destructure `Self` in generated `Debug` impls.

[programmerjake]

Generally when I write a manual Debug implementation, I almost always destructure self so I know to update the implementation when I add/remove/rename fields. I think when rust-analyzer generates a manual Debug implementation, it should always do that or at least have an option to make it always do that.

This came up on Zulip here.

So, to be clear, I don't want it to destructure self in the function argument if that becomes a feature, instead it should generate code like:

struct MyStruct<T: ?Sized> {
    field1: Foo,
    field2: Bar,
    field3: Baz,
    field4: T,
}

impl<T: ?Sized + fmt::Debug> fmt::Debug for MyStruct<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let Self {
            field1,
            field2,
            field3,
            field4,
        } = self;
        f.debug_struct("MyStruct")
            .field("field1", field1)
            .field("field2", field2)
            .field("field3", field3)
            .field("field4", &field4) // reference needed since T: ?Sized
            .finish()
    }
}

[programmerjake]

This should probably also be done for other generated impls for PartialEq, PartialOrd, Ord, and Hash. It's not necessary for Default or Clone since the return value is Self { ... } so already lists the fields so will error when those fields change.

[Veykril]

I think this is a classic case of feature discovery issues because we have an assist on the self identifier in the parameter position to do a struct destructure which would turn

impl fmt::Debug for Chunk {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Chunk")
            .field("distance", &self.distance)
            .field("material", &self.material)
            .field("dirty", &self.dirty)
            .finish()
    }
}

into

impl fmt::Debug for Chunk {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let Chunk { distance, material, dirty } = self;
        f.debug_struct("Chunk")
            .field("distance", distance)
            .field("material", material)
            .field("dirty", dirty)
            .finish()
    }
}

Which imo already solves this problem, I'd rather not add an assist specific knob here.