Implement List.find

`List.find : List elem, (elem -> Bool) -> Result elem [ NotFound ]*`
behaves as follows:

```
>>> List.find [1, 2, 3] (\n -> n > 2)
Ok 2
>>> List.find [1, 2, 3] (\n -> n > 4)
Err NotFound
```

We implement this as builtin in two phases. First, we call out to a
pure-llvm-lowlevel `ListFindUnsafe` that returns a record indicating
whether a satisfying element was found, and the value of that element
(the value is all null bytes if the element wasn't found). Then, we lift
that record to a `Result` via a standard construction of the can AST.

Closes #1909

compiler/mono/src/alias_analysis.rs

@@ -1093,6 +1093,42 @@ fn call_spec(
 
                     add_loop(builder, block, state_type, init_state, loop_body)
                 }
+                ListFindUnsafe { xs } => {
+                    let list = env.symbols[xs];
+
+                    // Mark the list as being used by the "find" predicate function.
+                    // It may be the case that all elements in the list are used by the predicate.
+                    // Since `bag_get` assumes items are picked non-deterministically, this is
+                    // (probably?) enough to express that usage.
+                    let bag = builder.add_get_tuple_field(block, list, LIST_BAG_INDEX)?;
+                    let cell = builder.add_get_tuple_field(block, list, LIST_CELL_INDEX)?;
+                    let element = builder.add_bag_get(block, bag)?;
+                    let _bool = call_function!(builder, block, [element]);
+
+                    // ListFindUnsafe returns { value: v, found: Bool=Int1 }
+                    let output_layouts =
+                        vec![arg_layouts[0].clone(), Layout::Builtin(Builtin::Int1)];
+                    let output_layout = Layout::Struct(&output_layouts);
+                    let output_type = layout_spec(builder, &output_layout)?;
+
+                    // We may or may not use the element we got from the list in the output struct,
+                    // depending on whether we found the element to satisfy the "find" predicate.
+                    let found_branch = builder.add_block();
+                    let output_with_element =
+                        builder.add_unknown_with(found_branch, &[element], output_type)?;
+
+                    let not_found_branch = builder.add_block();
+                    let output_without_element =
+                        builder.add_unknown_with(not_found_branch, &[], output_type)?;
+
+                    builder.add_choice(
+                        block,
+                        &[
+                            BlockExpr(found_branch, output_with_element),
+                            BlockExpr(not_found_branch, output_without_element),
+                        ],
+                    )
+                }
             }
         }
     }

compiler/mono/src/borrow.rs

@@ -618,7 +618,8 @@ impl<'a> BorrowInfState<'a> {
                     | ListKeepIf { xs }
                     | ListKeepOks { xs }
                     | ListKeepErrs { xs }
-                    | ListAny { xs } => {
+                    | ListAny { xs }
+                    | ListFindUnsafe { xs } => {
                         // own the list if the function wants to own the element
                         if !function_ps[0].borrow {
                             self.own_var(*xs);
@@ -959,6 +960,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
             arena.alloc_slice_copy(&[owned, owned, function, closure_data])
         }
         ListSortWith => arena.alloc_slice_copy(&[owned, function, closure_data]),
+        ListFindUnsafe => arena.alloc_slice_copy(&[owned, function, closure_data]),
 
         // TODO when we have lists with capacity (if ever)
         // List.append should own its first argument

compiler/mono/src/inc_dec.rs

@@ -531,7 +531,8 @@ impl<'a> Context<'a> {
                     | ListKeepIf { xs }
                     | ListKeepOks { xs }
                     | ListKeepErrs { xs }
-                    | ListAny { xs } => {
+                    | ListAny { xs }
+                    | ListFindUnsafe { xs } => {
                         let borrows = [function_ps[0].borrow, FUNCTION, CLOSURE_DATA];
 
                         let b = self.add_dec_after_lowlevel(arguments, &borrows, b, b_live_vars);

compiler/mono/src/ir.rs

@@ -4164,6 +4164,11 @@ pub fn with_hole<'a>(
 
                     match_on_closure_argument!(ListMap4, [xs, ys, zs, ws])
                 }
+                ListFindUnsafe => {
+                    debug_assert_eq!(arg_symbols.len(), 2);
+                    let xs = arg_symbols[0];
+                    match_on_closure_argument!(ListFindUnsafe, [xs])
+                }
                 _ => {
                     let call = self::Call {
                         call_type: CallType::LowLevel {

compiler/mono/src/low_level.rs

@@ -50,6 +50,9 @@ pub enum HigherOrder {
     ListAny {
         xs: Symbol,
     },
+    ListFindUnsafe {
+        xs: Symbol,
+    },
     DictWalk {
         xs: Symbol,
         state: Symbol,
@@ -71,6 +74,7 @@ impl HigherOrder {
             HigherOrder::ListKeepOks { .. } => 1,
             HigherOrder::ListKeepErrs { .. } => 1,
             HigherOrder::ListSortWith { .. } => 2,
+            HigherOrder::ListFindUnsafe { .. } => 1,
             HigherOrder::DictWalk { .. } => 2,
             HigherOrder::ListAny { .. } => 1,
         }