Add StructuralSort skeleton

This is modeling Compare after Eq/NotEq, which I'm not entirely sure
makes sense. Do we want to implement Sort for arbitrary types like
records and lists? I'm going to get folks' thoughts on Zulip.

crates/compiler/builtins/roc/List.roc

@@ -1329,3 +1329,7 @@ iterBackwardsHelp = \list, state, f, prevIndex ->
 
 Sort implements
     compare : a, a -> [LessThan, Equal, GreaterThan] where a implements Sort
+
+# INTERNAL COMPILER USE ONLY: used to lower calls to `compare` to structural
+# compare via the `Sort` low-level for derived types.
+structuralCompare : a, a -> [LessThan, Equal, GreaterThan]

crates/compiler/can/src/builtins.rs

@@ -216,6 +216,7 @@ map_symbol_to_lowlevel_and_arity! {
     And; BOOL_AND; 2,
     Or; BOOL_OR; 2,
     Not; BOOL_NOT; 1,
+    Compare; LIST_STRUCTURAL_COMPARE; 2,
     BoxExpr; BOX_BOX_FUNCTION; 1,
     UnboxExpr; BOX_UNBOX; 1,
     Unreachable; LIST_UNREACHABLE; 1,

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

@@ -1269,6 +1269,9 @@ pub(crate) fn run_low_level<'a, 'ctx>(
             let bool_val = env.builder.new_build_not(arg.into_int_value(), "bool_not");
             BasicValueEnum::IntValue(bool_val)
         }
+        Compare => {
+            panic!("TODO: implement this")
+        }
         Hash => {
             unimplemented!()
         }

crates/compiler/gen_wasm/src/low_level.rs

@@ -2087,6 +2087,10 @@ impl<'a> LowLevelCall<'a> {
 
             Eq | NotEq => self.eq_or_neq(backend),
 
+            Compare => {
+                panic!("TODO: implement this")
+            }
+
             BoxExpr | UnboxExpr => {
                 unreachable!("The {:?} operation is turned into mono Expr", self.lowlevel)
             }

crates/compiler/module/src/low_level.rs

@@ -113,6 +113,7 @@ pub enum LowLevel {
     And,
     Or,
     Not,
+    Compare,
     Hash,
     PtrCast,
     PtrStore,
@@ -352,6 +353,7 @@ map_symbol_to_lowlevel! {
     And <= BOOL_AND;
     Or <= BOOL_OR;
     Not <= BOOL_NOT;
+    Compare <= LIST_STRUCTURAL_COMPARE;
     Unreachable <= LIST_UNREACHABLE;
     DictPseudoSeed <= DICT_PSEUDO_SEED;
 }

crates/compiler/module/src/symbol.rs

@@ -112,7 +112,7 @@ impl Symbol {
             self,
             // The `structuralEq` call used deriving structural equality, which will wrap the `Eq`
             // low-level implementation.
-            &Self::BOOL_STRUCTURAL_EQ
+            &Self::BOOL_STRUCTURAL_EQ | &Self::LIST_STRUCTURAL_COMPARE
         )
     }
 
@@ -1743,6 +1743,7 @@ define_builtins! {
     16 SORT: "Sort" => {
         0 LIST_SORT: "Sort"
         1 LIST_COMPARE: "compare"
+        unexposed 2 LIST_STRUCTURAL_COMPARE: "structuralCompare"
     }
 
     num_modules: 17 // Keep this count up to date by hand! (TODO: see the mut_map! macro for how we could determine this count correctly in the macro)

crates/compiler/mono/src/drop_specialization.rs

@@ -1558,7 +1558,7 @@ fn low_level_no_rc(lowlevel: &LowLevel) -> RC {
         | ListReleaseExcessCapacity
         | StrReleaseExcessCapacity => RC::Rc,
 
-        Eq | NotEq => RC::NoRc,
+        Eq | NotEq | Compare => RC::NoRc,
 
         And | Or | NumAdd | NumAddWrap | NumAddChecked | NumAddSaturated | NumSub | NumSubWrap
         | NumSubChecked | NumSubSaturated | NumMul | NumMulWrap | NumMulSaturated

crates/compiler/mono/src/inc_dec.rs

@@ -1310,7 +1310,7 @@ pub(crate) fn lowlevel_borrow_signature(op: LowLevel) -> &'static [Ownership] {
         ListReleaseExcessCapacity => &[OWNED],
         StrReleaseExcessCapacity => &[OWNED],
 
-        Eq | NotEq => &[BORROWED, BORROWED],
+        Eq | NotEq | Compare => &[BORROWED, BORROWED],
 
         And | Or | NumAdd | NumAddWrap | NumAddChecked | NumAddSaturated | NumSub | NumSubWrap
         | NumSubChecked | NumSubSaturated | NumMul | NumMulWrap | NumMulSaturated