Tracking issue for 128-bit integer support (RFC 1504)

[nikomatsakis]

Tracking issue for rust-lang/rfcs#1504.

cc @Amanieu

Blocking stabilization:

• Casting u128::MAX to f32 is undefined #41799 (Casting u128::MAX to f32 is undefined)
• Interaction with FFI? (Tracking issue for 128-bit integer support (RFC 1504) #35118 (comment)) - rustc: Flag {i,u}128 as unsafe for FFI #44261
• separately feature gate repr(i128) - rustc: Separately feature gate repr(i128) #44262
• Implement i128 lowering for backends without stable i128 support  #45676 (u/i)128 lowering for backends without native support
  • lowering still does not work for emscripten even with the work around
• Enums with 128-bit discriminant: repr128 feature

[durka]

Is #[repr(u128)] enum SuchWideVeryDiscriminantWow { ... } allowed?

[Amanieu]

That's a good point, I don't see any reason why it shouldn't be allowed.

[durka]

The return type of the discriminant_value intrinsic needs to be updated, then :)

[nagisa]

Intrinsics are stuff internal to the compiler, therefore changes to them doesn’t need discussion in the RFCs.

[durka]

I know, I just wanted to mention it here since I didn't see enums discussed in the RFC.