> To me, a #![no_std] library is useful for contexts where there isn't a platform libc (embedded systems, kernels, etc.) and you can't assume things like the existence of stdin or processes.
"Could be useful for" (FTFY).
Unfortunately, for the reasons mentioned above, most #![no_std] libraries aren't useful for that, because they link libc and other libraries.
Most people don't do this intentionally. The compiler doesn't complain about:
#![no_std]
extern "C" { fn malloc(...) -> ...; }
so when somebody does it accidentally, everything works and they get no feedback.
When you then go and build a #![no_std] binary, and run it in a platform when libc is not available, only then you get an error. And at that point, good luck figuring out which of your 200 dependencies has the error.
In particular, if you are running `#[test]`s, doing that links standard, so if your program implicitly depends on libc somewhere, tests won't reveal that, because while testing, libc will be linked.
