Function expand_module_measured 

pub fn expand_module_measured(
    module: &Node,
    diags: &mut Diagnostics,
) -> (Node, u64)

Expand a module, and say how much of the interpreter’s step budget is left.

The second half is what makes interp::MAX_STEPS’s doc comment a measurement rather than an assertion: macro_interp.rs expands the most expensive macro body here and prints what it cost. Nothing in the compiler reads it.