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, CharClasses, FullCodeCharKind, RichChar};
  69 use config::IndentStyle;
  70 use expr::rewrite_call;
  71 use lists::extract_pre_comment;
  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, rewrite_ident,
  78     trimmed_last_line_width, 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!(".{}", rewrite_ident(context, ident)),
 194             ChainItemKind::TupleField(ident, nested) => format!(
 195                 "{}.{}",
 196                 if nested { " " } else { "" },
 197                 rewrite_ident(context, ident)
 198             ),
 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, _) | ast::ExprKind::Try(ref subexpr) => {
 398                 Some(Self::convert_try(subexpr, context))
 399             }
 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 => Box::new(ChainFormatterBlock::new(self)) as Box<ChainFormatter>,
 424             IndentStyle::Visual => Box::new(ChainFormatterVisual::new(self)) as Box<ChainFormatter>,
 425         };
 426
 427         formatter.format_root(&self.parent, context, shape)?;
 428         if let Some(result) = formatter.pure_root() {
 429             return wrap_str(result, context.config.max_width(), shape);
 430         }
 431
 432         // Decide how to layout the rest of the chain.
 433         let child_shape = formatter.child_shape(context, shape)?;
 434
 435         formatter.format_children(context, child_shape)?;
 436         formatter.format_last_child(context, shape, child_shape)?;
 437
 438         let result = formatter.join_rewrites(context, child_shape)?;
 439         wrap_str(result, context.config.max_width(), shape)
 440     }
 441 }
 442
 443 // There are a few types for formatting chains. This is because there is a lot
 444 // in common between formatting with block vs visual indent, but they are
 445 // different enough that branching on the indent all over the place gets ugly.
 446 // Anything that can format a chain is a ChainFormatter.
 447 trait ChainFormatter {
 448     // Parent is the first item in the chain, e.g., `foo` in `foo.bar.baz()`.
 449     // Root is the parent plus any other chain items placed on the first line to
 450     // avoid an orphan. E.g.,
 451     // ```
 452     // foo.bar
 453     //     .baz()
 454     // ```
 455     // If `bar` were not part of the root, then foo would be orphaned and 'float'.
 456     fn format_root(
 457         &mut self,
 458         parent: &ChainItem,
 459         context: &RewriteContext,
 460         shape: Shape,
 461     ) -> Option<()>;
 462     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape>;
 463     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()>;
 464     fn format_last_child(
 465         &mut self,
 466         context: &RewriteContext,
 467         shape: Shape,
 468         child_shape: Shape,
 469     ) -> Option<()>;
 470     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String>;
 471     // Returns `Some` if the chain is only a root, None otherwise.
 472     fn pure_root(&mut self) -> Option<String>;
 473 }
 474
 475 // Data and behaviour that is shared by both chain formatters. The concrete
 476 // formatters can delegate much behaviour to `ChainFormatterShared`.
 477 struct ChainFormatterShared<'a> {
 478     // The current working set of child items.
 479     children: &'a [ChainItem],
 480     // The current rewrites of items (includes trailing `?`s, but not any way to
 481     // connect the rewrites together).
 482     rewrites: Vec<String>,
 483     // Whether the chain can fit on one line.
 484     fits_single_line: bool,
 485     // The number of children in the chain. This is not equal to `self.children.len()`
 486     // because `self.children` will change size as we process the chain.
 487     child_count: usize,
 488 }
 489
 490 impl<'a> ChainFormatterShared<'a> {
 491     fn new(chain: &'a Chain) -> ChainFormatterShared<'a> {
 492         ChainFormatterShared {
 493             children: &chain.children,
 494             rewrites: Vec::with_capacity(chain.children.len() + 1),
 495             fits_single_line: false,
 496             child_count: chain.children.len(),
 497         }
 498     }
 499
 500     fn pure_root(&mut self) -> Option<String> {
 501         if self.children.is_empty() {
 502             assert_eq!(self.rewrites.len(), 1);
 503             Some(self.rewrites.pop().unwrap())
 504         } else {
 505             None
 506         }
 507     }
 508
 509     // Rewrite the last child. The last child of a chain requires special treatment. We need to
 510     // know whether 'overflowing' the last child make a better formatting:
 511     //
 512     // A chain with overflowing the last child:
 513     // ```
 514     // parent.child1.child2.last_child(
 515     //     a,
 516     //     b,
 517     //     c,
 518     // )
 519     // ```
 520     //
 521     // A chain without overflowing the last child (in vertical layout):
 522     // ```
 523     // parent
 524     //     .child1
 525     //     .child2
 526     //     .last_child(a, b, c)
 527     // ```
 528     //
 529     // In particular, overflowing is effective when the last child is a method with a multi-lined
 530     // block-like argument (e.g. closure):
 531     // ```
 532     // parent.child1.child2.last_child(|a, b, c| {
 533     //     let x = foo(a, b, c);
 534     //     let y = bar(a, b, c);
 535     //
 536     //     // ...
 537     //
 538     //     result
 539     // })
 540     // ```
 541     fn format_last_child(
 542         &mut self,
 543         may_extend: bool,
 544         context: &RewriteContext,
 545         shape: Shape,
 546         child_shape: Shape,
 547     ) -> Option<()> {
 548         let last = self.children.last()?;
 549         let extendable = may_extend && last_line_extendable(&self.rewrites[0]);
 550         let prev_last_line_width = last_line_width(&self.rewrites[0]);
 551
 552         // Total of all items excluding the last.
