Auto merge of #10993 - Centri3:iter_nth_zero, r=Manishearth

Don't lint [`iter_nth_zero`] in `next`

Closes #9820
This also *slightlyy* modifies the output of `iter_nth`, as I noticed the types' names weren't in backticks

changelog: [`iter_nth_zero`]: No longer lints in implementations of `Iterator::next`

tests/ui/iter_nth.stderr

@@ -1,4 +1,4 @@
-error: called `.iter().nth()` on a Vec
+error: called `.iter().nth()` on a `Vec`
   --> $DIR/iter_nth.rs:34:23
    |
 LL |         let bad_vec = some_vec.iter().nth(3);
@@ -23,7 +23,7 @@ LL |         let bad_boxed_slice = boxed_slice.iter().nth(3);
    |
    = help: calling `.get()` is both faster and more readable
 
-error: called `.iter().nth()` on a VecDeque
+error: called `.iter().nth()` on a `VecDeque`
   --> $DIR/iter_nth.rs:37:29
    |
 LL |         let bad_vec_deque = some_vec_deque.iter().nth(3);
@@ -31,7 +31,7 @@ LL |         let bad_vec_deque = some_vec_deque.iter().nth(3);
    |
    = help: calling `.get()` is both faster and more readable
 
-error: called `.iter_mut().nth()` on a Vec
+error: called `.iter_mut().nth()` on a `Vec`
   --> $DIR/iter_nth.rs:42:23
    |
 LL |         let bad_vec = some_vec.iter_mut().nth(3);
@@ -47,7 +47,7 @@ LL |         let bad_slice = &some_vec[..].iter_mut().nth(3);
    |
    = help: calling `.get_mut()` is both faster and more readable
 
-error: called `.iter_mut().nth()` on a VecDeque
+error: called `.iter_mut().nth()` on a `VecDeque`
   --> $DIR/iter_nth.rs:48:29
    |
 LL |         let bad_vec_deque = some_vec_deque.iter_mut().nth(3);

tests/ui/iter_nth_zero.fixed

@@ -29,3 +29,18 @@ fn main() {
     let mut iter2 = s3.iter();
     let _unwrapped = iter2.next().unwrap();
 }
+
+struct Issue9820;
+
+impl Iterator for Issue9820 {
+    type Item = ();
+
+    fn nth(&mut self, _n: usize) -> Option<Self::Item> {
+        todo!()
+    }
+
+    // Don't lint in implementations of `next`, as calling `next` in `next` is incorrect
+    fn next(&mut self) -> Option<Self::Item> {
+        self.nth(0)
+    }
+}

tests/ui/iter_nth_zero.rs

@@ -29,3 +29,18 @@ fn main() {
     let mut iter2 = s3.iter();
     let _unwrapped = iter2.nth(0).unwrap();
 }
+
+struct Issue9820;
+
+impl Iterator for Issue9820 {
+    type Item = ();
+
+    fn nth(&mut self, _n: usize) -> Option<Self::Item> {
+        todo!()
+    }
+
+    // Don't lint in implementations of `next`, as calling `next` in `next` is incorrect
+    fn next(&mut self) -> Option<Self::Item> {
+        self.nth(0)
+    }
+}