Fix btree CursorMut::insert_after check

[marc0246]

Fixes a check inside BTreeMap's CursorMut::insert_after, where it would peek the previous element to check whether the inserted key is below the next one, instead of peeking the next element.

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[cuviper]

Would you be willing to add #[should_panic] tests for insert_before and insert_after?

[marc0246]

Sure thing, where would the place for that be?

[cuviper]

In library/alloc/src/collections/btree/map/tests.rs, put them at the end near the positive test_cursor_mut. There are other examples of should_panic in that file -- remember that you only get one panic per test that way. Or you can write a single test that manually uses catch_unwind around each panic, if that feels better.

[marc0246]

Alrighty, I think I did it right, let's see what the CI thinks.

[cuviper]

Thanks!

@bors r+ rollup

[bors]

📌 Commit cdd9829 has been approved by cuviper

It is now in the queue for this repository.