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use crate::ty::context::TyCtxt;
use crate::ty::{DefId, DefIdTree};
use rustc_span::def_id::CRATE_DEF_ID;
use smallvec::SmallVec;
use std::mem;
use DefIdForest::*;
/// Represents a forest of `DefId`s closed under the ancestor relation. That is,
/// if a `DefId` representing a module is contained in the forest then all
/// `DefId`s defined in that module or submodules are also implicitly contained
/// in the forest.
///
/// This is used to represent a set of modules in which a type is visibly
/// uninhabited.
///
/// We store the minimal set of `DefId`s required to represent the whole set. If A and B are
/// `DefId`s in the `DefIdForest`, and A is a parent of B, then only A will be stored. When this is
/// used with `type_uninhabited_from`, there will very rarely be more than one `DefId` stored.
#[derive(Copy, Clone, HashStable, Debug)]
pub enum DefIdForest<'a> {
Empty,
Single(DefId),
/// This variant is very rare.
/// Invariant: >1 elements
Multiple(&'a [DefId]),
}
/// Tests whether a slice of roots contains a given DefId.
#[inline]
fn slice_contains<'tcx>(tcx: TyCtxt<'tcx>, slice: &[DefId], id: DefId) -> bool {
slice.iter().any(|root_id| tcx.is_descendant_of(id, *root_id))
}
impl<'tcx> DefIdForest<'tcx> {
/// Creates an empty forest.
pub fn empty() -> DefIdForest<'tcx> {
DefIdForest::Empty
}
/// Creates a forest consisting of a single tree representing the entire
/// crate.
#[inline]
pub fn full() -> DefIdForest<'tcx> {
DefIdForest::from_id(CRATE_DEF_ID.to_def_id())
}
/// Creates a forest containing a `DefId` and all its descendants.
pub fn from_id(id: DefId) -> DefIdForest<'tcx> {
DefIdForest::Single(id)
}
fn as_slice(&self) -> &[DefId] {
match self {
Empty => &[],
Single(id) => std::slice::from_ref(id),
Multiple(root_ids) => root_ids,
}
}
// Only allocates in the rare `Multiple` case.
fn from_vec(tcx: TyCtxt<'tcx>, root_ids: SmallVec<[DefId; 1]>) -> DefIdForest<'tcx> {
match &root_ids[..] {
[] => Empty,
[id] => Single(*id),
_ => DefIdForest::Multiple(tcx.arena.alloc_from_iter(root_ids)),
}
}
/// Tests whether the forest is empty.
pub fn is_empty(&self) -> bool {
match self {
Empty => true,
Single(..) | Multiple(..) => false,
}
}
/// Iterate over the set of roots.
fn iter(&self) -> impl Iterator<Item = DefId> + '_ {
self.as_slice().iter().copied()
}
/// Tests whether the forest contains a given DefId.
pub fn contains(&self, tcx: TyCtxt<'tcx>, id: DefId) -> bool {
slice_contains(tcx, self.as_slice(), id)
}
/// Calculate the intersection of a collection of forests.
pub fn intersection<I>(tcx: TyCtxt<'tcx>, iter: I) -> DefIdForest<'tcx>
where
I: IntoIterator<Item = DefIdForest<'tcx>>,
{
let mut iter = iter.into_iter();
let mut ret: SmallVec<[_; 1]> = if let Some(first) = iter.next() {
SmallVec::from_slice(first.as_slice())
} else {
return DefIdForest::full();
};
let mut next_ret: SmallVec<[_; 1]> = SmallVec::new();
for next_forest in iter {
// No need to continue if the intersection is already empty.
if ret.is_empty() || next_forest.is_empty() {
return DefIdForest::empty();
}
// We keep the elements in `ret` that are also in `next_forest`.
next_ret.extend(ret.iter().copied().filter(|&id| next_forest.contains(tcx, id)));
// We keep the elements in `next_forest` that are also in `ret`.
next_ret.extend(next_forest.iter().filter(|&id| slice_contains(tcx, &ret, id)));
mem::swap(&mut next_ret, &mut ret);
next_ret.clear();
}
DefIdForest::from_vec(tcx, ret)
}
/// Calculate the union of a collection of forests.
pub fn union<I>(tcx: TyCtxt<'tcx>, iter: I) -> DefIdForest<'tcx>
where
I: IntoIterator<Item = DefIdForest<'tcx>>,
{
let mut ret: SmallVec<[_; 1]> = SmallVec::new();
let mut next_ret: SmallVec<[_; 1]> = SmallVec::new();
for next_forest in iter {
// Union with the empty set is a no-op.
if next_forest.is_empty() {
continue;
}
// We add everything in `ret` that is not in `next_forest`.
next_ret.extend(ret.iter().copied().filter(|&id| !next_forest.contains(tcx, id)));
// We add everything in `next_forest` that we haven't added yet.
for id in next_forest.iter() {
if !slice_contains(tcx, &next_ret, id) {
next_ret.push(id);
}
}
mem::swap(&mut next_ret, &mut ret);
next_ret.clear();
}
DefIdForest::from_vec(tcx, ret)
}
}
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