mozilla-central/third_party/rust/self_cell/README.md

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# `self_cell!`
Use the macro-rules macro: `self_cell!` to create safe-to-use self-referential
structs in stable Rust, without leaking the struct internal lifetime.
In a nutshell, the API looks *roughly* like this:
```rust
// 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 expect the user to 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.
from experienced Rust users.
### Fast compile times
```
$ rm -rf target && cargo +nightly build -Z timings
Compiling self_cell v0.9.0
Completed self_cell v0.9.0 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]);
}
```
```
$ 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 a more in-depth API overview and advanced examples:
### Installing
## Running the tests
```
cargo test
cargo miri test
```
### Related projects
## 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.
## Contributing
Please respect the [CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md) when contributing.
## Versioning
We use [SemVer](http://semver.org/) for versioning. For the versions available,
see the [tags on this repository](https://github.com/Voultapher/self_cell/tags).
## Authors
* **Lukas Bergdoll** - *Initial work* - [Voultapher](https://github.com/Voultapher)
See also the list of [contributors](https://github.com/Voultapher/self_cell/contributors)
who participated in this project.
## License
This project is licensed under the Apache License, Version 2.0 -
see the [LICENSE.md](LICENSE.md) file for details.