Deprecate per-target editions

[nyurik]

Update: @epage created an RFC for this too

As a big surprise even to seasoned Rust developers, each target in a package could have their own edition, different from the edition of the package (docs):

The edition field defines the Rust edition the target will use. If not specified, it defaults to the edition field for the [package]. This field should usually not be set, and is only intended for advanced scenarios such as incrementally transitioning a large package to a new edition.

This creates significant mental burden, for example making it significantly more difficult to implement such basic features as providing an env var to the build script.

• Cargo should tell build.rs what the Rust edition is #6408

Has there been any useful examples of multiple editions per package, and if not, can we deprecate this?

[epage]

In case you aren't aware, I recently posted rust-lang/rfcs#3772

[nyurik]

Thanks! Funny how we both went back to that issue at nearly the same time :) I updated the description. I think this issue should help track any changes in Cargo once the RFC is approved?

[oriongonza]

What does it mean for Cargo to deprecate a feature? Will the behavior ever hard error?
What advantages does it have aside form generally discouraging the behavior?

[epage]

See rust-lang/rfcs#3772 for what I've proposed the deprecation to mean