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
41// FIXME(eddyb) this could be `trait` impls except for the `const fn` requirement.
42macro_rules! define_reify_functions {
43    ($(
44        fn $name:ident $(<$($param:ident),*>)?
45            for $(extern $abi:tt)? fn($($arg:ident: $arg_ty:ty),*) -> $ret_ty:ty;
46    )+) => {
47        $(pub(super) const fn $name<
48            $($($param,)*)?
49            F: Fn($($arg_ty),*) -> $ret_ty + Copy
50        >(f: F) -> $(extern $abi)? fn($($arg_ty),*) -> $ret_ty {
51            // FIXME(eddyb) describe the `F` type (e.g. via `type_name::<F>`) once panic
52            // formatting becomes possible in `const fn`.
53            assert!(size_of::<F>() == 0, "selfless_reify: closure must be zero-sized");
54
55            $(extern $abi)? fn wrapper<
56                $($($param,)*)?
57                F: Fn($($arg_ty),*) -> $ret_ty + Copy
58            >($($arg: $arg_ty),*) -> $ret_ty {
59                let f = unsafe {
60                    // SAFETY: `F` satisfies all criteria for "out of thin air"
61                    // reconstructability (see module-level doc comment).
62                    mem::MaybeUninit::<F>::uninit().assume_init()
63                };
64                f($($arg),*)
65            }
66            let _f_proof = f;
67            wrapper::<
68                $($($param,)*)?
69                F
70            >
71        })+
72    }
73}
74
75define_reify_functions! {
76    fn _reify_to_extern_c_fn_unary<A, R> for extern "C" fn(arg: A) -> R;
77
78    // HACK(eddyb) this abstraction is used with `for<'a> fn(BridgeConfig<'a>)
79    // -> T` but that doesn't work with just `reify_to_extern_c_fn_unary`
80    // because of the `fn` pointer type being "higher-ranked" (i.e. the
81    // `for<'a>` binder).
82    // FIXME(eddyb) try to remove the lifetime from `BridgeConfig`, that'd help.
83    fn reify_to_extern_c_fn_hrt_bridge<R> for extern "C" fn(bridge: super::BridgeConfig<'_>) -> R;
84}