Implement basic List.drop that satisfies tests

Issues with uniqueness and copying which leaks memory obviously needs to be fixed.

compiler/builtins/bitcode/src/list.zig

@@ -665,6 +665,33 @@ pub fn listAppend(list: RocList, alignment: usize, element: Opaque, element_widt
     return output;
 }
 
+pub fn listDrop(list: RocList, alignment: usize, element_width: usize, count: usize) callconv(.C) RocList {
+    const size = list.len();
+    if (size <= count) {
+        return RocList.empty();
+    }
+    if (list.bytes) |source_ptr| {
+        var i: usize = 0;
+
+        const output = RocList.allocate(std.heap.c_allocator, alignment, size - count, element_width);
+        const target_ptr = output.bytes orelse unreachable;
+
+        while (i < size - count) : (i += 1) {
+            @memcpy(target_ptr + (i * element_width), source_ptr + ((i + count) * element_width), element_width);
+        }
+
+        // if (list.isUnique()) {
+        //     std.heap.c_allocator.free(source_ptr);
+        // } else {
+        //     utils.decref(std.heap.c_allocator, alignment, list.bytes, size * element_width);
+        // }
+
+        return output;
+    } else {
+        return RocList.empty();
+    }
+}
+
 pub fn listRange(width: utils.IntWidth, low: Opaque, high: Opaque) callconv(.C) RocList {
     const allocator = std.heap.c_allocator;
     const IntWidth = utils.IntWidth;

compiler/builtins/bitcode/src/main.zig

@@ -25,6 +25,7 @@ comptime {
     exportListFn(list.listReverse, "reverse");
     exportListFn(list.listSortWith, "sort_with");
     exportListFn(list.listConcat, "concat");
+    exportListFn(list.listDrop, "drop");
 }
 
 // Dict Module

compiler/builtins/src/bitcode.rs

@@ -76,6 +76,7 @@ pub const LIST_WALK_BACKWARDS: &str = "roc_builtins.list.walk_backwards";
 pub const LIST_CONTAINS: &str = "roc_builtins.list.contains";
 pub const LIST_REPEAT: &str = "roc_builtins.list.repeat";
 pub const LIST_APPEND: &str = "roc_builtins.list.append";
+pub const LIST_DROP: &str = "roc_builtins.list.drop";
 pub const LIST_SINGLE: &str = "roc_builtins.list.single";
 pub const LIST_JOIN: &str = "roc_builtins.list.join";
 pub const LIST_RANGE: &str = "roc_builtins.list.range";

compiler/builtins/src/std.rs

@@ -848,6 +848,13 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         Box::new(list_type(flex(TVAR1))),
     );
 
+    // drop : List elem, Nat -> List elem
+    add_top_level_function_type!(
+        Symbol::LIST_DROP,
+        vec![list_type(flex(TVAR1)), nat_type()],
+        Box::new(list_type(flex(TVAR1))),
+    );
+
     // prepend : List elem, elem -> List elem
     add_top_level_function_type!(
         Symbol::LIST_PREPEND,

compiler/can/src/builtins.rs

@@ -84,6 +84,7 @@ pub fn builtin_defs_map(symbol: Symbol, var_store: &mut VarStore) -> Option<Def>
         LIST_MAP => list_map,
         LIST_MAP2 => list_map2,
         LIST_MAP3 => list_map3,
+        LIST_DROP => list_drop,
         LIST_MAP_WITH_INDEX => list_map_with_index,
         LIST_KEEP_IF => list_keep_if,
         LIST_KEEP_OKS => list_keep_oks,
@@ -1881,6 +1882,28 @@ fn list_set(symbol: Symbol, var_store: &mut VarStore) -> Def {
         list_ret_var,
     )
 }
+/// List.drop : List elem, Nat -> List elem
+fn list_drop(symbol: Symbol, var_store: &mut VarStore) -> Def {
+    let list_var = var_store.fresh();
+    let index_var = var_store.fresh();
+
+    let body = RunLowLevel {
+        op: LowLevel::ListDrop,
+        args: vec![
+            (list_var, Var(Symbol::ARG_1)),
+            (index_var, Var(Symbol::ARG_2)),
+        ],
+        ret_var: list_var,
+    };
+
+    defn(
+        symbol,
+        vec![(list_var, Symbol::ARG_1), (index_var, Symbol::ARG_2)],
+        var_store,
+        body,
+        list_var,
+    )
+}
 
