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Identify subpatterns by the path to them instead of spans

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This commit is contained in:
Nadrieril 2021-01-01 22:14:35 +00:00
parent 0162d603b3
commit 307a278d5c
6 changed files with 253 additions and 133 deletions
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2
compiler/rustc_mir_build/src/thir/pattern/check_match.rs
View File

@ -434,7 +434,7 @@ fn report_arm_reachability<'p, 'tcx>(
Reachable(unreachables) if unreachables.is_empty() => {}
// The arm is reachable, but contains unreachable subpatterns (from or-patterns).
Reachable(unreachables) => {
let mut unreachables: Vec<_> = unreachables.iter().collect();
let mut unreachables = unreachables.clone();
// Emit lints in the order in which they occur in the file.
unreachables.sort_unstable();
for span in unreachables {

343
compiler/rustc_mir_build/src/thir/pattern/usefulness.rs
View File

@ -288,6 +288,7 @@ use super::{Pat, PatKind};
use super::{PatternFoldable, PatternFolder};
use rustc_data_structures::captures::Captures;
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::sync::OnceCell;
use rustc_arena::TypedArena;
@ -618,82 +619,236 @@ impl<'p, 'tcx> FromIterator<PatStack<'p, 'tcx>> for Matrix<'p, 'tcx> {
}
}
/// Represents a set of `Span`s closed under the containment relation. That is, if a `Span` is
/// contained in the set then all `Span`s contained in it are also implicitly contained in the set.
/// In particular this means that when intersecting two sets, taking the intersection of some span
/// and one of its subspans returns the subspan, whereas a simple `HashSet` would have returned an
/// empty intersection.
/// It is assumed that two spans don't overlap without one being contained in the other; in other
/// words, that the inclusion structure forms a tree and not a DAG.
/// Intersection is not very efficient. It compares everything pairwise. If needed it could be made
/// faster by sorting the `Span`s and merging cleverly.
#[derive(Debug, Clone, Default)]
pub(crate) struct SpanSet {
/// The minimal set of `Span`s required to represent the whole set. If A and B are `Span`s in
/// the `SpanSet`, and A is a descendant of B, then only B will be in `root_spans`.
/// Invariant: the spans are disjoint.
root_spans: Vec<Span>,
/// Given a pattern or a pattern-stack, this struct captures a set of its subpattern branches. We
/// use that to track unreachable sub-patterns arising from or-patterns. In the absence of
/// or-patterns this will always be either `Empty` or `Full`.
/// We support a limited set of operations, so not all possible sets of subpatterns can be
/// represented. That's ok, we only want the ones that make sense to capture unreachable
/// subpatterns.
/// What we're trying to do is illustrated by this:
/// ```
/// match (true, true) {
/// (true, true) => {}
/// (true | false, true | false) => {}
/// }
/// ```
/// When we try the alternatives of the first or-pattern, the last `true` is unreachable in the
/// first alternative but no the other. So we don't want to report it as unreachable. Therefore we
/// intersect sets of unreachable patterns coming from different alternatives in order to figure
/// out which subpatterns are overall unreachable.
#[derive(Debug, Clone)]
enum SubPatSet<'p, 'tcx> {
/// The set containing the full pattern.
Full,
/// The empty set.
Empty,
/// If the pattern is a pattern with a constructor or a pattern-stack, we store a set for each
/// of its subpatterns. Missing entries in the map are implicitly empty.
Seq { subpats: FxHashMap<usize, SubPatSet<'p, 'tcx>> },
/// If the pattern is an or-pattern, we store a set for each of its alternatives. Missing
/// entries in the map are implicitly full. Note: we always flatten nested or-patterns.
Alt {
subpats: FxHashMap<usize, SubPatSet<'p, 'tcx>>,
/// Counts the total number of alternatives in the pattern
alt_count: usize,
/// We keep the pattern around to retrieve spans.
pat: &'p Pat<'tcx>,
},
}
impl SpanSet {
/// Creates an empty set.
fn new() -> Self {
Self::default()
impl<'p, 'tcx> SubPatSet<'p, 'tcx> {
fn empty() -> Self {
SubPatSet::Empty
}
fn full() -> Self {
SubPatSet::Full
}
/// Tests whether the set is empty.
pub(crate) fn is_empty(&self) -> bool {
self.root_spans.is_empty()
fn is_full(&self) -> bool {
match self {
SubPatSet::Full => true,
