ +//! │ │ │ │ Specialize with `true`: +//! │ │ │ ├─┐ Patterns: +//! │ │ │ │ │ 1. `[]` +//! │ │ │ │ │ 3. `[]` // now exploring irrelevant cases +//! │ │ │ │ │ +//! │ │ │ │ │ Row 1 is therefore useful. +//! │ │ │ ├─┘ +//! <etc...> +//! ``` +//! +//! Relevancy allowed us to skip the case `(true, true, _)` entirely. In some cases this pruning can +//! give drastic speedups. The case this was built for is the following (#118437): +//! +//! ```ignore(illustrative) +//! match foo { +//! (true, _, _, _, ..) => 1, +//! (_, true, _, _, ..) => 2, +//! (_, _, true, _, ..) => 3, +//! (_, _, _, true, ..) => 4, +//! ... +//! } +//! ``` +//! +//! Without considering relevancy, we would explore all 2^n combinations of the `true` and `Missing` +//! constructors. Relevancy tells us that e.g. `(true, true, false, false, false, ...)` is +//! irrelevant for all the rows. This allows us to skip all cases with more than one `true` +//! constructor, changing the runtime from exponential to linear. +//! +//! +//! ## Relevancy and exhaustiveness +//! +//! For exhaustiveness, we do something slightly different w.r.t relevancy: we do not report +//! witnesses of non-exhaustiveness that are irrelevant for the virtual wildcard row. For example, +//! in: +//! +//! ```ignore(illustrative) +//! match foo { +//! (true, true) => {} +//! } +//! ``` +//! +//! we only report `(false, _)` as missing. This was a deliberate choice made early in the +//! development of rust, for diagnostic and performance purposes. As showed in the previous section, +//! ignoring irrelevant cases preserves usefulness, so this choice still correctly computes whether +//! a match is exhaustive. +//! +//! +//! +//! # Or-patterns +//! +//! What we have described so far works well if there are no or-patterns. To handle them, if the +//! first pattern of any row in the matrix is an or-pattern, we expand it by duplicating the rest of +//! the row as necessary. For code reuse, this is implemented as "specializing with the `Or` +//! constructor". +//! +//! This makes usefulness tracking subtle, because we also want to compute whether an alternative of +//! an or-pattern is redundant, e.g. in `Some(_) | Some(0)`. We therefore track usefulness of each +//! subpattern of the match. +//! +//! +//! +//! # Constants and opaques +//! +//! There are two kinds of constants in patterns: +//! +//! * literals (`1`, `true`, `"foo"`) +//! * named or inline consts (`FOO`, `const { 5 + 6 }`) +//! +//! The latter are converted into the corresponding patterns by a previous phase. For example +//! `const_to_pat(const { [1, 2, 3] })` becomes an `Array(vec![Const(1), Const(2), Const(3)])` +//! pattern. This gets problematic when comparing the constant via `==` would behave differently +//! from matching on the constant converted to a pattern. The situation around this is currently +//! unclear and the lang team is working on clarifying what we want to do there. In any case, there +//! are constants we will not turn into patterns. We capture these with `Constructor::Opaque`. These +//! `Opaque` patterns do not participate in exhaustiveness, specialization or overlap checking. +//! +//! +//! +//! # Usefulness vs reachability, validity, and empty patterns +//! +//! This is likely the subtlest aspect of the algorithm. To be fully precise, a match doesn't +//! operate on a value, it operates on a place. In certain unsafe circumstances, it is possible for +//! a place to not contain valid data for its type. This has subtle consequences for empty types. +//! Take the following: +//! +//! ```rust +//! enum Void {} +//! let x: u8 = 0; +//! let ptr: *const Void = &x as *const u8 as *const Void; +//! unsafe { +//! match *ptr { +//! _ => println!("Reachable!"), +//! } +//! } +//! ``` +//! +//! In this example, `ptr` is a valid pointer pointing to a place with invalid data. The `_` pattern +//! does not look at the contents of `*ptr`, so this is ok and the arm is taken. In other words, +//! despite the place we are inspecting being of type `Void`, there is a reachable arm. If the +//! arm had a binding however: +//! +//! ```rust +//! # #[derive(Copy, Clone)] +//! # enum Void {} +//! # let x: u8 = 0; +//! # let ptr: *const Void = &x as *const u8 as *const Void; +//! # unsafe { +//! match *ptr { +//! _a => println!("Unreachable!"), +//! } +//! # } +//! ``` +//! +//! Here the binding loads the value of type `Void` from the `*ptr` place. In this example, this +//! causes UB since the data is not valid. In the general case, this asserts validity of the data at +//! `*ptr`. Either way, this arm will never be taken. +//! +//! Finally, let's consider the empty match `match *ptr {}`. If we consider this exhaustive, then +//! having invalid data at `*ptr` is invalid. In other words, the empty match is semantically +//! equivalent to the `_a => ...` match. In the interest of explicitness, we prefer the case with an +//! arm, hence we won't tell the user to remove the `_a` arm. In other words, the `_a` arm is +//! unreachable yet not redundant. This is why we lint on redundant arms rather than unreachable +//! arms, despite the fact that the lint says "unreachable". +//! +//! These considerations only affects certain places, namely those that can contain non-valid data +//! without UB. These are: pointer dereferences, reference dereferences, and union field accesses. +//! We track in the algorithm whether a given place is known to contain valid data. This is done +//! first by inspecting the scrutinee syntactically (which gives us `cx.known_valid_scrutinee`), and +//! then by tracking validity of each column of the matrix (which correspond to places) as we +//! recurse into subpatterns. That second part is done through [`PlaceValidity`], most notably +//! [`PlaceValidity::specialize`]. +//! +//! Having said all that, in practice we don't fully follow what's been presented in this section. +//! Let's call "toplevel exception" the case where the match scrutinee itself has type `!