wasmparser static TypeId assert can trigger with -Zrandomize-layout

[repi]

The following wasmparser code can fail if compiling with nightly and -Zrandomize-layout due to a static assert triggering on the size of TypeId when the field order gets reshuffled.

/// Represents a unique identifier for a type known to a [`crate::Validator`].
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub struct TypeId {
    /// The index into the global list of types.
    pub(crate) index: usize,
    /// The effective type size for the type.
    ///
    /// This is stored as part of the ID to avoid having to recurse through
    /// the global type list when calculating type sizes.
    pub(crate) type_size: u32,
    /// A unique integer assigned to this type.
    ///
    /// The purpose of this field is to ensure that two different `TypeId`
    /// representations can be handed out for two different aliased types within
    /// a component that actually point to the same underlying type (as pointed
    /// to by the `index` field).
    unique_id: u32,
}

// The size of `TypeId` was seen to have a large-ish impact in #844, so this
// assert ensures that it stays relatively small.
const _: () = {
    assert!(std::mem::size_of::<TypeId>() <= 16);
};

should we increase the limit her, remove the assert, or use repr(C) on TypeId to enforce the field order of this type?

We ran into this when building our application with nightly and -Zrandomize-layout which builds all dependent crates including wasmtime and wasmparser with it as well and this failed to build. Another option could be to move the assert to a unit test so it only gets triggered in the development of wasmparser and not in usages of the crate?

[alexcrichton]

Oh dear sorry for the breakage! I think it's probably best to use #[repr(C)] here which I think should fix the issue for you. If you'd like to send a PR that'd be appreciated!