Rollup merge of #76677 - RalfJung:stable-pointers, r=jonas-schievink

note that test_stable_pointers does not reflect a stable guarantee

Just to be sure...

library/alloc/tests/vec.rs

@@ -1511,6 +1511,9 @@ fn test_stable_pointers() {
     // Test that, if we reserved enough space, adding and removing elements does not
     // invalidate references into the vector (such as `v0`).  This test also
     // runs in Miri, which would detect such problems.
+    // Note that this test does *not* constitute a stable guarantee that all these functions do not
+    // reallocate! Only what is explicitly documented at
+    // <https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html#guarantees> is stably guaranteed.
     let mut v = Vec::with_capacity(128);
     v.push(13);