v3: sizeof(heap-promoted struct variable) returns pointer size instead of struct size #29305

Describe the bug

For a heap-promoted local struct, sizeof(variable) uses its generated C pointer representation rather than its V value type. This can silently truncate raw struct copies.

Reproduction Steps

Save as repro.v:

struct Record {
mut:
    id u32
    values [1024]u32
}

fn update(mut record Record) {
    record.id = 7
}

fn main() {
    mut record := Record{}
    update(mut record)
    assert sizeof(record) == sizeof(Record)
}

Compile and execute the binary directly:

v -new-compiler -cc tcc -o repro repro.v
./repro

Also reproduces with -cc gcc.

Expected Behavior

The assertion passes: both sizes describe the 4100-byte Record value.

Current Behavior

The binary exits 1:

left value: sizeof(record) = 8
right value: sizeof(Record) = 4100
V panic: Assertion failed...

Relevant generated C:

main__Record* record = (main__Record*)memdup(&(main__Record){0}, sizeof(main__Record));
update(record);
if (!(sizeof(record) == sizeof(main__Record))) {
    // assertion fails
}

Workaround and Impact

Use sizeof(Record) explicitly instead of sizeof(record). This avoids truncated SPS/PPS/slice-header storage in the public antono2/v_vulkan_video player.

V version and Environment

V 0.5.2, upstream commit 0dc6a692ed5333cbe3f05ff63ae0766f0333038a (current master when retested), clean source-built checkout. Ubuntu 24.04 x86-64. Reproduces with strict -new-compiler using both bundled TinyCC and GCC 13.3.0. Released V 0.5.2 (7647ce1) with TinyCC passes.

No imports, native dependencies, or GPU are needed for this reproducer.