Suggest the use of the std lib blsi instruction

What it does

It spots another coding pattern that's better replaced by a single std lib function.

Advantage

It avoids casts, and makes the code intent clear, removing the need of some code comments too.

Drawbacks

Translating Rust code to other languages could require a bit more work.

Example

fn foo1(i: u64) -> u64 {
    let si = i.cast_signed();
    i - (-si & si).cast_unsigned()
}

Could be written with the explicit call to the blsi CPU instruction (sometimes LLVM doesn't use blsi in similar cases):

#![feature(isolate_most_least_significant_one)]

fn foo2(i: u64) -> u64 {
    i - i.isolate_least_significant_one()
}

No files, tests, or entry points are named. Start by locating related Clippy lints for bit-operation patterns and their tests, then use the two Rust examples to define the expected match and suggestion; done means the lint handles the shown pattern and verifies its diagnostic.