| attr.rs |
|
4414 |
| expand.rs |
|
16601 |
| fmt.rs |
|
4243 |
| lib.rs |
This library provides a convenient derive macro for the standard library's
[`core::fmt::Display`] trait.
[`core::fmt::Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html
```toml
[dependencies]
displaydoc = "0.2"
```
*Compiler support: requires rustc 1.56+*
<br>
## Example
*Demonstration alongside the [`Error`][std::error::Error] derive macro from [`thiserror`](https://docs.rs/thiserror/1.0.25/thiserror/index.html),
to propagate source locations from [`io::Error`][std::io::Error] with the `#[source]` attribute:*
```rust
use std::io;
use displaydoc::Display;
use thiserror::Error;
#[derive(Display, Error, Debug)]
pub enum DataStoreError {
/// data store disconnected
Disconnect(#[source] io::Error),
/// the data for key `{0}` is not available
Redaction(String),
/// invalid header (expected {expected:?}, found {found:?})
InvalidHeader {
expected: String,
found: String,
},
/// unknown data store error
Unknown,
}
let error = DataStoreError::Redaction("CLASSIFIED CONTENT".to_string());
assert!("the data for key `CLASSIFIED CONTENT` is not available" == &format!("{}", error));
```
*Note that although [`io::Error`][std::io::Error] implements `Display`, we do not add it to the
generated message for `DataStoreError::Disconnect`, since it is already made available via
`#[source]`. See further context on avoiding duplication in error reports at the rust blog
[here](https://github.com/yaahc/blog.rust-lang.org/blob/master/posts/inside-rust/2021-05-15-What-the-error-handling-project-group-is-working-towards.md#duplicate-information-issue).*
<br>
## Details
- A `fmt::Display` impl is generated for your enum if you provide
a docstring comment on each variant as shown above in the example. The
`Display` derive macro supports a shorthand for interpolating fields from
the error:
- `/// {var}` ⟶ `write!("{}", self.var)`
- `/// {0}` ⟶ `write!("{}", self.0)`
- `/// {var:?}` ⟶ `write!("{:?}", self.var)`
- `/// {0:?}` ⟶ `write!("{:?}", self.0)`
- This also works with structs and [generic types][crate::Display#generic-type-parameters]:
```rust
# use displaydoc::Display;
/// oh no, an error: {0}
#[derive(Display)]
pub struct Error<E>(pub E);
let error: Error<&str> = Error("muahaha i am an error");
assert!("oh no, an error: muahaha i am an error" == &format!("{}", error));
```
- Two optional attributes can be added to your types next to the derive:
- `#[ignore_extra_doc_attributes]` makes the macro ignore any doc
comment attributes (or `///` lines) after the first. Multi-line
comments using `///` are otherwise treated as an error, so use this
attribute or consider switching to block doc comments (`/** */`).
- `#[prefix_enum_doc_attributes]` combines the doc comment message on
your enum itself with the messages for each variant, in the format
“enum: variant”. When added to an enum, the doc comment on the enum
becomes mandatory. When added to any other type, it has no effect.
- In case you want to have an independent doc comment, the
`#[displaydoc("...")` atrribute may be used on the variant or struct to
override it.
<br>
## FAQ
1. **Is this crate `no_std` compatible?**
* Yes! This crate implements the [`core::fmt::Display`] trait, not the [`std::fmt::Display`] trait, so it should work in `std` and `no_std` environments. Just add `default-features = false`.
2. **Does this crate work with `Path` and `PathBuf` via the `Display` trait?**
* Yuuup. This crate uses @dtolnay's [autoref specialization technique](https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md) to add a special trait for types to get the display impl. It then specializes for `Path` and `PathBuf`, and when either of these types are found, it calls `self.display()` to get a `std::path::Display<'_>` type which can be used with the `Display` format specifier! |
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