vendor/anyhow/README.md

   1 Anyhow&ensp;¯\\\_(°ペ)\_/¯
   2 ==========================
   3
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   8
   9 This library provides [`anyhow::Error`][Error], a trait object based error type
  10 for easy idiomatic error handling in Rust applications.
  11
  12 [Error]: https://docs.rs/anyhow/1.0/anyhow/struct.Error.html
  13
  14 ```toml
  15 [dependencies]
  16 anyhow = "1.0"
  17 ```
  18
  19 *Compiler support: requires rustc 1.34+*
  20
  21 <br>
  22
  23 ## Details
  24
  25 - Use `Result<T, anyhow::Error>`, or equivalently `anyhow::Result<T>`, as the
  26   return type of any fallible function.
  27
  28   Within the function, use `?` to easily propagate any error that implements the
  29   `std::error::Error` trait.
  30
  31   ```rust
  32   use anyhow::Result;
  33
  34   fn get_cluster_info() -> Result<ClusterMap> {
  35       let config = std::fs::read_to_string("cluster.json")?;
  36       let map: ClusterMap = serde_json::from_str(&config)?;
  37       Ok(map)
  38   }
  39   ```
  40
  41 - Attach context to help the person troubleshooting the error understand where
  42   things went wrong. A low-level error like "No such file or directory" can be
  43   annoying to debug without more context about what higher level step the
  44   application was in the middle of.
  45
  46   ```rust
  47   use anyhow::{Context, Result};
  48
  49   fn main() -> Result<()> {
  50       ...
  51       it.detach().context("Failed to detach the important thing")?;
  52
  53       let content = std::fs::read(path)
  54           .with_context(|| format!("Failed to read instrs from {}", path))?;
  55       ...
  56   }
  57   ```
  58
  59   ```console
  60   Error: Failed to read instrs from ./path/to/instrs.json
  61
  62   Caused by:
  63       No such file or directory (os error 2)
  64   ```
  65
  66 - Downcasting is supported and can be by value, by shared reference, or by
  67   mutable reference as needed.
  68
  69   ```rust
  70   // If the error was caused by redaction, then return a
  71   // tombstone instead of the content.
  72   match root_cause.downcast_ref::<DataStoreError>() {
  73       Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
  74       None => Err(error),
  75   }
  76   ```
  77
  78 - If using the nightly channel, or stable with `features = ["backtrace"]`, a
  79   a backtrace is captured and printed with the error if the underlying error
  80   type does not already provide its own. In order to see backtraces, they must
  81   be enabled through the environment variables described in [`std::backtrace`]:
  82
  83   - If you want panics and errors to both have backtraces, set
  84     `RUST_BACKTRACE=1`;
  85   - If you want only errors to have backtraces, set `RUST_LIB_BACKTRACE=1`;
  86   - If you want only panics to have backtraces, set `RUST_BACKTRACE=1` and
  87     `RUST_LIB_BACKTRACE=0`.
  88
  89   The tracking issue for this feature is [rust-lang/rust#53487].
  90
  91   [`std::backtrace`]: https://doc.rust-lang.org/std/backtrace/index.html#environment-variables
  92   [rust-lang/rust#53487]: https://github.com/rust-lang/rust/issues/53487
  93
  94 - Anyhow works with any error type that has an impl of `std::error::Error`,
  95   including ones defined in your crate. We do not bundle a `derive(Error)` macro
  96   but you can write the impls yourself or use a standalone macro like
  97   [thiserror].
  98
  99   ```rust
 100   use thiserror::Error;
 101
 102   #[derive(Error, Debug)]
 103   pub enum FormatError {
 104       #[error("Invalid header (expected {expected:?}, got {found:?})")]
 105       InvalidHeader {
 106           expected: String,
 107           found: String,
 108       },
 109       #[error("Missing attribute: {0}")]
 110       MissingAttribute(String),
 111   }
 112   ```
 113
 114 - One-off error messages can be constructed using the `anyhow!` macro, which
 115   supports string interpolation and produces an `anyhow::Error`.
 116
 117   ```rust
 118   return Err(anyhow!("Missing attribute: {}", missing));
 119   ```
 120
 121   A `bail!` macro is provided as a shorthand for the same early return.
 122
 123   ```rust
 124   bail!("Missing attribute: {}", missing);
 125   ```
 126
 127 <br>
 128
 129 ## No-std support
 130
 131 In no_std mode, the same API is almost all available and works the same way. To
 132 depend on Anyhow in no_std mode, disable our default enabled "std" feature in
 133 Cargo.toml. A global allocator is required.
 134
 135 ```toml
 136 [dependencies]
 137 anyhow = { version = "1.0", default-features = false }
 138 ```
 139
 140 Since the `?`-based error conversions would normally rely on the
 141 `std::error::Error` trait which is only available through std, no_std mode will
 142 require an explicit `.map_err(Error::msg)` when working with a non-Anyhow error
 143 type inside a function that returns Anyhow's error type.
 144
 145 <br>
 146
 147 ## Comparison to failure
 148
 149 The `anyhow::Error` type works something like `failure::Error`, but unlike
 150 failure ours is built around the standard library's `std::error::Error` trait
 151 rather than a separate trait `failure::Fail`. The standard library has adopted
 152 the necessary improvements for this to be possible as part of [RFC 2504].
 153
 154 [RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
 155
 156 <br>
 157
 158 ## Comparison to thiserror
 159
 160 Use Anyhow if you don't care what error type your functions return, you just
 161 want it to be easy. This is common in application code. Use [thiserror] if you
 162 are a library that wants to design your own dedicated error type(s) so that on
 163 failures the caller gets exactly the information that you choose.
 164
 165 [thiserror]: https://github.com/dtolnay/thiserror
 166
 167 <br>
 168
 169 #### License
 170
 171 <sup>
 172 Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
 173 2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
 174 </sup>
 175
 176 <br>
 177
 178 <sub>
 179 Unless you explicitly state otherwise, any contribution intentionally submitted
 180 for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
 181 be dual licensed as above, without any additional terms or conditions.
 182 </sub>