SubPatSet::Empty => false,
// If any subpattern in a sequence is unreachable, the whole pattern is unreachable.
SubPatSet::Seq { subpats } => subpats.values().any(|set| set.is_full()),
SubPatSet::Alt { subpats, .. } => subpats.values().all(|set| set.is_full()),
}
}
/// Iterate over the disjoint list of spans at the roots of this set.
pub(crate) fn iter<'a>(&'a self) -> impl Iterator<Item = Span> + Captures<'a> {
self.root_spans.iter().copied()
fn is_empty(&self) -> bool {
match self {
SubPatSet::Full => false,
SubPatSet::Empty => true,
SubPatSet::Seq { subpats } => subpats.values().all(|sub_set| sub_set.is_empty()),
SubPatSet::Alt { subpats, alt_count, .. } => {
subpats.len() == *alt_count && subpats.values().all(|set| set.is_empty())
}
}
}
/// Tests whether the set contains a given Span.
fn contains(&self, span: Span) -> bool {
self.iter().any(|root_span| root_span.contains(span))
}
/// Add a span to the set if we know the span has no intersection in this set.
fn push_nonintersecting(&mut self, new_span: Span) {
self.root_spans.push(new_span);
}
fn intersection_mut(&mut self, other: &Self) {
if self.is_empty() || other.is_empty() {
*self = Self::new();
/// Intersect `self` with `other`, mutating `self`.
fn intersect(&mut self, other: Self) {
use SubPatSet::*;
// Intersecting with empty stays empty; intersecting with full changes nothing.
if self.is_empty() || other.is_full() {
return;
} else if self.is_full() {
*self = other;
return;
} else if other.is_empty() {
*self = Empty;
return;
}
// Those that were in `self` but not contained in `other`
let mut leftover = SpanSet::new();
// We keep the elements in `self` that are also in `other`.
self.root_spans.retain(|span| {
let retain = other.contains(*span);
if !retain {
leftover.root_spans.push(*span);
match (&mut *self, other) {
(Seq { subpats: s_set }, Seq { subpats: mut o_set }) => {
s_set.retain(|i, s_sub_set| {
// Missing entries count as empty.
let o_sub_set = o_set.remove(&i).unwrap_or(Empty);
s_sub_set.intersect(o_sub_set);
// We drop empty entries.
!s_sub_set.is_empty()
});
// Everything left in `o_set` is missing from `s_set`, i.e. counts as empty. Since
// intersecting with empty returns empty, we can drop those entries.
}
retain
});
// We keep the elements in `other` that are also in the original `self`. You might think
// this is not needed because `self` already contains the intersection. But those aren't
// just sets of things. If `self = [a]`, `other = [b]` and `a` contains `b`, then `b`
// belongs in the intersection but we didn't catch it in the filtering above. We look at
// `leftover` instead of the full original `self` to avoid duplicates.
for span in other.iter() {
if leftover.contains(span) {
self.root_spans.push(span);
(Alt { subpats: s_set, .. }, Alt { subpats: mut o_set, .. }) => {
s_set.retain(|i, s_sub_set| {
// Missing entries count as full.
let o_sub_set = o_set.remove(&i).unwrap_or(Full);
s_sub_set.intersect(o_sub_set);
// We drop full entries.
!s_sub_set.is_full()
});
// Everything left in `o_set` is missing from `s_set`, i.e. counts as full. Since
// intersecting with full changes nothing, we can take those entries as is.
s_set.extend(o_set);
}
_ => bug!(),
}
if self.is_empty() {
*self = Empty;
}
}
/// Returns a list of the spans of the unreachable subpatterns. If `self` is full we return
/// `None`.
fn to_spans(&self) -> Option<Vec<Span>> {
/// Panics if `set.is_full()`.
fn fill_spans(set: &SubPatSet<'_, '_>, spans: &mut Vec<Span>) {
match set {
SubPatSet::Full => bug!(),
SubPatSet::Empty => {}
SubPatSet::Seq { subpats } => {
for (_, sub_set) in subpats {
fill_spans(sub_set, spans);
}
}
SubPatSet::Alt { subpats, pat, alt_count, .. } => {
let expanded = pat.expand_or_pat();
for i in 0..*alt_count {
let sub_set = subpats.get(&i).unwrap_or(&SubPatSet::Full);
if sub_set.is_full() {
spans.push(expanded[i].span);
} else {
fill_spans(sub_set, spans);
}
}
}
}
}
if self.is_full() {
return None;
}
if self.is_empty() {
return Some(Vec::new());
}
let mut spans = Vec::new();
fill_spans(self, &mut spans);
Some(spans)
}
/// When `self` refers to a patstack that was obtained from specialization, after running
/// `unspecialize` it will refer to the original patstack before specialization.
fn unspecialize(self, arity: usize) -> Self {
use SubPatSet::*;
match self {
Full => Full,
Empty => Empty,
Seq { subpats } => {
