Essentially there are a few popular Rust libraries that add functions in patch versions (or have done so in the past). This makes it necessary for users of that library to include the patch number when specifying the minimal version that they need because they can't assume an earlier patch will include all the functions they use.

The original forum poster said such libraries were in "violation" of the semver.org spec. This blog post disagrees on using the term "violation" because this is a minor issue (no pun intended... or was it).