Function expressions_used 

fn expressions_used(
    module: &Module,
    function: &Function,
    block: &Block,
    extra_roots: impl IntoIterator<Item = Handle<Expression>>,
    stop_at: &HandleSet<Expression>,
) -> HandleSet<Expression>

Return the expressions of function that the code generated for block and extra_roots will refer to, directly or indirectly.

This is wider than what block needs the values of, because we do no dead code elimination: Writer::put_block writes out a definition for every expression it bakes, whether or not anything reads it, and that definition refers to the expression’s operands. Hence trace_block_including_emits rather than trace_block. Miss those and a dead definition would leave its operands un-hoisted, and continuing would re-evaluate them in place - or, for a statement result, reach the unreachable! in Writer::put_expression, which has no inline form to fall back on.

The search does not continue past an expression in stop_at: such an expression is marked if something refers to it, but the search does not follow it to its operands. Anything reachable only through it is therefore left out.