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Auto merge of #71930 - Nadrieril:exhaustiveness-remove-tyerr, r=varkor 

De-abuse TyKind::Error in exhaustiveness checking

Replaces https://github.com/rust-lang/rust/pull/71074. Context: https://github.com/rust-lang/rust/issues/70866.

In order to remove the use of `TyKind::Error`, I had to make sure we skip over those fields whose inhabitedness should not be observed. This is potentially error-prone however, since we must be careful not to mix filtered and unfiltered lists of patterns. I managed to hide away most of the filtering behind a new `Fields` struct, that I used everywhere relevant. I quite like the result; I think the twin concepts of `Constructor` and `Fields` make a good mental model.

As usual, I tried to separate commits that shuffle code around from commits that require more thought to review.

cc @varkor @Centril
This commit is contained in:
bors 2020-05-21 18:52:46 +00:00
parent 148c125b1b d7e1d5f0c2
commit 9310e3bd4f
4 changed files with 427 additions and 260 deletions
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661
src/librustc_mir_build/hair/pattern/_match.rs
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@ -1,5 +1,5 @@
/// Note: most tests relevant to this file can be found (at the time of writing)
/// in src/tests/ui/pattern/usefulness.
/// Note: most of the tests relevant to this file can be found (at the time of writing) in
/// src/tests/ui/pattern/usefulness.
///
/// This file includes the logic for exhaustiveness and usefulness checking for
/// pattern-matching. Specifically, given a list of patterns for a type, we can
@ -13,6 +13,8 @@
/// summarise the algorithm here to hopefully save time and be a little clearer
/// (without being so rigorous).
///
/// # Premise
///
/// The core of the algorithm revolves about a "usefulness" check. In particular, we
/// are trying to compute a predicate `U(P, p)` where `P` is a list of patterns (we refer to this as
/// a matrix). `U(P, p)` represents whether, given an existing list of patterns
@ -27,8 +29,51 @@
/// pattern to those that have come before it doesn't increase the number of values
/// we're matching).
///
/// # Core concept
///
/// The idea that powers everything that is done in this file is the following: a value is made
/// from a constructor applied to some fields. 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`). Fields are just a (possibly empty) list of values.
///
/// Some of the constructors listed above might feel weird: `None` and `2` don't take any
/// arguments. This is part of what makes constructors so general: we will consider plain values
/// like numbers and string literals to be constructors that take no arguments, also called "0-ary
/// constructors"; they are the simplest case of constructors. This allows us to see any value as
/// made up from a tree of constructors, each having a given number of children. For example:
/// `(None, Ok(0))` is made from 4 different constructors.
///
/// This idea can be extended to patterns: a pattern captures a set of possible values, and we can
/// describe this set using constructors. For example, `Err(_)` captures all values of the type
/// `Result<T, E>` that start with the `Err` constructor (for some choice of `T` and `E`). The
/// wildcard `_` captures all values of the given type starting with any of the constructors for
/// that type.
///
/// We use this to compute whether different patterns might capture a same value. Do the patterns
/// `Ok("foo")` and `Err(_)` capture a common value? The answer is no, because the first pattern
/// captures only values starting with the `Ok` constructor and the second only values starting
/// with the `Err` constructor. Do the patterns `Some(42)` and `Some(1..10)` intersect? They might,
/// since they both capture values starting with `Some`. To be certain, we need to dig under the
/// `Some` constructor and continue asking the question. This is the main idea behind the
/// exhaustiveness algorithm: by looking at patterns constructor-by-constructor, we can efficiently
/// figure out if some new pattern might capture a value that hadn't been captured by previous
/// patterns.
///
/// Constructors are represented by the `Constructor` enum, and its fields by the `Fields` enum.
/// Most of the complexity of this file resides in transforming between patterns and
/// (`Constructor`, `Fields`) pairs, handling all the special cases correctly.
///
/// Caveat: this constructors/fields distinction doesn't quite cover every Rust value. For example
/// a value of type `Rc<u64>` doesn't fit this idea very well, nor do various other things.
/// However, this idea covers most of the cases that are relevant to exhaustiveness checking.
///
///
/// # Algorithm
///
/// Recall that `U(P, p)` represents whether, given an existing list of patterns (aka matrix) `P`,
/// adding a new pattern `p` will cover previously-uncovered values of the type.
/// During the course of the algorithm, the rows of the matrix won't just be individual patterns,
/// but rather partially-deconstructed patterns in the form of a list of patterns. The paper
/// but rather partially-deconstructed patterns in the form of a list of fields. The paper
/// calls those pattern-vectors, and we will call them pattern-stacks. The same holds for the
/// new pattern `p`.
