- Start Date: 2015-01-20
- RFC PR: [rust-lang/rfcs#640](https://github.com/rust-lang/rfcs/pull/640)
- Rust Issue: [rust-lang/rust#23083](https://github.com/rust-lang/rust/issues/23083)

## Summary

The `Debug` trait is intended to be implemented by every type and display
useful runtime information to help with debugging. This RFC proposes two
additions to the fmt API, one of which aids implementors of `Debug`, and one
which aids consumers of the output of `Debug`. Specifically, the `#` format
specifier modifier will cause `Debug` output to be "pretty printed", and some
utility builder types will be added to the `std::fmt` module to make it easier
to implement `Debug` manually.

## Motivation

### Pretty printing

The conventions for `Debug` format state that output should resemble Rust
struct syntax, without added line breaks. This can make output difficult to
read in the presence of complex and deeply nested structures:
```rust
HashMap { "foo": ComplexType { thing: Some(BufferedReader { reader: FileStream { path: "/home/sfackler/rust/README.md", mode: R }, buffer: 1013/65536 }), other_thing: 100 }, "bar": ComplexType { thing: Some(BufferedReader { reader: FileStream { path: "/tmp/foobar", mode: R }, buffer: 0/65536 }), other_thing: 0 } }
```
This can be made more readable by adding appropriate indentation:
```rust
HashMap {
    "foo": ComplexType {
        thing: Some(
            BufferedReader {
                reader: FileStream {
                    path: "/home/sfackler/rust/README.md",
                    mode: R
                },
                buffer: 1013/65536
            }
        ),
        other_thing: 100
    },
    "bar": ComplexType {
        thing: Some(
            BufferedReader {
                reader: FileStream {
                    path: "/tmp/foobar",
                    mode: R
                },
                buffer: 0/65536
            }
        ),
        other_thing: 0
    }
}
```
However, we wouldn't want this "pretty printed" version to be used by default,
since it's significantly more verbose.

### Helper types

For many Rust types, a Debug implementation can be automatically generated by
`#[derive(Debug)]`. However, many encapsulated types cannot use the
derived implementation. For example, the types in std::io::buffered all have
manual `Debug` impls.  They all maintain a byte buffer that is both extremely
large (64k by default) and full of uninitialized memory. Printing it in the
`Debug` impl would be a terrible idea. Instead, the implementation prints the
size of the buffer as well as how much data is in it at the moment:
https://github.com/rust-lang/rust/blob/0aec4db1c09574da2f30e3844de6d252d79d4939/src/libstd/io/buffered.rs#L48-L60

```rust
pub struct BufferedStream<S> {
    inner: BufferedReader<InternalBufferedWriter<S>>
}

impl<S> fmt::Debug for BufferedStream<S> where S: fmt::Debug {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        let reader = &self.inner;
        let writer = &self.inner.inner.0;
        write!(fmt, "BufferedStream {{ stream: {:?}, write_buffer: {}/{}, read_buffer: {}/{} }}",
               writer.inner,
               writer.pos, writer.buf.len(),
               reader.cap - reader.pos, reader.buf.len())
    }
}
```

A purely manual implementation is tedious to write and error prone. These
difficulties become even more pronounced with the introduction of the "pretty
printed" format described above. If `Debug` is too painful to manually
implement, developers of libraries will create poor implementations or omit
them entirely. Some simple structures to automatically create the correct
output format can significantly help ease these implementations:
```rust
impl<S> fmt::Debug for BufferedStream<S> where S: fmt::Debug {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        let reader = &self.inner;
        let writer = &self.inner.inner.0;
        fmt.debug_struct("BufferedStream")
            .field("stream", writer.inner)
            .field("write_buffer", &format_args!("{}/{}", writer.pos, writer.buf.len()))
            .field("read_buffer", &format_args!("{}/{}", reader.cap - reader.pos, reader.buf.len()))
            .finish()
    }
}
```

## Detailed design

### Pretty printing

The `#` modifier (e.g. `{:#?}`) will be interpreted by `Debug` implementations
as a request for "pretty printed" output:

* Non-compound output is unchanged from normal `Debug` output: e.g. `10`,
  `"hi"`, `None`.
* Array, set and map output is printed with one element per line, indented four
    spaces, and entries printed with the `#` modifier as well: e.g.
```rust
[
    "a",
    "b",
    "c"
]
```
```rust
HashSet {
    "a",
    "b",
    "c"
}
```
```rust
HashMap {
    "a": 1,
    "b": 2,
    "c": 3
}
```
* Struct and tuple struct output is printed with one field per line, indented
    four spaces, and fields printed with the `#` modifier as well: e.g.
```rust
Foo {
    field1: "hi",
    field2: 10,
    field3: false
}
```
```rust
Foo(
    "hi",
    10,
    false
)
```

In all cases, pretty printed and non-pretty printed output should differ *only*
in the addition of newlines and whitespace.

### Helper types

Types will be added to `std::fmt` corresponding to each of the common `Debug`
output formats. They will provide a builder-like API to create correctly
formatted output, respecting the `#` flag as needed. A full implementation can
be found at https://gist.github.com/sfackler/6d6610c5d9e271146d11. (Note that
there's a lot of almost-but-not-quite duplicated code in the various impls.
It can probably be cleaned up a bit). For convenience, methods will be added
to `Formatter` which create them. An example of use of the `debug_struct`
method is shown in the Motivation section. In addition, the `padded` method
returns a type implementing `fmt::Writer` that pads input passed to it. This
is used inside of the other builders, but is provided here for use by `Debug`
implementations that require formats not provided with the other helpers.
```rust
impl Formatter {
    pub fn debug_struct<'a>(&'a mut self, name: &str) -> DebugStruct<'a> { ... }
    pub fn debug_tuple<'a>(&'a mut self, name: &str) -> DebugTuple<'a> { ... }
    pub fn debug_set<'a>(&'a mut self, name: &str) -> DebugSet<'a> { ... }
    pub fn debug_map<'a>(&'a mut self, name: &str) -> DebugMap<'a> { ... }

    pub fn padded<'a>(&'a mut self) -> PaddedWriter<'a> { ... }
}
```

## Drawbacks

The use of the `#` modifier adds complexity to `Debug` implementations.

The builder types are adding extra `#[stable]` surface area to the standard
library that will have to be maintained.

## Alternatives

We could take the helper structs alone without the pretty printing format.
They're still useful even if a library author doesn't have to worry about the
second format.

## Unresolved questions

The indentation level is currently hardcoded to 4 spaces. We could allow that
to be configured as well by using the width or precision specifiers, for
example, `{:2#?}` would pretty print with a 2-space indent. It's not totally
clear to me that this provides enough value to justify the extra complexity.
