GitHub - dtolnay/zmij: Fast floating point to string conversion · GitHub seems to only have performance measurements, not binary size or cache usage measurements. Looking at zmij/src/lib.rs at master · dtolnay/zmij · GitHub I would be surprised if it compiled to compact code. And in real world use cases that do other things apart from parse floats, less icache and dcache usage often wins.
I would like to see the comprehensive measurements you base your statements above on, as well as benchmarks in real world usage (rather than just microbenchmarks). Also, benchmarks for something like this should be performed across a few different types of systems/architectures to make sure it is not a major regression on any, including at least one soft-fp configuration (representing embedded / kernel development).
It seems variations of this come up every so often, so I'll also link back to my statement from one of the previous times: [algorithm] new float/double to string algorithm - #2 by Vorpal (as well as the conversion that follows after and then runs out into the sand)
Note: I'm not saying that the library is bad, just that the bar from std is high, especially for code that ends up in no-std. It needs to work for every use case then.