diff --git a/library/std/src/sys/wasm/alloc.rs b/library/std/src/sys/wasm/alloc.rs
index 32b8b5bdaea..e12af19718a 100644
--- a/library/std/src/sys/wasm/alloc.rs
+++ b/library/std/src/sys/wasm/alloc.rs
@@ -25,25 +25,25 @@ unsafe impl GlobalAlloc for System {
     #[inline]
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
         let _lock = lock::lock();
-        DLMALLOC.malloc(layout.size(), layout.align())
+        unsafe { DLMALLOC.malloc(layout.size(), layout.align()) }
     }
 
     #[inline]
     unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
         let _lock = lock::lock();
-        DLMALLOC.calloc(layout.size(), layout.align())
+        unsafe { DLMALLOC.calloc(layout.size(), layout.align()) }
     }
 
     #[inline]
     unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
         let _lock = lock::lock();
-        DLMALLOC.free(ptr, layout.size(), layout.align())
+        unsafe { DLMALLOC.free(ptr, layout.size(), layout.align()) }
     }
 
     #[inline]
     unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
         let _lock = lock::lock();
-        DLMALLOC.realloc(ptr, layout.size(), layout.align(), new_size)
+        unsafe { DLMALLOC.realloc(ptr, layout.size(), layout.align(), new_size) }
     }
 }
 
diff --git a/library/std/src/sys/wasm/condvar_atomics.rs b/library/std/src/sys/wasm/condvar_atomics.rs
index a96bb18e6ef..b9133e9fb7d 100644
--- a/library/std/src/sys/wasm/condvar_atomics.rs
+++ b/library/std/src/sys/wasm/condvar_atomics.rs
@@ -44,13 +44,18 @@ impl Condvar {
 
     pub unsafe fn notify_one(&self) {
         self.cnt.fetch_add(1, SeqCst);
-        wasm32::memory_atomic_notify(self.ptr(), 1);
+        // SAFETY: ptr() is always valid
+        unsafe {
+            wasm32::memory_atomic_notify(self.ptr(), 1);
+        }
     }
 
     #[inline]
     pub unsafe fn notify_all(&self) {
-        self.cnt.fetch_add(1, SeqCst);
-        wasm32::memory_atomic_notify(self.ptr(), u32::MAX); // -1 == "wake everyone"
+        unsafe {
+            self.cnt.fetch_add(1, SeqCst);
+            wasm32::memory_atomic_notify(self.ptr(), u32::MAX); // -1 == "wake everyone"
+        }
     }
 
     pub unsafe fn wait(&self, mutex: &Mutex) {
diff --git a/library/std/src/sys/wasm/mod.rs b/library/std/src/sys/wasm/mod.rs
index 11c6896f050..82683c0f624 100644
--- a/library/std/src/sys/wasm/mod.rs
+++ b/library/std/src/sys/wasm/mod.rs
@@ -14,6 +14,8 @@
 //! compiling for wasm. That way it's a compile time error for something that's
 //! guaranteed to be a runtime error!
 
+#![deny(unsafe_op_in_unsafe_fn)]
+
 pub mod alloc;
 pub mod args;
 #[path = "../unsupported/cmath.rs"]
diff --git a/library/std/src/sys/wasm/mutex_atomics.rs b/library/std/src/sys/wasm/mutex_atomics.rs
index 2970fcf806c..11d7f4c389d 100644
--- a/library/std/src/sys/wasm/mutex_atomics.rs
+++ b/library/std/src/sys/wasm/mutex_atomics.rs
@@ -28,11 +28,14 @@ impl Mutex {
 
     pub unsafe fn lock(&self) {
         while !self.try_lock() {
-            let val = wasm32::memory_atomic_wait32(
-                self.ptr(),
-                1,  // we expect our mutex is locked
-                -1, // wait infinitely
-            );
+            // SAFETY: the caller must uphold the safety contract for `memory_atomic_wait32`.
+            let val = unsafe {
+                wasm32::memory_atomic_wait32(
+                    self.ptr(),
+                    1,  // we expect our mutex is locked
+                    -1, // wait infinitely
+                )
+            };
             // we should have either woke up (0) or got a not-equal due to a
             // race (1). We should never time out (2)
             debug_assert!(val == 0 || val == 1);
@@ -47,7 +50,7 @@ impl Mutex {
 
     #[inline]
     pub unsafe fn try_lock(&self) -> bool {
-        self.locked.compare_exchange(0, 1, SeqCst, SeqCst).is_ok()
+        unsafe { self.locked.compare_exchange(0, 1, SeqCst, SeqCst).is_ok() }
     }
 
     #[inline]
@@ -83,7 +86,7 @@ unsafe impl Sync for ReentrantMutex {}
 
 impl ReentrantMutex {
     pub const unsafe fn uninitialized() -> ReentrantMutex {
-        ReentrantMutex { owner: AtomicU32::new(0), recursions: UnsafeCell::new(0) }
+        unsafe { ReentrantMutex { owner: AtomicU32::new(0), recursions: UnsafeCell::new(0) } }
     }
 
     pub unsafe fn init(&self) {
@@ -93,19 +96,20 @@ impl ReentrantMutex {
     pub unsafe fn lock(&self) {
         let me = thread::my_id();
         while let Err(owner) = self._try_lock(me) {
-            let val = wasm32::memory_atomic_wait32(self.ptr(), owner as i32, -1);
+            // SAFETY: the caller must gurantee that `self.ptr()` and `owner` are valid i32.
+            let val = unsafe { wasm32::memory_atomic_wait32(self.ptr(), owner as i32, -1) };
             debug_assert!(val == 0 || val == 1);
         }
     }
 
     #[inline]
     pub unsafe fn try_lock(&self) -> bool {
-        self._try_lock(thread::my_id()).is_ok()
+        unsafe { self._try_lock(thread::my_id()).is_ok() }
     }
 
     #[inline]
     unsafe fn _try_lock(&self, id: u32) -> Result<(), u32> {
-        let id = id.checked_add(1).unwrap(); // make sure `id` isn't 0
+        let id = id.checked_add(1).unwrap();
         match self.owner.compare_exchange(0, id, SeqCst, SeqCst) {
             // we transitioned from unlocked to locked
             Ok(_) => {
@@ -132,7 +136,10 @@ impl ReentrantMutex {
         match *self.recursions.get() {
             0 => {
                 self.owner.swap(0, SeqCst);
-                wasm32::memory_atomic_notify(self.ptr() as *mut i32, 1); // wake up one waiter, if any
+                // SAFETY: the caller must gurantee that `self.ptr()` is valid i32.
+                unsafe {
+                    wasm32::atomic_notify(self.ptr() as *mut i32, 1);
+                } // wake up one waiter, if any
             }
             ref mut n => *n -= 1,
         }