Target-specific workspace default-members (or excludes)

[ratijas]

Describe the problem you are trying to solve

I want to exclude (or rather, specify different subset of default members) for certain platforms in cargo toml/config.

Describe the solution you'd like

Just like there are triples-specific overrides in .cargo/config.toml, I'd like to see workspace-related keys there.

My primary use case is to disable certain example (implemented as a full-blown workspace member) when compiling on Windows + MinGW toolchain, because certain libraries (namely, QtWebEngine) are not easily available there. So, I'd expect something like this in my workspace root Cargo.toml:

[workspace]
members = [
    'qmetaobject',
    'examples/todos',
    'examples/webengine',
]

[target.i686-pc-windows-gnu]
default-members = [
    'qmetaobject',
    'examples/todos',
]

Notes

I tried tweaking in such way both Cargo.toml and .cargo/config.toml, and none of that currently works. The only workaround for now is manually commenting out or disabling a package via --workspace --exclude <SPEC>.

See woboq/qmetaobject-rs#167

[epage]

This looks to be another variant of #6179. Closing in favor of that so we keep the conversation in one place for how to solve the need, rather than focus on each individual idea separately. If there is a reason for us to reconsider that, let us know!