Simulate OOM for the try_oom_error test

We can create the expected error manually, rather than trying to produce
a real one, so the error conversion test can run on all targets. Before,
it was only running on 64-bit and not miri.

In Fedora, we also found that s390x was not getting the expected error,
"successfully" allocating the huge size because it was optimizing the
real `malloc` call away. It's possible to counter that by looking at the
pointer in any way, like a debug print, but it's more robust to just
deal with errors directly, since this test is only about conversion.

library/std/src/io/tests.rs

@@ -811,13 +811,17 @@ fn read_to_end_error() {
 }
 
 #[test]
-// Miri does not support signalling OOM
-#[cfg_attr(miri, ignore)]
-// 64-bit only to be sure the allocator will fail fast on an impossible to satisfy size
-#[cfg(target_pointer_width = "64")]
 fn try_oom_error() {
-    let mut v = Vec::<u8>::new();
-    let reserve_err = v.try_reserve(isize::MAX as usize - 1).unwrap_err();
+    use alloc::alloc::Layout;
+    use alloc::collections::{TryReserveError, TryReserveErrorKind};
+
+    // We simulate a `Vec::try_reserve` error rather than attempting a huge size for real. This way
+    // we're not subject to the whims of optimization that might skip the actual allocation, and it
+    // also works for 32-bit targets and miri that might not OOM at all.
+    let layout = Layout::new::<u8>();
+    let kind = TryReserveErrorKind::AllocError { layout, non_exhaustive: () };
+    let reserve_err = TryReserveError::from(kind);
+
     let io_err = io::Error::from(reserve_err);
     assert_eq!(io::ErrorKind::OutOfMemory, io_err.kind());
 }