I got a question how does this compile.

fn this_function<'a, 'b>(arg: &'a mut &'b u8) -> 
&'b mut &'a u8
{
    let a: &'a mut &'a u8 = arg;
    a
}

Is there some implicit lifetime bound, why able to “swap” 2 lifetimes
Also if we introduce 2 new lifetimes

fn functionthing<'a, 'b, 'c, 'd>(arg: &'a mut &'b u8) -> 
&'b mut &'a u8
where
'b: 'c, 'c: 'b,
'b: 'd, 'd: 'b,
{
    let a: &'d mut &'c u8 = arg;
    a
}

but at the bounds you can replace the bounds to

‘b: ‘c, ‘c: ‘a,
‘b: ‘c, ‘c: ‘b

No compile error so extrapolating to (and changing &’c … to &’d …)

fn functionthing<'a, 'b, 'c, 'd>(arg: &'a mut &'b u8) -> 
&'b mut &'a u8
where
/*’b or ‘a*/: 'c, 'c: /*’b or ‘a*/,
/*’b or ‘a*/: 'd, 'd: /*’b or ‘a*/,
{
    let a: &/*’c or ‘d*/ mut &/*’c or’d*/u8 = arg;
    a
}

I tested some combination in rust playground but why does this work when compiling,
and does changing bounds, change the usability of this function

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