They're released on a schedule, not by features. They're useful to be able to refer to periods of time in Rust's rapidly changing lifetime, but not much beyond that.
Specifically, maturity #5 - production ready: https://github.com/mozilla/rust/issues?milestone=16&state=op...
> The purpose of this milestone is to represent a high degree of confidence the
> language's suitability for industrial use due to a high level of measured
> correctness and performance in the tests.
> In other words, for each of the aspects described in the "well covered"
> milestone, and for each supported target platform, test success and benchmark
> performance has reached a level deemed suitable for production use in
> environments with substantial business consequences for major defects or low
> performance. Where possible, performance benchmarks are linked to equivalent
> benchmarks in competitive languages, and rust is sufficiently close to
> equivalent in performance. > maturity #2 - backwards compatible
> we are comfortable making a long-term support commitment to downstream
> users (servo in particular) to keep the set of symbols, definitions
> and passing-tests from contractingNot a critique by any means--I admire the language and the work being done. More of a wish that the core language syntax would settle down soon.
That doesn't mean that your sentiment is wrong; if you don't want to be keeping up with the langauge's changes, certainly don't write projects in Rust. That said, there are more libraries than you'd expect, including a few that are several thousands of lines.
Yeah, only the entire for loop syntax changed. ;)
In this regard it couldn't be a better time to try it out.
I wouldn't devote a crucial project, school assignment, or startup to rust. But for some toy side projects, or small programs, it's not so bad right now.
It is of course not mandatory, and won't really be used to distribute binary versions of Rust programs, just the source.