Panics with `assertion failed: core::mem::size_of::<VariationTuple>() <= 80`

[lovelymono]

Same as rust-windowing/winit#4433.

You appear to be hitting a debug_assert in https://github.com/harfbuzz/ttf-parser. I suspect due to the nightly rustc version you're using increasing the size of the struct the assert runs on. You may wish to consider filing an issue there.

Description

Running a winit application fails with

thread 'main' (50531) panicked at /home/stella/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ttf-parser-0.25.1/src/tables/gvar.rs:283:5:
assertion failed: core::mem::size_of::<VariationTuple>() <= 80

Platform details

Rust version:

rustc 1.93.0-nightly (646a3f8c1 2025-12-02)
cargo 1.93.0-nightly (2a7c49606 2025-11-25)

Operating system, windowing system:

NixOS, nixos-unstable github:NixOS/nixpkgs/2d293cbfa5a793b4c50d17c05ef9e385b90edf6c
GNOME 49, Wayland

Additional information

Output of RUST_BACKTRACE=full https://gist.github.com/lovelymono/92b8899270d610a25ef0e5378701924a.

Source code:

#[must_use]
fn real_main() -> iced::Result {
    iced::application("Howl", Howl::update, Howl::view).run()
}

fn main() -> std::process::ExitCode {
    match real_main() {
        Ok(()) => std::process::ExitCode::SUCCESS,
        Err(e) => {
            eprintln!("{e:?}");
            std::process::ExitCode::FAILURE
        },
    }
}

#[derive(Debug)]
enum Message {
}

#[derive(Default)]
struct Howl {}

impl Howl {
    fn update(&mut self, _: Message) {
        
    }

    fn view(&self) -> iced::Element<'_, Message> {
        "waow".into()
    }
}

[lovelymono]

Forgot to add, and it seems pretty relevant:

I'm using:

RUSTFLAGS=(
  "-Clink-arg=-fuse-ld=${nixos-unstable.lib.getExe pkgs.mold-wrapped}"
  "-Ctarget-cpu=native"
  "-Zunstable-options"
  "-Zrandomize-layout"
  "-Zthreads=8"
)

and -Zrandomize-layout probably causes this issue (as expected, which is its purpose.) VariationTuple should probably use #[repr(C)] to guarantee the struct layout and size.

https://doc.rust-lang.org/stable/unstable-book/compiler-flags/randomize-layout.html

[RazrFalcon]

I would say this is out of scope. There are no plans to support -Zrandomize-layout.
And yes, some parts of ttf-parser are very sensitive to the struct sizes.

[lovelymono]

I think you misunderstand what -Zrandomize-layout does: it emulates some part of what Rust compiler is allowed to do anyway.

The -Zrandomize-layout flag changes the layout algorithm for repr(Rust) types defined in the current crate from its normal optimization goals to pseudorandomly rearranging fields within the degrees of freedom provided by the largely unspecified default representation. This also affects type sizes and padding.

There is no guarantee that a future version of rustc isn't going to rearrange the fields (for example, for optimization purposes) and break this code the same way.

[RazrFalcon]

I understand. And if it would break in the future - we would fix it.