clap-rs/clap v2.21.1 on GitHub

ArgRequiredElseHelp: fixes the precedence of this error to prioritize over other error messages (74b751ff, closes #895)
Positionals: fixes some regression bugs resulting from old asserts in debug mode. (9a3bc98e, closes #896)

v2.21.0 (2017-03-09)

doesn't run arg_post_processing on multiple values anymore (ec516182)
changes internal use of VecMap to Vec for matched values of Args (22bf137a)
vastly reduces the amount of cloning when adding non-global args minus when they're added from App::args which is forced to clone (8da0303b)
refactor to remove unneeded vectors and allocations and checks for significant performance increases (0efa4119)

when AppSettings::SubcommandsNegateReqs and ArgsNegateSubcommands are used, a new more accurate double line usage string is shown (50f02300, closes #871)

Arg::last: adds the ability to mark a positional argument as 'last' which means it should be used with -- syntax and can be accessed early (6a7aea90, closes #888)
provides default_value_os and default_value_if[s]_os (0f2a3782, closes #849)
provides App::help_message and App::version_message which allows one to override the auto-generated help/version flag associated help (389c413, closes #889)

InferSubcommands: adds a setting to allow one to infer shortened subcommands or aliases (i.e. for subcommmand "test", "t", "te", or "tes" would be allowed assuming no other ambiguities) (11602032, closes #863)

doesn't print the argument sections in the help message if all args in that section are hidden (ce5ee5f5)
doesn't include the various [ARGS] [FLAGS] or [OPTIONS] if the only ones available are hidden (7b4000af, closes #882)
now correctly shows subcommand as required in the usage string when AppSettings::SubcommandRequiredElseHelp is used (8f0884c1)
fixes some memory leaks when an error is detected and clap exits (8c2dd287)
fixes a trait that's marked private accidentlly, but should be crate internal public (1ae21108)
Completions: fixes a bug that tried to propogate global args multiple times when generating multiple completion scripts (5e9b9cf4, closes #846)

Options: adds the ability to require the equals syntax with options --opt=val (f002693d, closes #833)