` or +//! `EmptyEnum`. First, on stable rust, we require `_` patterns for empty types in all cases apart +//! from the toplevel exception. The `exhaustive_patterns` and `min_exaustive_patterns` allow +//! omitting patterns in the cases described above. There's a final detail: in the toplevel +//! exception or with the `exhaustive_patterns` feature, we ignore place validity when checking +//! whether a pattern is required for exhaustiveness. I (Nadrieril) hope to deprecate this behavior. +//! +//! +//! +//! # Full example +//! +//! We illustrate a full run of the algorithm on the following match. +//! +//! ```compile_fail,E0004 +//! # struct Pair(Option<u32>, bool); +//! # fn foo(x: Pair) -> u32 { +//! match x { +//! Pair(Some(0), _) => 1, +//! Pair(_, false) => 2, +//! Pair(Some(0), false) => 3, +//! } +//! # } +//! ``` +//! +//! We keep track of the original row for illustration purposes, this is not what the algorithm +//! actually does (it tracks usefulness as a boolean on each row). +//! +//! ```text +//! ┐ Patterns: +//! │ 1. `[Pair(Some(0), _)]` +//! │ 2. `[Pair(_, false)]` +//! │ 3. `[Pair(Some(0), false)]` +//! │ +//! │ Specialize with `Pair`: +//! ├─┐ Patterns: +//! │ │ 1. `[Some(0), _]` +//! │ │ 2. `[_, false]` +//! │ │ 3. `[Some(0), false]` +//! │ │ +//! │ │ Specialize with `Some`: +//! │ ├─┐ Patterns: +//! │ │ │ 1. `[0, _]` +//! │ │ │ 2. `[_, false]` +//! │ │ │ 3. `[0, false]` +//! │ │ │ +//! │ │ │ Specialize with `0`: +//! │ │ ├─┐ Patterns: +//! │ │ │ │ 1. `[_]` +//! │ │ │ │ 3. `[false]` +//! │ │ │ │ +//! │ │ │ │ Specialize with `true`: +//! │ │ │ ├─┐ Patterns: +//! │ │ │ │ │ 1. `[]` +//! │ │ │ │ │ +//! │ │ │ │ │ We note arm 1 is useful (by `Pair(Some(0), true)`). +//! │ │ │ ├─┘ +//! │ │ │ │ +//! │ │ │ │ Specialize with `false`: +//! │ │ │ ├─┐ Patterns: +//! │ │ │ │ │ 1. `[]` +//! │ │ │ │ │ 3. `[]` +//! │ │ │ │ │ +//! │ │ │ │ │ We note arm 1 is useful (by `Pair(Some(0), false)`). +//! │ │ │ ├─┘ +//! │ │ ├─┘ +//! │ │ │ +//! │ │ │ Specialize with `1..`: +//! │ │ ├─┐ Patterns: +//! │ │ │ │ 2. `[false]` +//! │ │ │ │ +//! │ │ │ │ Specialize with `true`: +//! │ │ │ ├─┐ Patterns: +//! │ │ │ │ │ // no rows left +//! │ │ │ │ │ +//! │ │ │ │ │ We have found an unmatched value (`Pair(Some(1..), true)`)! This gives us a witness. +//! │ │ │ │ │ New witnesses: +//! │ │ │ │ │ `[]` +//! │ │ │ ├─┘ +//! │ │ │ │ Unspecialize new witnesses with `true`: +//! │ │ │ │ `[true]` +//! │ │ │ │ +//! │ │ │ │ Specialize with `false`: +//! │ │ │ ├─┐ Patterns: +//! │ │ │ │ │ 2. `[]` +//! │ │ │ │ │ +//! │ │ │ │ │ We note arm 2 is useful (by `Pair(Some(1..), false)`). +//! │ │ │ ├─┘ +//! │ │ │ │ +//! │ │ │ │ Total witnesses for `1..`: +//! │ │ │ │ `[true]` +//! │ │ ├─┘ +//! │ │ │ Unspecialize new witnesses with `1..`: +//! │ │ │ `[1.., true]` +//! │ │ │ +//! │ │ │ Total witnesses for `Some`: +//! │ │ │ `[1.., true]` +//! │ ├─┘ +//! │ │ Unspecialize new witnesses with `Some`: +//! │ │ `[Some(1..), true]` +//! │ │ +//! │ │ Specialize with `None`: +//! │ ├─┐ Patterns: +//! │ │ │ 2. `[false]` +//! │ │ │ +//! │ │ │ Specialize with `true`: +//! │ │ ├─┐ Patterns: +//! │ │ │ │ // no rows left +//! │ │ │ │ +//! │ │ │ │ We have found an unmatched value (`Pair(None, true)`)! This gives us a witness. +//! │ │ │ │ New witnesses: +//! │ │ │ │ `[]` +//! │ │ ├─┘ +//! │ │ │ Unspecialize new witnesses with `true`: +//! │ │ │ `[true]` +//! │ │ │ +//! │ │ │ Specialize with `false`: +//! │ │ ├─┐ Patterns: +//! │ │ │ │ 2. `[]` +//! │ │ │ │ +//! │ │ │ │ We note arm 2 is useful (by `Pair(None, false)`). +//! │ │ ├─┘ +//! │ │ │ +//! │ │ │ Total witnesses for `None`: +//! │ │ │ `[true]` +//! │ ├─┘ +//! │ │ Unspecialize new witnesses with `None`: +//! │ │ `[None, true]` +//! │ │ +//! │ │ Total witnesses for `Pair`: +//! │ │ `[Some(1..), true]` +//! │ │ `[None, true]` +//! ├─┘ +//! │ Unspecialize new witnesses with `Pair`: +//! │ `[Pair(Some(1..), true)]` +//! │ `[Pair(None, true)]` +//! │ +//! │ Final witnesses: +//! │ `[Pair(Some(1..), true)]` +//! │ `[Pair(None, true)]` +//! ┘ +//! ``` +//! +//! We conclude: +//! - Arm 3 is redundant (it was never marked as useful); +//! - The match is not exhaustive; +//! - Adding arms with `Pair(Some(1..), true)` and `Pair(None, true)` would make the match exhaustive. +//! +//! Note that when we're deep in the algorithm, we don't know what specialization steps got us here. +//! We can only figure out what our witnesses correspond to by unspecializing back up the stack. +//! +//! +//! # Tests +//! +//! Note: tests specific to this file can be found in: +//! +//! - `ui/pattern/usefulness` +//! - `ui/or-patterns` +//! - `ui/consts/const_in_pattern` +//! - `ui/rfc-2008-non-exhaustive` +//! - `ui/half-open-range-patterns` +//! - probably many others +//! +//! I (Nadrieril) prefer to put new tests in `ui/pattern/usefulness` unless there's a specific +//! reason not to, for example if they crucially depend on a particular feature like `or_patterns`. + +use std::fmt; + +#[cfg(feature = "rustc")] +use rustc_data_structures::stack::ensure_sufficient_stack; +use rustc_hash::{FxHashMap, FxHashSet}; +use rustc_index::bit_set::BitSet; +use smallvec::{smallvec, SmallVec}; +use tracing::{debug, instrument}; + +use self::PlaceValidity::*; +use crate::constructor::{Constructor, ConstructorSet, IntRange}; +use crate::pat::{DeconstructedPat, PatId, PatOrWild, WitnessPat}; +use crate::{Captures, MatchArm, PatCx, PrivateUninhabitedField}; +#[cfg(not(feature = "rustc"))] +pub fn ensure_sufficient_stack<R>(f: impl FnOnce() -> R) -> R { + f() +} + +/// A pattern is a "branch" if it is the immediate child of an or-pattern, or if it is the whole +/// pattern of a match arm. These are the patterns that can be meaningfully considered "redundant", +/// since e.g. `0` in `(0, 1)` cannot be redundant on its own. +/// +/// We track for each branch pattern whether it is useful, and if not why. +struct BranchPatUsefulness<'p, Cx: PatCx> { + /// Whether this pattern is useful. + useful: bool, + /// A set of patterns that: + /// - come before this one in the match; + /// - intersect this one; + /// - at the end of the algorithm, if `!self.useful`, their union covers this pattern. + covered_by: FxHashSet<&'p DeconstructedPat<Cx>>, +} + +impl<'p, Cx: PatCx> BranchPatUsefulness<'p, Cx> { + /// Update `self` with the usefulness information found in `row`. + fn update(&mut self, row: &MatrixRow<'p, Cx>, matrix: &Matrix<'p, Cx>) { + self.useful |= row.useful; + // This deserves an explanation: `intersects_at_least` does not contain all intersections + // because we skip irrelevant values (see the docs for `intersects_at_least` for an + // example). Yet we claim this suffices to build a covering set. + // + // Let `p` be our pattern. Assume it is found not useful. For a value `v`, if the value was + // relevant then we explored that value and found that there was another pattern `q` before + // `p` that matches it too. We therefore recorded an intersection with `q`. If `v` was + // irrelevant, we know there's another value `v2` that matches strictly fewer rows (while + // still matching our row) and is relevant. Since `p` is not useful, there must have been a + // `q` before `p` that matches `v2`, and we recorded that intersection. Since `v2` matches + // strictly fewer rows than `v`, `q` also matches `v`. In either case, we recorded in + // `intersects_at_least` a