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//! # Overview
//!
//! `self_cell` provides one macro-rules macro: [`self_cell`]. With this macro
//! you can create self-referential structs that are safe-to-use in stable Rust,
//! without leaking the struct internal lifetime.
//!
//! In a nutshell, the API looks *roughly* like this:
//!
//! ```ignore
//! // User code:
//!
//! self_cell!(
//! struct NewStructName {
//! owner: Owner,
//!
//! #[covariant]
//! dependent: Dependent,
//! }
//!
//! impl {Debug}
//! );
//!
//! // Generated by macro:
//!
//! struct NewStructName(...);
//!
//! impl NewStructName {
//! fn new(
//! owner: Owner,
//! dependent_builder: impl for<'a> FnOnce(&'a Owner) -> Dependent<'a>
//! ) -> NewStructName { ... }
//! fn borrow_owner<'a>(&'a self) -> &'a Owner { ... }
//! fn borrow_dependent<'a>(&'a self) -> &'a Dependent<'a> { ... }
//! }
//!
//! impl Debug for NewStructName { ... }
//! ```
//!
//! Self-referential structs are currently not supported with safe vanilla Rust.
//! The only reasonable safe alternative is to have the user juggle 2 separate
//! data structures which is a mess. The library solution ouroboros is really
//! expensive to compile due to its use of procedural macros.
//!
//! This alternative is `no_std`, uses no proc-macros, some self contained
//! unsafe and works on stable Rust, and is miri tested. With a total of less
//! than 300 lines of implementation code, which consists mostly of type and
//! trait implementations, this crate aims to be a good minimal solution to the
//! problem of self-referential structs.
//!
//! It has undergone [community code
//! review](https://users.rust-lang.org/t/experimental-safe-to-use-proc-macro-free-self-referential-structs-in-stable-rust/52775)
//! from experienced Rust users.
//!
//! ### Fast compile times
//!
//! ```txt
//! $ rm -rf target && cargo +nightly build -Z timings
//!
//! Compiling self_cell v0.7.0
//! Completed self_cell v0.7.0 in 0.2s
//! ```
//!
//! Because it does **not** use proc-macros, and has 0 dependencies
//! compile-times are fast.
//!
//! Measurements done on a slow laptop.
//!
//! ### A motivating use case
//!
//! ```rust
//! use self_cell::self_cell;
//!
//! #[derive(Debug, Eq, PartialEq)]
//! struct Ast<'a>(pub Vec<&'a str>);
//!
//! self_cell!(
//! struct AstCell {
//! owner: String,
//!
//! #[covariant]
//! dependent: Ast,
//! }
//!
//! impl {Debug, Eq, PartialEq}
//! );
//!
//! fn build_ast_cell(code: &str) -> AstCell {
//! // Create owning String on stack.
//! let pre_processed_code = code.trim().to_string();
//!
//! // Move String into AstCell, then build Ast inplace.
//! AstCell::new(
//! pre_processed_code,
//! |code| Ast(code.split(' ').filter(|word| word.len() > 1).collect())
//! )
//! }
//!
//! fn main() {
//! let ast_cell = build_ast_cell("fox = cat + dog");
//!
//! println!("ast_cell -> {:?}", &ast_cell);
//! println!("ast_cell.borrow_owner() -> {:?}", ast_cell.borrow_owner());
//! println!("ast_cell.borrow_dependent().0[1] -> {:?}", ast_cell.borrow_dependent().0[1]);
//! }
//! ```
//!
//! ```txt
//! $ cargo run
//!
//! ast_cell -> AstCell { owner: "fox = cat + dog", dependent: Ast(["fox", "cat", "dog"]) }
//! ast_cell.borrow_owner() -> "fox = cat + dog"
//! ast_cell.borrow_dependent().0[1] -> "cat"
//! ```
//!
//! There is no way in safe Rust to have an API like `build_ast_cell`, as soon
//! as `Ast` depends on stack variables like `pre_processed_code` you can't
//! return the value out of the function anymore. You could move the
//! pre-processing into the caller but that gets ugly quickly because you can't
//! encapsulate things anymore. Note this is a somewhat niche use case,
//! self-referential structs should only be used when there is no good
//! alternative.
//!
//! Under the hood, it heap allocates a struct which it initializes first by
//! moving the owner value to it and then using the reference to this now
//! Pin/Immovable owner to construct the dependent inplace next to it. This
//! makes it safe to move the generated SelfCell but you have to pay for the
//! heap allocation.
//!
//! See the documentation for [`self_cell`] to dive further into the details.
//!
