Fix clippy warnings

[ArifRoktim]

Should close #10

[tesuji]

Unlike rustfmt, clippy uses conditional compilation to check the code.
So it should be run on all windows, mac and linux.

[ArifRoktim]

Clippy on latest stable suggests replacing lines like

self.executable_dir.as_ref().map(|p| p.as_path())

with

self.executable_dir.as_deref()

Option::as_deref was introduced in 1.40.0. I'm thinking about adding
#![allow(clippy::option_as_ref_deref)] with a comment to remove it when msrv is bumped past 1.40.

[tesuji]

In the meantime, we are having plan to downgrade Rust version to 1.34.
I would review this PR after we decide.

[ArifRoktim]

Different clippy versions were tested by running:

mkdir clippy_versions
for i in 1.{34..42}.0 stable; do
    find -name '*.rs' -execdir touch {} \;
    cargo +${i} clippy --all --all-targets &> clippy_versions/clippy-${i}.txt
done

---

clippy::option_as_ref_deref, introduced with the clippy that comes with rust 1.42.0, is not compatible with the MSRV until the MSRV is bumped past 1.40.0, so I ignore the lint.
But clippy from rust MSRV until rust 1.41.0 will complain about the lint being unknown, so I also ignore clippy::unknown_clippy_lints.

---

clippy::redundant_closure in rust 1.34.0 was overzealous and included warnings for closures containing method calls. Since 1.35.0, this lint has been split into two by rust-lang/rust-clippy/pull/4101 and there is now a separate clippy::redundant_closure_for_method_calls that is ignored by default.

[tesuji]

I suggest using command line flag to allow clippy lint.

[ArifRoktim]

Clippy on the command line doesn't seem to heed clippy::unknown_clippy_lints:

$ cargo +1.42.0 clippy --all --all-targets -- -A clippy::option_as_ref_deref
    Checking dirs-sys-next v0.1.0 (/home/arif/git/dirs/dirs-sys)
    Checking directories-next v1.0.0 (/home/arif/git/dirs/directories)
    Checking dirs-next v1.0.0 (/home/arif/git/dirs)
    Finished dev [unoptimized + debuginfo] target(s) in 0.45s

$ cargo +1.41.0 clippy --all --all-targets -- -A clippy::option_as_ref_deref
    Checking dirs-sys-next v0.1.0 (/home/arif/git/dirs/dirs-sys)
error[E0602]: unknown lint: `clippy::option_as_ref_deref`
  |
  = help: did you mean: `clippy::option_unwrap_used`
  = note: requested on the command line with `-A clippy::option_as_ref_deref`

error[E0602]: unknown lint: `clippy::option_as_ref_deref`
  |
  = help: did you mean: `clippy::option_unwrap_used`
  = note: requested on the command line with `-A clippy::option_as_ref_deref`

error: aborting due to previous error
<snip>

$ cargo +1.41.0 clippy --all --all-targets -- -A clippy::unknown_clippy_lints -A clippy::option_as_ref_deref
    Checking dirs-sys-next v0.1.0 (/home/arif/git/dirs/dirs-sys)
error[E0602]: unknown lint: `clippy::option_as_ref_deref`
  |
  = help: did you mean: `clippy::option_unwrap_used`
  = note: requested on the command line with `-A clippy::option_as_ref_deref`

error[E0602]: unknown lint: `clippy::option_as_ref_deref`
  |
  = help: did you mean: `clippy::option_unwrap_used`
  = note: requested on the command line with `-A clippy::option_as_ref_deref`

error: aborting due to previous error
<snip>

Plus if in the code itself, the output when users run clippy in the repo will be the same as that of the CI.

[tesuji]

How about using clippy of nightly toolchain? This way we don't have to allow clippy::unknown_clippy_lints.

[ArifRoktim]

Whoops, I think I misunderstood what you wanted. I was trying to make it so that users who ran clippy on the shell using any version from the MSRV to the current stable didn't get extraneous warnings, which is why I added the #![allow(clippy::<lint>)] in the source files.

I think you're only concerned about the CI. If that's the case, then the latest stable is all that's needed. All of os: [ubuntu-latest, macos-latest, windows-latest] use 1.43.0+, so it should be fine to just do:
cargo clippy --all --all-targets -- -A clippy::option_as_ref_deref.

[tesuji]

Yes, I don't think it is useful to use old clippy for linting.

[tesuji]

Thanks @ArifRoktim

[tesuji]

bors has often crashed recently. Merge manually I think.