Fix clippy unused_unit warnings in host_bindgen macro for void return functions

[Copilot]

The host_bindgen macro was generating clippy unused_unit warnings when creating bindings for WIT functions with no return value (void functions). This occurred because the macro generated explicit () return statements that clippy correctly identified as redundant.

Problem

When using WIT like:

interface adder {
  do-something: func(number: u32);  // No return value
}

The macro generated code like:

fn do_something(&mut self, number: u32) -> () {
    let ret = ::hyperlight_host::sandbox::Callable::call::<::std::vec::Vec<u8>>(&mut self.sb, /*...*/);
    let ::std::result::Result::Ok(ret) = ret else { panic!("bad return from guest {:?}", ret) };
    ()  // ← This explicit () triggers clippy::unused_unit
}

Solution

1. Modified emit_hl_unmarshal_result in src/hyperlight_component_util/src/hl.rs:

• Changed from quote! { () } to TokenStream::new() for empty result types
• Functions with void return now generate no explicit return statement

2. Updated guest code generation in src/hyperlight_component_util/src/guest.rs:

• Made the #[allow(clippy::unused_unit)] attribute conditional
• Only emit the attribute when there's actually an unmarshal expression to suppress

Result

Now generates clean code that doesn't trigger clippy warnings:

fn do_something(&mut self, number: u32) -> () {
    let ret = ::hyperlight_host::sandbox::Callable::call::<::std::vec::Vec<u8>>(&mut self.sb, /*...*/);
    let ::std::result::Result::Ok(ret) = ret else { panic!("bad return from guest {:?}", ret) };
    // No explicit () - function implicitly returns ()
}

Testing

• ✅ Builds successfully
• ✅ Clippy passes (including with -D clippy::unused-unit)
• ✅ Existing functionality unchanged
• ✅ Guest bindings still compile correctly
• ✅ Tested with the exact WIT example from the issue

Fixes #653.

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[jsturtevant]

It would be nice to see a test case added to the wit file that has a function that has no returns

[jsturtevant]

I don't think this will be needed since this use case is handled by return an empty quote

[Copilot]

You're right! Since emit_hl_unmarshal_result returns TokenStream::new() for void functions, the conditional logic wasn't needed. Simplified the code to use unmarshal directly. (412ab9f)

[jsturtevant]

Where did this come from? Don't include thes right now.

[Copilot]

Removed the proptest-regressions file that was accidentally included. (412ab9f)

[jsturtevant]

let's not do dependency updates in this PR and leave that for independent PRs.

[Copilot]

Reverted the Cargo.lock dependency updates to keep this PR focused only on the clippy fix. (412ab9f)

[Copilot]

It would be nice to see a test case added to the wit file that has a function that has no returns

Added a test case for void return function to guest.wit: do-something: func(number: u32); with corresponding implementation. Verified the fix works correctly with this test case and passes clippy with -D clippy::unused_unit. (412ab9f)

[syntactically]

I don't like this as a solution, because it changes the contract for this helper to something a lot less intuitive:

The resultant token stream will be an expression which typechecks at the Rust type ...

becomes

The resultant token stream will typecheck at the Rust type of the unnamed type of the result, or the empty stream if named results are used

which means that it can't generally be used to uniformly generate a return value. Right now, this expression is only used in the tail position of a function definition, so it works "by coincidence"---but this change makes the interface more brittle (e.g. if you wanted to refactor the macro to trace the return value, you would need special cases to not accidentally generate an empty let statement).