 /// List.append : List elem, elem -> List elem
 fn list_append(symbol: Symbol, var_store: &mut VarStore) -> Def {

compiler/gen/src/llvm/build.rs

@@ -6,9 +6,9 @@ use crate::llvm::build_dict::{
 use crate::llvm::build_hash::generic_hash;
 use crate::llvm::build_list::{
     allocate_list, empty_list, empty_polymorphic_list, list_append, list_concat, list_contains,
-    list_get_unsafe, list_join, list_keep_errs, list_keep_if, list_keep_oks, list_len, list_map,
-    list_map2, list_map3, list_map_with_index, list_prepend, list_range, list_repeat, list_reverse,
-    list_set, list_single, list_sort_with, list_walk_help,
+    list_drop, list_get_unsafe, list_join, list_keep_errs, list_keep_if, list_keep_oks, list_len,
+    list_map, list_map2, list_map3, list_map_with_index, list_prepend, list_range, list_repeat,
+    list_reverse, list_set, list_single, list_sort_with, list_walk_help,
 };
 use crate::llvm::build_str::{
     empty_str, str_concat, str_count_graphemes, str_ends_with, str_from_float, str_from_int,
@@ -3883,6 +3883,24 @@ fn run_low_level<'a, 'ctx, 'env>(
 
             list_append(env, inplace, original_wrapper, elem, elem_layout)
         }
+        ListDrop => {
+            // List.drop : List elem, Nat -> List elem
+            debug_assert_eq!(args.len(), 2);
+
+            let (list, list_layout) = load_symbol_and_layout(scope, &args[0]);
+            let original_wrapper = list.into_struct_value();
+
+            let count = load_symbol(scope, &args[1]);
+            let inplace = get_inplace_from_layout(layout);
+
+            list_drop(
+                env,
+                inplace,
+                original_wrapper,
+                count.into_int_value(),
+                list_layout,
+            )
+        }
         ListPrepend => {
             // List.prepend : List elem, elem -> List elem
             debug_assert_eq!(args.len(), 2);

compiler/gen/src/llvm/build_list.rs

@@ -322,6 +322,42 @@ pub fn list_append<'a, 'ctx, 'env>(
     )
 }
 
+/// List.drop : List elem, Nat -> List elem
+pub fn list_drop<'a, 'ctx, 'env>(
+    env: &Env<'a, 'ctx, 'env>,
+    _inplace: InPlace,
+    original_wrapper: StructValue<'ctx>,
+    count: IntValue<'ctx>,
+    list_layout: &Layout<'a>,
+) -> BasicValueEnum<'ctx> {
+    let (_, element_layout) = match *list_layout {
+        Layout::Builtin(Builtin::EmptyList) => (
+            InPlace::InPlace,
+            // this pointer will never actually be dereferenced
+            Layout::Builtin(Builtin::Int64),
+        ),
+        Layout::Builtin(Builtin::List(memory_mode, elem_layout)) => (
+            match memory_mode {
+                MemoryMode::Unique => InPlace::InPlace,
+                MemoryMode::Refcounted => InPlace::Clone,
+            },
+            *elem_layout,
+        ),
+
+        _ => unreachable!("Invalid layout {:?} in List.drop", list_layout),
+    };
+    call_bitcode_fn_returns_list(
+        env,
+        &[
+            pass_list_as_i128(env, original_wrapper.into()),
+            alignment_intvalue(env, &element_layout),
+            layout_width(env, &element_layout),
+            count.into(),
+        ],
+        &bitcode::LIST_DROP,
+    )
+}
+
 /// List.set : List elem, Int, elem -> List elem
 pub fn list_set<'a, 'ctx, 'env>(
     parent: FunctionValue<'ctx>,

compiler/module/src/low_level.rs

@@ -39,6 +39,7 @@ pub enum LowLevel {
     ListKeepOks,
     ListKeepErrs,
     ListSortWith,
+    ListDrop,
     DictSize,
     DictEmpty,
     DictInsert,

compiler/module/src/symbol.rs

@@ -924,6 +924,7 @@ define_builtins! {
         29 LIST_WALK_UNTIL: "walkUntil"
         30 LIST_RANGE: "range"
         31 LIST_SORT_WITH: "sortWith"
+        32 LIST_DROP: "drop"
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias

compiler/mono/src/borrow.rs

@@ -665,6 +665,7 @@ pub fn lowlevel_borrow_signature(arena: &Bump, op: LowLevel) -> &[bool] {
         // TODO when we have lists with capacity (if ever)
         // List.append should own its first argument
         ListAppend => arena.alloc_slice_copy(&[owned, owned]),
+        ListDrop => arena.alloc_slice_copy(&[owned, irrelevant]),
 
         Eq | NotEq => arena.alloc_slice_copy(&[borrowed, borrowed]),
 

compiler/test_gen/src/gen_list.rs

@@ -147,6 +147,17 @@ fn list_append() {
     );
 }
 
+#[test]
+fn list_drop() {
+    assert_evals_to!(
+        "List.drop [1,2,3] 2",
+        RocList::from_slice(&[3]),
+        RocList<i64>
+    );
+    assert_evals_to!("List.drop [] 1", RocList::from_slice(&[]), RocList<i64>);
+    assert_evals_to!("List.drop [1,2] 5", RocList::from_slice(&[]), RocList<i64>);
+}
+
 #[test]
 fn list_append_to_empty_list() {
     assert_evals_to!("List.append [] 3", RocList::from_slice(&[3]), RocList<i64>);