Add wasi-preview2 os module

Signed-off-by: Ryan Levick <me@ryanlevick.com>

Author: rylev

library/std/src/os/mod.rs

@@ -82,9 +82,12 @@ pub mod linux;
         all(target_vendor = "fortanix", target_env = "sgx")
     )
 )))]
-#[cfg(any(target_os = "wasi", target_os = "wasi-preview2", doc))]
+#[cfg(any(target_family = "wasi", doc))]
 pub mod wasi;
 
+#[cfg(any(target_os = "wasi-preview2", doc))]
+pub mod wasi_preview2;
+
 // windows
 #[cfg(not(all(
     doc,
@@ -155,7 +158,7 @@ pub(crate) mod watchos;
 #[cfg(target_os = "xous")]
 pub mod xous;
 
-#[cfg(any(unix, target_os = "wasi", target_os = "wasi-preview2", doc))]
+#[cfg(any(unix, target_family = "wasi", doc))]
 pub mod fd;
 
 #[cfg(any(target_os = "linux", target_os = "android", doc))]

library/std/src/os/wasi_preview2/mod.rs

@@ -0,0 +1,5 @@
+//! Platform-specific extensions to `std` for Preview 2 of the WebAssembly System Interface (WASI).
+//!
+//! This module is currently empty, but will be filled over time as wasi-libc support for WASI Preview 2 is stabilized.
+
+#![stable(feature = "raw_ext", since = "1.1.0")]

src/bootstrap/src/core/build_steps/compile.rs

@@ -343,7 +343,10 @@ fn copy_self_contained_objects(
         let srcdir = builder
             .wasi_root(target)
             .unwrap_or_else(|| {
-                panic!("Target {:?} does not have a \"wasi-root\" key", target.triple)
+                panic!(
+                    "Target {:?} does not have a \"wasi-root\" key in Config.toml",
+                    target.triple
+                )
             })
             .join("lib")
             .join(target.to_string().replace("-preview1", "").replace("-preview2", ""));