imp: make Arg::validator more flexible (v3)

[durka]

From #1088.

Makes the validator functions more flexible by changing the return type from Result<(), String> to Result<O, E> where O is anything and E is anything convertible to a String.

This allows, for example, using the same function for validating and parsing your argument.

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This change is 

[mention-bot]

@durka, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kbknapp to be a potential reviewer.

[durka]

I don't think it requires additional turbofishes but I didn't test very extensively.

[kbknapp]

Here's the test failure so you don't have to dig through the logs:

Running `/home/travis/build/kbknapp/clap-rs/target/debug/deps/env-a56ad90d3ded717c`
running 15 tests
test env_os ... ok
test env ... ok
test multiple_no_delimiter ... ok
test multiple_one ... ok
test no_env ... ok
test multiple_three ... ok
test not_possible_value ... ok
test opt_user_override ... ok
test positionals_user_override ... ok
test positionals ... ok
test possible_value ... ok
test validator ... ok
test validator_invalid ... ok
test validator_output ... FAILED
test with_default ... ok
failures:
---- validator_output stdout ----
	thread 'validator_output' panicked at 'called `Result::unwrap()` on an `Err` value: Error { message: "error: Invalid value for \'<arg>\': invalid digit found in string", kind: ValueValidation, info: None }', libcore/result.rs:945:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.
failures:
    validator_output
test result: FAILED. 14 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
error: test failed, to rerun pass '--test env'

[durka]

WTF, that test passes locally. I just copied it from the validator test.

     Running target/debug/deps/env-02b35348c36b6f4b

running 15 tests
test multiple_one ... ok
test env ... ok
test env_os ... ok
test multiple_no_delimiter ... ok
test multiple_three ... ok
test no_env ... ok
test not_possible_value ... ok
test positionals ... ok
test opt_user_override ... ok
test positionals_user_override ... ok
test possible_value ... ok
test validator ... ok
test validator_invalid ... ok
test validator_output ... ok
test with_default ... ok

test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

[kbknapp]

Yeah super strange, cause it passes locally for me too! I'll restart the travis build.

[kbknapp]

kevin@beefcake: ~/Projects/clap-rs 
➜ cargo test --test env -- validator_output
    Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
     Running target/debug/deps/env-c0e4ef4fb869aef2

running 1 test
test validator_output ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 14 filtered out

[durka]

Looks good now.

[kbknapp]

Thanks 👍