From de597fca40d129435c53a69c6662d7bfac29771d Mon Sep 17 00:00:00 2001
From: Mara Bos <m-ou.se@m-ou.se>
Date: Sun, 27 Sep 2020 15:35:35 +0200
Subject: [PATCH] Optimize set_{panic,print}(None).

---
 library/std/src/io/stdio.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/library/std/src/io/stdio.rs b/library/std/src/io/stdio.rs
index df83c329829..36b49401591 100644
--- a/library/std/src/io/stdio.rs
+++ b/library/std/src/io/stdio.rs
@@ -905,6 +905,10 @@ impl fmt::Debug for StderrLock<'_> {
 #[doc(hidden)]
 pub fn set_panic(sink: Option<Box<dyn Write + Send>>) -> Option<Box<dyn Write + Send>> {
     use crate::mem;
+    if sink.is_none() && !LOCAL_STREAMS.load(Ordering::Relaxed) {
+        // LOCAL_STDERR is definitely None since LOCAL_STREAMS is false.
+        return None;
+    }
     let s = LOCAL_STDERR.with(move |slot| mem::replace(&mut *slot.borrow_mut(), sink)).and_then(
         |mut s| {
             let _ = s.flush();
@@ -932,6 +936,10 @@ pub fn set_panic(sink: Option<Box<dyn Write + Send>>) -> Option<Box<dyn Write +
 #[doc(hidden)]
 pub fn set_print(sink: Option<Box<dyn Write + Send>>) -> Option<Box<dyn Write + Send>> {
     use crate::mem;
+    if sink.is_none() && !LOCAL_STREAMS.load(Ordering::Relaxed) {
+        // LOCAL_STDOUT is definitely None since LOCAL_STREAMS is false.
+        return None;
+    }
     let s = LOCAL_STDOUT.with(move |slot| mem::replace(&mut *slot.borrow_mut(), sink)).and_then(
         |mut s| {
             let _ = s.flush();