Rollup merge of #133435 - RalfJung:test_downgrade_observe, r=tgross35

miri: disable test_downgrade_observe test on macOS

Due to #121950, this test can fail on Miri. The test is also quite slow on Miri (taking more than 30s) due to the high iteration count (a total of 2000), so let's reduce that a little.

Fixes #133421

library/std/src/sync/rwlock/tests.rs

@@ -511,12 +511,15 @@ fn test_downgrade_basic() {
 }
 
 #[test]
+// FIXME: On macOS we use a provenance-incorrect implementation and Miri catches that issue.
+// See <https://github.com/rust-lang/rust/issues/121950> for details.
+#[cfg_attr(all(miri, target_os = "macos"), ignore)]
 fn test_downgrade_observe() {
     // Taken from the test `test_rwlock_downgrade` from:
     // https://github.com/Amanieu/parking_lot/blob/master/src/rwlock.rs
 
     const W: usize = 20;
-    const N: usize = 100;
+    const N: usize = if cfg!(miri) { 40 } else { 100 };
 
     // This test spawns `W` writer threads, where each will increment a counter `N` times, ensuring
     // that the value they wrote has not changed after downgrading.