[mypyc] Don't copy items twice in list(<generator expression>)

mypyc/irbuild/specialize.py

@@ -148,7 +148,7 @@
     string_writer_range_check_op,
 )
 from mypyc.primitives.librt_vecs_ops import isinstance_vec
-from mypyc.primitives.list_ops import isinstance_list, new_list_set_item_op
+from mypyc.primitives.list_ops import isinstance_list
 from mypyc.primitives.misc_ops import (
     isinstance_bool,
     isinstance_complex,
@@ -208,6 +208,14 @@ def _apply_specialization(
     # TODO: Allow special cases to have default args or named args. Currently they don't since
     #       they check that everything in arg_kinds is ARG_POS.
 
+    # Specializers translate generator expression arguments inline, which doesn't support
+    # unpacking (Python 3.15+). Use the generic path, where transform_generator_expr
+    # reports an error.
+    if any(
+        isinstance(arg, GeneratorExpr) and isinstance(arg.left_expr, StarExpr) for arg in expr.args
+    ):
+        return None
+
     # If there is a specializer for this function, try calling it.
     # Return the first successful one.
     if name and (name, typ) in specializers:
@@ -431,31 +439,17 @@ def dict_methods_fast_path(builder: IRBuilder, expr: CallExpr, callee: RefExpr)
 def translate_list_from_generator_call(
     builder: IRBuilder, expr: CallExpr, callee: RefExpr
 ) -> Value | None:
-    """Special case for simplest list comprehension.
+    """Special case for list(<generator expression>), which acts like a list comprehension.
 
     For example:
-        list(f(x) for x in some_list/some_tuple/some_str)
-    'translate_list_comprehension()' would take care of other cases
-    if this fails.
+        list(f(x) for x in some_iterable)
     """
     if (
         len(expr.args) == 1
         and expr.arg_kinds[0] == ARG_POS
         and isinstance(expr.args[0], GeneratorExpr)
-        and can_preallocate_from_generator(builder, expr.args[0])
     ):
-        gen = expr.args[0]
-
-        def set_item(x: Value, y: Value, z: Value, line: int) -> None:
-            builder.call_c(new_list_set_item_op, [x, y, z], line)
-
-        with comprehension_scope(builder, gen):
-            return sequence_from_generator_preallocate_helper(
-                builder,
-                gen,
-                empty_op_llbuilder=builder.builder.new_list_op_with_length,
-                set_item_op=set_item,
-            )
+        return translate_generator_to_list(builder, expr.args[0])
     return None
 
 

mypyc/test-data/irbuild-lists.test

@@ -447,6 +447,46 @@ L8:
     b = r11
     return 1
 
+[case testListFromGeneratorExpr]
+def f(n: int) -> list[int]:
+    return list(x + 1 for x in range(n) if x != 2)
+[out]
+def f(n):
+    n :: int
+    r0 :: list
+    r1 :: int
+    r2 :: bit
+    x :: int
+    r3 :: bit
+    r4 :: int
+    r5 :: object
+    r6 :: i32
+    r7 :: bit
+    r8 :: int
+L0:
+    r0 = PyList_New(0)
+    r1 = 0
+L1:
+    r2 = int_lt r1, n
+    if r2 goto L2 else goto L6 :: bool
+L2:
+    x = r1
+    r3 = int_ne x, 4
+    if r3 goto L4 else goto L3 :: bool
+L3:
+    goto L5
+L4:
+    r4 = CPyTagged_Add(x, 2)
+    r5 = box(int, r4)
+    r6 = PyList_Append(r0, r5)
+    r7 = r6 >= 0 :: signed
+L5:
+    r8 = CPyTagged_Add(r1, 2)
+    r1 = r8
+    goto L1
+L6:
+    return r0
+
 [case testGeneratorNext_withgil]
 from typing import List, Optional
 

mypyc/test-data/irbuild-python315.test

@@ -1,6 +1,17 @@
 [case testPEP798FailGracefully_python3_15]
+from typing import Iterator
+
 x = [[1]]
 y = [*l for l in x]  # E: Unpacking in comprehensions is not supported yet
 
 dx = [{1: 1}]
 dy = {**d for d in dx}  # E: Unpacking in comprehensions is not supported yet
+
+def f(it: Iterator[list[int]], x: list[list[int]], xs: list[list[str]]) -> None:
+    list(*l for l in it)  # E: Unpacking in comprehensions is not supported yet
+    list(*l for l in x)  # E: Unpacking in comprehensions is not supported yet
+    tuple(*l for l in x)  # E: Unpacking in comprehensions is not supported yet
+    sorted(*l for l in x)  # E: Unpacking in comprehensions is not supported yet
+    any(*l for l in x)  # E: Unpacking in comprehensions is not supported yet
+    ",".join(*l for l in xs)  # E: Unpacking in comprehensions is not supported yet
+    sorted((*l for l in x), key=abs)  # E: Unpacking in comprehensions is not supported yet

mypyc/test-data/run-lists.test

@@ -557,7 +557,7 @@ def test_in_operator_various_cases() -> None:
     assert list_in_mixed(type)
 
 [case testListBuiltFromGenerator]
-from typing import Final
+from typing import Final, Iterator
 abc: Final = "abc"
 def test_from_gen() -> None:
     source_a = ["a", "b", "c"]
@@ -583,6 +583,21 @@ def test_known_length() -> None:
     built = [str(x) for x in [*abc, *"def", *b"ghi", ("j", "k"), *("l", "m", "n")]]
     assert built == ['a', 'b', 'c', 'd', 'e', 'f', '103', '104', '105', "('j', 'k')", 'l', 'm', 'n']
 
+def gen(n: int) -> Iterator[int]:
+    for i in range(n):
+        yield i
+
+def test_from_gen_unknown_length() -> None:
+    n = 4
+    assert list(x + 1 for x in range(n)) == [1, 2, 3, 4]
+    assert list(x for x in range(n) if x % 2) == [1, 3]
+    assert list(x * y for x in range(3) for y in range(2)) == [0, 0, 0, 1, 0, 2]
+    assert list(k for k in {"a": 1, "b": 2}) == ["a", "b"]
+    assert list(x for x in gen(3)) == [0, 1, 2]
+    assert list(x for x in iter([5, 6])) == [5, 6]
+    empty: list[int] = []
+    assert list(x for x in empty) == []
+
 [case testNext]
 from typing import List