Pretty formating

[SimonSapin]

It would be helpful for debugging to have a string formatting "style" that is for "{:?}" what extra::json::Json::to_pretty_str is to extra::json::Json::to_str. That is, it does the same (uses introspection to dump the content of various data structures) but adds newlines and indentation as appropriate for readability.

This pretty formatting would be used by assert_eq!().

For example, I am writing tests like this for Selector parsing in Servo:

    assert_eq!(parse("e.foo #bar"), Some(~[Selector{
        compound_selectors: CompoundSelector {
            simple_selectors: ~[LocalNameSelector(~"e"),
                                ClassSelector(~"foo")],
            next: Some((~CompoundSelector {
                simple_selectors: ~[IDSelector(~"baz")],
                next: None,
            }, Descendant)),
        },
        pseudo_element: None,
        specificity: specificity(1, 1, 1),
    }]))

At the moment, a failing test gives this output:

task '<unnamed>' failed at 'assertion failed: `(left == right) && (right ==
 left)` (left: `Some(~[selectors::Selector{compound_selectors: selectors::C
ompoundSelector{simple_selectors: ~[IDSelector(~"bar")], next: Some((~selec
tors::CompoundSelector{simple_selectors: ~[LocalNameSelector(~"e"), ClassSe
lector(~"foo")], next: None}, Descendant))}, pseudo_element: None, specific
ity: 1049601u32}])`, right: `Some(~[selectors::Selector{compound_selectors:
 selectors::CompoundSelector{simple_selectors: ~[LocalNameSelector(~"e"), C
lassSelector(~"foo")], next: Some((~selectors::CompoundSelector{simple_sele
ctors: ~[IDSelector(~"bar")], next: None}, Descendant))}, pseudo_element: N
one, specificity: 1049601u32}])`)', /home/simon/projects/servo/src/componen
ts/style/selectors.rs:550

Not exactly easy to read. Ideally, the output should be closer to this:

task '<unnamed>' failed at 'assertion failed: `(left == right) && (right == left)`
left: `Some(~[
    selectors::Selector{
        compound_selectors: selectors::CompoundSelector{
            simple_selectors: ~[
                IDSelector(~"bar")
            ],
            next: Some((
                ~selectors::CompoundSelector{
                    simple_selectors: ~[
                        LocalNameSelector(~"e"),
                        ClassSelector(~"foo")
                    ],
                    next: None
                },
                Descendant
            ))
        },
        pseudo_element: None,
        specificity: 1049601u32
    }
])`,
right: `Some(~[
    selectors::Selector{
        compound_selectors: selectors::CompoundSelector{
            simple_selectors: ~[
                LocalNameSelector(~"e"),
                ClassSelector(~"foo")
            ],
            next: Some((
                ~selectors::CompoundSelector{
                    simple_selectors: ~[
                        IDSelector(~"bar")
                    ],
                    next: None
                },
                Descendant
            ))
        },
        pseudo_element: None,
        specificity: 1049601u32
    }
])`'
/home/simon/projects/servo/src/components/style/selectors.rs:550

See also Python’s pprint module: http://docs.python.org/3/library/pprint.html
http://hg.python.org/cpython/file/3.3/Lib/pprint.py

[klutzy]

Reflection is going to removed. What could be done for this?

[sfackler]

Seems reasonable to leave it open, as we'd presumably want the same thing for #[deriving(Show)] or whatever new thing is added for debug output.

[SimonSapin]

This issue will soon be one year old. Meanwhile, assert_eq! does not use {:?} anymore but relies on the compared type implementing Show. So… what @sfackler said.

[tomjakubowski]

Visiting after the fmt::Show/fmt::String split. Now that Show is exclusively for debugging output, uses {:?}, and should be derived and therefore have a predictable structure, a debugging Show pretty printer would be more useful than ever!

[SimonSapin]

Fixed in #23162 (RFC #640). Use with {:#?}.