Import ouroboros crate

Bug: 267375624
Test: None. No build files added yet
Change-Id: I3c4a94c36b791c2829b47aeb2597497e902496f4

11 files changed, 784 insertions, 0 deletions

diff --git a/.cargo_vcs_info.json b/.cargo_vcs_info.json
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+++ b/.cargo_vcs_info.json
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+{
+  "git": {
+    "sha1": "79bac55f29edbb44f7205246ab5c3f706cbc4647"
+  },
+  "path_in_vcs": "ouroboros"
+}
\ No newline at end of file
diff --git a/Cargo.toml b/Cargo.toml
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--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,31 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2018"
+name = "ouroboros"
+version = "0.15.5"
+authors = ["Joshua Maros <joshua-maros@github.com>"]
+description = "Easy, safe self-referential struct generation."
+documentation = "https://docs.rs/ouroboros"
+readme = "README.md"
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/joshua-maros/ouroboros"
+
+[dependencies.aliasable]
+version = "0.1.3"
+
+[dependencies.ouroboros_macro]
+version = "0.15.5"
+
+[features]
+default = ["std"]
+std = ["ouroboros_macro/std"]
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
new file mode 100644
index 0000000..73d97ad
--- /dev/null
+++ b/Cargo.toml.orig
@@ -0,0 +1,18 @@
+[package]
+name = "ouroboros"
+version = "0.15.5"
+authors = ["Joshua Maros <joshua-maros@github.com>"]
+edition = "2018"
+license = "MIT OR Apache-2.0"
+description = "Easy, safe self-referential struct generation."
+readme = "../README.md"
+documentation = "https://docs.rs/ouroboros"
+repository = "https://github.com/joshua-maros/ouroboros"
+
+[dependencies]
+aliasable = "0.1.3"
+ouroboros_macro = { version = "0.15.5", path = "../ouroboros_macro" }
+
+[features]
+default = ["std"]
+std = ["ouroboros_macro/std"]
diff --git a/LICENSE b/LICENSE
new file mode 120000
index 0000000..6d46dcc
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1 @@
+LICENSE_MIT
\ No newline at end of file
diff --git a/LICENSE_APACHE b/LICENSE_APACHE
new file mode 100644
index 0000000..574cb18
--- /dev/null
+++ b/LICENSE_APACHE
@@ -0,0 +1,202 @@
+                                 Apache License
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+   END OF TERMS AND CONDITIONS
+
+   APPENDIX: How to apply the Apache License to your work.
+
+      To apply the Apache License to your work, attach the following
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+   Copyright 2021 Joshua Maros
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
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+
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+   Unless required by applicable law or agreed to in writing, software
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diff --git a/LICENSE_MIT b/LICENSE_MIT
new file mode 100644
index 0000000..9ed5b2a
--- /dev/null
+++ b/LICENSE_MIT
@@ -0,0 +1,22 @@
+MIT License
+
+Copyright (c) 2021 Joshua Maros
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
diff --git a/METADATA b/METADATA
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--- /dev/null
+++ b/METADATA
@@ -0,0 +1,19 @@
+name: "ouroboros"
+description: "Easy, safe self-referential struct generation."
+third_party {
+  url {
+    type: HOMEPAGE
+    value: "https://crates.io/crates/ouroboros"
+  }
+  url {
+    type: ARCHIVE
+    value: "https://static.crates.io/crates/ouroboros/ouroboros-0.15.5.crate"
+  }
+  version: "0.15.5"
+  license_type: NOTICE
+  last_upgrade_date {
+    year: 2023
+    month: 2
+    day: 15
+  }
+}
diff --git a/MODULE_LICENSE_APACHE2 b/MODULE_LICENSE_APACHE2
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index 0000000..e69de29
--- /dev/null
+++ b/MODULE_LICENSE_APACHE2
diff --git a/OWNERS b/OWNERS
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--- /dev/null
+++ b/OWNERS
@@ -0,0 +1 @@
+include platform/prebuilts/rust:master:/OWNERS
diff --git a/README.md b/README.md
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+++ b/README.md
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+# Ouroboros
+
+[![Ouroboros on Crates.IO](https://img.shields.io/crates/v/ouroboros)](https://crates.io/crates/ouroboros)
+[![Documentation](https://img.shields.io/badge/documentation-link-success)](https://docs.rs/ouroboros)
+
+
+Easy self-referential struct generation for Rust. 
