specr-transpile: Indexing a `List` in `assert_eq!()` does not desugar to `.index_at(...)`

[essickmango]

Encountered in minirust/spec/lang/representation.md, encode function of enums when trying to assert_eq!(AbstractByte::Uninit, bytes[offset.bytes()]); while writing the discriminant.

The rust compiler receives the same indexing method as in the markdown, which doesn't compile since List doesn't implement Index. Usually in the above example bytes[offset.bytes()] would get desugared into bytes.index_at(offset.bytes()) by specr-transpile.

[essickmango]

The easiest fix would be to implement Index<Int> for the List, and drop the desugaring entirely, or am I wrong and something prevents this? Int is sized, right?

[RalfJung]

Index returns a reference, and I don't think we can return references into List due to the GC setup.