Find a way to update `serde_derive`

[eqrion]

From #159

I thought we had an issue filed with exactly what is going on here, but we don't exactly. I'll summarize it here for reference and file an issue later.

cbindgen depends on syn which depends on proc-macro-2 because syn is geared to be used in proc-macros not random binaries for parsing rust code. proc-macro-2 is the problematic library that causes this crash because it depends on internal compiler libs.

cbindgen doesn't need any of the functionality from syn that uses proc-macro-2 though, so we had a feature added to syn called proc-macro that we could use to disable this dependency. But disabling that feature is not enough to prevent proc-macro-2 from being linked.

Unfortunately proc-macro-2 is used by serde_derive as a build time proc-macro crate, and cargo does feature flag resolution once for build dependencies and binary dependencies. So our feature flag gets overridden because serde_derive needs that flag enabled. Even though serde_derive only happens at compile time, and we want the feature disabled at run time.

But not all serde_derive's have this dependency. Which is why we pin to a version which doesn't yet have it.

That's what rust-lang/cargo#2589 is about, and why it's still relevant. If that's fixed we could use the feature flag and update serde_derive.

I don't think we could easily drop the serde_derive requirement either. It's used for parsing cbindgen.toml which isn't required if all you care about is build.rs. But it is used for parsing Cargo.lock which is required for most uses of the crate.

This has been a major pain, and I'd love to find an answer to it.

[cuviper]

I've updated the pinned version to 1.0.58 in #207, but the core problem is not resolved.

Would you be open to a PR that manually deserializes the necessary structs in cbindgen? I know there are quite a few, but I'm somewhat inclined to just see how it goes...

[sylvestre]

@eqrion @cuviper This issue is preventing us to upload cbindgen in Debian & Ubuntu.
We have serde_derive 1.0.70 in Debian and it would be tricky for us to backport 1.0.58

[eqrion]

@sylvestre I wrote #222 to drop the serde_derive dependency using a hack. Hopefully this resolves the issue for you.

[infinity0]

What's wrong with relaxing the dependency to [dependencies.serde_derive] version = 1.0? I couldn't figure this out from the huge explanation above.

[eqrion]

What's wrong with relaxing the dependency to [dependencies.serde_derive] version = 1.0? I couldn't figure this out from the huge explanation above.

If we have a dependency on proc-macro-2 with feature 'proc-macro' enabled, the cbindgen binary will crash because it can't find compiler libraries. We have a dependency on syn which uses 'proc-macro-2' with that feature disabled, so we're fine.

If we update serde_derive, it will add a dependency on 'proc-macro-2' with that feature enabled causing the linked version of 'proc-macro-2' to enable that feature causing us to crash. serde_derive only needs this at build time, but cargo doesn't generate separate dependency graphs for build dependencies and runtime dependencies., so the features are unioned together.

[infinity0]

the cbindgen binary will crash because it can't find compiler libraries.

why does it crash, and can we make it not crash?

[eqrion]

It crashes because libproc-macro (a rustc internal library) is added as a linker dependency, and cbindgen can't find it when run outside of cargo run.

% target/debug/cbindgen -h
target/debug/cbindgen: error while loading shared libraries: libproc_macro-76e776d93c7dc9f0.so: cannot open shared object file: No such file or directory
%

There's no guarantee that the rustc libraries should even exist to run cbindgen either.

[infinity0]

I see, I think we can probably work around it in Debian since it's possible for us to keep multiple versions of the rustc internal libs around on the system. We already ship rust internal libs in /usr/lib so the linker can pick it up at runtime without any extra flags:

$ ls -gG /usr/lib/x86_64-linux-gnu/libproc_macro-f1cfd9a3f1fc2e7c.so 
-rw-r--r-- 1 556464 Sep 23 10:16 /usr/lib/x86_64-linux-gnu/libproc_macro-f1cfd9a3f1fc2e7c.so

Then the rust-cbindgen package will automatically get a Depends: libstd-rust-X.YY which can be co-installed with any future rustc compiler libs. Of course we'd want to do a rebuild against the newest rustc, but things will remain working until we do.

@sylvestre TL;DR I'm pretty sure things with Just Work if we relax the dependency to 1.0 in Debian.

[infinity0]

cbindgen$ cargo build
 Downloading serde_derive v1.0.80
[..]
   Compiling serde_derive v1.0.80
[..]
   Compiling cbindgen v0.6.4 (file:///home/infinity0/var/lib/rust/debcargo-build/cbindgen)
    Finished dev [unoptimized + debuginfo] target(s) in 44.44s
$ ldd -r target/debug/cbindgen
	linux-vdso.so.1 (0x00007ffdc812f000)
	libproc_macro-f1cfd9a3f1fc2e7c.so => /usr/lib/x86_64-linux-gnu/libproc_macro-f1cfd9a3f1fc2e7c.so (0x00007f102f6e6000)
	libsyntax-7c988e0bb240f29d.so => /usr/lib/x86_64-linux-gnu/libsyntax-7c988e0bb240f29d.so (0x00007f102f2c8000)
[.. etc ..]
$ dpkg-shlibdeps target/debug/cbindgen
dpkg-shlibdeps: warning: can't extract name and version from library name 'libproc_macro-f1cfd9a3f1fc2e7c.so'
[..]
dpkg-shlibdeps: warning: can't extract name and version from library name 'libstd-adf5983c479e1d65.so'
dpkg-shlibdeps: warning: binaries to analyze should already be installed in their package's directory
dpkg-shlibdeps: warning: package could avoid a useless dependency if target/debug/cbindgen was not linked against libm.so.6 (it uses none of the library's symbols)
$ cat debian/substvars 
shlibs:Depends=libc6 (>= 2.14), libgcc1 (>= 1:3.0), libstd-rust-1.29 (>= 1.29.0+dfsg1)

Yeah this is totally fine. The dependency on libstd-rust-1.29 will keep this package around on the system even if the user installs rustc 1.30. We don't even need to modify any packaging files, on top of the patch to relax the dependency from =1.0.58 to 1.0.