///
@ -242,7 +287,7 @@ use rustc_hir::{HirId, RangeEnd};
use rustc_middle::mir::interpret::{truncate, AllocId, ConstValue, Pointer, Scalar};
use rustc_middle::mir::Field;
use rustc_middle::ty::layout::IntegerExt;
use rustc_middle::ty::{self, Const, Ty, TyCtxt, TypeFoldable, VariantDef};
use rustc_middle::ty::{self, Const, Ty, TyCtxt};
use rustc_session::lint;
use rustc_span::{Span, DUMMY_SP};
use rustc_target::abi::{Integer, Size, VariantIdx};
@ -441,13 +486,11 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
&self,
cx: &mut MatchCheckCtxt<'p, 'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &'p [Pat<'tcx>],
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Option<PatStack<'p, 'tcx>> {
let new_heads = specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns);
new_heads.map(|mut new_head| {
new_head.0.extend_from_slice(&self.0[1..]);
new_head
})
let new_fields =
specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns)?;
Some(new_fields.push_on_patstack(&self.0[1..]))
}
}
@ -503,7 +546,7 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
&self,
cx: &mut MatchCheckCtxt<'p, 'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &'p [Pat<'tcx>],
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Matrix<'p, 'tcx> {
self.0
.iter()
@ -593,7 +636,7 @@ impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
}
}
// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
/// Returns whether the given type is an enum from another crate declared `#[non_exhaustive]`.
crate fn is_foreign_non_exhaustive_enum(&self, ty: Ty<'tcx>) -> bool {
match ty.kind {
ty::Adt(def, ..) => {
@ -602,15 +645,6 @@ impl<'a, 'tcx> MatchCheckCtxt<'a, 'tcx> {
_ => false,
}
}
// Returns whether the given variant is from another crate and has its fields declared
// `#[non_exhaustive]`.
fn is_foreign_non_exhaustive_variant(&self, ty: Ty<'tcx>, variant: &VariantDef) -> bool {
match ty.kind {
ty::Adt(def, ..) => variant.is_field_list_non_exhaustive() && !def.did.is_local(),
_ => false,
}
}
}
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
@ -722,10 +756,17 @@ impl Slice {
}
}
/// 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_one_pattern` 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(Clone, Debug, PartialEq)]
enum Constructor<'tcx> {
/// The constructor of all patterns that don't vary by constructor,
/// e.g., struct patterns and fixed-length arrays.
/// The constructor for patterns that have a single constructor, like tuples, struct patterns
/// and fixed-length arrays.
Single,
/// Enum variants.
Variant(DefId),
@ -850,107 +891,10 @@ impl<'tcx> Constructor<'tcx> {
}
}
/// This returns one wildcard pattern for each argument to this constructor.
///
/// This must be consistent with `apply`, `specialize_one_pattern`, and `arity`.
fn wildcard_subpatterns<'a>(
&self,
cx: &MatchCheckCtxt<'a, 'tcx>,
ty: Ty<'tcx>,
) -> Vec<Pat<'tcx>> {
debug!("wildcard_subpatterns({:#?}, {:?})", self, ty);
match self {
Single | Variant(_) => match ty.kind {
ty::Tuple(ref fs) => {
fs.into_iter().map(|t| t.expect_ty()).map(Pat::wildcard_from_ty).collect()
}
ty::Ref(_, rty, _) => vec![Pat::wildcard_from_ty(rty)],
ty::Adt(adt, substs) => {
if adt.is_box() {
// Use T as the sub pattern type of Box<T>.
vec![Pat::wildcard_from_ty(substs.type_at(0))]
} else {
let variant = &adt.variants[self.variant_index_for_adt(cx, adt)];
let is_non_exhaustive = cx.is_foreign_non_exhaustive_variant(ty, variant);
variant
.fields
.iter()
.map(|field| {
let is_visible = adt.is_enum()
|| field.vis.is_accessible_from(cx.module, cx.tcx);
let is_uninhabited = cx.is_uninhabited(field.ty(cx.tcx, substs));
match (is_visible, is_non_exhaustive, is_uninhabited) {
// Treat all uninhabited types in non-exhaustive variants as
// `TyErr`.
(_, true, true) => cx.tcx.types.err,
// Treat all non-visible fields as `TyErr`. They can't appear
// in any other pattern from this match (because they are
// private), so their type does not matter - but we don't want
// to know they are uninhabited.
