wast lexer size assert not compatible with randomized struct layouts

[the8472]

wasm-tools/crates/wast/src/lexer.rs

Lines 64 to 66 in d68b382

	const _: () = {
	assert!(std::mem::size_of::<Token>() <= std::mem::size_of::<u64>() * 2);
	};

breaks when compiling with layout randomization

If the assert is necessary for correctness then repr(C) should be used to get a deterministic layout.
If it's just an optimization then it's probably better to just check it in tests/CI.

Found this while trying to build the rust compiler with randomized layouts. rust-lang/rust#101339

[alexcrichton]

Thanks for the report, and sorry for the breakage! Is this something that you'd prefer to see urgently fixed? If so I can work on getting a patch release out for rust-lang/rust to update to. (it's a bit nonstandard to do patch releases in this repo unfortunately but I can make it work)

[the8472]

Not urgent, I can probably work around it for now by excluding a bunch of things from randomization.
I see that wast is updated monthly or so, that should be fine.

[the8472]

Looks like that wasn't sufficient, there are another compile time size assert:

wasm-tools/crates/wast/src/core/expr.rs

Lines 1211 to 1215 in 537ca81

	const _: () = {
	let size = std::mem::size_of::<Instruction<'_>>();
	let pointer = std::mem::size_of::<u64>();
	assert!(size <= pointer * 11);
	};

[the8472]

There are more

wasm-tools/crates/wasmparser/src/validator/types.rs

Lines 109 to 111 in 537ca81

	const _: () = {
	assert!(core::mem::size_of::<CoreTypeId>() <= 4);
	};

generally all such asserts should be moved to tests, otherwise the crates break under layout randomization.

[alexcrichton]

Oops, thanks for pointing that out! I've submitted #1894 to relax some more assertions.

To confirm though, in the second case you mentioned, that assert is technically ok to leave as a const-assert right? It's an assertion about a #[repr(C)] structure which -Zrandomize-layouts doesn't affect, right?

[the8472]

Oh yeah, I didn't look at the struct declaration. That one is fine.