Documented syntax for core::env is wrong

[lukaslueg]

The documentation for the compiler-built-in macro core::env / std::env currently reads:

macro_rules! env {
    ($name:expr $(,)?) => { ... };
}

The text and example state that a second parameter can be given to customize the error message in case the environment variable is not defined. This second parameter is not reflected in the syntax above. The following is valid:

fn main() {
    let x: &'static str = env!("CARGO_MANIFEST_DIR", "oh no!");  // this should not match `$(name:expr $(,)?)`
}

Looking at the implementation, the syntax should most likely be documented as

macro_rules! env {
    ($name:expr) => { ... };
    ($name:expr, $error:expr) => { ... };
}

Am I missing something? I can commit a PR if so desired.

[jyn514]

@lukaslueg I think you're right; probably the compiler should be checking that the declared macro and the built-in macro accept the same syntax, although that could be hard to enforce. Changing the declared macro seems like a good first step :)