Add safety comment to StableAddress impl for Mmap

compiler/rustc_data_structures/src/memmap.rs

@@ -40,4 +40,8 @@ impl Deref for Mmap {
     }
 }
 
+// SAFETY: On architectures other than WASM, mmap is used as backing storage. The address of this
+// memory map is stable. On WASM, `Vec<u8>` is used as backing storage. The `Mmap` type doesn't
+// export any function that can cause the `Vec` to be re-allocated. As such the address of the
+// bytes inside this `Vec` is stable.
 unsafe impl StableAddress for Mmap {}