Don't document impossible to call default trait items on impls

[compiler-errors]

Closes #100176

This only skips documenting default trait items on impls, not ones that are written inside the impl block. This is a conservative approach, since I think we should document all items written in an impl block (I guess unless hidden or whatever), but the existence of this new query I added makes this easy to extend to other rustdoc cases.

[rust-highfive]

r? @lcnr

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[compiler-errors]

cc @notriddle, would appreciate a review on the rustdoc part!

[lcnr]

the impl looks good, leaving the final review to rustdoc

r? rustdoc

[notriddle]

The code seems fine to me, but I’d like confirmation from at least one other rustdoc team member that this is the right solution at all. That we actually want to hide these methods, instead of doing something else, like slapping a label on them.

[camelid]

This seems like the right approach to me. From the user's perspective, it's as if the method doesn't exist for that type. I think still displaying the method but adding a label would be more confusing.

[camelid]

@bors r=lcnr,notriddle,camelid

[bors]

📌 Commit b3b23aa has been approved by lcnr,notriddle,camelid

It is now in the queue for this repository.

[camelid]

This only skips documenting default trait items on impls, not ones that are written inside the impl block.

Inherent methods with impossible bounds (e.g. Self: Sized) seem to be disallowed by rustc:

pub struct Bar([u8]);

impl Bar {
    fn needs_sized(&self) where Self: Sized {}
}

error[[E0277]](https://doc.rust-lang.org/stable/error-index.html#E0277): the size for values of type `[u8]` cannot be known at compilation time
 --> src/lib.rs:4:33
  |
4 |     fn needs_sized(&self) where Self: Sized {}
  |                                 ^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: within `Bar`, the trait `Sized` is not implemented for `[u8]`
note: required because it appears within the type `Bar`
 --> src/lib.rs:1:12
  |
1 | pub struct Bar([u8]);
  |            ^^^
  = help: see issue #48214

[compiler-errors]

@camelid: I actually meant trait items provided by an impl explicitly, like:

trait Foo {
  fn needs_sized() where Self: Sized;
}

struct Unsized([u8]);

impl Foo for Unsized {
  fn needs_sized() where Self: Sized {}
}

But that's also impossible to write currently, lol

error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
 --> src/lib.rs:8:26
  |
8 |   fn needs_sized() where Self: Sized {}
  |                          ^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: within `Unsized`, the trait `Sized` is not implemented for `[u8]`
note: required because it appears within the type `Unsized`
 --> src/lib.rs:5:8
  |
5 | struct Unsized([u8]);
  |        ^^^^^^^
  = help: see issue #48214
  = help: [add `#![feature(trivial_bounds)]`](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#) to the crate attributes to enable