cargo: Meson is not able to automatically handle path dependencies in workspaces

[TitouanReal]

Describe the bug
Take two crates in a workspace, foo and bar. foo depends on bar through a path dependency. During setup, Meson isn't able to make the path dep a subproject, and the setup fails. However, Cargo is able to build the project.

To Reproduce
See the minimal reproducer attached.

meson-cargo-path-dep-reproducer.zip

Expected behavior
Meson should be able to build the library

system parameters

• Native build
• Linux (GNOME OS)
• Python 3.14.7
• Meson version: master

[TitouanReal]

This might be of interest to you @bonzini .

[bonzini]

Does it work if bar is in the subprojects/ directory?

[TitouanReal]

I tried to copy bar in:

• subprojects/bar
• subprojects/bar-0.1
• subprojects/bar-0.1.0
• subprojects/bar-0.1-rs

And in all those cases the setup fails, with

The Meson build system
Version: 1.12.99
Source dir: /home/titouan/Projets/meson-cargo-path-dep-reproducer
Build dir: /home/titouan/Projets/meson-cargo-path-dep-reproducer/build
Build type: native build
Project name: meson-cargo-path-dep-reproducer
Project version: 0.1.0
Rust compiler for the host machine: rustc -C linker=cc (rustc 1.98.0 "1.98.0-nightly")
Rust linker for the host machine:  ld.bfd 2.47.20260726
Host machine cpu family: x86_64
Host machine cpu: x86_64

foo/meson.build:6:6: ERROR: dependency "bar-0.1-rs" was not overridden for the host machine

A full log can be found at /home/titouan/Projets/meson-cargo-path-dep-reproducer/build/meson-logs/meson-log.txt

[TitouanReal]

Only workaround I found is to create subprojects/bar-0.1.wrap containing:

[wrap-git]
url = ../
revision = HEAD

[provide]
dependency_names = bar-0.1-rs

Then in bar/meson.build I need to add

crate = workspace.package('bar')
lib = crate.library(rust_abi: 'rust')
crate.override_dependency(
    declare_dependency(link_with: lib, version: crate.version()),
    rust_abi: 'rust',
)

And finally in foo/meson.build I need to add

dependency(
    'bar-0.1-rs',
    version: '>=0.1',
),

Then it works, but I completely bypassed Meson automatic crate resolve system.

[bonzini]

Ok, thanks for the small reproducer. I will give it a look.

[bonzini]

bar/meson.build is missing both the override_dependency call and the rust ABI library, something like:

bar_cdylib = crate.library(rust_abi: 'c')
crate.override_dependency(declare_dependency(link_with: bar_cdylib), rust_abi: 'c')
bar_rlib = crate.library(rust_abi: 'rust')
crate.override_dependency(declare_dependency(link_with: bar_rlib), rust_abi: 'rust')

Of course you can leave out one of the two ABIs if you don't need it.

[TitouanReal]

Well yes, as I said here I can make it work this way. But it needs also the dependency call in foo/meson.build and the wrap file.

I would expect Meson to be able to treat path dependencies the same as normal crates for remote registries, and for my minimal reproducer to work without any change. Am I misunderstanding something?

[bonzini]

Path dependencies are within your workspace, and for everything in your workspace you need to write the rules by hand. Only subprojects have automatically generated meson.build, because meson can't know which libraries have dependencies on C code, or have generated sources, etc.

Typically it's just three lines of code (pkg = cargo.package(...), lib = cargo.library(...), cargo.override_dependency(...)), or five if you have both C and Rust ABI libraries.

Leaving this open as a documentation issue.

[TitouanReal]

I am not sure I understand how I am supposed to do this. Could you please send a patch file over the minimal example I sent that shows what the solution should be?

I don't unnderstand if you want me to add all the changes I detailed in my comment above or if I can do it in a simpler way.

