diff --git a/crates/compiler/builtins/bitcode/src/main.zig b/crates/compiler/builtins/bitcode/src/main.zig
index 6f0b59468d..672e1224ec 100644
--- a/crates/compiler/builtins/bitcode/src/main.zig
+++ b/crates/compiler/builtins/bitcode/src/main.zig
@@ -174,6 +174,7 @@ comptime {
     exportUtilsFn(utils.test_panic, "test_panic");
     exportUtilsFn(utils.increfC, "incref");
     exportUtilsFn(utils.decrefC, "decref");
+    exportUtilsFn(utils.isUnique, "is_unique");
     exportUtilsFn(utils.decrefCheckNullC, "decref_check_null");
     exportUtilsFn(utils.allocateWithRefcountC, "allocate_with_refcount");
 
diff --git a/crates/compiler/builtins/bitcode/src/utils.zig b/crates/compiler/builtins/bitcode/src/utils.zig
index 841b143863..79ae4200fa 100644
--- a/crates/compiler/builtins/bitcode/src/utils.zig
+++ b/crates/compiler/builtins/bitcode/src/utils.zig
@@ -231,6 +231,22 @@ inline fn decref_ptr_to_refcount(
     }
 }
 
+pub export fn isUnique(
+    bytes_or_null: ?[*]u8,
+) bool {
+    var bytes = bytes_or_null orelse return true;
+
+    const ptr = @ptrToInt(bytes);
+    const tag_mask: usize = if (@sizeOf(usize) == 8) 0b111 else 0b11;
+    const masked_ptr = ptr & ~tag_mask;
+
+    const isizes: [*]isize = @intToPtr([*]isize, masked_ptr);
+
+    const refcount = (isizes - 1)[0];
+
+    return refcount == REFCOUNT_ONE_ISIZE;
+}
+
 // We follow roughly the [fbvector](https://github.com/facebook/folly/blob/main/folly/docs/FBVector.md) when it comes to growing a RocList.
 // Here is [their growth strategy](https://github.com/facebook/folly/blob/3e0525988fd444201b19b76b390a5927c15cb697/folly/FBVector.h#L1128) for push_back:
 //
diff --git a/crates/compiler/builtins/src/bitcode.rs b/crates/compiler/builtins/src/bitcode.rs
index d96496eae9..5eefcde053 100644
--- a/crates/compiler/builtins/src/bitcode.rs
+++ b/crates/compiler/builtins/src/bitcode.rs
@@ -377,6 +377,7 @@ pub const UTILS_TEST_PANIC: &str = "roc_builtins.utils.test_panic";
 pub const UTILS_ALLOCATE_WITH_REFCOUNT: &str = "roc_builtins.utils.allocate_with_refcount";
 pub const UTILS_INCREF: &str = "roc_builtins.utils.incref";
 pub const UTILS_DECREF: &str = "roc_builtins.utils.decref";
+pub const UTILS_IS_UNIQUE: &str = "roc_builtins.utils.is_unique";
 pub const UTILS_DECREF_CHECK_NULL: &str = "roc_builtins.utils.decref_check_null";
 
 pub const UTILS_EXPECT_FAILED_START_SHARED_BUFFER: &str =
diff --git a/crates/compiler/can/src/builtins.rs b/crates/compiler/can/src/builtins.rs
index aafbff85a7..2dcf24a271 100644
--- a/crates/compiler/can/src/builtins.rs
+++ b/crates/compiler/can/src/builtins.rs
@@ -87,6 +87,7 @@ macro_rules! map_symbol_to_lowlevel_and_arity {
                 LowLevel::PtrCast => unimplemented!(),
                 LowLevel::RefCountInc => unimplemented!(),
                 LowLevel::RefCountDec => unimplemented!(),
+                LowLevel::RefCountIsUnique => unimplemented!(),
 
                 // these are not implemented, not sure why
                 LowLevel::StrFromInt => unimplemented!(),
diff --git a/crates/compiler/gen_llvm/src/llvm/lowlevel.rs b/crates/compiler/gen_llvm/src/llvm/lowlevel.rs
index e7c115e003..d57253de2a 100644
--- a/crates/compiler/gen_llvm/src/llvm/lowlevel.rs
+++ b/crates/compiler/gen_llvm/src/llvm/lowlevel.rs
@@ -1251,6 +1251,11 @@ pub(crate) fn run_low_level<'a, 'ctx, 'env>(
             unreachable!("Not used in LLVM backend: {:?}", op);
         }
 
+        RefCountIsUnique => {
+            arguments!(input);
+            call_bitcode_fn(env, &[input], &bitcode::UTILS_IS_UNIQUE)
+        }
+
         Unreachable => match RocReturn::from_layout(env, layout_interner, layout) {
             RocReturn::Return => {
                 let basic_type = basic_type_from_layout(env, layout_interner, layout);
diff --git a/crates/compiler/gen_wasm/src/low_level.rs b/crates/compiler/gen_wasm/src/low_level.rs
index 6cfd49bea3..6e248275ee 100644
--- a/crates/compiler/gen_wasm/src/low_level.rs
+++ b/crates/compiler/gen_wasm/src/low_level.rs
@@ -1949,6 +1949,7 @@ impl<'a> LowLevelCall<'a> {
             }
             RefCountInc => self.load_args_and_call_zig(backend, bitcode::UTILS_INCREF),
             RefCountDec => self.load_args_and_call_zig(backend, bitcode::UTILS_DECREF),
+            RefCountIsUnique => self.load_args_and_call_zig(backend, bitcode::UTILS_IS_UNIQUE),
 
             PtrCast => {
                 let code_builder = &mut backend.code_builder;
diff --git a/crates/compiler/module/src/low_level.rs b/crates/compiler/module/src/low_level.rs
index 7c72a0ec3d..4f9f8dab9a 100644
--- a/crates/compiler/module/src/low_level.rs
+++ b/crates/compiler/module/src/low_level.rs
@@ -117,6 +117,7 @@ pub enum LowLevel {
     PtrCast,
     RefCountInc,
     RefCountDec,
+    RefCountIsUnique,
     BoxExpr,
     UnboxExpr,
     Unreachable,
@@ -221,6 +222,7 @@ macro_rules! map_symbol_to_lowlevel {
                 LowLevel::PtrCast => unimplemented!(),
                 LowLevel::RefCountInc => unimplemented!(),
                 LowLevel::RefCountDec => unimplemented!(),
+                LowLevel::RefCountIsUnique => unimplemented!(),
 
                 // these are not implemented, not sure why
                 LowLevel::StrFromInt => unimplemented!(),
diff --git a/crates/compiler/mono/src/borrow.rs b/crates/compiler/mono/src/borrow.rs
index 85861f859f..229d7e360c 100644
--- a/crates/compiler/mono/src/borrow.rs
+++ b/crates/compiler/mono/src/borrow.rs
@@ -1031,7 +1031,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[Ownership] {
             unreachable!("These lowlevel operations are turned into mono Expr's")
         }
 
-        PtrCast | RefCountInc | RefCountDec => {
+        PtrCast | RefCountInc | RefCountDec | RefCountIsUnique => {
             unreachable!("Only inserted *after* borrow checking: {:?}", op);
         }
     }