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Implement #[error(fmt = ...)]#367

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dtolnay merged 2 commits intomasterfrom
fmt
Nov 5, 2024
Merged

Implement #[error(fmt = ...)]#367
dtolnay merged 2 commits intomasterfrom
fmt

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@dtolnay dtolnay commented Nov 5, 2024

Before:

#[derive(Error, Debug)]
pub enum Error {
    #[error("{code}{}", match .message {
        Some(msg) => format!(" - {}", &msg),
        None => "".to_owned(),
    })]
    Demo { code: u16, message: Option<String> },
}

After:

#[derive(Error, Debug)]
pub enum Error {
    #[error(fmt = demo_fmt)]
    Demo { code: u16, message: Option<String> },
}

fn demo_fmt(code: &u16, message: &Option<String>, formatter: &mut fmt::Formatter) -> fmt::Result {
    write!(formatter, "{code}")?;
    if let Some(msg) = message {
        write!(formatter, " - {msg}")?;
    }
    Ok(())
}

This avoids forcing allocations (or the even worse workarounds in #201) and keeps elaborate formatting logic out of the data structure.

The parameter order is designed to be usable with generic methods.

#[derive(Error, Debug)]
#[error(fmt = core::fmt::Octal::fmt)]
pub enum Error {
    I32(i32),
    I64(i64),
}

@dtolnay dtolnay linked an issue Nov 5, 2024 that may be closed by this pull request
Consider exposing formatter in display attribute #201
Closed
@dtolnay
Ignore trivially_copy_pass_by_ref pedantic clippy lint in test
8311380 
    warning: this argument (4 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
       --> tests/test_display.rs:388:16
        |
    388 |     fn pair(k: &i32, v: &i32, formatter: &mut fmt::Formatter) -> fmt::Result {
        |                ^^^^ help: consider passing by value instead: `i32`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref
        = note: `-W clippy::trivially-copy-pass-by-ref` implied by `-W clippy::pedantic`
        = help: to override `-W clippy::pedantic` add `#[allow(clippy::trivially_copy_pass_by_ref)]`

    warning: this argument (4 byte) is passed by reference, but would be more efficient if passed by value (limit: 8 byte)
       --> tests/test_display.rs:388:25
        |
    388 |     fn pair(k: &i32, v: &i32, formatter: &mut fmt::Formatter) -> fmt::Result {
        |                         ^^^^ help: consider passing by value instead: `i32`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#trivially_copy_pass_by_ref
@dtolnay dtolnay merged commit 184c100 into master Nov 5, 2024
@dtolnay dtolnay deleted the fmt branch November 5, 2024 22:37
@kaivol
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kaivol commented Sep 4, 2025

I just came across this, looks like a very useful feature!

However, this doesn't seem to be documented anywhere except in the patch notes.
I would have expected a mention here.

jimsynz pushed a commit to jimsynz/outrun that referenced this pull request Oct 13, 2025
@jimsynz
fix(deps): update rust crate thiserror to v2 (#81)
44e9083 
This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [thiserror](https://github.com/dtolnay/thiserror) | dependencies | major | `1.0` -> `2.0` |

---

### Release Notes

<details>
<summary>dtolnay/thiserror (thiserror)</summary>

### [`v2.0.17`](https://github.com/dtolnay/thiserror/releases/tag/2.0.17)

[Compare Source](dtolnay/thiserror@2.0.16...2.0.17)

- Use differently named \_\_private module per patch release ([#&#8203;434](dtolnay/thiserror#434))

### [`v2.0.16`](https://github.com/dtolnay/thiserror/releases/tag/2.0.16)

[Compare Source](dtolnay/thiserror@2.0.15...2.0.16)

- Add to "no-std" crates.io category ([#&#8203;429](dtolnay/thiserror#429))

### [`v2.0.15`](https://github.com/dtolnay/thiserror/releases/tag/2.0.15)

[Compare Source](dtolnay/thiserror@2.0.14...2.0.15)

- Prevent `Error::provide` API becoming unavailable from a future new compiler lint ([#&#8203;427](dtolnay/thiserror#427))

### [`v2.0.14`](https://github.com/dtolnay/thiserror/releases/tag/2.0.14)

[Compare Source](dtolnay/thiserror@2.0.13...2.0.14)

- Allow build-script cleanup failure with NFSv3 output directory to be non-fatal ([#&#8203;426](dtolnay/thiserror#426))

