vm_exceptionref is only implemented for the sys backend

Error message

vm_exceptionref is only implemented for the sys backend

What it means

Exception::vm_exceptionref yields the raw VMExceptionRef for engine interop, dispatching only to the sys variant; other backends panic with unimplemented!().

Source

Thrown at lib/api/src/entities/exception/inner.rs:76

    }

    /// Gets the exception payload values.
    #[inline]
    pub fn payload(&self, store: &mut impl AsStoreMut) -> Vec<Value> {
        match self {
            #[cfg(feature = "sys")]
            Self::Sys(s) => s.payload(store),
            _ => unimplemented!("payload is only implemented for the sys backend"),
        }
    }

    /// Get the `VMExceptionRef` corresponding to this `Exception`.
    #[inline]
    pub fn vm_exceptionref(&self) -> VMExceptionRef {
        match self {
            #[cfg(feature = "sys")]
            Self::Sys(s) => VMExceptionRef::Sys(s.exnref()),
            _ => unimplemented!("vm_exceptionref is only implemented for the sys backend"),
        }
    }

    /// Creates a new `Exception` from a `VMExceptionRef`.
    #[inline]
    pub fn from_vm_exceptionref(exnref: VMExceptionRef) -> Self {
        match exnref {
            #[cfg(feature = "sys")]
            VMExceptionRef::Sys(s) => {
                Self::Sys(crate::backend::sys::exception::Exception::from_exnref(s))
            }
            _ => unimplemented!("from_vm_exceptionref is only implemented for the sys backend"),
        }
    }
}

View on GitHub (pinned to 8c4b9ee9d3)

Solutions

Use the sys backend wherever vm_exceptionref is required
Remove raw VMExceptionRef paths from backend-agnostic code
Feature-gate the interop code to the sys backend
Implement vm_exceptionref for the remaining backends

Defensive patterns

Strategy: validation

Validate before calling

if !exceptions_supported(&store) { return Err(...); } // backend must be Sys

Type guard

fn is_sys_exception(ex: &ExceptionInner) -> bool {
    matches!(ex, ExceptionInner::Sys(_))
}

Try / catch

let exn_ref = std::panic::catch_unwind(|| exn.vm_exceptionref()).ok();

Prevention

Isolate raw VMExceptionRef usage in sys-gated modules
Do not propagate exceptions across engines without backend checks
Keep engine-interop layers per-backend instead of sharing them

When it happens

Trigger: Calling Exception::vm_exceptionref() when converting exceptions into VM-level representations (e.g. rethrowing, cross-boundary propagation) on a non-sys backend.

Common situations: Embedding layers that trap/rethrow wasm exceptions internally; engine interop glue written against the sys backend.

Related errors

AI-assisted analysis of wasmerio/wasmer@8c4b9ee9d3 (2026-09-01). Data as JSON: /api/errors/baa33c6dc2910a3f. Report an issue: GitHub.