Add a lint for missing implementations of Error

[PatchMixolydic]

What it does

This lint lints against local types which are used as a Result's E type, but don't implement the Error trait.

Lint Name

missing_error_impls

Category

style, pedantic, or restriction

Advantage

• Allows the error to be passed to functions expecting dyn Error/impl Error
• Enables the use of ? on the Result type in functions/try blocks that return Result<_, Box<dyn Error>> or anyhow::Result<_>

Drawbacks

• Requires up to two trait implementations and a derive to resolve (or a dependency if the programmer opts to use thiserror)
• On no_std, Error is only available with the error_in_core feature gate

Example

pub struct MyError;

pub fn foo() -> Result<(), MyError> {
    Ok(())
}

Could be written as:

#[derive(Debug)]
pub struct MyError;

impl std::error::Error for MyError {}

impl std::fmt::Display for MyError {
    fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
        todo!()
    }
}

pub fn foo() -> Result<(), MyError> {
    Ok(())
}

Or as:

#[derive(Debug, thiserror::Error)]
#[error("todo")]
pub struct MyError;

pub fn foo() -> Result<(), MyError> {
    Ok(())
}

[lukaslueg]

Same as #6409?

[PatchMixolydic]

Seems like it; my bad 😅