diagnostics: use rustc_on_unimplemented to recommend [].iter()

To make this work, the `#[rustc_on_unimplemented]` data needs to be used to
report method resolution errors, which is most of what this commit does.

Fixes rust-lang#94581

Author: notriddle

core/src/iter/traits/iterator.rs

@@ -34,6 +34,11 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
         note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
               to have a bounded `RangeInclusive`: `0..=end`"
     ),
+    on(
+        _Self = "[]",
+        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
+    ),
+    on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
     on(
         _Self = "&str",
         label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"