Generated meson.build from Cargo crate references undefined variable

[2xsaiko]

Describe the bug
Meson's cargo importer generates a meson.build that is referencing an undefined variable for the crate libflate_lz77, which is a transitive dependency of the crate I'm trying to import.

Might be related to these two crates depending on each other, or dev-dependencies: libflate depends on libflate_lz77 and libflate_lz77 has libflate in its dev-dependencies.

Executing subproject matfile:libflate-2-rs:libflate_lz77-2-rs method cargo

libflate_lz77-2-rs| Generated Meson AST: /Users/saiko/Src/test/build/subprojects/libflate_lz77-2.1.0/meson.build
libflate_lz77-2-rs| Project name: libflate_lz77
libflate_lz77-2-rs| Project version: 2.1.0
libflate_lz77-2-rs| Rust compiler for the host machine: rustc -C linker=cc (rustc 1.82.0)
libflate_lz77-2-rs| Rust linker for the host machine: rustc -C linker=cc ld.lld 19.1.2

subprojects/libflate_lz77-2.1.0/meson.build:25:-1: ERROR: Unknown variable "libflate_options".

The first couple lines from the meson.build in question (note libflate_options not being defined unlike the others):

project(
  'libflate_lz77',
  'rust',
  version : '2.1.0',
  meson_version : '>= 1.6.0',
  default_options : ['rust_std=2021'],
  license : 'MIT'
)
rust = import('rust')
features = {}
core2_options = {}
hashbrown_options = {}
rle_decode_fast_options = {}
required_deps = {}
if get_option('feature-default')
  required_deps += {'core2' : true, 'libflate' : true}
  features += {'std' : true, 'default' : true}
  core2_options += {'feature-std' : true}
  libflate_options += {'feature-std' : true}
endif
if get_option('feature-std')
  required_deps += {'core2' : true, 'libflate' : true}
  features += {'std' : true}
  core2_options += {'feature-std' : true}
  libflate_options += {'feature-std' : true}
endif

To Reproduce
meson.build:

project(
    'test',
    'rust',
    default_options: {'rust_std': '2021'},
    meson_version: '1.6.0',
)

matfile_proj = subproject('matfile')

matfile.wrap:

[wrap-file]
method = cargo
directory = matfile-0.5.0

source_url = https://crates.io/api/v1/crates/matfile/0.5.0/download
source_filename = matfile-0.5.0.tar.gz
source_hash = b22724209845c600f095bca5be902d636b4fc4f0b27fe5bb5d8314b5c117d0d6

[provide]
dependency_names = matfile

Expected behavior
The generated meson.build for libflate_lz77 does not reference an undefined variable.

system parameters

• Is this a cross build or just a plain native build (for the same computer)? Native build
• what operating system (e.g. MacOS Catalina, Windows 10, CentOS 8.0, Ubuntu 18.04, etc.) Gentoo Linux
• what Python version are you using? 3.12.7
• what meson --version 1.6.0 (with patch from 5769074)
• what ninja --version if it's a Ninja build? 1.12.1

[peterdelevoryas]

I also have this issue, with a different project and dependency here. It's kind of blocking me from using Meson to build Rust, because there doesn't seem to be an option to disable default features, like --no-default-features would? Based on this comment.

[eli-schwartz]

Poking around a bit in the libflate example, I notice that libflate depends on libflate_lz77.

libflate itself has these features:

[features]
default = ["std"]
std = ["libflate_lz77/std", "core2/std"]

Meanwhile, libflate_lz77 has these features:

[features]
default = ["std"]
std = ["core2/std", "libflate/std"]

It looks like... each one is trying to enforce on the other, to build with a consistent set of features? And lz77 wants to force the feature active for its own parent crate, not a dependency. Which is apparently legal. Is this only the case because libflate via path ../ is a dev-dependency, or is this generally legal?

I guess meson needs to handle arbitrary features getting set on things that aren't in dependencies.

[peterdelevoryas]

Yeah I suspect libflate_lz77 has a dev-dependency on libflate because it's using it in doc-comment examples, which actually get compiled for some tests. I'm also surprised that works, because you would expect circular dependencies, but perhaps in dev-dependency mode it resolves without circular references.

https://github.com/sile/libflate/blob/6676d36d381eec60f376337e528d658b16c87c7a/libflate_lz77/src/lib.rs#L117-L118

I think the issue is this code in meson is assuming that a dependency should be added whenever a crate tries to enable a feature on a dependency crate using self_feature = ["dep/dep_feature"] syntax:

https://github.com/mesonbuild/meson/blob/master/mesonbuild/cargo/interpreter.py#L581-L582

Reference: https://doc.rust-lang.org/cargo/reference/features.html#dependency-features

I haven't quite gotten the fix right though, so, maybe I'm mistaken somehow.

[xclaesse]

First step at fixing this: #13976.

Configure passes but it has a bunch of build time errors, like:

FAILED: subprojects/zerocopy-0.7.32/libzerocopy.rlib 
rustc -C linker=cc --color=always -C debug-assertions=yes -C overflow-checks=no --crate-type rlib --edition=2018 -g --crate-name zerocopy --emit dep-info=subprojects/zerocopy-0.7.32/zerocopy.d --emit link=subprojects/zerocopy-0.7.32/libzerocopy.rlib --out-dir subprojects/zerocopy-0.7.32/libzerocopy.rlib.p -C metadata=adcab9b@@zerocopy@sta --cfg 'feature="simd"' ../subprojects/zerocopy-0.7.32/src/lib.rs
error: environment variable `CARGO_PKG_VERSION` not defined at compile time
    --> ../subprojects/zerocopy-0.7.32/src/lib.rs:1356:58
     |
1356 |     doc = concat!("[derive]: https://docs.rs/zerocopy/", env!("CARGO_PKG_VERSION"), "/zerocopy/derive.FromZeroes.html"),
     |                                                          ^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = help: Cargo sets build script variables at run time. Use `std::env::var("CARGO_PKG_VERSION")` instead
     = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

That requires using the rustc --set-env CLI. I think it got merged upstream but I'm not sure it's stable yet.

[xclaesse]

Seems rustc decided to close the issue.... someone will have to push them harder... rust-lang/rust#119926