+Dual licensed under MIT / Apache 2.0.
+
+While this crate is `no_std` compatible, it still requires the `alloc` crate.
+
+Version notes:
+- Version `0.13.0` and later contain checks for additional situations which
+  cause undefined behavior if not caught.
+- Version `0.11.0` and later place restrictions on derive macros, earlier
+  versions allowed using them in ways which could lead to undefined behavior if
+  not used properly.
+- Version `0.10.0` and later automatically box every field. This is done
+  to prevent undefined behavior, but has the side effect of making the library
+  easier to work with.
+
+Tests are located in the examples/ folder because they need to be in a crate
+outside of `ouroboros` for the `self_referencing` macro to work properly.
+
+```rust
+use ouroboros::self_referencing;
+
+#[self_referencing]
+struct MyStruct {
+    int_data: i32,
+    float_data: f32,
+    #[borrows(int_data)]
+    // the 'this lifetime is created by the #[self_referencing] macro
+    // and should be used on all references marked by the #[borrows] macro
+    int_reference: &'this i32,
+    #[borrows(mut float_data)]
+    float_reference: &'this mut f32,
+}
+
+fn main() {
+    // The builder is created by the #[self_referencing] macro 
+    // and is used to create the struct
+    let mut my_value = MyStructBuilder {
+        int_data: 42,
+        float_data: 3.14,
+
+        // Note that the name of the field in the builder 
+        // is the name of the field in the struct + `_builder` 
+        // ie: {field_name}_builder
+        // the closure that assigns the value for the field will be passed 
+        // a reference to the field(s) defined in the #[borrows] macro
+	
+        int_reference_builder: |int_data: &i32| int_data,
+        float_reference_builder: |float_data: &mut f32| float_data,
+    }.build();
+
+    // The fields in the original struct can not be accesed directly
+    // The builder creates accessor methods which are called borrow_{field_name}()
+
+    // Prints 42
+    println!("{:?}", my_value.borrow_int_data());
+    // Prints 3.14
+    println!("{:?}", my_value.borrow_float_reference());
+    // Sets the value of float_data to 84.0
+    my_value.with_mut(|fields| {
+        **fields.float_reference = (**fields.int_reference as f32) * 2.0;
+    });
+
+    // We can hold on to this reference...
+    let int_ref = *my_value.borrow_int_reference();
+    println!("{:?}", *int_ref);
+    // As long as the struct is still alive.
+    drop(my_value);
+    // This will cause an error!
+    // println!("{:?}", *int_ref);
+}
+```
diff --git a/src/lib.rs b/src/lib.rs
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+++ b/src/lib.rs
@@ -0,0 +1,407 @@
+//! A crate for creating safe self-referencing structs.
+//!
+//! See the documentation of [`ouroboros_examples`](https://docs.rs/ouroboros_examples) for
+//! sample documentation of structs which have had the macro applied to them.
+
+#![no_std]
+#![allow(clippy::needless_doctest_main)]
+
+/// This macro is used to turn a regular struct into a self-referencing one. An example:
+/// ```rust
+/// use ouroboros::self_referencing;
+///
+/// #[self_referencing]
+/// struct MyStruct {
+///     int_data: i32,
+///     float_data: f32,
+///     #[borrows(int_data)]
+///     // the 'this lifetime is created by the #[self_referencing] macro
+///     // and should be used on all references marked by the #[borrows] macro
+///     int_reference: &'this i32,
+///     #[borrows(mut float_data)]
+///     float_reference: &'this mut f32,
+/// }
+///
+/// fn main() {
+///     // The builder is created by the #[self_referencing] macro
+///     // and is used to create the struct
+///     let mut my_value = MyStructBuilder {
+///         int_data: 42,
+///         float_data: 3.14,
+///
+///         // Note that the name of the field in the builder
+///         // is the name of the field in the struct + `_builder`
+///         // ie: {field_name}_builder
+///         // the closure that assigns the value for the field will be passed
+///         // a reference to the field(s) defined in the #[borrows] macro
+///
+///         int_reference_builder: |int_data: &i32| int_data,
+///         float_reference_builder: |float_data: &mut f32| float_data,
+///     }.build();
+///
+///     // The fields in the original struct can not be accesed directly
+///     // The builder creates accessor methods which are called borrow_{field_name}()
+///
+///     // Prints 42
+///     println!("{:?}", my_value.borrow_int_data());
+///     // Prints 3.14
+///     println!("{:?}", my_value.borrow_float_reference());
+///     // Sets the value of float_data to 84.0
+///     my_value.with_mut(|fields| {
+///         **fields.float_reference = (**fields.int_reference as f32) * 2.0;
+///     });
+///
+///     // We can hold on to this reference...
