Auto merge of #2489 - RalfJung:srw-merging, r=saethlin

add test that we do not merge neighboring SRW

Turns out that interior_mut2 also already tests this, but that also involves `UnsafeCell` so the new test still seems more clear. Basically the new test is the same as the old except that it uses raw pointers rather than `&UnsafeCell`. (When the old test was written, raw pointers were still untagged, so no such test would have been possible.)

I verified that both of these fail when we remove mutable references rather than disabling them.
Here is the patch I used for that:

<details>

```diff
diff --git a/Cargo.toml b/Cargo.toml
index 208b3a76..f9d1b0ac 100644
--- a/Cargo.toml
+++ b/Cargo.toml
`@@` -53,7 +53,7 `@@` name = "compiletest"
 harness = false

 [features]
-default = ["stack-cache"]
+default = []
 stack-cache = []

 # Be aware that this file is inside a workspace when used via the
diff --git a/src/lib.rs b/src/lib.rs
index ba337f28..2a3066f4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
`@@` -9,6 +9,7 `@@`
 #![feature(is_some_with)]
 #![feature(nonzero_ops)]
 #![feature(local_key_cell_methods)]
+#![feature(drain_filter)]
 // Configure clippy and other lints
 #![allow(
     clippy::collapsible_else_if,
diff --git a/src/stacked_borrows/stack.rs b/src/stacked_borrows/stack.rs
index 4a9a13d3..37246df7 100644
--- a/src/stacked_borrows/stack.rs
+++ b/src/stacked_borrows/stack.rs
`@@` -351,6 +351,9 `@@` impl<'tcx> Stack {
         #[cfg(all(feature = "stack-cache", debug_assertions))]
         self.verify_cache_consistency();

+        // HACK -- now just delete all disabled things.
+        self.borrows.drain_filter(|b| matches!(b.perm(), Permission::Disabled));
+
         Ok(())
     }
```

</details>

r? `@saethlin`

0 files changed, 0 insertions, 0 deletions