### [`v2.0.13`](https://github.com/dtolnay/thiserror/releases/tag/2.0.13)

[Compare Source](dtolnay/thiserror@2.0.12...2.0.13)

- Documentation improvements

### [`v2.0.12`](https://github.com/dtolnay/thiserror/releases/tag/2.0.12)

[Compare Source](dtolnay/thiserror@2.0.11...2.0.12)

- Prevent elidable\_lifetime\_names pedantic clippy lint in generated impl ([#&#8203;413](dtolnay/thiserror#413))

### [`v2.0.11`](https://github.com/dtolnay/thiserror/releases/tag/2.0.11)

[Compare Source](dtolnay/thiserror@2.0.10...2.0.11)

- Add feature gate to tests that use std ([#&#8203;409](dtolnay/thiserror#409), [#&#8203;410](dtolnay/thiserror#410), thanks [@&#8203;Maytha8](https://github.com/Maytha8))

### [`v2.0.10`](https://github.com/dtolnay/thiserror/releases/tag/2.0.10)

[Compare Source](dtolnay/thiserror@2.0.9...2.0.10)

- Support errors containing a generic type parameter's associated type in a field ([#&#8203;408](dtolnay/thiserror#408))

### [`v2.0.9`](https://github.com/dtolnay/thiserror/releases/tag/2.0.9)

[Compare Source](dtolnay/thiserror@2.0.8...2.0.9)

- Work around `missing_inline_in_public_items` clippy restriction being triggered in macro-generated code ([#&#8203;404](dtolnay/thiserror#404))

### [`v2.0.8`](https://github.com/dtolnay/thiserror/releases/tag/2.0.8)

[Compare Source](dtolnay/thiserror@2.0.7...2.0.8)

- Improve support for macro-generated `derive(Error)` call sites ([#&#8203;399](dtolnay/thiserror#399))

### [`v2.0.7`](https://github.com/dtolnay/thiserror/releases/tag/2.0.7)

[Compare Source](dtolnay/thiserror@2.0.6...2.0.7)

- Work around conflict with #\[deny(clippy::allow\_attributes)] ([#&#8203;397](dtolnay/thiserror#397), thanks [@&#8203;zertosh](https://github.com/zertosh))

### [`v2.0.6`](https://github.com/dtolnay/thiserror/releases/tag/2.0.6)

[Compare Source](dtolnay/thiserror@2.0.5...2.0.6)

- Suppress deprecation warning on generated From impls ([#&#8203;396](dtolnay/thiserror#396))

### [`v2.0.5`](https://github.com/dtolnay/thiserror/releases/tag/2.0.5)

[Compare Source](dtolnay/thiserror@2.0.4...2.0.5)

- Prevent deprecation warning on generated impl for deprecated type ([#&#8203;394](dtolnay/thiserror#394))

### [`v2.0.4`](https://github.com/dtolnay/thiserror/releases/tag/2.0.4)

[Compare Source](dtolnay/thiserror@2.0.3...2.0.4)

- Eliminate needless\_lifetimes clippy lint in generated `From` impls ([#&#8203;391](dtolnay/thiserror#391), thanks [@&#8203;matt-phylum](https://github.com/matt-phylum))

### [`v2.0.3`](https://github.com/dtolnay/thiserror/releases/tag/2.0.3)

[Compare Source](dtolnay/thiserror@2.0.2...2.0.3)

- Support the same Path field being repeated in both Debug and Display representation in error message ([#&#8203;383](dtolnay/thiserror#383))
- Improve error message when a format trait used in error message is not implemented by some field ([#&#8203;384](dtolnay/thiserror#384))

### [`v2.0.2`](https://github.com/dtolnay/thiserror/releases/tag/2.0.2)

[Compare Source](dtolnay/thiserror@2.0.1...2.0.2)

- Fix hang on invalid input inside #\[error(...)] attribute ([#&#8203;382](dtolnay/thiserror#382))

### [`v2.0.1`](https://github.com/dtolnay/thiserror/releases/tag/2.0.1)

[Compare Source](dtolnay/thiserror@2.0.0...2.0.1)

- Support errors that contain a dynamically sized final field ([#&#8203;375](dtolnay/thiserror#375))
- Improve inference of trait bounds for fields that are interpolated multiple times in an error message ([#&#8203;377](dtolnay/thiserror#377))

