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Rollup merge of rust-lang#66967 - Nadrieril:remove-or-pat-hack, r=varkor
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Remove hack for top-level or-patterns in match checking

Follow-up to rust-lang#66612.

Or-patterns are now truly first-class in match checking. As a side-effect, redundant subpatterns are linted as such, making the `unreachable_patterns` lint a bit more general.

cc rust-lang#54883

r? @varkor
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@Centril
Centril authored Dec 3, 2019
2 parents 29e5b0a + 1c1bec2 commit cc3b75a
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Showing 7 changed files with 355 additions and 177 deletions.
105 changes: 56 additions & 49 deletions src/librustc_mir/hair/pattern/_match.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -425,16 +425,12 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
}

/// This computes `S(constructor, self)`. See top of the file for explanations.
fn specialize_constructor<'a, 'q>(
fn specialize_constructor(
&self,
cx: &mut MatchCheckCtxt<'a, 'tcx>,
cx: &mut MatchCheckCtxt<'p, 'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &[&'q Pat<'tcx>],
) -> Option<PatStack<'q, 'tcx>>
where
'a: 'q,
'p: 'q,
{
ctor_wild_subpatterns: &'p [Pat<'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..]);
Expand All @@ -459,6 +455,7 @@ impl<'p, 'tcx> FromIterator<&'p Pat<'tcx>> for PatStack<'p, 'tcx> {
}

/// A 2D matrix.
#[derive(Clone)]
pub struct Matrix<'p, 'tcx>(Vec<PatStack<'p, 'tcx>>);

impl<'p, 'tcx> Matrix<'p, 'tcx> {
Expand Down Expand Up @@ -486,16 +483,12 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
}

/// This computes `S(constructor, self)`. See top of the file for explanations.
fn specialize_constructor<'a, 'q>(
fn specialize_constructor(
&self,
cx: &mut MatchCheckCtxt<'a, 'tcx>,
cx: &mut MatchCheckCtxt<'p, 'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &[&'q Pat<'tcx>],
) -> Matrix<'q, 'tcx>
where
'a: 'q,
'p: 'q,
{
ctor_wild_subpatterns: &'p [Pat<'tcx>],
) -> Matrix<'p, 'tcx> {
self.0
.iter()
.filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
Expand Down Expand Up @@ -1033,17 +1026,19 @@ impl<'tcx> Constructor<'tcx> {
}

#[derive(Clone, Debug)]
pub enum Usefulness<'tcx> {
Useful,
pub enum Usefulness<'tcx, 'p> {
/// Carries a list of unreachable subpatterns. Used only in the presence of or-patterns.
Useful(Vec<&'p Pat<'tcx>>),
/// Carries a list of witnesses of non-exhaustiveness.
UsefulWithWitness(Vec<Witness<'tcx>>),
NotUseful,
}

impl<'tcx> Usefulness<'tcx> {
impl<'tcx, 'p> Usefulness<'tcx, 'p> {
fn new_useful(preference: WitnessPreference) -> Self {
match preference {
ConstructWitness => UsefulWithWitness(vec![Witness(vec![])]),
LeaveOutWitness => Useful,
LeaveOutWitness => Useful(vec![]),
}
}

Expand Down Expand Up @@ -1604,13 +1599,13 @@ impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
/// relation to preceding patterns, it is not reachable) and exhaustiveness
/// checking (if a wildcard pattern is useful in relation to a matrix, the
/// matrix isn't exhaustive).
pub fn is_useful<'p, 'a, 'tcx>(
cx: &mut MatchCheckCtxt<'a, 'tcx>,
pub fn is_useful<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
matrix: &Matrix<'p, 'tcx>,
v: &PatStack<'_, 'tcx>,
v: &PatStack<'p, 'tcx>,
witness_preference: WitnessPreference,
hir_id: HirId,
) -> Usefulness<'tcx> {
) -> Usefulness<'tcx, 'p> {
let &Matrix(ref rows) = matrix;
debug!("is_useful({:#?}, {:#?})", matrix, v);

