Suggest replacing .map(f).flatten() with .and_then(f) on Options

[scottmcm]

What it does

What does this lint do?
Checks for usage of _.map(_).flatten(),

This is like https://rust-lang.github.io/rust-clippy/master/index.html#map_flatten but for Options instead of Iterators

Categories (optional)

• Kind: clippy::style/complexity

Easier to follow what's happening, shorter

Drawbacks

None.

Example

Inspired by @steffahn's suggestion in Nadrieril/dhall-rust#213 (comment)

[giraffate]

Do you mean this case?

rust-clippy/tests/ui/map_flatten.stderr

Lines 33 to 39 in 4c8d248

	error: called `map(..).flatten()` on an `Option`
	--> $DIR/map_flatten.rs:25:39
	|
	LL | let _: Option<_> = (Some(Some(1))).map(|x| x).flatten();
	| ^^^^^^^^^^^^^^^^^^^^^ help: try using `and_then` instead: `.and_then(|x| x)`
	error: aborting due to 6 previous errors

[scottmcm]

Ah, so I did! The description in the docs made me think it's only for Iterator::map.

[steffahn]

Which means the description should be fixed, right?

[giraffate]

Yes, I think so. I will prepare a fix.

[scottmcm]

Thanks, @giraffate!

[jyn514]

Hmm, this doesn't seem to be working: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=d9855ae1539cc222b88fa4baa7b46b92

    let _: Option<&str> = x.as_ref().map(|pb| pb.to_str()).flatten();

doesn't get a warning.

[steffahn]

@jyn514 It’s a pedantic lint; not enabled by default.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=1e75f19dfbb4f5b35b2f2ef4a9d8b200

[jyn514]

Ah ok, that makes sense. What do you think about making it a style lint instead?

[steffahn]

Seems reasonable to me to have it be a style lint. I’m not really sure I understand the distinction anyways; I see a lot of similar types of lints on both sides of the distinction, e.g. option_map_or_none is a style lint that also suggests and_then for a different type of pattern, and there are lots of iterator-related style lints for cases where a combination of iterator methods can be simplified, e.g. iter_skip_next.

More pedantic lints other than map_flatten that seem similar in spirit to other style lints include filter_map_next and manual_ok_or.

[orhun]

I think this should be a style lint too. I didn't know about this lint until I figure out I can use and_then instead of map & flatten so it would be good to be warned about this by clippy, as default.