From 4b0caec04bb1dd73373b0660841c14b72507bede Mon Sep 17 00:00:00 2001
From: Mahdi Ali-Raihan <asder8215@gmail.com>
Date: Sat, 28 Mar 2026 19:19:10 -0400
Subject: [PATCH 1/2] Panic/return false on overflow in no_threads
 read/try_read impl

---
 library/std/src/sys/sync/rwlock/no_threads.rs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/library/std/src/sys/sync/rwlock/no_threads.rs b/library/std/src/sys/sync/rwlock/no_threads.rs
index 6b919bde80bb..81202ba8a3ae 100644
--- a/library/std/src/sys/sync/rwlock/no_threads.rs
+++ b/library/std/src/sys/sync/rwlock/no_threads.rs
@@ -17,6 +17,10 @@ pub const fn new() -> RwLock {
     #[inline]
     pub fn read(&self) {
         let m = self.mode.get();
+
+        // Check for overflow.
+        assert!(m == isize::MAX, "too many active read locks on RwLock");
+
         if m >= 0 {
             self.mode.set(m + 1);
         } else {
@@ -28,6 +32,9 @@ pub fn read(&self) {
     pub fn try_read(&self) -> bool {
         let m = self.mode.get();
         if m >= 0 {
+            if m == isize::MAX {
+                return false;
+            }
             self.mode.set(m + 1);
             true
         } else {

From 1d7d435d08c59f917b3fc266a2844fe002cfa7d7 Mon Sep 17 00:00:00 2001
From: Mahdi Ali-Raihan <asder8215@gmail.com>
Date: Sun, 29 Mar 2026 20:06:25 -0400
Subject: [PATCH 2/2] Panic on unlocking a non-read locked RwLock + provided
 better error msgs

---
 library/std/src/sys/sync/rwlock/no_threads.rs | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/library/std/src/sys/sync/rwlock/no_threads.rs b/library/std/src/sys/sync/rwlock/no_threads.rs
index 81202ba8a3ae..6de5577504c4 100644
--- a/library/std/src/sys/sync/rwlock/no_threads.rs
+++ b/library/std/src/sys/sync/rwlock/no_threads.rs
@@ -17,12 +17,8 @@ pub const fn new() -> RwLock {
     #[inline]
     pub fn read(&self) {
         let m = self.mode.get();
-
-        // Check for overflow.
-        assert!(m == isize::MAX, "too many active read locks on RwLock");
-
         if m >= 0 {
-            self.mode.set(m + 1);
+            self.mode.set(m.checked_add(1).expect("rwlock overflowed read locks"));
         } else {
             rtabort!("rwlock locked for writing");
         }
@@ -63,16 +59,19 @@ pub fn try_write(&self) -> bool {
 
     #[inline]
     pub unsafe fn read_unlock(&self) {
-        self.mode.set(self.mode.get() - 1);
+        assert!(
+            self.mode.replace(self.mode.get() - 1) > 0,
+            "rwlock has not been locked for reading"
+        );
     }
 
     #[inline]
     pub unsafe fn write_unlock(&self) {
-        assert_eq!(self.mode.replace(0), -1);
+        assert_eq!(self.mode.replace(0), -1, "rwlock has not been locked for writing");
     }
 
     #[inline]
     pub unsafe fn downgrade(&self) {
-        assert_eq!(self.mode.replace(1), -1);
+        assert_eq!(self.mode.replace(1), -1, "rwlock has not been locked for writing");
     }
 }