(false, ..) => cx.tcx.types.err,
(true, ..) => {
let ty = field.ty(cx.tcx, substs);
match ty.kind {
// If the field type returned is an array of an unknown
// size return an TyErr.
ty::Array(_, len)
if len
.try_eval_usize(cx.tcx, cx.param_env)
.is_none() =>
{
cx.tcx.types.err
}
_ => ty,
}
}
}
})
.map(Pat::wildcard_from_ty)
.collect()
}
}
_ => vec![],
},
Slice(_) => match ty.kind {
ty::Slice(ty) | ty::Array(ty, _) => {
let arity = self.arity(cx, ty);
(0..arity).map(|_| Pat::wildcard_from_ty(ty)).collect()
}
_ => bug!("bad slice pattern {:?} {:?}", self, ty),
},
ConstantValue(..) | FloatRange(..) | IntRange(..) | NonExhaustive => vec![],
}
}
/// This computes the arity of a constructor. The arity of a constructor
/// is how many subpattern patterns of that constructor should be expanded to.
///
/// For instance, a tuple pattern `(_, 42, Some([]))` has the arity of 3.
/// A struct pattern's arity is the number of fields it contains, etc.
///
/// This must be consistent with `wildcard_subpatterns`, `specialize_one_pattern`, and `apply`.
fn arity<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> u64 {
debug!("Constructor::arity({:#?}, {:?})", self, ty);
match self {
Single | Variant(_) => match ty.kind {
ty::Tuple(ref fs) => fs.len() as u64,
ty::Slice(..) | ty::Array(..) => bug!("bad slice pattern {:?} {:?}", self, ty),
ty::Ref(..) => 1,
ty::Adt(adt, _) => {
adt.variants[self.variant_index_for_adt(cx, adt)].fields.len() as u64
}
_ => 0,
},
Slice(slice) => slice.arity(),
ConstantValue(..) | FloatRange(..) | IntRange(..) | NonExhaustive => 0,
}
}
/// Apply a constructor to a list of patterns, yielding a new pattern. `pats`
/// must have as many elements as this constructor's arity.
///
/// This must be consistent with `wildcard_subpatterns`, `specialize_one_pattern`, and `arity`.
/// This is roughly the inverse of `specialize_one_pattern`.
///
/// Examples:
/// `self`: `Constructor::Single`
@ -962,13 +906,13 @@ impl<'tcx> Constructor<'tcx> {
/// `ty`: `Option<bool>`
/// `pats`: `[false]`
/// returns `Some(false)`
fn apply<'a>(
fn apply<'p>(
&self,
cx: &MatchCheckCtxt<'a, 'tcx>,
cx: &MatchCheckCtxt<'p, 'tcx>,
ty: Ty<'tcx>,
pats: impl IntoIterator<Item = Pat<'tcx>>,
fields: Fields<'p, 'tcx>,
) -> Pat<'tcx> {
let mut subpatterns = pats.into_iter();
let mut subpatterns = fields.all_patterns();
let pat = match self {
Single | Variant(_) => match ty.kind {
@ -1033,8 +977,263 @@ impl<'tcx> Constructor<'tcx> {
/// Like `apply`, but where all the subpatterns are wildcards `_`.
fn apply_wildcards<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {
let subpatterns = self.wildcard_subpatterns(cx, ty).into_iter().rev();
self.apply(cx, ty, subpatterns)
self.apply(cx, ty, Fields::wildcards(cx, self, ty))
}
}
/// Some fields need to be explicitly hidden away in certain cases; see the comment above the
/// `Fields` struct. This struct represents such a potentially-hidden field. When a field is hidden
/// we still keep its type around.
#[derive(Debug, Copy, Clone)]
enum FilteredField<'p, 'tcx> {
Kept(&'p Pat<'tcx>),
Hidden(Ty<'tcx>),
}
impl<'p, 'tcx> FilteredField<'p, 'tcx> {
fn kept(self) -> Option<&'p Pat<'tcx>> {
match self {
FilteredField::Kept(p) => Some(p),
FilteredField::Hidden(_) => None,
}
}
fn to_pattern(self) -> Pat<'tcx> {
match self {
FilteredField::Kept(p) => p.clone(),
FilteredField::Hidden(ty) => Pat::wildcard_from_ty(ty),
}
}
}
/// A value can be decomposed into a constructor applied to some fields. This struct represents
/// those fields, generalized to allow patterns in each field. See also `Constructor`.
///
/// If a private or `non_exhaustive` field is uninhabited, the code mustn't observe that it is
/// uninhabited. For that, we filter these fields out of the matrix. This is subtle because we
/// still need to have those fields back when going to/from a `Pat`. Most of this is handled
/// automatically in `Fields`, but when constructing or deconstructing `Fields` you need to be
/// careful. As a rule, when going to/from the matrix, use the filtered field list; when going
/// to/from `Pat`, use the full field list.