pattern that matches `v`. Hence using `intersects_at_least` is + // sufficient to build a covering set. + for row_id in row.intersects_at_least.iter() { + let row = &matrix.rows[row_id]; + if row.useful && !row.is_under_guard { + if let PatOrWild::Pat(intersecting) = row.head() { + self.covered_by.insert(intersecting); + } + } + } + } + + /// Check whether this pattern is redundant, and if so explain why. + fn is_redundant(&self) -> Option<RedundancyExplanation<'p, Cx>> { + if self.useful { + None + } else { + // We avoid instability by sorting by `uid`. The order of `uid`s only depends on the + // pattern structure. + #[cfg_attr(feature = "rustc", allow(rustc::potential_query_instability))] + let mut covered_by: Vec<_> = self.covered_by.iter().copied().collect(); + covered_by.sort_by_key(|pat| pat.uid); // sort to avoid instability + Some(RedundancyExplanation { covered_by }) + } + } +} + +impl<'p, Cx: PatCx> Default for BranchPatUsefulness<'p, Cx> { + fn default() -> Self { + Self { useful: Default::default(), covered_by: Default::default() } + } +} + +/// Context that provides information for usefulness checking. +struct UsefulnessCtxt<'a, 'p, Cx: PatCx> { + /// The context for type information. + tycx: &'a Cx, + /// Track information about the usefulness of branch patterns (see definition of "branch + /// pattern" at [`BranchPatUsefulness`]). + branch_usefulness: FxHashMap<PatId, BranchPatUsefulness<'p, Cx>>, + complexity_limit: Option<usize>, + complexity_level: usize, +} + +impl<'a, 'p, Cx: PatCx> UsefulnessCtxt<'a, 'p, Cx> { + fn increase_complexity_level(&mut self, complexity_add: usize) -> Result<(), Cx::Error> { + self.complexity_level += complexity_add; + if self + .complexity_limit + .is_some_and(|complexity_limit| complexity_limit < self.complexity_level) + { + return self.tycx.complexity_exceeded(); + } + Ok(()) + } +} + +/// Context that provides information local to a place under investigation. +struct PlaceCtxt<'a, Cx: PatCx> { + cx: &'a Cx, + /// Type of the place under investigation. + ty: &'a Cx::Ty, +} + +impl<'a, Cx: PatCx> Copy for PlaceCtxt<'a, Cx> {} +impl<'a, Cx: PatCx> Clone for PlaceCtxt<'a, Cx> { + fn clone(&self) -> Self { + Self { cx: self.cx, ty: self.ty } + } +} + +impl<'a, Cx: PatCx> fmt::Debug for PlaceCtxt<'a, Cx> { + fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { + fmt.debug_struct("PlaceCtxt").field("ty", self.ty).finish() + } +} + +impl<'a, Cx: PatCx> PlaceCtxt<'a, Cx> { + fn ctor_arity(&self, ctor: &Constructor<Cx>) -> usize { + self.cx.ctor_arity(ctor, self.ty) + } + fn wild_from_ctor(&self, ctor: Constructor<Cx>) -> WitnessPat<Cx> { + WitnessPat::wild_from_ctor(self.cx, ctor, self.ty.clone()) + } +} + +/// Track whether a given place (aka column) is known to contain a valid value or not. +#[derive(Debug, Copy, Clone, PartialEq, Eq)] +pub enum PlaceValidity { + ValidOnly, + MaybeInvalid, +} + +impl PlaceValidity { + pub fn from_bool(is_valid_only: bool) -> Self { + if is_valid_only { ValidOnly } else { MaybeInvalid } + } + + fn is_known_valid(self) -> bool { + matches!(self, ValidOnly) + } + + /// If the place has validity given by `self` and we read that the value at the place has + /// constructor `ctor`, this computes what we can assume about the validity of the constructor + /// fields. + /// + /// Pending further opsem decisions, the current behavior is: validity is preserved, except + /// inside `&` and union fields where validity is reset to `MaybeInvalid`. + fn specialize<Cx: PatCx>(self, ctor: &Constructor<Cx>) -> Self { + // We preserve validity except when we go inside a reference or a union field. + if matches!(ctor, Constructor::Ref | Constructor::UnionField) { + // Validity of `x: &T` does not imply validity of `*x: T`. + MaybeInvalid + } else { + self + } + } +} + +impl fmt::Display for PlaceValidity { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + let s = match self { + ValidOnly => "✓", + MaybeInvalid => "?", + }; + write!(f, "{s}") + } +} + +/// Data about a place under investigation. Its methods contain a lot of the logic used to analyze +/// the constructors in the matrix. +struct PlaceInfo<Cx: PatCx> { + /// The type of the place. + ty: Cx::Ty, + /// Whether the place is a private uninhabited field. If so we skip this field during analysis + /// so that we don't observe its emptiness. + private_uninhabited: bool, + /// Whether the place is known to contain valid data. + validity: PlaceValidity, + /// Whether the place is the scrutinee itself or a subplace of it. + is_scrutinee: bool, +} + +impl<Cx: PatCx> PlaceInfo<Cx> { + /// Given a constructor for the current place, we return one `PlaceInfo` for each field of the + /// constructor. + fn specialize<'a>( + &'a self, + cx: &'a Cx, + ctor: &'a Constructor<Cx>, + ) -> impl Iterator<Item = Self> + ExactSizeIterator + Captures<'a> { + let ctor_sub_tys = cx.ctor_sub_tys(ctor, &self.ty); + let ctor_sub_validity = self.validity.specialize(ctor); + ctor_sub_tys.map(move |(ty, PrivateUninhabitedField(private_uninhabited))| PlaceInfo { + ty, + private_uninhabited, + validity: ctor_sub_validity, + is_scrutinee: false, + }) + } + + /// This analyzes a column of constructors corresponding to the current place. It returns a pair + /// `(split_ctors, missing_ctors)`. + /// + /// `split_ctors` is a splitted list of constructors that cover the whole type. This will be + /// used to specialize the matrix. + /// + /// `missing_ctors` is a list of the constructors not found in the column, for reporting + /// purposes. + fn split_column_ctors<'a>( + &self, + cx: &Cx, + ctors: impl Iterator<Item = &'a Constructor<Cx>> + Clone, + ) -> Result<(SmallVec<[Constructor<Cx>; 1]>, Vec<Constructor<Cx>>), Cx::Error> + where + Cx: 'a, + { + debug!(?self.ty); + if self.private_uninhabited { + // Skip the whole column + return Ok((smallvec![Constructor::PrivateUninhabited], vec![])); + } + + if ctors.clone().any(|c| matches!(c, Constructor::Or)) { + // If any constructor is `Or`, we expand or-patterns. + return Ok((smallvec![Constructor::Or], vec![])); + } + + let ctors_for_ty = cx.ctors_for_ty(&self.ty)?; + debug!(?ctors_for_ty); + + // We treat match scrutinees of type `!` or `EmptyEnum` differently. + let is_toplevel_exception = + self.is_scrutinee && matches!