Extend option-map-unwrap-or-else to Result

[oli-obk]

opt.map(f).unwrap_or_else(g) is linted to suggest opt.map_or_else(g, f), but res.map(f).unwrap_or_else(g) is not linted even though it could be res.ok().map_or_else(|_| g(), f)

Somewhat ugly suggestion, so should be allow-by-default, maybe we should poke rustc to add Result::map_or_else

[Arnavion]

This was implemented in #2189, so it could be closed, though I'll note that both the OP suggestion and the implementation of the lint are wrong in the general case. res.map(f).unwrap_or_else(g) cannot be replaced with res.ok().map_or_else(g, f) if g uses the Err value. The lint should be tightened to only fire if g is |_|

[mathstuf]

Why is it wrong? f doesn't get called if res is Err, so it goes right to g. If it is Ok, g isn't getting called at all. Maybe you confused it with and_then where g could get res or f's Err depending on the behavior of f?

map followed by unwrap_or_else:

let intermediate = match res {
  Ok(v) => Ok(f(v)),
  Err(err) => Err(err),
};
match intermediate {
  Ok(v) => v,
  Err(err) => g(err),
}

map_or_else:

match res {
  Ok(v) => f(v),
  Err(err) => g(err),
}

These look isomorphic to me.

@oli-obk Good to close?

[Arnavion]

@mathstuf If you were responding to my comment, you should read more closely what I wrote and the lint is suggesting you to use. Notice that it's not suggesting Result::map_or_else

[mathstuf]

Ah, missed the .ok() that's in there. If we just get rid of that in the suggestion, this is resolved then, right?