Type Alias Fallback 

pub type Fallback = Box<dyn Fn() -> Result<Vec<u8>, FileCacheError>>;

Ported from the Fallback function type. Go’s version takes a context.Context; this port’s fallback signature omits it for now (see crate::exec::context’s discussion of ambient context) and will need revisiting once cancellation is threaded through in Phase 4.

Aliased Type

pub struct Fallback(/* private fields */);