Only add implementation for Arc if the target supports it

[nn1ks]

Currently the Encode and Decode traits are always implemented for alloc::sync::Arc regardless of whether the Arc type is actually available. When compiling to a target that does not have atomic pointers (e.g. thumbv6m-none-eabi) the compiler emits the following error message:

error[E0433]: failed to resolve: could not find `sync` in `alloc`
   --> src/derive/impls.rs:112:26
    |
112 | impl_smart_ptr!(::alloc::sync::Arc);
    |                          ^^^^ could not find `sync` in `alloc`
    |
note: found an item that was configured out
   --> /nix/store/awlsysagj2w76yjz13w9sqfh5ws4zv73-rust-nightly/lib/rustlib/src/rust/library/alloc/src/lib.rs:228:9
    |
228 | pub mod sync;
    |         ^^^^
note: the item is gated here
   --> /nix/store/awlsysagj2w76yjz13w9sqfh5ws4zv73-rust-nightly/lib/rustlib/src/rust/library/alloc/src/lib.rs:227:1
    |
227 | #[cfg(all(not(no_rc), not(no_sync), target_has_atomic = "ptr"))]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0433`.
error: could not compile `bitcode` (lib) due to 1 previous error

With this change the traits are only implemented if the Arc type exists.

[finnbear]

Thanks very much! Is it possible to add a CI step that checks compilation on that target? I'd like to avoid breaking this in the future.

[nn1ks]

I tried to add it but it seems like zstd doesn't work on that target. Should I handle this specific target similiar to miri and add it to the cfg here:

bitcode/Cargo.toml

Lines 35 to 37 in 1604d52

	# zstd doesn't compile with miri big-endian.
	[target.'cfg(not(miri))'.dev-dependencies]
	zstd = "0.13.0"

Or what do you think is the best solution?

[nn1ks]

I had a better idea now: Just run cargo check instead of cargo test since it's a compile error so check is sufficient here :)