crates/prettyplease/README.md

prettyplease::unparse
=====================

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A minimal `syn` syntax tree pretty-printer.

<br>

## Overview

This is a pretty-printer to turn a `syn` syntax tree into a `String` of
well-formatted source code. In contrast to rustfmt, this library is intended to
be suitable for arbitrary generated code.

Rustfmt prioritizes high-quality output that is impeccable enough that you'd be
comfortable spending your career staring at its output &mdash; but that means
some heavyweight algorithms, and it has a tendency to bail out on code that is
hard to format (for example [rustfmt#3697], and there are dozens more issues
like it). That's not necessarily a big deal for human-generated code because
when code gets highly nested, the human will naturally be inclined to refactor
into more easily formattable code. But for generated code, having the formatter
just give up leaves it totally unreadable.

[rustfmt#3697]: https://github.com/rust-lang/rustfmt/issues/3697

This library is designed using the simplest possible algorithm and data
structures that can deliver about 95% of the quality of rustfmt-formatted
output. In my experience testing real-world code, approximately 97-98% of output
lines come out identical between rustfmt's formatting and this crate's. The rest
have slightly different linebreak decisions, but still clearly follow the
dominant modern Rust style.

The tradeoffs made by this crate are a good fit for generated code that you will
*not* spend your career staring at. For example, the output of `bindgen`, or the
output of `cargo-expand`. In those cases it's more important that the whole
thing be formattable without the formatter giving up, than that it be flawless.

<br>

## Feature matrix

Here are a few superficial comparisons of this crate against the AST
pretty-printer built into rustc, and rustfmt. The sections below go into more
detail comparing the output of each of these libraries.

| | prettyplease | rustc | rustfmt |
|:---|:---:|:---:|:---:|
| non-pathological behavior on big or generated code | �� | ❌ | ❌ |
| idiomatic modern formatting ("locally indistinguishable from rustfmt") | �� | ❌ | �� |
| throughput | 60 MB/s | 39 MB/s | 2.8 MB/s |
| number of dependencies | 3 | 72 | 66 |
| compile time including dependencies | 2.4 sec | 23.1 sec | 29.8 sec |
| buildable using a stable Rust compiler | �� | ❌ | ❌ |
| published to crates.io | �� | ❌ | ❌ |
| extensively configurable output | ❌ | ❌ | �� |
| intended to accommodate hand-maintained source code | ❌ | ❌ | �� |

<br>

## Comparison to rustfmt

- [input.rs](https://github.com/dtolnay/prettyplease/blob/0.1.0/examples/input.rs)
- [output.prettyplease.rs](https://github.com/dtolnay/prettyplease/blob/0.1.0/examples/output.prettyplease.rs)
- [output.rustfmt.rs](https://github.com/dtolnay/prettyplease/blob/0.1.0/examples/output.rustfmt.rs)

If you weren't told which output file is which, it would be practically
impossible to tell &mdash; **except** for line 435 in the rustfmt output, which
is more than 1000 characters long because rustfmt just gave up formatting that
part of the file:

```rust
            match segments[5] {
                0 => write!(f, "::{}", ipv4),
                0xffff => write!(f, "::ffff:{}", ipv4),
                _ => unreachable!(),
            }
        } else { # [derive (Copy , Clone , Default)] struct Span { start : usize , len : usize , } let zeroes = { let mut longest = Span :: default () ; let mut current = Span :: default () ; for (i , & segment) in segments . iter () . enumerate () { if segment == 0 { if current . len == 0 { current . start = i ; } current . len += 1 ; if current . len > longest . len { longest = current ; } } else { current = Span :: default () ; } } longest } ; # [doc = " Write a colon-separated part of the address"] # [inline] fn fmt_subslice (f : & mut fmt :: Formatter < '_ > , chunk : & [u16]) -> fmt :: Result { if let Some ((first , tail)) = chunk . split_first () { write ! (f , "{:x}" , first) ? ; for segment in tail { f . write_char (':') ? ; write ! (f , "{:x}" , segment) ? ; } } Ok (()) } if zeroes . len > 1 { fmt_subslice (f , & segments [.. zeroes . start]) ? ; f . write_str ("::") ? ; fmt_subslice (f , & segments [zeroes . start + zeroes . len ..]) } else { fmt_subslice (f , & segments) } }
    } else {
        const IPV6_BUF_LEN: usize = (4 * 8) + 7;
        let mut buf = [0u8; IPV6_BUF_LEN];
        let mut buf_slice = &mut buf[..];
```

This is a pretty typical manifestation of rustfmt bailing out in generated code
&mdash; a chunk of the input ends up on one line. The other manifestation is
that you're working on some code, running rustfmt on save like a conscientious
developer, but after a while notice it isn't doing anything. You introduce an
intentional formatting issue, like a stray indent or semicolon, and run rustfmt
to check your suspicion. Nope, it doesn't get cleaned up &mdash; rustfmt is just
not formatting the part of the file you are working on.