Expand All @@ -1631,11 +1626,26 @@ pub fn is_useful<'p, 'a, 'tcx>(

// If the first pattern is an or-pattern, expand it.
if let Some(vs) = v.expand_or_pat() {
return vs
.into_iter()
.map(|v| is_useful(cx, matrix, &v, witness_preference, hir_id))
.find(|result| result.is_useful())
.unwrap_or(NotUseful);
// We need to push the already-seen patterns into the matrix in order to detect redundant
// branches like `Some(_) | Some(0)`. We also keep track of the unreachable subpatterns.
let mut matrix = matrix.clone();
let mut unreachable_pats = Vec::new();
let mut any_is_useful = false;
for v in vs {
let res = is_useful(cx, &matrix, &v, witness_preference, hir_id);
match res {
Useful(pats) => {
any_is_useful = true;
unreachable_pats.extend(pats);
}
NotUseful => unreachable_pats.push(v.head()),
UsefulWithWitness(_) => {
bug!("Encountered or-pat in `v` during exhaustiveness checking")
}
}
matrix.push(v);
}
return if any_is_useful { Useful(unreachable_pats) } else { NotUseful };
}

let (ty, span) = matrix
Expand Down Expand Up @@ -1768,21 +1778,21 @@ pub fn is_useful<'p, 'a, 'tcx>(

/// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
/// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
fn is_useful_specialized<'p, 'a, 'tcx>(
cx: &mut MatchCheckCtxt<'a, 'tcx>,
fn is_useful_specialized<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
matrix: &Matrix<'p, 'tcx>,
v: &PatStack<'_, 'tcx>,
v: &PatStack<'p, 'tcx>,
ctor: Constructor<'tcx>,
lty: Ty<'tcx>,
witness_preference: WitnessPreference,
hir_id: HirId,
) -> Usefulness<'tcx> {
) -> Usefulness<'tcx, 'p> {
debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);

let ctor_wild_subpatterns_owned: Vec<_> = ctor.wildcard_subpatterns(cx, lty);
let ctor_wild_subpatterns: Vec<_> = ctor_wild_subpatterns_owned.iter().collect();
let matrix = matrix.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns);
v.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns)
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)
.map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id))
.map(|u| u.apply_constructor(cx, &ctor, lty))
.unwrap_or(NotUseful)
Expand Down Expand Up @@ -2250,13 +2260,13 @@ fn constructor_covered_by_range<'tcx>(
if intersects { Some(()) } else { None }
}

fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
cx: &mut MatchCheckCtxt<'a, 'tcx>,
fn patterns_for_variant<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
subpatterns: &'p [FieldPat<'tcx>],
ctor_wild_subpatterns: &[&'p Pat<'tcx>],
ctor_wild_subpatterns: &'p [Pat<'tcx>],
is_non_exhaustive: bool,
) -> PatStack<'p, 'tcx> {
let mut result = SmallVec::from_slice(ctor_wild_subpatterns);
let mut result: SmallVec<_> = ctor_wild_subpatterns.iter().collect();

for subpat in subpatterns {
if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
Expand All @@ -2280,11 +2290,11 @@ fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
/// different patterns.
/// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
/// fields filled with wild patterns.
fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
cx: &mut MatchCheckCtxt<'a, 'tcx>,
pat: &'q Pat<'tcx>,
fn specialize_one_pattern<'p, 'tcx>(
cx: &mut MatchCheckCtxt<'p, 'tcx>,
pat: &'p Pat<'tcx>,
constructor: &Constructor<'tcx>,
ctor_wild_subpatterns: &[&'p Pat<'tcx>],
ctor_wild_subpatterns: &'p [Pat<'tcx>],
) -> Option<PatStack<'p, 'tcx>> {
if let NonExhaustive = constructor {
// Only a wildcard pattern can match the special extra constructor
Expand All @@ -2294,9 +2304,7 @@ fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
let result = match *pat.kind {
PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`

PatKind::Binding { .. } | PatKind::Wild => {
Some(PatStack::from_slice(ctor_wild_subpatterns))
}
PatKind::Binding { .. } | PatKind::Wild => Some(ctor_wild_subpatterns.iter().collect()),

PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
let ref variant = adt_def.variants[variant_index];
Expand Down Expand Up @@ -2406,7 +2414,6 @@ fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
.chain(
ctor_wild_subpatterns
.iter()
.map(|p| *p)
.skip(prefix.len())
.take(slice_count)
.chain(suffix.iter()),
Expand Down
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