+///     let int_ref = *my_value.borrow_int_reference();
+///     println!("{:?}", *int_ref);
+///     // As long as the struct is still alive.
+///     drop(my_value);
+///     // This will cause an error!
+///     // println!("{:?}", *int_ref);
+/// }
+/// ```
+/// To explain the features and limitations of this crate, some definitions are necessary:
+/// # Definitions
+/// - **immutably borrowed field**: a field which is immutably borrowed by at least one other field.
+/// - **mutably borrowed field**: a field which is mutably borrowed by exactly one other field.
+/// - **self-referencing field**: a field which borrows at least one other field.
+/// - **head field**: a field which does not borrow any other fields, I.E. not self-referencing.
+///   This does not include fields with empty borrows annotations (`#[borrows()]`.)
+/// - **tail field**: a field which is not borrowed by any other fields.
+///
+/// # Usage
+/// To make a self-referencing struct, you must write a struct definition and place
+/// `#[self_referencing]` on top. For every field that borrows other fields, you must place
+/// `#[borrows()]` on top and place inside the parenthesis a list of fields that it borrows. Mut can
+/// be prefixed to indicate that a mutable borrow is required. For example,
+/// `#[borrows(a, b, mut c)]` indicates that the first two fields need to be borrowed immutably and
+/// the third needs to be borrowed mutably. You can also use `#[borrows()]` without any arguments to
+/// indicate a field that will eventually borrow from the struct, but does not borrow anything when
+/// first created. For example, you could use this on a field like `error: Option<&'this str>`.
+///
+/// # You must comply with these limitations
+/// - Fields must be declared before the first time they are borrowed.
+/// - Normal borrowing rules apply, E.G. a field cannot be borrowed mutably twice.
+/// - Fields that use the `'this` lifetime must have a corresponding `#[borrows()]` annotation.
+///   The error for this needs some work, currently you will get an error saying that `'this` is
+///   undefined at the location it was illegally used in.
+///
+/// Violating them will result in an error message directly pointing out the violated rule.
+///
+/// # Flexibility of this crate
+/// The example above uses plain references as the self-referencing part of the struct, but you can
+/// use anything that is dependent on lifetimes of objects inside the struct. For example, you could
+/// do something like this:
+/// ```rust
+/// use ouroboros::self_referencing;
+///
+/// pub struct ComplexData<'a, 'b> {
+///     aref: &'a i32,
+///     bref: &'b mut i32,
+///     number: i32,
+/// }
+///
+/// impl<'a, 'b> ComplexData<'a, 'b> {
+///     fn new(aref: &'a i32, bref: &'b mut i32, number: i32) -> Self {
+///         Self { aref, bref, number }
+///     }
+///
+///     /// Copies the value aref points to into what bref points to.
+///     fn transfer(&mut self) {
+///         *self.bref = *self.aref;
+///     }
+///
+///     /// Prints the value bref points to.
+///     fn print_bref(&self) {
+///         println!("{}", *self.bref);
+///     }
+/// }
+///
+/// fn main() {
+///     #[self_referencing]
+///     struct DataStorage {
+///         immutable: i32,
+///         mutable: i32,
+///         #[borrows(immutable, mut mutable)]
+///         #[covariant]
+///         complex_data: ComplexData<'this, 'this>,
+///     }
+///
+///     let mut data_storage = DataStorageBuilder {
+///         immutable: 10,
+///         mutable: 20,
+///         complex_data_builder: |i: &i32, m: &mut i32| ComplexData::new(i, m, 12345),
+///     }.build();
+///     data_storage.with_complex_data_mut(|data| {
+///         // Copies the value in immutable into mutable.
+///         data.transfer();
+///         // Prints 10
+///         data.print_bref();
+///     });
+/// }
+/// ```
+///
+/// # Note on memory leaks
+/// Currently, if a builder panics when creating a field, all previous fields will be leaked. This
+/// does not cause any undefined behavior. This behavior may be resolved in the future so that all
+/// previous fields are dropped when a builder panics.
