src/chains.rs

   1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Formatting of chained expressions, i.e. expressions which are chained by
  12 //! dots: struct and enum field access, method calls, and try shorthand (?).
  13 //!
  14 //! Instead of walking these subexpressions one-by-one, as is our usual strategy
  15 //! for expression formatting, we collect maximal sequences of these expressions
  16 //! and handle them simultaneously.
  17 //!
  18 //! Whenever possible, the entire chain is put on a single line. If that fails,
  19 //! we put each subexpression on a separate, much like the (default) function
  20 //! argument function argument strategy.
  21 //!
  22 //! Depends on config options: `chain_indent` is the indent to use for
  23 //! blocks in the parent/root/base of the chain (and the rest of the chain's
  24 //! alignment).
  25 //! E.g., `let foo = { aaaa; bbb; ccc }.bar.baz();`, we would layout for the
  26 //! following values of `chain_indent`:
  27 //! Block:
  28 //!
  29 //! ```ignore
  30 //! let foo = {
  31 //!     aaaa;
  32 //!     bbb;
  33 //!     ccc
  34 //! }.bar
  35 //!     .baz();
  36 //! ```
  37 //!
  38 //! Visual:
  39 //!
  40 //! ```ignore
  41 //! let foo = {
  42 //!               aaaa;
  43 //!               bbb;
  44 //!               ccc
  45 //!           }
  46 //!           .bar
  47 //!           .baz();
  48 //! ```
  49 //!
  50 //! If the first item in the chain is a block expression, we align the dots with
  51 //! the braces.
  52 //! Block:
  53 //!
  54 //! ```ignore
  55 //! let a = foo.bar
  56 //!     .baz()
  57 //!     .qux
  58 //! ```
  59 //!
  60 //! Visual:
  61 //!
  62 //! ```ignore
  63 //! let a = foo.bar
  64 //!            .baz()
  65 //!            .qux
  66 //! ```
  67
  68 use comment::rewrite_comment;
  69 use config::IndentStyle;
  70 use expr::rewrite_call;
  71 use lists::{extract_post_comment, extract_pre_comment, get_comment_end};
  72 use macros::convert_try_mac;
  73 use rewrite::{Rewrite, RewriteContext};
  74 use shape::Shape;
  75 use source_map::SpanUtils;
  76 use utils::{
  77     first_line_width, last_line_extendable, last_line_width, mk_sp, trimmed_last_line_width,
  78     wrap_str,
  79 };
  80
  81 use std::borrow::Cow;
  82 use std::cmp::min;
  83 use std::iter;
  84
  85 use syntax::source_map::{BytePos, Span};
  86 use syntax::{ast, ptr};
  87
  88 pub fn rewrite_chain(expr: &ast::Expr, context: &RewriteContext, shape: Shape) -> Option<String> {
  89     let chain = Chain::from_ast(expr, context);
  90     debug!("rewrite_chain {:?} {:?}", chain, shape);
  91
  92     // If this is just an expression with some `?`s, then format it trivially and
  93     // return early.
  94     if chain.children.is_empty() {
  95         return chain.parent.rewrite(context, shape);
  96     }
  97
  98     chain.rewrite(context, shape)
  99 }
 100
 101 #[derive(Debug)]
 102 enum CommentPosition {
 103     Back,
 104     Top,
 105 }
 106
 107 // An expression plus trailing `?`s to be formatted together.
 108 #[derive(Debug)]
 109 struct ChainItem {
 110     kind: ChainItemKind,
 111     tries: usize,
 112     span: Span,
 113 }
 114
 115 // FIXME: we can't use a reference here because to convert `try!` to `?` we
 116 // synthesise the AST node. However, I think we could use `Cow` and that
 117 // would remove a lot of cloning.
