clippy doesn't properly model whether a type is allowed to be uninit

[Noratrieb]

Summary

The uninit_vec and uninit_assumed_init lints use a helper to check whether a type may be uninit.

rust-clippy/clippy_utils/src/ty.rs

Lines 541 to 548 in 3d193fa

	pub fn is_uninit_value_valid_for_ty(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
	match *ty.kind() {
	ty::Array(component, _) => is_uninit_value_valid_for_ty(cx, component),
	ty::Tuple(types) => types.iter().all(|ty| is_uninit_value_valid_for_ty(cx, ty)),
	ty::Adt(adt, _) => cx.tcx.lang_items().maybe_uninit() == Some(adt.did()),
	_ => false,
	}
	}

This helper is incorrect for newtypes around MaybeUninit<T>, ZSTs and unions in general.

This should be fixed by just deleting this helper function and using rustc's check_validity_of_init (the name may change in rust-lang/rust#108505). It doesn't currently support uninit checks but that should be really easy to add upstream (and needed for rust-lang/rust#100423 anyways).

Lint Name

uninit_vec,uninit_assumed_init

Reproducer

I tried this code:

use core::mem::MaybeUninit;

#[repr(transparent)]
struct Transparent<T>(MaybeUninit<T>);

pub fn foo<T>() {
    let mut vec = Vec::<Transparent<T>>::new();
    vec.reserve(20);
    unsafe { vec.set_len(20) };
}

I saw this happen:

error: calling `set_len()` immediately after reserving a buffer creates uninitialized values
 --> src/lib.rs:8:5
  |
8 |     vec.reserve(20);
  |     ^^^^^^^^^^^^^^^^
9 |     unsafe { vec.set_len(20) };
  |              ^^^^^^^^^^^^^^^
  |
  = help: initialize the buffer or wrap the content in `MaybeUninit`
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninit_vec
  = note: `#[deny(clippy::uninit_vec)]` on by default

I expected to see this happen: Everything is fine

Version

rustc 1.69.0-nightly (34e6673a0 2023-02-25)
binary: rustc
commit-hash: 34e6673a0473e90ef01a18eb575392c9e3859747
commit-date: 2023-02-25
host: x86_64-unknown-linux-gnu
release: 1.69.0-nightly
LLVM version: 15.0.7

Additional Labels

No response

[Noratrieb]

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