Provide an impl error::Error for FromPlantumlError

[drmoose]

This request adds std::fmt::Display and std::error::Error boilerplate to FromPlantumlError, making it compatible with anyhow and the ? operator.

[maksugr]

Hi, @drmoose! Sorry for not noticing the PR for so long, I’ll take a look at it in the next few hours. If you don’t mind me asking, one quick question: what are you using this package for, if it’s not a secret?

[drmoose]

It's not a secret, but it's also not very interesting. Gitlab wiki supports inline plantuml, and I was editing a large wiki page on my company's intranet that had a lot of diagrams in it. Because I wasn't very familiar with the syntax, I wanted a way to quickly preview all my diagrams, so I wrote a bash script that extracted all the ```plantuml blocks and rendered the diagrams to a browser.

Originally, I was using the official plantuml jar to encode URLs:

time perl -ne '
  if ($f) {
    if (/^```/) { 
      $f=0; 
      $ENV{uml}=$n; 
      @argv=("zsh", "-c", "java -jar plantuml.jar -encodeurl =(echo \"\$uml\")");
      system(@argv) 
    } else { 
      $n .= $_ 
    } 
  } elsif (/^```plantuml/) {
     $f = 1 
  }
' < notes.md

But, java -jar has a very long startup time, so this took perl -ne < notes.md 6.18s user per render. A 6 sec cycle time may not seem like much, but I don't know the plantuml syntax that well and was guessing one byte at a time, so I went looking for something faster, found your library, and made it into a standalone program called planturl.

fn main() -> anyhow::Result<()> {
	let mut had_arg = false;
	for argument in env::args().skip(1) {
		had_arg = true;
		if argument == "-" {
			encode_stdin()?;
		} else {
			let encoded = encode_plantuml_deflate(argument)?;
			println!("{}", encoded);
		}
	}
	if had_arg { Ok(()) } else { encode_stdin() }
}

fn encode_stdin() -> anyhow::Result<()> {
	let mut stdin = io::stdin();
	let mut buf = String::new();
	stdin.read_to_string(&mut buf)?;
	let encoded = encode_plantuml_deflate(buf)?;
	println!("{}", encoded);

	Ok(())
}

With this in my $PATH, I replaced my perl script above with this one

time perl -ne '
  if ($f) {
    if (/^```/) { 
      $f=0; 
      @argv=("planturl", "$n");
      system(@argv) 
    } else { 
      $n .= $_ 
    } 
  } elsif (/^```plantuml/) {
     $f = 1 
  }
' < notes.md

And got a glorious perl -ne < notes.md 0.01s user, which let me have a per-keystroke live preview of the diagrams in my document, which made it possible for me to guess the correct syntax :)