src/chains.rs

   1 //! Formatting of chained expressions, i.e., expressions that are chained by
   2 //! dots: struct and enum field access, method calls, and try shorthand (`?`).
   3 //!
   4 //! Instead of walking these subexpressions one-by-one, as is our usual strategy
   5 //! for expression formatting, we collect maximal sequences of these expressions
   6 //! and handle them simultaneously.
   7 //!
   8 //! Whenever possible, the entire chain is put on a single line. If that fails,
   9 //! we put each subexpression on a separate, much like the (default) function
  10 //! argument function argument strategy.
  11 //!
  12 //! Depends on config options: `chain_indent` is the indent to use for
  13 //! blocks in the parent/root/base of the chain (and the rest of the chain's
  14 //! alignment).
  15 //! E.g., `let foo = { aaaa; bbb; ccc }.bar.baz();`, we would layout for the
  16 //! following values of `chain_indent`:
  17 //! Block:
  18 //!
  19 //! ```ignore
  20 //! let foo = {
  21 //!     aaaa;
  22 //!     bbb;
  23 //!     ccc
  24 //! }.bar
  25 //!     .baz();
  26 //! ```
  27 //!
  28 //! Visual:
  29 //!
  30 //! ```ignore
  31 //! let foo = {
  32 //!               aaaa;
  33 //!               bbb;
  34 //!               ccc
  35 //!           }
  36 //!           .bar
  37 //!           .baz();
  38 //! ```
  39 //!
  40 //! If the first item in the chain is a block expression, we align the dots with
  41 //! the braces.
  42 //! Block:
  43 //!
  44 //! ```ignore
  45 //! let a = foo.bar
  46 //!     .baz()
  47 //!     .qux
  48 //! ```
  49 //!
  50 //! Visual:
  51 //!
  52 //! ```ignore
  53 //! let a = foo.bar
  54 //!            .baz()
  55 //!            .qux
  56 //! ```
  57
  58 use std::borrow::Cow;
  59 use std::cmp::min;
  60
  61 use syntax::source_map::{BytePos, Span};
  62 use syntax::{ast, ptr};
  63
  64 use crate::comment::{rewrite_comment, CharClasses, FullCodeCharKind, RichChar};
  65 use crate::config::IndentStyle;
  66 use crate::expr::rewrite_call;
  67 use crate::lists::extract_pre_comment;
  68 use crate::macros::convert_try_mac;
  69 use crate::rewrite::{Rewrite, RewriteContext};
  70 use crate::shape::Shape;
  71 use crate::source_map::SpanUtils;
  72 use crate::utils::{
  73     self, first_line_width, last_line_extendable, last_line_width, mk_sp, rewrite_ident,
  74     trimmed_last_line_width, wrap_str,
  75 };
  76
  77 pub(crate) fn rewrite_chain(
  78     expr: &ast::Expr,
  79     context: &RewriteContext<'_>,
  80     shape: Shape,
  81 ) -> Option<String> {
  82     let chain = Chain::from_ast(expr, context);
  83     debug!("rewrite_chain {:?} {:?}", chain, shape);
  84
  85     // If this is just an expression with some `?`s, then format it trivially and
  86     // return early.
  87     if chain.children.is_empty() {
  88         return chain.parent.rewrite(context, shape);
  89     }
  90
  91     chain.rewrite(context, shape)
  92 }
  93
  94 #[derive(Debug)]
  95 enum CommentPosition {
  96     Back,
  97     Top,
  98 }
  99
 100 // An expression plus trailing `?`s to be formatted together.
 101 #[derive(Debug)]
 102 struct ChainItem {
 103     kind: ChainItemKind,
 104     tries: usize,
 105     span: Span,
 106 }
 107
 108 // FIXME: we can't use a reference here because to convert `try!` to `?` we
 109 // synthesise the AST node. However, I think we could use `Cow` and that
 110 // would remove a lot of cloning.