+///
+/// # Covariance
+/// Many types in Rust have a property called "covariance". In practical tearms, this means that a
+/// covariant type like `Box<&'this i32>` can be used as a `Box<&'a i32>` as long as `'a` is
+/// smaller than `'this`. Since the lifetime is smaller, it does not violate the lifetime specified
+/// by the original type. Contrast this to `Fn(&'this i32)`, which is not covariant. You cannot give
+/// this function a reference with a lifetime shorter than `'this` as the function needs something
+/// that lives at *least* as long as `'this`. Unfortunately, there is no easy way to determine
+/// whether or not a type is covariant from inside the macro. As such, you may
+/// receive a compiler error letting you know that the macro is uncertain if a particular field
+/// uses a covariant type. Adding `#[covariant]` or `#[not_covariant]` will resolve this issue.
+///
+/// These annotations control whether or not a `borrow_*` method is generated for that field.
+/// Incorrectly using one of these tags will result in a compilation error. It is impossible to
+/// use them unsoundly.
+///
+/// # Async usage
+/// All self-referencing structs can be initialized asynchronously by using either the
+/// `MyStruct::new_async()` function or the `MyStructAsyncBuilder` builder. Due to limitations of
+/// the rust compiler you closures must return a Future trait object wrapped in a `Pin<Box<_>>`.
+///
+/// Here is the same example as above in its async version:
+///
+/// ```ignore
+/// use ouroboros::self_referencing;
+///
+/// #[self_referencing]
+/// struct MyStruct {
+///     int_data: i32,
+///     float_data: f32,
+///     #[borrows(int_data)]
+///     int_reference: &'this i32,
+///     #[borrows(mut float_data)]
+///     float_reference: &'this mut f32,
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut my_value = MyStructAsyncBuilder {
+///         int_data: 42,
+///         float_data: 3.14,
+///         int_reference_builder: |int_data: &i32| Box::pin(async move { int_data }),
+///         float_reference_builder: |float_data: &mut f32| Box::pin(async move { float_data }),
+///     }.build().await;
+///
+///     // Prints 42
+///     println!("{:?}", my_value.borrow_int_data());
+///     // Prints 3.14
+///     println!("{:?}", my_value.borrow_float_reference());
+///     // Sets the value of float_data to 84.0
+///     my_value.with_mut(|fields| {
+///         **fields.float_reference = (**fields.int_reference as f32) * 2.0;
+///     });
+///
+///     // We can hold on to this reference...
+///     let int_ref = *my_value.borrow_int_reference();
+///     println!("{:?}", *int_ref);
+///     // As long as the struct is still alive.
+///     drop(my_value);
+///     // This will cause an error!
+///     // println!("{:?}", *int_ref);
+/// }
+/// ```
+///
+/// # Async Send
+/// When Send trait is needed, the Send variant of async methods and builders is available.
+///
+/// Here is the same example as above in its async send version:
+///
+/// ```ignore
+/// use ouroboros::self_referencing;
+///
+/// #[self_referencing]
+/// struct MyStruct {
+///     int_data: i32,
+///     float_data: f32,
+///     #[borrows(int_data)]
+///     int_reference: &'this i32,
+///     #[borrows(mut float_data)]
+///     float_reference: &'this mut f32,
+/// }
+///
+/// #[tokio::main]
+/// async fn main() {
+///     let mut my_value = MyStructAsyncSendBuilder {
+///         int_data: 42,
+///         float_data: 3.14,
+///         int_reference_builder: |int_data: &i32| Box::pin(async move { int_data }),
+///         float_reference_builder: |float_data: &mut f32| Box::pin(async move { float_data }),
+///     }.build().await;
+///
+///     // Prints 42
+///     println!("{:?}", my_value.borrow_int_data());
+///     // Prints 3.14
+///     println!("{:?}", my_value.borrow_float_reference());
+///     // Sets the value of float_data to 84.0
+///     my_value.with_mut(|fields| {
+///         **fields.float_reference = (**fields.int_reference as f32) * 2.0;
+///     });
+///
+///     // We can hold on to this reference...
+///     let int_ref = *my_value.borrow_int_reference();
+///     println!("{:?}", *int_ref);
+///     // As long as the struct is still alive.
+///     drop(my_value);
+///     // This will cause an error!
+///     // println!("{:?}", *int_ref);
+/// }
+/// ```
+///
+/// # What does the macro generate?
