List.unreachable

crates/compiler/builtins/roc/List.roc

@@ -790,3 +790,5 @@ iterHelp = \list, state, f, index, length ->
                 Break b
     else
         Continue state
+
+unreachable : [] -> a

crates/compiler/can/src/builtins.rs

@@ -100,6 +100,7 @@ pub fn builtin_defs_map(symbol: Symbol, var_store: &mut VarStore) -> Option<Def>
         STR_TO_I16 => str_to_num,
         STR_TO_U8 => str_to_num,
         STR_TO_I8 => str_to_num,
+        LIST_UNREACHABLE => roc_unreachable,
         LIST_LEN => list_len,
         LIST_WITH_CAPACITY => list_with_capacity,
         LIST_GET_UNSAFE => list_get_unsafe,
@@ -2475,6 +2476,11 @@ fn list_walk_until(symbol: Symbol, var_store: &mut VarStore) -> Def {
     lowlevel_3(symbol, LowLevel::ListWalkUntil, var_store)
 }
 
+/// List.unreachable : [] -> a
+fn roc_unreachable(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    lowlevel_1(symbol, LowLevel::Unreachable, var_store)
+}
+
 /// List.map : List before, (before -> after) -> List after
 fn list_map(symbol: Symbol, var_store: &mut VarStore) -> Def {
     lowlevel_2(symbol, LowLevel::ListMap, var_store)

crates/compiler/gen_llvm/src/llvm/build.rs

@@ -6068,6 +6068,20 @@ fn run_low_level<'a, 'ctx, 'env>(
         PtrCast | RefCountInc | RefCountDec => {
             unreachable!("Not used in LLVM backend: {:?}", op);
         }
+
+        Unreachable => match RocReturn::from_layout(env, layout) {
+            RocReturn::Return => {
+                let basic_type = basic_type_from_layout(env, layout);
+                basic_type.const_zero()
+            }
+            RocReturn::ByPointer => {
+                let basic_type = basic_type_from_layout(env, layout);
+                let ptr = env.builder.build_alloca(basic_type, "unreachable_alloca");
+                env.builder.build_store(ptr, basic_type.const_zero());
+
+                ptr.into()
+            }
+        },
     }
 }
 

crates/compiler/gen_wasm/src/low_level.rs

@@ -1631,6 +1631,17 @@ impl<'a> LowLevelCall<'a> {
             BoxExpr | UnboxExpr => {
                 unreachable!("The {:?} operation is turned into mono Expr", self.lowlevel)
             }
+
+            Unreachable => match self.ret_storage {
+                StoredValue::VirtualMachineStack { value_type, .. }
+                | StoredValue::Local { value_type, .. } => match value_type {
+                    ValueType::I32 => backend.code_builder.i32_const(0),
+                    ValueType::I64 => backend.code_builder.i64_const(0),
+                    ValueType::F32 => backend.code_builder.f32_const(0.0),
+                    ValueType::F64 => backend.code_builder.f64_const(0.0),
+                },
+                StoredValue::StackMemory { .. } => { /* do nothing */ }
+            },
         }
     }
 

crates/compiler/module/src/low_level.rs

@@ -119,6 +119,7 @@ pub enum LowLevel {
     RefCountDec,
     BoxExpr,
     UnboxExpr,
+    Unreachable,
 }
 
 macro_rules! higher_order {

crates/compiler/module/src/symbol.rs

@@ -1255,6 +1255,7 @@ define_builtins! {
         61 LIST_REPLACE_UNSAFE: "replaceUnsafe"
         62 LIST_WITH_CAPACITY: "withCapacity"
         63 LIST_ITERATE: "iterate"
+        64 LIST_UNREACHABLE: "unreachable"
     }
     6 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" // the Result.Result type alias

crates/compiler/mono/src/borrow.rs

@@ -890,6 +890,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
     // - arguments that we may want to update destructively must be Owned
     // - other refcounted arguments are Borrowed
     match op {
+        Unreachable => arena.alloc_slice_copy(&[irrelevant]),
         ListLen | StrIsEmpty | StrCountGraphemes => arena.alloc_slice_copy(&[borrowed]),
         ListWithCapacity => arena.alloc_slice_copy(&[irrelevant]),
         ListReplaceUnsafe => arena.alloc_slice_copy(&[owned, irrelevant, irrelevant]),