not exactly an embedded problem per-se, but rustc complains about some code where i really don't understand why:
```
$ cargo build
warning: allow(unreachable_code) incompatible with previous forbid
   --> src\lib.rs:257:9
    |
20  |   #![forbid(unused)]
    |             ------ `forbid` level set here
...
257 | /         self.pwm
258 | |             .set_duty_cycle_percent(speed)
259 | |             .map_err(|_| DriveError::InvalidSpeed)?;
    | |___________________________________________________^ overruled by previous forbid
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #81670 <https://github.com/rust-lang/rust/issues/81670>
    = note: `#[warn(forbidden_lint_groups)]` on by default

warning: `tb6612fng` (lib) generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
```

in the [referenced issue](https://github.com/rust-lang/rust/issues/81670#issuecomment-1558795417) others mention the same problem but there's no reply as to what's actually causing the warning.
does anyone know what's causing this?

after all, it seems to complain about the fact that i use `?` there - which should be perfectly fine?! i don't see how that can be unreachable code.

the affected code is here:
https://github.com/rursprung/tb6612fng-rs/pull/25/files#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R257-R259

thanks for any help!