This is an internal improvement to the solver so now when a dependency can't be solved it will give the reason (via an extra :reason part after the usual missed). For example, in the output of alr with --solve:

Dependencies (external):
   hello(=1.0) & (=1.0.1) (direct,missed:conflict)
   libhello^777 (direct,missed:unavailable)
   unobtanium* (direct,missed:unindexed)
Dependencies (graph):
   some_dep=0.1.0-dev --> hello=1.0.1 
   xxx=0.1.0-dev      --> hello=1.0    
   xxx=0.1.0-dev      --> libhello^777
   xxx=0.1.0-dev      --> unobtanium*

• Unindexed: the crate doesn't exist at all in our indexes
• Unavailable: the requested version does not exist (but the crate does)
• Conflict: two dependencies on the same crate cannot be reconciled. This can be further diagnosed in the graph portion of the solution (no change on the info shown there).