// We gather the first `arity` subpatterns together and shift the remaining ones.
let mut new_subpats = FxHashMap::default();
let mut new_subpats_first_col = FxHashMap::default();
for (i, sub_set) in subpats {
if i < arity {
// The first `arity` indices are now part of the pattern in the first
// column.
new_subpats_first_col.insert(i, sub_set);
} else {
// Indices after `arity` are simply shifted
new_subpats.insert(i - arity + 1, sub_set);
}
}
if !new_subpats_first_col.is_empty() {
new_subpats.insert(0, Seq { subpats: new_subpats_first_col });
}
Seq { subpats: new_subpats }
}
Alt { .. } => bug!(), // `self` is a patstack
}
}
/// When `self` refers to a patstack that was obtained from splitting an or-pattern, after
/// running `unspecialize` it will refer to the original patstack before splitting.
///
/// This case is subtle. Consider:
/// ```
/// match Some(true) {
/// Some(true) => {}
/// None | Some(true | false) => {}
/// }
/// ```
/// Imagine we naively preserved the sets of unreachable subpatterns. Here `None` would return
/// the empty set and `Some(true | false)` would return the set containing `true`. Intersecting
/// those two would return the empty set, so we'd miss that the last `true` is unreachable.
/// To fix that, when specializing a given alternative of an or-pattern, we consider all other
/// alternatives as unreachable. That way, intersecting the results will not unduly discard
/// unreachable subpatterns coming from the other alternatives. This is what this function does
/// (remember that missing entries in the `Alt` case count as full; in other words alternatives
/// other than `alt_id` count as unreachable).
fn unsplit_or_pat(mut self, alt_id: usize, alt_count: usize, pat: &'p Pat<'tcx>) -> Self {
use SubPatSet::*;
if self.is_full() {
return Full;
}
let set_first_col = match &mut self {
Empty => Empty,
Seq { subpats } => subpats.remove(&0).unwrap_or(Empty),
Full => unreachable!(),
Alt { .. } => bug!(), // `self` is a patstack
};
let mut subpats_first_col = FxHashMap::default();
subpats_first_col.insert(alt_id, set_first_col);
let set_first_col = Alt { subpats: subpats_first_col, pat, alt_count };
let mut subpats = match self {
Empty => FxHashMap::default(),
Seq { subpats } => subpats,
Full => unreachable!(),
Alt { .. } => bug!(), // `self` is a patstack
};
subpats.insert(0, set_first_col);
Seq { subpats }
}
}
#[derive(Clone, Debug)]
enum Usefulness<'tcx> {
enum Usefulness<'p, 'tcx> {
/// Potentially carries a set of sub-branches that have been found to be unreachable. Used
/// only in the presence of or-patterns, otherwise it stays empty.
NoWitnesses(SpanSet),
NoWitnesses(SubPatSet<'p, 'tcx>),
/// When not carrying witnesses, indicates that the whole pattern is unreachable.
NoWitnessesFull,
/// Carries a list of witnesses of non-exhaustiveness. Non-empty.
@ -702,11 +857,11 @@ enum Usefulness<'tcx> {
WithWitnessesEmpty,
}
impl<'tcx> Usefulness<'tcx> {
impl<'p, 'tcx> Usefulness<'p, 'tcx> {
fn new_useful(preference: WitnessPreference) -> Self {
match preference {
ConstructWitness => WithWitnesses(vec![Witness(vec![])]),
LeaveOutWitness => NoWitnesses(Default::default()),
LeaveOutWitness => NoWitnesses(SubPatSet::empty()),
}
}
fn new_not_useful(preference: WitnessPreference) -> Self {
@ -718,33 +873,13 @@ impl<'tcx> Usefulness<'tcx> {
/// Combine usefulnesses from two branches. This is an associative operation.
fn extend(&mut self, other: Self) {
// If we have detected some unreachable sub-branches, we only want to keep them when they
// were unreachable in _all_ branches. Eg. in the following, the last `true` is unreachable
// in the second branch of the first or-pattern, but not otherwise. Therefore we don't want
// to lint that it is unreachable.
// ```
// match (true, true) {
// (true, true) => {}
// (false | true, false | true) => {}
// }
// ```
// Here however we _do_ want to lint that the last `false` is unreachable. In order to
// handle that correctly, each branch of an or-pattern marks the other branches as
// unreachable (see `unsplit_or_pat`). That way, intersecting the results will correctly
// identify unreachable sub-patterns.
// ```
// match None {
// Some(false) => {}
// None | Some(true | false) => {}
// }
// ```
match (&mut *self, other) {
(WithWitnesses(s), WithWitnesses(o)) => s.extend(o),
(WithWitnessesEmpty, WithWitnesses(o)) => *self = WithWitnesses(o),
(WithWitnesses(_), WithWitnessesEmpty) => {}
