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#[cfg(doc)] use crate::StackBox; /** Helper macro to define custom `StackBox<dyn …>` trait objects. Mainly useful for [`FnOnce`], **when higher-order lifetimes** are needed. <details><summary><b>What are higher-order lifetimes?</b></summary> Consider the following example: you are able to create some local value within a function, and want to call some provided callback on a _borrow_ of it. ```rust # use ::core::cell::RefCell; # fn with_nested_refcell ( nested: &'_ RefCell<RefCell<str>>, f: fn(&'_ str), ) { f(&*nested.borrow().borrow()) } ``` The main question then is: > what is the lifetime of that borrow? 1. If we try to unsugar each elided lifetime (`'_`) with a lifetime parameter on the function, the code no longer compiles: ```rust,compile_fail # use ::core::cell::RefCell; # fn with_nested_refcell<'nested, 'arg> ( nested: &'nested RefCell<RefCell<str>>, f: fn(&'arg str), ) { f(&*nested.borrow().borrow()) } ``` <details><summary>Error message</summary> ```text error[E0716]: temporary value dropped while borrowed --> src/lib.rs:8:9 | 3 | fn with_nested_refcell<'nested, 'arg> ( | ---- lifetime `'arg` defined here ... 8 | f(&*nested.borrow().borrow()) | ^^^^^^^^^^^^^^^--------- | | | creates a temporary which is freed while still in use | argument requires that borrow lasts for `'arg` 9 | } | - temporary value is freed at the end of this statement error[E0716]: temporary value dropped while borrowed --> src/lib.rs:8:9 | 3 | fn with_nested_refcell<'nested, 'arg> ( | ---- lifetime `'arg` defined here ... 8 | f(&*nested.borrow().borrow()) | ----^^^^^^^^^^^^^^^^^^^^^^^^- | | | | | creates a temporary which is freed while still in use | argument requires that borrow lasts for `'arg` 9 | } | - temporary value is freed at the end of this statement ``` ___ </details> 1. So Rust considers that **generic lifetime parameters represent lifetimes that span beyond the end of a function's body** (_c.f._ the previous error message). Or in other words, since generic parameters, including generic lifetime parameters, are _chosen by the caller_, and since the body of a callee is opaque to the caller, it is impossible for the caller to provide / choose a lifetime that is able to match an internal scope. 1. Because of that, in order for a callback to be able to use a borrow with some existential but unnameable lifetime, only one solution remains: > **That the caller provide a callback able to handle _all_ the lifetimes / _any lifetime possible_**. - Imaginary syntax: ```rust # macro_rules! ignore {($($t:tt)*) => ()} ignore! { fn with_nested_refcell<'nested> ( nested: &'nested RefCell<RefCell<str>>, f: fn<'arg>(&'arg str), ) { f(&*nested.borrow().borrow()) } # } fn main () {} ``` the difference then being: <!-- ```diff - fn with_nested_refcell<'nested, 'arg> ( + fn with_nested_refcell<'nested> ( nested: &'nested RefCell<RefCell<str>>, - f: fn(&'arg str), + f: fn<'arg>(&'arg str), ) ``` --> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/> <style type="text/css"> pre { white-space: pre-wrap; } .ef0,.f0 { color: #000000; } .eb0,.b0 { background-color: #000000; } .ef1,.f1 { color: #AA0000; } .eb1,.b1 { background-color: #AA0000; } .ef2,.f2 { color: #00AA00; } .eb2,.b2 { background-color: #00AA00; 