make UI test OS-agnostic

[japaric]

the internal representation of std::sync::Mutex depends on the compilation target. due to this, the compiler produces different number of errors for UI test issue-17431-6.rs depending on the compilation target.

for example, when compiling the UI test to an *-apple-* or *-qnx7* target, the "cycle detected" error is not reported

$ cat src/lib.rs
use std::sync::Mutex;

enum Foo {
    X(Mutex<Option<Foo>>),
}

impl Foo {
    fn bar(self) {}
}

fn main() {}

$ cargo check --target x86_64-apple-ios 2>&1 | rg '^error\['
error[E0072]: recursive type `Foo` has infinite size

whereas rustc produces two errors for other OSes, like Linux, which is what the UI test expects

$ cargo check --target x86_64-unknown-linux-gnu 2>&1 | rg '^error\['
error[E0072]: recursive type `Foo` has infinite size
error[E0391]: cycle detected when computing when `Foo` needs drop

this commit replaces the problematic Mutex with UnsafeCell, which has the same internal representation regardless of the compilation target. with that change, rustc reports two errors for all compilation targets.

$ cat src/lib.rs
use std::cell::UnsafeCell;

enum Foo {
    X(UnsafeCell<Option<Foo>>),
}

impl Foo {
    fn bar(self) {}
}

fn main() {}

$ cargo check --target x86_64-apple-ios 2>&1 | rg '^error\['
error[E0072]: recursive type `Foo` has infinite size
error[E0391]: cycle detected when computing when `Foo` needs drop

with this change, we can remove the ignore-apple directive as the UI test now also passes on apple targets.

[rustbot]

r? @cjgillot

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[Noratrieb]

hey, this even makes the test a lot better and easier to understand, nice!
@bors r+ rollup

[bors]

📌 Commit 7f02756 has been approved by Noratrieb

It is now in the queue for this repository.