(WithWitnessesEmpty, WithWitnessesEmpty) => {}
(NoWitnesses(s), NoWitnesses(o)) => s.intersection_mut(&o),
(NoWitnesses(s), NoWitnesses(o)) => s.intersect(o),
(NoWitnessesFull, NoWitnesses(o)) => *self = NoWitnesses(o),
(NoWitnesses(_), NoWitnessesFull) => {}
(NoWitnessesFull, NoWitnessesFull) => {}
@ -761,8 +896,8 @@ impl<'tcx> Usefulness<'tcx> {
let mut ret = Self::new_not_useful(pref);
for u in usefulnesses {
ret.extend(u);
if let NoWitnesses(spans) = &ret {
if spans.is_empty() {
if let NoWitnesses(subpats) = &ret {
if subpats.is_empty() {
// Once we reach the empty set, more intersections won't change the result.
return ret;
}
@ -773,30 +908,19 @@ impl<'tcx> Usefulness<'tcx> {
/// After calculating the usefulness for a branch of an or-pattern, call this to make this
/// usefulness mergeable with those from the other branches.
fn unsplit_or_pat(self, this_span: Span, or_pat_spans: &[Span]) -> Self {
match self {
NoWitnesses(mut spans) => {
// We register the spans of the other branches of this or-pattern as being
// unreachable from this one. This ensures that intersecting together the sets of
// spans returns what we want.
// Until we optimize `SpanSet` however, intersecting this entails a number of
// comparisons quadratic in the number of branches.
for &span in or_pat_spans {
if span != this_span {
spans.push_nonintersecting(span);
}
}
NoWitnesses(spans)
}
NoWitnessesFull => NoWitnessesFull,
fn unsplit_or_pat(self, alt_id: usize, alt_count: usize, pat: &'p Pat<'tcx>) -> Self {
let subpats = match self {
NoWitnesses(subpats) => subpats,
NoWitnessesFull => SubPatSet::full(),
WithWitnesses(_) | WithWitnessesEmpty => bug!(),
}
};
NoWitnesses(subpats.unsplit_or_pat(alt_id, alt_count, pat))
}
/// After calculating usefulness after a specialization, call this to recontruct a usefulness
/// that makes sense for the matrix pre-specialization. This new usefulness can then be merged
/// with the results of specializing with the other constructors.
fn apply_constructor<'p>(
fn apply_constructor(
self,
pcx: PatCtxt<'_, 'p, 'tcx>,
matrix: &Matrix<'p, 'tcx>, // used to compute missing ctors
@ -836,7 +960,9 @@ impl<'tcx> Usefulness<'tcx> {
};
WithWitnesses(new_witnesses)
}
x => x,
NoWitnesses(subpats) => NoWitnesses(subpats.unspecialize(ctor_wild_subpatterns.len())),
NoWitnessesFull => NoWitnessesFull,
WithWitnessesEmpty => WithWitnessesEmpty,
}
}
}
@ -953,7 +1079,7 @@ fn is_useful<'p, 'tcx>(
hir_id: HirId,
is_under_guard: bool,
is_top_level: bool,
) -> Usefulness<'tcx> {
) -> Usefulness<'p, 'tcx> {
debug!("matrix,v={:?}{:?}", matrix, v);
let Matrix { patterns: rows, .. } = matrix;
@ -981,13 +1107,13 @@ fn is_useful<'p, 'tcx>(
// If the first pattern is an or-pattern, expand it.
let ret = if v.head().is_or_pat() {
debug!("expanding or-pattern");
let v_head = v.head();
let vs: Vec<_> = v.expand_or_pat().collect();
let subspans: Vec<_> = vs.iter().map(|v| v.head().span).collect();
let alt_count = vs.len();
// We expand the or pattern, trying each of its branches in turn and keeping careful track
// of possible unreachable sub-branches.
let mut matrix = matrix.clone();
let usefulnesses = vs.into_iter().map(|v| {
let v_span = v.head().span;
let usefulnesses = vs.into_iter().enumerate().map(|(i, v)| {
let usefulness =
is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false);
// If pattern has a guard don't add it to the matrix.
@ -996,7 +1122,7 @@ fn is_useful<'p, 'tcx>(
// branches like `Some(_) | Some(0)`.
matrix.push(v);
}
usefulness.unsplit_or_pat(v_span, &subspans)
usefulness.unsplit_or_pat(i, alt_count, v_head)
});
Usefulness::merge(witness_preference, usefulnesses)
} else {
@ -1045,7 +1171,7 @@ crate struct MatchArm<'p, 'tcx> {
crate enum Reachability {
/// Potentially carries a set of sub-branches that have been found to be unreachable. Used only
/// in the presence of or-patterns, otherwise it stays empty.
Reachable(SpanSet),
Reachable(Vec<Span>),
Unreachable,
}
@ -1081,7 +1207,8 @@ crate fn compute_match_usefulness<'p, 'tcx>(
matrix.push(v);
}
let reachability = match usefulness {
NoWitnesses(spans) => Reachability::Reachable(spans),
NoWitnesses(subpats) if subpats.is_full() => Reachability::Unreachable,
NoWitnesses(subpats) => Reachability::Reachable(subpats.to_spans().unwrap()),
NoWitnessesFull => Reachability::Unreachable,
WithWitnesses(..) | WithWitnessesEmpty => bug!(),
};

