Use Unsupported on platforms where available_concurrency is not implemented.

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

@@ -1,5 +1,6 @@
 #![allow(dead_code)]
 
+use super::unsupported;
 use crate::ffi::CStr;
 use crate::io;
 use crate::mem;
@@ -97,10 +98,7 @@ impl Thread {
 }
 
 pub fn available_concurrency() -> io::Result<NonZeroUsize> {
-    Err(io::Error::new_const(
-        io::ErrorKind::NotFound,
-        &"The number of hardware threads is not known for the target platform",
-    ))
+    unsupported()
 }
 
 pub mod guard {

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

@@ -1,4 +1,5 @@
 #![cfg_attr(test, allow(dead_code))] // why is this necessary?
+use super::unsupported;
 use crate::ffi::CStr;
 use crate::io;
 use crate::num::NonZeroUsize;
@@ -137,10 +138,7 @@ impl Thread {
 }
 
 pub fn available_concurrency() -> io::Result<NonZeroUsize> {
-    Err(io::Error::new_const(
-        io::ErrorKind::NotFound,
-        &"The number of hardware threads is not known for the target platform",
-    ))
+    unsupported()
 }
 
 pub mod guard {

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

@@ -276,7 +276,7 @@ pub fn available_concurrency() -> io::Result<NonZeroUsize> {
             Ok(unsafe { NonZeroUsize::new_unchecked(cpus as usize) })
         } else {
             // FIXME: implement on vxWorks, Redox, Haiku, l4re
-            Err(io::Error::new_const(io::ErrorKind::NotFound, &"The number of hardware threads is not known for the target platform"))
+            Err(io::Error::new_const(io::ErrorKind::Unsupported, &"Getting the number of hardware threads is not supported on the target platform"))
         }
     }
 }

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

@@ -32,10 +32,7 @@ impl Thread {
 }
 
 pub fn available_concurrency() -> io::Result<NonZeroUsize> {
-    Err(io::Error::new_const(
-        io::ErrorKind::NotFound,
-        &"The number of hardware threads is not known for the target platform",
-    ))
+    unsupported()
 }
 
 pub mod guard {

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

@@ -65,10 +65,7 @@ impl Thread {
 }
 
 pub fn available_concurrency() -> io::Result<NonZeroUsize> {
-    Err(io::Error::new_const(
-        io::ErrorKind::NotFound,
-        &"The number of hardware threads is not known for the target platform",
-    ))
+    unsupported()
 }
 
 pub mod guard {

library/std/src/sys/wasm/atomics/thread.rs

@@ -1,3 +1,4 @@
+use super::unsupported;
 use crate::ffi::CStr;
 use crate::io;
 use crate::num::NonZeroUsize;
@@ -41,10 +42,7 @@ impl Thread {
 }
 
 pub fn available_concurrency() -> io::Result<NonZeroUsize> {
-    Err(io::Error::new_const(
-        io::ErrorKind::NotFound,
-        &"The number of hardware threads is not known for the target platform",
-    ))
+    unsupported()
 }
 
 pub mod guard {