 111 #[derive(Debug)]
 112 enum ChainItemKind {
 113     Parent(ast::Expr),
 114     MethodCall(
 115         ast::PathSegment,
 116         Vec<ast::GenericArg>,
 117         Vec<ptr::P<ast::Expr>>,
 118     ),
 119     StructField(ast::Ident),
 120     TupleField(ast::Ident, bool),
 121     Await,
 122     Comment(String, CommentPosition),
 123 }
 124
 125 impl ChainItemKind {
 126     fn is_block_like(&self, context: &RewriteContext<'_>, reps: &str) -> bool {
 127         match self {
 128             ChainItemKind::Parent(ref expr) => utils::is_block_expr(context, expr, reps),
 129             ChainItemKind::MethodCall(..)
 130             | ChainItemKind::StructField(..)
 131             | ChainItemKind::TupleField(..)
 132             | ChainItemKind::Await
 133             | ChainItemKind::Comment(..) => false,
 134         }
 135     }
 136
 137     fn is_tup_field_access(expr: &ast::Expr) -> bool {
 138         match expr.node {
 139             ast::ExprKind::Field(_, ref field) => {
 140                 field.name.to_string().chars().all(|c| c.is_digit(10))
 141             }
 142             _ => false,
 143         }
 144     }
 145
 146     fn from_ast(context: &RewriteContext<'_>, expr: &ast::Expr) -> (ChainItemKind, Span) {
 147         let (kind, span) = match expr.node {
 148             ast::ExprKind::MethodCall(ref segment, ref expressions) => {
 149                 let types = if let Some(ref generic_args) = segment.args {
 150                     if let ast::GenericArgs::AngleBracketed(ref data) = **generic_args {
 151                         data.args.clone()
 152                     } else {
 153                         vec![]
 154                     }
 155                 } else {
 156                     vec![]
 157                 };
 158                 let span = mk_sp(expressions[0].span.hi(), expr.span.hi());
 159                 let kind = ChainItemKind::MethodCall(segment.clone(), types, expressions.clone());
 160                 (kind, span)
 161             }
 162             ast::ExprKind::Field(ref nested, field) => {
 163                 let kind = if Self::is_tup_field_access(expr) {
 164                     ChainItemKind::TupleField(field, Self::is_tup_field_access(nested))
 165                 } else {
 166                     ChainItemKind::StructField(field)
 167                 };
 168                 let span = mk_sp(nested.span.hi(), field.span.hi());
 169                 (kind, span)
 170             }
 171             ast::ExprKind::Await(_, ref nested) => {
 172                 let span = mk_sp(nested.span.hi(), expr.span.hi());
 173                 (ChainItemKind::Await, span)
 174             }
 175             _ => return (ChainItemKind::Parent(expr.clone()), expr.span),
 176         };
 177
 178         // Remove comments from the span.
 179         let lo = context.snippet_provider.span_before(span, ".");
 180         (kind, mk_sp(lo, span.hi()))
 181     }
 182 }
 183
 184 impl Rewrite for ChainItem {
 185     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
 186         let shape = shape.sub_width(self.tries)?;
 187         let rewrite = match self.kind {
 188             ChainItemKind::Parent(ref expr) => expr.rewrite(context, shape)?,
 189             ChainItemKind::MethodCall(ref segment, ref types, ref exprs) => {
 190                 Self::rewrite_method_call(segment.ident, types, exprs, self.span, context, shape)?
 191             }
 192             ChainItemKind::StructField(ident) => format!(".{}", rewrite_ident(context, ident)),
 193             ChainItemKind::TupleField(ident, nested) => format!(
 194                 "{}.{}",
 195                 if nested { " " } else { "" },
 196                 rewrite_ident(context, ident)
 197             ),
 198             ChainItemKind::Await => ".await".to_owned(),
 199             ChainItemKind::Comment(ref comment, _) => {
 200                 rewrite_comment(comment, false, shape, context.config)?
