Function collect_reads 

fn collect_reads<'hir>(
    ctx: &LintContext<'_, '_>,
    hir: &'hir Hir<'hir>,
    expr: &'hir Expr<'hir>,
    pending: &mut HashMap<VariableId, Span>,
)

Remove any state variable mentioned in expr from pending (it was read). For nested assignments, delegates to process_expr so writes are handled correctly.