Function integer_const 

pub fn integer_const(
    cx: &LateContext<'_>,
    expr: &Expr<'_>,
    ctxt: SyntaxContext,
) -> Option<u128>

If expr evaluates to an integer constant, return its value.

The context argument is the context used to view the evaluated expression. e.g. when evaluating the argument in f(m!(1)) the context of the call expression should be used. This is need so the const evaluator can see the m macro and marke the evaluation as non-local independant of what the macro expands to.