 118 #[derive(Debug)]
 119 enum ChainItemKind {
 120     Parent(ast::Expr),
 121     MethodCall(
 122         ast::PathSegment,
 123         Vec<ast::GenericArg>,
 124         Vec<ptr::P<ast::Expr>>,
 125     ),
 126     StructField(ast::Ident),
 127     TupleField(ast::Ident, bool),
 128     Comment(String, CommentPosition),
 129 }
 130
 131 impl ChainItemKind {
 132     fn is_block_like(&self, context: &RewriteContext, reps: &str) -> bool {
 133         match self {
 134             ChainItemKind::Parent(ref expr) => is_block_expr(context, expr, reps),
 135             ChainItemKind::MethodCall(..) => reps.contains('\n'),
 136             ChainItemKind::StructField(..)
 137             | ChainItemKind::TupleField(..)
 138             | ChainItemKind::Comment(..) => false,
 139         }
 140     }
 141
 142     fn is_tup_field_access(expr: &ast::Expr) -> bool {
 143         match expr.node {
 144             ast::ExprKind::Field(_, ref field) => {
 145                 field.name.to_string().chars().all(|c| c.is_digit(10))
 146             }
 147             _ => false,
 148         }
 149     }
 150
 151     fn from_ast(context: &RewriteContext, expr: &ast::Expr) -> (ChainItemKind, Span) {
 152         let (kind, span) = match expr.node {
 153             ast::ExprKind::MethodCall(ref segment, ref expressions) => {
 154                 let types = if let Some(ref generic_args) = segment.args {
 155                     if let ast::GenericArgs::AngleBracketed(ref data) = **generic_args {
 156                         data.args.clone()
 157                     } else {
 158                         vec![]
 159                     }
 160                 } else {
 161                     vec![]
 162                 };
 163                 let span = mk_sp(expressions[0].span.hi(), expr.span.hi());
 164                 let kind = ChainItemKind::MethodCall(segment.clone(), types, expressions.clone());
 165                 (kind, span)
 166             }
 167             ast::ExprKind::Field(ref nested, field) => {
 168                 let kind = if Self::is_tup_field_access(expr) {
 169                     ChainItemKind::TupleField(field, Self::is_tup_field_access(nested))
 170                 } else {
 171                     ChainItemKind::StructField(field)
 172                 };
 173                 let span = mk_sp(nested.span.hi(), field.span.hi());
 174                 (kind, span)
 175             }
 176             _ => return (ChainItemKind::Parent(expr.clone()), expr.span),
 177         };
 178
 179         // Remove comments from the span.
 180         let lo = context.snippet_provider.span_before(span, ".");
 181         (kind, mk_sp(lo, span.hi()))
 182     }
 183 }
 184
 185 impl Rewrite for ChainItem {
 186     fn rewrite(&self, context: &RewriteContext, shape: Shape) -> Option<String> {
 187         let shape = shape.sub_width(self.tries)?;
 188         let rewrite = match self.kind {
 189             ChainItemKind::Parent(ref expr) => expr.rewrite(context, shape)?,
 190             ChainItemKind::MethodCall(ref segment, ref types, ref exprs) => {
 191                 Self::rewrite_method_call(segment.ident, types, exprs, self.span, context, shape)?
 192             }
 193             ChainItemKind::StructField(ident) => format!(".{}", ident.name),
 194             ChainItemKind::TupleField(ident, nested) => {
 195                 format!("{}.{}", if nested { " " } else { "" }, ident.name)
 196             }
 197             ChainItemKind::Comment(ref comment, _) => {
 198                 rewrite_comment(comment, false, shape, context.config)?
