How many fns the MIR walker can emit
standalone vs how many need HIR fallback. Pre-wire-up signal
so callers can track walker reach across the shipped corpus
without altering the codegen path.
Walker-side emit context. Holds the slice of the
CodegenContext the MIR-to-Rust walker reads — kept explicit
so future CodegenContext refactors don’t ripple through the
walker, and so other backends (wasm-gc, wasip2) can introduce
their own emit-ctx structs without inheriting Rust-specific
fields.
Walk every fn in program and report walker reach. For each
fn, calls [emit_mir_expr] on the body and counts
Some / None. Suitable for --explain-mir-coverage–style
diagnostics; the codegen path itself is untouched.