No way to reset a large variant object without corrupting the stack #25438
Description
Nim Version
Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2026-01-14
Copyright (c) 2006-2026 by Andreas Rumpf
git hash: c1e381a
active boot switches: -d:release
Description
type
Foo = object
case isBig: bool
of true:
bigArray: array[100000000, byte]
of false:
discard
Bar = ref object
f: Foo
proc makeFooBig(f: var Foo, isBig: bool) =
f = Foo(isBig: isBig) # How to change discriminant???
var b = Bar()
echo "Bar allocated"
makeFooBig(b.f, true) # Stack overflow
echo "OK"nim c -r --passC="-flto -fstack-usage -Werror=stack-usage=524288" --passL="-flto -fstack-usage -Werror=stack-usage=524288" test.nimC code for makeFooBig:
N_LIB_PRIVATE N_NIMCALL(void, makeFooBig__test7_u8)(tyObject_Foo__rTC9cDxGwsb1FuzDuHHpVrg* f_p0, NIM_BOOL isBig_p1) {
NIM_BOOL colontmpD_;
tyObject_Foo__rTC9cDxGwsb1FuzDuHHpVrg T1_; // STACK ALLOCATION!
colontmpD_ = ((NIM_BOOL) 0);
nimZeroMem(((void*) ((&T1_))), sizeof(tyObject_Foo__rTC9cDxGwsb1FuzDuHHpVrg));
colontmpD_ = isBig_p1;
T1_.isBig = colontmpD_;
(*f_p0) = T1_;
}Current Output
.../.cache/nim/test7_d/@mtest7.nim.c: In function ‘makeFooBig__test7_u8’:
.../.cache/nim/test7_d/@mtest7.nim.c:128:15: error: stack usage is 10000112 bytes [-Werror=stack-usage=]
128 | N_LIB_PRIVATE N_NIMCALL(void, makeFooBig__test7_u8)(tyObject_Foo__rTC9cDxGwsb1FuzDuHHpVrg* f_p0, NIM_BOOL isBig_p1) {
| ^
lto1: some warnings being treated as errors
lto-wrapper: fatal error: gcc returned 1 exit status
Expected Output
Compile and run successfully
Known Workarounds
proc makeFooBig(f: var Foo, isBig: bool) =
reset(f)
{.push checks: off.}
f.isBig = isBig
{.pop.}No response
Additional Information
Relevant issue: #25287