/// This filtering is uncommon in practice, because uninhabited fields are rarely used, so we avoid
/// it when possible to preserve performance.
#[derive(Debug, Clone)]
enum Fields<'p, 'tcx> {
/// Lists of patterns that don't contain any filtered fields.
/// `Slice` and `Vec` behave the same; the difference is only to avoid allocating and
/// triple-dereferences when possible. Frankly this is premature optimization, I (Nadrieril)
/// have not measured if it really made a difference.
Slice(&'p [Pat<'tcx>]),
Vec(SmallVec<[&'p Pat<'tcx>; 2]>),
/// Patterns where some of the fields need to be hidden. `len` caches the number of non-hidden
/// fields.
Filtered {
fields: SmallVec<[FilteredField<'p, 'tcx>; 2]>,
len: usize,
},
}
impl<'p, 'tcx> Fields<'p, 'tcx> {
fn empty() -> Self {
Fields::Slice(&[])
}
/// Construct a new `Fields` from the given pattern. Must not be used if the pattern is a field
/// of a struct/tuple/variant.
fn from_single_pattern(pat: &'p Pat<'tcx>) -> Self {
Fields::Slice(std::slice::from_ref(pat))
}
/// Construct a new `Fields` from the given patterns. You must be sure those patterns can't
/// contain fields that need to be filtered out. When in doubt, prefer `replace_fields`.
fn from_slice_unfiltered(pats: &'p [Pat<'tcx>]) -> Self {
Fields::Slice(pats)
}
/// Convenience; internal use.
fn wildcards_from_tys(
cx: &MatchCheckCtxt<'p, 'tcx>,
tys: impl IntoIterator<Item = Ty<'tcx>>,
) -> Self {
let wilds = tys.into_iter().map(Pat::wildcard_from_ty);
let pats = cx.pattern_arena.alloc_from_iter(wilds);
Fields::Slice(pats)
}
/// Creates a new list of wildcard fields for a given constructor.
fn wildcards(
cx: &MatchCheckCtxt<'p, 'tcx>,
constructor: &Constructor<'tcx>,
ty: Ty<'tcx>,
) -> Self {
debug!("Fields::wildcards({:#?}, {:?})", constructor, ty);
let wildcard_from_ty = |ty| &*cx.pattern_arena.alloc(Pat::wildcard_from_ty(ty));
match constructor {
Single | Variant(_) => match ty.kind {
ty::Tuple(ref fs) => {
Fields::wildcards_from_tys(cx, fs.into_iter().map(|ty| ty.expect_ty()))
}
ty::Ref(_, rty, _) => Fields::from_single_pattern(wildcard_from_ty(rty)),
ty::Adt(adt, substs) => {
if adt.is_box() {
// Use T as the sub pattern type of Box<T>.
Fields::from_single_pattern(wildcard_from_ty(substs.type_at(0)))
} else {
let variant = &adt.variants[constructor.variant_index_for_adt(cx, adt)];
// Whether we must not match the fields of this variant exhaustively.
let is_non_exhaustive =
variant.is_field_list_non_exhaustive() && !adt.did.is_local();
let field_tys = variant.fields.iter().map(|field| field.ty(cx.tcx, substs));
// In the following cases, we don't need to filter out any fields. This is
// the vast majority of real cases, since uninhabited fields are uncommon.
let has_no_hidden_fields = (adt.is_enum() && !is_non_exhaustive)
|| !field_tys.clone().any(|ty| cx.is_uninhabited(ty));
if has_no_hidden_fields {
Fields::wildcards_from_tys(cx, field_tys)
} else {
let mut len = 0;
let fields = variant
.fields
.iter()
.map(|field| {
let ty = field.ty(cx.tcx, substs);
let is_visible = adt.is_enum()
|| field.vis.is_accessible_from(cx.module, cx.tcx);
let is_uninhabited = cx.is_uninhabited(ty);
// In the cases of either a `#[non_exhaustive]` field list
// or a non-public field, we hide uninhabited fields in
// order not to reveal the uninhabitedness of the whole
// variant.
