jemalloc is supposed to be enabled on macos for rustc, but with all the testing I've done, it does not appear to work.

Noted by Thom Chiovoloni on Zulip here, the jemalloc-sys crate prevents using unprefixed symbols here, which IIUC, means that it is building with symbols like __rjem_malloc, which AFAIK are never actually used.

If it indeed does not work, then I think it would be good to consider either looking into fixing it (which I suspect may not be easy), or stop building it on CI.

Tested with most recent nightly (rustc 1.52.0-nightly (3e826bb11 2021-02-21)) and master (a15f484).