build a result unwrap_or's fallback lazily

Makefile

@@ -1032,6 +1032,7 @@ MEMCHECK_CASES := \
 	tests/cases/test_channel_fanout_ownership \
 	tests/cases/test_empty_list_field_ownership \
 	tests/cases/test_list_bytes_ownership \
+	tests/cases/test_result_unwrap_or_fallback_leak \
 	tests/cases/test_struct_closure_field tests/cases/test_generic_struct_field_dtor \
 	tests/cases/test_iterator_drain tests/cases/test_method_fresh_string_return \
 	tests/cases/test_lazy_adapter_pipeline tests/cases/test_named_local_adapter_chain \

self-host/bootstrap/ir_driver.ir

@@ -146854,81 +146854,81 @@ load 24 node
 iconst 25 0
 call 26 ir_emitter_core_ir_method_call_arg_expr int 2 24 25
 store arg_idx 26
-load 27 arg_idx
-call 28 ir_emitter_core_ir_expr int 1 27
-store fallback_r 28
-load 29 ok_l
-call 30 ir_builder_ir_label string 0
-call 31 pith_cstring_release void 1 29
-store ok_l 30
-load 32 fallback_l
-call 33 ir_builder_ir_label string 0
-call 34 pith_cstring_release void 1 32
-store fallback_l 33
-load 35 merge_l
-call 36 ir_builder_ir_label string 0
-call 37 pith_cstring_release void 1 35
-store merge_l 36
-strref 38 m46s829
-load 39 cond_r
-call 40 int_to_string string 1 39
-concat 41 38 40
-call 42 pith_cstring_release void 1 38
-call 43 pith_cstring_release void 1 40
-strref 44 m46s333
-concat 45 41 44
-call 46 pith_cstring_release void 1 41
-call 47 pith_cstring_release void 1 44
-load 48 ok_l
-concat 49 45 48
-call 50 pith_cstring_release void 1 45
-strref 51 m46s333
-concat 52 49 51
-call 53 pith_cstring_release void 1 49
-call 54 pith_cstring_release void 1 51
-load 55 fallback_l
-concat 56 52 55
-call 57 pith_cstring_release void 1 52
-call 58 ir_builder_ir_emit void 1 56
-call 59 pith_cstring_release void 1 56
-load 60 ok_l
-call 61 ir_call_emit_ir_emit_label void 1 60
-load 62 recv_r
-load 63 recv_tid
-strref 64 m46s182
-call 65 ir_result_abi_ir_emit_result_value_field int 3 62 63 64
-call 66 pith_cstring_release void 1 64
-store ok_value_r 65
-call 67 ir_builder_ir_reg int 0
-strref 68 m46s893
-strref 69 m46s823
-load 70 recv_r
-call 71 ir_call_emit_ir_emit_call1 void 4 67 68 69 70
-call 72 pith_cstring_release void 1 68
-call 73 pith_cstring_release void 1 69
-strref 74 m46s825
-load 75 temp_name
-concat 76 74 75
-call 77 pith_cstring_release void 1 74
-strref 78 m46s333
-concat 79 76 78
-call 80 pith_cstring_release void 1 76
-call 81 pith_cstring_release void 1 78
-load 82 ok_value_r
-call 83 int_to_string string 1 82
-concat 84 79 83
-call 85 pith_cstring_release void 1 79
-call 86 pith_cstring_release void 1 83
-call 87 ir_builder_ir_emit void 1 84
-call 88 pith_cstring_release void 1 84
-strref 89 m46s828
-load 90 merge_l
-concat 91 89 90
+load 27 ok_l
+call 28 ir_builder_ir_label string 0
+call 29 pith_cstring_release void 1 27
+store ok_l 28
+load 30 fallback_l
+call 31 ir_builder_ir_label string 0
+call 32 pith_cstring_release void 1 30
+store fallback_l 31
+load 33 merge_l
+call 34 ir_builder_ir_label string 0
+call 35 pith_cstring_release void 1 33
+store merge_l 34
+strref 36 m46s829
+load 37 cond_r
+call 38 int_to_string string 1 37
+concat 39 36 38
+call 40 pith_cstring_release void 1 36
+call 41 pith_cstring_release void 1 38