+/// The `#[self_referencing]` struct will replace your definition with an unsafe self-referencing
+/// struct with a safe public interface. Many functions will be generated depending on your original
+/// struct definition. Documentation is generated for all items, so building documentation for
+/// your project allows accessing detailed information about available functions. Using
+/// `#[self_referencing(no_doc)]` will hide the generated items from documentation if it is becoming
+/// too cluttered.
+///
+/// ### A quick note on visibility
+/// The visibility of generated items is dependent on one of two things. If the
+/// generated item is related to a specific field of the struct, it uses the visibility of the
+/// original field. (The actual field in the struct will be made private since accessing it could cause
+/// undefined behavior.) If the generated item is not related to any particular field, it will by
+/// default only be visible to the module the struct is declared in. (This includes things like
+/// `new()` and `with()`.) You can use `#[self_referencing(pub_extras)]` to make these items have the
+/// same visibility as the struct itself.
+///
+/// # List of generated items
+/// ### `MyStruct::new(fields...) -> MyStruct`
+/// A basic constructor. It accepts values for each field in the order you declared them in. For
+/// **head fields**, you only need to pass in what value it should have and it will be moved in
+/// to the output. For **self-referencing fields**, you must provide a function or closure which creates
+/// the value based on the values it borrows. A field using the earlier example of
+/// `#[borrow(a, b, mut c)]` would require a function typed as
+/// `FnOnce(a: &_, b: &_, c: &mut _) -> _`. Fields which have an empty borrows annotation
+/// (`#[borrows()]`) should have their value directly passed in. A field using the earlier example
+/// of `Option<&'this str>` would require an input of `None`. Do not pass a function. Do not collect
+/// 200 dollars.
+/// ### `MyStruct::new_async(fields...) -> MyStruct`
+/// A basic async constructor. It works identically to the sync constructor differing only in the
+/// type of closures it expects. Whenever a closure is required it is expected to return a Pinned
+/// and Boxed Future that Outputs the same type as the synchronous version.
+/// ### `MyStruct::new_async_send(fields...) -> MyStruct`
+/// An async send constructor. It works identically to the sync constructor differing only in the
+/// Send trait being specified in the return type.
+/// ### `MyStructBuilder`
+/// This is the preferred way to create a new instance of your struct. It is similar to using the
+/// `MyStruct { a, b, c, d }` syntax instead of `MyStruct::new(a, b, c, d)`. It contains one field
+/// for every argument in the actual constructor. **Head fields** have the same name that you
+/// originally defined them with. **self-referencing fields** are suffixed with `_builder` since you need
+/// to provide a function instead of a value. Fields with an empty borrows annotation are not
+/// initialized using builders. Calling `.build()` on an instance of `MyStructBuilder`
+/// will convert it to an instance of `MyStruct` by calling all `_builder` functions in the order that
+/// they were declared and storing their results.
+/// ### `MyStructAsyncBuilder`
+/// This is the preferred way to asynchronously create a new instance of your struct. It works
+/// identically to the synchronous builder differing only in the type of closures it expects. In
+/// particular, all builder functions are called serially in the order that they were declared.
+/// Whenever a closure is required it is expected to return a Pinned and Boxed Future that Outputs
+/// the same type as the synchronous version.
+/// ### `MyStructAsyncSendBuilder`
+/// Same as MyStructAsyncBuilder, but with Send trait specified in the return type.
+/// ### `MyStruct::try_new<E>(fields...) -> Result<MyStruct, E>`
+/// Similar to the regular `new()` function, except the functions wich create values for all
+/// **self-referencing fields** can return `Result<>`s. If any of those are `Err`s, that error will be
+/// returned instead of an instance of `MyStruct`. The preferred way to use this function is through
+/// `MyStructTryBuilder` and its `try_build()` function.
+/// ### `MyStruct::try_new_async<E>(fields...) -> Result<MyStruct, E>`
+/// Similar to the regular `new_async()` function, except the functions wich create values for all
+/// **self-referencing fields** can return `Result<>`s. If any of those are `Err`s, that error will be
+/// returned instead of an instance of `MyStruct`. The preferred way to use this function is through
+/// `MyStructAsyncTryBuilder` and its `try_build()` function.
+/// ### `MyStruct::try_new_async_send<E>(fields...) -> Result<MyStruct, E>`
+/// Same as `new_async()` function, but with Send trait specified in the return type.
+/// ### `MyStruct::try_new_or_recover_async<E>(fields...) -> Result<MyStruct, (E, Heads)>`
+/// Similar to the `try_new_async()` function, except that all the **head fields** are returned along side
+/// the original error in case of an error. The preferred way to use this function is through
+/// `MyStructAsyncTryBuilder` and its `try_build_or_recover()` function.
