What exactly are the guarantees that jemalloc's malloc provides in terms of alignment?

The docs say

The allocated space is suitably aligned (after possible pointer coercion) for storage of any type of object.

However, in rust-lang/rust#45955 we noticed that this is not correct: at least with GCC/clang extensions, one can define a type of size 8 that has alignment 16. However, jemalloc has been observed handing out allocations of size 8 that are just 8-aligned.

System allocator functions usually seem to guarantee that everything is at least 16-byte aligned on an x86-64 system -- at least that's what comments in the Rust source say, but I do not know where that information is coming from. However, jemalloc violates that expectation. It would be useful to know what exactly is guaranteed in terms of alignment for small allocations (including small non-power-of-2 allocations).

EDIT: This value of 16 seems to originate from https://www.gnu.org/software/libc/manual/html_node/Aligned-Memory-Blocks.html#Aligned-Memory-Blocks (Ctrl-F "sixteen"). Of course that is not normative for jemalloc, but it will catch applications by surprise when jemalloc is used as a drop-in replacement.