possible bug in vtable formation

[nikomatsakis]

While working on #112355 I think I found a bug. Given this program:

#![feature(rustc_attrs)]

#[rustc_dump_vtable]
pub trait What: Send + Sync {
    fn a(&self) {}
}

impl What for () {}

fn main() {
    (&() as &dyn What).a();
}

rustc outputs the following vtable layout:

 [
    MetadataDropInPlace,
    MetadataSize,
    MetadataAlign,
    TraitVPtr(<() as Sync>),
    Method(<() as What>::a),
]

Which is surprising, because TraitVPtr(<() as Sync>) is unnecessary — Send does not have any methods, so Sync vtable can be inlined...

I think we should be smarter here:

rust/compiler/rustc_trait_selection/src/traits/vtable.rs

Lines 143 to 144 in cb882fa

	// Other than the left-most path, vptr should be emitted for each trait.
	emit_vptr_on_new_entry = true;

(for the record, I'm pretty sure this happens in practice, I've found this with log::Log)

Originally posted by @WaffleLapkin in #65991 (comment)

[nikomatsakis]

cc @crlf0710

[crlf0710]

sorry been slow a little recently... This is a nice improvement.