Implement env!() builtin macro

[CohenArthur]

env!() is part of the list of builtin macros enumerated here

Its description is as follows:

Inspects an environment variable at compile time

The goal is to fetch environment variables passed to gccrs, and expand them into the user's compiled code.

arthur@platypus ~/G/gccrs (add-assert-macro)> cat test.rs
fn main() {
    let s = env!("CUSTOM");

    println!("{}", s);
}
arthur@platypus ~/G/gccrs (add-assert-macro)> rustc test.rs
error: environment variable `CUSTOM` not defined
 --> test.rs:2:13
  |
2 |     let s = env!("CUSTOM");
  |             ^^^^^^^^^^^^^^
  |
  = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

arthur@platypus ~/G/gccrs (add-assert-macro) [1]> CUSTOM='hi!' rustc test.rs
arthur@platypus ~/G/gccrs (add-assert-macro)> ./test
hi!

The macro should not expand to a function call that fetches environment variables during the user's program's execution.

arthur@platypus ~/G/gccrs (add-assert-macro)> ./test
hi!
arthur@platypus ~/G/gccrs (add-assert-macro)> CUSTOM='something else!' ./test
hi!

The original macro contains two match arms, including one with a custom error message:

    macro_rules! env {
        ($name:expr $(,)?) => {{ /* compiler built-in */ }};
        ($name:expr, $error_msg:expr $(,)?) => {{ /* compiler built-in */ }};
    }

You should try and support the first form first, by simply fetching an environment variable and creating an associated string using the make_string function defined in gcc/rust/expand/rust-macro-builtins.cc

The second form could be implemented in a later PR for simplicity

[bjorn3]

Rustc has a dep file extension showing which env vars are read and if valid UTF-8 what value saod env vars had. This is used by cargo to rebuild if the env vars change.

[CohenArthur]

Oh that's really smart. I think this is still a little far for gccrs but we should keep it in mind. I'm guessing this is what this line is for, then. We'll definitely need to look into it before compiling actual projects

[bjorn3]

Yes, and this is where it is emitted: https://github.com/rust-lang/rust/blob/45e2c2881d11324d610815bfff097e25c412199e/compiler/rustc_interface/src/passes.rs#L703-L706 And this is the PR originally implementing it: rust-lang/rust#71858

[omachota]

Hi, I would like to work on this issue.

[CohenArthur]

Hi @omachota, and welcome! I've assigned you the issue. Feel free to ask any questions here