//! Or take a look at the advanced examples:
//! - [Example how to handle dependent construction that can
//! fail](https://github.com/Voultapher/self_cell/tree/main/examples/fallible_dependent_construction)
//!
//! - [How to build a lazy AST with
//!
//! - [How to use an owner type with
//!
//! ### Min required rustc version
//!
//! By default the minimum required rustc version is 1.51.
//!
//! There is an optional feature you can enable called "old_rust" that enables
//! support down to rustc version 1.36. However this requires polyfilling std
//! library functionality for older rustc with technically UB versions. Testing
//! does not show older rustc versions (ab)using this. Use at your own risk.
//!
//! The minimum versions are a best effor and may change with any new major
//! release.
#![no_std]
#[doc(hidden)]
pub extern crate alloc;
#[doc(hidden)]
pub mod unsafe_self_cell;
/// This macro declares a new struct of `$StructName` and implements traits
/// based on `$AutomaticDerive`.
///
/// ### Example:
///
/// ```rust
/// use self_cell::self_cell;
///
/// #[derive(Debug, Eq, PartialEq)]
/// struct Ast<'a>(Vec<&'a str>);
///
/// self_cell!(
/// #[doc(hidden)]
/// struct PackedAstCell {
/// owner: String,
///
/// #[covariant]
/// dependent: Ast,
/// }
///
/// impl {Debug, PartialEq, Eq, Hash}
/// );
/// ```
///
/// See the crate overview to get a get an overview and a motivating example.
///
/// ### Generated API:
///
/// The macro implements these constructors:
///
/// ```ignore
/// fn new(
/// owner: $Owner,
/// dependent_builder: impl for<'a> FnOnce(&'a $Owner) -> $Dependent<'a>
/// ) -> Self
/// ```
///
/// ```ignore
/// fn try_new<Err>(
/// owner: $Owner,
/// dependent_builder: impl for<'a> FnOnce(&'a $Owner) -> Result<$Dependent<'a>, Err>
/// ) -> Result<Self, Err>
/// ```
///
/// ```ignore
/// fn try_new_or_recover<Err>(
/// owner: $Owner,
/// dependent_builder: impl for<'a> FnOnce(&'a $Owner) -> Result<$Dependent<'a>, Err>
/// ) -> Result<Self, ($Owner, Err)>
/// ```
///
/// The macro implements these methods:
///
/// ```ignore
/// fn borrow_owner<'a>(&'a self) -> &'a $Owner
/// ```
///
/// ```ignore
/// // Only available if dependent is covariant.
/// fn borrow_dependent<'a>(&'a self) -> &'a $Dependent<'a>
/// ```
///
/// ```ignore
/// fn with_dependent<'outer_fn, Ret>(
/// &'outer_fn self,
/// func: impl for<'a> FnOnce(&'a $Owner, &'outer_fn $Dependent<'a>
/// ) -> Ret) -> Ret
/// ```
///
/// ```ignore
/// fn with_dependent_mut<'outer_fn, Ret>(
/// &'outer_fn mut self,
/// func: impl for<'a> FnOnce(&'a $Owner, &'outer_fn mut $Dependent<'a>) -> Ret
/// ) -> Ret
/// ```
///
/// ```ignore
/// fn into_owner(self) -> $Owner
/// ```
///
///
/// ### Parameters:
///
/// - `$Vis:vis struct $StructName:ident` Name of the struct that will be
/// declared, this needs to be unique for the relevant scope. Example: `struct
/// AstCell` or `pub struct AstCell`. `$Vis` can be used to mark the struct
/// and all functions implemented by the macro as public.
///
/// `$(#[$StructMeta:meta])*` allows you specify further meta items for this
/// struct, eg. `#[doc(hidden)] struct AstCell`.
///
/// - `$Owner:ty` Type of owner. This has to have a `'static` lifetime. Example:
/// `String`.
///
/// - `$Dependent:ident` Name of the dependent type without specified lifetime.
/// This can't be a nested type name. As workaround either create a type alias
/// `type Dep<'a> = Option<Vec<&'a str>>;` or create a new-type `struct
/// Dep<'a>(Option<Vec<&'a str>>);`. Example: `Ast`.
///
/// `$Covariance:ident` Marker declaring if `$Dependent` is
/// Possible Values:
///
/// * **covariant**: This generates the direct reference accessor function
/// `borrow_dependent`. This is only safe to do if this compiles `fn
/// _assert_covariance<'x: 'y, 'y>(x: $Dependent<'x>) -> $Dependent<'y>
/// {x}`. Otherwise you could choose a lifetime that is too short for types
/// with interior mutability like `Cell`, which can lead to UB in safe code.
