proc_macro/bridge/
selfless_reify.rs

1//! Abstraction for creating `fn` pointers from any callable that *effectively*
2//! has the equivalent of implementing `Default`, even if the compiler neither
3//! provides `Default` nor allows reifying closures (i.e. creating `fn` pointers)
4//! other than those with absolutely no captures.
5//!
6//! More specifically, for a closure-like type to be "effectively `Default`":
7//! * it must be a ZST (zero-sized type): no information contained within, so
8//!   that `Default`'s return value (if it were implemented) is unambiguous
9//! * it must be `Copy`: no captured "unique ZST tokens" or any other similar
10//!   types that would make duplicating values at will unsound
11//!   * combined with the ZST requirement, this confers a kind of "telecopy"
12//!     ability: similar to `Copy`, but without keeping the value around, and
13//!     instead "reconstructing" it (a noop given it's a ZST) when needed
14//! * it must be *provably* inhabited: no captured uninhabited types or any
15//!   other types that cannot be constructed by the user of this abstraction
16//!   * the proof is a value of the closure-like type itself, in a sense the
17//!     "seed" for the "telecopy" process made possible by ZST + `Copy`
18//!   * this requirement is the only reason an abstraction limited to a specific
19//!     usecase is required: ZST + `Copy` can be checked with *at worst* a panic
20//!     at the "attempted `::default()` call" time, but that doesn't guarantee
21//!     that the value can be soundly created, and attempting to use the typical
22//!     "proof ZST token" approach leads yet again to having a ZST + `Copy` type
23//!     that is not proof of anything without a value (i.e. isomorphic to a
24//!     newtype of the type it's trying to prove the inhabitation of)
25//!
26//! A more flexible (and safer) solution to the general problem could exist once
27//! `const`-generic parameters can have type parameters in their types:
28//!
29//! ```rust,ignore (needs future const-generics)
30//! extern "C" fn ffi_wrapper<
31//!     A, R,
32//!     F: Fn(A) -> R,
33//!     const f: F, // <-- this `const`-generic is not yet allowed
34//! >(arg: A) -> R {
35//!     f(arg)
36//! }
37//! ```
38
39use std::mem;
40
41pub(super) const fn reify_to_extern_c_fn_hrt_bridge<
42    R,
43    F: Fn(super::BridgeConfig<'_>) -> R + Copy,
44>(
45    f: F,
46) -> extern "C" fn(super::BridgeConfig<'_>) -> R {
47    // FIXME(eddyb) describe the `F` type (e.g. via `type_name::<F>`) once panic
48    // formatting becomes possible in `const fn`.
49    const {
50        assert!(size_of::<F>() == 0, "selfless_reify: closure must be zero-sized");
51    }
52    extern "C" fn wrapper<R, F: Fn(super::BridgeConfig<'_>) -> R + Copy>(
53        bridge: super::BridgeConfig<'_>,
54    ) -> R {
55        let f = unsafe {
56            // SAFETY: `F` satisfies all criteria for "out of thin air"
57            // reconstructability (see module-level doc comment).
58            mem::conjure_zst::<F>()
59        };
60        f(bridge)
61    }
62    let _f_proof = f;
63    wrapper::<R, F>
64}
proc_macro/bridge/selfless_reify.rs

proc_macro/bridge/
selfless_reify.rs
