map_err mapping to same error type

What it does

Detect when map_err maps from error type E to error type E. In those situations the mapping is redundant

Advantage

• this is like a redundant operation
• mapping the error one from enum variant to another is likely not what people intend in most cases

Drawbacks

No response

Example

enum Error {
  A,
  B
}

fn source() -> Result<(), Error> { Err(Error::A) }

fn main() -> Result<(), Error> {
  source().map_err(|_| Error::A)
}

Could be written as:

fn main() -> Result<(), Error> {
  source()
}

Start with the two Rust examples in the issue and trace how Clippy handles the source and mapped error types. Done means a lint identifies redundant map_err calls where both types are the same, while distinguishing this from mappings between different enum variants.