/// Which would violate the promise of this library that it is safe-to-use.
/// If you accidentally mark a type that is not covariant as covariant, you
/// will get a compile time error.
///
/// * **not_covariant**: This generates no additional code but you can use the
/// `with_dependent` function. See [How to build a lazy AST with
/// for a usage example.
///
/// In both cases you can use the `with_dependent_mut` function to mutate the
/// dependent value. This is safe to do because notionally you are replacing
/// pointers to a value not the other way around.
///
/// - `impl {$($AutomaticDerive:ident),*},` Optional comma separated list of
/// optional automatic trait implementations. Possible Values:
///
/// * **Debug**: Prints the debug representation of owner and dependent.
/// Example: `AstCell { owner: "fox = cat + dog", dependent: Ast(["fox",
/// "cat", "dog"]) }`
///
/// * **PartialEq**: Logic `*self.borrow_owner() == *other.borrow_owner()`,
/// this assumes that `Dependent<'a>::From<&'a Owner>` is deterministic, so
/// that only comparing owner is enough.
///
/// * **Eq**: Will implement the trait marker `Eq` for `$StructName`. Beware
/// if you select this `Eq` will be implemented regardless if `$Owner`
/// implements `Eq`, that's an unfortunate technical limitation.
///
/// * **Hash**: Logic `self.borrow_owner().hash(state);`, this assumes that
/// `Dependent<'a>::From<&'a Owner>` is deterministic, so that only hashing
/// owner is enough.
///
/// All `AutomaticDerive` are optional and you can implement you own version
/// of these traits. The declared struct is part of your module and you are
/// free to implement any trait in any way you want. Access to the unsafe
/// internals is only possible via unsafe functions, so you can't accidentally
/// use them in safe code.
///
/// There is limited nested cell support. Eg, having an owner with non static
/// references. Eg `struct ChildCell<'a> { owner: &'a String, ...`. You can
/// use any lifetime name you want, except `_q` and only a single lifetime is
/// supported, and can only be used in the owner. Due to macro_rules
/// limitations, no `AutomaticDerive` are supported if an owner lifetime is
/// provided.
///
#[macro_export]
macro_rules! self_cell {
(
$(#[$StructMeta:meta])*
$Vis:vis struct $StructName:ident $(<$OwnerLifetime:lifetime>)? {
owner: $Owner:ty,
#[$Covariance:ident]
dependent: $Dependent:ident,
}
$(impl {$($AutomaticDerive:ident),*})?
) => {
#[repr(transparent)]
$(#[$StructMeta])*
$Vis struct $StructName $(<$OwnerLifetime>)? {
unsafe_self_cell: $crate::unsafe_self_cell::UnsafeSelfCell<
$StructName$(<$OwnerLifetime>)?,
$Owner,
$Dependent<'static>
>,
$(owner_marker: $crate::_covariant_owner_marker!($Covariance, $OwnerLifetime) ,)?
}
impl $(<$OwnerLifetime>)? $StructName $(<$OwnerLifetime>)? {
$Vis fn new(
owner: $Owner,
dependent_builder: impl for<'_q> FnOnce(&'_q $Owner) -> $Dependent<'_q>
) -> Self {
use core::ptr::NonNull;
unsafe {
// All this has to happen here, because there is not good way
// of passing the appropriate logic into UnsafeSelfCell::new
// short of assuming Dependent<'static> is the same as
// Dependent<'_q>, which I'm not confident is safe.
// For this API to be safe there has to be no safe way to
// capture additional references in `dependent_builder` and then
// return them as part of Dependent. Eg. it should be impossible
// to express: '_q should outlive 'x here `fn
// bad<'_q>(outside_ref: &'_q String) -> impl for<'x> FnOnce(&'x
// Owner) -> Dependent<'x>`.
type JoinedCell<'_q $(, $OwnerLifetime)?> =
$crate::unsafe_self_cell::JoinedCell<$Owner, $Dependent<'_q>>;
let layout = $crate::alloc::alloc::Layout::new::<JoinedCell>();
assert!(layout.size() != 0);
let joined_void_ptr = NonNull::new($crate::alloc::alloc::alloc(layout)).unwrap();
let mut joined_ptr = core::mem::transmute::<NonNull<u8>, NonNull<JoinedCell>>(
joined_void_ptr
);
let (owner_ptr, dependent_ptr) = JoinedCell::_field_pointers(joined_ptr.as_ptr());
// Move owner into newly allocated space.
owner_ptr.write(owner);
// Drop guard that cleans up should building the dependent panic.
let drop_guard =
$crate::unsafe_self_cell::OwnerAndCellDropGuard::new(joined_ptr);
// Initialize dependent with owner reference in final place.
dependent_ptr.write(dependent_builder(&*owner_ptr));
core::mem::forget(drop_guard);
Self {
unsafe_self_cell: $crate::unsafe_self_cell::UnsafeSelfCell::new(
joined_void_ptr,
),
$(owner_marker: $crate::_covariant_owner_marker_ctor!($OwnerLifetime) ,)?