(ctors_for_ty, ConstructorSet::NoConstructors); + // Whether empty patterns are counted as useful or not. We only warn an empty arm unreachable if + // it is guaranteed unreachable by the opsem (i.e. if the place is `known_valid`). + let empty_arms_are_unreachable = self.validity.is_known_valid() + && (is_toplevel_exception + || cx.is_exhaustive_patterns_feature_on() + || cx.is_min_exhaustive_patterns_feature_on()); + // Whether empty patterns can be omitted for exhaustiveness. We ignore place validity in the + // toplevel exception and `exhaustive_patterns` cases for backwards compatibility. + let can_omit_empty_arms = empty_arms_are_unreachable + || is_toplevel_exception + || cx.is_exhaustive_patterns_feature_on(); + + // Analyze the constructors present in this column. + let mut split_set = ctors_for_ty.split(ctors); + debug!(?split_set); + let all_missing = split_set.present.is_empty(); + + // Build the set of constructors we will specialize with. It must cover the whole type, so + // we add `Missing` to represent the missing ones. This is explained under "Constructor + // Splitting" at the top of this file. + let mut split_ctors = split_set.present; + if !(split_set.missing.is_empty() + && (split_set.missing_empty.is_empty() || empty_arms_are_unreachable)) + { + split_ctors.push(Constructor::Missing); + } + + // Which empty constructors are considered missing. We ensure that + // `!missing_ctors.is_empty() => split_ctors.contains(Missing)`. The converse usually holds + // except when `!self.validity.is_known_valid()`. + let mut missing_ctors = split_set.missing; + if !can_omit_empty_arms { + missing_ctors.append(&mut split_set.missing_empty); + } + + // Whether we should report "Enum::A and Enum::C are missing" or "_ is missing". At the top + // level we prefer to list all constructors. + let report_individual_missing_ctors = self.is_scrutinee || !all_missing; + if !missing_ctors.is_empty() && !report_individual_missing_ctors { + // Report `_` as missing. + missing_ctors = vec![Constructor::Wildcard]; + } else if missing_ctors.iter().any(|c| c.is_non_exhaustive()) { + // We need to report a `_` anyway, so listing other constructors would be redundant. + // `NonExhaustive` is displayed as `_` just like `Wildcard`, but it will be picked + // up by diagnostics to add a note about why `_` is required here. + missing_ctors = vec![Constructor::NonExhaustive]; + } + + Ok((split_ctors, missing_ctors)) + } +} + +impl<Cx: PatCx> Clone for PlaceInfo<Cx> { + fn clone(&self) -> Self { + Self { + ty: self.ty.clone(), + private_uninhabited: self.private_uninhabited, + validity: self.validity, + is_scrutinee: self.is_scrutinee, + } + } +} + +/// Represents a pattern-tuple under investigation. +// The three lifetimes are: +// - 'p coming from the input +// - Cx global compilation context +struct PatStack<'p, Cx: PatCx> { + // Rows of len 1 are very common, which is why `SmallVec[_; 2]` works well. + pats: SmallVec<[PatOrWild<'p, Cx>; 2]>, + /// Sometimes we know that as far as this row is concerned, the current case is already handled + /// by a different, more general, case. When the case is irrelevant for all rows this allows us + /// to skip a case entirely. This is purely an optimization. See at the top for details. + relevant: bool, +} + +impl<'p, Cx: PatCx> Clone for PatStack<'p, Cx> { + fn clone(&self) -> Self { + Self { pats: self.pats.clone(), relevant: self.relevant } + } +} + +impl<'p, Cx: PatCx> PatStack<'p, Cx> { + fn from_pattern(pat: &'p DeconstructedPat<Cx>) -> Self { + PatStack { pats: smallvec![PatOrWild::Pat(pat)], relevant: true } + } + + fn len(&self) -> usize { + self.pats.len() + } + + fn head(&self) -> PatOrWild<'p, Cx> { + self.pats[0] + } + + fn iter(&self) -> impl Iterator<Item = PatOrWild<'p, Cx>> + Captures<'_> { + self.pats.iter().copied() + } + + // Expand the first or-pattern into its subpatterns. Only useful if the pattern is an + // or-pattern. Panics if `self` is empty. + fn expand_or_pat(&self) -> impl Iterator<Item = PatStack<'p, Cx>> + Captures<'_> { + self.head().expand_or_pat().into_iter().map(move |pat| { + let mut new = self.clone(); + new.pats[0] = pat; + new + }) + } + + /// This computes `specialize(ctor, self)`. See top of the file for explanations. + /// Only call if `ctor.is_covered_by(self.head().ctor())` is true. + fn pop_head_constructor( + &self, + cx: &Cx, + ctor: &Constructor<Cx>, + ctor_arity: usize, + ctor_is_relevant: bool, + ) -> Result<PatStack<'p, Cx>, Cx::Error> { + let head_pat = self.head(); + if head_pat.as_pat().is_some_and(|pat| pat.arity() > ctor_arity) { + // Arity can be smaller in case of variable-length slices, but mustn't be larger. + return Err(cx.bug(format_args!( + "uncaught type error: pattern {:?} has inconsistent arity (expected arity <= {ctor_arity})", + head_pat.as_pat().unwrap() + ))); + } + // We pop the head pattern and push the new fields extracted from the arguments of + // `self.head()`. + let mut new_pats = head_pat.specialize(ctor, ctor_arity); + new_pats.extend_from_slice(&self.pats[1..]); + // `ctor` is relevant for this row if it is the actual constructor of this row, or if the + // row has a wildcard and `ctor` is relevant for wildcards. + let ctor_is_relevant = + !matches!(self.head().ctor(), Constructor::Wildcard) || ctor_is_relevant; + Ok(PatStack { pats: new_pats, relevant: self.relevant && ctor_is_relevant }) + } +} + +impl<'p, Cx: PatCx> fmt::Debug for PatStack<'p, Cx> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + // We pretty-print similarly to the `Debug` impl of `Matrix`. + write!(f, "+")?; + for pat in self.iter() { + write!(f, " {pat:?} +")?; + } + Ok(()) + } +} + +/// A row of the matrix. +#[derive(Clone)] +struct MatrixRow<'p, Cx: PatCx> { + // The patterns in the row. + pats: PatStack<'p, Cx>, + /// Whether the original arm had a guard. This is inherited when specializing. + is_under_guard: bool, + /// When we specialize, we remember which row of the original matrix produced a given row of the + /// specialized matrix. When we unspecialize, we use this to propagate usefulness back up the + /// callstack. On creation, this stores the index of the original match arm. + parent_row: usize, + /// False when the matrix is just built. This is set to `true` by + /// [`compute_exhaustiveness_and_usefulness`] if the arm is found to be useful. + /// This is reset to `false` when specializing. + useful: bool, + /// Tracks some rows above this one that have an intersection with this one, i.e. such that + /// there is a value that matches both rows. + /// Because of relevancy we may miss some intersections. The intersections we do find are + /// correct. In other words, this is an underapproximation of the real set of intersections. + /// + /// For example: + /// ```rust,ignore(illustrative) + /// match ... { + /// (true, _, _) => {} // `intersects_at_least = []` + /// (_, true, 0..