### [`v2.0.0`](https://github.com/dtolnay/thiserror/releases/tag/2.0.0)

[Compare Source](dtolnay/thiserror@1.0.69...2.0.0)

##### Breaking changes

- Referencing keyword-named fields by a raw identifier like `{r#type}` inside a format string is no longer accepted; simply use the unraw name like `{type}` ([#&#8203;347](dtolnay/thiserror#347))

  This aligns thiserror with the standard library's formatting macros, which gained support for implicit argument capture later than the release of this feature in thiserror 1.x.

  ```rust
  #[derive(Error, Debug)]
  #[error("... {type} ...")]  // Before: {r#type}
  pub struct Error {
      pub r#type: Type,
  }
  ```

- Trait bounds are no longer inferred on fields whose value is shadowed by an explicit named argument in a format message ([#&#8203;345](dtolnay/thiserror#345))

  ```rust
  // Before: impl<T: Octal> Display for Error<T>
  // After: impl<T> Display for Error<T>
  #[derive(Error, Debug)]
  #[error("{thing:o}", thing = "...")]
  pub struct Error<T> {
      thing: T,
  }
  ```

- Tuple structs and tuple variants can no longer use numerical `{0}` `{1}` access at the same time as supplying extra positional arguments for a format message, as this makes it ambiguous whether the number refers to a tuple field vs a different positional arg ([#&#8203;354](dtolnay/thiserror#354))

  ```rust
  #[derive(Error, Debug)]
  #[error("ambiguous: {0} {}", $N)]
  //                  ^^^ Not allowed, use #[error("... {0} {n}", n = $N)]
  pub struct TupleError(i32);
  ```

- Code containing invocations of thiserror's `derive(Error)` must now have a direct dependency on the `thiserror` crate regardless of the error data structure's contents ([#&#8203;368](dtolnay/thiserror#368), [#&#8203;369](dtolnay/thiserror#369), [#&#8203;370](dtolnay/thiserror#370), [#&#8203;372](dtolnay/thiserror#372))

##### Features

- Support disabling thiserror's standard library dependency by disabling the default "std" Cargo feature: `thiserror = { version = "2", default-features = false }` ([#&#8203;373](dtolnay/thiserror#373))

- Support using `r#source` as field name to opt out of a field named "source" being treated as an error's `Error::source()` ([#&#8203;350](dtolnay/thiserror#350))

  ```rust
  #[derive(Error, Debug)]
  #[error("{source} ==> {destination}")]
  pub struct Error {
      r#source: char,
      destination: char,
  }

  let error = Error { source: 'S', destination: 'D' };
  ```

- Infinite recursion in a generated Display impl now produces an `unconditional_recursion` warning ([#&#8203;359](dtolnay/thiserror#359))

  ```rust
  #[derive(Error, Debug)]
  #[error("??? {self}")]
  pub struct Error;
  ```

- A new attribute `#[error(fmt = path::to::myfmt)]` can be used to write formatting logic for an enum variant out-of-line ([#&#8203;367](dtolnay/thiserror#367))

  ```rust
  #[derive(Error, Debug)]
  pub enum Error {
      #[error(fmt = demo_fmt)]
      Demo { code: u16, message: Option<String> },
  }

  fn demo_fmt(code: &u16, message: &Option<String>, formatter: &mut fmt::Formatter) -> fmt::Result {
      write!(formatter, "{code}")?;
      if let Some(msg) = message {
          write!(formatter, " - {msg}")?;
      }
      Ok(())
  }
  ```

- Enums with an enum-level format message are now able to have individual variants that are `transparent` to supersede the enum-level message ([#&#8203;366](dtolnay/thiserror#366))

  ```rust
  #[derive(Error, Debug)]
  #[error("my error {0}")]
  pub enum Error {
      Json(#[from] serde_json::Error),
      Yaml(#[from] serde_yaml::Error),
      #[error(transparent)]
      Other(#[from] anyhow::Error),
  }
  ```

</details>

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takumi-earth pushed a commit to earthlings-dev/thiserror that referenced this pull request Jan 27, 2026
@dtolnay
Merge pull request dtolnay#367 from dtolnay/fmt
ddeca37 
Implement #[error(fmt = ...)]
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Consider exposing formatter in display attribute

2 participants

@dtolnay @kaivol