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//! As explained in [`crate::usefulness`], values and patterns are made from constructors applied to
//! fields. This file defines a `Constructor` enum and various operations to manipulate them.
//!
//! There are two important bits of core logic in this file: constructor inclusion and constructor
//! splitting. Constructor inclusion, i.e. whether a constructor is included in/covered by another,
//! is straightforward and defined in [`Constructor::is_covered_by`].
//!
//! Constructor splitting is mentioned in [`crate::usefulness`] but not detailed. We describe it
//! precisely here.
//!
//!
//!
//! # Constructor grouping and splitting
//!
//! As explained in the corresponding section in [`crate::usefulness`], to make usefulness tractable
//! we need to group together constructors that have the same effect when they are used to
//! specialize the matrix.
//!
//! Example:
//! ```compile_fail,E0004
//! match (0, false) {
//! (0 ..=100, true) => {}
//! (50..=150, false) => {}
//! (0 ..=200, _) => {}
//! }
//! ```
//!
//! In this example we can restrict specialization to 5 cases: `0..50`, `50..=100`, `101..=150`,
//! `151..=200` and `200..`.
//!
//! In [`crate::usefulness`], we had said that `specialize` only takes value-only constructors. We
//! now relax this restriction: we allow `specialize` to take constructors like `0..50` as long as
//! we're careful to only do that with constructors that make sense. For example, `specialize(0..50,
//! (0..=100, true))` is sensible, but `specialize(50..=200, (0..=100, true))` is not.
//!
//! Constructor splitting looks at the constructors in the first column of the matrix and constructs
//! such a sensible set of constructors. Formally, we want to find a smallest disjoint set of
//! constructors:
//! - Whose union covers the whole type, and
//! - That have no non-trivial intersection with any of the constructors in the column (i.e. they're
//! each either disjoint with or covered by any given column constructor).
//!
//! We compute this in two steps: first [`PatCx::ctors_for_ty`] determines the
//! set of all possible constructors for the type. Then [`ConstructorSet::split`] looks at the
//! column of constructors and splits the set into groups accordingly. The precise invariants of
//! [`ConstructorSet::split`] is described in [`SplitConstructorSet`].
//!
//! Constructor splitting has two interesting special cases: integer range splitting (see
//! [`IntRange::split`]) and slice splitting (see [`Slice::split`]).
//!
//!
//!
//! # The `Missing` constructor
//!
//! We detail a special case of constructor splitting that is a bit subtle. Take the following:
//!
//! ```
//! enum Direction { North, South, East, West }
//! # let wind = (Direction::North, 0u8);
//! match wind {
//! (Direction::North, 50..) => {}
//! (_, _) => {}
//! }
//! ```
//!
//! Here we expect constructor splitting to output two cases: `North`, and "everything else". This
//! "everything else" is represented by [`Constructor::Missing`]. Unlike other constructors, it's a
//! bit contextual: to know the exact list of constructors it represents we have to look at the
//! column. In practice however we don't need to, because by construction it only matches rows that
//! have wildcards. This is how this constructor is special: the only constructor that covers it is
//! `Wildcard`.
//!
//! The only place where we care about which constructors `Missing` represents is in diagnostics
//! (see `crate::usefulness::WitnessMatrix::apply_constructor`).
//!
//! We choose whether to specialize with `Missing` in
//! `crate::usefulness::compute_exhaustiveness_and_usefulness`.
//!
//!
//!
//! ## Empty types, empty constructors, and the `exhaustive_patterns` feature
//!
//! An empty type is a type that has no valid value, like `!`, `enum Void {}`, or `Result<!, !>`.
//! They require careful handling.
//!
//! First, for soundness reasons related to the possible existence of invalid values, by default we
//! don't treat empty types as empty. We force them to be matched with wildcards. Except if the
//! `exhaustive_patterns` feature is turned on, in which case we do treat them as empty. And also
//! except if the type has no constructors (like `enum Void {}` but not like `Result<!, !>`), we
//! specifically allow `match void {}` to be exhaustive. There are additionally considerations of
//! place validity that are handled in `crate::usefulness`. Yes this is a bit tricky.
//!
//! The second thing is that regardless of the above, it is always allowed to use all the
//! constructors of a type. For example, all the following is ok:
//!
//! ```rust,ignore(example)
//! # #![feature(never_type)]
//! # #![feature(exhaustive_patterns)]
//! fn foo(x: Option<!>) {
//! match x {
//! None => {}
//! Some(_) => {}
//! }
//! }
//! fn bar(x: &[!]) -> u32 {
//! match x {
//! [] => 1,
//! [_] => 2,
//! [_, _] => 3,
//! }
//! }
//! ```
//!
//! Moreover, take the following:
//!
//! ```rust
//! # #![feature(never_type)]
//! # #![feature(exhaustive_patterns)]
//! # let x = None::<!>;
//! match x {
//! None => {}
//! }
//! ```
//!
//! On a normal type, we would identify `Some` as missing and tell the user. If `x: Option<!>`
//! however (and `exhaustive_patterns` is on), it's ok to omit `Some`. When listing the constructors
//! of a type, we must therefore track which can be omitted.
//!
//! Let's call "empty" a constructor that matches no valid value for the type, like `Some` for the
//! type `Option<!>`. What this all means is that `ConstructorSet` must know which constructors are
//! empty. The difference between empty and nonempty constructors is that empty constructors need
//! not be present for the match to be exhaustive.