+/// ### `MyStruct::try_new_or_recover_async_send<E>(fields...) -> Result<MyStruct, (E, Heads)>`
+/// Same as `try_new_or_recover_async()` function, but with Send trait specified in the return type.
+/// ### `MyStruct::with_FIELD<R>(&self, user: FnOnce(field: &FieldType) -> R) -> R`
+/// This function is generated for every **tail and immutably-borrowed field** in your struct. It
+/// allows safely accessing
+/// a reference to that value. The function generates the reference and passes it to `user`. You
+/// can do anything you want with the reference, it is constructed to not outlive the struct.
+/// ### `MyStruct::borrow_FIELD(&self) -> &FieldType`
+/// This function is generated for every **tail and immutably-borrowed field** in your struct. It
+/// is equivalent to calling `my_struct.with_FIELD(|field| field)`. It is only generated for types
+/// which are known to be covariant, either through the macro being able to detect it or through the
+/// programmer adding the `#[covariant]` annotation to the field.
+/// There is no `borrow_FIELD_mut`, unfortunately, as Rust's
+/// borrow checker is currently not capable of ensuring that such a method would be used safely.
+/// ### `MyStruct::with_FIELD_mut<R>(&mut self, user: FnOnce(field: &mut FieldType) -> R) -> R`
+/// This function is generated for every **tail field** in your struct. It is the mutable version
+/// of `with_FIELD`.
+/// ### `MyStruct::with<R>(&self, user: FnOnce(fields: AllFields) -> R) -> R`
+/// Allows borrowing all **tail and immutably-borrowed fields** at once. Functions similarly to
+/// `with_FIELD`.
+/// ### `MyStruct::with_mut<R>(&self, user: FnOnce(fields: AllFields) -> R) -> R`
+/// Allows mutably borrowing all **tail fields** and immutably borrowing all **immutably-borrowed**
+/// fields at once. Functions similarly to `with_FIELD_mut`, except that you can borrow multiple
+/// fields as mutable at the same time and also have immutable access to any remaining fields.
+/// ### `MyStruct::into_heads(self) -> Heads`
+/// Drops all self-referencing fields and returns a struct containing all **head fields**.
+pub use ouroboros_macro::self_referencing;
+
+#[doc(hidden)]
+pub mod macro_help {
+    pub extern crate alloc;
+
+    pub use aliasable::boxed::AliasableBox;
+    use aliasable::boxed::UniqueBox;
+
+    pub struct CheckIfTypeIsStd<T>(core::marker::PhantomData<T>);
+
+    macro_rules! std_type_check {
+        ($fn_name:ident $T:ident $check_for:ty) => {
+            impl<$T: ?Sized> CheckIfTypeIsStd<$check_for> {
+                pub fn $fn_name() {}
+            }
+        };
+    }
+
+    std_type_check!(is_std_box_type T alloc::boxed::Box<T>);
+    std_type_check!(is_std_arc_type T alloc::sync::Arc<T>);
+    std_type_check!(is_std_rc_type T alloc::rc::Rc<T>);
+
+    pub fn aliasable_boxed<T>(data: T) -> AliasableBox<T> {
+        AliasableBox::from_unique(UniqueBox::new(data))
+    }
+
+    pub fn unbox<T>(boxed: AliasableBox<T>) -> T {
+        *AliasableBox::into_unique(boxed)
+    }
+
+    /// Converts a reference to an object to a static reference This is
+    /// obviously unsafe because the compiler can no longer guarantee that the
+    /// data outlives the reference.  It is up to the consumer to get rid of the
+    /// reference before the container is dropped. The + 'static ensures that
+    /// whatever we are referring to will remain valid indefinitely, that there
+    /// are no limitations on how long the pointer itself can live.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the returned reference is not used after the originally passed
+    /// reference would become invalid.
+    pub unsafe fn change_lifetime<'old, 'new: 'old, T: 'new>(data: &'old T) -> &'new T {
+        &*(data as *const _)
+    }
+
+    /// Like change_lifetime, but for mutable references.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the returned reference is not used after the originally passed
+    /// reference would become invalid.
+    pub unsafe fn change_lifetime_mut<'old, 'new: 'old, T: 'new>(data: &'old mut T) -> &'new mut T {
+        &mut *(data as *mut _)
+    }
+}