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} .blink { text-decoration: blink; } /* Avoid pixels between adjacent span elements. Note this only works for lines less than 80 chars where we close span elements on the same line. span { display: inline-block; } */ </style> </head> <body class="f9 b9"> <pre> <span class="f1">- fn with_nested_refcell<'nested, 'arg> (</span> <span class="f2">+ fn with_nested_refcell<'nested> (</span> nested: &'nested RefCell<RefCell<str>>, <span class="f1">- f: fn(&'arg str),</span> <span class="f2">+ f: fn<'arg>(&'arg str),</span> ) </pre> </body> </html> This is actually a real property with real syntax that can be expressed in Rust, and thus that Rust can check, and is precisely the mechanism that enables to have closures (and other objects involving custom traits with lifetimes, such as [`Deserialize<'de>`]( https://serde.rs/lifetimes.html)) operate on borrow over callee locals. The real syntax for this, is: ```rust # use ::core::cell::RefCell; # fn with_nested_refcell<'nested> ( nested: &'nested RefCell<RefCell<str>>, f: for<'arg> fn(&'arg str), // ≈ fn<'arg> (&'arg str), ) { f(&*nested.borrow().borrow()) } ``` - For function pointer types (`fn`): ```rust # macro_rules! ignore {($($t:tt)*) => ()} ignore! { for<'lifetimes...> fn(args...) -> Ret # } fn main () {} ``` - For trait bounds: ```rust # macro_rules! ignore {($($t:tt)*) => ()} ignore! { for<'lifetimes...> Type : Bounds , // e.g., for<'arg> F : Fn(&'arg str) , # } fn main () {} ``` or: ```rust # macro_rules! ignore {($($t:tt)*) => ()} ignore! { Type : for<'lifetimes...> Bounds // e.g., F : for<'arg> Fn(&'arg str) # } fn main () {} ``` Back to the `::serde::Deserialize` example, it can be interesting to observe that `DeserializeOwned` is defined as the following simple trait alias: ```rust # macro_rules! ignore {($($t:tt)*) => ()} ignore! { DeserializeOwned = for<'any> Deserialize<'any> # } fn main () {} ``` > **This whole thing is called Higher-Rank Trait Bounds ([HRTB]) or > higher-order lifetimes.** [HRTB]: https://doc.rust-lang.org/nomicon/hrtb.html ___ Ok, time to go back to the question at hand, that of using the [`custom_dyn!`] macro in the context of [`StackBox`]es: </details> So the problem is that the currently defined types and impls in this crate do not support higher-order lifetimes. Indeed, although it is possible to write an impl that supports a specific higher-order signature, it is currently impossible in Rust to write generic code that covers all the possible higher-order signatures: 1. For instance, given an `impl<A> Trait for fn(A) { … }`, we won't have `fn(&str)` implementing `Trait`, since `fn(&str) = for<'any> fn(&'any str) ≠ fn(A)`. 1. And even if we wrote `impl<A : ?Sized> Trait for fn(&A) { … }`, we won't have `fn(&&str)` implementing `Trait`, since ```rust # macro_rules! ignore {($($t:tt)*) => ()} ignore! { fn(&&str) = for<'a, 'b> fn(&'a &'b str) != for<'c> fn(&'c A) = fn(&A) # } fn main () {} ``` That's where [`custom_dyn!