}
}
}
$Vis fn try_new<Err>(
owner: $Owner,
dependent_builder:
impl for<'_q> FnOnce(&'_q $Owner) -> core::result::Result<$Dependent<'_q>, Err>
) -> core::result::Result<Self, Err> {
use core::ptr::NonNull;
unsafe {
// See fn new for more explanation.
type JoinedCell<'_q $(, $OwnerLifetime)?> =
$crate::unsafe_self_cell::JoinedCell<$Owner, $Dependent<'_q>>;
let layout = $crate::alloc::alloc::Layout::new::<JoinedCell>();
assert!(layout.size() != 0);
let joined_void_ptr = NonNull::new($crate::alloc::alloc::alloc(layout)).unwrap();
let mut joined_ptr = core::mem::transmute::<NonNull<u8>, NonNull<JoinedCell>>(
joined_void_ptr
);
let (owner_ptr, dependent_ptr) = JoinedCell::_field_pointers(joined_ptr.as_ptr());
// Move owner into newly allocated space.
owner_ptr.write(owner);
// Drop guard that cleans up should building the dependent panic.
let mut drop_guard =
$crate::unsafe_self_cell::OwnerAndCellDropGuard::new(joined_ptr);
match dependent_builder(&*owner_ptr) {
Ok(dependent) => {
dependent_ptr.write(dependent);
core::mem::forget(drop_guard);
Ok(Self {
unsafe_self_cell: $crate::unsafe_self_cell::UnsafeSelfCell::new(
joined_void_ptr,
),
$(owner_marker: $crate::_covariant_owner_marker_ctor!($OwnerLifetime) ,)?
})
}
Err(err) => Err(err)
}
}
}
$Vis fn try_new_or_recover<Err>(
owner: $Owner,
dependent_builder:
impl for<'_q> FnOnce(&'_q $Owner) -> core::result::Result<$Dependent<'_q>, Err>
) -> core::result::Result<Self, ($Owner, Err)> {
use core::ptr::NonNull;
unsafe {
// See fn new for more explanation.
type JoinedCell<'_q $(, $OwnerLifetime)?> =
$crate::unsafe_self_cell::JoinedCell<$Owner, $Dependent<'_q>>;
let layout = $crate::alloc::alloc::Layout::new::<JoinedCell>();
assert!(layout.size() != 0);
let joined_void_ptr = NonNull::new($crate::alloc::alloc::alloc(layout)).unwrap();
let mut joined_ptr = core::mem::transmute::<NonNull<u8>, NonNull<JoinedCell>>(
joined_void_ptr
);
let (owner_ptr, dependent_ptr) = JoinedCell::_field_pointers(joined_ptr.as_ptr());
// Move owner into newly allocated space.
owner_ptr.write(owner);
// Drop guard that cleans up should building the dependent panic.
let mut drop_guard =
$crate::unsafe_self_cell::OwnerAndCellDropGuard::new(joined_ptr);
match dependent_builder(&*owner_ptr) {
Ok(dependent) => {
dependent_ptr.write(dependent);
core::mem::forget(drop_guard);
Ok(Self {
unsafe_self_cell: $crate::unsafe_self_cell::UnsafeSelfCell::new(
joined_void_ptr,
),
$(owner_marker: $crate::_covariant_owner_marker_ctor!($OwnerLifetime) ,)?
})
}
Err(err) => {
// In contrast to into_owner ptr::read, here no dependent
// ever existed in this function and so we are sure its
// drop impl can't access owner after the read.
// And err can't return a reference to owner.
let owner_on_err = core::ptr::read(owner_ptr);
// Allowing drop_guard to finish would let it double free owner.