 201             }
 202         };
 203         Some(format!("{}{}", rewrite, "?".repeat(self.tries)))
 204     }
 205 }
 206
 207 impl ChainItem {
 208     fn new(context: &RewriteContext<'_>, expr: &ast::Expr, tries: usize) -> ChainItem {
 209         let (kind, span) = ChainItemKind::from_ast(context, expr);
 210         ChainItem { kind, tries, span }
 211     }
 212
 213     fn comment(span: Span, comment: String, pos: CommentPosition) -> ChainItem {
 214         ChainItem {
 215             kind: ChainItemKind::Comment(comment, pos),
 216             tries: 0,
 217             span,
 218         }
 219     }
 220
 221     fn is_comment(&self) -> bool {
 222         match self.kind {
 223             ChainItemKind::Comment(..) => true,
 224             _ => false,
 225         }
 226     }
 227
 228     fn rewrite_method_call(
 229         method_name: ast::Ident,
 230         types: &[ast::GenericArg],
 231         args: &[ptr::P<ast::Expr>],
 232         span: Span,
 233         context: &RewriteContext<'_>,
 234         shape: Shape,
 235     ) -> Option<String> {
 236         let type_str = if types.is_empty() {
 237             String::new()
 238         } else {
 239             let type_list = types
 240                 .iter()
 241                 .map(|ty| ty.rewrite(context, shape))
 242                 .collect::<Option<Vec<_>>>()?;
 243
 244             format!("::<{}>", type_list.join(", "))
 245         };
 246         let callee_str = format!(".{}{}", rewrite_ident(context, method_name), type_str);
 247         rewrite_call(context, &callee_str, &args[1..], span, shape)
 248     }
 249 }
 250
 251 #[derive(Debug)]
 252 struct Chain {
 253     parent: ChainItem,
 254     children: Vec<ChainItem>,
 255 }
 256
 257 impl Chain {
 258     fn from_ast(expr: &ast::Expr, context: &RewriteContext<'_>) -> Chain {
 259         let subexpr_list = Self::make_subexpr_list(expr, context);
 260
 261         // Un-parse the expression tree into ChainItems
 262         let mut rev_children = vec![];
 263         let mut sub_tries = 0;
 264         for subexpr in &subexpr_list {
 265             match subexpr.node {
 266                 ast::ExprKind::Try(_) => sub_tries += 1,
 267                 _ => {
 268                     rev_children.push(ChainItem::new(context, subexpr, sub_tries));
 269                     sub_tries = 0;
 270                 }
 271             }
 272         }
 273
 274         fn is_tries(s: &str) -> bool {
 275             s.chars().all(|c| c == '?')
 276         }
 277
 278         fn is_post_comment(s: &str) -> bool {
 279             let comment_start_index = s.chars().position(|c| c == '/');
 280             if comment_start_index.is_none() {
 281                 return false;
 282             }
 283
 284             let newline_index = s.chars().position(|c| c == '\n');
 285             if newline_index.is_none() {
 286                 return true;
 287             }
 288
 289             comment_start_index.unwrap() < newline_index.unwrap()
 290         }
 291
 292         fn handle_post_comment(
 293             post_comment_span: Span,
 294             post_comment_snippet: &str,
 295             prev_span_end: &mut BytePos,
 296             children: &mut Vec<ChainItem>,
 297         ) {
 298             let white_spaces: &[_] = &[' ', '\t'];
 299             if post_comment_snippet
 300                 .trim_matches(white_spaces)
 301                 .starts_with('\n')
 302             {
 303                 // No post comment.
 304                 return;
 305             }
 306             let trimmed_snippet = trim_tries(post_comment_snippet);
 307             if is_post_comment(&trimmed_snippet) {
 308                 children.push(ChainItem::comment(
 309                     post_comment_span,
 310                     trimmed_snippet.trim().to_owned(),
 311                     CommentPosition::Back,
 312                 ));
 313                 *prev_span_end = post_comment_span.hi();
 314             }
 315         }
 316
 317         let parent = rev_children.pop().unwrap();
 318         let mut children = vec![];
 319         let mut prev_span_end = parent.span.hi();
 320         let mut iter = rev_children.into_iter().rev().peekable();
 321         if let Some(first_chain_item) = iter.peek() {
 322             let comment_span = mk_sp(prev_span_end, first_chain_item.span.lo());
 323             let comment_snippet = context.snippet(comment_span);
 324             if !is_tries(comment_snippet.trim()) {
 325                 handle_post_comment(
 326                     comment_span,
 327                     comment_snippet,
 328                     &mut prev_span_end,
 329                     &mut children,
 330                 );
 331             }
 332         }
 333         while let Some(chain_item) = iter.next() {
 334             let comment_snippet = context.snippet(chain_item.span);
 335             // FIXME: Figure out the way to get a correct span when converting `try!` to `?`.
