Fix thread::available_parallelism on WASI targets with threads

library/std/src/sys/thread/unix.rs

@@ -155,6 +155,7 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
             target_os = "aix",
             target_vendor = "apple",
             target_os = "cygwin",
+            all(target_os = "wasi", target_feature = "atomics"),
         ) => {
             #[allow(unused_assignments)]
             #[allow(unused_mut)]
@@ -314,6 +315,10 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
                 Ok(NonZero::new_unchecked(set.count_ones() as usize))
             }
         }
+        all(target_os = "wasi", not(target_feature = "atomics")) => {
+            // Single-threaded WASM always has exactly one thread of execution.
+            Ok(NonZero::new(1).unwrap())
+        }
         _ => {
             // FIXME: implement on Redox, l4re
             Err(io::const_error!(io::ErrorKind::Unsupported, "getting the number of hardware threads is not supported on the target platform"))

library/std/tests/thread.rs

@@ -85,7 +85,6 @@ fn thread_local_hygeiene() {
         target_env = "sgx",
         target_os = "solid_asp3",
         target_os = "teeos",
-        target_os = "wasi"
     ),
     should_panic
 )]