if is_uninhabited && (!is_visible || is_non_exhaustive) {
FilteredField::Hidden(ty)
} else {
len += 1;
FilteredField::Kept(wildcard_from_ty(ty))
}
})
.collect();
Fields::Filtered { fields, len }
}
}
}
_ => Fields::empty(),
},
Slice(slice) => match ty.kind {
ty::Slice(ty) | ty::Array(ty, _) => {
let arity = slice.arity();
Fields::wildcards_from_tys(cx, (0..arity).map(|_| ty))
}
_ => bug!("bad slice pattern {:?} {:?}", constructor, ty),
},
ConstantValue(..) | FloatRange(..) | IntRange(..) | NonExhaustive => Fields::empty(),
}
}
fn len(&self) -> usize {
match self {
Fields::Slice(pats) => pats.len(),
Fields::Vec(pats) => pats.len(),
Fields::Filtered { len, .. } => *len,
}
}
/// Returns the complete list of patterns, including hidden fields.
fn all_patterns(self) -> impl Iterator<Item = Pat<'tcx>> {
let pats: SmallVec<[_; 2]> = match self {
Fields::Slice(pats) => pats.iter().cloned().collect(),
Fields::Vec(pats) => pats.into_iter().cloned().collect(),
Fields::Filtered { fields, .. } => {
// We don't skip any fields here.
fields.into_iter().map(|p| p.to_pattern()).collect()
}
};
pats.into_iter()
}
/// Overrides some of the fields with the provided patterns. Exactly like
/// `replace_fields_indexed`, except that it takes `FieldPat`s as input.
fn replace_with_fieldpats(
&self,
new_pats: impl IntoIterator<Item = &'p FieldPat<'tcx>>,
) -> Self {
self.replace_fields_indexed(
new_pats.into_iter().map(|pat| (pat.field.index(), &pat.pattern)),
)
}
/// Overrides some of the fields with the provided patterns. This is used when a pattern
/// defines some fields but not all, for example `Foo { field1: Some(_), .. }`: here we start with a
/// `Fields` that is just one wildcard per field of the `Foo` struct, and override the entry
/// corresponding to `field1` with the pattern `Some(_)`. This is also used for slice patterns
/// for the same reason.
fn replace_fields_indexed(
&self,
new_pats: impl IntoIterator<Item = (usize, &'p Pat<'tcx>)>,
) -> Self {
let mut fields = self.clone();
if let Fields::Slice(pats) = fields {
fields = Fields::Vec(pats.iter().collect());
}
match &mut fields {
Fields::Vec(pats) => {
for (i, pat) in new_pats {
pats[i] = pat
}
}
Fields::Filtered { fields, .. } => {
for (i, pat) in new_pats {
if let FilteredField::Kept(p) = &mut fields[i] {
*p = pat
}
}
}
Fields::Slice(_) => unreachable!(),
}
fields
}
/// Replaces contained fields with the given filtered list of patterns, e.g. taken from the
/// matrix. There must be `len()` patterns in `pats`.
fn replace_fields(
&self,
cx: &MatchCheckCtxt<'p, 'tcx>,
pats: impl IntoIterator<Item = Pat<'tcx>>,
) -> Self {
let pats: &[_] = cx.pattern_arena.alloc_from_iter(pats);
match self {
Fields::Filtered { fields, len } => {
let mut pats = pats.iter();
let mut fields = fields.clone();
for f in &mut fields {
if let FilteredField::Kept(p) = f {
// We take one input pattern for each `Kept` field, in order.
*p = pats.next().unwrap();
}
}
Fields::Filtered { fields, len: *len }
}
_ => Fields::Slice(pats),
}
}
fn push_on_patstack(self, stack: &[&'p Pat<'tcx>]) -> PatStack<'p, 'tcx> {
let pats: SmallVec<_> = match self {
Fields::Slice(pats) => pats.iter().chain(stack.iter().copied()).collect(),
Fields::Vec(mut pats) => {
pats.extend_from_slice(stack);
pats
}
Fields::Filtered { fields, .. } => {
// We skip hidden fields here
fields.into_iter().filter_map(|p| p.kept()).chain(stack.iter().copied()).collect()
}
};
PatStack::from_vec(pats)
}
}
@ -1064,15 +1263,16 @@ impl<'tcx, 'p> Usefulness<'tcx, 'p> {
fn apply_constructor(
self,
cx: &MatchCheckCtxt<'_, 'tcx>,
cx: &MatchCheckCtxt<'p, 'tcx>,
ctor: &Constructor<'tcx>,
ty: Ty<'tcx>,
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Self {
match self {
UsefulWithWitness(witnesses) => UsefulWithWitness(
witnesses
.into_iter()
.map(|witness| witness.apply_constructor(cx, &ctor, ty))
.map(|witness| witness.apply_constructor(cx, &ctor, ty, ctor_wild_subpatterns))
.collect(),
),
x => x,
@ -1192,17 +1392,19 @@ impl<'tcx> Witness<'tcx> {
///
/// left_ty: struct X { a: (bool, &'static str), b: usize}
/// pats: [(false, "foo"), 42] => X { a: (false, "foo"), b: 42 }
fn apply_constructor<'a>(
fn apply_constructor<'p>(
mut self,
cx: &MatchCheckCtxt<'a, 'tcx>,
cx: &MatchCheckCtxt<'p, 'tcx>,
ctor: &Constructor<'tcx>,
ty: Ty<'tcx>,
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Self {
let arity = ctor.arity(cx, ty);
let pat = {
let len = self.0.len() as u64;
let pats = self.0.drain((len - arity) as usize..).rev();
ctor.apply(cx, ty, pats)
let len = self.0.len();
let arity = ctor_wild_subpatterns.len();
let pats = self.0.drain((len - arity)..).rev();
let fields = ctor_wild_subpatterns.replace_fields(cx, pats);
ctor.apply(cx, ty, fields)
};
self.0.push(pat);
@ -1600,11 +1802,7 @@ impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
/// to a set of such vectors `m` - this is defined as there being a set of
/// inputs that will match `v` but not any of the sets in `m`.
