New lint: map to Option followed by flatten

[leudz]

Calling map to wrap the value in an Option and flatten right after is equivalent to filter_map.

[flip1995]

Playground

fn main() {
    let v = [10, 20, 30];

    let filtered1 = v
        .iter()
        .map(|x| if *x > 10 { Some(x) } else { None })
        .flatten()
        .collect::<Vec<_>>();

    let filtered2 = v
        .iter()
        .filter_map(|x| if *x > 10 { Some(x) } else { None })
        .collect::<Vec<_>>();

    assert_eq!(filtered1, filtered2);
}

This can be implemented in clippy_lints/src/methods/mod.rs, similar to multiple other lints in this module.

[derivmug]

I would like to work on this as a first issue.

[flip1995]

Awesome, don't hesitate to ask questions, if you have any!

[derivmug]

I am not sure whether this lint would be EarlyLintPass or LateLintPass. I guess EarlyLintPass as it doesn't need to check any types, is that right?

[flip1995]

You don't really have to care about that. You can just add another case here

rust-clippy/clippy_lints/src/methods/mod.rs

Lines 1069 to 1072 in abbb7ee

	match method_names.as_slice() {
	["unwrap", "get"] => lint_get_unwrap(cx, expr, arg_lists[1], false),
	["unwrap", "get_mut"] => lint_get_unwrap(cx, expr, arg_lists[1], true),
	["unwrap", ..] => lint_unwrap(cx, expr, arg_lists[0]),

and implement the lint from there.

You should create a new file in the methods module and implement it there instead of mod.rs directly. See:

rust-clippy/clippy_lints/src/methods/mod.rs

Lines 1108 to 1112 in abbb7ee

	["unwrap_or", arith @ "checked_add"]
	| ["unwrap_or", arith @ "checked_sub"]
	| ["unwrap_or", arith @ "checked_mul"] => {
	manual_saturating_arithmetic::lint(cx, expr, &arg_lists, &arith["checked_".len()..])
	},

manual_saturating_arithmetic

---

But to answer your question: This should be a LateLintPass, since you have to make sure, that the type map is called on is an Iterator

[derivmug]

Ok, thank you for the explanation.
I have added the following to the match statement and also created the lint using declare_clippy_lint! and appended it to declare_lint_pass! inside the mod.rs file, however it doesn't seem to match over my test. I have used the example above as the test.

            ["map", "flatten"] => {
                println!("LINT CALLED");
                map_flatten_filtermap::lint(cx, expr, arg_lists[0])
            }

(I am relatively new to Rust and this is the first time I am writing a lint, sorry for asking so much)

[flip1995]

From https://github.com/rust-lang/rust-clippy/blob/master/doc/adding_lints.md#lint-declaration:

Next we need to run util/dev update_lints to register the lint in various places, mainly in clippy_lints/src/lib.rs.

Have you done this? If not, run util/dev update_lints and try running your test again. If you have, I don't really know what could cause this from your description, so please open a WIP PR, so I can have a look at your code.

[derivmug]

I ran it but still no luck, I have opened a WIP PR here: #4521
Thank you for all your help :)

Do we need a new lint for this? map_flatten already lints on this code. I think this would have to be modification to that lint. Just check if the map closure produces Options and recommend filter_map instead of flat_map.

[derivmug]

I have noticed that the map_flatten lint already lints my example. I'll try to modify the other lint instead.