Enable test-only code via feature instead of target_arch.

[Dirbaio]

Fixes #223

• Replace target_arch = "x86_64" with feature = "unstable-test" everywhere
• Add features to Cargo.tomls
• Add --features unstable-test in CI script

[japaric]

The approach looks good to me.

I think you meant to put the cfg instead the quote! macros though. See my inline comments.

[japaric]

I don't think the feature is necessary in this crate. The proc macro code is emitting cfg(feature = "unstable-test") but not using the Cargo feature.

EDIT: after reading the rest of the PR, I see that we do want this

[japaric]

this (cfg inside quote!) seems fine because internp is an implementation detail and only used within defmt non-public / non-user-facing code.

[japaric]

this cfg however will end in the code emitted by the logging macros, write!, intern!, etc. I'm not sure if that's the behavior we want.

The other option is make the defmt macros always emit the fetch_add_string_index version if its "unstable-test" feature is enabled.

EDIT: OK, after reading the rest of the PR. I think this should be:

if cfg!(feature = "unstable-test") {
    quote!(/* .. */) // fetch add string version
} else {
    quote!(/* .. */) // linker version
}

[japaric]

nit: this module should be renamed as it's no longer x86 only

[japaric]

in addition to this I think we should also run: RUSTFLAGS='--deny warnings' cargo check --features unstable-test (there's a previous build step that does this without the --features)

[Dirbaio]

Thanks for the review. Nice catch about the macros, I didn't think about that at all.
Review comments adressed, plus I fixed some comments and docs to reflect that defmt is now usable on x64.

[japaric]

This is great 💯 Thanks for implementing this!