 199             }
 200         };
 201         Some(format!("{}{}", rewrite, "?".repeat(self.tries)))
 202     }
 203 }
 204
 205 impl ChainItem {
 206     fn new(context: &RewriteContext, expr: &ast::Expr, tries: usize) -> ChainItem {
 207         let (kind, span) = ChainItemKind::from_ast(context, expr);
 208         ChainItem { kind, tries, span }
 209     }
 210
 211     fn comment(span: Span, comment: String, pos: CommentPosition) -> ChainItem {
 212         ChainItem {
 213             kind: ChainItemKind::Comment(comment, pos),
 214             tries: 0,
 215             span,
 216         }
 217     }
 218
 219     fn is_comment(&self) -> bool {
 220         match self.kind {
 221             ChainItemKind::Comment(..) => true,
 222             _ => false,
 223         }
 224     }
 225
 226     fn rewrite_method_call(
 227         method_name: ast::Ident,
 228         types: &[ast::GenericArg],
 229         args: &[ptr::P<ast::Expr>],
 230         span: Span,
 231         context: &RewriteContext,
 232         shape: Shape,
 233     ) -> Option<String> {
 234         let type_str = if types.is_empty() {
 235             String::new()
 236         } else {
 237             let type_list = types
 238                 .iter()
 239                 .map(|ty| ty.rewrite(context, shape))
 240                 .collect::<Option<Vec<_>>>()?;
 241
 242             format!("::<{}>", type_list.join(", "))
 243         };
 244         let callee_str = format!(".{}{}", method_name, type_str);
 245         rewrite_call(context, &callee_str, &args[1..], span, shape)
 246     }
 247 }
 248
 249 #[derive(Debug)]
 250 struct Chain {
 251     parent: ChainItem,
 252     children: Vec<ChainItem>,
 253 }
 254
 255 impl Chain {
 256     fn from_ast(expr: &ast::Expr, context: &RewriteContext) -> Chain {
 257         let subexpr_list = Self::make_subexpr_list(expr, context);
 258
 259         // Un-parse the expression tree into ChainItems
 260         let mut rev_children = vec![];
 261         let mut sub_tries = 0;
 262         for subexpr in &subexpr_list {
 263             match subexpr.node {
 264                 ast::ExprKind::Try(_) => sub_tries += 1,
 265                 _ => {
 266                     rev_children.push(ChainItem::new(context, subexpr, sub_tries));
 267                     sub_tries = 0;
 268                 }
 269             }
 270         }
 271
 272         fn is_tries(s: &str) -> bool {
 273             s.chars().all(|c| c == '?')
 274         }
 275
 276         fn handle_post_comment(
 277             post_comment_span: Span,
 278             post_comment_snippet: &str,
 279             prev_span_end: &mut BytePos,
 280             children: &mut Vec<ChainItem>,
 281         ) {
 282             let white_spaces: &[_] = &[' ', '\t'];
 283             if post_comment_snippet
 284                 .trim_matches(white_spaces)
 285                 .starts_with('\n')
 286             {
 287                 // No post comment.
 288                 return;
 289             }
 290             // HACK: Treat `?`s as separators.
 291             let trimmed_snippet = post_comment_snippet.trim_matches('?');
 292             let comment_end = get_comment_end(trimmed_snippet, "?", "", false);
 293             let maybe_post_comment = extract_post_comment(trimmed_snippet, comment_end, "?")
 294                 .and_then(|comment| {
 295                     if comment.is_empty() {
 296                         None
 297                     } else {
 298                         Some((comment, comment_end))
 299                     }
 300                 });
 301
 302             if let Some((post_comment, comment_end)) = maybe_post_comment {
 303                 children.push(ChainItem::comment(
 304                     post_comment_span,
 305                     post_comment,
 306                     CommentPosition::Back,
 307                 ));
 308                 *prev_span_end = *prev_span_end + BytePos(comment_end as u32);
 309             }
 310         }
 311
 312         let parent = rev_children.pop().unwrap();
 313         let mut children = vec![];
 314         let mut prev_span_end = parent.span.hi();
 315         let mut iter = rev_children.into_iter().rev().peekable();
 316         if let Some(first_chain_item) = iter.peek() {
 317             let comment_span = mk_sp(prev_span_end, first_chain_item.span.lo());
 318             let comment_snippet = context.snippet(comment_span);
 319             if !is_tries(comment_snippet.trim()) {
 320                 handle_post_comment(
 321                     comment_span,
 322                     comment_snippet,
 323                     &mut prev_span_end,
 324                     &mut children,
 325                 );
 326             }
 327         }
 328         while let Some(chain_item) = iter.next() {
 329             let comment_snippet = context.snippet(chain_item.span);
 330             // FIXME: Figure out the way to get a correct span when converting `try!` to `?`.
