Warning when linking Python extension module written in Rust

[dnicolodi]

The linking of a Python extension module written in Rust results in this warning:

 warning: linker stdout:    Creating library rust_pyo3.cp315-win_amd64.dll.lib and object rust_pyo3.cp315-win_amd64.dll.exp
    |
    = note: `#[warn(linker_messages)]` on by default

  warning: 1 warning emitted

This is the command that produces the warning (wrapped for readability):

"rustc" \
  "-C" "linker=link" \
  "--color=always" \
  "-C" "debug-assertions=no" \
  "-C" "overflow-checks=no" \
  "-C" "embed-bitcode=no" \
  "--crate-type" "cdylib" \
  "-C" "link-arg=/release" \
  "-C" "link-arg=/nologo" \
  "-Cdefault-linker-libraries" \
  "-Clink-arg=/nodefaultlib:libcmt" \
  "-Clink-arg=/defaultlib:msvcrt" \
  "--edition=2024" \
  "-C" "opt-level=3" \
  "--crate-name" "rust_pyo3_cp315_win_amd64" \
  "--emit" "dep-info=rust_pyo3.cp315-win_amd64.pyd.p\rust_pyo3.cp315-win_amd64.d" \
  "--emit" "link=rust_pyo3.cp315-win_amd64.pyd" \
  "-C" "metadata=rust_pyo3.cp315-win_amd64.pyd@sha" \
  "--check-cfg" "cfg(docsrs)" \
  "--check-cfg" "cfg(test)" \
  "--cfg" "feature=\"default\"" \
  "-Z" "unstable-options" \
  "--env-set" "OUT_DIR=D:\a\rust-pyo3-meson\rust-pyo3-meson\.mesonpy-833bppcm\meson" \
  "--env-set" "CARGO_MANIFEST_DIR=D:\a\rust-pyo3-meson\rust-pyo3-meson\\" \
  "--env-set" "CARGO_MANIFEST_PATH=D:\a\rust-pyo3-meson\rust-pyo3-meson\Cargo.toml" \
  "--env-set" "CARGO_PKG_VERSION=1.0.0" \
  "--env-set" "CARGO_PKG_VERSION_MAJOR=1" \
  "--env-set" "CARGO_PKG_VERSION_MINOR=0" \
  "--env-set" "CARGO_PKG_VERSION_PATCH=0" \
  "--env-set" "CARGO_PKG_VERSION_PRE=" \
  "--env-set" "CARGO_PKG_AUTHORS=" \
  "--env-set" "CARGO_PKG_NAME=rust-pyo3" \
  "--env-set" "CARGO_PKG_HOMEPAGE=" \
  "--env-set" "CARGO_PKG_REPOSITORY=" \
  "--env-set" "CARGO_PKG_LICENSE=" \
  "--env-set" "CARGO_PKG_LICENSE_FILE=" \
  "--env-set" "CARGO_PKG_RUST_VERSION=" \
  "--env-set" "CARGO_PKG_README=" \
  "--env-set" "CARGO_CRATE_NAME=rust_pyo3" \
  "--extern" "pyo3=subprojects\pyo3-0.29.2\libpyo3+0_29.rlib" \
  "-Clink-arg=C:\hostedtoolcache\windows\Python\3.15.0-rc.1\x64\libs\python315.lib" \
  "-Lsubprojects\pyo3-0.29.2" \
  "-Lsubprojects\libc-0.2.189" \
  "-Lsubprojects\once_cell-1.21.4" \
  "-Lsubprojects\pyo3-ffi-0.29.2" \
  "-Lsubprojects\portable-atomic-1.15.0" \
  "-Lsubprojects\pyo3-macros-0.29.2" \
  "-LC:\hostedtoolcache\windows\Python\3.15.0-rc.1\x64\libs" \
  "-C" "link-arg=/SUBSYSTEM:CONSOLE" 
  ../src/lib.rs

To Reproduce
See the project in this repository https://github.com/dnicolodi/rust-pyo3-meson The warning can be seen here https://github.com/dnicolodi/rust-pyo3-meson/actions/runs/31894115936/job/95034642886#step:5:269

system parameters

• what operating system: Windows (there is no warning on Linux or macOS)
• what meson --version: 1.12.0

[bonzini]

This is a known issue in Rust, it should be silenced but it isn't: see the checklist at rust-lang/rust#136096

[eli-schwartz]

Doesn't look like a known issue to me. Upstream rust claims to have fixed this rustc bug in rust 1.96.0 -- I do agree that it should be reported to them, though.

