New lint: iter_count

[TaKO8Ki]

This pull request adds a new lint named iter_count.

---

closes #6262

changelog: new lint iter_count

[rust-highfive]

r? @flip1995

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[matthiaskrgr]

Should this also lint for into_iter().count() ?

[TaKO8Ki]

@matthewjasper
Oh, I forgot to do this. I'm going to fix the lint.

[TaKO8Ki]

@flip1995 @matthewjasper @Y-Nak
I'm ready for your review!

[matthiaskrgr]

Seems there is a false positive with code inside the cargo repo.
Reduced:

needs a cargo.toml with

[dependencies]
im-rc = "*"

main.rs:

fn main() {
    let mut g: Graph<i32, i32> = Graph::new();
    g.iter().count();
}

impl<N: Eq + Ord + Clone, E: Default + Clone> Graph<N, E> {
    pub fn new() -> Graph<N, E> {
        Graph {
            nodes: im_rc::OrdMap::new(),
        }
    }

    pub fn iter(&self) -> impl Iterator<Item = &N> {
        self.nodes.keys()
    }
}

pub struct Graph<N: Clone, E: Clone> {
    nodes: im_rc::OrdMap<N, im_rc::OrdMap<N, E>>,
}

Your lint warns

warning: called `.iter().count()` on a `std::iter::Iterator`
 --> src/main.rs:3:5
  |
3 |     g.iter().count();
  |     ^^^^^^^^^^^^^^^^ help: try: `g.len()`

but there is no corresponding .len() method

error[E0599]: no method named `len` found for struct `Graph<i32, i32>` in the current scope
  --> src/main.rs:3:7
   |
3  |     g.len();
   |       ^^^ method not found in `Graph<i32, i32>`
...
18 | pub struct Graph<N: Clone, E: Clone> {
   | ------------------------------------ method `len` not found for this
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following traits define an item `len`, perhaps you need to implement one of them:
           candidate #1: `std::iter::ExactSizeIterator`
           candidate #2: `typenum::type_operators::Len`
           candidate #3: `bitmaps::types::BitOps`

[TaKO8Ki]

@matthewjasper
I've just solved this false positive.

[bors]

☔ The latest upstream changes (presumably #6730) made this pull request unmergeable. Please resolve the merge conflicts.

[matthiaskrgr]

Thanks!

[matthiaskrgr]

@bors r+

[bors]

📌 Commit 6041365 has been approved by matthiaskrgr

[bors]

⌛ Testing commit 6041365 with merge eb04beb...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: matthiaskrgr
Pushing eb04beb to master...