 331             let handle_comment =
 332                 !(context.config.use_try_shorthand() || is_tries(comment_snippet.trim()));
 333
 334             // Pre-comment
 335             if handle_comment {
 336                 let pre_comment_span = mk_sp(prev_span_end, chain_item.span.lo());
 337                 let pre_comment_snippet = context.snippet(pre_comment_span);
 338                 let pre_comment_snippet = pre_comment_snippet.trim().trim_matches('?');
 339                 let (pre_comment, _) = extract_pre_comment(pre_comment_snippet);
 340                 match pre_comment {
 341                     Some(ref comment) if !comment.is_empty() => {
 342                         children.push(ChainItem::comment(
 343                             pre_comment_span,
 344                             comment.to_owned(),
 345                             CommentPosition::Top,
 346                         ));
 347                     }
 348                     _ => (),
 349                 }
 350             }
 351
 352             prev_span_end = chain_item.span.hi();
 353             children.push(chain_item);
 354
 355             // Post-comment
 356             if !handle_comment || iter.peek().is_none() {
 357                 continue;
 358             }
 359
 360             let next_lo = iter.peek().unwrap().span.lo();
 361             let post_comment_span = mk_sp(prev_span_end, next_lo);
 362             let post_comment_snippet = context.snippet(post_comment_span);
 363             handle_post_comment(
 364                 post_comment_span,
 365                 post_comment_snippet,
 366                 &mut prev_span_end,
 367                 &mut children,
 368             );
 369         }
 370
 371         Chain { parent, children }
 372     }
 373
 374     // Returns a Vec of the prefixes of the chain.
 375     // E.g., for input `a.b.c` we return [`a.b.c`, `a.b`, 'a']
 376     fn make_subexpr_list(expr: &ast::Expr, context: &RewriteContext) -> Vec<ast::Expr> {
 377         let mut subexpr_list = vec![expr.clone()];
 378
 379         while let Some(subexpr) = Self::pop_expr_chain(subexpr_list.last().unwrap(), context) {
 380             subexpr_list.push(subexpr.clone());
 381         }
 382
 383         subexpr_list
 384     }
 385
 386     // Returns the expression's subexpression, if it exists. When the subexpr
 387     // is a try! macro, we'll convert it to shorthand when the option is set.
 388     fn pop_expr_chain(expr: &ast::Expr, context: &RewriteContext) -> Option<ast::Expr> {
 389         match expr.node {
 390             ast::ExprKind::MethodCall(_, ref expressions) => {
 391                 Some(Self::convert_try(&expressions[0], context))
 392             }
 393             ast::ExprKind::Field(ref subexpr, _) | ast::ExprKind::Try(ref subexpr) => {
 394                 Some(Self::convert_try(subexpr, context))
 395             }
 396             _ => None,
 397         }
 398     }
 399
 400     fn convert_try(expr: &ast::Expr, context: &RewriteContext) -> ast::Expr {
 401         match expr.node {
 402             ast::ExprKind::Mac(ref mac) if context.config.use_try_shorthand() => {
 403                 if let Some(subexpr) = convert_try_mac(mac, context) {
 404                     subexpr
 405                 } else {
 406                     expr.clone()
 407                 }
 408             }
 409             _ => expr.clone(),
 410         }
 411     }
 412 }
 413
 414 impl Rewrite for Chain {
 415     fn rewrite(&self, context: &RewriteContext, shape: Shape) -> Option<String> {
 416         debug!("rewrite chain {:?} {:?}", self, shape);
 417
 418         let mut formatter = match context.config.indent_style() {
 419             IndentStyle::Block => Box::new(ChainFormatterBlock::new(self)) as Box<ChainFormatter>,
 420             IndentStyle::Visual => Box::new(ChainFormatterVisual::new(self)) as Box<ChainFormatter>,
 421         };
 422
 423         formatter.format_root(&self.parent, context, shape)?;
 424         if let Some(result) = formatter.pure_root() {
 425             return wrap_str(result, context.config.max_width(), shape);
 426         }
 427
 428         // Decide how to layout the rest of the chain.
 429         let child_shape = formatter.child_shape(context, shape)?;
 430
 431         formatter.format_children(context, child_shape)?;
 432         formatter.format_last_child(context, shape, child_shape)?;
 433
 434         let result = formatter.join_rewrites(context, child_shape)?;
 435         wrap_str(result, context.config.max_width(), shape)
 436     }
 437 }
 438
 439 // There are a few types for formatting chains. This is because there is a lot
 440 // in common between formatting with block vs visual indent, but they are
 441 // different enough that branching on the indent all over the place gets ugly.
