rustc_middle/query/arena_cached.rs
1/// Helper trait that allows `arena_cache` queries to return `Option<&T>`
2/// instead of `&Option<T>`, and avoid allocating `None` in the arena.
3///
4/// An arena-cached query must be declared to return a type that implements
5/// this trait, i.e. either `&'tcx T` or `Option<&'tcx T>`. This trait then
6/// determines the types returned by the provider and stored in the arena,
7/// and provides a function to bridge between the three types.
8pub trait ArenaCached<'tcx>: Sized {
9 /// Type that is returned by the query provider.
10 type Provided;
11 /// Type that is stored in the arena.
12 type Allocated: 'tcx;
13
14 /// Takes a provided value, and allocates it in the arena (if appropriate)
15 /// with the help of the given `arena_alloc` closure.
16 fn alloc_in_arena(
17 arena_alloc: impl Fn(Self::Allocated) -> &'tcx Self::Allocated,
18 value: Self::Provided,
19 ) -> Self;
20}
21
22impl<'tcx, T> ArenaCached<'tcx> for &'tcx T {
23 type Provided = T;
24 type Allocated = T;
25
26 fn alloc_in_arena(
27 arena_alloc: impl Fn(Self::Allocated) -> &'tcx Self::Allocated,
28 value: Self::Provided,
29 ) -> Self {
30 // Just allocate in the arena normally.
31 arena_alloc(value)
32 }
33}
34
35impl<'tcx, T> ArenaCached<'tcx> for Option<&'tcx T> {
36 type Provided = Option<T>;
37 /// The provide value is `Option<T>`, but we only store `T` in the arena.
38 type Allocated = T;
39
40 fn alloc_in_arena(
41 arena_alloc: impl Fn(Self::Allocated) -> &'tcx Self::Allocated,
42 value: Self::Provided,
43 ) -> Self {
44 // Don't store None in the arena, and wrap the allocated reference in Some.
45 value.map(arena_alloc)
46 }
47}