Non-local impl errors in latest rust

[leighmcculloch]

error: non-local `impl` definition, `impl` blocks should be written at the same level as their item
  --> /home/runner/work/rs-soroban-sdk/rs-soroban-sdk/soroban-sdk/src/auth.rs:11:1
   |
11 | #[contracttype(crate_path = "crate", export = false)]
   | ^----------------------------------------------------
   | |
   | `Arbitrary` is not local
   | move the `impl` block outside of this constant `_` and up 2 bodies
12 | pub enum Context {
   |          ------- `ArbitraryContext` is not local
   |
   = note: the derive macro `crate::testutils::arbitrary::arbitrary::Arbitrary` defines the non-local `impl`, and may need to be changed
   = note: the derive macro `crate::testutils::arbitrary::arbitrary::Arbitrary` may come from an old version of the `derive_arbitrary` crate, try updating your dependency with `cargo update -p derive_arbitrary`
   = note: an `impl` is never scoped, even when it is nested inside an item, as it may impact type checking outside of that item, which can be the case if neither the trait or the self type are at the same nesting level as the `impl`
   = note: items in an anonymous const item (`const _: () = { ... }`) are treated as in the same scope as the anonymous const's declaration for the purpose of this lint
   = note: `-D non-local-definitions` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(non_local_definitions)]`
   = note: this error originates in the derive macro `crate::testutils::arbitrary::arbitrary::Arbitrary` which comes from the expansion of the attribute macro `contracttype` (in Nightly builds, run with -Z macro-backtrace for more info)

Ref: https://github.com/stellar/rs-soroban-sdk/actions/runs/11286263771/job/31861055751

[leighmcculloch]

This compiler error is showing up because the lint has been added:

• Tracking Issue for RFC 3373: Avoid non-local definitions in functions rust-lang/rust#120363

[leighmcculloch]

Actually, my comment above is incorrect. The issue is a bug in the compiler in nightly. It's being tracked and discussed in:

• Warning about non-local impl-definition in derive-macro output rust-lang/rust#131643