Suggest replacing pattern matching with `map` or `map_err`

[lkts]

What it does

This is in some way an extension to redundant_pattern_matching lint. The lint flags pattern matching that can be trivially converted to map or map_err invocations.

Lint Name

extend redundant_pattern_matching or prefer_map_over_pattern_match

Category

style

Advantage

• Code is shorter
• Less indentation
• Reduced boilerplate of mapping enum cases as is
• In my experience this is a common pattern people new to Rust follow and it helps to discover the alternative

Drawbacks

Depending on the code base this lint can be very intrusive.

Example

• let foo = match some_result_returning_fn() {
  Ok(val) => Ok(some_fn(val)),
  Err(e) => Err(e)
  }

Could be written as:

let foo = some_result_returning_fn().map(some_fn);

• let foo = match some_option_returning_fn() {
  Some(val) => Some(some_fn(val)),
  None => None
  }

Could be written as:

let foo = some_option_returning_fn().map(some_fn);

• let foo = match some_result_returning_fn() {
  Ok(val) => Ok(val),
  Err(e) => Err(some_fn(e))
  }

Could be written as:

let foo = some_option_returning_fn().map_err(some_fn);

[giraffate]

It would be better to improve the manual_map.