fix: remove trait bounds from where clause in IntoWasmAbi expansion

[dsgallups]

This PR fixes an issue with trait implementation where you have

use tsify::Tsify;

pub trait Constraint {}

#[derive(Tsify)]
#[tsify(into_wasm_abi, from_wasm_abi)]
pub struct GenericStruct<T: Constraint> {
    x: T,
}

error[E0229]: associated item constraints are not allowed here
  |
  | pub struct GenericStruct<T: Constraint> {
  |                          ^^^^^^^^^^^^^ associated item constraint not allowed here

This is because the implementation of traits would have an invalidate generic constraint in the where clause:

    #[automatically_derived]
    impl<T: Constraint> IntoWasmAbi for &GenericStruct<T>
    where
        GenericStruct<T: Constraint>: _serde::Serialize,

This has been fixed:

    #[automatically_derived]
    impl<T: Constraint> IntoWasmAbi for &GenericStruct<T>
    where
        GenericStruct<T>: _serde::Serialize,

[madonoharu]

Sorry this sat for so long — thanks for tracking the bug down. The diagnosis is exactly right: wasm_bindgen.rs interpolated a raw syn::Generics into a type position, and Generics::to_tokens emits the declaration form (bounds included), which is what trips E0229.

Two things came up while getting it ready to land:

1. split_for_impl().1 handles all three parameter kinds. The hand-built stripped Generics clears bounds on type params only, so <'a: 'b, 'b> and <const N: usize> still reach the type position. I reproduced that against your branch — the derive panics with expected ',' on both:

#[derive(Tsify)] #[tsify(into_wasm_abi, from_wasm_abi)]
pub struct GenericLifetime<'a: 'b, 'b> { x: &'a str, y: &'b str }

#[derive(Tsify)] #[tsify(into_wasm_abi, from_wasm_abi)]
pub struct GenericConst<const N: usize> { x: u32 }

TypeGenerics is syn's type-position rendering, so materialising the predicate first fixes all three at once and drops the hand-rolled block:

let predicate: syn::WherePredicate = {
    let (_, ty_generics, _) = generics.split_for_impl();
    parse_quote!(#ident #ty_generics: #serde_path::Serialize)
};
generics.make_where_clause().predicates.push(predicate);

2. The expected output needs regenerating. main has since landed the expandtest fix (#87), so the snapshots are now fully expanded — generic_constrained_struct.expanded.rs grows from 498 to ~2160 lines.

Since this branch doesn't have "Allow edits from maintainers" enabled, I can't push these here, so I've opened #89 with your commits rebased onto main and the two changes above on top. Your commits keep their authorship. Happy to close that one instead if you'd rather finish this PR — just say the word.

[madonoharu]

Landed in #89 (269b3dd) — your commits carried through with co-author credit. Thanks again for finding this, and sorry it took so long to get in.

Closing this one as superseded.