+strref 42 m46s333
+concat 43 39 42
+call 44 pith_cstring_release void 1 39
+call 45 pith_cstring_release void 1 42
+load 46 ok_l
+concat 47 43 46
+call 48 pith_cstring_release void 1 43
+strref 49 m46s333
+concat 50 47 49
+call 51 pith_cstring_release void 1 47
+call 52 pith_cstring_release void 1 49
+load 53 fallback_l
+concat 54 50 53
+call 55 pith_cstring_release void 1 50
+call 56 ir_builder_ir_emit void 1 54
+call 57 pith_cstring_release void 1 54
+load 58 ok_l
+call 59 ir_call_emit_ir_emit_label void 1 58
+load 60 recv_r
+load 61 recv_tid
+strref 62 m46s182
+call 63 ir_result_abi_ir_emit_result_value_field int 3 60 61 62
+call 64 pith_cstring_release void 1 62
+store ok_value_r 63
+call 65 ir_builder_ir_reg int 0
+strref 66 m46s893
+strref 67 m46s823
+load 68 recv_r
+call 69 ir_call_emit_ir_emit_call1 void 4 65 66 67 68
+call 70 pith_cstring_release void 1 66
+call 71 pith_cstring_release void 1 67
+strref 72 m46s825
+load 73 temp_name
+concat 74 72 73
+call 75 pith_cstring_release void 1 72
+strref 76 m46s333
+concat 77 74 76
+call 78 pith_cstring_release void 1 74
+call 79 pith_cstring_release void 1 76
+load 80 ok_value_r
+call 81 int_to_string string 1 80
+concat 82 77 81
+call 83 pith_cstring_release void 1 77
+call 84 pith_cstring_release void 1 81
+call 85 ir_builder_ir_emit void 1 82
+call 86 pith_cstring_release void 1 82
+strref 87 m46s828
+load 88 merge_l
+concat 89 87 88
+call 90 pith_cstring_release void 1 87
+call 91 ir_builder_ir_emit void 1 89
 call 92 pith_cstring_release void 1 89
-call 93 ir_builder_ir_emit void 1 91
-call 94 pith_cstring_release void 1 91
-load 95 fallback_l
-call 96 ir_call_emit_ir_emit_label void 1 95
+load 93 fallback_l
+call 94 ir_call_emit_ir_emit_label void 1 93
+load 95 arg_idx
+call 96 ir_emitter_core_ir_expr int 1 95
+store fallback_r 96
 load 97 recv_r
 load 98 recv_tid
 call 99 ir_emitter_core_ir_emit_release_result_error void 2 97 98
@@ -172849,18 +172849,18 @@ store __legacy_result_25_101 34
 label L103
 load 35 __legacy_result_25_101
 field 36 35 0 bool is_ok
-strref 37 m48s69
 brif 36 L104 L105
 label L104
-field 38 35 8 string ok
-call 39 pith_struct_release void 1 35
-store __unwrap_or_37_104 38
+field 37 35 8 string ok
+call 38 pith_struct_release void 1 35
+store __unwrap_or_37_104 37
 jmp L106
 label L105
+strref 39 m48s69
 field 40 35 16 string err
 call 41 pith_cstring_release void 1 40
 call 42 pith_struct_release void 1 35
-store __unwrap_or_37_104 37
+store __unwrap_or_37_104 39
 label L106
 load 43 __unwrap_or_37_104
 call 44 pith_cstring_retain void 1 43
@@ -173276,18 +173276,18 @@ store __legacy_result_28_152 37
 label L154
 load 38 __legacy_result_28_152
 field 39 38 0 bool is_ok
-strref 40 m48s69
 brif 39 L155 L156
 label L155
-field 41 38 8 string ok
-call 42 pith_struct_release void 1 38
-store __unwrap_or_40_155 41
+field 40 38 8 string ok
+call 41 pith_struct_release void 1 38
+store __unwrap_or_40_155 40
 jmp L157
 label L156
+strref 42 m48s69
 field 43 38 16 string err
 call 44 pith_cstring_release void 1 43
 call 45 pith_struct_release void 1 38
-store __unwrap_or_40_155 40
+store __unwrap_or_40_155 42
 label L157
 load 46 __unwrap_or_40_155
 call 47 pith_cstring_retain void 1 46

