On 8/17/18 1:37 PM, Jan Beich wrote:
Emilio Cobos Álvarez <[email protected]>
writes:
Hi,
In bug 1478813 (now on inbound), I've added a new toolchain job and
code to install cbindgen[1] via `mach bootstrap`, and it will become a
hard build dependency in the near future.
We use it on the style system to generate C++ structs / enums from the
Rust definitions (as opposed to bindgen[2], which we use for the
opposite thing).
Some downstream questions:
Why bindgen is built as part of Firefox but cbindgen isn't? It's not hard
to package both cbindgen and bindgen but the inconsistency looks ad hoc.
The reason is somewhat nontrivial.
We can run cbindgen the same way as bindgen in theory (in the style
system's build script), but the C++ headers it generates need to be
available way earlier in the build process (because the headers need to
be available for the rest of Gecko).
So using it from build.rs is not an option, because we'd need to move
the whole style system build before export (which I suspect wouldn't
work because of bindgen, plus it'd be really odd, and it'd slow down the
build time a lot I guess).
We could theoretically vendor it and build it ourselves ensuring it's on
the host, but as of right now there's no way to build a host rust crate
before the export of a header that I know of. We have very limited
support for that for C++ [1]. We can only build single-file C++ files
and such.
Adding support for general crates.io crates to be built this way and
appear somewhere on the objdir seemed way more build system hackery than
what I could do in a reasonable timeframe with my limited knowledge of
the build system. So requiring the binary to be there was the easiest
way I got it to work.
That being said, if some build system peer is able to make something
like that work, it'd be easy to switch to it if wanted.
How stable cbindgen output across versions? Would Firefox XY (e.g., ESR68)
still build with a version released a year after (in future)?
I think it should be reasonably stable (as in, existing options usually
remain untouched and keep generating the same kind of code).
We can force a locked down version of cbindgen at configure / bootstrap
time if needed to ensure it works if it's wanted.
Hope it answers your questions :)
Thanks,
-- Emilio
[1]:
https://searchfox.org/mozilla-central/rev/246f2b4fab2c1a6cca99418bc2e4d73d1102cc38/layout/style/test/moz.build#26
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