4
src/test/ui/or-patterns/exhaustiveness-unreachable-pattern.rs
View File

@ -103,8 +103,8 @@ fn main() {
}
macro_rules! t_or_f {
() => {
(true // FIXME: should be unreachable
| false)
(true //~ ERROR unreachable
| false)
};
}
match (true, None) {

13
src/test/ui/or-patterns/exhaustiveness-unreachable-pattern.stderr
View File

@ -131,6 +131,17 @@ error: unreachable pattern
LL | (true | false, None | Some(true
| ^^^^
error: unreachable pattern
--> $DIR/exhaustiveness-unreachable-pattern.rs:106:14
|
LL | (true
| ^^^^
...
LL | (true | false, None | Some(t_or_f!())) => {}
| --------- in this macro invocation
|
= note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
error: unreachable pattern
--> $DIR/exhaustiveness-unreachable-pattern.rs:117:14
|
@ -155,5 +166,5 @@ error: unreachable pattern
LL | | true,
| ^^^^
error: aborting due to 25 previous errors
error: aborting due to 26 previous errors

6
src/test/ui/pattern/usefulness/issue-80501-or-pat-and-macro.rs
View File

@ -1,5 +1,5 @@
// check-pass
#![deny(unreachable_patterns)]
//~^ NOTE: lint level is defined here
pub enum TypeCtor {
Slice,
Array,
@ -9,13 +9,13 @@ pub struct ApplicationTy(TypeCtor);
macro_rules! ty_app {
($ctor:pat) => {
ApplicationTy($ctor) //~ ERROR unreachable pattern
ApplicationTy($ctor)
};
}
fn _foo(ty: ApplicationTy) {
match ty {
ty_app!(TypeCtor::Array) | ty_app!(TypeCtor::Slice) => {} //~ NOTE: in this expansion
ty_app!(TypeCtor::Array) | ty_app!(TypeCtor::Slice) => {}
}
// same as above, with the macro expanded

18
src/test/ui/pattern/usefulness/issue-80501-or-pat-and-macro.stderr
View File

@ -1,18 +0,0 @@
error: unreachable pattern
--> $DIR/issue-80501-or-pat-and-macro.rs:12:9
|
LL | ApplicationTy($ctor)
| ^^^^^^^^^^^^^^^^^^^^
...
LL | ty_app!(TypeCtor::Array) | ty_app!(TypeCtor::Slice) => {}
| ------------------------ in this macro invocation
|
note: the lint level is defined here
--> $DIR/issue-80501-or-pat-and-macro.rs:1:9
|
LL | #![deny(unreachable_patterns)]
| ^^^^^^^^^^^^^^^^^^^^
= note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
error: aborting due to previous error