`] comes into play: - I, the crate author, cannot know which higher-order signature(s) you, the user of the library, will be using, and sadly cannot cover that with a generic impl. - But since _you_ know which kind of signature you need, I can "let you impl yourself". Sadly, the actual impl is complex and error-prone (involves `unsafe` and VTables!), so instead I provide you this handy macro that will take care of all the nitty-gritty details for you. In a way, since I am hitting a limitation of the too-early-type-checked Rust metaprogramming tool (generics), I am falling back to the duck-typed / only-type-checked-when-instanced metaprogramming tool (macros), thus acting as a C++ template of sorts, we could say 😄 ## Example: `dyn FnOnce(&str) = dyn for<'any> FnOnce(&'any str)` The following example fails to compile: ```rust,compile_fail use ::stackbox::prelude::*; // `f: StackBox<dyn FnOnce(&'arg str) -> ()>` fn call_with_local<'arg> (f: StackBoxDynFnOnce_1<&'arg str, ()>) { let local = format!("..."); f.call(&local) } fn main () { stackbox!(let f = |_: &str| ()); call_with_local(f.into_dyn()); } ``` <details><summary>Error message</summary> ```text error[E0597]: `local` does not live long enough --> src/some/file.rs:8:12 | 5 | fn call_with_local<'arg> (f: StackBoxDynFnOnce_1<&'arg str, ()>) | ---- lifetime `'arg` defined here ... 8 | f.call(&local) | -------^^^^^^- | | | | | borrowed value does not live long enough | argument requires that `local` is borrowed for `'arg` 9 | } | - `local` dropped here while still borrowed ``` This is exactly the same problem we had when I was explaining higher-order signatures and we had `fn nested_refcells<'nested, 'arg>...`: the lifetime of the parameter is an _outer_ (fixed) generic lifetime parameter, and it can thus not work with a local / callee-specific lifetime. ___ </details> The solution is to define a new `DynFnOnce...` trait, which involves a higher-order lifetime in the signature: ```rust use ::stackbox::prelude::*; custom_dyn! { pub dyn FnOnceStr : FnOnce(&str) { fn call (self: Self, arg: &str) { self(arg) } } } // `f: StackBox<dyn FnOnce(&str) -> ()>` fn call_with_local (f: StackBoxDynFnOnceStr) { let local = format!("..."); f.call(&local) } fn main () { stackbox!(let f = |_: &str| ()); call_with_local(f.into_dyn()); } ``` */ #[macro_export] macro_rules! custom_dyn {( #![dollar = $__:tt] $( #[doc = $doc:expr] )* $pub:vis dyn $Trait:ident $( < $($lt:lifetime),* $(,)? $($T:ident),* $(,)? > )? : $super:path $( where { $($wc:tt)* } )? { $( fn $method:ident ( $self:ident : $( & $($ref:lifetime)? $(mut $(@$mut:tt)?)? )? Self $(, $arg_name:ident : $ArgTy:ty )* $(,)? ) $(-> $RetTy:ty)? { $($body:tt)* } )* } ) => ($crate::__::paste! { $( #[doc = $doc] )* $pub struct [<StackBoxDyn $Trait>] < '__frame, $($($lt : '__frame ,)* $($T : '__frame ,)*)? __AutoTraits : ?$crate::__::Sized + $crate::__::Sendness + $crate::__::Syncness = $crate::__::NoAutoTraits, > $( where $($wc)* )? { ptr: $crate::__::ErasedPtr, vtable: &'__frame <Self as $crate::__::GetVTable>::VTable, _auto_traits: $crate::__::PhantomData<__AutoTraits>, } const _: () = { trait $Trait<$($($lt ,)* $($T ,)*)?> : $crate::__::Sized + $super $( where $($wc)* )? { $( #[inline(always)] fn [<__ $method>] ( // underscored to avoid conflicts $self : $( & $($ref)? $(mut $(@$mut)?)? )? Self $(, $arg_name : $ArgTy )* ) $(-> $RetTy)? { $($body)* } )* } impl<$($($lt ,)* $($T ,)*)? __Self : $super> $Trait<$($($lt ,)* $($T ,)*)?> for __Self $(where $($wc)* )? {} pub struct __VTable<$($($lt ,)* $($T ,)*)?