Need `--cfg=libc_align` in v0.3.16 but not v0.3.15; why?

[wchargin]

Hi! I transitively use socket2 as part of a Rust project built with
Bazel. I’m noting that socket2 builds successfully for me at v0.3.15,
but not at version v0.3.16 (released today) unless I specify the extra
flag --cfg=libc_align. I’m a bit hesitant to do so because it feels
low-level and platform specific, so I wanted to check and see whether
this is expected or whether there’s a better solution?

Here is a repro repo. You do need to have Bazel to repro, but you
don’t need any additional setup. The error is:

INFO: From Compiling Rust lib socket2 v0.3.16 (7 files):
error[E0063]: missing field `__align` in initializer of `libc::in6_addr`
   --> external/raze__socket2__0_3_16/src/sockaddr.rs:243:25
    |
243 |         let sin6_addr = in6_addr {
    |                         ^^^^^^^^ missing `__align`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0063`.
ERROR: /HOMEDIR/.cache/bazel/_bazel_wchargin/e6b2d93af64d3456b38c9c9d2131e939/external/raze__socket2__0_3_16/BUILD.bazel:33:13: output 'external/raze__socket2__0_3_16/libsocket2--1560485693.rlib' was not created
ERROR: /HOMEDIR/.cache/bazel/_bazel_wchargin/e6b2d93af64d3456b38c9c9d2131e939/external/raze__socket2__0_3_16/BUILD.bazel:33:13: not all outputs were created or valid
Target @raze__socket2__0_3_16//:socket2 failed to build
Use --verbose_failures to see the command lines of failed build steps.
INFO: Elapsed time: 0.707s, Critical Path: 0.20s
INFO: 2 processes: 1 internal, 1 linux-sandbox.
FAILED: Build did NOT complete successfully

I dug far enough into the libc structure to see the align/no_align
modules and guess that this cfg flag would probably fix it; it does.

The failing Bazel action, with directory structure and command line
invocation, is here (in a Gist because it has some long paths):
https://gist.github.com/wchargin/4da947cf43d2a241cfffb89873788a82

In all cases, the libc dependency is at version 0.2.80. But I note
that socket2 v0.3.15 depends on libc with default features, whereas
in v0.3.16 it depends on the align feature:

$ cargo tree -e features --manifest-path socket2-0.3.15/Cargo.toml
socket2-libc-align-demo v0.0.0 (/HOMEDIR/git/socket2-libc-align-demo/socket2-0.3.15)
└── socket2 feature "default"
    └── socket2 v0.3.15
        ├── cfg-if feature "default"
        │   └── cfg-if v0.1.10
        └── libc feature "default"
            ├── libc v0.2.80
            └── libc feature "std"
                └── libc v0.2.80
$ cargo tree -e features --manifest-path socket2-0.3.16-default/Cargo.toml
socket2-libc-align-demo v0.0.0 (/HOMEDIR/git/socket2-libc-align-demo/socket2-0.3.16-default)
└── socket2 feature "default"
    └── socket2 v0.3.16
        ├── cfg-if feature "default"
        │   └── cfg-if v0.1.10
        ├── libc feature "align"
        │   └── libc v0.2.80
        └── libc feature "default"
            ├── libc v0.2.80
            └── libc feature "std"
                └── libc v0.2.80
$ cargo tree -e features --manifest-path socket2-0.3.16-libc-align/Cargo.toml
socket2-libc-align-demo v0.0.0 (/HOMEDIR/git/socket2-libc-align-demo/socket2-0.3.16-libc-align)
└── socket2 feature "default"
    └── socket2 v0.3.16
        ├── cfg-if feature "default"
        │   └── cfg-if v0.1.10
        ├── libc feature "align"
        │   └── libc v0.2.80
        └── libc feature "default"
            ├── libc v0.2.80
            └── libc feature "std"
                └── libc v0.2.80

So I’m hoping that you can help me understand what caused this change,
why it requires me to build with --cfg=libc_align, (maybe) why it’s
needed with Bazel but not with Cargo (though I understand if that’s
outside your purview/expertise), and whether I should feel comfortable
doing so? I want to build for non-Linux platforms, too, eventually.

I’m on rustc 1.47.0 (18bf6b4f0 2020-10-07) on Debian-like Linux.

Thanks much!

[Thomasdezeeuw]

This was done in #120, maybe @faern can elaborate why it's needed a bit.

[faern]

It is very strange that you get this on rustc 1.47.0. According to the libc build script it should automatically activate libc_align when you compile it on rustc 1.25 or newer: https://github.com/rust-lang/libc/blob/master/build.rs#L56-L59

And also having socket2 activate the feature should also make the need for the __align field go away.

I have never used Bazel. Maybe it does not support features or build scripts enough?

[faern]

Or maybe it simply does not parse the build script output of cargo:rustc-cfg=libc_align!? That would make kind of sense since the line includes cargo:. But then again, would not almost everything break if it did not support those? :O

[wchargin]

Ah! Thank you, thank you. Indeed, it can parse cfg=... outputs from
build.rs, but only if you specify gen_buildrs = true (by design, to
make it explicit where the build isn’t necessarily hermetic), and
I hadn’t done so for this crate. Silly of me; sorry for the trouble, and
thanks for the pointer. :-)