diff --git a/library/std/src/sys/sync/rwlock/no_threads.rs b/library/std/src/sys/sync/rwlock/no_threads.rs
index 6b919bde80bb..6de5577504c4 100644
--- a/library/std/src/sys/sync/rwlock/no_threads.rs
+++ b/library/std/src/sys/sync/rwlock/no_threads.rs
@@ -18,7 +18,7 @@ pub const fn new() -> RwLock {
     pub fn read(&self) {
         let m = self.mode.get();
         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");
         }
@@ -28,6 +28,9 @@ 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 {
@@ -56,16 +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");
     }
 }