Official API for reading cargo env variables

[epage]

Problem

Having a Rust API would help with several problems

• Parsing multi-values env variables, like author
• Avoid people making type mistakes
• Bring documentation more inline with people's workflow, e.g. IDEs auto-completing and rendering doc comments
• Help improve discoverability of fields
• Ensures a feedback loop with the cargo team, hoping it avoids some of the failure modes where compatibiluty is broken
• Reduce duplicated effort
  • clap provides an API
  • shadow-rs
  • bugreport
  • versionisator

Proposed Solution

A crate that provides macros for various compile-time env variables set by cargo

Notes

Things to work out

• Name
• If we co-maintain, how repo and publish permissions work out
• How do we handle the MSRV because the cargo repo currently only handles "latest" (see also fix: Set MSRV for internal packages #12381)

[joshtriplett]

We discussed this in today's @rust-lang/cargo meeting. We're broadly in support of this, though it'll need some design for the API.