Suppress ``link-arg`` in dependency

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1

I’m building a library for FFI users that depends on a sibling library
which is also available through FFI. To illustrate the project layout:

.
├── dependency
│   ├── build.rs
│   ├── Cargo.toml
│   └── src
│       └── lib.rs
└── main
    ├── build.rs
    ├── Cargo.toml
    └── src
        └── lib.rs

Here both main and dependency are libraries of crate-type = ["lib", "cdylib"].
(In the actual product they’re part of a workspace but the problem
occurs without a workspace as well so let’s not complicate matters.)

main depends on dependency like so:

[dependencies]
dependency = { path = "../dependency" }

dependency is a leaf library. Both main and dependency get
their DT_SONAME embedded like so:

$ cat dependency/build.rs
fn main () { println!("cargo:rustc-cdylib-link-arg=-Wl,-soname,libdependency.so.0"); }
$ cat main/build.rs
fn main () { println!("cargo:rustc-cdylib-link-arg=-Wl,-soname,libmain.so.0"); }

(Actually I’m using the cdylib-link-lines crate to accomplish this
but it boils down to the same issue.)

This works fine for libdependency.so. However, when running
cargo run in ./main/, I obtain a libmain.so with the
SONAME of libdependency.so embedded:

$ readelf -d target/debug/libmain.so |grep SONAME
 0x000000000000000e (SONAME)             Library soname: [libdependency.so.0]

This is obviously wrong, it should be libmain.so.0.. :wink: The reason
for this is that the rustc-cdylib-link-arg lines get executed from
the build.rs of both libraries. Thus both of them end up
getting appended to the rustc command line:

rustc \
    --crate-name main \
    --edition=2021 src/lib.rs \
    --error-format=json \
    --json=diagnostic-rendered-ansi,artifacts,future-incompat \
    --crate-type lib \
    --crate-type cdylib \
    --emit=dep-info,link \
    -C embed-bitcode=no \
    -C debuginfo=2 \
    -C metadata=e0d0d2eeaf7bb2d0 \
    --out-dir /home/phg/src/ugh-cargo/soname/main/target/debug/deps \
    -C incremental=/home/phg/src/ugh-cargo/soname/main/target/debug/incremental \
    -L dependency=/home/phg/src/ugh-cargo/soname/main/target/debug/deps \
    --extern dependency=/home/phg/src/ugh-cargo/soname/main/target/debug/deps/libdependency.rlib \
    -C link-arg=-Wl,-soname,libmain.so.0 \
    -C link-arg=-Wl,-soname,libdependency.so.0

Now since there is only one SONAME field per object file, this
would actually work if the order of link-args were reversed. As
it is, later arguments override earlier ones and bam, libmain.so
wrongly claims it’s libdependency.so.0!

Ideally Cargo would support setting the SONAME directly in
Cargo.toml.

Alternatively, I’d be happy with a mechanism to suppress the bad -C link-arg in the above example when building libmain.so. However,
conditionally disabling the println! in dependency/build.rs
results in the SONAME being missing from libdependency.so.
Is there a way to filter rustc arguments added by dependencies?

I guess if I could tell cargo to put the output of main/build.rs
last, that would work too.

2

FWIW I also tried adding to the linker line in .cargo/config.toml
but the values there get prepended to those from build.rs.

EDIT: So do arguments to cargo on the command line (--config='build.rustflags = [ "-C", "link-arg=-Wl,-soname,lib__ARGV__.so.0" ]').

3

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