New lint to validate that E in public Result<_, E> implements std::error::Error

What it does

For public functions that return a Result<_, E> where E is not (), it should warn if E does not implement std::error::Error. Inspired by https://github.com/rustls/rustls/pull/837.

When propagating errors across crate boundaries, you almost always want to be able to propagate errors, and usually this works best if the error type implements Error.

Categories (optional)

• Kind: style, I guess?

Drawbacks

More code to write; might not be useful in all contexts; might apply only in library code.

Example

#[derive(Debug)]
pub struct SignError(());

Could be written as:

#[derive(Debug)]
pub struct SignError(());

impl fmt::Display for SignError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.write_str("sign error")
    }
}

impl StdError for SignError {}