=10) => {} // `intersects_at_least = []` + /// (_, true, 5..15) => {} // `intersects_at_least = [1]` + /// } + /// ``` + /// Here the `(true, true)` case is irrelevant. Since we skip it, we will not detect that row 0 + /// intersects rows 1 and 2. + intersects_at_least: BitSet<usize>, + /// Whether the head pattern is a branch (see definition of "branch pattern" at + /// [`BranchPatUsefulness`]) + head_is_branch: bool, +} + +impl<'p, Cx: PatCx> MatrixRow<'p, Cx> { + fn new(arm: &MatchArm<'p, Cx>, arm_id: usize) -> Self { + MatrixRow { + pats: PatStack::from_pattern(arm.pat), + parent_row: arm_id, + is_under_guard: arm.has_guard, + useful: false, + intersects_at_least: BitSet::new_empty(0), // Initialized in `Matrix::push`. + // This pattern is a branch because it comes from a match arm. + head_is_branch: true, + } + } + + fn len(&self) -> usize { + self.pats.len() + } + + fn head(&self) -> PatOrWild<'p, Cx> { + self.pats.head() + } + + fn iter(&self) -> impl Iterator<Item = PatOrWild<'p, Cx>> + Captures<'_> { + self.pats.iter() + } + + // Expand the first or-pattern (if any) into its subpatterns. Panics if `self` is empty. + fn expand_or_pat( + &self, + parent_row: usize, + ) -> impl Iterator<Item = MatrixRow<'p, Cx>> + Captures<'_> { + let is_or_pat = self.pats.head().is_or_pat(); + self.pats.expand_or_pat().map(move |patstack| MatrixRow { + pats: patstack, + parent_row, + is_under_guard: self.is_under_guard, + useful: false, + intersects_at_least: BitSet::new_empty(0), // Initialized in `Matrix::push`. + head_is_branch: is_or_pat, + }) + } + + /// This computes `specialize(ctor, self)`. See top of the file for explanations. + /// Only call if `ctor.is_covered_by(self.head().ctor())` is true. + fn pop_head_constructor( + &self, + cx: &Cx, + ctor: &Constructor<Cx>, + ctor_arity: usize, + ctor_is_relevant: bool, + parent_row: usize, + ) -> Result<MatrixRow<'p, Cx>, Cx::Error> { + Ok(MatrixRow { + pats: self.pats.pop_head_constructor(cx, ctor, ctor_arity, ctor_is_relevant)?, + parent_row, + is_under_guard: self.is_under_guard, + useful: false, + intersects_at_least: BitSet::new_empty(0), // Initialized in `Matrix::push`. + head_is_branch: false, + }) + } +} + +impl<'p, Cx: PatCx> fmt::Debug for MatrixRow<'p, Cx> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + self.pats.fmt(f) + } +} + +/// A 2D matrix. Represents a list of pattern-tuples under investigation. +/// +/// Invariant: each row must have the same length, and each column must have the same type. +/// +/// Invariant: the first column must not contain or-patterns. This is handled by +/// [`Matrix::push`]. +/// +/// In fact each column corresponds to a place inside the scrutinee of the match. E.g. after +/// specializing `(,)` and `Some` on a pattern of type `(Option<u32>, bool)`, the first column of +/// the matrix will correspond to `scrutinee.0.Some.0` and the second column to `scrutinee.1`. +#[derive(Clone)] +struct Matrix<'p, Cx: PatCx> { + /// Vector of rows. The rows must form a rectangular 2D array. Moreover, all the patterns of + /// each column must have the same type. Each column corresponds to a place within the + /// scrutinee. + rows: Vec<MatrixRow<'p, Cx>>, + /// Track info about each place. Each place corresponds to a column in `rows`, and their types + /// must match. + place_info: SmallVec<[PlaceInfo<Cx>; 2]>, + /// Track whether the virtual wildcard row used to compute exhaustiveness is relevant. See top + /// of the file for details on relevancy. + wildcard_row_is_relevant: bool, +} + +impl<'p, Cx: PatCx> Matrix<'p, Cx> { + /// Pushes a new row to the matrix. Internal method, prefer [`Matrix::new`]. + fn push(&mut self, mut row: MatrixRow<'p, Cx>) { + row.intersects_at_least = BitSet::new_empty(self.rows.len()); + self.rows.push(row); + } + + /// Build a new matrix from an iterator of `MatchArm`s. + fn new(arms: &[MatchArm<'p, Cx>], scrut_ty: Cx::Ty, scrut_validity: PlaceValidity) -> Self { + let place_info = PlaceInfo { + ty: scrut_ty, + private_uninhabited: false, + validity: scrut_validity, + is_scrutinee: true, + }; + let mut matrix = Matrix { + rows: Vec::with_capacity(arms.len()), + place_info: smallvec![place_info], + wildcard_row_is_relevant: true, + }; + for (arm_id, arm) in arms.iter().enumerate() { + matrix.push(MatrixRow::new(arm, arm_id)); + } + matrix + } + + fn head_place(&self) -> Option<&PlaceInfo<Cx>> { + self.place_info.first() + } + fn column_count(&self) -> usize { + self.place_info.len() + } + + fn rows( + &self, + ) -> impl Iterator<Item = &MatrixRow<'p, Cx>> + Clone + DoubleEndedIterator + ExactSizeIterator + { + self.rows.iter() + } + fn rows_mut( + &mut self, + ) -> impl Iterator<Item = &mut MatrixRow<'p, Cx>> + DoubleEndedIterator + ExactSizeIterator + { + self.rows.iter_mut() + } + + /// Iterate over the first pattern of each row. + fn heads(&self) -> impl Iterator<Item = PatOrWild<'p, Cx>> + Clone + Captures<'_> { + self.rows().map(|r| r.head()) + } + + /// This computes `specialize(ctor, self)`. See top of the file for explanations. + fn specialize_constructor( + &self, + pcx: &PlaceCtxt<'_, Cx>, + ctor: &Constructor<Cx>, + ctor_is_relevant: bool, + ) -> Result<Matrix<'p, Cx>, Cx::Error> { + if matches!(ctor, Constructor::Or) { + // Specializing with `Or` means expanding rows with or-patterns. + let mut matrix = Matrix { + rows: Vec::new(), + place_info: self.place_info.clone(), + wildcard_row_is_relevant: self.wildcard_row_is_relevant, + }; + for (i, row) in self.rows().enumerate() { + for new_row in row.expand_or_pat(i) { + matrix.push(new_row); + } + } + Ok(matrix) + } else { + let subfield_place_info = self.place_info[0].specialize(pcx.cx, ctor); + let arity = subfield_place_info.len(); + let specialized_place_info = + subfield_place_info.chain(self.place_info[1..].iter().cloned()).collect(); + let mut matrix = Matrix { + rows: Vec::new(), + place_info: specialized_place_info, + wildcard_row_is_relevant: self.wildcard_row_is_relevant && ctor_is_relevant, + }; + for (i, row) in self.rows().enumerate() { + if ctor.is_covered_by(pcx.cx, row.head().ctor())? { + let new_row = + row.pop_head_constructor(pcx.cx, ctor, arity, ctor_is_relevant, i)?; + matrix.push(new_row); + } + } + Ok(matrix) + } + } + + /// Recover row usefulness and intersection information from a processed specialized matrix. + /// `specialized` must come from `self.specialize_constructor`. + fn unspecialize(&mut self, specialized: Self) { + for child_row in specialized.rows() { + let parent_row_id = child_row.parent_row; + let parent_row = &mut self.rows[parent_row_id]; + // A parent row is useful if any of its children is. + parent_row.useful |= child_row.useful; + for child_intersection in child_row.intersects_at_least.iter() { + // Convert the intersecting ids into ids for the parent matrix. + let parent_intersection = specialized.rows[child_intersection].parent_row; + // Note: self-intersection can happen with or-patterns. + if parent_intersection != parent_row_id { + parent_row.intersects_at_least.insert(parent_intersection); + } + } + } + } +} + +/// Pretty-printer for matrices of patterns, example: +/// +/// ```text +/// + _ + [] + +/// + true + [First] + +/// + true + [Second(true)] + +/// + false + [_] + +/// + _ + [_, _, tail @ ..] + +/// | ✓ | ? | // validity +/// ``` +impl<'p, Cx: PatCx> fmt::Debug for Matrix<'p, Cx> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + write!