Why do non-lexical lifetimes not work in this case?

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7

Something that this other thread made me realise:

This can be avoided with the unsafe (and unstable) #[may_dangle] attribute:

#![feature(dropck_eyepatch)]

pub
struct Foo<'a, T> /* = */ (&'a mut T);

unsafe impl<#[may_dangle] 'a, T : 'a>
    Drop for Foo<'a, T>
{
    fn drop (self: &'_ mut Self)
    {
        // this does not dereference self.0 (it just prints the raw address of the pointee), so it is fine
        println!("Dropping Foo({:p})", self.0);
    } 
}

fn main ()
{
    let mut x = 5;
    println!("x at {:p}", &x);

    let foo = Foo(&mut x);
    println!("Created Foo(&mut x = {:p})", foo.0);

    println!("Accessing x directly: {}", x);
}

Which outputs:

x at 0x7fff656e20d4
Created Foo(&mut x = 0x7fff656e20d4)
Accessing x directly: 5
Dropping Foo(0x7fff656e20d4)