feat: host_bindgen! returns Result from guest-implemented functions

[jsturtevant]

fixes: #1316

host_bindgen! export wrappers no longer panic when Callable::call() returns Err (guest abort/trap/timeout). Host callers now receive the HyperlightError.

Breaking change

Calls to guest exports return hyperlight_host::Result. Implementations of host imports are unchanged.

Example WIT:

world test {
  import roundtrip;
  export roundtrip;
}

interface roundtrip {
  echo: func(x: string) -> string;
}

// Implementing an import: unchanged.
impl test::wit::Roundtrip for Host {
    fn echo(&mut self, x: String) -> String { x }
}

// Calling an export: now fallible.
let value: Result<String, HyperlightError> = sandbox.roundtrip().echo("hi".into());

Interface direction

The trait and its two markers live in hyperlight_host::component, so nothing is emitted into the wit namespaces where a package name could collide:

pub trait InterfaceDirection: Sealed {
    type CallResult<T>;      // Imported: T      Exported: hyperlight_host::Result<T>
    type Borrow<'a, T: 'a>;  // Imported: guard  Exported: &'a T
}
pub struct Imported;  // host implements it, calls run guest to host
pub struct Exported;  // guest implements it, calls run host to guest

Generated traits are written against that parameter:

pub trait Roundtrip<D: InterfaceDirection = Imported> {
    fn echo(&mut self, x: String) -> <D as InterfaceDirection>::CallResult<String>;
}

[Copilot]

Pull request overview

This PR updates host_bindgen!-generated host bindings so that calls into guest-implemented exports return hyperlight_host::Result<T> instead of panicking when the guest traps/aborts/timeouts. It also introduces a direction-marker mechanism so a single generated Rust trait can represent both imported and exported occurrences of the same WIT interface while selecting the appropriate return/borrow shapes at each use site.

Changes:

• Make host-side export wrappers propagate Callable::call() failures as HyperlightError instead of panicking.
• Add hyperlight_host::component::{InterfaceDirection, Imported, Exported} and update codegen to parameterize instance/resource traits over direction.
• Extend tests and WIT fixtures with a deliberately trapping guest export to assert the new non-panicking behavior, and document the breaking change in CHANGELOG.md.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/tests/rust_guests/witguest/src/main.rs	Adds a guest failable.will_trap() implementation that panics to simulate guest abort/trap.
src/tests/rust_guests/witguest/guest.wit	Declares the new exported failable interface with will-trap.
src/hyperlight_host/tests/wit_test.rs	Updates callers to unwrap fallible export calls and adds a test asserting guest traps return Err(HyperlightError::GuestAborted(..)).
src/hyperlight_host/src/lib.rs	Exposes the new public component module for bindgen support types.
src/hyperlight_host/src/component.rs	Adds the sealed InterfaceDirection trait plus Imported/Exported markers defining call/borrow shapes.
src/hyperlight_component_util/src/rtypes.rs	Updates type emission to use direction markers for borrow and call-result shapes, and ensures resource direction is chosen correctly.
src/hyperlight_component_util/src/host.rs	Updates generated host export wrappers to use ? (no panic) and return Ok(unmarshal(..)).
src/hyperlight_component_util/src/guest.rs	Removes now-obsolete is_export manipulation in guest emission.
src/hyperlight_component_util/src/emit.rs	Replaces is_export with direction tracking and adds helpers for emitting direction markers/params.
CHANGELOG.md	Records the breaking API change for host_bindgen! export call sites.

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[jsturtevant]

@syntactically, please feel free to push any changes you would perfer directly here. I did my best with how I understand components but it isn't at the same level as you 😃