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Diffstat (limited to 'library/proc_macro/src/bridge/selfless_reify.rs')
| -rw-r--r-- | library/proc_macro/src/bridge/selfless_reify.rs | 84 |
1 files changed, 84 insertions, 0 deletions
diff --git a/library/proc_macro/src/bridge/selfless_reify.rs b/library/proc_macro/src/bridge/selfless_reify.rs new file mode 100644 index 000000000..907ad256e --- /dev/null +++ b/library/proc_macro/src/bridge/selfless_reify.rs @@ -0,0 +1,84 @@ +//! Abstraction for creating `fn` pointers from any callable that *effectively* +//! has the equivalent of implementing `Default`, even if the compiler neither +//! provides `Default` nor allows reifying closures (i.e. creating `fn` pointers) +//! other than those with absolutely no captures. +//! +//! More specifically, for a closure-like type to be "effectively `Default`": +//! * it must be a ZST (zero-sized type): no information contained within, so +//! that `Default`'s return value (if it were implemented) is unambiguous +//! * it must be `Copy`: no captured "unique ZST tokens" or any other similar +//! types that would make duplicating values at will unsound +//! * combined with the ZST requirement, this confers a kind of "telecopy" +//! ability: similar to `Copy`, but without keeping the value around, and +//! instead "reconstructing" it (a noop given it's a ZST) when needed +//! * it must be *provably* inhabited: no captured uninhabited types or any +//! other types that cannot be constructed by the user of this abstraction +//! * the proof is a value of the closure-like type itself, in a sense the +//! "seed" for the "telecopy" process made possible by ZST + `Copy` +//! * this requirement is the only reason an abstraction limited to a specific +//! usecase is required: ZST + `Copy` can be checked with *at worst* a panic +//! at the "attempted `::default()` call" time, but that doesn't guarantee +//! that the value can be soundly created, and attempting to use the typical +//! "proof ZST token" approach leads yet again to having a ZST + `Copy` type +//! that is not proof of anything without a value (i.e. isomorphic to a +//! newtype of the type it's trying to prove the inhabitation of) +//! +//! A more flexible (and safer) solution to the general problem could exist once +//! `const`-generic parameters can have type parameters in their types: +//! +//! ```rust,ignore (needs future const-generics) +//! extern "C" fn ffi_wrapper< +//! A, R, +//! F: Fn(A) -> R, +//! const f: F, // <-- this `const`-generic is not yet allowed +//! >(arg: A) -> R { +//! f(arg) +//! } +//! ``` + +use std::mem; + +// FIXME(eddyb) this could be `trait` impls except for the `const fn` requirement. +macro_rules! define_reify_functions { + ($( + fn $name:ident $(<$($param:ident),*>)? + for $(extern $abi:tt)? fn($($arg:ident: $arg_ty:ty),*) -> $ret_ty:ty; + )+) => { + $(pub const fn $name< + $($($param,)*)? + F: Fn($($arg_ty),*) -> $ret_ty + Copy + >(f: F) -> $(extern $abi)? fn($($arg_ty),*) -> $ret_ty { + // FIXME(eddyb) describe the `F` type (e.g. via `type_name::<F>`) once panic + // formatting becomes possible in `const fn`. + assert!(mem::size_of::<F>() == 0, "selfless_reify: closure must be zero-sized"); + + $(extern $abi)? fn wrapper< + $($($param,)*)? + F: Fn($($arg_ty),*) -> $ret_ty + Copy + >($($arg: $arg_ty),*) -> $ret_ty { + let f = unsafe { + // SAFETY: `F` satisfies all criteria for "out of thin air" + // reconstructability (see module-level doc comment). + mem::MaybeUninit::<F>::uninit().assume_init() + }; + f($($arg),*) + } + let _f_proof = f; + wrapper::< + $($($param,)*)? + F + > + })+ + } +} + +define_reify_functions! { + fn _reify_to_extern_c_fn_unary<A, R> for extern "C" fn(arg: A) -> R; + + // HACK(eddyb) this abstraction is used with `for<'a> fn(BridgeConfig<'a>) + // -> T` but that doesn't work with just `reify_to_extern_c_fn_unary` + // because of the `fn` pointer type being "higher-ranked" (i.e. the + // `for<'a>` binder). + // FIXME(eddyb) try to remove the lifetime from `BridgeConfig`, that'd help. + fn reify_to_extern_c_fn_hrt_bridge<R> for extern "C" fn(bridge: super::BridgeConfig<'_>) -> R; +} |