`--emit metadata` produces less error messages with some targets

Hello. I was looking into a UI test failure with the aarch64-unknown-nto-qnx710 target. (but this is not a QNX specific issue as noted below)

The test is question is /tests/ui/structs-enums/enum-rec/issue-17431-6.rs. The contents of the file have been copied below for your convenience.

//@ ignore-apple: cycle error does not appear on apple

use std::sync::Mutex;

enum Foo { X(Mutex<Option<Foo>>)  }
//~^ ERROR recursive type `Foo` has infinite size
//~| ERROR cycle detected

impl Foo { fn bar(self) {}  }

fn main() {}

The UI test expects the compiler to produce TWO error messages (see ERROR comments in the code) but the compiler only produces ONE when the compilation target is aarch64-unknown-nto-qnx710.

NOTE The issue can also be reproduced without a QNX toolchain on a Linux host using the aarch64-apple-darwin target.

I have narrowed down the issue to compiletest's internal use of the --emit metadata flag. In other words, rustc --emit metadata --target aarch64-apple-darwin issue-17431-6.rs produces ONE error; whereas rustc --target aarch64-apple-darwin issue-17431-6.rs (no --emit flag) produces the TWO error messages that the UI test expects.

I'm not sure why --emit metadata triggers the issue but I suspect that Mutex having OS / platform specific implementations (i.e. cfg-style conditional compilation is involved) is part of the problem. After all, the error that's not reported is about a cycle being found when computing the layout of Foo; the error includes notes that list all the types beneath Foo: that is Mutex, UnsafeCell, etc.

A workaround that fixes the UI test (when target = QNX) is to replace the Mutex type with UnsafeCell, that is

use std::cell::UnsafeCell;

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

This change may let you drop the ignore-apple compiletest attribute from the test but I haven't tested this change with an apple target.

Again, I suspect that this modified version does not run into the problem because UnsafeCell does NOT have a target specific implementation / layout.

Meta

rustc --version --verbose:

rustc 1.82.0-nightly (636d7ff91 2024-08-19)
binary: rustc
commit-hash: 636d7ff91b9847d6d43c7bbe023568828f6e3246
commit-date: 2024-08-19
host: x86_64-unknown-linux-gnu
release: 1.82.0-nightly
LLVM version: 19.1.0

Start with tests/ui/structs-enums/enum-rec/issue-17431-6.rs and reproduce the target-specific behavior using the two rustc commands shown for aarch64-apple-darwin. Compare diagnostics with and without --emit metadata, including the Mutex and UnsafeCell variants. Done means the UI test consistently receives both expected errors for affected targets.