1derive(Display) /// `From<docs>`
2===============
3
4[![Latest Version](https://img.shields.io/crates/v/displaydoc.svg)](https://crates.io/crates/displaydoc)
5[![Rust Documentation](https://img.shields.io/badge/api-rustdoc-blue.svg)](https://docs.rs/displaydoc)
6
7This library provides a convenient derive macro for the standard library's
8[`core::fmt::Display`] trait.
9
10[`core::fmt::Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html
11
12```toml
13[dependencies]
14displaydoc = "0.2"
15```
16
17*Compiler support: requires rustc 1.56+*
18
19<br>
20
21### Example
22
23*Demonstration alongside the [`Error`][std::error::Error] derive macro from [`thiserror`](https://docs.rs/thiserror/1.0.25/thiserror/index.html),
24to propagate source locations from [`io::Error`][std::io::Error] with the `#[source]` attribute:*
25```rust
26use std::io;
27use displaydoc::Display;
28use thiserror::Error;
29
30#[derive(Display, Error, Debug)]
31pub enum DataStoreError {
32    /// data store disconnected
33    Disconnect(#[source] io::Error),
34    /// the data for key `{0}` is not available
35    Redaction(String),
36    /// invalid header (expected {expected:?}, found {found:?})
37    InvalidHeader {
38        expected: String,
39        found: String,
40    },
41    /// unknown data store error
42    Unknown,
43}
44
45let error = DataStoreError::Redaction("CLASSIFIED CONTENT".to_string());
46assert!("the data for key `CLASSIFIED CONTENT` is not available" == &format!("{}", error));
47```
48*Note that although [`io::Error`][std::io::Error] implements `Display`, we do not add it to the
49generated message for `DataStoreError::Disconnect`, since it is already made available via
50`#[source]`. See further context on avoiding duplication in error reports at the rust blog
51[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).*
52
53<br>
54
55### Details
56
57- A `fmt::Display` impl is generated for your enum if you provide
58  a docstring comment on each variant as shown above in the example. The
59  `Display` derive macro supports a shorthand for interpolating fields from
60  the error:
61    - `/// {var}` ⟶ `write!("{}", self.var)`
62    - `/// {0}` ⟶ `write!("{}", self.0)`
63    - `/// {var:?}` ⟶ `write!("{:?}", self.var)`
64    - `/// {0:?}` ⟶ `write!("{:?}", self.0)`
65- This also works with structs and [generic types][crate::Display#generic-type-parameters]:
66```rust
67/// oh no, an error: {0}
68#[derive(Display)]
69pub struct Error<E>(pub E);
70
71let error: Error<&str> = Error("muahaha i am an error");
72assert!("oh no, an error: muahaha i am an error" == &format!("{}", error));
73```
74
75- Two optional attributes can be added to your types next to the derive:
76
77    - `#[ignore_extra_doc_attributes]` makes the macro ignore any doc
78      comment attributes (or `///` lines) after the first. Multi-line
79      comments using `///` are otherwise treated as an error, so use this
80      attribute or consider switching to block doc comments (`/** */`).
81
82    - `#[prefix_enum_doc_attributes]` combines the doc comment message on
83      your enum itself with the messages for each variant, in the format
84      “enum: variant”. When added to an enum, the doc comment on the enum
85      becomes mandatory. When added to any other type, it has no effect.
86
87- In case you want to have an independent doc comment, the
88  `#[displaydoc("...")` atrribute may be used on the variant or struct to
89  override it.
90
91<br>
92
93### FAQ
94
951. **Is this crate `no_std` compatible?**
96    * 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`.
97
982. **Does this crate work with `Path` and `PathBuf` via the `Display` trait?**
99    * 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!
100
101
102#### License
103
104<sup>
105Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
1062.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
107</sup>
108
109<br>
110
111<sub>
112Unless you explicitly state otherwise, any contribution intentionally submitted
113for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
114be dual licensed as above, without any additional terms or conditions.
115</sub>
116