Add support for chained associated types and returned associated types

[mootz12]

What

Resolve chained associated types and return-type associated types in #[contractimpl] macro expansion. The flatten_associated_items_in_impl_fns function now iteratively resolves associated type definitions that reference other associated types (e.g., type End = Self::Foo where type Foo = Self::Base) and substitutes Self::* in function return types, not just input parameters.

Why

Using chained associated types and returning associated types from interfaces is valid Rust syntax but resulted in unexpected compiler errors.

Known limitations

None

[mootz12]

This is a (sorta unrelated) change Claude recommended. Calling it out explicitly to see if we intentionally didn't flatten function return values before for a reason I am missing.

[Copilot]

Pull request overview

This PR extends the Soroban SDK procedural macros to correctly flatten/resolve chained associated type aliases (e.g., type End = Self::Foo; type Foo = Self::Base;) when generating contract function signatures, and adds a regression test contract + snapshot to validate the behavior.

Changes:

• Enhance flatten_associated_items_in_impl_fns to iteratively resolve chained Self::AssociatedType aliases and apply resolution to both function inputs and return types.
• Add a new test contract (associated_type_chained) covering chained associated type resolution, along with its snapshot and committed macro-expansion outputs.
• Update Cargo.lock to include the newly added test crate.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/associated_type_chained/src/lib.rs	New regression test contract exercising chained associated types in a #[contractimpl] trait impl.
tests/associated_type_chained/Cargo.toml	Adds the new test crate to the workspace (via tests/*) with appropriate SDK deps.
tests/associated_type_chained/test_snapshots/test/test_chained.1.json	Snapshot validating resulting ledger state after invoking the contract via generated client.
tests-expanded/test_associated_types_chained_wasm32v1-none.rs	Committed expanded output for the wasm build of the new test contract.
tests-expanded/test_associated_types_chained_tests.rs	Committed expanded output for the test build, exercising the generated client calls.
soroban-sdk-macros/src/syn_ext.rs	Implements iterative resolution for chained associated types and applies it to fn inputs + outputs.
Cargo.lock	Registers the new test_associated_types_chained package in the lockfile.

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[leighmcculloch]

Will this change also make it work where you reference an associated type through an associated type trait?

[mootz12]

Closing this PR in favor of just issuing compiler errors.

There are a lot of edge cases with resolving supertrait associated types, and given this is not a requested feature, we should just make the constraint clear for now.