PR #5288 implements the function references proposal modulo a few missing bits. The missing bits are:

• Support for function references in the embedder API.
• Failing tests:
  • type-equivalence.wast: type canonicalisation is currently broken due to #[derive(hash)] not considering that two distinct typed function reference indices may point to structurally equivalent types.
  • br_table.wast: this fails due to incomplete support for the various syntactic sugar in the wasm-tools/wast crate. See Support for Wast table initialisation syntactic sugar with typeful references wasm-tools#952.
  • ref_null.wast and local_init.wast: fail due to lack of support in the embedder API.
  • return_call_ref.wast, return_call_indirect.wast, and return_call.wast: fail due to lack of tail call support.

At the time of writing the implementation does not take advantage of the static non-null safety to elide null checks when executing call_ref: