feat: Autocomplete Cargo-defined env vars in env! and option_env! (#12448)

[btwotwo]

Closes #12448

Important to know:

• Variables are taken from https://doc.rust-lang.org/cargo/reference/environment-variables.html and hardcoded as a const array.
• For the sake of simplicity I didn't include the autocompletion of CARGO_BIN_EXE_<name> and OUT_DIR since it would require information about build.rs and binary name. If somebody knows an easy way of obtaining them I can add those vars as well :)

[WaffleLapkin]

A possible follow up is to check that the macro is build-in, to guard against shadowing. Something like #13356

[btwotwo]

Good point, thanks! Will add that check

[btwotwo]

Done

[Veykril]

For the sake of simplicity I didn't include the autocompletion of CARGO_BIN_EXE_ and OUT_DIR since it would require information about build.rs and binary name. If somebody knows an easy way of obtaining them I can add those vars as well :)

OUT_DIR should be feasible in theory, we already store it when evaluating build scripts. But we aren't exposing it yet, though I am unsure how we would want to make it accessible. Probably through the DefMap would need to think about that a bit.

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit 2b2b9f8 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2b2b9f8 with merge a0ab61f...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing a0ab61f to master...

[lnicola]

env-completions.mp4