Reduce size of ReferenceId and SymbolId in AST

[overlookmotel]

ReferenceId and SymbolId are u32. In the AST, they're stored as Cell<Option<Id>>.

https://github.com/oxc-project/oxc/blob/b9d69ad665ab386233f940e9f1a9b3095a2238dc/crates/oxc_ast/src/ast/js.rs#L428
https://github.com/oxc-project/oxc/blob/b9d69ad665ab386233f940e9f1a9b3095a2238dc/crates/oxc_ast/src/ast/js.rs#L456

semantic's ScopeTree also contains parent_ids: IndexVec<ScopeId, Option<ScopeId>>

https://github.com/oxc-project/oxc/blob/b9d69ad665ab386233f940e9f1a9b3095a2238dc/crates/oxc_semantic/src/scope.rs#L23

u32 has no niche, so Option<u32> is 8 bytes.

This makes oxc::ast::IdentifierReference 8 bytes larger than it needs to be, because its reference_flag: ReferenceFlag field is an extra byte, but gets followed by 7 bytes of padding. IdentifierReference is very common in the AST.

We should either:

1. Make these IDs NonZeroU32 to give them a niche.
   or
2. Don't use Cell<Option<Id>>. Just use Cell<Id> and use 0 as a default value. 0 can mean "global scope" or "undefined".

I prefer option 2. Then we don't have to unwrap the Option all the time, which may remove panic code.

[Boshen]

Can I use https://crates.io/crates/nonmax

This didn't exist 6 months so I was too lazy to add a NonZeroU32 due to its awkward API.

rustc has an internal NonMax* type.

[Boshen]

I hate rust

define_index_type! {
    pub struct ReferenceId = NonMaxU32;
}

error[E0599]: no function or associated item named `max_value` found for struct `NonMaxU32` in the current scope
   --> crates/oxc_syntax/src/reference.rs:7:1
    |
7   | / define_index_type! {
8   | |     pub struct ReferenceId = NonMaxU32;
9   | | }
    | |_^ function or associated item not found in `NonMaxU32`
    |
note: if you're trying to build a new `NonMaxU32` consider using one of the following associated functions:
      NonMaxU32::new
      NonMaxU32::new_unchecked
   --> /Users/boshen/.cargo/registry/src/index.crates.io-6f17d22bba15001f/nonmax-0.5.5/src/lib.rs:395:1
    |
395 | nonmax!(unsigned, NonMaxU32, NonZeroU32, u32);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in the macro `define_index_type` which comes from the expansion of the macro `nonmax` (in Nightly builds, run with -Z macro-backtrace for more info)

Same error for NonZeroU32.

[Boshen]

Ahhh

https://github.com/thomcc/index_vec/tree/master?tab=readme-ov-file#what-features-are-planned

Allow index types such as NonZeroU32 and such, if it can be done sanely.

[Boshen]

Good news is we forked index_vec :-)

[Boshen]

Open to public oxc-project/oxc#3318