 336             let handle_comment =
 337                 !(context.config.use_try_shorthand() || is_tries(comment_snippet.trim()));
 338
 339             // Pre-comment
 340             if handle_comment {
 341                 let pre_comment_span = mk_sp(prev_span_end, chain_item.span.lo());
 342                 let pre_comment_snippet = trim_tries(context.snippet(pre_comment_span));
 343                 let (pre_comment, _) = extract_pre_comment(&pre_comment_snippet);
 344                 match pre_comment {
 345                     Some(ref comment) if !comment.is_empty() => {
 346                         children.push(ChainItem::comment(
 347                             pre_comment_span,
 348                             comment.to_owned(),
 349                             CommentPosition::Top,
 350                         ));
 351                     }
 352                     _ => (),
 353                 }
 354             }
 355
 356             prev_span_end = chain_item.span.hi();
 357             children.push(chain_item);
 358
 359             // Post-comment
 360             if !handle_comment || iter.peek().is_none() {
 361                 continue;
 362             }
 363
 364             let next_lo = iter.peek().unwrap().span.lo();
 365             let post_comment_span = mk_sp(prev_span_end, next_lo);
 366             let post_comment_snippet = context.snippet(post_comment_span);
 367             handle_post_comment(
 368                 post_comment_span,
 369                 post_comment_snippet,
 370                 &mut prev_span_end,
 371                 &mut children,
 372             );
 373         }
 374
 375         Chain { parent, children }
 376     }
 377
 378     // Returns a Vec of the prefixes of the chain.
 379     // E.g., for input `a.b.c` we return [`a.b.c`, `a.b`, 'a']
 380     fn make_subexpr_list(expr: &ast::Expr, context: &RewriteContext<'_>) -> Vec<ast::Expr> {
 381         let mut subexpr_list = vec![expr.clone()];
 382
 383         while let Some(subexpr) = Self::pop_expr_chain(subexpr_list.last().unwrap(), context) {
 384             subexpr_list.push(subexpr.clone());
 385         }
 386
 387         subexpr_list
 388     }
 389
 390     // Returns the expression's subexpression, if it exists. When the subexpr
 391     // is a try! macro, we'll convert it to shorthand when the option is set.
 392     fn pop_expr_chain(expr: &ast::Expr, context: &RewriteContext<'_>) -> Option<ast::Expr> {
 393         match expr.node {
 394             ast::ExprKind::MethodCall(_, ref expressions) => {
 395                 Some(Self::convert_try(&expressions[0], context))
 396             }
 397             ast::ExprKind::Field(ref subexpr, _)
 398             | ast::ExprKind::Try(ref subexpr)
 399             | ast::ExprKind::Await(_, ref subexpr) => Some(Self::convert_try(subexpr, context)),
 400             _ => None,
 401         }
 402     }
 403
 404     fn convert_try(expr: &ast::Expr, context: &RewriteContext<'_>) -> ast::Expr {
 405         match expr.node {
 406             ast::ExprKind::Mac(ref mac) if context.config.use_try_shorthand() => {
 407                 if let Some(subexpr) = convert_try_mac(mac, context) {
 408                     subexpr
 409                 } else {
 410                     expr.clone()
 411                 }
 412             }
 413             _ => expr.clone(),
 414         }
 415     }
 416 }
 417
 418 impl Rewrite for Chain {
 419     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
 420         debug!("rewrite chain {:?} {:?}", self, shape);
 421
 422         let mut formatter = match context.config.indent_style() {
 423             IndentStyle::Block => {
 424                 Box::new(ChainFormatterBlock::new(self)) as Box<dyn ChainFormatter>
 425             }
 426             IndentStyle::Visual => {
 427                 Box::new(ChainFormatterVisual::new(self)) as Box<dyn ChainFormatter>
 428             }
 429         };
 430
 431         formatter.format_root(&self.parent, context, shape)?;
 432         if let Some(result) = formatter.pure_root() {
 433             return wrap_str(result, context.config.max_width(), shape);
 434         }
 435
 436         // Decide how to layout the rest of the chain.
