> At no point is it invalidated, and the new arena will now start allocating new stuff from the start of the chunk. Right? And my old pointer still works, and the data inside it is at some point overwritten.
It doesn't allocate at the start of the chunk, it just picks up wherever the last one left off. That allocated memory in the chunk from previous arena allocations is not reused until the chunk as a whole is unmapped and the GC can confirm that no more pointers point into that chunk's address space (if you leave a dangling pointer, you only waste address space). This points-into property is cheap to check, it's equivalent to whether the chunk has been marked by the GC.
Again, I think that MSAN/ASAN should probably just be more strict with these kinds of use-after-frees. You won't crash, but it's still technically incorrect. (Not much can be done about the "GC is in the mark phase" case, unfortunately. Otherwise MSAN/ASAN will complain when the GC inevitably tries to access a pointer into a delayed chunk.)
It doesn't allocate at the start of the chunk, it just picks up wherever the last one left off. That allocated memory in the chunk from previous arena allocations is not reused until the chunk as a whole is unmapped and the GC can confirm that no more pointers point into that chunk's address space (if you leave a dangling pointer, you only waste address space). This points-into property is cheap to check, it's equivalent to whether the chunk has been marked by the GC.
Again, I think that MSAN/ASAN should probably just be more strict with these kinds of use-after-frees. You won't crash, but it's still technically incorrect. (Not much can be done about the "GC is in the mark phase" case, unfortunately. Otherwise MSAN/ASAN will complain when the GC inevitably tries to access a pointer into a delayed chunk.)