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Therefore, these fields need to have their lifetime erased. A naive implementation would be to replace their lifetimes with `'static`. However, doing so is unsound for multiple reasons: * Users may directly access such field with field access syntax, and get exposed with wrong lifetime; * Auto trait implementations will cause the struct to be implementing auto traits when the type only implements the auto trait for specific lifetime. This is similar to how specialization can be unsound if specialized on lifetime. Create a `Erase` type, which has `for<'a> fn(&'a ()) -> Foo<'a>` as generic parameter. Internally, it uses a helper trait to resolve that to `Foo<'static>`. The first issue is solved by not exposing any public accessor on that type. The second issue is solved by add custom `Send` and `Sync` implementations that requires `Send` to be implemented for all lifetimes, thus closing the lifetime specialization hole. The actual implementation is a bit more convoluted because it supports erasing multiple lifetimes. This is more or less a stable polyfill of the unstable `unsafe_binder` feature, without `unsafe_binders`'s no drop glue requirement. Signed-off-by: Gary Guo --- rust/pin-init/internal/src/pin_data.rs | 16 +++++++ rust/pin-init/src/__internal.rs | 77 ++++++++++++++++++++++++++++++++++ 2 files changed, 93 insertions(+) diff --git a/rust/pin-init/internal/src/pin_data.rs b/rust/pin-init/internal/src/pin_data.rs index cf6142cd656d..868701c9a986 100644 --- a/rust/pin-init/internal/src/pin_data.rs +++ b/rust/pin-init/internal/src/pin_data.rs @@ -417,6 +417,22 @@ fn generate_struct_def(info: &StructInfo) -> TokenStream { ty, } = &field.field; + let mut ty = ty.to_token_stream(); + + // Replace lifetime for self-referential fields. + if !field.captures.is_empty() { + // Build a chain `for<'a> fn(&'a ()) -> ... -> (Ty,)`. Such type will have a `EraseTy` + // implementation and thus may be used inside `Erased`. + ty = quote!((#ty,)); + + for borrow in field.captures.iter().rev() { + let lt = &borrow.lifetime; + ty = quote!(for<#lt> fn(&#lt()) -> #ty); + } + + ty = quote!(::pin_init::__internal::Erase<#ty>); + }; + quote! { #(#attrs)* #vis #ident #colon_token #ty } diff --git a/rust/pin-init/src/__internal.rs b/rust/pin-init/src/__internal.rs index cba53b8c3c94..80c5624d78c9 100644 --- a/rust/pin-init/src/__internal.rs +++ b/rust/pin-init/src/__internal.rs @@ -385,3 +385,80 @@ unsafe fn __init(self, _slot: *mut T) -> Result<(), ()> { Err(()) } } +/// Polyfill of `FnOnce` trait to be able to reference output via associated type. +pub trait FnOutput { + type Output; +} + +macro_rules! impl_fn_output { + () => {}; + ($ret:ident, $($arg:ident,)*) => { + impl FnOutput<($($arg,)*)> for This + where + This: FnOnce($($arg,)*) -> $ret, + { + type Output = $ret; + } + impl_fn_output!($($arg,)*); + }; +} + +impl_fn_output!(A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U,); + +/// Lifetime erasure facility. +/// +/// Say we have `exists<'a, 'b> Foo<'a, 'b>` and we want to store it. There's no concrete +/// lifetimes we can use, so we want to erase the lifetime. +/// +/// Such erasure can be encoded as +/// `Erased fn(&'a ()) -> for<'b> fn(&'b()) -> (Foo<'a, 'b>,)`. +/// +/// This can be considered the stable version of Rust's `unsafe_binder` feature, without the +/// no-drop-glue requirement. +#[repr(transparent)] +#[allow(private_bounds)] +pub struct Erase(F::Erased); + +/// Helper trait to resolve the erased lifetime. +trait EraseLt { + type Erased; +} + +impl EraseLt for (T,) { + type Erased = T; +} + +impl EraseLt for T +where + T: for<'a> FnOutput<(&'a (),), Output: EraseLt>, +{ + type Erased = <>::Output as EraseLt>::Erased; +} + +// The default `Send` and `Sync` are not sufficient, because one can use lifetime specialization to +// implement `Send` or `Sync` for a concrete instance of lifetime. Use HRTB to ensure that the type +// will only implement `Send` or `Sync` if it's implemented for *all* erased lifetimes. + +// SAFETY: Trivial, no lifetime to erase. +unsafe impl Send for Erase<(T,)> {} + +// SAFETY: If we erased a lifetime, then the type needs to be `Send` for across *all* that +// lifetimes. +unsafe impl Send for Erase +where + F: for<'a> FnOutput<(&'a (),), Output: EraseLt>, + for<'a> Erase<>::Output>: Send, +{ +} + +// SAFETY: Trivial, no lifetime to erase. +unsafe impl Sync for Erase<(T,)> {} + +// SAFETY: If we erased a lifetime, then the type needs to be `Send` for across *all* that +// lifetimes. +unsafe impl Sync for Erase +where + F: for<'a> FnOutput<(&'a (),), Output: EraseLt>, + for<'a> Erase<>::Output>: Sync, +{ +} -- 2.54.0