manual_split_once lint causes a change in behaviour

[Marwes]

Lint name: manual_split_once

I tried this code:

path.rsplitn(2, '/').next().unwrap()

I expected to see this happen: cargo clippy should not suggest I change it to rsplit_once as that changes the behaviour.

Instead, this happened:

manual implementation of `rsplit_once`
   --> flux-core/src/semantic/nodes.rs:465:21
    |
465 |             None => path.rsplitn(2, '/').next().unwrap(),
    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try this: `path.rsplit_once('/').unwrap().1`
    |

cargo clippy --fix rewrote this to path.rsplit_once('/').unwrap().1 which is not equivalent code as rsplitn will always return at least one item, whereas rsplit_once will return None if the separator does not exist in the string causing the unwrap to panic.

path.rsplit_once('/').map_or(path, |t| t.1) would be a valid transformation.

Meta

Rust version (rustc -Vv):

rustc 1.58.0-nightly (e269e6bf4 2021-10-26)
binary: rustc
commit-hash: e269e6bf47f40c9046cd44ab787881d700099252
commit-date: 2021-10-26
host: x86_64-unknown-linux-gnu
release: 1.58.0-nightly
LLVM version: 13.0.0