From 82b02847060aa01a42c559a4db3849563a0237cb Mon Sep 17 00:00:00 2001
From: Spotandjake <spotandjake@hotmail.com>
Date: Fri, 3 Apr 2026 00:40:17 -0400
Subject: [PATCH] feat(compiler): Stub panic with unreachable to prevent bad
 codegen

This pr stubs panic with unreachable when we are compiling in `@runtimeMode`.

The reason for this change is that when working in `@runtimeMode` we don't have the panic module available however if you want to use something like `arr[x]` you are out of luck as it has a dependency on panic, this results in bad wasm codegen.

My solution to this is we just fail with `unreachable` the downside is we don't get a nice error but if someone is working in the runtime I feel like thats a reasonable sacrifice, to be able to write the higher level code in the first place.

Closes: #2379
fix: skip test in js mode
---
 compiler/src/codegen/compcore.re | 28 ++++++++++++++++------------
 compiler/test/suites/runtime.re  | 12 ++++++++++++
 2 files changed, 28 insertions(+), 12 deletions(-)

diff --git a/compiler/src/codegen/compcore.re b/compiler/src/codegen/compcore.re
index d2cef90bca..1fe1f2ed4c 100644
--- a/compiler/src/codegen/compcore.re
+++ b/compiler/src/codegen/compcore.re
@@ -149,20 +149,24 @@ let get_metadata_ptr = wasm_mod =>
 let get_grain_imported_name = (mod_, name) => Ident.unique_name(name);
 
 let call_panic_handler = (wasm_mod, env, args) => {
-  let args = [
-    Expression.Global_get.make(
+  switch (env.compilation_mode) {
+  | Runtime => Expression.Unreachable.make(wasm_mod)
+  | _ =>
+    let args = [
+      Expression.Global_get.make(
+        wasm_mod,
+        resolve_global(~env, panic_with_exception_name),
+        ref_any(),
+      ),
+      ...args,
+    ];
+    Expression.Call.make(
       wasm_mod,
-      resolve_global(~env, panic_with_exception_name),
+      resolve_func(~env, panic_with_exception_name),
+      args,
       ref_any(),
-    ),
-    ...args,
-  ];
-  Expression.Call.make(
-    wasm_mod,
-    resolve_func(~env, panic_with_exception_name),
-    args,
-    ref_any(),
-  );
+    );
+  };
 };
 
 let call_equal = (wasm_mod, env, args) =>
diff --git a/compiler/test/suites/runtime.re b/compiler/test/suites/runtime.re
index 35cc3ec8ce..ed7c060c0e 100644
--- a/compiler/test/suites/runtime.re
+++ b/compiler/test/suites/runtime.re
@@ -5,7 +5,19 @@ describe("runtime", ({test, testSkip}) => {
   let test_or_skip =
     Sys.backend_type == Other("js_of_ocaml") ? testSkip : test;
 
+  let assertRunError = makeErrorRunner(test_or_skip);
   let assertRuntime = makeRuntimeRunner(test_or_skip);
+
+  assertRunError(
+    ~config_fn=() => {Grain_utils.Config.compilation_mode := Runtime},
+    "runtime_mode_panics",
+    {|
+      let arr = [> 1, 2, 3]
+      provide let x = arr[10]
+    |},
+    "RuntimeError: unreachable",
+  );
+
   assertRuntime("numbers.test");
 
   assertRuntime("unsafe/wasmf32.test");