Jemalloc added over a megabyte to every project for only questionable gains, and it was awkward and unwieldy to remove it. While there are good reasons to use a different allocator depending on the project, Rust defaulting to this type of behavior failed a certain personal litmus test on what it wanted to be as a language in that it felt like it was fighting the system rather than integrating with it.

It also does not give a good first impression at all to newcomers to see their hello world project built in release mode take up almost 2MiB of space. Today it's a much more (subjectively) tolerable 136kiB on Windows (considering that Rust std is statically linked).

Seems like overstepping bounds. I expect a language runtime to make use of system provided facilities wherever feasible. I also expect it to provide hooks for me to link in alternatives.

If nothing else doing otherwise is likely to cause compatibility issues for someone at some point. For example see all the problems Go had with DIY syscalls pretty much everywhere except for Linux.

There's a legitimate question of whether the kernel ABI or the libc API qualifies as the system provided facilities on a linux box. But that uncertainty only furthers my latter point about compatibility.

There are embedded Linux systems with a total image size of ~10MB (not image as in docker "image" but an actual disk image). It would be quite hard to justify the additional dependency.