> $( where $($wc)* )? { drop_in_place: unsafe fn ($crate::__::ErasedPtr), $( $method: unsafe fn( __ptr: $crate::__::ErasedPtr $(, $arg_name: $ArgTy )* ) $(-> $RetTy)? , )* } impl< '__frame, $($($lt ,)* $($T : '__frame ,)*)? __AutoTraits : ?$crate::__::Sized + $crate::__::Sendness + $crate::__::Syncness, > $crate::__::GetVTable for [<StackBoxDyn $Trait>]<'__frame, $($($lt ,)* $($T ,)*)? __AutoTraits> $( where $($wc)* )? { type VTable = __VTable<$($($lt ,)* $($T ,)*)?>; } trait HasVTable< '__frame, $($($lt ,)* $($T : '__frame ,)*)? > : $crate::__::Sized $( where $($wc)* )? { const VTABLE: __VTable<$($($lt ,)* $($T ,)*)?>; } impl< '__frame, $($($lt ,)* $($T : '__frame ,)*)? __Self : $super, > HasVTable<'__frame, $($($lt ,)* $($T ,)*)?> for __Self $( where $($wc)* )? { const VTABLE: __VTable<$($($lt ,)* $($T ,)*)?> = __VTable { drop_in_place: $crate::__::drop_in_place::<Self>, $( $method: { | __ptr: $crate::__::ErasedPtr $(, $arg_name: $ArgTy )* | $( -> $RetTy )? { unsafe { // Safety: immediately coerced to an `unsafe fn` let _convert = |ptr: $crate::__::ErasedPtr| -> Self { let ptr: $crate::StackBox<'_, Self> = $crate::__::transmute(ptr) ; ptr.into_inner() } ; { $( let _convert = $crate::__::transmute::< $crate::__::ErasedPtr, &'_ $(mut $($mut)?)? Self, > ; )? _convert(__ptr).[<__ $method>]($($arg_name),*) } }} }, )* }; } define_coercions! { [$crate::__::Send] => dyn $crate::__::Send, [$crate::__::Sync] => dyn $crate::__::Sync, [$crate::__::Send, $crate::__::Sync] => dyn $crate::__::Send + $crate::__::Sync, [] => $crate::__::NoAutoTraits, } macro_rules! define_coercions {( $__( [$__($AutoTrait:path),* $__(,)?] => $Marker:ty ),* $__(,)? ) => ( $__( impl< '__frame, $($($lt : '__frame ,)* $($T : '__frame ,)*)? __Pointee : $super > $crate::__::DynCoerce<$crate::StackBox<'__frame, __Pointee>> for [<StackBoxDyn $Trait>]<'__frame, $($($lt ,)* $($T ,)*)? $Marker> where $__( __Pointee : $AutoTrait, )* $($($wc)*)? { fn fatten (it: $crate::StackBox<'__frame, __Pointee>) -> Self { Self { vtable: &<__Pointee as HasVTable<'__frame, $($($lt ,)* $($T ,)*)?>>::VTABLE, ptr: unsafe { $crate::__::transmute(it) }, _auto_traits: $crate::__::PhantomData, } } } $__( unsafe // Safety: from the `DynCoerce` bound added at construction site. impl<'__frame, $($($lt : '__frame ,)* $($T : '__frame ,)*)?> $AutoTrait for [<StackBoxDyn $Trait>]<'__frame, $($($lt ,)* $($T ,)*)? $Marker> {} )* )* )} use define_coercions; impl< '__frame, $($($lt : '__frame ,)* $($T : '__frame ,)*)? __AutoTraits : ?$crate::__::Sized + $crate::__::Sendness + $crate::__::Syncness, > [<StackBoxDyn $Trait>]<'__frame, $($($lt ,)* $($T ,)*)? __AutoTraits> $(where $($wc)* )? { $( $pub fn $method ( self: $( & $($ref)? $(mut $($mut)?)? )? Self $(, $arg_name: $ArgTy )* ) $(-> $RetTy)? { unsafe { (self.vtable.$method)( $crate::__::ManuallyDrop::new(self).ptr $(, $arg_name )* ) } } )* } impl< '__frame, $($($lt : '__frame ,)* $($T : '__frame ,)*)? __AutoTraits : ?$crate::__::Sized + $crate::__::Sendness + $crate::__::Syncness, > $crate::__::Drop for [<StackBoxDyn $Trait>]<'__frame, $($($lt ,)* $($T ,)*)? __AutoTraits> $(where $($wc)* )? { fn drop (self: &'_ mut Self) { unsafe { (self.vtable.drop_in_place)(self.ptr) } } } }; }); ( // Missing dollar case $( #[doc = $doc:expr ] )* $pub:vis dyn $($rest:tt)* ) => ($crate::custom_dyn! { #![dollar = $] $( #[doc = $doc] )* $pub dyn $($rest)* })}