So I have a case where I need to pin a dependency to its exact version on one platform and use a different version of that crate for all others.

This is what I have in my Cargo.toml:

[target.'cfg(not(target_os = "macos"))'.dependencies]
cxx = { version = "^1.0.187", features = ["c++20"] }

[target.'cfg(target_os = "macos")'.dependencies]
cxx = { version = "=1.0.129", features = ["c++20"] }

However cargo update is complaining now:

cargo update cxx
    Updating crates.io index
error: failed to select a version for `cxx`.
    ... required by package `copper-showdown-emulator-sys v0.1.0 (/home/mop/projects/akronyme-analogiker/copper-showdown-editor/crates/copper-showdown-emulator-sys)`
    ... which satisfies path dependency `copper-showdown-emulator-sys` (locked to 0.1.0) of package `copper-showdown v0.3.0 (/home/mop/projects/akronyme-analogiker/copper-showdown-editor/crates/copper-showdown)`
versions that meet the requirements `=1.0.129` are: 1.0.129

all possible versions conflict with previously selected packages.

  previously selected package `cxx v1.0.187`
    ... which satisfies dependency `cxx = "^1.0.187"` of package `copper-showdown-emulator-sys v0.1.0 (/home/mop/projects/akronyme-analogiker/copper-showdown-editor/crates/copper-showdown-emulator-sys)`
    ... which satisfies path dependency `copper-showdown-emulator-sys` (locked to 0.1.0) of package `copper-showdown v0.3.0 (/home/mop/projects/akronyme-analogiker/copper-showdown-editor/crates/copper-showdown)`

failed to select a version for `cxx` which could resolve this conflict

I am puzzled. What am I doing wrong? I thought I can specify mutually exclusive version changes like this

More background on this for the curious:

With a newer rustc the cxx package generates dozens of warnings inside a macro (not sure if I can somehow allow them - didn't dive into that yet).
Upgrading the cxx package to the latest version worked and I got rid of the warnings.

However I am not using a mac but instead cross compile it for mac in a docker container which has all the mac compiler toolchains onboard (which I need because this is basically a crate that creates a Rust<->C++ bridge). However the new version of cxx adds some static asserts etc that don't work with this toolchain

So I want to use the latest cxx for normal operations. for the cases I am cross compiling to mac I don't care about the new warnings and want to pin it down to the version I know is compatible with this specific toolchain. It should just work.

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