In uninit checking, add fallback for polymorphic types

[Noratrieb]

After #10520, we always assumed that polymorphic types do not allow to be left uninitialized. But we can do better, by peeking into polymorphic types and adding a few special cases for going through tuples, arrays (because the length may be polymorphic) and blanket allowing all unions (like MaybeUninit).

fixes #10551

changelog: [uninit_vec]: fix false positive for polymorphic types
changelog: [uninit_assumed_init]: fix false positive for polymorphic types

[rustbot]

r? @xFrednet

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[Alexendoo]

Would be good to add a test for #10565 as well

[Alexendoo]

Thanks for the follow up, looks good to me

@bors r+

[bors]

📌 Commit 51b4d2a has been approved by Alexendoo

It is now in the queue for this repository.

[bors]

⌛ Testing commit 51b4d2a with merge c5011e9...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Alexendoo
Pushing c5011e9 to master...

[pickfire]

I believe this still does not fully fix the issue, monoio still get false positive. https://github.com/bytedance/monoio/actions/runs/4635132256/jobs/8205680770?pr=157

struct Page<T> {
    // continued buffer of fixed size
    slots: Box<[MaybeUninit<Entry<T>>]>,
}

impl<T> Page<T> {
    fn new(size: usize, prev_len: usize) -> Self {
        let mut buffer = Vec::with_capacity(size);
        unsafe { buffer.set_len(size) };
        let slots = buffer.into_boxed_slice();
        Self {
            slots,
        }
    }
}

I am using clippy 0.1.70 (23ee2af 2023-04-07).

[Alexendoo]

The fix is not yet available on nightly, it will be in the one after rust-lang/rust#110003 is merged

I'll add a beta-nominated label to ensure it's in the same release as #10520, not sure how that works out timing wise

[flip1995]

This will get into the same beta/stable version as #10520. No action required AFAICT.