(f, "\n")?; + + let mut pretty_printed_matrix: Vec<Vec<String>> = self + .rows + .iter() + .map(|row| row.iter().map(|pat| format!("{pat:?}")).collect()) + .collect(); + pretty_printed_matrix + .push(self.place_info.iter().map(|place| format!("{}", place.validity)).collect()); + + let column_count = self.column_count(); + assert!(self.rows.iter().all(|row| row.len() == column_count)); + assert!(self.place_info.len() == column_count); + let column_widths: Vec<usize> = (0..column_count) + .map(|col| pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)) + .collect(); + + for (row_i, row) in pretty_printed_matrix.into_iter().enumerate() { + let is_validity_row = row_i == self.rows.len(); + let sep = if is_validity_row { "|" } else { "+" }; + write!(f, "{sep}")?; + for (column, pat_str) in row.into_iter().enumerate() { + write!(f, " ")?; + write!(f, "{:1$}", pat_str, column_widths[column])?; + write!(f, " {sep}")?; + } + if is_validity_row { + write!(f, " // validity")?; + } + write!(f, "\n")?; + } + Ok(()) + } +} + +/// A witness-tuple of non-exhaustiveness for error reporting, represented as a list of patterns (in +/// reverse order of construction). +/// +/// This mirrors `PatStack`: they function similarly, except `PatStack` contains user patterns we +/// are inspecting, and `WitnessStack` contains witnesses we are constructing. +/// FIXME(Nadrieril): use the same order of patterns for both. +/// +/// A `WitnessStack` should have the same types and length as the `PatStack`s we are inspecting +/// (except we store the patterns in reverse order). The same way `PatStack` starts with length 1, +/// at the end of the algorithm this will have length 1. In the middle of the algorithm, it can +/// contain multiple patterns. +/// +/// For example, if we are constructing a witness for the match against +/// +/// ```compile_fail,E0004 +/// struct Pair(Option<(u32, u32)>, bool); +/// # fn foo(p: Pair) { +/// match p { +/// Pair(None, _) => {} +/// Pair(_, false) => {} +/// } +/// # } +/// ``` +/// +/// We'll perform the following steps (among others): +/// ```text +/// - Start with a matrix representing the match +/// `PatStack(vec![Pair(None, _)])` +/// `PatStack(vec![Pair(_, false)])` +/// - Specialize with `Pair` +/// `PatStack(vec![None, _])` +/// `PatStack(vec![_, false])` +/// - Specialize with `Some` +/// `PatStack(vec![_, false])` +/// - Specialize with `_` +/// `PatStack(vec![false])` +/// - Specialize with `true` +/// // no patstacks left +/// - This is a non-exhaustive match: we have the empty witness stack as a witness. +/// `WitnessStack(vec![])` +/// - Apply `true` +/// `WitnessStack(vec![true])` +/// - Apply `_` +/// `WitnessStack(vec![true, _])` +/// - Apply `Some` +/// `WitnessStack(vec![true, Some(_)])` +/// - Apply `Pair` +/// `WitnessStack(vec![Pair(Some(_), true)])` +/// ``` +/// +/// The final `Pair(Some(_), true)` is then the resulting witness. +/// +/// See the top of the file for more detailed explanations and examples. +#[derive(Debug)] +struct WitnessStack<Cx: PatCx>(Vec<WitnessPat<Cx>>); + +impl<Cx: PatCx> Clone for WitnessStack<Cx> { + fn clone(&self) -> Self { + Self(self.0.clone()) + } +} + +impl<Cx: PatCx> WitnessStack<Cx> { + /// Asserts that the witness contains a single pattern, and returns it. + fn single_pattern(self) -> WitnessPat<Cx> { + assert_eq!(self.0.len(), 1); + self.0.into_iter().next().unwrap() + } + + /// Reverses specialization by the `Missing` constructor by pushing a whole new pattern. + fn push_pattern(&mut self, pat: WitnessPat<Cx>) { + self.0.push(pat); + } + + /// Reverses specialization. Given a witness obtained after specialization, this constructs a + /// new witness valid for before specialization. See the section on `unspecialize` at the top of + /// the file. + /// + /// Examples: + /// ```text + /// ctor: tuple of 2 elements + /// pats: [false, "foo", _, true] + /// result: [(false, "foo"), _, true] + /// + /// ctor: Enum::Variant { a: (bool, &'static str), b: usize} + /// pats: [(false, "foo"), _, true] + /// result: [Enum::Variant { a: (false, "foo"), b: _ }, true] + /// ``` + fn apply_constructor( + mut self, + pcx: &PlaceCtxt<'_, Cx>, + ctor: &Constructor<Cx>, + ) -> SmallVec<[Self; 1]> { + let len = self.0.len(); + let arity = pcx.ctor_arity(ctor); + let fields: Vec<_> = self.0.drain((len - arity)..).rev().collect(); + if matches!(ctor, Constructor::UnionField) + && fields.iter().filter(|p| !matches!(p.ctor(), Constructor::Wildcard)).count() >= 2 + { + // Convert a `Union { a: p, b: q }` witness into `Union { a: p }` and `Union { b: q }`. + // First add `Union { .. }` to `self`. + self.0.push(WitnessPat::wild_from_ctor(pcx.cx, ctor.clone(), pcx.ty.clone())); + fields + .into_iter() + .enumerate() + .filter(|(_, p)| !matches!(p.ctor(), Constructor::Wildcard)) + .map(|(i, p)| { + let mut ret = self.clone(); + // Fill the `i`th field of the union with `p`. + ret.0.last_mut().unwrap().fields[i] = p; + ret + }) + .collect() + } else { + self.0.push(WitnessPat::new(ctor.clone(), fields, pcx.ty.clone())); + smallvec![self] + } + } +} + +/// Represents a set of pattern-tuples that are witnesses of non-exhaustiveness for error +/// reporting. This has similar invariants as `Matrix` does. +/// +/// The `WitnessMatrix` returned by [`compute_exhaustiveness_and_usefulness`] obeys the invariant +/// that the union of the input `Matrix` and the output `WitnessMatrix` together matches the type +/// exhaustively. +/// +/// Just as the `Matrix` starts with a single column, by the end of the algorithm, this has a single +/// column, which contains the patterns that are missing for the match to be exhaustive. +#[derive(Debug)] +struct WitnessMatrix<Cx: PatCx>(Vec<WitnessStack<Cx>>); + +impl<Cx: PatCx> Clone for WitnessMatrix<Cx> { + fn clone(&self) -> Self { + Self(self.0.clone()) + } +} + +impl<Cx: PatCx> WitnessMatrix<Cx> { + /// New matrix with no witnesses. + fn empty() -> Self { + WitnessMatrix(Vec::new()) + } + /// New matrix with one `()` witness, i.e. with no columns. + fn unit_witness() -> Self { + WitnessMatrix(vec![WitnessStack(Vec::new())]) + } + + /// Whether this has any witnesses. + fn is_empty(&self) -> bool { + self.0.is_empty() + } + /// Asserts that there is a single column and returns the patterns in it. + fn single_column(self) -> Vec<WitnessPat<Cx>> { + self.0.into_iter().map(|w| w.single_pattern()).collect() + } + + /// Reverses specialization by the `Missing` constructor by pushing a whole new pattern. + fn push_pattern(&mut self, pat: WitnessPat<Cx>) { + for witness in self.0.iter_mut() { + witness.push_pattern(pat.clone()) + } + } + + /// Reverses specialization by `ctor`. See the section on `unspecialize` at the top of the file. + fn apply_constructor( + &mut self, + pcx: &PlaceCtxt<'_, Cx>, + missing_ctors: &[Constructor<Cx>], + ctor: &Constructor<Cx>, + ) { + // The `Or` constructor indicates that we expanded or-patterns. This doesn't affect + // witnesses. + if self.is_empty() || matches!