Add shims for modules that we can't implement on CloudABI.

As discussed in #47268, libstd isn't ready to have certain functionality
disabled yet. Follow wasm's approach of adding no-op modules for all of
the features that we can't implement.

I've placed all of those shims in a shims/ subdirectory, so we (the
CloudABI folks) can experiment with removing them more easily. It also
ensures that the code that does work doesn't get polluted with lots of
useless boilerplate code.

src/libstd/lib.rs

@@ -473,21 +473,16 @@ pub mod f64;
 pub mod thread;
 pub mod ascii;
 pub mod collections;
-#[cfg(not(target_os = "cloudabi"))]
 pub mod env;
 pub mod error;
 pub mod ffi;
-#[cfg(not(target_os = "cloudabi"))]
 pub mod fs;
 pub mod io;
-#[cfg(not(target_os = "cloudabi"))]
 pub mod net;
 pub mod num;
 pub mod os;
 pub mod panic;
-#[cfg(not(target_os = "cloudabi"))]
 pub mod path;
-#[cfg(not(target_os = "cloudabi"))]
 pub mod process;
 pub mod sync;
 pub mod time;

src/libstd/sys/cloudabi/args.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+pub use sys::cloudabi::shims::args::*;
+
 #[allow(dead_code)]
 pub fn init(_: isize, _: *const *const u8) {}
 

src/libstd/sys/cloudabi/mod.rs

@@ -34,6 +34,9 @@ pub mod time;
 
 mod abi;
 
+mod shims;
+pub use self::shims::*;
+
 #[allow(dead_code)]
 pub fn init() {}
 

src/libstd/sys/cloudabi/os.rs

@@ -12,6 +12,8 @@ use ffi::CStr;
 use libc::{self, c_int};
 use str;
 
+pub use sys::cloudabi::shims::os::*;
+
 pub fn errno() -> i32 {
     extern "C" {
         #[thread_local]
@@ -29,3 +31,7 @@ pub fn error_string(errno: i32) -> String {
         .unwrap()
         .to_owned()
 }
+
+pub fn exit(code: i32) -> ! {
+    unsafe { libc::exit(code as c_int) }
+}

src/libstd/sys/cloudabi/shims/args.rs

@@ -0,0 +1,45 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use ffi::OsString;
+
+pub struct Args(());
+
+impl Args {
+    pub fn inner_debug(&self) -> &[OsString] {
+        &[]
+    }
+}
+
+impl Iterator for Args {
+    type Item = OsString;
+    fn next(&mut self) -> Option<OsString> {
+        None
+    }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (0, Some(0))
+    }
+}
+
+impl ExactSizeIterator for Args {
+    fn len(&self) -> usize {
+        0
+    }
+}
+
+impl DoubleEndedIterator for Args {
+    fn next_back(&mut self) -> Option<OsString> {
+        None
+    }
+}
+
+pub fn args() -> Args {
+    Args(())
+}

src/libstd/sys/cloudabi/shims/env.rs

@@ -0,0 +1,19 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+pub mod os {
+    pub const FAMILY: &'static str = "cloudabi";
+    pub const OS: &'static str = "cloudabi";
+    pub const DLL_PREFIX: &'static str = "lib";
+    pub const DLL_SUFFIX: &'static str = ".so";
+    pub const DLL_EXTENSION: &'static str = "so";
+    pub const EXE_SUFFIX: &'static str = "";
+    pub const EXE_EXTENSION: &'static str = "";
+}