WIP: add unnecessary_split_off lint

[sebhernandezr]

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fixes #4953

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changelog:

[andre-digiexam]

r? @llogiq

[llogiq]

The latter should be enough. Each literal is also a const

[llogiq]

I'd also like to test

let zero = 0;
let vec6 = vec1.split_off(zero);

[llogiq]

So you should look into clippy_utils/src/sym.rs which has a generate! macro call containing all our pre-interned symbols. You can add split_off there and directly use it to compare, which saves the .as_str() overhead. Also please edit the PR description to add a changelog line and squash your commits.