 437         let child_shape = formatter.child_shape(context, shape)?;
 438
 439         formatter.format_children(context, child_shape)?;
 440         formatter.format_last_child(context, shape, child_shape)?;
 441
 442         let result = formatter.join_rewrites(context, child_shape)?;
 443         wrap_str(result, context.config.max_width(), shape)
 444     }
 445 }
 446
 447 // There are a few types for formatting chains. This is because there is a lot
 448 // in common between formatting with block vs visual indent, but they are
 449 // different enough that branching on the indent all over the place gets ugly.
 450 // Anything that can format a chain is a ChainFormatter.
 451 trait ChainFormatter {
 452     // Parent is the first item in the chain, e.g., `foo` in `foo.bar.baz()`.
 453     // Root is the parent plus any other chain items placed on the first line to
 454     // avoid an orphan. E.g.,
 455     // ```ignore
 456     // foo.bar
 457     //     .baz()
 458     // ```
 459     // If `bar` were not part of the root, then foo would be orphaned and 'float'.
 460     fn format_root(
 461         &mut self,
 462         parent: &ChainItem,
 463         context: &RewriteContext<'_>,
 464         shape: Shape,
 465     ) -> Option<()>;
 466     fn child_shape(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<Shape>;
 467     fn format_children(&mut self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<()>;
 468     fn format_last_child(
 469         &mut self,
 470         context: &RewriteContext<'_>,
 471         shape: Shape,
 472         child_shape: Shape,
 473     ) -> Option<()>;
 474     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String>;
 475     // Returns `Some` if the chain is only a root, None otherwise.
 476     fn pure_root(&mut self) -> Option<String>;
 477 }
 478
 479 // Data and behaviour that is shared by both chain formatters. The concrete
 480 // formatters can delegate much behaviour to `ChainFormatterShared`.
 481 struct ChainFormatterShared<'a> {
 482     // The current working set of child items.
 483     children: &'a [ChainItem],
 484     // The current rewrites of items (includes trailing `?`s, but not any way to
 485     // connect the rewrites together).
 486     rewrites: Vec<String>,
 487     // Whether the chain can fit on one line.
 488     fits_single_line: bool,
 489     // The number of children in the chain. This is not equal to `self.children.len()`
 490     // because `self.children` will change size as we process the chain.
 491     child_count: usize,
 492 }
 493
 494 impl<'a> ChainFormatterShared<'a> {
 495     fn new(chain: &'a Chain) -> ChainFormatterShared<'a> {
 496         ChainFormatterShared {
 497             children: &chain.children,
 498             rewrites: Vec::with_capacity(chain.children.len() + 1),
 499             fits_single_line: false,
 500             child_count: chain.children.len(),
 501         }
 502     }
 503
 504     fn pure_root(&mut self) -> Option<String> {
 505         if self.children.is_empty() {
 506             assert_eq!(self.rewrites.len(), 1);
 507             Some(self.rewrites.pop().unwrap())
 508         } else {
 509             None
 510         }
 511     }
 512
 513     // Rewrite the last child. The last child of a chain requires special treatment. We need to
 514     // know whether 'overflowing' the last child make a better formatting:
 515     //
 516     // A chain with overflowing the last child:
 517     // ```ignore
 518     // parent.child1.child2.last_child(
 519     //     a,
 520     //     b,
 521     //     c,
 522     // )
 523     // ```
 524     //
 525     // A chain without overflowing the last child (in vertical layout):
 526     // ```ignore
 527     // parent
 528     //     .child1
 529     //     .child2
 530     //     .last_child(a, b, c)
 531     // ```
 532     //
 533     // In particular, overflowing is effective when the last child is a method with a multi-lined
 534     // block-like argument (e.g., closure):
 535     // ```ignore
 536     // parent.child1.child2.last_child(|a, b, c| {
 537     //     let x = foo(a, b, c);
 538     //     let y = bar(a, b, c);
 539     //
 540     //     // ...
