Rust resolver panics on empty source (index out of bounds)

[dqnykamp]

Description

PublicDoenetMLCore panics with an index-out-of-bounds error when set_source() / return_dast() is called on empty (or whitespace-only) DoenetML source.

Steps to Reproduce

1. Initialize a PublicDoenetMLCore instance
2. Call core.set_flags("{}")
3. Call core.set_source(dast, "") where dast is the parsed result of an empty string
4. Call core.return_dast()

This triggers a Rust panic:

panicked at typed-index-collections-3.3.0/src/slice/slice_index.rs:108:10:
index out of bounds: the len is 0 but the index is 0

Full stack trace includes:

core::panicking::panic_bounds_check
<K as typed_index_collections::slice::slice_index::TiSliceIndex<K,V>>::index

Context

This was discovered while wiring the Rust WASM resolver into the LSP for autocompletion. When a user selects all text in the editor and deletes it, the LSP calls set_source with the empty document, which triggers the panic.

Current Workaround

The RustResolverAdapter in @doenet/lsp-tools now guards against empty source before calling into the Rust core, skipping the sync and falling back to JS-only autocompletion.