[dnicolodi]

I forgot an important detail: the warning is not there when building with cargo. This is the reason why I reported the issue here. The command line options passed to rustc by cargo and meson are different, but I don't know which combination makes a difference.

[bonzini]

Can you add a CI action that compiles on Windows with cargo build -v, or include here the rustc options used by cargo?

[dnicolodi]

This is the linking command issue by cargo, wrapped for readability:

set CARGO=C:\Users\runneradmin\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\cargo.exe&&
set CARGO_CRATE_NAME=rust_pyo3&&
set CARGO_MANIFEST_DIR=D:\a\rust-pyo3-meson\rust-pyo3-meson&&
set CARGO_MANIFEST_PATH=D:\a\rust-pyo3-meson\rust-pyo3-meson\Cargo.toml&&
set CARGO_PKG_AUTHORS=""&&
set CARGO_PKG_DESCRIPTION=""&&
set CARGO_PKG_HOMEPAGE=""&&
set CARGO_PKG_LICENSE=""&&
set CARGO_PKG_LICENSE_FILE=""&&
set CARGO_PKG_NAME=rust-pyo3&&
set CARGO_PKG_README=""&&
set CARGO_PKG_REPOSITORY=""&&
set CARGO_PKG_RUST_VERSION=""&&
set CARGO_PKG_VERSION=1.0.0&&
set CARGO_PKG_VERSION_MAJOR=1&&
set CARGO_PKG_VERSION_MINOR=0&&
set CARGO_PKG_VERSION_PATCH=0&&
set CARGO_PKG_VERSION_PRE=""&&
set CARGO_PRIMARY_PACKAGE=1&&
set PATH="C:\Program Files\PowerShell\7;C:\Users\runneradmin\AppData\Roaming\Python\Python314\Scripts;C:\hostedtoolcache\windows\Python\3.14.7\x64\Scripts;C:\hostedtoolcache\windows\Python\3.14.7\x64;C:\Program Files\MongoDB\Server\7.0\bin;C:\vcpkg;C:\tools\zstd;C:\hostedtoolcache\windows\stack\3.11.1\x64;C:\cabal\bin;C:\\ghcup\bin;C:\mingw64\bin;C:\Program Files\dotnet;C:\Program Files\MySQL\MySQL Server 8.0\bin;C:\Program Files\R\R-4.6.1\bin\x64;C:\SeleniumWebDrivers\GeckoDriver;C:\SeleniumWebDrivers\EdgeDriver\;C:\SeleniumWebDrivers\ChromeDriver;C:\Program Files (x86)\sbt\bin;C:\Program Files (x86)\GitHub CLI;C:\Program Files\Git\bin;C:\Program Files (x86)\pipx_bin;C:\npm\prefix;C:\hostedtoolcache\windows\go\1.24.13\x64\bin;C:\hostedtoolcache\windows\Python\3.12.10\x64\Scripts;C:\hostedtoolcache\windows\Python\3.12.10\x64;C:\hostedtoolcache\windows\Ruby\3.3.12\x64\bin;C:\Program Files\OpenSSL\bin;C:\tools\kotlinc\bin;C:\hostedtoolcache\windows\Java_Temurin-Hotspot_jdk\17.0.20-8\x64\bin;C:\Program Files\ImageMagick-7.1.2-Q16-HDRI;C:\Program Files\Microsoft SDKs\Azure\CLI2\wbin;C:\ProgramData\kind;C:\ProgramData\Chocolatey\bin;C:\Windows\system32;C:\Windows;C:\Windows\System32\Wbem;C:\Windows\System32\WindowsPowerShell\v1.0\;C:\Windows\System32\OpenSSH\;C:\Program Files\PowerShell\7\;C:\Program Files\Microsoft\Web Platform Installer\;C:\Program Files\Microsoft SQL Server\170\Tools\Binn\;C:\Program Files\Microsoft SQL Server\Client SDK\ODBC\170\Tools\Binn\;C:\Program Files\dotnet\;C:\Program Files (x86)\Windows Kits\10\Windows Performance Toolkit\;C:\Program Files (x86)\WiX Toolset v3.14\bin;C:\Program Files\Microsoft SQL Server\130\DTS\Binn\;C:\Program Files\Microsoft SQL Server\140\DTS\Binn\;C:\Program Files\Microsoft SQL Server\150\DTS\Binn\;C:\Program Files\Microsoft SQL Server\160\DTS\Binn\;C:\Program Files\Microsoft SQL Server\170\DTS\Binn\;C:\ProgramData\chocolatey\lib\pulumi\tools\Pulumi\bin;C:\Program