Clarify serde_derive platform support

[DarkKirb]

Originally posted by @dtolnay in #2538 (comment)

[…] The precompiled implementation is the only supported way to use the macros that are published in serde_derive. […]

This does seem to imply that platforms where the precompiled implementation is not available are not officially supported. However the rust compiler, which uses serde_derive, is supported on many platforms, most of which are listed in the rust documentation.

[fogti]

I'm also quite irritated that this way was chosen instead of e.g. compiling the macro into some form of byte-code like WASM or such (which wouldn't have the platform support problems to such an extent), while still providing some form of build speedup (as I suppose that the build time of serde_derive is the problem).

[zopsicle]

Compiling to bytecode would make little sense, because then a bytecode interpreter must be shipped in addition to serde-derive, which would itself most certainly take even longer to compile.

[pacak]

Compiling to bytecode would make little sense, because then a bytecode interpreter must be shipped in addition to serde-derive, which would itself most certainly take even longer to compile.

There's https://github.com/dtolnay/watt and readme suggests that compiling a bytecode interpreter is faster than complex macro with dependencies, what's more - it uses serde as an example.

[DarkKirb]

watt would work for my use-case (cargo2nix), but it is still incompatible with corporate or distro policy

[dtolnay]

Thanks for clarifying the unclear wording — this specifically refers to platforms on which cargo build involves the precompiled implementation in the first place. The crate works on all host platforms on which rustc supports proc macros; not all of those involve a precompiled macro. It also only refers to support extended by a serde maintainer. Support obtained from elsewhere (notably distro packagers) would be free to support different build modes, as always.