//!
//! A final remark: empty constructors of arity 0 break specialization, we must avoid them. The
//! reason is that if we specialize by them, nothing remains to witness the emptiness; the rest of
//! the algorithm can't distinguish them from a nonempty constructor. The only known case where this
//! could happen is the `[..]` pattern on `[!; N]` with `N > 0` so we must take care to not emit it.
//!
//! This is all handled by [`PatCx::ctors_for_ty`] and
//! [`ConstructorSet::split`]. The invariants of [`SplitConstructorSet`] are also of interest.
//!
//!
//! ## Unions
//!
//! Unions allow us to match a value via several overlapping representations at the same time. For
//! example, the following is exhaustive because when seeing the value as a boolean we handled all
//! possible cases (other cases such as `n == 3` would trigger UB).
//!
//! ```rust
//! # fn main() {
//! union U8AsBool {
//! n: u8,
//! b: bool,
//! }
//! let x = U8AsBool { n: 1 };
//! unsafe {
//! match x {
//! U8AsBool { n: 2 } => {}
//! U8AsBool { b: true } => {}
//! U8AsBool { b: false } => {}
//! }
//! }
//! # }
//! ```
//!
//! Pattern-matching has no knowledge that e.g. `false as u8 == 0`, so the values we consider in the
//! algorithm look like `U8AsBool { b: true, n: 2 }`. In other words, for the most part a union is
//! treated like a struct with the same fields. The difference lies in how we construct witnesses of
//! non-exhaustiveness.
//!
//!
//! ## Opaque patterns
//!
//! Some patterns, such as constants that are not allowed to be matched structurally, cannot be
//! inspected, which we handle with `Constructor::Opaque`. Since we know nothing of these patterns,
//! we assume they never cover each other. In order to respect the invariants of
//! [`SplitConstructorSet`], we give each `Opaque` constructor a unique id so we can recognize it.
use std::cmp::{self, max, min, Ordering};
use std::fmt;
use std::iter::once;
use smallvec::SmallVec;
use rustc_apfloat::ieee::{DoubleS, IeeeFloat, SingleS};
use rustc_index::bit_set::{BitSet, GrowableBitSet};
use rustc_index::IndexVec;
use self::Constructor::*;
use self::MaybeInfiniteInt::*;
use self::SliceKind::*;
use crate::PatCx;
/// Whether we have seen a constructor in the column or not.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
enum Presence {
Unseen,
Seen,
}
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum RangeEnd {
Included,
Excluded,
}
impl fmt::Display for RangeEnd {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str(match self {
RangeEnd::Included => "..=",
RangeEnd::Excluded => "..",
})
}
}
/// A possibly infinite integer. Values are encoded such that the ordering on `u128` matches the
/// natural order on the original type. For example, `-128i8` is encoded as `0` and `127i8` as
/// `255`. See `signed_bias` for details.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
pub enum MaybeInfiniteInt {
NegInfinity,
/// Encoded value. DO NOT CONSTRUCT BY HAND; use `new_finite_{int,uint}`.
#[non_exhaustive]
Finite(u128),
PosInfinity,
}
impl MaybeInfiniteInt {
pub fn new_finite_uint(bits: u128) -> Self {
Finite(bits)
}
pub fn new_finite_int(bits: u128, size: u64) -> Self {
// Perform a shift if the underlying types are signed, which makes the interval arithmetic
// type-independent.
let bias = 1u128 << (size - 1);
Finite(bits ^ bias)
}
pub fn as_finite_uint(self) -> Option<u128> {
match self {
Finite(bits) => Some(bits),
_ => None,
}
}
pub fn as_finite_int(self, size: u64) -> Option<u128> {
// We decode the shift.
match self {
Finite(bits) => {
let bias = 1u128 << (size - 1);
Some(bits ^ bias)
}
_ => None,
}
}
/// Note: this will not turn a finite value into an infinite one or vice-versa.
pub fn minus_one(self) -> Option<Self> {
match self {
Finite(n) => n.checked_sub(1).map(Finite),
x => Some(x),
}
}
/// Note: this will turn `u128::MAX` into `PosInfinity`. This means `plus_one` and `minus_one`
/// are not strictly inverses, but that poses no problem in our use of them.
/// this will not turn a finite value into an infinite one or vice-versa.
pub fn plus_one(self) -> Option<Self> {
match self {
Finite(n) => match n.checked_add(1) {
Some(m) => Some(Finite(m)),
None => Some(PosInfinity),
},
x => Some(x),
}
}
}
/// An exclusive interval, used for precise integer exhaustiveness checking. `IntRange`s always
/// store a contiguous range.
///
/// `IntRange` is never used to encode an empty range or a "range" that wraps around the (offset)
/// space: i.e., `range.lo < range.hi`.
#[derive(Clone, Copy, PartialEq, Eq)]
pub struct IntRange {
pub lo: MaybeInfiniteInt, // Must not be `PosInfinity`.
pub hi: MaybeInfiniteInt, // Must not be `NegInfinity`.
}
impl IntRange {
/// Best effort; will not know that e.g. `255u8..` is a singleton.
pub fn is_singleton(&self) -> bool {
// Since `lo` and `hi` can't be the same `Infinity` and `plus_one` never changes from finite
// to infinite, this correctly only detects ranges that contain exacly one `Finite(x)`.
self.lo.plus_one() == Some(self.hi)
}
/// Construct a singleton range.
