`type_complexity` fires on type that cannot be simplified

Summary

I have a type that is admittedly complicated, but due to its structure,
I believe that there is no way to factor it apart.

Lint Name

type_complexity

Reproducer

I tried this code:

pub struct Filters<'f, E> {
    pub widgets: Box<dyn for<'a> FnOnce(&'a Widget) -> Result<(), E> + 'f>,
    pub gadgets: Box<dyn for<'a> FnOnce(Gadget<'a>) -> Result<(), E> + 'f>,
}
pub struct Widget;
pub struct Gadget<'a>(&'a ());

I saw this happen:

warning: very complex type used. Consider factoring parts into `type` definitions
 --> src/lib.rs:2:18
  |
2 |     pub widgets: Box<dyn for<'a> FnOnce(&'a Widget) -> Result<(), E> + 'f>,
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_complexity
  = note: `#[warn(clippy::type_complexity)]` on by default

warning: very complex type used. Consider factoring parts into `type` definitions
 --> src/lib.rs:3:18
  |
3 |     pub gadgets: Box<dyn for<'a> FnOnce(Gadget<'a>) -> Result<(), E> + 'f>,
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#type_complexity

warning: `playground` (lib) generated 2 warnings

See playground link.

I expected to see this happen:

No warning, since the type cannot be simplified.

I would love to simplify these types. Ideally, I would write something like:

// does not compile!
pub type Filter<'f, E, T> = Box<dyn for<'a> FnOnce(T<'a>) -> Result<(), E> + 'f>;

pub struct Filters<'f, E> {
    pub widgets: Filter<'f, E, &_ Widget>, // made-up syntax!
    pub gadgets: Filter<'f, E, Gadget<_>>, // made-up syntax!
}
pub struct Widget;
pub struct Gadget<'a>(&'a ());

But of course this does not work. I don't see a way to break down any
meaningful part of this type expression. (Even breaking out the type parameter
to Box<_> would require trait aliases, which aren't available on stable.)

Clippy shouldn't prompt me to improve code along an axis that admits no further
improvements.

Version

rustc 1.70.0 (90c541806 2023-05-31)
binary: rustc
commit-hash: 90c541806f23a127002de5b4038be731ba1458ca
commit-date: 2023-05-31
host: x86_64-unknown-linux-gnu
release: 1.70.0
LLVM version: 16.0.2

Additional Labels

No response

Start at the type_complexity lint entry point and reproduce the report with the Rust snippet in the issue. Add a regression case covering the two higher-ranked FnOnce fields; the work is done when the lint no longer recommends factoring types that cannot be meaningfully simplified.