Support for std::env::vars() (and ::vars_os)

[srijs]

Hi!

I came across an issue trying to run a proptest-based test suite via miri. proptest relies on the std::env::vars function to read a snapshot of all environment variables.

This fails on my OSX machine with the following error:

417 |     _NSGetEnviron()
    |     ^^^^^^^^^^^^^^^ Miri evaluation error: can't call foreign function: _NSGetEnviron
    |
    = note: inside call to `std::sys::unix::os::environ` at /Users/sam/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/sys/unix/os.rs:438:28
    = note: inside call to `std::sys::unix::os::env` at /Users/sam/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/src/rust/src/libstd/env.rs:143:21
    = note: inside call to `std::env::vars_os` at /Users/sam/.cargo/registry/src/gh.loli.garden-1ecc6299db9ec823/proptest-0.9.4/src/test_runner/config.rs:78:9

Reading through the implementation of std::env::vars, it will call the _NSGetEnviron function on OSX, and on all other unix machines it will rely on an external symbol (extern char **environ) to read the environment.

Looking at the current shim code, I can see how it would be possible to implement for OSX, but it's not clear to me how you would implement this for other UNIX systems, as there doesn't seem to be an existing way in miri to hook into external symbols that are not functions.

[oli-obk]

This is being worked on right now: #928

The way rustc exposes statics to us is being adjusted to support this case

[srijs]

Neat! Not sure how I missed that issue, thanks!

[RalfJung]

Well what is being worked on is the Linux version. Not sure how much extra work the macOS version needs and if @christianpoveda is up for that. ;)

[RalfJung]

But indeed this is a duplicate of #756.

[srijs]

I'm subscribed to #756 now, and will happily tackle the OSX side of it once it works for Linux :)

[pvdrz]

Well what is being worked on is the Linux version. Not sure how much extra work the macOS version needs and if @christianpoveda is up for that. ;)

iirc there shouldn't be too much problem implementing this for macos after finishing linux (which will happen in the "near" future).

[pvdrz]

I'm subscribed to #756 now, and will happily tackle the OSX side of it once it works for Linux :)

I'll ping you when the linux implementation is done :)