self-host/ir_emitter_core.pith

@@ -3712,7 +3712,6 @@ fn ir_emit_result_unwrap_or(node: Node) -> Int:
     cond_r := ir_emit_result_flag_field(recv_r, "is_ok")
     temp_name := ir_new_temp_name("unwrap_or")
     arg_idx := ir_method_call_arg_expr(node, 0)
-    fallback_r := ir_expr(arg_idx)
     ok_l := ir_label()
     fallback_l := ir_label()
     merge_l := ir_label()
@@ -3723,6 +3722,11 @@ fn ir_emit_result_unwrap_or(node: Node) -> Int:
     ir_emit("store " + temp_name + " " + ok_value_r.to_string())
     ir_emit("jmp " + merge_l)
     ir_emit_label(fallback_l)
+    # build the fallback INSIDE the error arm: on the ok path it is never
+    # needed, and evaluating it eagerly before the branch leaked a heap default
+    # (`r.unwrap_or(bytes.empty())`) every ok iteration. this mirrors the
+    # optional unwrap_or, which already defers its default into the none arm.
+    fallback_r := ir_expr(arg_idx)
     ir_emit_release_result_error(recv_r, recv_tid)
     ir_emit_call1(ir_reg(), "pith_struct_release", "void", recv_r)
     ir_emit("store " + temp_name + " " + fallback_r.to_string())

tests/cases/test_result_unwrap_or_fallback_leak.pith

@@ -0,0 +1,45 @@
+# a result's `unwrap_or` must not leak its fallback on the ok path.
+#
+# the fallback used to be built before the ok/err branch, so a heap default
+# like `unwrap_or(bytes.empty())` was allocated every call and, whenever the
+# result was ok, discarded without release. the optional `unwrap_or` already
+# deferred its default into the none arm; the result path now matches. this
+# runs the ok path many times over and checks the process footprint stays flat.
+
+import std.bytes as bytes
+import std.fs as fs
+import std.strings as strings
+
+fn make(i: Int) -> Bytes!:
+    if i < 0:
+        fail "negative"
+    return bytes.from_string_utf8("a payload wide enough to notice if it leaks")
+
+fn hwm_kb() -> Int:
+    status := fs.read_file("/proc/self/status") catch ""
+    for line in status.split(chr(10)):
+        if line.starts_with("VmHWM"):
+            for part in line.split(" "):
+                v := strings.parse_int(part).unwrap_or(0 - 1)
+                if v > 0:
+                    return v
+    return 0
+
+fn main():
+    mut total := 0
+    mut i := 0
+    while i < 200000:
+        # inline (owned) operand: `unwrap_or` moves its ok payload out and frees
+        # the fresh result shell itself, so this exercises only the fallback,
+        # not the separate borrowed-operand ownership question. the fallback
+        # allocates every iteration and, on the ok path, must never be kept.
+        total = total + make(i).unwrap_or(bytes.empty()).len()
+        i = i + 1
+    peak := hwm_kb()
+    # a per-iteration leak of the ~48-byte empty-bytes fallback would add
+    # ~9 mb over 200k iterations; flat sits near 3 mb. 20 mb is a wide margin
+    # that a real leak blows through and normal variance never reaches.
+    if peak < 20000:
+        print("flat total=" + total.to_string())
+    else:
+        print("leaked peak_kb=" + peak.to_string())

tests/expected/test_result_unwrap_or_fallback_leak.txt

@@ -0,0 +1 @@
+flat total=8600000