Adds a new --env-set command line option on rustc. It allows to set environment variables value at compile-time to be used by env! and option_env! macros.

When retrieving and environment variable value, the one specified by --env-set will take precedence. For example if you want have PATH=a in your environment and pass --env-set PATH=env, then you will have:

assert_eq!(env!("PATH"), "env");

Steps

• Implement the MCP -> Done in Implement --env compiler flag (without tracked_env support) #118368, in Add support for --env on tracked_env::var #118830 and in Rename --env option flag to --env-set #119884
• Stabilization PR - Stabilize --env-set option #119926

Unresolved questions

• What is the desired behavior if the same --env variable is passed twice?