Function is_received_selector 

fn is_received_selector<'hir>(
    gcx: Gcx<'hir>,
    hir: &'hir Hir<'hir>,
    expr: &Expr<'hir>,
) -> bool

Whether an expression is the accepting answer. Comparing the hook’s answer against anything else mints to a recipient that answered something the ERC721 receiver interface calls a refusal, so only an operand this reads and finds equal to the selector is credited: the literal, a conversion of it, a constant holding it, or a selector member resolving to the receiver hook itself. The member is resolved rather than matched by name: spelled on a same-name function of another shape, .selector is a different value, which a recipient answering it never accepted with. A value settled at deployment or at runtime, an immutable or a state variable, is unknown here and does not exempt.