// So we dealloc the JoinedCell here manually.
core::mem::forget(drop_guard);
$crate::alloc::alloc::dealloc(joined_void_ptr.as_ptr(), layout);
Err((owner_on_err, err))
}
}
}
}
$Vis fn borrow_owner<'_q>(&'_q self) -> &'_q $Owner {
unsafe { self.unsafe_self_cell.borrow_owner::<$Dependent<'_q>>() }
}
$Vis fn with_dependent<'outer_fn, Ret>(
&'outer_fn self,
func: impl for<'_q> FnOnce(&'_q $Owner, &'outer_fn $Dependent<'_q>
) -> Ret) -> Ret {
unsafe {
func(
self.unsafe_self_cell.borrow_owner::<$Dependent>(),
self.unsafe_self_cell.borrow_dependent()
)
}
}
$Vis fn with_dependent_mut<'outer_fn, Ret>(
&'outer_fn mut self,
func: impl for<'_q> FnOnce(&'_q $Owner, &'outer_fn mut $Dependent<'_q>) -> Ret
) -> Ret {
let (owner, dependent) = unsafe {
self.unsafe_self_cell.borrow_mut()
};
func(owner, dependent)
}
$crate::_covariant_access!($Covariance, $Vis, $Dependent);
$Vis fn into_owner(self) -> $Owner {
// This is only safe to do with repr(transparent).
let unsafe_self_cell = unsafe { core::mem::transmute::<
Self,
$crate::unsafe_self_cell::UnsafeSelfCell<
$StructName$(<$OwnerLifetime>)?,
$Owner,
$Dependent<'static>
>
>(self) };
let owner = unsafe { unsafe_self_cell.into_owner::<$Dependent>() };
owner
}
}
impl $(<$OwnerLifetime>)? Drop for $StructName $(<$OwnerLifetime>)? {
fn drop(&mut self) {
unsafe {
self.unsafe_self_cell.drop_joined::<$Dependent>();
}
}
}
// The user has to choose which traits can and should be automatically
// implemented for the cell.
$($(
$crate::_impl_automatic_derive!($AutomaticDerive, $StructName);
)*)*
};
}
#[doc(hidden)]
#[macro_export]
macro_rules! _covariant_access {
(covariant, $Vis:vis, $Dependent:ident) => {
$Vis fn borrow_dependent<'_q>(&'_q self) -> &'_q $Dependent<'_q> {
fn _assert_covariance<'x: 'y, 'y>(x: $Dependent<'x>) -> $Dependent<'y> {
// This function only compiles for covariant types.
x // Change the macro invocation to not_covariant.
}
unsafe { self.unsafe_self_cell.borrow_dependent() }
}
};
(not_covariant, $Vis:vis, $Dependent:ident) => {
// For types that are not covariant it's unsafe to allow
// returning direct references.
// For example a lifetime that is too short could be chosen:
};
($x:ident, $Vis:vis, $Dependent:ident) => {
compile_error!("This macro only accepts `covariant` or `not_covariant`");
};
}
#[doc(hidden)]
#[macro_export]
macro_rules! _covariant_owner_marker {
(covariant, $OwnerLifetime:lifetime) => {
// Ensure that contravariant owners don't imply covariance
core::marker::PhantomData<&$OwnerLifetime ()>
};
(not_covariant, $OwnerLifetime:lifetime) => {
//
// If the dependent is non_covariant, mark the owner as invariant over its
// lifetime. Otherwise unsound use is possible.
core::marker::PhantomData<fn(&$OwnerLifetime ()) -> &$OwnerLifetime ()>
};
($x:ident, $OwnerLifetime:lifetime) => {
compile_error!("This macro only accepts `covariant` or `not_covariant`");
};
}
#[doc(hidden)]
#[macro_export]
macro_rules! _covariant_owner_marker_ctor {
($OwnerLifetime:lifetime) => {
// Helper to optionally expand into PhantomData for construction.
core::marker::PhantomData
};
}
#[doc(hidden)]
#[macro_export]
macro_rules! _impl_automatic_derive {
(Debug, $StructName:ident) => {
impl core::fmt::Debug for $StructName {
fn fmt(
&self,
fmt: &mut core::fmt::Formatter,
) -> core::result::Result<(), core::fmt::Error> {
self.with_dependent(|owner, dependent| {
fmt.debug_struct(stringify!($StructName))
.field("owner", owner)
.field("dependent", dependent)
.finish()
})
}
}
};
(PartialEq, $StructName:ident) => {
impl core::cmp::PartialEq for $StructName {
fn eq(&self, other: &Self) -> bool {
*self.borrow_owner() == *other.borrow_owner()
}
}
};
(Eq, $StructName:ident) => {
// TODO this should only be allowed if owner is Eq.
impl core::cmp::Eq for $StructName {}
};
(Hash, $StructName:ident) => {
impl core::hash::Hash for $StructName {
fn hash<H: core::hash::Hasher>(&self, state: &mut H) {
self.borrow_owner().hash(state);
}
}
};
($x:ident, $StructName:ident) => {
compile_error!(concat!(
"No automatic trait impl for trait: ",
stringify!($x)
));
};
}