 442 // Anything that can format a chain is a ChainFormatter.
 443 trait ChainFormatter {
 444     // Parent is the first item in the chain, e.g., `foo` in `foo.bar.baz()`.
 445     // Root is the parent plus any other chain items placed on the first line to
 446     // avoid an orphan. E.g.,
 447     // ```
 448     // foo.bar
 449     //     .baz()
 450     // ```
 451     // If `bar` were not part of the root, then foo would be orphaned and 'float'.
 452     fn format_root(
 453         &mut self,
 454         parent: &ChainItem,
 455         context: &RewriteContext,
 456         shape: Shape,
 457     ) -> Option<()>;
 458     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape>;
 459     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()>;
 460     fn format_last_child(
 461         &mut self,
 462         context: &RewriteContext,
 463         shape: Shape,
 464         child_shape: Shape,
 465     ) -> Option<()>;
 466     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String>;
 467     // Returns `Some` if the chain is only a root, None otherwise.
 468     fn pure_root(&mut self) -> Option<String>;
 469 }
 470
 471 // Data and behaviour that is shared by both chain formatters. The concrete
 472 // formatters can delegate much behaviour to `ChainFormatterShared`.
 473 struct ChainFormatterShared<'a> {
 474     // The current working set of child items.
 475     children: &'a [ChainItem],
 476     // The current rewrites of items (includes trailing `?`s, but not any way to
 477     // connect the rewrites together).
 478     rewrites: Vec<String>,
 479     // Whether the chain can fit on one line.
 480     fits_single_line: bool,
 481     // The number of children in the chain. This is not equal to `self.children.len()`
 482     // because `self.children` will change size as we process the chain.
 483     child_count: usize,
 484 }
 485
 486 impl<'a> ChainFormatterShared<'a> {
 487     fn new(chain: &'a Chain) -> ChainFormatterShared<'a> {
 488         ChainFormatterShared {
 489             children: &chain.children,
 490             rewrites: Vec::with_capacity(chain.children.len() + 1),
 491             fits_single_line: false,
 492             child_count: chain.children.len(),
 493         }
 494     }
 495
 496     fn pure_root(&mut self) -> Option<String> {
 497         if self.children.is_empty() {
 498             assert_eq!(self.rewrites.len(), 1);
 499             Some(self.rewrites.pop().unwrap())
 500         } else {
 501             None
 502         }
 503     }
 504
 505     // Rewrite the last child. The last child of a chain requires special treatment. We need to
 506     // know whether 'overflowing' the last child make a better formatting:
 507     //
 508     // A chain with overflowing the last child:
 509     // ```
 510     // parent.child1.child2.last_child(
 511     //     a,
 512     //     b,
 513     //     c,
 514     // )
 515     // ```
 516     //
 517     // A chain without overflowing the last child (in vertical layout):
 518     // ```
 519     // parent
 520     //     .child1
 521     //     .child2
 522     //     .last_child(a, b, c)
 523     // ```
 524     //
 525     // In particular, overflowing is effective when the last child is a method with a multi-lined
 526     // block-like argument (e.g. closure):
 527     // ```
 528     // parent.child1.child2.last_child(|a, b, c| {
 529     //     let x = foo(a, b, c);
 530     //     let y = bar(a, b, c);
 531     //
 532     //     // ...
 533     //
 534     //     result
 535     // })
 536     // ```
 537     fn format_last_child(
 538         &mut self,
 539         may_extend: bool,
 540         context: &RewriteContext,
 541         shape: Shape,
 542         child_shape: Shape,
 543     ) -> Option<()> {
 544         let last = self.children.last()?;
 545         let extendable = may_extend && last_line_extendable(&self.rewrites[0]);
 546         let prev_last_line_width = last_line_width(&self.rewrites[0]);
 547
 548         // Total of all items excluding the last.
