#[link(name = "...")] is poorly supported

[Ralith]

Some FFI crates define bindings with the form #[link(name = "foo")] extern "C" { ... }, and rely on the buildscript to emit a cargo:rustc-link-search directive to find the foo library, whose unqualified name is forwarded directly to the linker by rustc. Reindeer silently drops the rustc-link-search directive, leading to linker failures when those FFI definitions are statically used. Major examples include pyo3 and windows-sys.

A workaround is to place these libraries in a filegroup, then arrange for that filegroup to be added to the linker's search path via linker_flags in downstream targets or the toolchain. I haven't managed to get linker_flags in a fixup to do anything.

[Ralith]

A reasonable solution might be to have a fixup (or default behavior...) to propagate rustc-link-search paths to downstream targets, perhaps raising a warning if they're not hermetic.

[Ralith]

It looks like -Z link-native-libraries=no is the intended solution for this situation on the rustc side. Unfortunately not usable with stable Rust... de-facto requiring nightly Rust for nontrivial buck2 Rust builds is unfortunate.

[capickett]

Unfortunately not usable with stable Rust... de-facto requiring nightly Rust for nontrivial buck2 Rust builds is unfortunate.

We do have support in the buck2 prelude to enable nightly features on non-nightly toolchains. Setting nightly_features on the rust_toolchain to True adds the RUSTC_BOOTSTRAP=1 environment variable, which allows for then using -Z flags and
unstable features. We use this extensively in Meta.

https://github.com/facebook/buck2-prelude/blob/main/rust/rust_toolchain.bzl#L128

[Ralith]

Thanks! That should work well in practice. Ideally we could set this flag without also globally enabling nightly features, which might not otherwise be desired, but that's a fairly small footgun.

[Ralith]

For posterity: Passing -Z link-native-libraries=no also disables the dependencies Rust's std pulls in on Windows, at https://github.com/rust-lang/rust/blob/master/library/std/src/sys/pal/windows/c/windows_sys.rs. With a system CXX toolchain, these additional rustc_flags get things working again:
"-Clink-arg=msvcrt.lib", "-Clink-arg=ws2_32.lib", "-Clink-arg=ntdll.lib", "-Clink-arg=userenv.lib". More may be necessary depending on what you call.