3 files changed, 21 insertions, 15 deletions
diff --git a/library/std/src/env.rs b/library/std/src/env.rs
index 27f4daba44b..043747d0bc5 100644
--- a/library/std/src/env.rs
+++ b/library/std/src/env.rs
@@ -608,20 +608,16 @@ impl Error for JoinPathsError {
 ///
 /// # Windows
 ///
-/// - Returns the value of the 'HOME' environment variable if it is set
-///   (including to an empty string).
-/// - Otherwise, returns the value of the 'USERPROFILE' environment variable if it is set
-///   (including to an empty string).
-/// - If both do not exist, [`GetUserProfileDirectory`][msdn] is used to return the path.
+/// - Returns the value of the 'USERPROFILE' environment variable if it is set, and is not an empty string.
+/// - Otherwise, [`GetUserProfileDirectory`][msdn] is used to return the path. This may change in the future.
 ///
 /// [msdn]: https://docs.microsoft.com/en-us/windows/win32/api/userenv/nf-userenv-getuserprofiledirectorya
 ///
-/// # Deprecation
+/// In UWP (Universal Windows Platform) targets this function is unimplemented and always returns `None`.
 ///
-/// This function is deprecated because the behavior on Windows is not correct.
-/// The 'HOME' environment variable is not standard on Windows, and may not produce
-/// desired results; for instance, under Cygwin or Mingw it will return `/home/you`
-/// when it should return `C:\Users\you`.
+/// Before Rust CURRENT_RUSTC_VERSION, this function used to return the value of the 'HOME' environment variable
+/// on Windows, which in Cygwin or Mingw environments could return non-standard paths like `/home/you`
+/// instead of `C:\Users\you`.
 ///
 /// # Examples
 ///
diff --git a/library/std/src/sys/pal/wasi/thread.rs b/library/std/src/sys/pal/wasi/thread.rs
index 4b83870fdea..f5e19f26bfe 100644
--- a/library/std/src/sys/pal/wasi/thread.rs
+++ b/library/std/src/sys/pal/wasi/thread.rs
@@ -2,7 +2,6 @@
 
 use crate::ffi::CStr;
 use crate::num::NonZero;
-use crate::sys::unsupported;
 use crate::time::Duration;
 use crate::{io, mem};
 
@@ -34,6 +33,8 @@ cfg_if::cfg_if! {
             #[allow(non_camel_case_types)]
             pub type pthread_t = *mut ffi::c_void;
 
+            pub const _SC_NPROCESSORS_ONLN: ffi::c_int = 84;
+
             extern "C" {
                 pub fn pthread_create(
                     native: *mut pthread_t,
@@ -121,7 +122,7 @@ impl Thread {
             }
         } else {
             pub unsafe fn new(_stack: usize, _p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-                unsupported()
+                crate::sys::unsupported()
             }
         }
     }
@@ -187,5 +188,14 @@ impl Thread {
 }
 
 pub fn available_parallelism() -> io::Result<NonZero<usize>> {
-    unsupported()
+    cfg_if::cfg_if! {
+        if #[cfg(target_feature = "atomics")] {
+            match unsafe { libc::sysconf(libc::_SC_NPROCESSORS_ONLN) } {
+                -1 => Err(io::Error::last_os_error()),
+                cpus => NonZero::new(cpus as usize).ok_or(io::Error::UNKNOWN_THREAD_COUNT),
+            }
+        } else {
+            crate::sys::unsupported()
+        }
+    }
 }
diff --git a/library/std/src/sys/pal/windows/os.rs b/library/std/src/sys/pal/windows/os.rs
index 5242bc9da31..5231a34469a 100644
--- a/library/std/src/sys/pal/windows/os.rs
+++ b/library/std/src/sys/pal/windows/os.rs
@@ -377,8 +377,8 @@ fn home_dir_crt() -> Option<PathBuf> {
 }
 
 pub fn home_dir() -> Option<PathBuf> {
-    crate::env::var_os("HOME")
-        .or_else(|| crate::env::var_os("USERPROFILE"))
+    crate::env::var_os("USERPROFILE")
+        .filter(|s| !s.is_empty())
         .map(PathBuf::from)
         .or_else(home_dir_crt)
 }