Function no_snapshot_conformance 

pub fn no_snapshot_conformance(value: &Value, span: Span) -> RuntimeError

What a value that implements no Snapshot conformance is refused with.

Only the interpreter reaches it: a struct or an enum whose type wrote none, met while walking a Vector whose element type is not known until the value is. The linear-memory backend has no runtime version of this question — a Vector’s element type is part of its layout, so whether it snapshots itself or calls a conformance is decided when the walk is lowered, not when it runs.