Adding to this, rustc's parser is both correct and fault-tolerant, yet rust-analyzer ended up building its own for different reasons (needing a CST and not an AST).

I settled on starting with a CST using rowan, doing semantic analysis, having an agnostic editor services, exposing both an LSP interface with tower, and a WASM interface (for Monaco in the browser [non-LSP!!]) and having the runtime binary doing the interpretation/compilation with the same pathway, with a slight jog, as the editor services. Phew!