 549         let almost_total = if extendable {
 550             prev_last_line_width
 551         } else {
 552             self.rewrites.iter().fold(0, |a, b| a + b.len())
 553         } + last.tries;
 554         let one_line_budget = if self.child_count == 1 {
 555             shape.width
 556         } else {
 557             min(shape.width, context.config.width_heuristics().chain_width)
 558         }.saturating_sub(almost_total);
 559
 560         let all_in_one_line = !self.children.iter().any(ChainItem::is_comment)
 561             && self.rewrites.iter().all(|s| !s.contains('\n'))
 562             && one_line_budget > 0;
 563         let last_shape = if all_in_one_line {
 564             shape.sub_width(last.tries)?
 565         } else if extendable {
 566             child_shape.sub_width(last.tries)?
 567         } else {
 568             child_shape.sub_width(shape.rhs_overhead(context.config) + last.tries)?
 569         };
 570
 571         let mut last_subexpr_str = None;
 572         if all_in_one_line || extendable {
 573             // First we try to 'overflow' the last child and see if it looks better than using
 574             // vertical layout.
 575             if let Some(one_line_shape) = last_shape.offset_left(almost_total) {
 576                 if let Some(rw) = last.rewrite(context, one_line_shape) {
 577                     // We allow overflowing here only if both of the following conditions match:
 578                     // 1. The entire chain fits in a single line except the last child.
 579                     // 2. `last_child_str.lines().count() >= 5`.
 580                     let line_count = rw.lines().count();
 581                     let could_fit_single_line = first_line_width(&rw) <= one_line_budget;
 582                     if could_fit_single_line && line_count >= 5 {
 583                         last_subexpr_str = Some(rw);
 584                         self.fits_single_line = all_in_one_line;
 585                     } else {
 586                         // We could not know whether overflowing is better than using vertical
 587                         // layout, just by looking at the overflowed rewrite. Now we rewrite the
 588                         // last child on its own line, and compare two rewrites to choose which is
 589                         // better.
 590                         let last_shape = child_shape
 591                             .sub_width(shape.rhs_overhead(context.config) + last.tries)?;
 592                         match last.rewrite(context, last_shape) {
 593                             Some(ref new_rw) if !could_fit_single_line => {
 594                                 last_subexpr_str = Some(new_rw.clone());
 595                             }
 596                             Some(ref new_rw) if new_rw.lines().count() >= line_count => {
 597                                 last_subexpr_str = Some(rw);
 598                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 599                             }
 600                             new_rw @ Some(..) => {
 601                                 last_subexpr_str = new_rw;
 602                             }
 603                             _ => {
 604                                 last_subexpr_str = Some(rw);
 605                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 606                             }
 607                         }
 608                     }
 609                 }
 610             }
 611         }
 612
 613         last_subexpr_str = last_subexpr_str.or_else(|| last.rewrite(context, last_shape));
 614         self.rewrites.push(last_subexpr_str?);
 615         Some(())
 616     }
 617
 618     fn join_rewrites(
 619         &self,
 620         context: &RewriteContext,
 621         child_shape: Shape,
 622         block_like_iter: impl Iterator<Item = bool>,
 623     ) -> Option<String> {
 624         let connector = if self.fits_single_line {
 625             // Yay, we can put everything on one line.
 626             Cow::from("")
 627         } else {
 628             // Use new lines.
 629             if *context.force_one_line_chain.borrow() {
 630                 return None;
 631             }
 632             child_shape.to_string_with_newline(context.config)
 633         };
 634
 635         let mut rewrite_iter = self.rewrites.iter();
 636         let mut result = rewrite_iter.next().unwrap().clone();
 637         let children_iter = self.children.iter();
 638         let iter = rewrite_iter.zip(block_like_iter).zip(children_iter);
 639
 640         for ((rewrite, prev_is_block_like), chain_item) in iter {
 641             match chain_item.kind {
 642                 ChainItemKind::Comment(_, CommentPosition::Back) => result.push(' '),
 643                 ChainItemKind::Comment(_, CommentPosition::Top) => result.push_str(&connector),
 644                 _ => {
 645                     if !prev_is_block_like {
 646                         result.push_str(&connector);
 647                     }
 648                 }
 649             }
 650             result.push_str(&rewrite);
 651         }
 652
 653         Some(result)
 654     }
 655 }
 656
 657 // Formats a chain using block indent.
