style: Avoid linear search to check for custom property presence

The test-case in the next patch from 10s to a couple hundred
milliseconds on my machine. No behavior change other than that.

Differential Revision: https://phabricator.services.mozilla.com/D175612

components/to_shmem/lib.rs

@@ -17,7 +17,6 @@ extern crate thin_vec;
 use std::alloc::Layout;
 #[cfg(debug_assertions)]
 use std::any::TypeId;
-#[cfg(debug_assertions)]
 use std::collections::HashSet;
 use std::ffi::CString;
 use std::marker::PhantomData;
@@ -426,6 +425,21 @@ impl<T: ToShmem> ToShmem for Option<T> {
     }
 }
 
+impl<T: ToShmem, S> ToShmem for HashSet<T, S>
+where
+    Self: Default,
+{
+    fn to_shmem(&self, _builder: &mut SharedMemoryBuilder) -> Result<Self> {
+        if !self.is_empty() {
+            return Err(format!(
+                "ToShmem failed for HashSet: We only support empty sets \
+                 (we don't expect custom properties in UA sheets, they're observable by content)",
+            ))
+        }
+        Ok(ManuallyDrop::new(Self::default()))
+    }
+}
+
 impl<T: 'static + ToShmem> ToShmem for Arc<T> {
     fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> Result<Self> {
         // Assert that we don't encounter any shared references to values we