Improve panic message of autocfg::new

[cuviper]

Do you have an example of how this looks with a real error, before and after?

If the problem is just the verbosity of derive(Debug), we could implement that manually to look more like Display.

[taiki-e]

Do you have an example of how this looks with a real error, before and after?

Here is the current output and the output after this PR has been applied:

• ErrorKind::Io:

  Before:
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" }) }', autocfg/src/lib.rs:137:28
  After:
  thread 'main' panicked at 'No such file or directory (os error 2)', autocfg/src/lib.rs:146:47

• ErrorKind::Num:

  Before:
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: Num(ParseIntError { kind: InvalidDigit }) }', autocfg/src/lib.rs:137:28
  After:
  thread 'main' panicked at 'invalid digit found in string', autocfg/src/lib.rs:146:47

• ErrorKind::Utf8:

  Before:
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: Utf8(Utf8Error { valid_up_to: 1, error_len: Some(1) }) }', autocfg/src/lib.rs:137:28
  After:
  thread 'main' panicked at 'invalid utf-8 sequence of 1 bytes from index 1', autocfg/src/lib.rs:146:47

• ErrorKind::Other:

  Before:
  thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { kind: Other("could not find rustc release") }', autocfg/src/lib.rs:137:28
  After:
  thread 'main' panicked at 'could not find rustc release', autocfg/src/lib.rs:146:47

---

If the problem is just the verbosity of derive(Debug), we could implement that manually to look more like Display.

I think verbosity is the main problem, but I think the output of .unwrap (i.e., "called Result::unwrap() on an Err value:") is also verbose. Here is output if use fmt::Display::fmt in Debug impl:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: No such file or directory (os error 2)', autocfg/src/lib.rs:137:28
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: invalid digit found in string', autocfg/src/lib.rs:137:28
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: invalid utf-8 sequence of 1 bytes from index 1', autocfg/src/lib.rs:137:28
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: could not find rustc release', autocfg/src/lib.rs:137:28

[taiki-e]

Closing -- I no longer need this as I replaced all use of autocfg::new with autocfg::AutoCfg::new like https://github.com/crossbeam-rs/crossbeam/pull/604.