 658 struct ChainFormatterBlock<'a> {
 659     shared: ChainFormatterShared<'a>,
 660     // For each rewrite, whether the corresponding item is block-like.
 661     is_block_like: Vec<bool>,
 662 }
 663
 664 impl<'a> ChainFormatterBlock<'a> {
 665     fn new(chain: &'a Chain) -> ChainFormatterBlock<'a> {
 666         ChainFormatterBlock {
 667             shared: ChainFormatterShared::new(chain),
 668             is_block_like: Vec::with_capacity(chain.children.len() + 1),
 669         }
 670     }
 671 }
 672
 673 impl<'a> ChainFormatter for ChainFormatterBlock<'a> {
 674     fn format_root(
 675         &mut self,
 676         parent: &ChainItem,
 677         context: &RewriteContext,
 678         shape: Shape,
 679     ) -> Option<()> {
 680         let mut root_rewrite: String = parent.rewrite(context, shape)?;
 681
 682         let mut root_ends_with_block = parent.kind.is_block_like(context, &root_rewrite);
 683         let tab_width = context.config.tab_spaces().saturating_sub(shape.offset);
 684
 685         while root_rewrite.len() <= tab_width && !root_rewrite.contains('\n') {
 686             let item = &self.shared.children[0];
 687             if let ChainItemKind::Comment(..) = item.kind {
 688                 break;
 689             }
 690             let shape = shape.offset_left(root_rewrite.len())?;
 691             match &item.rewrite(context, shape) {
 692                 Some(rewrite) => root_rewrite.push_str(rewrite),
 693                 None => break,
 694             }
 695
 696             root_ends_with_block = item.kind.is_block_like(context, &root_rewrite);
 697
 698             self.shared.children = &self.shared.children[1..];
 699             if self.shared.children.is_empty() {
 700                 break;
 701             }
 702         }
 703         self.is_block_like.push(root_ends_with_block);
 704         self.shared.rewrites.push(root_rewrite);
 705         Some(())
 706     }
 707
 708     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape> {
 709         Some(
 710             if self.is_block_like[0] {
 711                 shape.block_indent(0)
 712             } else {
 713                 shape.block_indent(context.config.tab_spaces())
 714             }.with_max_width(context.config),
 715         )
 716     }
 717
 718     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()> {
 719         for item in &self.shared.children[..self.shared.children.len() - 1] {
 720             let rewrite = item.rewrite(context, child_shape)?;
 721             self.is_block_like
 722                 .push(item.kind.is_block_like(context, &rewrite));
 723             self.shared.rewrites.push(rewrite);
 724         }
 725         Some(())
 726     }
 727
 728     fn format_last_child(
 729         &mut self,
 730         context: &RewriteContext,
 731         shape: Shape,
 732         child_shape: Shape,
 733     ) -> Option<()> {
 734         self.shared
 735             .format_last_child(true, context, shape, child_shape)
 736     }
 737
 738     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 739         self.shared
 740             .join_rewrites(context, child_shape, self.is_block_like.iter().cloned())
 741     }
 742
 743     fn pure_root(&mut self) -> Option<String> {
 744         self.shared.pure_root()
 745     }
 746 }
 747
 748 // Format a chain using visual indent.