/// `x` must be a `Finite(_)` value.
#[inline]
pub fn from_singleton(x: MaybeInfiniteInt) -> IntRange {
// `unwrap()` is ok on a finite value
IntRange { lo: x, hi: x.plus_one().unwrap() }
}
/// Construct a range with these boundaries.
/// `lo` must not be `PosInfinity`. `hi` must not be `NegInfinity`.
#[inline]
pub fn from_range(lo: MaybeInfiniteInt, mut hi: MaybeInfiniteInt, end: RangeEnd) -> IntRange {
if end == RangeEnd::Included {
hi = hi.plus_one().unwrap();
}
if lo >= hi {
// This should have been caught earlier by E0030.
panic!("malformed range pattern: {lo:?}..{hi:?}");
}
IntRange { lo, hi }
}
fn is_subrange(&self, other: &Self) -> bool {
other.lo <= self.lo && self.hi <= other.hi
}
fn intersection(&self, other: &Self) -> Option<Self> {
if self.lo < other.hi && other.lo < self.hi {
Some(IntRange { lo: max(self.lo, other.lo), hi: min(self.hi, other.hi) })
} else {
None
}
}
/// Partition a range of integers into disjoint subranges. This does constructor splitting for
/// integer ranges as explained at the top of the file.
///
/// This returns an output that covers `self`. The output is split so that the only
/// intersections between an output range and a column range are inclusions. No output range
/// straddles the boundary of one of the inputs.
///
/// Additionally, we track for each output range whether it is covered by one of the column ranges or not.
///
/// The following input:
/// ```text
/// (--------------------------) // `self`
/// (------) (----------) (-)
/// (------) (--------)
/// ```
/// is first intersected with `self`:
/// ```text
/// (--------------------------) // `self`
/// (----) (----------) (-)
/// (------) (--------)
/// ```
/// and then iterated over as follows:
/// ```text
/// (-(--)-(-)-(------)-)--(-)-
/// ```
/// where each sequence of dashes is an output range, and dashes outside parentheses are marked
/// as `Presence::Missing`.
///
/// ## `isize`/`usize`
///
/// Whereas a wildcard of type `i32` stands for the range `i32::MIN..=i32::MAX`, a `usize`
/// wildcard stands for `0..PosInfinity` and a `isize` wildcard stands for
/// `NegInfinity..PosInfinity`. In other words, as far as `IntRange` is concerned, there are
/// values before `isize::MIN` and after `usize::MAX`/`isize::MAX`.
/// This is to avoid e.g. `0..(u32::MAX as usize)` from being exhaustive on one architecture and
/// not others. This was decided in <https://github.com/rust-lang/rfcs/pull/2591>.
///
/// These infinities affect splitting subtly: it is possible to get `NegInfinity..0` and
/// `usize::MAX+1..PosInfinity` in the output. Diagnostics must be careful to handle these
/// fictitious ranges sensibly.
fn split(
&self,
column_ranges: impl Iterator<Item = IntRange>,
) -> impl Iterator<Item = (Presence, IntRange)> {
// The boundaries of ranges in `column_ranges` intersected with `self`.
// We do parenthesis matching for input ranges. A boundary counts as +1 if it starts
// a range and -1 if it ends it. When the count is > 0 between two boundaries, we
// are within an input range.
let mut boundaries: Vec<(MaybeInfiniteInt, isize)> = column_ranges
.filter_map(|r| self.intersection(&r))
.flat_map(|r| [(r.lo, 1), (r.hi, -1)])
.collect();
// We sort by boundary, and for each boundary we sort the "closing parentheses" first. The
// order of +1/-1 for a same boundary value is actually irrelevant, because we only look at
// the accumulated count between distinct boundary values.
boundaries.sort_unstable();
// Accumulate parenthesis counts.
let mut paren_counter = 0isize;
// Gather pairs of adjacent boundaries.
let mut prev_bdy = self.lo;
boundaries
.into_iter()
// End with the end of the range. The count is ignored.
.chain(once((self.hi, 0)))
// List pairs of adjacent boundaries and the count between them.
.map(move |(bdy, delta)| {
// `delta` affects the count as we cross `bdy`, so the relevant count between
// `prev_bdy` and `bdy` is untouched by `delta`.
let ret = (prev_bdy, paren_counter, bdy);
prev_bdy = bdy;
paren_counter += delta;
ret
})
// Skip empty ranges.
.filter(|&(prev_bdy, _, bdy)| prev_bdy != bdy)
// Convert back to ranges.
.map(move |(prev_bdy, paren_count, bdy)| {
use Presence::*;
let presence = if paren_count > 0 { Seen } else { Unseen };
let range = IntRange { lo: prev_bdy, hi: bdy };
(presence, range)
})
}
}
/// Note: this will render signed ranges incorrectly. To render properly, convert to a pattern
/// first.
impl fmt::Debug for IntRange {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
if self.is_singleton() {
// Only finite ranges can be singletons.
let Finite(lo) = self.lo else { unreachable!() };
write!(f, "{lo}")?;
} else {
if let Finite(lo) = self.lo {
write!(f, "{lo}")?;
}
write!(f, "{}", RangeEnd::Excluded)?;
if let Finite(hi) = self.hi {
write!(f, "{hi}")?;
}
}
Ok(())
}
}
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub enum SliceKind {
/// Patterns of length `n` (`[x, y]`).
