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/// Helper trait that allows `arena_cache` queries to return `Option<&T>`
/// instead of `&Option<T>`, and avoid allocating `None` in the arena.
///
/// An arena-cached query must be declared to return a type that implements
/// this trait, i.e. either `&'tcx T` or `Option<&'tcx T>`. This trait then
/// determines the types returned by the provider and stored in the arena,
/// and provides a function to bridge between the three types.
pub trait ArenaCached<'tcx>: Sized {
/// Type that is returned by the query provider.
type Provided;
/// Type that is stored in the arena.
type Allocated: 'tcx;
/// Takes a provided value, and allocates it in the arena (if appropriate)
/// with the help of the given `arena_alloc` closure.
fn alloc_in_arena(
arena_alloc: impl Fn(Self::Allocated) -> &'tcx Self::Allocated,
value: Self::Provided,
) -> Self;
}
impl<'tcx, T> ArenaCached<'tcx> for &'tcx T {
type Provided = T;
type Allocated = T;
fn alloc_in_arena(
arena_alloc: impl Fn(Self::Allocated) -> &'tcx Self::Allocated,
value: Self::Provided,
) -> Self {
// Just allocate in the arena normally.
arena_alloc(value)
}
}
impl<'tcx, T> ArenaCached<'tcx> for Option<&'tcx T> {
type Provided = Option<T>;
/// The provide value is `Option<T>`, but we only store `T` in the arena.
type Allocated = T;
fn alloc_in_arena(
arena_alloc: impl Fn(Self::Allocated) -> &'tcx Self::Allocated,
value: Self::Provided,
) -> Self {
// Don't store None in the arena, and wrap the allocated reference in Some.
value.map(arena_alloc)
}
}
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