5 files changed, 41 insertions, 0 deletions

diff --git a/embassy-sync/Cargo.toml b/embassy-sync/Cargo.toml
index 1e2ea8ea1..9e5c39f5e 100644
--- a/embassy-sync/Cargo.toml
+++ b/embassy-sync/Cargo.toml
@@ -43,3 +43,4 @@ futures-util = { version = "0.3.17", features = [ "channel", "sink" ] }
 # Enable critical-section implementation for std, for tests
 critical-section = { version = "1.1", features = ["std"] }
 static_cell = { version = "2" }
+trybuild = "1.0.105"
diff --git a/embassy-sync/tests/ui.rs b/embassy-sync/tests/ui.rs
new file mode 100644
index 000000000..9ef0d4c67
--- /dev/null
+++ b/embassy-sync/tests/ui.rs
@@ -0,0 +1,13 @@
+// #[cfg(not(miri))]
+#[test]
+fn ui() {
+    let t = trybuild::TestCases::new();
+
+    // These test cases should fail to compile since OnceLock and LazyLock should not unconditionally implement sync
+    // for all types. These tests are regression tests against the following issues:
+    // * https://github.com/embassy-rs/embassy/issues/4307
+    // * https://github.com/embassy-rs/embassy/issues/3904
+    t.compile_fail("tests/ui/sync_impl/lazy_lock_function.rs");
+    t.compile_fail("tests/ui/sync_impl/lazy_lock_type.rs");
+    t.compile_fail("tests/ui/sync_impl/once_lock.rs");
+}
diff --git a/embassy-sync/tests/ui/sync_impl/lazy_lock_function.rs b/embassy-sync/tests/ui/sync_impl/lazy_lock_function.rs
new file mode 100644
index 000000000..c6e6f7e64
--- /dev/null
+++ b/embassy-sync/tests/ui/sync_impl/lazy_lock_function.rs
@@ -0,0 +1,15 @@
+use embassy_sync::lazy_lock::LazyLock;
+
+fn main() {
+    let x = 128u8;
+    let x_ptr: *const u8 = core::ptr::addr_of!(x);
+
+    let closure_capturing_non_sync_variable = || {
+        unsafe {
+            core::ptr::read(x_ptr)
+        }
+    };
+
+    // This should fail to compile because the closure captures a non-Sync variable: x_ptr.
+    let _lazy_u8: LazyLock<u8, _> = LazyLock::new(closure_capturing_non_sync_variable);
+}
diff --git a/embassy-sync/tests/ui/sync_impl/lazy_lock_type.rs b/embassy-sync/tests/ui/sync_impl/lazy_lock_type.rs
new file mode 100644
index 000000000..4e1383143
--- /dev/null
+++ b/embassy-sync/tests/ui/sync_impl/lazy_lock_type.rs
@@ -0,0 +1,6 @@
+use embassy_sync::lazy_lock::LazyLock;
+
+// *mut u8 is not Sync, so LazyLock should not implement Sync for this type. This should fail to compile.
+static GLOBAL: LazyLock<*mut u8> = LazyLock::new(|| core::ptr::null_mut());
+
+fn main() {}
diff --git a/embassy-sync/tests/ui/sync_impl/once_lock.rs b/embassy-sync/tests/ui/sync_impl/once_lock.rs
new file mode 100644
index 000000000..8f50d583b
--- /dev/null
+++ b/embassy-sync/tests/ui/sync_impl/once_lock.rs
@@ -0,0 +1,6 @@
+use embassy_sync::once_lock::OnceLock;
+
+// *mut u8 is not Sync, so OnceLock should not implement Sync for this type. This should fail to compile.
+static GLOBAL: OnceLock<*mut u8> = OnceLock::new();
+
+fn main() {}