(ctor, Constructor::Or) { + return; + } + if matches!(ctor, Constructor::Missing) { + // We got the special `Missing` constructor that stands for the constructors not present + // in the match. For each missing constructor `c`, we add a `c(_, _, _)` witness + // appropriately filled with wildcards. + let mut ret = Self::empty(); + for ctor in missing_ctors { + let pat = pcx.wild_from_ctor(ctor.clone()); + // Clone `self` and add `c(_, _, _)` to each of its witnesses. + let mut wit_matrix = self.clone(); + wit_matrix.push_pattern(pat); + ret.extend(wit_matrix); + } + *self = ret; + } else { + // Any other constructor we unspecialize as expected. + for witness in std::mem::take(&mut self.0) { + self.0.extend(witness.apply_constructor(pcx, ctor)); + } + } + } + + /// Merges the witnesses of two matrices. Their column types must match. + fn extend(&mut self, other: Self) { + self.0.extend(other.0) + } +} + +/// Collect ranges that overlap like `lo..=overlap`/`overlap..=hi`. Must be called during +/// exhaustiveness checking, if we find a singleton range after constructor splitting. This reuses +/// row intersection information to only detect ranges that truly overlap. +/// +/// If two ranges overlapped, the split set will contain their intersection as a singleton. +/// Specialization will then select rows that match the overlap, and exhaustiveness will compute +/// which rows have an intersection that includes the overlap. That gives us all the info we need to +/// compute overlapping ranges without false positives. +/// +/// We can however get false negatives because exhaustiveness does not explore all cases. See the +/// section on relevancy at the top of the file. +fn collect_overlapping_range_endpoints<'p, Cx: PatCx>( + cx: &Cx, + overlap_range: IntRange, + matrix: &Matrix<'p, Cx>, + specialized_matrix: &Matrix<'p, Cx>, +) { + let overlap = overlap_range.lo; + // Ranges that look like `lo..=overlap`. + let mut prefixes: SmallVec<[_; 1]> = Default::default(); + // Ranges that look like `overlap..=hi`. + let mut suffixes: SmallVec<[_; 1]> = Default::default(); + // Iterate on patterns that contained `overlap`. We iterate on `specialized_matrix` which + // contains only rows that matched the current `ctor` as well as accurate intersection + // information. It doesn't contain the column that contains the range; that can be found in + // `matrix`. + for (child_row_id, child_row) in specialized_matrix.rows().enumerate() { + let PatOrWild::Pat(pat) = matrix.rows[child_row.parent_row].head() else { continue }; + let Constructor::IntRange(this_range) = pat.ctor() else { continue }; + // Don't lint when one of the ranges is a singleton. + if this_range.is_singleton() { + continue; + } + if this_range.lo == overlap { + // `this_range` looks like `overlap..=this_range.hi`; it overlaps with any + // ranges that look like `lo..=overlap`. + if !prefixes.is_empty() { + let overlaps_with: Vec<_> = prefixes + .iter() + .filter(|&&(other_child_row_id, _)| { + child_row.intersects_at_least.contains(other_child_row_id) + }) + .map(|&(_, pat)| pat) + .collect(); + if !overlaps_with.is_empty() { + cx.lint_overlapping_range_endpoints(pat, overlap_range, &overlaps_with); + } + } + suffixes.push((child_row_id, pat)) + } else if Some(this_range.hi) == overlap.plus_one() { + // `this_range` looks like `this_range.lo..=overlap`; it overlaps with any + // ranges that look like `overlap..=hi`. + if !suffixes.is_empty() { + let overlaps_with: Vec<_> = suffixes + .iter() + .filter(|&&(other_child_row_id, _)| { + child_row.intersects_at_least.contains(other_child_row_id) + }) + .map(|&(_, pat)| pat) + .collect(); + if !overlaps_with.is_empty() { + cx.lint_overlapping_range_endpoints(pat, overlap_range, &overlaps_with); + } + } + prefixes.push((child_row_id, pat)) + } + } +} + +/// Collect ranges that have a singleton gap between them. +fn collect_non_contiguous_range_endpoints<'p, Cx: PatCx>( + cx: &Cx, + gap_range: &IntRange, + matrix: &Matrix<'p, Cx>, +) { + let gap = gap_range.lo; + // Ranges that look like `lo..gap`. + let mut onebefore: SmallVec<[_; 1]> = Default::default(); + // Ranges that start on `gap+1` or singletons `gap+1`. + let mut oneafter: SmallVec<[_; 1]> = Default::default(); + // Look through the column for ranges near the gap. + for pat in matrix.heads() { + let PatOrWild::Pat(pat) = pat else { continue }; + let Constructor::IntRange(this_range) = pat.ctor() else { continue }; + if gap == this_range.hi { + onebefore.push(pat) + } else if gap.plus_one() == Some(this_range.lo) { + oneafter.push(pat) + } + } + + for pat_before in onebefore { + cx.lint_non_contiguous_range_endpoints(pat_before, *gap_range, oneafter.as_slice()); + } +} + +/// The core of the algorithm. +/// +/// This recursively computes witnesses of the non-exhaustiveness of `matrix` (if any). Also tracks +/// usefulness of each row in the matrix (in `row.useful`). We track usefulness of subpatterns in +/// `mcx.branch_usefulness`. +/// +/// The input `Matrix` and the output `WitnessMatrix` together match the type exhaustively. +/// +/// The key steps are: +/// - specialization, where we dig into the rows that have a specific constructor and call ourselves +/// recursively; +/// - unspecialization, where we lift the results from the previous step into results for this step +/// (using `apply_constructor` and by updating `row.useful` for each parent row). +/// This is all explained at the top of the file. +#[instrument(level = "debug", skip(mcx), ret)] +fn compute_exhaustiveness_and_usefulness<'a, 'p, Cx: PatCx>( + mcx: &mut UsefulnessCtxt<'a, 'p, Cx>, + matrix: &mut Matrix<'p, Cx>, +) -> Result<WitnessMatrix<Cx>, Cx::Error> { + debug_assert!