Consider separate lints for "hierarchical" unused lints (or some way to opt-out)

Description

Consider the example:

fn unused_foo() {
    used_bar_but_just_here();
}

fn used_bar_but_just_here() {}

In the past (I can't pinpoint when), cargo check would emit a "unused" warning for unused_foo, but not used_bar_but_just_here, like so:

warning: function `unused_foo` is never used
 --> src/main.rs:6:4
  |
6 | fn unused_foo() { used_bar_but_just_here(); }
  |    ^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

Nowadays, cargo check emits unused warnings for both:

warning: function `unused_foo` is never used
 --> src/main.rs:6:4
  |
6 | fn unused_foo() { used_bar_but_just_here(); }
  |    ^^^^^^^^^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: function `used_bar_but_just_here` is never used
 --> src/main.rs:8:4
  |
8 | fn used_bar_but_just_here() {}
  |    ^^^^^^^^^^^^^^^^^^^^^^

The new behavior is really disruptive and annoying, as it marks your entire method "tree" with unused instead of just the root, which litters the codebase with warnings during prototype stages. It also fails to pinpoint the "real culprit".

Another point: As a user, I expect that removing unused items shouldn't cause compiler errors, in the example, cargo check complains that used_bar_but_just_here is never used, yet removing it would cause a compiler error since unused_foo does use that method.

If I may suggest, one of these options would really be appreciated (I'm willing to make the PR myself if any of these is accepted):

• Revert to the old behavior (I personally can't think of a situation where I would miss the new behavior).
• Move the new behavior to a different lint (something like unused_chained?) or add that new lint to represent the old behavior.

Version

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7

Reproduce the reported example with cargo check using the rustc 1.86.0 details in the issue, then read the existing dead_code and unused-lint behavior. Compare the proposed revert, separate lint, and opt-out approaches; done means the desired hierarchical warning behavior is agreed and covered by appropriate compiler tests.