 541     //
 542     //     result
 543     // })
 544     // ```
 545     fn format_last_child(
 546         &mut self,
 547         may_extend: bool,
 548         context: &RewriteContext<'_>,
 549         shape: Shape,
 550         child_shape: Shape,
 551     ) -> Option<()> {
 552         let last = self.children.last()?;
 553         let extendable = may_extend && last_line_extendable(&self.rewrites[0]);
 554         let prev_last_line_width = last_line_width(&self.rewrites[0]);
 555
 556         // Total of all items excluding the last.
 557         let almost_total = if extendable {
 558             prev_last_line_width
 559         } else {
 560             self.rewrites.iter().map(String::len).sum()
 561         } + last.tries;
 562         let one_line_budget = if self.child_count == 1 {
 563             shape.width
 564         } else {
 565             min(shape.width, context.config.width_heuristics().chain_width)
 566         }
 567         .saturating_sub(almost_total);
 568
 569         let all_in_one_line = !self.children.iter().any(ChainItem::is_comment)
 570             && self.rewrites.iter().all(|s| !s.contains('\n'))
 571             && one_line_budget > 0;
 572         let last_shape = if all_in_one_line {
 573             shape.sub_width(last.tries)?
 574         } else if extendable {
 575             child_shape.sub_width(last.tries)?
 576         } else {
 577             child_shape.sub_width(shape.rhs_overhead(context.config) + last.tries)?
 578         };
 579
 580         let mut last_subexpr_str = None;
 581         if all_in_one_line || extendable {
 582             // First we try to 'overflow' the last child and see if it looks better than using
 583             // vertical layout.
 584             let one_line_shape = if context.use_block_indent() {
 585                 last_shape.offset_left(almost_total)
 586             } else {
 587                 last_shape
 588                     .visual_indent(almost_total)
 589                     .sub_width(almost_total)
 590             };
 591
 592             if let Some(one_line_shape) = one_line_shape {
 593                 if let Some(rw) = last.rewrite(context, one_line_shape) {
 594                     // We allow overflowing here only if both of the following conditions match:
 595                     // 1. The entire chain fits in a single line except the last child.
 596                     // 2. `last_child_str.lines().count() >= 5`.
 597                     let line_count = rw.lines().count();
 598                     let could_fit_single_line = first_line_width(&rw) <= one_line_budget;
 599                     if could_fit_single_line && line_count >= 5 {
 600                         last_subexpr_str = Some(rw);
 601                         self.fits_single_line = all_in_one_line;
 602                     } else {
 603                         // We could not know whether overflowing is better than using vertical
 604                         // layout, just by looking at the overflowed rewrite. Now we rewrite the
 605                         // last child on its own line, and compare two rewrites to choose which is
 606                         // better.
 607                         let last_shape = child_shape
 608                             .sub_width(shape.rhs_overhead(context.config) + last.tries)?;
 609                         match last.rewrite(context, last_shape) {
 610                             Some(ref new_rw) if !could_fit_single_line => {
 611                                 last_subexpr_str = Some(new_rw.clone());
 612                             }
 613                             Some(ref new_rw) if new_rw.lines().count() >= line_count => {
 614                                 last_subexpr_str = Some(rw);
 615                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 616                             }
 617                             new_rw @ Some(..) => {
 618                                 last_subexpr_str = new_rw;
 619                             }
 620                             _ => {
 621                                 last_subexpr_str = Some(rw);
 622                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 623                             }
 624                         }
 625                     }
 626                 }
 627             }
 628         }
 629
 630         let last_shape = if context.use_block_indent() {
 631             last_shape
 632         } else {
 633             child_shape.sub_width(shape.rhs_overhead(context.config) + last.tries)?
 634         };
 635
 636         last_subexpr_str = last_subexpr_str.or_else(|| last.rewrite(context, last_shape));
 637         self.rewrites.push(last_subexpr_str?);
 638         Some(())
 639     }
 640
 641     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String> {
 642         let connector = if self.fits_single_line {
 643             // Yay, we can put everything on one line.
 644             Cow::from("")
 645         } else {
 646             // Use new lines.