(matrix.rows().all(|r| r.len() == matrix.column_count())); + + if !matrix.wildcard_row_is_relevant && matrix.rows().all(|r| !r.pats.relevant) { + // Here we know that nothing will contribute further to exhaustiveness or usefulness. This + // is purely an optimization: skipping this check doesn't affect correctness. See the top of + // the file for details. + return Ok(WitnessMatrix::empty()); + } + + let Some(place) = matrix.head_place() else { + mcx.increase_complexity_level(matrix.rows().len())?; + // The base case: there are no columns in the matrix. We are morally pattern-matching on (). + // A row is useful iff it has no (unguarded) rows above it. + let mut useful = true; // Whether the next row is useful. + for (i, row) in matrix.rows_mut().enumerate() { + row.useful = useful; + row.intersects_at_least.insert_range(0..i); + // The next rows stays useful if this one is under a guard. + useful &= row.is_under_guard; + } + return if useful && matrix.wildcard_row_is_relevant { + // The wildcard row is useful; the match is non-exhaustive. + Ok(WitnessMatrix::unit_witness()) + } else { + // Either the match is exhaustive, or we choose not to report anything because of + // relevancy. See at the top for details. + Ok(WitnessMatrix::empty()) + }; + }; + + // Analyze the constructors present in this column. + let ctors = matrix.heads().map(|p| p.ctor()); + let (split_ctors, missing_ctors) = place.split_column_ctors(mcx.tycx, ctors)?; + + let ty = &place.ty.clone(); // Clone it out so we can mutate `matrix` later. + let pcx = &PlaceCtxt { cx: mcx.tycx, ty }; + let mut ret = WitnessMatrix::empty(); + for ctor in split_ctors { + // Dig into rows that match `ctor`. + debug!("specialize({:?})", ctor); + // `ctor` is *irrelevant* if there's another constructor in `split_ctors` that matches + // strictly fewer rows. In that case we can sometimes skip it. See the top of the file for + // details. + let ctor_is_relevant = matches!(ctor, Constructor::Missing) || missing_ctors.is_empty(); + let mut spec_matrix = matrix.specialize_constructor(pcx, &ctor, ctor_is_relevant)?; + let mut witnesses = ensure_sufficient_stack(|| { + compute_exhaustiveness_and_usefulness(mcx, &mut spec_matrix) + })?; + + // Transform witnesses for `spec_matrix` into witnesses for `matrix`. + witnesses.apply_constructor(pcx, &missing_ctors, &ctor); + // Accumulate the found witnesses. + ret.extend(witnesses); + + // Detect ranges that overlap on their endpoints. + if let Constructor::IntRange(overlap_range) = ctor { + if overlap_range.is_singleton() + && spec_matrix.rows.len() >= 2 + && spec_matrix.rows.iter().any(|row| !row.intersects_at_least.is_empty()) + { + collect_overlapping_range_endpoints(mcx.tycx, overlap_range, matrix, &spec_matrix); + } + } + + matrix.unspecialize(spec_matrix); + } + + // Detect singleton gaps between ranges. + if missing_ctors.iter().any(|c| matches!(c, Constructor::IntRange(..))) { + for missing in &missing_ctors { + if let Constructor::IntRange(gap) = missing { + if gap.is_singleton() { + collect_non_contiguous_range_endpoints(mcx.tycx, gap, matrix); + } + } + } + } + + // Record usefulness of the branch patterns. + for row in matrix.rows() { + if row.head_is_branch { + if let PatOrWild::Pat(pat) = row.head() { + mcx.branch_usefulness.entry(pat.uid).or_default().update(row, matrix); + } + } + } + + Ok(ret) +} + +/// Indicates why a given pattern is considered redundant. +#[derive(Clone, Debug)] +pub struct RedundancyExplanation<'p, Cx: PatCx> { + /// All the values matched by this pattern are already matched by the given set of patterns. + /// This list is not guaranteed to be minimal but the contained patterns are at least guaranteed + /// to intersect this pattern. + pub covered_by: Vec<&'p DeconstructedPat<Cx>>, +} + +/// Indicates whether or not a given arm is useful. +#[derive(Clone, Debug)] +pub enum Usefulness<'p, Cx: PatCx> { + /// The arm is useful. This additionally carries a set of or-pattern branches that have been + /// found to be redundant despite the overall arm being useful. Used only in the presence of + /// or-patterns, otherwise it stays empty. + Useful(Vec<(&'p DeconstructedPat<Cx>, RedundancyExplanation<'p, Cx>)>), + /// The arm is redundant and can be removed without changing the behavior of the match + /// expression. + Redundant(RedundancyExplanation<'p, Cx>), +} + +/// The output of checking a match for exhaustiveness and arm usefulness. +pub struct UsefulnessReport<'p, Cx: PatCx> { + /// For each arm of the input, whether that arm is useful after the arms above it. + pub arm_usefulness: Vec<(MatchArm<'p, Cx>, Usefulness<'p, Cx>)>, + /// If the match is exhaustive, this is empty. If not, this contains witnesses for the lack of + /// exhaustiveness. + pub non_exhaustiveness_witnesses: Vec<WitnessPat<Cx>>, + /// For each arm, a set of indices of arms above it that have non-empty intersection, i.e. there + /// is a value matched by both arms. This may miss real intersections. + pub arm_intersections: Vec<BitSet<usize>>, +} + +/// Computes whether a match is exhaustive and which of its arms are useful. +#[instrument(skip(tycx, arms), level = "debug")] +pub fn compute_match_usefulness<'p, Cx: PatCx>( + tycx: &Cx, + arms: &[MatchArm<'p, Cx>], + scrut_ty: Cx::Ty, + scrut_validity: PlaceValidity, + complexity_limit: Option<usize>, +) -> Result<UsefulnessReport<'p, Cx>, Cx::Error> { + let mut cx = UsefulnessCtxt { + tycx, + branch_usefulness: FxHashMap::default(), + complexity_limit, + complexity_level: 0, + }; + let mut matrix = Matrix::new(arms, scrut_ty, scrut_validity); + let non_exhaustiveness_witnesses = compute_exhaustiveness_and_usefulness(&mut cx, &mut matrix)?; + + let non_exhaustiveness_witnesses: Vec<_> = non_exhaustiveness_witnesses.single_column(); + let arm_usefulness: Vec<_> = arms + .iter() + .copied() + .map(|arm| { + debug!(?arm); + let usefulness = cx.branch_usefulness.get(&arm.pat.uid).unwrap(); + let usefulness = if let Some(explanation) = usefulness.is_redundant() { + Usefulness::Redundant(explanation) + } else { + let mut redundant_subpats = Vec::new(); + arm.pat.walk(&mut |subpat| { + if let Some(u) = cx.branch_usefulness.get(&subpat.uid) { + if let Some(explanation) = u.is_redundant() { + redundant_subpats.push((subpat, explanation)); + false // stop recursing + } else { + true // keep recursing + } + } else { + true // keep recursing + } + }); + Usefulness::Useful(redundant_subpats) + }; + debug!(?usefulness); + (arm, usefulness) + }) + .collect(); + + let arm_intersections: Vec<_> = + matrix.rows().map(|row| row.intersects_at_least.clone()).collect(); + + Ok(UsefulnessReport { arm_usefulness, non_exhaustiveness_witnesses, arm_intersections }) +} |