FixedLen(usize),
/// Patterns using the `..` notation (`[x, .., y]`).
/// Captures any array constructor of `length >= i + j`.
/// In the case where `array_len` is `Some(_)`,
/// this indicates that we only care about the first `i` and the last `j` values of the array,
/// and everything in between is a wildcard `_`.
VarLen(usize, usize),
}
impl SliceKind {
pub fn arity(self) -> usize {
match self {
FixedLen(length) => length,
VarLen(prefix, suffix) => prefix + suffix,
}
}
/// Whether this pattern includes patterns of length `other_len`.
fn covers_length(self, other_len: usize) -> bool {
match self {
FixedLen(len) => len == other_len,
VarLen(prefix, suffix) => prefix + suffix <= other_len,
}
}
}
/// A constructor for array and slice patterns.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub struct Slice {
/// `None` if the matched value is a slice, `Some(n)` if it is an array of size `n`.
pub(crate) array_len: Option<usize>,
/// The kind of pattern it is: fixed-length `[x, y]` or variable length `[x, .., y]`.
pub(crate) kind: SliceKind,
}
impl Slice {
pub fn new(array_len: Option<usize>, kind: SliceKind) -> Self {
let kind = match (array_len, kind) {
// If the middle `..` has length 0, we effectively have a fixed-length pattern.
(Some(len), VarLen(prefix, suffix)) if prefix + suffix == len => FixedLen(len),
(Some(len), VarLen(prefix, suffix)) if prefix + suffix > len => panic!(
"Slice pattern of length {} longer than its array length {len}",
prefix + suffix
),
_ => kind,
};
Slice { array_len, kind }
}
pub fn arity(self) -> usize {
self.kind.arity()
}
/// See `Constructor::is_covered_by`
fn is_covered_by(self, other: Self) -> bool {
other.kind.covers_length(self.arity())
}
/// This computes constructor splitting for variable-length slices, as explained at the top of
/// the file.
///
/// A slice pattern `[x, .., y]` behaves like the infinite or-pattern `[x, y] | [x, _, y] | [x,
/// _, _, y] | etc`. The corresponding value constructors are fixed-length array constructors of
/// corresponding lengths. We obviously can't list this infinitude of constructors.
/// Thankfully, it turns out that for each finite set of slice patterns, all sufficiently large
/// array lengths are equivalent.
///
/// Let's look at an example, where we are trying to split the last pattern:
/// ```
/// # fn foo(x: &[bool]) {
/// match x {
/// [true, true, ..] => {}
/// [.., false, false] => {}
/// [..] => {}
/// }
/// # }
/// ```
/// Here are the results of specialization for the first few lengths:
/// ```
/// # fn foo(x: &[bool]) { match x {
/// // length 0
/// [] => {}
/// // length 1
/// [_] => {}
/// // length 2
/// [true, true] => {}
/// [false, false] => {}
/// [_, _] => {}
/// // length 3
/// [true, true, _ ] => {}
/// [_, false, false] => {}
/// [_, _, _ ] => {}
/// // length 4
/// [true, true, _, _ ] => {}
/// [_, _, false, false] => {}
/// [_, _, _, _ ] => {}
/// // length 5
/// [true, true, _, _, _ ] => {}
/// [_, _, _, false, false] => {}
/// [_, _, _, _, _ ] => {}
/// # _ => {}
/// # }}
/// ```
///
/// We see that above length 4, we are simply inserting columns full of wildcards in the middle.
/// This means that specialization and witness computation with slices of length `l >= 4` will
/// give equivalent results regardless of `l`. This applies to any set of slice patterns: there
/// will be a length `L` above which all lengths behave the same. This is exactly what we need
/// for constructor splitting.
///
/// A variable-length slice pattern covers all lengths from its arity up to infinity. As we just
/// saw, we can split this in two: lengths below `L` are treated individually with a
/// fixed-length slice each; lengths above `L` are grouped into a single variable-length slice
/// constructor.
///
/// For each variable-length slice pattern `p` with a prefix of length `plₚ` and suffix of
/// length `slₚ`, only the first `plₚ` and the last `slₚ` elements are examined. Therefore, as
/// long as `L` is positive (to avoid concerns about empty types), all elements after the
/// maximum prefix length and before the maximum suffix length are not examined by any
/// variable-length pattern, and therefore can be ignored. This gives us a way to compute `L`.
///
/// Additionally, if fixed-length patterns exist, we must pick an `L` large enough to miss them,
/// so we can pick `L = max(max(FIXED_LEN)+1, max(PREFIX_LEN) + max(SUFFIX_LEN))`.
/// `max_slice` below will be made to have this arity `L`.
///
/// If `self` is fixed-length, it is returned as-is.
///
/// Additionally, we track for each output slice whether it is covered by one of the column slices or not.
fn split(
self,
column_slices: impl Iterator<Item = Slice>,
) -> impl Iterator<Item = (Presence, Slice)> {
// Range of lengths below `L`.
let smaller_lengths;
let arity = self.arity();
let mut max_slice = self.kind;
// Tracks the smallest variable-length slice we've seen. Any slice arity above it is
// therefore `Presence::Seen` in the column.
let mut min_var_len = usize::MAX;
// Tracks the fixed-length slices we've seen, to mark them as `Presence::Seen`.
let mut seen_fixed_lens = GrowableBitSet::new_empty();
match &mut max_slice {
VarLen(max_prefix_len, max_suffix_len) => {
// A length larger than any fixed-length slice encountered.