Files\CMake\bin;C:\Strawberry\c\bin;C:\Strawberry\perl\site\bin;C:\Strawberry\perl\bin;C:\ProgramData\chocolatey\lib\maven\apache-maven-3.9.16\bin;C:\Program Files\Microsoft Service Fabric\bin\Fabric\Fabric.Code;C:\Program Files\Microsoft SDKs\Service Fabric\Tools\ServiceFabricLocalClusterManager;C:\Program Files\nodejs\;C:\Program Files\Git\cmd;C:\Program Files\Git\mingw64\bin;C:\Program Files\Git\usr\bin;C:\Program Files\GitHub CLI\;c:\tools\php;C:\Program Files (x86)\sbt\bin;C:\Program Files\Amazon\AWSCLIV2\;C:\Program Files\Amazon\SessionManagerPlugin\bin\;C:\Program Files\Amazon\AWSSAMCLI\bin\;C:\Program Files\Microsoft SQL Server\130\Tools\Binn\;C:\Program Files\mongosh\;C:\Program Files\LLVM\bin;C:\Program Files (x86)\LLVM\bin;C:\Users\runneradmin\.dotnet\tools;C:\Users\runneradmin\.cargo\bin;C:\Users\runneradmin\AppData\Local\Microsoft\WindowsApps;C:\Users\runneradmin\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin"&&
C:\Users\runneradmin\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\bin\rustc.exe
--check-cfg "cfg(feature, values())"
--check-cfg cfg(docsrs,test)
--crate-name rust_pyo3
--crate-type cdylib
--edition=2024
--emit=dep-info,link
--error-format=json
--extern pyo3=D:\a\rust-pyo3-meson\rust-pyo3-meson\target\debug\build\pyo3/2850e24e56b13d67\out\libpyo3-2850e24e56b13d67.rlib
--json=diagnostic-rendered-ansi,artifacts,future-incompat
--out-dir D:\a\rust-pyo3-meson\rust-pyo3-meson\target\debug\build\rust-pyo3/af41ba4157500a70\out
-C debuginfo=2
-C embed-bitcode=no
-C metadata=97ad6b2255686032
-L dependency=D:\a\rust-pyo3-meson\rust-pyo3-meson\target\debug\build\libc/afb5a94062130244\out
-L dependency=D:\a\rust-pyo3-meson\rust-pyo3-meson\target\debug\build\once_cell/0b0ec8523699b41a\out
-L dependency=D:\a\rust-pyo3-meson\rust-pyo3-meson\target\debug\build\pyo3-ffi/6754c58aad09a998\out
-L dependency=D:\a\rust-pyo3-meson\rust-pyo3-meson\target\debug\build\pyo3-macros/330655a2301bb89e\out
-L dependency=D:\a\rust-pyo3-meson\rust-pyo3-meson\target\debug\build\pyo3/2850e24e56b13d67\out
src\lib.rs

I got this from a local copy of the compilation log I was using to investigate this issue. It is likely that I sorted the options passed to rustc alphabetically to make it easier to compare them with the compilation log from Meson, but I don't think that this makes a difference.

[bonzini]

Ok, let's leave it open for investigation in case we can work around it. Looking at the rustc code it's possible that -C linker=link vs -C linker=link.exe makes a difference.

Can you test it with a sed (or Python str.replace) tweak on build.ninja, which you can run even within meson-python via postconf hooks? Or maybe even just with rust_ld=link.exe in the project options, I am not sure.

[dnicolodi]

I reported this as a Rust issue rust-lang/rust#161164

[dnicolodi]

Looking at the rustc code it's possible that -C linker=link vs -C linker=link.exe makes a difference.

Good catch. With rustc -C linker=link.exe there is no warning.

[dnicolodi]

@bonzini Do you see any value in applying a work-around in Meson for this? If so, I can see how difficult it would be to turn -C linker=link into -C linker=link.exe. Another possibility would be to do not specify the linker if it is the default for the platform (which I think it is what Cargo does).