Remain sorted
=============
[](https://travis-ci.com/dtolnay/remain)
[](https://crates.io/crates/remain)
[](https://docs.rs/remain)
This crate provides an attribute macro to check at compile time that the
variants of an enum or the arms of a match expression are written in sorted
order.
```toml
[dependencies]
remain = "0.1"
```
## Syntax
Place a `#[remain::sorted]` attribute on enums, on match-expressions, or on
let-statements whose value is a match-expression.
Alternatively, import as `use remain::sorted;` and use `#[sorted]` as the
attribute.
```rust
#[remain::sorted]
#[derive(Debug)]
pub enum Error {
BlockSignal(signal::Error),
CreateCrasClient(libcras::Error),
CreateEventFd(sys_util::Error),
CreateSignalFd(sys_util::SignalFdError),
CreateSocket(io::Error),
DetectImageType(qcow::Error),
DeviceJail(io_jail::Error),
NetDeviceNew(virtio::NetError),
SpawnVcpu(io::Error),
}
impl Display for Error {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
use self::Error::*;
#[remain::sorted]
match self {
BlockSignal(e) => write!(f, "failed to block signal: {}", e),
CreateCrasClient(e) => write!(f, "failed to create cras client: {}", e),
CreateEventFd(e) => write!(f, "failed to create eventfd: {}", e),
CreateSignalFd(e) => write!(f, "failed to create signalfd: {}", e),
CreateSocket(e) => write!(f, "failed to create socket: {}", e),
DetectImageType(e) => write!(f, "failed to detect disk image type: {}", e),
DeviceJail(e) => write!(f, "failed to jail device: {}", e),
NetDeviceNew(e) => write!(f, "failed to set up virtio networking: {}", e),
SpawnVcpu(e) => write!(f, "failed to spawn VCPU thread: {}", e),
}
}
}
```
If an enum variant or match arm is inserted out of order,
```diff
NetDeviceNew(virtio::NetError),
SpawnVcpu(io::Error),
+ AaaUhOh(Box<dyn StdError>),
}
```
then the macro produces a compile error.
```console
error: AaaUhOh should sort before BlockSignal
--> tests/stable.rs:49:5
|
49 | AaaUhOh(Box<dyn StdError>),
| ^^^^^^^
```
## Compiler support
The attribute on enums is supported on any rustc version 1.31+.
Rust does not yet have stable support for user-defined attributes within a
function body, so the attribute on match-expressions and let-statements requires
a nightly compiler and the following two features enabled:
```rust
#![feature(proc_macro_hygiene, stmt_expr_attributes)]
```
As a stable alternative, this crate provides a function-level attribute called
`#[remain::check]` which makes match-expression and let-statement attributes
work on any rustc version 1.31+. Place this attribute on any function containing
`#[sorted]` to make them work on a stable compiler.
```rust
impl Display for Error {
#[remain::check]
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
use self::Error::*;
#[sorted]
match self {
/* ... */
}
}
}
```
<br>
#### License
<sup>
Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
</sup>
<br>
<sub>
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.
</sub>