// We start at 1 in case the subtype is empty because in that case the zero-length
// slice must be treated separately from the rest.
let mut fixed_len_upper_bound = 1;
// We grow `max_slice` to be larger than all slices encountered, as described above.
// `L` is `max_slice.arity()`. For diagnostics, we keep the prefix and suffix
// lengths separate.
for slice in column_slices {
match slice.kind {
FixedLen(len) => {
fixed_len_upper_bound = cmp::max(fixed_len_upper_bound, len + 1);
seen_fixed_lens.insert(len);
}
VarLen(prefix, suffix) => {
*max_prefix_len = cmp::max(*max_prefix_len, prefix);
*max_suffix_len = cmp::max(*max_suffix_len, suffix);
min_var_len = cmp::min(min_var_len, prefix + suffix);
}
}
}
// If `fixed_len_upper_bound >= L`, we set `L` to `fixed_len_upper_bound`.
if let Some(delta) =
fixed_len_upper_bound.checked_sub(*max_prefix_len + *max_suffix_len)
{
*max_prefix_len += delta
}
// We cap the arity of `max_slice` at the array size.
match self.array_len {
Some(len) if max_slice.arity() >= len => max_slice = FixedLen(len),
_ => {}
}
smaller_lengths = match self.array_len {
// The only admissible fixed-length slice is one of the array size. Whether `max_slice`
// is fixed-length or variable-length, it will be the only relevant slice to output
// here.
Some(_) => 0..0, // empty range
// We need to cover all arities in the range `(arity..infinity)`. We split that
// range into two: lengths smaller than `max_slice.arity()` are treated
// independently as fixed-lengths slices, and lengths above are captured by
// `max_slice`.
None => self.arity()..max_slice.arity(),
};
}
FixedLen(_) => {
// No need to split here. We only track presence.
for slice in column_slices {
match slice.kind {
FixedLen(len) => {
if len == arity {
seen_fixed_lens.insert(len);
}
}
VarLen(prefix, suffix) => {
min_var_len = cmp::min(min_var_len, prefix + suffix);
}
}
}
smaller_lengths = 0..0;
}
};
smaller_lengths.map(FixedLen).chain(once(max_slice)).map(move |kind| {
let arity = kind.arity();
let seen = if min_var_len <= arity || seen_fixed_lens.contains(arity) {
Presence::Seen
} else {
Presence::Unseen
};
(seen, Slice::new(self.array_len, kind))
})
}
}
/// A globally unique id to distinguish `Opaque` patterns.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct OpaqueId(u32);
impl OpaqueId {
pub fn new() -> Self {
use std::sync::atomic::{AtomicU32, Ordering};
static OPAQUE_ID: AtomicU32 = AtomicU32::new(0);
OpaqueId(OPAQUE_ID.fetch_add(1, Ordering::SeqCst))
}
}
/// A value can be decomposed into a constructor applied to some fields. This struct represents
/// the constructor. See also `Fields`.
///
/// `pat_constructor` retrieves the constructor corresponding to a pattern.
/// `specialize_constructor` returns the list of fields corresponding to a pattern, given a
/// constructor. `Constructor::apply` reconstructs the pattern from a pair of `Constructor` and
/// `Fields`.
#[derive(Debug)]
pub enum Constructor<Cx: PatCx> {
/// Tuples and structs.
Struct,
/// Enum variants.
Variant(Cx::VariantIdx),
/// References
Ref,
/// Array and slice patterns.
Slice(Slice),
/// Union field accesses.
UnionField,
/// Booleans
Bool(bool),
/// Ranges of integer literal values (`2`, `2..=5` or `2..5`).
IntRange(IntRange),
/// Ranges of floating-point literal values (`2.0..=5.2`).
F32Range(IeeeFloat<SingleS>, IeeeFloat<SingleS>, RangeEnd),
F64Range(IeeeFloat<DoubleS>, IeeeFloat<DoubleS>, RangeEnd),
/// String literals. Strings are not quite the same as `&[u8]` so we treat them separately.
Str(Cx::StrLit),
/// Constants that must not be matched structurally. They are treated as black boxes for the
/// purposes of exhaustiveness: we must not inspect them, and they don't count towards making a
/// match exhaustive.
/// Carries an id that must be unique within a match. We need this to ensure the invariants of
/// [`SplitConstructorSet`].
Opaque(OpaqueId),
/// Or-pattern.
Or,
/// Wildcard pattern.
Wildcard,
/// Never pattern. Only used in `WitnessPat`. An actual never pattern should be lowered as
/// `Wildcard`.
Never,
/// Fake extra constructor for enums that aren't allowed to be matched exhaustively. Also used
/// for those types for which we cannot list constructors explicitly, like `f64` and `str`. Only
/// used in `WitnessPat`.
NonExhaustive,
/// Fake extra constructor for variants that should not be mentioned in diagnostics. We use this
/// for variants behind an unstable gate as well as `#[doc(hidden)]` ones. Only used in
/// `WitnessPat`.
Hidden,
/// Fake extra constructor for constructors that are not seen in the matrix, as explained at the
/// top of the file. Only used for specialization.
