diff --git a/Earthfile b/Earthfile
index d0ab4f1977..8a1be4e1c6 100644
--- a/Earthfile
+++ b/Earthfile
@@ -89,6 +89,9 @@ test-rust:
     # gen-wasm has some multithreading problems to do with the wasmer runtime. Run it single-threaded as a separate job
     RUN --mount=type=cache,target=$SCCACHE_DIR \
         cargo test --locked --release --package test_gen --no-default-features --features gen-wasm -- --test-threads=1 && sccache --show-stats
+    # run `roc test` on Str builtins
+    RUN --mount=type=cache,target=$SCCACHE_DIR \
+        cargo run --release -- test crates/compiler/builtins/roc/Str.roc && sccache --show-stats
     # repl_test: build the compiler for wasm target, then run the tests on native target
     RUN --mount=type=cache,target=$SCCACHE_DIR \
         crates/repl_test/test_wasm.sh && sccache --show-stats
diff --git a/crates/compiler/builtins/roc/Str.roc b/crates/compiler/builtins/roc/Str.roc
index 166f5f9a60..43cd4bec8a 100644
--- a/crates/compiler/builtins/roc/Str.roc
+++ b/crates/compiler/builtins/roc/Str.roc
@@ -330,6 +330,15 @@ splitLast = \haystack, needle ->
         None ->
             Err NotFound
 
+# splitLast when needle isn't in haystack
+expect Str.splitLast "foo" "z" == Err NotFound
+
+# splitLast when haystack ends with needle repeated
+expect Str.splitLast "foo" "o" == Ok { before: "fo", after: "" }
+
+# splitLast with multi-byte needle
+expect Str.splitLast "hullabaloo" "ab" == Ok { before: "hull", after: "aloo" }
+
 lastMatch : Str, Str -> [Some Nat, None]
 lastMatch = \haystack, needle ->
     haystackLength = Str.countUtf8Bytes haystack
diff --git a/crates/compiler/load_internal/src/file.rs b/crates/compiler/load_internal/src/file.rs
index 0151e3a130..444343f3a8 100644
--- a/crates/compiler/load_internal/src/file.rs
+++ b/crates/compiler/load_internal/src/file.rs
@@ -445,6 +445,7 @@ fn start_phase<'a>(
 
                 BuildTask::BuildPendingSpecializations {
                     layout_cache,
+                    execution_mode: state.exec_mode,
                     module_id,
                     module_timing,
                     solved_subs,
@@ -1090,6 +1091,7 @@ enum BuildTask<'a> {
     },
     BuildPendingSpecializations {
         module_timing: ModuleTiming,
+        execution_mode: ExecutionMode,
         layout_cache: LayoutCache<'a>,
         solved_subs: Solved<Subs>,
         imported_module_thunks: &'a [Symbol],
@@ -4736,6 +4738,7 @@ fn make_specializations<'a>(
 #[allow(clippy::too_many_arguments)]
 fn build_pending_specializations<'a>(
     arena: &'a Bump,
+    execution_mode: ExecutionMode,
     solved_subs: Solved<Subs>,
     imported_module_thunks: &'a [Symbol],
     home: ModuleId,
@@ -4993,6 +4996,12 @@ fn build_pending_specializations<'a>(
                 // the declarations of this group will be treaded individually by later iterations
             }
             Expectation => {
+                // skip expectations if we're not going to run them
+                match execution_mode {
+                    ExecutionMode::Test => { /* fall through */ }
+                    ExecutionMode::Check | ExecutionMode::Executable => continue,
+                }
+
                 // mark this symbol as a top-level thunk before any other work on the procs
                 module_thunks.push(symbol);
 
@@ -5265,6 +5274,7 @@ fn run_task<'a>(
         )),
         BuildPendingSpecializations {
             module_id,
+            execution_mode,
             ident_ids,
             decls,
             module_timing,
@@ -5277,6 +5287,7 @@ fn run_task<'a>(
             derived_module,
         } => Ok(build_pending_specializations(
             arena,
+            execution_mode,
             solved_subs,
             imported_module_thunks,
             module_id,
diff --git a/crates/repl_expect/src/lib.rs b/crates/repl_expect/src/lib.rs
index 78a044d4c2..5240bb8a1f 100644
--- a/crates/repl_expect/src/lib.rs
+++ b/crates/repl_expect/src/lib.rs
@@ -108,7 +108,7 @@ mod test {
             target_info,
             render: RenderTarget::ColorTerminal,
             threading: Threading::Single,
-            exec_mode: ExecutionMode::Executable,
+            exec_mode: ExecutionMode::Test,
         };
         let loaded = roc_load::load_and_monomorphize_from_str(
             arena,