[bonzini]

You can do it lik this:

diff --git a/bar/meson.build b/bar/meson.build
index ff37eea..829fbae 100644
--- a/bar/meson.build
+++ b/bar/meson.build
@@ -1,6 +1,7 @@
-rust = import('rust')
+bar = workspace.package('bar')
 
-workspace = rust.workspace()
-crate = workspace.package('bar')
+bar_rlib = bar.library(rust_abi: 'rust')
+bar.override_dependency(declare_dependency(link_with: bar_rlib), rust_abi: 'rust')
 
-crate.library(rust_abi: 'c')
+bar_cdylib = bar.library(rust_abi: 'c')
+bar.override_dependency(declare_dependency(link_with: bar_cdylib), rust_abi: 'c')
diff --git a/foo/meson.build b/foo/meson.build
index 132f286..25650dd 100644
--- a/foo/meson.build
+++ b/foo/meson.build
@@ -1,6 +1,7 @@
-rust = import('rust')
+foo = workspace.package('foo')
 
-workspace = rust.workspace()
-crate = workspace.package('foo')
+foo_rlib = foo.library(rust_abi: 'rust')
+foo.override_dependency(declare_dependency(link_with: foo_rlib), rust_abi: 'rust')
 
-crate.library(rust_abi: 'c')
+foo_cdylib = foo.library(rust_abi: 'c')
+foo.override_dependency(declare_dependency(link_with: foo_cdylib), rust_abi: 'c')
diff --git a/meson.build b/meson.build
index b2fba9a..4518d50 100644
--- a/meson.build
+++ b/meson.build
@@ -5,5 +5,7 @@ project(
     meson_version: '>=1.11',
 )
 
+rust = import('rust')
+workspace = rust.workspace()
 subdir('bar')
 subdir('foo')

The build results:

-rw-r--r--. 1 pbonzini users    5562 Aug 24 07:15 bar/libbar+0_1.rlib
-rwxr-xr-x. 1 pbonzini users 4296416 Aug 24 07:15 bar/libbar.so
-rw-r--r--. 1 pbonzini users    5714 Aug 24 07:15 foo/libfoo+0_1.rlib
-rwxr-xr-x. 1 pbonzini users 4296952 Aug 24 07:15 foo/libfoo.so

Moving the import and rust.workspace line is not necessary, but it is cleaner.

[dnicolodi]

@bonzini How do you see this working in the case of crates that are intended to be distributed via crates.io and used as subprojects in other Meson projects? I am specifically thinking to the case of pyo3. There pyo3 is the top level workspace crate with a number of dependent crates in sub-directories. IIIUC, compiling pyo3 with the code organized as in its repository or included via subprojects requires different meson.build, not only for the top level crate. The usual disclaimer about me not being very familiar with Rust concepts apply.

[bonzini]

When uploading to crates.io, the Cargo.toml file is rewritten and the workspace is removed. Something like

[dependencies]
foo = { path = "../foo", version = "0.18" }

is rewritten to

[dependencies.foo]
version = "0.18"

So each crate can be parsed independently, either by generating a meson.build file during meson setup, or by using the one provided by the project.

On the other hand, there is a real issue in testng PyO3/pyo3 from its CI. In that case you would indeed need to hand-write meson.build files that are normally generated by Meson, unless you use PyO3/pyo3 somehow as a subproject. Probably the simplest solution would be to add a separate git repository https://github.com/PyO3/pyo3-meson-example, clone it from the CI job, but I haven't looked at it.

[TitouanReal]

I have a similar question for git dependencies that targets workspaces as well. I have this example in my Cargo.toml:

clepsydre = { package = "clepsydre-rebind", git = "https://gitlab.gnome.org/TitouanReal/clepsydre.git" }
clepsydre-android = { package = "clepsydre-android", git = "https://gitlab.gnome.org/TitouanReal/clepsydre.git", optional = true }
clepsydre-eds = { package = "clepsydre-eds", git = "https://gitlab.gnome.org/TitouanReal/clepsydre.git", optional = true }
clepsydre-mock = { package = "clepsydre-mock", git = "https://gitlab.gnome.org/TitouanReal/clepsydre.git", optional = true }
clepsydre-p2panda = { package = "clepsydre-p2panda", git = "https://gitlab.gnome.org/TitouanReal/clepsydre.git", optional = true }

Is it the same issue? Then the changes required are more pervasive, because I actually need wrap files for each of them, and dependency calls.