Missing,
/// Fake extra constructor that indicates and empty field that is private. When we encounter one
/// we skip the column entirely so we don't observe its emptiness. Only used for specialization.
PrivateUninhabited,
}
impl<Cx: PatCx> Clone for Constructor<Cx> {
fn clone(&self) -> Self {
match self {
Constructor::Struct => Constructor::Struct,
Constructor::Variant(idx) => Constructor::Variant(*idx),
Constructor::Ref => Constructor::Ref,
Constructor::Slice(slice) => Constructor::Slice(*slice),
Constructor::UnionField => Constructor::UnionField,
Constructor::Bool(b) => Constructor::Bool(*b),
Constructor::IntRange(range) => Constructor::IntRange(*range),
Constructor::F32Range(lo, hi, end) => Constructor::F32Range(lo.clone(), *hi, *end),
Constructor::F64Range(lo, hi, end) => Constructor::F64Range(lo.clone(), *hi, *end),
Constructor::Str(value) => Constructor::Str(value.clone()),
Constructor::Opaque(inner) => Constructor::Opaque(inner.clone()),
Constructor::Or => Constructor::Or,
Constructor::Never => Constructor::Never,
Constructor::Wildcard => Constructor::Wildcard,
Constructor::NonExhaustive => Constructor::NonExhaustive,
Constructor::Hidden => Constructor::Hidden,
Constructor::Missing => Constructor::Missing,
Constructor::PrivateUninhabited => Constructor::PrivateUninhabited,
}
}
}
impl<Cx: PatCx> Constructor<Cx> {
pub(crate) fn is_non_exhaustive(&self) -> bool {
matches!(self, NonExhaustive)
}
pub(crate) fn as_variant(&self) -> Option<Cx::VariantIdx> {
match self {
Variant(i) => Some(*i),
_ => None,
}
}
fn as_bool(&self) -> Option<bool> {
match self {
Bool(b) => Some(*b),
_ => None,
}
}
pub(crate) fn as_int_range(&self) -> Option<&IntRange> {
match self {
IntRange(range) => Some(range),
_ => None,
}
}
fn as_slice(&self) -> Option<Slice> {
match self {
Slice(slice) => Some(*slice),
_ => None,
}
}
/// The number of fields for this constructor. This must be kept in sync with
/// `Fields::wildcards`.
pub(crate) fn arity(&self, cx: &Cx, ty: &Cx::Ty) -> usize {
cx.ctor_arity(self, ty)
}
/// Returns whether `self` is covered by `other`, i.e. whether `self` is a subset of `other`.
/// For the simple cases, this is simply checking for equality. For the "grouped" constructors,
/// this checks for inclusion.
// We inline because this has a single call site in `Matrix::specialize_constructor`.
#[inline]
pub(crate) fn is_covered_by(&self, cx: &Cx, other: &Self) -> Result<bool, Cx::Error> {
Ok(match (self, other) {
(Wildcard, _) => {
return Err(cx.bug(format_args!(
"Constructor splitting should not have returned `Wildcard`"
)));
}
// Wildcards cover anything
(_, Wildcard) => true,
// `PrivateUninhabited` skips everything.
(PrivateUninhabited, _) => true,
// Only a wildcard pattern can match these special constructors.
(Missing { .. } | NonExhaustive | Hidden, _) => false,
(Struct, Struct) => true,
(Ref, Ref) => true,
(UnionField, UnionField) => true,
(Variant(self_id), Variant(other_id)) => self_id == other_id,
(Bool(self_b), Bool(other_b)) => self_b == other_b,
(IntRange(self_range), IntRange(other_range)) => self_range.is_subrange(other_range),
(F32Range(self_from, self_to, self_end), F32Range(other_from, other_to, other_end)) => {
self_from.ge(other_from)
&& match self_to.partial_cmp(other_to) {
Some(Ordering::Less) => true,
Some(Ordering::Equal) => other_end == self_end,
_ => false,
}
}
(F64Range(self_from, self_to, self_end), F64Range(other_from, other_to, other_end)) => {
self_from.ge(other_from)
&& match self_to.partial_cmp(other_to) {
Some(Ordering::Less) => true,
Some(Ordering::Equal) => other_end == self_end,
_ => false,
}
}
(Str(self_val), Str(other_val)) => {
// FIXME Once valtrees are available we can directly use the bytes
// in the `Str` variant of the valtree for the comparison here.
self_val == other_val
}
(Slice(self_slice), Slice(other_slice)) => self_slice.is_covered_by(*other_slice),
// Opaque constructors don't interact with anything unless they come from the
// syntactically identical pattern.
(Opaque(self_id), Opaque(other_id)) => self_id == other_id,
(Opaque(..), _) | (_, Opaque(..)) => false,
_ => {
return Err(cx.bug(format_args!(
"trying to compare incompatible constructors {self:?} and {other:?}"
)));
}
})
}
pub(crate) fn fmt_fields(
&self,
f: &mut fmt::Formatter<'_>,
ty: &Cx::Ty,
mut fields: impl Iterator<Item = impl fmt::Debug>,
) -> fmt::Result {
let mut first = true;
let mut start_or_continue = |s| {
if first {
first = false;
""
} else {
s
}
};
let mut start_or_comma = || start_or_continue(", ");
match self {
Struct | Variant(_) | UnionField => {
Cx::write_variant_name(f, self, ty)?;
// Without `cx`, we can't know which field corresponds to which, so we can't
// get the names of the fields. Instead we just display everything as a tuple
// struct, which should be good enough.
write!(f, "(")?;
for p in fields {
write!(f, "{}{:?}", start_or_comma(), p)?;
}
write!(f, ")")?;
}
// Note: given the expansion of `&str` patterns done in `expand_pattern`, we should
// be careful to detect strings here. However a string literal pattern will never
// be reported as a non-exhaustiveness witness, so we can ignore this issue.
