Enable parsing config values in virtual manifests

[sondre-teigen]

Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

Cross should be able to parse config values from [workspace.metadata.cross] sections.

Describe why this would be a good inclusion for cross

It seems to be impossible to set per-workspace config values when using virtual manifests without editing Cross.toml, since [package.metadata] sections do not work in virtual manifests.

An applicable scenario would be when the CROSS_CONFIG variable is set to a global, constant, Cross.toml file.

[Alexhuszagh]

Related to #842.

[Emilgardis]

Hmm, this is an unfortunate situation.

Currently, as we support package.metadata.cross, we technically should support multiple of those and with this feature implemented workspace.metadata.cross. This however presents a problem.

Say I have the following workspace (which is a virtual manifest with foo and bar members)

# Cargo.toml

[package]
name = "foo"
# ...
[package.metadata.cross]
build.image = "my_foo_image"

[workspace]
members = "bar"
[workspace.metadata.cross]
build.image = "my_workspace_image"


# bar/Cargo.toml

[package]
name = "bar"
# ...
[package.metadata.cross]
build.image = "my_bar_image"

what image should we use on cross build, cross build -p foo and cross build -p bar?
This is even more confusing when you consider values which append when merging.

I think the solution is to only read workspace.metadata.cross, or package.metadata.cross, but only allow one package.metadata.cross