Why "The 'de lifetime should not appear in the type to which the Deserialize impl applies"?

[grelative]

Serde's documentation about lifetime mentioned this:

The 'de lifetime should not appear in the type to which the Deserialize impl applies.

 // Do not do this. Sooner or later you will be sad.
 impl<'de> Deserialize<'de> for Q<'de> {

 // Do this instead.
 impl<'de: 'a, 'a> Deserialize<'de> for Q<'a> {

But didn't give a explanation, I'm curious about what's the drawback of using 'de as the lifetime of type which Deserialize impl applies.

[Mingun]

Probably https://users.rust-lang.org/t/how-to-end-borrow-in-this-code/72719/3?u=mingun can give you an answer:

...

// `&'static [u8]` in particular implements
// `for<'any> BufferedInput<'any, ()>`
impl<'long: 'short, 'short> BufferedInput<'short, ()> for &'long [u8] {
    fn read(&mut self, _buf: ()) -> &'short [u8] {
        self
    }
}

... You may wonder why we had to spell this out, and the answer is that lifetimes in trait parameters are invariant -- they can't automatically shrink, like you're used to with &[u8], say. The above change makes the implementation covariant by breaking up the lifetime of the implemented trait ('short) and the lifetime of the implementer ('long) -- every concrete &'long [u8] gets an implementation for every 'short that is shorter. ...

But I totally agree that such an explanation should be placed in the documentation.

[grelative]

impl<'long: 'short, 'short> BufferedInput<'short, ()> for &'long [u8] {
    fn read(&mut self, _buf: ()) -> &'short [u8] {
        self
    }
}

Yeah, this imp of BufferedInput has the benefit of satisfying the HRTB for<'any> &'static [u8]: BufferedInput<'any, ()>.
But it does not apply to Deserialize, there is nothing can represent a lifetime smaller than anyone, so we can not form a HRTB for<'any> Q<'???>: Deserialize<'any> that the following impl will satisfy.

 impl<'de: 'a, 'a> Deserialize<'de> for Q<'a> {}