The prettyplease library is designed to have no pathological cases that force a
bail out; the entire input you give it will get formatted in some "good enough"
form.

Separately, rustfmt can be problematic to integrate into projects. It's written
using rustc's internal syntax tree, so it can't be built by a stable compiler.
Its releases are not regularly published to crates.io, so in Cargo builds you'd
need to depend on it as a git dependency, which precludes publishing your crate
to crates.io also. You can shell out to a `rustfmt` binary, but that'll be
whatever rustfmt version is installed on each developer's system (if any), which
can lead to spurious diffs in checked-in generated code formatted by different
versions. In contrast prettyplease is designed to be easy to pull in as a
library, and compiles fast.

<br>

## Comparison to rustc_ast_pretty

- [input.rs](https://github.com/dtolnay/prettyplease/blob/0.1.0/examples/input.rs)
- [output.prettyplease.rs](https://github.com/dtolnay/prettyplease/blob/0.1.0/examples/output.prettyplease.rs)
- [output.rustc.rs](https://github.com/dtolnay/prettyplease/blob/0.1.0/examples/output.rustc.rs)

This is the pretty-printer that gets used when rustc prints source code, such as
`rustc -Zunpretty=expanded`. It's used also by the standard library's
`stringify!` when stringifying an interpolated macro_rules AST fragment, like an
$:expr, and transitively by `dbg!` and many macros in the ecosystem.

Rustc's formatting is mostly okay, but does not hew closely to the dominant
contemporary style of Rust formatting. Some things wouldn't ever be written on
one line, like this `match` expression, and certainly not with a comma in front
of the closing brace:

```rust
fn eq(&self, other: &IpAddr) -> bool {
    match other { IpAddr::V4(v4) => self == v4, IpAddr::V6(_) => false, }
}
```

Some places use non-multiple-of-4 indentation, which is definitely not the norm:

```rust
pub const fn to_ipv6_mapped(&self) -> Ipv6Addr {
    let [a, b, c, d] = self.octets();
    Ipv6Addr{inner:
                 c::in6_addr{s6_addr:
                                 [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xFF,
                                  0xFF, a, b, c, d],},}
}
```

And although there isn't an egregious example of it in the link because the
input code is pretty tame, in general rustc_ast_pretty has pathological behavior
on generated code. It has a tendency to use excessive horizontal indentation and
rapidly run out of width:

```rust
::std::io::_print(::core::fmt::Arguments::new_v1(&[""],
                                                 &match (&msg,) {
                                                      _args =>
                                                      [::core::fmt::ArgumentV1::new(_args.0,
                                                                                    ::core::fmt::Display::fmt)],
                                                  }));
```

The snippets above are clearly different from modern rustfmt style. In contrast,
prettyplease is designed to have output that is practically indistinguishable
from rustfmt-formatted code.

<br>

## Example

```rust
// [dependencies]
// prettyplease = "0.2"
// syn = { version = "2", default-features = false, features = ["full", "parsing"] }

const INPUT: &str = stringify! {
    use crate::{
          lazy::{Lazy, SyncLazy, SyncOnceCell}, panic,
        sync::{ atomic::{AtomicUsize, Ordering::SeqCst},
            mpsc::channel, Mutex, },
      thread,
    };
    impl<T, U> Into<U> for T where U: From<T> {
        fn into(self) -> U { U::from(self) }
    }
};

fn main() {
    let syntax_tree = syn::parse_file(INPUT).unwrap();
    let formatted = prettyplease::unparse(&syntax_tree);
    print!("{}", formatted);
}
```

<br>

## Algorithm notes

The approach and terminology used in the implementation are derived from [*Derek
C. Oppen, "Pretty Printing" (1979)*][paper], on which rustc_ast_pretty is also
based, and from rustc_ast_pretty's implementation written by Graydon Hoare in
2011 (and modernized over the years by dozens of volunteer maintainers).

