needless error conversion

What it does

I think it may make sense to suggest map_err simplification here. This is especially useful because thiserror provides automatic #[from] implementation

Advantage

• shorter, simpler code

Drawbacks

No response

Example

pub enum MyErr {
    IoErr,
}

impl From<std::io::Error> for MyErr {
    fn from(_: std::io::Error) -> Self {
        MyErr::IoErr
    }
}

fn foo(e: Result<(), std::io::Error>) -> Result<(), MyErr> {
    // useless conversion here
    e.map_err(MyErr::from)?;
    // or e.map_err(|e| MyErr::from(e))?;
    // should be replaced with `e?`
    Ok(())
}

Comparison with existing lints

No response

Additional Context

No response

The issue does not identify implementation files or tests. Start by reviewing existing Clippy lints related to map_err and the thiserror #[from] pattern, then define which needless conversions should be diagnosed and how the example should be covered by tests.