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use std::future::Future;
use anyhow::Result;
use tracing::Span;
use super::VisitControlFlow;
/// A trait that allows a graph traversal to visit the edges of a node
/// transitively.
pub trait Visit<Node, Abort = !, Impl = ()> {
type Edge;
type EdgesIntoIter: IntoIterator<Item = Self::Edge>;
type EdgesFuture: Future<Output = Result<Self::EdgesIntoIter>> + Send;
/// Visits an edge to get to the neighbor node. Should return a
/// [`VisitControlFlow`] that indicates whether to:
/// * continue visiting the neighbor node edges;
/// * skip visiting the neighbor node's edges;
/// * abort the traversal entirely.
fn visit(&mut self, edge: Self::Edge) -> VisitControlFlow<Node, Abort>;
/// Returns a future that resolves to the outgoing edges of the given `node`.
///
/// Lifetimes:
/// - The returned future's lifetime cannot depend on the reference to self because there are
/// multiple `edges` futures created and awaited concurrently.
/// - The returned future's lifetime cannot depend on `node` because `GraphStore::insert`
/// returns a node reference that's only valid for the lifetime of its `&mut self` reference.
fn edges(&mut self, node: &Node) -> Self::EdgesFuture;
/// Returns a [Span] for the given `node`, under which all edges are
/// processed.
fn span(&mut self, node: &Node) -> Span {
let _ = node;
Span::current()
}
}
// The different `Impl*` here are necessary in order to avoid the `Conflicting
// implementations of trait` error when implementing `Visit` on different
// kinds of `FnMut`.
// See https://users.rust-lang.org/t/conflicting-implementation-when-implementing-traits-for-fn/53359/3
pub struct ImplRef;
impl<Node, VisitFn, NeighFut, NeighIt> Visit<Node, !, ImplRef> for VisitFn
where
VisitFn: FnMut(&Node) -> NeighFut,
NeighFut: Future<Output = Result<NeighIt>> + Send,
NeighIt: IntoIterator<Item = Node>,
{
type Edge = Node;
type EdgesIntoIter = NeighIt;
type EdgesFuture = NeighFut;
fn visit(&mut self, edge: Self::Edge) -> VisitControlFlow<Node> {
VisitControlFlow::Continue(edge)
}
fn edges(&mut self, node: &Node) -> Self::EdgesFuture {
(self)(node)
}
}
pub struct ImplValue;
impl<Node, VisitFn, NeighFut, NeighIt> Visit<Node, !, ImplValue> for VisitFn
where
Node: Clone,
VisitFn: FnMut(Node) -> NeighFut,
NeighFut: Future<Output = Result<NeighIt>> + Send,
NeighIt: IntoIterator<Item = Node>,
{
type Edge = Node;
type EdgesIntoIter = NeighIt;
type EdgesFuture = NeighFut;
fn visit(&mut self, edge: Self::Edge) -> VisitControlFlow<Node> {
VisitControlFlow::Continue(edge)
}
fn edges(&mut self, node: &Node) -> Self::EdgesFuture {
(self)(node.clone())
}
}
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