Problem

After #2756 rustup will just hang if it's not able to determine the effective max ram of the system. The differences seem to be:

1. The fallback for when effective_limits::memory_limit changed from 500MB to 32MB.
2. Rustup gets stuck waiting for some memory to free up here:

   rustup/src/dist/component/package.rs

   Lines 402 to 409 in a1d5912

   	while !io_executor.buffer_available(size as usize) {
   	flush_ios::<tar::Entry<'_, R>, _>(
   	&mut *io_executor,
   	&mut directories,
   	None,
   	&full_path,
   	)?;
   	}

I encountered this running on an illumos machine.

Steps
On a machine not supported by effective_limits v0.5.2`:

$ curl https://sh.rustup.rs -sSf | sh -s -- --profile minimal --default-toolchain nightly

Possible Solution(s)

1. Choose a larger default than 32MB (note, 64MB is still not enough for --profile complete).
2. v0.5.3-alpha of the effective-limits crate supports illumos but hasn't been released yet. (New release rbtcollins/effective-limits.rs#15)

Notes

Output of rustup --version: 1.24.2 (755e2b07e 2021-05-12)