fix arm64 build

[riederm]

cargo build --release --target=aarch64-unknown-linux-gnu fails with

the arm64 implementation of the filesystem seams to differ slightly with their types ?!

error[E0277]: `?` couldn't convert the error to `TryIntoFileAttrError`
   --> /opt/cargo/git/checkouts/polyfuse-cbcbf2a95fa40771/0301238/src/types.rs:422:42
    |
422 |             nlink: st.st_nlink.try_into()?,
    |                                ----------^ the trait `From<Infallible>` is not implemented for `TryIntoFileAttrError`
    |                                |
    |                                this can't be annotated with `?` because it has type `Result<_, Infallible>

[Copilot]

Pull request overview

Fixes an ARM64 build failure in FileAttr conversion by ensuring integer conversions produce an error type compatible with TryIntoFileAttrError, allowing ? to be used consistently across architectures.

Changes:

• Force st_nlink/st_blksize conversions through u64 before narrowing to u32 in TryFrom<&libc::stat> for FileAttr.
• Apply the same u64-then-narrow pattern for Metadata-derived nlink/blksize.

[Copilot]

Using (st.st_nlink as u64).try_into()? / (st.st_blksize as u64).try_into()? works around the Infallible error-type mismatch on platforms where the source field is already u32, but it does so by forcing an as cast. A more direct (and easier to reason about) fix is to make TryIntoFileAttrError convertible from std::convert::Infallible (e.g., via a From<Infallible> impl that is unreachable), or explicitly handle the Infallible case when calling try_into(). That would allow keeping the original try_into()? without widening casts and keeps the intent clearer across architectures.

[Copilot]

MetadataExt::nlink() and MetadataExt::blksize() are already u64 on Unix, so the added as u64 casts are redundant here and slightly obscure that the only potentially-failing step is the u64 -> u32 narrowing conversion. Consider dropping the cast (and if you add a From<Infallible> for TryIntoFileAttrError as suggested above, the libc::stat and Metadata conversions can stay consistent).

[ubnt-intrepid]

IMO, the fundamental problem is that the error type TryIntoFileAttrError doesn't implement From<Infallible>.
How about simply adding the following code?

impl From<Infallible> for TryIntoFileAttrError {
    fn from(never: Infallible) -> Self {
        match never {}
    }
}