Avoid overflow in component ABI size calculations

[subotac]

Fixes #13980.

Preserve the canonical ABI size overflow sentinel when aligning and composing record and variant layouts and when advancing field offsets.

Add regression coverage for dynamic and const record/variant layouts and 32-/64-bit field offsets.

Tests:

• cargo test -p wasmtime-environ --features component-model
• cargo clippy -p wasmtime-environ --features component-model --all-targets -- -D warnings
• cargo fmt -p wasmtime-environ -- --check

[alexcrichton]

This feel s a bit too coarse to me to just slap .saturating_* everywhere and return, for example, explicitly unaligned sizes on align_to on overflow. With your understanding of this issue do you know of a more targeted fix?

[subotac]

Thanks — I narrowed the fix in 3ca57dd. align_to and the next_field* helpers now retain their original strict behavior. Only cached record/variant ABI size composition uses u64 intermediates and saturates at
u32::MAX, matching the existing fixed-length-list size representation. I also replaced the broad unit test with the issue reproducer in the fixed-length-list WAST suite.

[alexcrichton]

Thanks for the updates, but reading over this again I continue to be left with a bit of unease about fixing this issue here. I think I'd personally prefer to go the route of WebAssembly/component-model#682 and fix this at the component-model validation layer rather than in a bit of an ad-hoc fashion within Wasmtime here. I'd ideally like to take that as an opportunity to flesh out the tests here and expand them, too.

I don't believe that this is is a major issue for preexisting components and currently the only issue is when dealing with fixed-length-lists, so I think it's ok to leave this un-fixed on main for a bit personally.