`multiple_supertrait_upcastable` lint is maybe overzealous for auto traits

I tried this code:

#![feature(multiple_supertrait_upcastable)]
#![warn(multiple_supertrait_upcastable)]

pub trait Foo: Send + Sync {}

I expected to see this happen: Send and Sync are auto traits, and thus do not (currently) take any extra space in the vtable for dyn Foo[^1], so the multiple_supertrait_upcastable lint should not fire. (similarly if Foo had one auto and one non-auto supertrait).

Instead, this happened: The multiple_supertrait_upcastable lint fires, since Foo has multiple supertraits (Send and Sync) (playground).

Tracking issue for multiple_supertrait_upcastable: #150833

This could be determined to not be a false positive, as the diagnostic is technically correct. However, the reason for the lint when it was introduced was:

To support upcasting with multiple supertraits, we need to store multiple vtables and
this can result in extra space overhead, even if no code actually uses upcasting. This
lint allows users to identify when such scenarios occur and to decide whether the
additional overhead is justified

Under that reasoning, I think Foo should not be linted on.

There could also be an argument for not linting on empty non-auto supertraits, but that is less convincing IMO.

pub trait Super1 {} // empty
pub trait Super2 {} // empty
pub trait Bar: Super1 + Super2 { /* ... */ }

Meta

rustc --version --verbose:

rustc 1.95.0-nightly (d940e5684 2026-01-19)
binary: rustc
commit-hash: d940e56841ddcc05671ead99290e35ff2e98369f
commit-date: 2026-01-19
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 21.1.8

(no backtrace)

[^1]: upcasting dyn Foo to dyn Send just re-uses the vtable since it has a common prefix of drop_in_place, size, align

Start by reproducing the multiple_supertrait_upcastable warning with the Foo: Send + Sync example from the issue, then read the lint's implementation and the tracking issue #150833. Done means reaching and testing a decision about whether combinations containing auto traits should avoid this warning, including the mixed auto/non-auto case.