Support drop-in replacements for cargo

[epage]

Like xargo, see roblabla/cargo-travis#34 (comment)

[epage]

Options

• Extra constructor for Cargo
  • Like with Should arg and build_with be unsafe? #6, should it be marked as unsafe?
• Environment variable
  • Seems like this should more so be a policy for clients to define?
  • But a consistent policy would be nice

[roblabla]

cargo_metadata uses the CARGO environment variable. https://github.com/oli-obk/cargo_metadata/blob/master/src/lib.rs#L302

[roblabla]

Note that while this works fine for overriding cargo in the cargo metadata case (it allows setting the path to the cargo binary), it's not such a great solution for cases where you want to override cargo build so it instead runs cargo xbuild for instance. I'm not sure how to best handle this.

[epage]

it's not such a great solution for cases where you want to override cargo build so it instead runs cargo xbuild for instance. I'm not sure how to best handle this.

I guess that could be a special environment variable for escargot? The only other option is a custom cargo wrapper that maps build to xbuild.