 749 struct ChainFormatterVisual<'a> {
 750     shared: ChainFormatterShared<'a>,
 751     // The extra offset from the chain's shape to the position of the `.`
 752     offset: usize,
 753 }
 754
 755 impl<'a> ChainFormatterVisual<'a> {
 756     fn new(chain: &'a Chain) -> ChainFormatterVisual<'a> {
 757         ChainFormatterVisual {
 758             shared: ChainFormatterShared::new(chain),
 759             offset: 0,
 760         }
 761     }
 762 }
 763
 764 impl<'a> ChainFormatter for ChainFormatterVisual<'a> {
 765     fn format_root(
 766         &mut self,
 767         parent: &ChainItem,
 768         context: &RewriteContext,
 769         shape: Shape,
 770     ) -> Option<()> {
 771         let parent_shape = shape.visual_indent(0);
 772         let mut root_rewrite = parent.rewrite(context, parent_shape)?;
 773         let multiline = root_rewrite.contains('\n');
 774         self.offset = if multiline {
 775             last_line_width(&root_rewrite).saturating_sub(shape.used_width())
 776         } else {
 777             trimmed_last_line_width(&root_rewrite)
 778         };
 779
 780         if !multiline || parent.kind.is_block_like(context, &root_rewrite) {
 781             let item = &self.shared.children[0];
 782             if let ChainItemKind::Comment(..) = item.kind {
 783                 self.shared.rewrites.push(root_rewrite);
 784                 return Some(());
 785             }
 786             let child_shape = parent_shape
 787                 .visual_indent(self.offset)
 788                 .sub_width(self.offset)?;
 789             let rewrite = item.rewrite(context, child_shape)?;
 790             match wrap_str(rewrite, context.config.max_width(), shape) {
 791                 Some(rewrite) => root_rewrite.push_str(&rewrite),
 792                 None => {
 793                     // We couldn't fit in at the visual indent, try the last
 794                     // indent.
 795                     let rewrite = item.rewrite(context, parent_shape)?;
 796                     root_rewrite.push_str(&rewrite);
 797                     self.offset = 0;
 798                 }
 799             }
 800
 801             self.shared.children = &self.shared.children[1..];
 802         }
 803
 804         self.shared.rewrites.push(root_rewrite);
 805         Some(())
 806     }
 807
 808     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape> {
 809         shape
 810             .with_max_width(context.config)
 811             .offset_left(self.offset)
 812             .map(|s| s.visual_indent(0))
 813     }
 814
 815     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()> {
 816         for item in &self.shared.children[..self.shared.children.len() - 1] {
 817             let rewrite = item.rewrite(context, child_shape)?;
 818             self.shared.rewrites.push(rewrite);
 819         }
 820         Some(())
 821     }
 822
 823     fn format_last_child(
 824         &mut self,
 825         context: &RewriteContext,
 826         shape: Shape,
 827         child_shape: Shape,
 828     ) -> Option<()> {
 829         self.shared
 830             .format_last_child(false, context, shape, child_shape)
 831     }
 832
 833     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 834         self.shared
 835             .join_rewrites(context, child_shape, iter::repeat(false))
 836     }
 837
 838     fn pure_root(&mut self) -> Option<String> {
 839         self.shared.pure_root()
 840     }
 841 }
 842
 843 // States whether an expression's last line exclusively consists of closing
 844 // parens, braces, and brackets in its idiomatic formatting.
 845 fn is_block_expr(context: &RewriteContext, expr: &ast::Expr, repr: &str) -> bool {
 846     match expr.node {
 847         ast::ExprKind::Mac(..)
 848         | ast::ExprKind::Call(..)
 849         | ast::ExprKind::MethodCall(..)
 850         | ast::ExprKind::Struct(..)
 851         | ast::ExprKind::While(..)
 852         | ast::ExprKind::WhileLet(..)
 853         | ast::ExprKind::If(..)
 854         | ast::ExprKind::IfLet(..)
 855         | ast::ExprKind::Block(..)
 856         | ast::ExprKind::Loop(..)
 857         | ast::ExprKind::ForLoop(..)
 858         | ast::ExprKind::Match(..) => repr.contains('\n'),
 859         ast::ExprKind::Paren(ref expr)
 860         | ast::ExprKind::Binary(_, _, ref expr)
 861         | ast::ExprKind::Index(_, ref expr)
 862         | ast::ExprKind::Unary(_, ref expr)
 863         | ast::ExprKind::Closure(_, _, _, _, ref expr, _)
 864         | ast::ExprKind::Try(ref expr)
 865         | ast::ExprKind::Yield(Some(ref expr)) => is_block_expr(context, expr, repr),
 866         // This can only be a string lit
 867         ast::ExprKind::Lit(_) => {
 868             repr.contains('\n') && trimmed_last_line_width(repr) <= context.config.tab_spaces()
 869         }
 870         _ => false,
 871     }
 872 }