privacy: Fix type privacy holes when it doesn't cause too much breakage

[petrochenkov]

A subset of #146470.

Private types in RPITITs now report hard errors.
Private types in bounds of associated types still only report the private_bounds lint due to unacceptable amount of breakage (#146470 (comment)).

Closes #144139

[petrochenkov]

@bors try

[petrochenkov]

TODO: @craterbot check p=1 crates=https://crater-reports.s3.amazonaws.com/pr-146470-3/retry-regressed-list.txt

[rustbot]

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[rust-bors]

☀️ Try build successful (CI)
Build commit: fcfb0ff (fcfb0ffc7ed7fc631df1136063939ef648434735, parent: 7ad4e69ad585d8ff214f7b42d01f1959eda08f40)

[petrochenkov]

@craterbot check p=1 crates=https://crater-reports.s3.amazonaws.com/pr-146470-3/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-152543 created and queued.
🤖 Automatically detected try build fcfb0ff
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-152543 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-152543 is completed!
📊 27 regressed and 0 fixed (5514 total)
📊 267 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-152543/retry-regressed-list.txt ↩

[petrochenkov]

Crater results are basically the same as #146470 without the second commit (see e.g. #146470 (comment)).

[petrochenkov]

Previous FCP - #146470 (comment).

[traviscross]

We talked about this in the lang call last week. @petrochenkov, it'd be helpful to us to have a brief write-up of what's being proposed here and what's changed since our FCP in #146470 (comment).

[petrochenkov]

and what's changed since our FCP in #146470 (comment).

Nothing is changed since the FCP in #146470 (comment).

[petrochenkov]

The previous summary (#146470 (comment)) is still relevant, except user-written private bounds on associated types are not turned into an error due to the amount breakage.

pub trait PubTrait {
  fn foo1() -> impl PrivTrait; // error now
  fn foo2() -> impl PubTrait<PrivType>; // error now

  type A1: PrivTrait; // not error, too much breakage (but reports `private_bounds` lint)
  type A2: PubTrait<PrivType>; // not error, too much breakage (but reports `private_bounds` lint)
}