`dead_code` warning when private trait is only used as type bound in another trait

[eduardosm]

Code

trait UInt: Copy + From<u8> {}

impl UInt for u16 {}

trait Int: Copy {
    type Unsigned: UInt;

    fn as_unsigned(self) -> Self::Unsigned;
}

impl Int for i16 {
    type Unsigned = u16;

    fn as_unsigned(self) -> u16 {
        self as _
    }
}

fn priv_func<T: Int>(x: u8, y: T) -> (T::Unsigned, T::Unsigned) {
    (T::Unsigned::from(x), y.as_unsigned())
}

pub fn pub_func(x: u8, y: i16) -> (u16, u16) {
    priv_func(x, y)
}

Current output

warning: trait `UInt` is never used
 --> src/lib.rs:1:7
  |
1 | trait UInt: Copy + From<u8> {}
  |       ^^^^
  |
  = note: `#[warn(dead_code)]` on by default

Desired output

<empty>

Rationale and extra context

The false positive happens when a private trait is only used as type bound in another trait.

Other cases

No response

Rust Version

rustc 1.80.1 (3f5fd8dd4 2024-08-06)
binary: rustc
commit-hash: 3f5fd8dd41153bc5fdca9427e9e05be2c767ba23
commit-date: 2024-08-06
host: x86_64-unknown-linux-gnu
release: 1.80.1
LLVM version: 18.1.7

Anything else?

No response

[compiler-errors]

This uncovers a regression from 1.78 in #118257, since we backported recent changes to dead code analysis. cc @mu001999