In Thread::new, add a comment that a panic could cause a memory leak.

src/libstd/sys/cloudabi/thread.rs

@@ -31,12 +31,15 @@ impl Thread {
         assert_eq!(libc::pthread_attr_setstacksize(&mut attr, stack_size), 0);
 
         let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
+        // Note: if the thread creation fails and this assert fails, then p will
+        // be leaked. However, an alternative design could cause double-free
+        // which is clearly worse.
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })

src/libstd/sys/hermit/thread.rs

@@ -61,7 +61,7 @@ impl Thread {
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::new(io::ErrorKind::Other, "Unable to create thread!"))
         } else {
             Ok(Thread { tid: tid })

src/libstd/sys/unix/thread.rs

@@ -64,12 +64,15 @@ impl Thread {
         };
 
         let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
+        // Note: if the thread creation fails and this assert fails, then p will
+        // be leaked. However, an alternative design could cause double-free
+        // which is clearly worse.
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })

src/libstd/sys/vxworks/thread.rs

@@ -52,12 +52,15 @@ impl Thread {
         };
 
         let ret = libc::pthread_create(&mut native, &attr, thread_start, p as *mut _);
+        // Note: if the thread creation fails and this assert fails, then p will
+        // be leaked. However, an alternative design could cause double-free
+        // which is clearly worse.
         assert_eq!(libc::pthread_attr_destroy(&mut attr), 0);
 
         return if ret != 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::from_raw_os_error(ret))
         } else {
             Ok(Thread { id: native })

src/libstd/sys/windows/thread.rs

@@ -41,7 +41,7 @@ impl Thread {
         return if ret as usize == 0 {
             // The thread failed to start and as a result p was not consumed. Therefore, it is
             // safe to reconstruct the box so that it gets deallocated.
-            let _ = Box::from_raw(p);
+            drop(Box::from_raw(p));
             Err(io::Error::last_os_error())
         } else {
             Ok(Thread { handle: Handle::new(ret) })