Ref => {
write!(f, "&{:?}", &fields.next().unwrap())?;
}
Slice(slice) => {
write!(f, "[")?;
match slice.kind {
SliceKind::FixedLen(_) => {
for p in fields {
write!(f, "{}{:?}", start_or_comma(), p)?;
}
}
SliceKind::VarLen(prefix_len, _) => {
for p in fields.by_ref().take(prefix_len) {
write!(f, "{}{:?}", start_or_comma(), p)?;
}
write!(f, "{}..", start_or_comma())?;
for p in fields {
write!(f, "{}{:?}", start_or_comma(), p)?;
}
}
}
write!(f, "]")?;
}
Bool(b) => write!(f, "{b}")?,
// Best-effort, will render signed ranges incorrectly
IntRange(range) => write!(f, "{range:?}")?,
F32Range(lo, hi, end) => write!(f, "{lo}{end}{hi}")?,
F64Range(lo, hi, end) => write!(f, "{lo}{end}{hi}")?,
Str(value) => write!(f, "{value:?}")?,
Opaque(..) => write!(f, "<constant pattern>")?,
Or => {
for pat in fields {
write!(f, "{}{:?}", start_or_continue(" | "), pat)?;
}
}
Never => write!(f, "!")?,
Wildcard | Missing | NonExhaustive | Hidden | PrivateUninhabited => {
write!(f, "_ : {:?}", ty)?
}
}
Ok(())
}
}
#[derive(Debug, Clone, Copy)]
pub enum VariantVisibility {
/// Variant that doesn't fit the other cases, i.e. most variants.
Visible,
/// Variant behind an unstable gate or with the `#[doc(hidden)]` attribute. It will not be
/// mentioned in diagnostics unless the user mentioned it first.
Hidden,
/// Variant that matches no value. E.g. `Some::<Option<!>>` if the `exhaustive_patterns` feature
/// is enabled. Like `Hidden`, it will not be mentioned in diagnostics unless the user mentioned
/// it first.
Empty,
}
/// Describes the set of all constructors for a type. For details, in particular about the emptiness
/// of constructors, see the top of the file.
///
/// In terms of division of responsibility, [`ConstructorSet::split`] handles all of the
/// `exhaustive_patterns` feature.
#[derive(Debug)]
pub enum ConstructorSet<Cx: PatCx> {
/// The type is a tuple or struct. `empty` tracks whether the type is empty.
Struct { empty: bool },
/// This type has the following list of constructors. If `variants` is empty and
/// `non_exhaustive` is false, don't use this; use `NoConstructors` instead.
Variants { variants: IndexVec<Cx::VariantIdx, VariantVisibility>, non_exhaustive: bool },
/// The type is `&T`.
Ref,
/// The type is a union.
Union,
/// Booleans.
Bool,
/// The type is spanned by integer values. The range or ranges give the set of allowed values.
/// The second range is only useful for `char`.
Integers { range_1: IntRange, range_2: Option<IntRange> },
/// The type is matched by slices. `array_len` is the compile-time length of the array, if
/// known. If `subtype_is_empty`, all constructors are empty except possibly the zero-length
/// slice `[]`.
Slice { array_len: Option<usize>, subtype_is_empty: bool },
/// The constructors cannot be listed, and the type cannot be matched exhaustively. E.g. `str`,
/// floats.
Unlistable,
/// The type has no constructors (not even empty ones). This is `!` and empty enums.
NoConstructors,
}
/// Describes the result of analyzing the constructors in a column of a match.
///
/// `present` is morally the set of constructors present in the column, and `missing` is the set of
/// constructors that exist in the type but are not present in the column.
///
/// More formally, if we discard wildcards from the column, this respects the following constraints:
/// 1. the union of `present`, `missing` and `missing_empty` covers all the constructors of the type
/// 2. each constructor in `present` is covered by something in the column
/// 3. no constructor in `missing` or `missing_empty` is covered by anything in the column
/// 4. each constructor in the column is equal to the union of one or more constructors in `present`
/// 5. `missing` does not contain empty constructors (see discussion about emptiness at the top of
/// the file);
/// 6. `missing_empty` contains only empty constructors
/// 7. constructors in `present`, `missing` and `missing_empty` are split for the column; in other
/// words, they are either fully included in or fully disjoint from each constructor in the
/// column. In yet other words, there are no non-trivial intersections like between `0..10` and
/// `5..15`.
///
/// We must be particularly careful with weird constructors like `Opaque`: they're not formally part
/// of the `ConstructorSet` for the type, yet if we forgot to include them in `present` we would be
/// ignoring any row with `Opaque`s in the algorithm. Hence the importance of point 4.