 647             if *context.force_one_line_chain.borrow() {
 648                 return None;
 649             }
 650             child_shape.to_string_with_newline(context.config)
 651         };
 652
 653         let mut rewrite_iter = self.rewrites.iter();
 654         let mut result = rewrite_iter.next().unwrap().clone();
 655         let children_iter = self.children.iter();
 656         let iter = rewrite_iter.zip(children_iter);
 657
 658         for (rewrite, chain_item) in iter {
 659             match chain_item.kind {
 660                 ChainItemKind::Comment(_, CommentPosition::Back) => result.push(' '),
 661                 ChainItemKind::Comment(_, CommentPosition::Top) => result.push_str(&connector),
 662                 _ => result.push_str(&connector),
 663             }
 664             result.push_str(&rewrite);
 665         }
 666
 667         Some(result)
 668     }
 669 }
 670
 671 // Formats a chain using block indent.
 672 struct ChainFormatterBlock<'a> {
 673     shared: ChainFormatterShared<'a>,
 674     root_ends_with_block: bool,
 675 }
 676
 677 impl<'a> ChainFormatterBlock<'a> {
 678     fn new(chain: &'a Chain) -> ChainFormatterBlock<'a> {
 679         ChainFormatterBlock {
 680             shared: ChainFormatterShared::new(chain),
 681             root_ends_with_block: false,
 682         }
 683     }
 684 }
 685
 686 impl<'a> ChainFormatter for ChainFormatterBlock<'a> {
 687     fn format_root(
 688         &mut self,
 689         parent: &ChainItem,
 690         context: &RewriteContext<'_>,
 691         shape: Shape,
 692     ) -> Option<()> {
 693         let mut root_rewrite: String = parent.rewrite(context, shape)?;
 694
 695         let mut root_ends_with_block = parent.kind.is_block_like(context, &root_rewrite);
 696         let tab_width = context.config.tab_spaces().saturating_sub(shape.offset);
 697
 698         while root_rewrite.len() <= tab_width && !root_rewrite.contains('\n') {
 699             let item = &self.shared.children[0];
 700             if let ChainItemKind::Comment(..) = item.kind {
 701                 break;
 702             }
 703             let shape = shape.offset_left(root_rewrite.len())?;
 704             match &item.rewrite(context, shape) {
 705                 Some(rewrite) => root_rewrite.push_str(rewrite),
 706                 None => break,
 707             }
 708
 709             root_ends_with_block = last_line_extendable(&root_rewrite);
 710
 711             self.shared.children = &self.shared.children[1..];
 712             if self.shared.children.is_empty() {
 713                 break;
 714             }
 715         }
 716         self.shared.rewrites.push(root_rewrite);
 717         self.root_ends_with_block = root_ends_with_block;
 718         Some(())
 719     }
 720
 721     fn child_shape(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<Shape> {
 722         Some(
 723             if self.root_ends_with_block {
 724                 shape.block_indent(0)
 725             } else {
 726                 shape.block_indent(context.config.tab_spaces())
 727             }
 728             .with_max_width(context.config),
 729         )
 730     }
 731
 732     fn format_children(&mut self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<()> {
 733         for item in &self.shared.children[..self.shared.children.len() - 1] {
 734             let rewrite = item.rewrite(context, child_shape)?;
 735             self.shared.rewrites.push(rewrite);
 736         }
 737         Some(())
 738     }
 739
 740     fn format_last_child(
 741         &mut self,
 742         context: &RewriteContext<'_>,
 743         shape: Shape,
 744         child_shape: Shape,
 745     ) -> Option<()> {
 746         self.shared
 747             .format_last_child(true, context, shape, child_shape)
 748     }
 749
 750     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String> {
 751         self.shared.join_rewrites(context, child_shape)
 752     }
 753
 754     fn pure_root(&mut self) -> Option<String> {
 755         self.shared.pure_root()
 756     }
 757 }
 758
 759 // Format a chain using visual indent.
