Make find_map lint be more conservative

[yaahc]

The find_map lint will trigger if you do a filter followed by a map but its not always preferable.

---- dogfood stdout ----
status: exit code: 101
stdout:
stderr:     Blocking waiting for file lock on package cache lock
    Checking clippy v0.0.212 (/home/jlusby/git/rust/clippy)
error: called `find(p).map(q)` on an `Iterator`. This is more succinctly expressed by calling `.find_map(..)` instead.
   --> src/driver.rs:129:9
    |
129 | /         LINT_LEVELS
130 | |             .iter()
131 | |             .find(|level_mapping| level_mapping.0 == lint.group)
132 | |             .map(|(_, level)| level)
    | |____________________________________^
    |
    = note: `-D clippy::find-map` implied by `-D clippy::pedantic`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#find_map

error: called `filter(p).map(q)` on an `Iterator`. This is more succinctly expressed by calling `.filter_map(..)` instead.
   --> src/driver.rs:205:24
    |
205 |               let desc = lints
    |  ________________________^
206 | |                 .iter()
207 | |                 .filter(|&lint| lint.group == group)
208 | |                 .map(|lint| lint.name)
    | |______________________________________^
    |
    = note: `-D clippy::filter-map` implied by `-D clippy::pedantic`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#filter_map

error: called `find(p).map(q)` on an `Iterator`. This is more succinctly expressed by calling `.find_map(..)` instead.
   --> src/driver.rs:129:9
    |
129 | /         LINT_LEVELS
130 | |             .iter()
131 | |             .find(|level_mapping| level_mapping.0 == lint.group)
132 | |             .map(|(_, level)| level)
    | |____________________________________^
    |
    = note: `-D clippy::find-map` implied by `-D clippy::pedantic`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#find_map

error: called `filter(p).map(q)` on an `Iterator`. This is more succinctly expressed by calling `.filter_map(..)` instead.
   --> src/driver.rs:205:24
    |
205 |               let desc = lints
    |  ________________________^
206 | |                 .iter()
207 | |                 .filter(|&lint| lint.group == group)
208 | |                 .map(|lint| lint.name)
    | |______________________________________^
    |
    = note: `-D clippy::filter-map` implied by `-D clippy::pedantic`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#filter_map

error: aborting due to 2 previous errors

In the case of the first lint at least, you'll run into lifetime issues if you try to replace lines 131 and 132 with a find_map call.

        LINT_LEVELS
            .iter()
            .find_map(|level_mapping| {
                if level_mapping.0 == lint.group {
                    Some(level_mapping.1)
                } else {
                    None
                }
            })
            .map(|level| match level {
                Level::Allow => "allow",
                Level::Warn => "warn",
                Level::Deny => "deny",
            })
            .unwrap()

master ✗ $ cargo check
    Checking clippy v0.0.212 (/home/jlusby/git/rust/clippy)
error[E0507]: cannot move out of `level_mapping.1` which is behind a shared reference
   --> src/driver.rs:133:26
    |
133 |                     Some(level_mapping.1)
    |                          ^^^^^^^^^^^^^^^ move occurs because `level_mapping.1` has type `lintlist::lint::Level`, which does not implement the `Copy` trait

error: aborting due to previous error

For more information about this error, try `rustc --explain E0507`.
error: Could not compile `clippy`.

To learn more, run the command again with --verbose.

And even if it didn't run into the borrow error I personally feel like the find_map version is less concise than the filter -> map version.

If the Item type of the iterator was already a Try type then I can see it being easy enough to write a concise find_map impl, but with the boolean as the only starting point for the closure outputting Some or None I don't think it works as nicely as filter and map.

[yaahc]

clippy::filter_map might also need the same treatment.

[ghost]

I opened issue #3188 for filter_map a while ago.

[jakubadamw]

Personally, I think this lint is not useful and should be removed. The find_map() combinator in place of the combination of find() and map() makes the code more succinct only in a certain class of cases. Clippy is not in a position to distinguish them from the majority, where using the combinator is not desired, and so I am not sure what making the lint more conservative would in fact entail.