[derivable_impls]: don't lint if default() call expr unsize-coerces to trait object

[y21]

Fixes #10158.

This fixes a FP where the derive-generated Default impl would have different behavior because of unsize coercion from Box<T> to Box<dyn Trait>:

struct S {
  x: Box<dyn std::fmt::Debug>
}
impl Default for S {
  fn default() -> Self {
    Self {
      x: Box::<()>::default()
     // ^~ Box<()> coerces to Box<dyn Debug>
     // #[derive(Default)] would call Box::<dyn Debug>::default()
    }
  }
}

(this intentionally only looks for trait objects dyn specifically, and not any unsize coercion, e.g. &[i32; 5] to &[i32], because that breaks existing tests and isn't actually problematic, as far as I can tell)

changelog: [derivable_impls]: don't lint if default() call expression unsize-coerces to trait object

[rustbot]

r? @llogiq

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[llogiq]

Thank you!

@bors r+

[bors]

📌 Commit 4795c91 has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit 4795c91 with merge ffe9525...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing ffe9525 to master...