`rustc` stack overflow when compiling rmk-types with Rust 1.92.0

[decaday]

After updating my stable toolchain to rustc 1.92.0 (ded5c06cf 2025-12-08) yesterday, I found that I could no longer compile any examples:

thread 'rustc' (6732) has overflowed its stack
error: could not compile `rmk-types` (lib)

The compilation only succeeds if I set the stack size to approximately 64MB via an environment variable: (pwsh)

$env:RUST_MIN_STACK = "67108864"
cargo build

or (bash):

export RUST_MIN_STACK=67108864
cargo build

I attempted to create a minimal reproduction and traced the issue to this specific location:

rmk/rmk-types/src/keycode.rs

Lines 14 to 17 in e7577ea

	#[derive(postcard::experimental::max_size::MaxSize)]
	pub enum KeyCode {
	/// Reserved, no-key.
	No = 0x0000,

The issue disappears if I either remove #[derive(postcard::experimental::max_size::MaxSize)] or reduce the overall size of the enum.

I suspect that Rust 1.92.0 might have introduced changes to macro expansion or other internals, as the compiler's default stack size remains unchanged:

https://github.com/rust-lang/rust/blob/123588a5bf9250f214e1931a761f814660e69620/compiler/rustc_interface/src/util.rs#L105

Could we perhaps file an issue with postcard or wait for the macro to stabilize, and maybe notify users that they might need to set RUST_MIN_STACK during compilation?

[HaoboGu]

Yes, this issue has been reported several times, @jamesmunns do you have any idea about this issue?

[decaday]

Yes, this issue has been reported several times, @jamesmunns do you have any idea about this issue?

I tried running cargo expand, but cargo-expand stack overflowed as well. After shrinking the enum size and successfully running the expansion, I found this, which I suspect is the culprit behind the rustc stack overflow:

(This is just a 8-layer illustration; each additional enum variant adds another layer of nesting.)

impl ::postcard::experimental::max_size::MaxSize for KeyCode {
        const POSTCARD_MAX_SIZE: usize = 1usize
            + {
                let lhs = {
                    let lhs = {
                        let lhs = {
                            let lhs = {
                                let lhs = {
                                    let lhs = {
                                        let lhs = {
                                            let lhs = 0;
                                            let rhs = 0;
                                            if lhs > rhs { lhs } else { rhs }
                                        };
                                        let rhs = 0;
                                        if lhs > rhs { lhs } else { rhs }
                                    };
                                    let rhs = 0;
                                    if lhs > rhs { lhs } else { rhs }
                                };
                                let rhs = 0;
                                if lhs > rhs { lhs } else { rhs }
                            };
                            let rhs = 0;
                            if lhs > rhs { lhs } else { rhs }
                        };
                        let rhs = 0;
                        if lhs > rhs { lhs } else { rhs }
                    };
                    let rhs = 0;
                    if lhs > rhs { lhs } else { rhs }
                };
                let rhs = 0;
                if lhs > rhs { lhs } else { rhs }
            };
    }

[jamesmunns]

Oh wow. Yeah, that's fun. I'll see if I can improve the codegen on that, it's just sort of silly right now.

That being said: KeyCode is a repr(u16) enum. For now, instead of the derive, you could just do:

impl ::postcard::experimental::max_size::MaxSize for KeyCode {
    const POSTCARD_MAX_SIZE: usize = 3usize;
}

As long as KeyCode is a u16, this is guaranteed by the spec: https://postcard.jamesmunns.com/wire-format#maximum-encoded-length

[HaoboGu]

As long as KeyCode is a u16, this is guaranteed by the spec: https://postcard.jamesmunns.com/wire-format#maximum-encoded-length

Thanks James, the solution is beautiful :P