Function walk_program_interpolated 

pub fn walk_program_interpolated(
    source: &str,
    program: &[SNode],
    visitor: &mut impl FnMut(&SNode),
)

Walk program INCLUDING the expressions inside ${...} string interpolation.

walk_program cannot reach them: the lexer stores a hole as unparsed source text plus a position, so Node::InterpolatedString is a leaf with no AST children by construction. Any analysis that asks “is this name used?” and walks with walk_program alone answers NO for a name used only inside a hole — an unsound answer, not merely an incomplete one.

That is not hypothetical. The whole-program capability solver computed a helper’s required capabilities that way, concluded random was unused because its only use was "...${harness.random.uuid_v7()}", and deleted it from the parameter type and every call site. The repair runs automatically in the fleet bump workflow, so it rewrote working code into code that does not compile (value of type nil has no method uuid_v7) and shipped it as a bump PR (harn-cloud#1469).

source must be the whole file program was parsed from, so re-parsed holes carry spans in the containing file’s coordinates and stay safe to edit against. A hole that fails to re-parse is skipped: it cannot be a well-typed use, and the containing parse already reported it.