#[derive(Debug)]
pub struct SplitConstructorSet<Cx: PatCx> {
pub present: SmallVec<[Constructor<Cx>; 1]>,
pub missing: Vec<Constructor<Cx>>,
pub missing_empty: Vec<Constructor<Cx>>,
}
impl<Cx: PatCx> ConstructorSet<Cx> {
/// This analyzes a column of constructors to 1/ determine which constructors of the type (if
/// any) are missing; 2/ split constructors to handle non-trivial intersections e.g. on ranges
/// or slices. This can get subtle; see [`SplitConstructorSet`] for details of this operation
/// and its invariants.
pub fn split<'a>(
&self,
ctors: impl Iterator<Item = &'a Constructor<Cx>> + Clone,
) -> SplitConstructorSet<Cx>
where
Cx: 'a,
{
let mut present: SmallVec<[_; 1]> = SmallVec::new();
// Empty constructors found missing.
let mut missing_empty = Vec::new();
// Nonempty constructors found missing.
let mut missing = Vec::new();
// Constructors in `ctors`, except wildcards and opaques.
let mut seen = Vec::new();
for ctor in ctors.cloned() {
match ctor {
Opaque(..) => present.push(ctor),
Wildcard => {} // discard wildcards
_ => seen.push(ctor),
}
}
match self {
ConstructorSet::Struct { empty } => {
if !seen.is_empty() {
present.push(Struct);
} else if *empty {
missing_empty.push(Struct);
} else {
missing.push(Struct);
}
}
ConstructorSet::Ref => {
if !seen.is_empty() {
present.push(Ref);
} else {
missing.push(Ref);
}
}
ConstructorSet::Union => {
if !seen.is_empty() {
present.push(UnionField);
} else {
missing.push(UnionField);
}
}
ConstructorSet::Variants { variants, non_exhaustive } => {
let mut seen_set = BitSet::new_empty(variants.len());
for idx in seen.iter().filter_map(|c| c.as_variant()) {
seen_set.insert(idx);
}
let mut skipped_a_hidden_variant = false;
for (idx, visibility) in variants.iter_enumerated() {
let ctor = Variant(idx);
if seen_set.contains(idx) {
present.push(ctor);
} else {
// We only put visible variants directly into `missing`.
match visibility {
VariantVisibility::Visible => missing.push(ctor),
VariantVisibility::Hidden => skipped_a_hidden_variant = true,
VariantVisibility::Empty => missing_empty.push(ctor),
}
}
}
if skipped_a_hidden_variant {
missing.push(Hidden);
}
if *non_exhaustive {
missing.push(NonExhaustive);
}
}
ConstructorSet::Bool => {
let mut seen_false = false;
let mut seen_true = false;
for b in seen.iter().filter_map(|ctor| ctor.as_bool()) {
if b {
seen_true = true;
} else {
seen_false = true;
}
}
if seen_false {
present.push(Bool(false));
} else {
missing.push(Bool(false));
}
if seen_true {
present.push(Bool(true));
} else {
missing.push(Bool(true));
}
}
ConstructorSet::Integers { range_1, range_2 } => {
let seen_ranges: Vec<_> =
seen.iter().filter_map(|ctor| ctor.as_int_range()).copied().collect();
for (seen, splitted_range) in range_1.split(seen_ranges.iter().cloned()) {
match seen {
Presence::Unseen => missing.push(IntRange(splitted_range)),
Presence::Seen => present.push(IntRange(splitted_range)),
}
}
if let Some(range_2) = range_2 {
for (seen, splitted_range) in range_2.split(seen_ranges.into_iter()) {
match seen {
Presence::Unseen => missing.push(IntRange(splitted_range)),
Presence::Seen => present.push(IntRange(splitted_range)),
}
}
}
}
ConstructorSet::Slice { array_len, subtype_is_empty } => {
let seen_slices = seen.iter().filter_map(|c| c.as_slice());
let base_slice = Slice::new(*array_len, VarLen(0, 0));
for (seen, splitted_slice) in base_slice.split(seen_slices) {
let ctor = Slice(splitted_slice);
match seen {
Presence::Seen => present.push(ctor),
Presence::Unseen => {
if *subtype_is_empty && splitted_slice.arity() != 0 {
// We have subpatterns of an empty type, so the constructor is
// empty.
missing_empty.push(ctor);
} else {
missing.push(ctor);
}
}
}
}
}
ConstructorSet::Unlistable => {
// Since we can't list constructors, we take the ones in the column. This might list
// some constructors several times but there's not much we can do.
present.extend(seen);
missing.push(NonExhaustive);
}
ConstructorSet::NoConstructors => {
// In a `MaybeInvalid` place even an empty pattern may be reachable. We therefore
// add a dummy empty constructor here, which will be ignored if the place is
// `ValidOnly`.
missing_empty.push(Never);
}
}
SplitConstructorSet { present, missing, missing_empty }
}
/// Whether this set only contains empty constructors.
pub(crate) fn all_empty(&self) -> bool {
match self {
ConstructorSet::Bool
| ConstructorSet::Integers { .. }
| ConstructorSet::Ref
| ConstructorSet::Union
| ConstructorSet::Unlistable => false,
ConstructorSet::NoConstructors => true,
ConstructorSet::Struct { empty } => *empty,
ConstructorSet::Variants { variants, non_exhaustive } => {
!*non_exhaustive
&& variants
.iter()
.all(|visibility| matches!(visibility, VariantVisibility::Empty))
}
ConstructorSet::Slice { array_len, subtype_is_empty } => {
*subtype_is_empty && matches!(array_len, Some(1..))
}
}
}
}