 760 struct ChainFormatterVisual<'a> {
 761     shared: ChainFormatterShared<'a>,
 762     // The extra offset from the chain's shape to the position of the `.`
 763     offset: usize,
 764 }
 765
 766 impl<'a> ChainFormatterVisual<'a> {
 767     fn new(chain: &'a Chain) -> ChainFormatterVisual<'a> {
 768         ChainFormatterVisual {
 769             shared: ChainFormatterShared::new(chain),
 770             offset: 0,
 771         }
 772     }
 773 }
 774
 775 impl<'a> ChainFormatter for ChainFormatterVisual<'a> {
 776     fn format_root(
 777         &mut self,
 778         parent: &ChainItem,
 779         context: &RewriteContext<'_>,
 780         shape: Shape,
 781     ) -> Option<()> {
 782         let parent_shape = shape.visual_indent(0);
 783         let mut root_rewrite = parent.rewrite(context, parent_shape)?;
 784         let multiline = root_rewrite.contains('\n');
 785         self.offset = if multiline {
 786             last_line_width(&root_rewrite).saturating_sub(shape.used_width())
 787         } else {
 788             trimmed_last_line_width(&root_rewrite)
 789         };
 790
 791         if !multiline || parent.kind.is_block_like(context, &root_rewrite) {
 792             let item = &self.shared.children[0];
 793             if let ChainItemKind::Comment(..) = item.kind {
 794                 self.shared.rewrites.push(root_rewrite);
 795                 return Some(());
 796             }
 797             let child_shape = parent_shape
 798                 .visual_indent(self.offset)
 799                 .sub_width(self.offset)?;
 800             let rewrite = item.rewrite(context, child_shape)?;
 801             match wrap_str(rewrite, context.config.max_width(), shape) {
 802                 Some(rewrite) => root_rewrite.push_str(&rewrite),
 803                 None => {
 804                     // We couldn't fit in at the visual indent, try the last
 805                     // indent.
 806                     let rewrite = item.rewrite(context, parent_shape)?;
 807                     root_rewrite.push_str(&rewrite);
 808                     self.offset = 0;
 809                 }
 810             }
 811
 812             self.shared.children = &self.shared.children[1..];
 813         }
 814
 815         self.shared.rewrites.push(root_rewrite);
 816         Some(())
 817     }
 818
 819     fn child_shape(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<Shape> {
 820         shape
 821             .with_max_width(context.config)
 822             .offset_left(self.offset)
 823             .map(|s| s.visual_indent(0))
 824     }
 825
 826     fn format_children(&mut self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<()> {
 827         for item in &self.shared.children[..self.shared.children.len() - 1] {
 828             let rewrite = item.rewrite(context, child_shape)?;
 829             self.shared.rewrites.push(rewrite);
 830         }
 831         Some(())
 832     }
 833
 834     fn format_last_child(
 835         &mut self,
 836         context: &RewriteContext<'_>,
 837         shape: Shape,
 838         child_shape: Shape,
 839     ) -> Option<()> {
 840         self.shared
 841             .format_last_child(false, context, shape, child_shape)
 842     }
 843
 844     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String> {
 845         self.shared.join_rewrites(context, child_shape)
 846     }
 847
 848     fn pure_root(&mut self) -> Option<String> {
 849         self.shared.pure_root()
 850     }
 851 }
 852
 853 /// Removes try operators (`?`s) that appear in the given string. If removing
 854 /// them leaves an empty line, remove that line as well unless it is the first
 855 /// line (we need the first newline for detecting pre/post comment).
 856 fn trim_tries(s: &str) -> String {
 857     let mut result = String::with_capacity(s.len());
 858     let mut line_buffer = String::with_capacity(s.len());
 859     for (kind, rich_char) in CharClasses::new(s.chars()) {
 860         match rich_char.get_char() {
 861             '\n' => {
 862                 if result.is_empty() || !line_buffer.trim().is_empty() {
 863                     result.push_str(&line_buffer);
 864                     result.push('\n')
 865                 }
 866                 line_buffer.clear();
 867             }
 868             '?' if kind == FullCodeCharKind::Normal => continue,
 869             c => line_buffer.push(c),
 870         }
 871     }
 872     if !line_buffer.trim().is_empty() {
 873         result.push_str(&line_buffer);
 874     }
 875     result
 876 }