Environment variables don't work if `main` isn't used

[alexcrichton]

We discovered an issue in the Rust wasm32-wasi target today where a wasm binary uses std::env but doesn't end up importing the requisite wasi items to actually get env vars working.

The broken example looks like this:

#[no_mangle]
pub extern fn foo() {
    println!("{:?}", std::env::var("FOO"));
}

And when compiled:

$ rustc +nightly foo.rs --target wasm32-wasi -O --crate-type cdylib
$ wasm2wat foo.wasm | grep import
  (import "wasi_snapshot_preview1" "fd_write" (func $_ZN4wasi13lib_generated22wasi_snapshot_preview18fd_write17h8cb541fbb70a6a5bE (type 7)))

This is showing that environ_get isn't imported at all!

When compiled as a binary though it does indeed work:

fn main() {
    println!("{:?}", std::env::var("FOO"));
}

$ rustc +nightly foo.rs --target wasm32-wasi -O
$ wasm2wat foo.wasm | grep import
  (import "wasi_snapshot_preview1" "proc_exit" (func $__wasi_proc_exit (type 1)))
  (import "wasi_snapshot_preview1" "fd_write" (func $_ZN4wasi13lib_generated22wasi_snapshot_preview18fd_write17h8cb541fbb70a6a5bE (type 9)))
  (import "wasi_snapshot_preview1" "fd_prestat_get" (func $__wasi_fd_prestat_get (type 3)))
  (import "wasi_snapshot_preview1" "fd_prestat_dir_name" (func $__wasi_fd_prestat_dir_name (type 8)))
  (import "wasi_snapshot_preview1" "environ_sizes_get" (func $__wasi_environ_sizes_get (type 3)))
  (import "wasi_snapshot_preview1" "environ_get" (func $__wasi_environ_get (type 3)))

I'm told that @sunfishcode is working on a fix!

[syrusakbary]

Not sure if this is related:
#181

I encountered in programs with main