[Jtsummers]

SPARK 2012 was what made SPARK viable for a broader audience. Earlier versions of SPARK were not really properly integrated into Ada (SPARK annotations were in comments). SPARK also restricts what is expressible to what is provable within SPARK (which expands over time), this has some important consequences for concurrency and parallelism (using the "concurrency is not parallelism" distinction). So it would have been hard for Graydon Hoare to use SPARK 2012 in 2006 when he started on Rust, and the limitations around concurrency in SPARK 2012 would have been a non-starter for the objectives of Rust as adopted by Mozilla.