///
/// All the patterns at each column of the `matrix ++ v` matrix must
/// have the same type, except that wildcard (PatKind::Wild) patterns
/// with type `TyErr` are also allowed, even if the "type of the column"
/// is not `TyErr`. That is used to represent private fields, as using their
/// real type would assert that they are inhabited.
/// All the patterns at each column of the `matrix ++ v` matrix must have the same type.
///
/// This is used both for reachability checking (if a pattern isn't useful in
/// relation to preceding patterns, it is not reachable) and exhaustiveness
@ -1668,34 +1866,7 @@ crate fn is_useful<'p, 'tcx>(
return if any_is_useful { Useful(unreachable_pats) } else { NotUseful };
}
let (ty, span) = matrix
.heads()
.map(|r| (r.ty, r.span))
.find(|(ty, _)| !ty.references_error())
.unwrap_or((v.head().ty, v.head().span));
let pcx = PatCtxt {
// TyErr is used to represent the type of wildcard patterns matching
// against inaccessible (private) fields of structs, so that we won't
// be able to observe whether the types of the struct's fields are
// inhabited.
//
// If the field is truly inaccessible, then all the patterns
// matching against it must be wildcard patterns, so its type
// does not matter.
//
// However, if we are matching against non-wildcard patterns, we
// need to know the real type of the field so we can specialize
// against it. This primarily occurs through constants - they
// can include contents for fields that are inaccessible at the
// location of the match. In that case, the field's type is
// inhabited - by the constant - so we can just use it.
//
// FIXME: this might lead to "unstable" behavior with macro hygiene
// introducing uninhabited patterns for inaccessible fields. We
// need to figure out how to model that.
ty,
span,
};
let pcx = PatCtxt { ty: v.head().ty, span: v.head().span };
debug!("is_useful_expand_first_col: pcx={:#?}, expanding {:#?}", pcx, v.head());
@ -1827,19 +1998,19 @@ fn is_useful_specialized<'p, 'tcx>(
matrix: &Matrix<'p, 'tcx>,
v: &PatStack<'p, 'tcx>,
ctor: Constructor<'tcx>,
lty: Ty<'tcx>,
ty: Ty<'tcx>,
witness_preference: WitnessPreference,
hir_id: HirId,
is_under_guard: bool,
) -> Usefulness<'tcx, 'p> {
debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, ty);
let ctor_wild_subpatterns =
cx.pattern_arena.alloc_from_iter(ctor.wildcard_subpatterns(cx, lty));
let matrix = matrix.specialize_constructor(cx, &ctor, ctor_wild_subpatterns);
v.specialize_constructor(cx, &ctor, ctor_wild_subpatterns)
// We cache the result of `Fields::wildcards` because it is used a lot.
let ctor_wild_subpatterns = Fields::wildcards(cx, &ctor, ty);
let matrix = matrix.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns);
v.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns)
.map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false))
.map(|u| u.apply_constructor(cx, &ctor, lty))
.map(|u| u.apply_constructor(cx, &ctor, ty, &ctor_wild_subpatterns))
.unwrap_or(NotUseful)
}
@ -2303,27 +2474,6 @@ fn constructor_covered_by_range<'tcx>(
if intersects { Some(()) } else { None }
}
fn patterns_for_variant<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
subpatterns: &'p [FieldPat<'tcx>],
ctor_wild_subpatterns: &'p [Pat<'tcx>],
is_non_exhaustive: bool,
) -> PatStack<'p, 'tcx> {
let mut result: SmallVec<_> = ctor_wild_subpatterns.iter().collect();
for subpat in subpatterns {
if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
result[subpat.field.index()] = &subpat.pattern;
}
}
debug!(
"patterns_for_variant({:#?}, {:#?}) = {:#?}",
subpatterns, ctor_wild_subpatterns, result
);
PatStack::from_vec(result)
}
/// This is the main specialization step. It expands the pattern
/// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
/// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
@ -2333,37 +2483,40 @@ fn patterns_for_variant<'p, 'tcx>(
/// different patterns.
/// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
/// fields filled with wild patterns.
///
/// This is roughly the inverse of `Constructor::apply`.
fn specialize_one_pattern<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
pat: &'p Pat<'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &'p [Pat<'tcx>],
) -> Option<PatStack<'p, 'tcx>> {
ctor_wild_subpatterns: &Fields<'p, 'tcx>,
) -> Option<Fields<'p, 'tcx>> {
if let NonExhaustive = constructor {
// Only a wildcard pattern can match the special extra constructor
return if pat.is_wildcard() { Some(PatStack::default()) } else { None };
if !pat.is_wildcard() {
return None;
}
return Some(Fields::empty());
}
let result = match *pat.kind {
PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
PatKind::Binding { .. } | PatKind::Wild => Some(ctor_wild_subpatterns.iter().collect()),
PatKind::Binding { .. } | PatKind::Wild => Some(ctor_wild_subpatterns.clone()),
PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
let variant = &adt_def.variants[variant_index];
let is_non_exhaustive = cx.is_foreign_non_exhaustive_variant(pat.ty, variant);
Some(Variant(variant.def_id))
.filter(|variant_constructor| variant_constructor == constructor)
.map(|_| {
patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, is_non_exhaustive)
})
if constructor != &Variant(variant.def_id) {
return None;
}
Some(ctor_wild_subpatterns.replace_with_fieldpats(subpatterns))
}
PatKind::Leaf { ref subpatterns } => {
Some(patterns_for_variant(cx, subpatterns, ctor_wild_subpatterns, false))
Some(ctor_wild_subpatterns.replace_with_fieldpats(subpatterns))
}
PatKind::Deref { ref subpattern } => Some(PatStack::from_pattern(subpattern)),
PatKind::Deref { ref subpattern } => Some(Fields::from_single_pattern(subpattern)),
PatKind::Constant { value } if constructor.is_slice() => {
// We extract an `Option` for the pointer because slices of zero
@ -2376,11 +2529,10 @@ fn specialize_one_pattern<'p, 'tcx>(
// Shortcut for `n == 0` where no matter what `alloc` and `offset` we produce,
// the result would be exactly what we early return here.
if n == 0 {
if ctor_wild_subpatterns.len() as u64 == 0 {
return Some(PatStack::from_slice(&[]));
} else {
if ctor_wild_subpatterns.len() as u64 != n {
return None;
}
return Some(Fields::empty());
}
match value.val {
ty::ConstKind::Value(ConstValue::ByRef { offset, alloc, .. }) => {
@ -2414,24 +2566,26 @@ fn specialize_one_pattern<'p, 'tcx>(
constructor,
),
};
if ctor_wild_subpatterns.len() as u64 == n {
// convert a constant slice/array pattern to a list of patterns.
let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
let ptr = Pointer::new(AllocId(0), offset);
(0..n)
.map(|i| {
let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
let scalar = scalar.not_undef().ok()?;
let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
let pattern =
Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
Some(&*cx.pattern_arena.alloc(pattern))
})
.collect()
} else {
None
if ctor_wild_subpatterns.len() as u64 != n {
return None;
}
// Convert a constant slice/array pattern to a list of patterns.
let layout = cx.tcx.layout_of(cx.param_env.and(ty)).ok()?;
let ptr = Pointer::new(AllocId(0), offset);
let pats = cx.pattern_arena.alloc_from_iter((0..n).filter_map(|i| {
let ptr = ptr.offset(layout.size * i, &cx.tcx).ok()?;
let scalar = alloc.read_scalar(&cx.tcx, ptr, layout.size).ok()?;
let scalar = scalar.not_undef().ok()?;
let value = ty::Const::from_scalar(cx.tcx, scalar, ty);
let pattern = Pat { ty, span: pat.span, kind: box PatKind::Constant { value } };
Some(pattern)
}));
// Ensure none of the dereferences failed.
if pats.len() as u64 != n {
return None;
}
Some(Fields::from_slice_unfiltered(pats))
}
PatKind::Constant { .. } | PatKind::Range { .. } => {
@ -2439,50 +2593,39 @@ fn specialize_one_pattern<'p, 'tcx>(
// - Single value: add a row if the pattern contains the constructor.
