`bevy build` does not work for complex config.toml

[selvmaya]

CLI version

main (1219ee2)
also a problem on v0.1.0-alpha.2

Relevant system information

• Rust version
  rustc 1.92.0-nightly (fa3155a64 2025-09-30)
• Operating system
  Arch Linux: 6.16.8-arch3-1

Contents of global config.toml

# [target.'cfg(target_arch = "x86_64")']
# rustflags = ["-Ctarget-cpu=native"]

[target.'cfg(and(toolchain = stable, target_arch = "x86_64"))']
linker = "/usr/bin/clang"
rustflags = [
  "-Ctarget-cpu=native",
  # use mold *even on stable*
  "-Clink-arg=--ld-path=/usr/bin/mold",
  "-Clink-arg=-fuse-ld=mold" # see bevy's config_fast_builds.toml
]

[target.'cfg(and(toolchain = nightly, target_arch = "x86_64"))']
linker = "/usr/bin/clang"
rustflags = [
  "-Ctarget-cpu=native",
  # "-Clink-arg=--ld-path=/usr/bin/mold", # use "mold" linker
  # "-Clink-arg=-fuse-ld=mold",
  "-Clink-arg=--ld-path=/usr/bin/wild", # use "wild" linker
  "-Zthreads=0", "-Zshare-generics=y" # use nightly-only stuff

What you did

• Inititalize new -t=2d bevy project
• Tried building with the subcommand.
  bevy b [-r]

What went wrong

• What I expected
  bevy_cli starts and finishes build (similarly to cargo build)

• What actually happened
  It immediately fails with this message:

error: and(toolchain = nightly, target_arch = "x86_64")
   ^ expected one of `=`, `,`, `)` here

(note: yes the error message seems to crop out some of the code)

Additional information

• Note
  It could be that my ~/.cargo/config.toml cannot be parsed properly (cfg stuff can be obscure and cargo/rust usually does not error even if that is the case).
  If this is the case, I'd love to know why this only happens for bevy_cli and not cargo, as well as how I could avoid this problem in the future. Maybe the error message could be improved?
• Workaround
  Assuming I shouldn't just fix it (it may be impossible to correctly parse), a fix is to temporarily make my global (~/.cargo/) config.toml inaccessible from cargo or the cli by renaming it (I renamed it config-2.toml).
  This seems to make bevy b work just like cargo b.
  Simply trying to shadow the global config with the local ./.cargo/config.toml (from config_fast_builds.toml) was not enough.
• Motivation for fix
  It seems I don't really need to use the cli to build, but since it's a command and there is some useful metadata and helpers inside the tool (e.g. bevy b web), I'd like for it to work.

Terminal screenshot:

[BD103]

I've been able to reproduce this bug locally, and have narrowed it down to an issue with cargo_config2.

$ RUST_BACKTRACE=1 bevy b --verbose
debug: running: `cargo metadata --format-version 1`
error: and(toolchain = nightly, target_arch = "x86_64")
   ^ expected one of `=`, `,`, `)` here

Stack backtrace:
   0: std::backtrace::Backtrace::create
   1: anyhow::error::<impl core::convert::From<E> for anyhow::Error>::from
   2: bevy_cli::config::CliConfig::append_cargo_config_rustflags
   3: bevy_cli::commands::build::build
   4: bevy::main
   5: std::sys::backtrace::__rust_begin_short_backtrace
   6: std::rt::lang_start::{{closure}}
   7: std::rt::lang_start_internal
   8: _main

This is the line that causes the error:

bevy_cli/src/config.rs

Line 228 in 1219ee2

if let Some(cargo_config_rustflags) = config.rustflags(target)? {

I don't think it's happy with the and( bit at the beginning, but I'll continue investigating. Thanks for filing a report!

[BD103]

Yup, using a debugger it looks like the error sources from here. I don't know enough about what syntax is supported in [target.*], so I'm going to look into that more.

[selvmaya]

I really wish the syntax for this was clearly explained somewhere. All my knowledge is crossreferences across different types of toml files and purposes/environments, and limitations are not easy to find written anywhere. I have tried running cargo config and eyeballing measurements in compile times and such but the stack of arguments is kind of interweaved and opaque.

Edit: Found this reference which doesn’t deal with our issue at hand but at least serves as a reference for the different major configurations in such a file: https://nnethercote.github.io/perf-book/build-configuration.html