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Diffstat (limited to 'third_party/rust/self_cell/src/lib.rs')
| -rw-r--r-- | third_party/rust/self_cell/src/lib.rs | 659 |
1 files changed, 659 insertions, 0 deletions
diff --git a/third_party/rust/self_cell/src/lib.rs b/third_party/rust/self_cell/src/lib.rs new file mode 100644 index 0000000000..e2d8f2ee62 --- /dev/null +++ b/third_party/rust/self_cell/src/lib.rs @@ -0,0 +1,659 @@ +//! # 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 +//! self_cell](https://github.com/Voultapher/self_cell/tree/main/examples/lazy_ast) +//! +//! - [How to use an owner type with +//! lifetime](https://github.com/Voultapher/self_cell/tree/main/examples/owner_with_lifetime) +//! +//! ### 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 +/// [covariant](https://doc.rust-lang.org/nightly/nomicon/subtyping.html). +/// 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>) -> $Dependent<'y> +/// {x}`. Otherwise you could choose a lifetime that is too short for types +/// with interior mutability like `Cell`, which can lead to UB in safe code. +/// Which would violate the promise of this library that it is safe-to-use. +/// If you accidentally mark a type that is not covariant as covariant, you +/// will get a compile time error. +/// +/// * **not_covariant**: This generates no additional code but you can use the +/// `with_dependent` function. See [How to build a lazy AST with +/// self_cell](https://github.com/Voultapher/self_cell/tree/main/examples/lazy_ast) +/// for a usage example. +/// +/// In both cases you can use the `with_dependent_mut` function to mutate the +/// dependent value. This is safe to do because notionally you are replacing +/// pointers to a value not the other way around. +/// +/// - `impl {$($AutomaticDerive:ident),*},` Optional comma separated list of +/// optional automatic trait implementations. Possible Values: +/// +/// * **Debug**: Prints the debug representation of owner and dependent. +/// Example: `AstCell { owner: "fox = cat + dog", dependent: Ast(["fox", +/// "cat", "dog"]) }` +/// +/// * **PartialEq**: Logic `*self.borrow_owner() == *other.borrow_owner()`, +/// this assumes that `Dependent<'a>::From<&'a Owner>` is deterministic, so +/// that only comparing owner is enough. +/// +/// * **Eq**: Will implement the trait marker `Eq` for `$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, $Dependent:ident) => { + // For types that are not covariant it's unsafe to allow + // returning direct references. + // For example a lifetime that is too short could be chosen: + // See https://github.com/Voultapher/self_cell/issues/5 + }; + ($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, $OwnerLifetime:lifetime) => { + // Ensure that contravariant owners don't imply covariance + // over the dependent. See issue https://github.com/Voultapher/self_cell/issues/18 + core::marker::PhantomData<&$OwnerLifetime ()> + }; + (not_covariant, $OwnerLifetime:lifetime) => { + // See the discussion in https://github.com/Voultapher/self_cell/pull/29 + // + // If the dependent is non_covariant, mark the owner as invariant over its + // lifetime. Otherwise unsound use is possible. + core::marker::PhantomData<fn(&$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) + )); + }; +} |