// - Range: add a row if the constructor intersects the pattern.
if let IntRange(ctor) = constructor {
match IntRange::from_pat(cx.tcx, cx.param_env, pat) {
Some(pat) => ctor.intersection(cx.tcx, &pat).map(|_| {
// Constructor splitting should ensure that all intersections we encounter
// are actually inclusions.
assert!(ctor.is_subrange(&pat));
PatStack::default()
}),
_ => None,
}
let pat = IntRange::from_pat(cx.tcx, cx.param_env, pat)?;
ctor.intersection(cx.tcx, &pat)?;
// Constructor splitting should ensure that all intersections we encounter
// are actually inclusions.
assert!(ctor.is_subrange(&pat));
} else {
// Fallback for non-ranges and ranges that involve
// floating-point numbers, which are not conveniently handled
// by `IntRange`. For these cases, the constructor may not be a
// range so intersection actually devolves into being covered
// by the pattern.
constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat)
.map(|()| PatStack::default())
constructor_covered_by_range(cx.tcx, cx.param_env, constructor, pat)?;
}
Some(Fields::empty())
}
PatKind::Array { ref prefix, ref slice, ref suffix }
| PatKind::Slice { ref prefix, ref slice, ref suffix } => match *constructor {
Slice(_) => {
// Number of subpatterns for this pattern
let pat_len = prefix.len() + suffix.len();
if let Some(slice_count) = ctor_wild_subpatterns.len().checked_sub(pat_len) {
if slice_count == 0 || slice.is_some() {
Some(
prefix
.iter()
.chain(
ctor_wild_subpatterns
.iter()
.skip(prefix.len())
.take(slice_count)
.chain(suffix.iter()),
)
.collect(),
)
} else {
None
}
} else {
None
// Number of subpatterns for this constructor
let arity = ctor_wild_subpatterns.len();
if (slice.is_none() && arity != pat_len) || pat_len > arity {
return None;
}
// Replace the prefix and the suffix with the given patterns, leaving wildcards in
// the middle if there was a subslice pattern `..`.
let prefix = prefix.iter().enumerate();
let suffix = suffix.iter().enumerate().map(|(i, p)| (arity - suffix.len() + i, p));
Some(ctor_wild_subpatterns.replace_fields_indexed(prefix.chain(suffix)))
}
ConstantValue(cv) => {
match slice_pat_covered_by_const(
@ -2494,7 +2637,7 @@ fn specialize_one_pattern<'p, 'tcx>(
suffix,
cx.param_env,
) {
Ok(true) => Some(PatStack::default()),
Ok(true) => Some(Fields::empty()),
Ok(false) => None,
Err(ErrorReported) => None,
}

2
src/librustc_mir_build/hair/pattern/check_match.rs
View File

@ -187,9 +187,9 @@ impl<'tcx> MatchVisitor<'_, 'tcx> {
let matrix = check_arms(&mut cx, &inlined_arms, source);
// Fifth, check if the match is exhaustive.
let scrut_ty = self.tables.node_type(scrut.hir_id);
// Note: An empty match isn't the same as an empty matrix for diagnostics purposes,
// since an empty matrix can occur when there are arms, if those arms all have guards.
let scrut_ty = self.tables.expr_ty_adjusted(scrut);
let is_empty_match = inlined_arms.is_empty();
check_exhaustive(&mut cx, scrut_ty, scrut.span, &matrix, scrut.hir_id, is_empty_match);
}

2
src/librustc_typeck/check/_match.rs
View File

@ -436,6 +436,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
if let Some(m) = contains_ref_bindings {
self.check_expr_with_needs(scrut, Needs::maybe_mut_place(m))
} else if arms.is_empty() {
self.check_expr(scrut)
} else {
// ...but otherwise we want to use any supertype of the
// scrutinee. This is sort of a workaround, see note (*) in

22
src/test/ui/pattern/usefulness/issue-71930-type-of-match-scrutinee.rs Normal file
View File

@ -0,0 +1,22 @@
// check-pass
// In PR 71930, it was discovered that the code to retrieve the inferred type of a match scrutinee
// was incorrect.
fn f() -> ! {
panic!()
}
fn g() -> usize {
match f() { // Should infer type `bool`
false => 0,
true => 1,
}
}
fn h() -> usize {
match f() { // Should infer type `!`
}
}
fn main() {}