Document `since` for cargo environment variables

[raldone01]

Problem

I tried to use CARGO_MANIFEST_PATH but it isn't available in older rustc versions.
It would be nice to add a since indication as is custom in std.

I am talking specifically about:
https://doc.rust-lang.org/cargo/reference/environment-variables.html

However, adding since to all new features and retroactively to older features would be nice.

Proposed Solution

Add since to the cargo environment variables.

[weihanglo]

Thanks for the idea. There were some discussions about expanding environment variables doc, though the previously proposed solution had some drawbacks and has yet been done. See https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/Anchors.20for.20each.20environment.20variable.20in.20The.20Cargo.20Book.

[raldone01]

Oh yeah linking to specific environment variables would be awesome too!

[weihanglo]

Yeah, and we have started adding MSRV note for some Cargo manifest fields recently like this. It is worth for every environment variable as well.

[raldone01]

By any chance do you know why CARGO_MANIFEST_PATH was added?
Isn't it always just CARGO_MANIFEST_DIR + /Cargo.toml?

Ahh it is for single file rust scripts. Cool!

[epage]

We just need a decent way of formatting this.

[weihanglo]

One of the problems of making a heading for each env var: variable name can conflict with each other. The unique id anchor would only link to the first occurrence.