GC panics while tracing a suspended stack-switching continuation

Start in crates/wasmtime/src/runtime/vm/traphandlers/backtrace.rs and follow trace_through_continuations through trace_suspended_continuation and trace_wasm_continuation_roots. Run the inline gc_traces_a_suspended_continuation reproduction with GC, stack switching, exceptions, and function references enabled; done means the test completes without the VMStackLimits panic and traces the suspended continuation successfully.

Wasmtime version

This reproduces with Wasmtime 48.0.1 and current main at
d8a0da6d661605713798c1c9c76be5c28e3159ff.

Reproduction

With function references, exceptions, stack switching, and GC enabled, the
following test panics when guest allocations trigger GC while a continuation is
suspended:

#[cfg_attr(any(asan, miri), ignore)]
#[test]
fn gc_traces_a_suspended_continuation() -> Result<()> {
    let wat = r#"
        (module
            (type $ft (func))
            (type $ct (cont $ft))
            (type $st (struct (field i32)))
            (tag $t)

            (func $suspend
                (suspend $t)
            )
            (elem declare func $suspend)

            (func (export "entry")
                (local $continuation (ref null $ct))
                (local $i i32)
                (block $handler (result (ref $ct))
                    (resume $ct
                        (on $t $handler)
                        (cont.new $ct (ref.func $suspend)))
                    (return)
                )
                (local.set $continuation)
                (loop $allocate
                    (drop (struct.new $st (i32.const 7)))
                    (local.set $i (i32.add (local.get $i) (i32.const 1)))
                    (br_if $allocate (i32.lt_u (local.get $i) (i32.const 20000)))
                )
                (resume $ct (local.get $continuation))
            )
        )
    "#;

    test_utils::Runner::new().run_test::<()>(wat, &[])
}

Actual result

panicked at crates/wasmtime/src/runtime/vm/traphandlers/backtrace.rs:340:18:
expected one more VMStackLimits than continuations

The stack proceeds through gc_alloc_raw, do_gc,
trace_wasm_continuation_roots, trace_suspended_continuation, and
trace_through_continuations.

Expected result

GC should trace the suspended continuation and entry should complete normally.

Analysis

trace_through_continuations handles the current activation before walking its
ancestors. It advances the stack-limits iterator past that activation, but does
not advance the matching continuation iterator. The remaining iterators
therefore have different lengths.

Advancing continuations_iter once before making it peekable aligns it with the
existing stack-limits advance. The test above fails before that change and
passes afterward.

This appears distinct from #13750, which concerns missing stack maps for
continuation payload values. The broader stack-switching tracking issue is
#10248.

Environment

• OS: Linux x86_64
• Rust: 1.97.1