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Let MissingConstructors handle the subtleties of missing constructors

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This commit is contained in:
Nadrieril 2020-10-26 17:35:59 +00:00
parent 1190e7275c
commit cdafd1e1bd
1 changed files with 72 additions and 89 deletions
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161
compiler/rustc_mir_build/src/thir/pattern/_match.rs
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@ -1309,11 +1309,6 @@ impl<'tcx> Constructor<'tcx> {
Pat { ty, span: DUMMY_SP, kind: Box::new(pat) }
}
/// Like `apply`, but where all the subpatterns are wildcards `_`.
fn apply_wildcards<'a>(&self, cx: &MatchCheckCtxt<'a, 'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {
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
@ -1649,35 +1644,15 @@ impl<'tcx> Usefulness<'tcx> {
}
}
fn apply_wildcard(self, ty: Ty<'tcx>) -> Self {
match self {
UsefulWithWitness(witnesses) => {
let wild = Pat::wildcard_from_ty(ty);
UsefulWithWitness(
witnesses
.into_iter()
.map(|mut witness| {
witness.0.push(wild.clone());
witness
})
.collect(),
)
}
x => x,
}
}
fn apply_missing_ctors(
fn apply_wildcard<'p>(
self,
cx: &MatchCheckCtxt<'_, 'tcx>,
ty: Ty<'tcx>,
missing_ctors: &MissingConstructors<'tcx>,
cx: &MatchCheckCtxt<'p, 'tcx>,
pcx: PatCtxt<'tcx>,
missing_ctors: MissingConstructors<'tcx>,
) -> Self {
match self {
UsefulWithWitness(witnesses) => {
let new_patterns: Vec<_> =
missing_ctors.iter().map(|ctor| ctor.apply_wildcards(cx, ty)).collect();
// Add the new patterns to each witness
let new_patterns = missing_ctors.report_patterns(cx, pcx);
UsefulWithWitness(
witnesses
.into_iter()
@ -2270,11 +2245,21 @@ impl<'tcx> std::cmp::PartialEq for IntRange<'tcx> {
struct MissingConstructors<'tcx> {
all_ctors: Vec<Constructor<'tcx>>,
used_ctors: Vec<Constructor<'tcx>>,
is_top_level: bool,
}
impl<'tcx> MissingConstructors<'tcx> {
fn new(all_ctors: Vec<Constructor<'tcx>>, used_ctors: Vec<Constructor<'tcx>>) -> Self {
MissingConstructors { all_ctors, used_ctors }
fn new<'p>(
cx: &MatchCheckCtxt<'p, 'tcx>,
pcx: PatCtxt<'tcx>,
matrix: &Matrix<'p, 'tcx>,
is_top_level: bool,
) -> Self {
let used_ctors: Vec<Constructor<'_>> =
matrix.head_ctors(cx).cloned().filter(|c| !c.is_wildcard()).collect();
let all_ctors = all_constructors(cx, pcx);
MissingConstructors { all_ctors, used_ctors, is_top_level }
}
fn into_inner(self) -> (Vec<Constructor<'tcx>>, Vec<Constructor<'tcx>>) {
@ -2284,16 +2269,64 @@ impl<'tcx> MissingConstructors<'tcx> {
fn is_empty(&self) -> bool {
self.iter().next().is_none()
}
/// Whether this contains all the constructors for the given type or only a
/// subset.
fn all_ctors_are_missing(&self) -> bool {
self.used_ctors.is_empty()
}
/// Iterate over all_ctors \ used_ctors
fn iter<'a>(&'a self) -> impl Iterator<Item = Constructor<'tcx>> + Captures<'a> {
self.all_ctors.iter().flat_map(move |req_ctor| req_ctor.subtract_ctors(&self.used_ctors))
}
/// List the patterns corresponding to the missing constructors. In some cases, instead of
/// listing all constructors of a given type, we prefer to simply report a wildcard.
fn report_patterns<'p>(
&self,
cx: &MatchCheckCtxt<'p, 'tcx>,
pcx: PatCtxt<'tcx>,
) -> SmallVec<[Pat<'tcx>; 1]> {
// There are 2 ways we can report a witness here.
// Commonly, we can report all the "free"
// constructors as witnesses, e.g., if we have:
//
// ```
// enum Direction { N, S, E, W }
// let Direction::N = ...;
// ```
//
// we can report 3 witnesses: `S`, `E`, and `W`.