[paper]: http://i.stanford.edu/pub/cstr/reports/cs/tr/79/770/CS-TR-79-770.pdf

The paper describes two language-agnostic interacting procedures `Scan()` and
`Print()`. Language-specific code decomposes an input data structure into a
stream of `string` and `break` tokens, and `begin` and `end` tokens for
grouping. Each `begin`&ndash;`end` range may be identified as either "consistent
breaking" or "inconsistent breaking". If a group is consistently breaking, then
if the whole contents do not fit on the line, *every* `break` token in the group
will receive a linebreak. This is appropriate, for example, for Rust struct
literals, or arguments of a function call. If a group is inconsistently
breaking, then the `string` tokens in the group are greedily placed on the line
until out of space, and linebroken only at those `break` tokens for which the
next string would not fit. For example, this is appropriate for the contents of
a braced `use` statement in Rust.

Scan's job is to efficiently accumulate sizing information about groups and
breaks. For every `begin` token we compute the distance to the matched `end`
token, and for every `break` we compute the distance to the next `break`. The
algorithm uses a ringbuffer to hold tokens whose size is not yet ascertained.
The maximum size of the ringbuffer is bounded by the target line length and does
not grow indefinitely, regardless of deep nesting in the input stream. That's
because once a group is sufficiently big, the precise size can no longer make a
difference to linebreak decisions and we can effectively treat it as "infinity".

Print's job is to use the sizing information to efficiently assign a "broken" or
"not broken" status to every `begin` token. At that point the output is easily
constructed by concatenating `string` tokens and breaking at `break` tokens
contained within a broken group.

Leveraging these primitives (i.e. cleverly placing the all-or-nothing consistent
breaks and greedy inconsistent breaks) to yield rustfmt-compatible formatting
for all of Rust's syntax tree nodes is a fun challenge.

Here is a visualization of some Rust tokens fed into the pretty printing
algorithm. Consistently breaking `begin`&mdash;`end` pairs are represented by
`«`&#8288;`»`, inconsistently breaking by `‹`&#8288;`›`, `break` by `·`, and the
rest of the non-whitespace are `string`.

```text
use crate::«{·
‹    lazy::«{·‹Lazy,· SyncLazy,· SyncOnceCell›·}»,·
    panic,·
    sync::«{·
‹        atomic::«{·‹AtomicUsize,· Ordering::SeqCst›·}»,·
        mpsc::channel,· Mutex›,·
    }»,·
    thread›,·
}»;·
«‹«impl<«·T‹›,· U‹›·»>» Into<«·U·»>· for T›·
where·
    U:‹ From<«·T·»>›,·
{·
«    fn into(·«·self·») -> U {·
‹        U::from(«·self·»)›·
»    }·
»}·
```

The algorithm described in the paper is not quite sufficient for producing
well-formatted Rust code that is locally indistinguishable from rustfmt's style.
The reason is that in the paper, the complete non-whitespace contents are
assumed to be independent of linebreak decisions, with Scan and Print being only
in control of the whitespace (spaces and line breaks). In Rust as idiomatically
formattted by rustfmt, that is not the case. Trailing commas are one example;
the punctuation is only known *after* the broken vs non-broken status of the
surrounding group is known:

```rust
let _ = Struct { x: 0, y: true };

let _ = Struct {
    x: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx,
    y: yyyyyyyyyyyyyyyyyyyyyyyyyyyyyy,   //<- trailing comma if the expression wrapped
};
```

The formatting of `match` expressions is another case; we want small arms on the
same line as the pattern, and big arms wrapped in a brace. The presence of the
brace punctuation, comma, and semicolon are all dependent on whether the arm
fits on the line:

```rust
match total_nanos.checked_add(entry.nanos as u64) {
    Some(n) => tmp = n,   //<- small arm, inline with comma
    None => {
        total_secs = total_secs
            .checked_add(total_nanos / NANOS_PER_SEC as u64)
            .expect("overflow in iter::sum over durations");
    }   //<- big arm, needs brace added, and also semicolon^
}
```

The printing algorithm implementation in this crate accommodates all of these
situations with conditional punctuation tokens whose selection can be deferred
and populated after it's known that the group is or is not broken.

<br>

#### License

<sup>
Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
</sup>

<br>

<sub>
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.
</sub>