Why the borrow checker is ok with `self:&'a` for a struct Foo<'a>

struct Foo<'a> {s: &'a str}

impl<'a> Foo<'a> {
    fn foo(&'a self) -> &'a str {. // <---- this is the line in question
        self.s
    }
}

fn main() {
   let f: Foo<'static> = Foo{s:"123"} ;
   {
       let bf = &f;
       let s = bf.foo();
   }
}

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The function foo signature should be fn foo(&self) -> &str {. I don't understand why the borrow checker is ok with fn foo(&'a self) -> &'a str {.

My understanding of lifetime is that they are types (with subtyping and variance). Here self is of type Foo<'static>. foo should not accept any reference with a shorter lifetime than 'static, based on how the code is written. But why bf, a non-static-lifetime reference, can still call foo?

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