//
// However, there is a case where we don't want
// to do this and instead report a single `_` witness:
// if the user didn't actually specify a constructor
// in this arm, e.g., in
//
// ```
// let x: (Direction, Direction, bool) = ...;
// let (_, _, false) = x;
// ```
//
// we don't want to show all 16 possible witnesses
// `(<direction-1>, <direction-2>, true)` - we are
// satisfied with `(_, _, true)`. In this case,
// `used_ctors` is empty.
// The exception is: if we are at the top-level, for example in an empty match, we
// sometimes prefer reporting the list of constructors instead of just `_`.
let report_when_all_missing = self.is_top_level && !IntRange::is_integral(pcx.ty);
if self.used_ctors.is_empty() && !report_when_all_missing {
// All constructors are unused. Report only a wildcard
// rather than each individual constructor.
smallvec![Pat::wildcard_from_ty(pcx.ty)]
} else {
// Construct for each missing constructor a "wild" version of this
// constructor, that matches everything that can be built with
// it. For example, if `ctor` is a `Constructor::Variant` for
// `Option::Some`, we get the pattern `Some(_)`.
self.iter()
.map(|missing_ctor| {
let fields = Fields::wildcards(cx, &missing_ctor, pcx.ty);
missing_ctor.apply(cx, pcx.ty, fields)
})
.collect()
}
}
}
impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
@ -2434,14 +2467,6 @@ crate fn is_useful<'p, 'tcx>(
} else {
debug!("is_useful - expanding wildcard");
let used_ctors: Vec<Constructor<'_>> =
matrix.head_ctors(cx).cloned().filter(|c| !c.is_wildcard()).collect();
debug!("is_useful_used_ctors = {:#?}", used_ctors);
// `all_ctors` are all the constructors for the given type, which
// should all be represented (or caught with the wild pattern `_`).
let all_ctors = all_constructors(cx, pcx);
debug!("is_useful_all_ctors = {:#?}", all_ctors);
// `missing_ctors` is the set of constructors from the same type as the
// first column of `matrix` that are matched only by wildcard patterns
// from the first column.
@ -2453,7 +2478,7 @@ crate fn is_useful<'p, 'tcx>(
// Missing constructors are those that are not matched by any non-wildcard patterns in the
// current column. We only fully construct them on-demand, because they're rarely used and
// can be big.
let missing_ctors = MissingConstructors::new(all_ctors, used_ctors);
let missing_ctors = MissingConstructors::new(cx, pcx, matrix, is_top_level);
debug!("is_useful_missing_ctors.empty()={:#?}", missing_ctors.is_empty(),);
@ -2485,49 +2510,7 @@ crate fn is_useful<'p, 'tcx>(
let usefulness =
is_useful(cx, &matrix, &v, witness_preference, hir_id, is_under_guard, false);
// In this case, there's at least one "free"
// constructor that is only matched against by
// wildcard patterns.
//
// There are 2 ways we can report a witness here.
// Commonly, we can report all the "free"
// constructors as witnesses, e.g., if we have:
//
// ```
// enum Direction { N, S, E, W }
// let Direction::N = ...;
// ```
//
// we can report 3 witnesses: `S`, `E`, and `W`.
//
// However, there is a case where we don't want
// to do this and instead report a single `_` witness:
// if the user didn't actually specify a constructor
// in this arm, e.g., in
//
// ```
// let x: (Direction, Direction, bool) = ...;
// let (_, _, false) = x;
// ```
//
// we don't want to show all 16 possible witnesses
// `(<direction-1>, <direction-2>, true)` - we are
// satisfied with `(_, _, true)`. In this case,
// `used_ctors` is empty.
// The exception is: if we are at the top-level, for example in an empty match, we
// sometimes prefer reporting the list of constructors instead of just `_`.
let report_ctors_rather_than_wildcard = is_top_level && !IntRange::is_integral(pcx.ty);
if missing_ctors.all_ctors_are_missing() && !report_ctors_rather_than_wildcard {
// All constructors are unused. Add a wild pattern
// rather than each individual constructor.
usefulness.apply_wildcard(pcx.ty)
} else {
// Construct for each missing constructor a "wild" version of this
// constructor, that matches everything that can be built with
// it. For example, if `ctor` is a `Constructor::Variant` for
// `Option::Some`, we get the pattern `Some(_)`.
usefulness.apply_missing_ctors(cx, pcx.ty, &missing_ctors)
}
usefulness.apply_wildcard(cx, pcx, missing_ctors)
}
};
debug!("is_useful::returns({:#?}, {:#?}) = {:?}", matrix, v, ret);