 553         let almost_total = if extendable {
 554             prev_last_line_width
 555         } else {
 556             self.rewrites.iter().fold(0, |a, b| a + b.len())
 557         } + last.tries;
 558         let one_line_budget = if self.child_count == 1 {
 559             shape.width
 560         } else {
 561             min(shape.width, context.config.width_heuristics().chain_width)
 562         }.saturating_sub(almost_total);
 563
 564         let all_in_one_line = !self.children.iter().any(ChainItem::is_comment)
 565             && self.rewrites.iter().all(|s| !s.contains('\n'))
 566             && one_line_budget > 0;
 567         let last_shape = if all_in_one_line {
 568             shape.sub_width(last.tries)?
 569         } else if extendable {
 570             child_shape.sub_width(last.tries)?
 571         } else {
 572             child_shape.sub_width(shape.rhs_overhead(context.config) + last.tries)?
 573         };
 574
 575         let mut last_subexpr_str = None;
 576         if all_in_one_line || extendable {
 577             // First we try to 'overflow' the last child and see if it looks better than using
 578             // vertical layout.
 579             if let Some(one_line_shape) = last_shape.offset_left(almost_total) {
 580                 if let Some(rw) = last.rewrite(context, one_line_shape) {
 581                     // We allow overflowing here only if both of the following conditions match:
 582                     // 1. The entire chain fits in a single line except the last child.
 583                     // 2. `last_child_str.lines().count() >= 5`.
 584                     let line_count = rw.lines().count();
 585                     let could_fit_single_line = first_line_width(&rw) <= one_line_budget;
 586                     if could_fit_single_line && line_count >= 5 {
 587                         last_subexpr_str = Some(rw);
 588                         self.fits_single_line = all_in_one_line;
 589                     } else {
 590                         // We could not know whether overflowing is better than using vertical
 591                         // layout, just by looking at the overflowed rewrite. Now we rewrite the
 592                         // last child on its own line, and compare two rewrites to choose which is
 593                         // better.
 594                         let last_shape = child_shape
 595                             .sub_width(shape.rhs_overhead(context.config) + last.tries)?;
 596                         match last.rewrite(context, last_shape) {
 597                             Some(ref new_rw) if !could_fit_single_line => {
 598                                 last_subexpr_str = Some(new_rw.clone());
 599                             }
 600                             Some(ref new_rw) if new_rw.lines().count() >= line_count => {
 601                                 last_subexpr_str = Some(rw);
 602                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 603                             }
 604                             new_rw @ Some(..) => {
 605                                 last_subexpr_str = new_rw;
 606                             }
 607                             _ => {
 608                                 last_subexpr_str = Some(rw);
 609                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 610                             }
 611                         }
 612                     }
 613                 }
 614             }
 615         }
 616
 617         let last_shape = if context.use_block_indent() {
 618             last_shape
 619         } else {
 620             child_shape.sub_width(shape.rhs_overhead(context.config) + last.tries)?
 621         };
 622
 623         last_subexpr_str = last_subexpr_str.or_else(|| last.rewrite(context, last_shape));
 624         self.rewrites.push(last_subexpr_str?);
 625         Some(())
 626     }
 627
 628     fn join_rewrites(
 629         &self,
 630         context: &RewriteContext,
 631         child_shape: Shape,
 632         block_like_iter: impl Iterator<Item = bool>,
 633     ) -> Option<String> {
 634         let connector = if self.fits_single_line {
 635             // Yay, we can put everything on one line.
 636             Cow::from("")
 637         } else {
 638             // Use new lines.
 639             if *context.force_one_line_chain.borrow() {
 640                 return None;
 641             }
 642             child_shape.to_string_with_newline(context.config)
 643         };
 644
 645         let mut rewrite_iter = self.rewrites.iter();
 646         let mut result = rewrite_iter.next().unwrap().clone();
 647         let children_iter = self.children.iter();
 648         let iter = rewrite_iter.zip(block_like_iter).zip(children_iter);
 649
 650         for ((rewrite, prev_is_block_like), chain_item) in iter {
 651             match chain_item.kind {
 652                 ChainItemKind::Comment(_, CommentPosition::Back) => result.push(' '),
 653                 ChainItemKind::Comment(_, CommentPosition::Top) => result.push_str(&connector),
 654                 _ => {
 655                     if !prev_is_block_like {
 656                         result.push_str(&connector);
 657                     }
 658                 }
 659             }
 660             result.push_str(&rewrite);
 661         }
 662
 663         Some(result)
 664     }
 665 }
 666
 667 // Formats a chain using block indent.
 668 struct ChainFormatterBlock<'a> {
 669     shared: ChainFormatterShared<'a>,
 670     // For each rewrite, whether the corresponding item is block-like.
 671     is_block_like: Vec<bool>,
 672 }
 673
 674 impl<'a> ChainFormatterBlock<'a> {
 675     fn new(chain: &'a Chain) -> ChainFormatterBlock<'a> {
 676         ChainFormatterBlock {
 677             shared: ChainFormatterShared::new(chain),
 678             is_block_like: Vec::with_capacity(chain.children.len() + 1),
 679         }
 680     }
 681 }
 682
 683 impl<'a> ChainFormatter for ChainFormatterBlock<'a> {
 684     fn format_root(
 685         &mut self,
 686         parent: &ChainItem,
 687         context: &RewriteContext,
 688         shape: Shape,
 689     ) -> Option<()> {
 690         let mut root_rewrite: String = parent.rewrite(context, shape)?;
 691
 692         let mut root_ends_with_block = parent.kind.is_block_like(context, &root_rewrite);
 693         let tab_width = context.config.tab_spaces().saturating_sub(shape.offset);
 694
 695         while root_rewrite.len() <= tab_width && !root_rewrite.contains('\n') {
 696             let item = &self.shared.children[0];
 697             if let ChainItemKind::Comment(..) = item.kind {
 698                 break;
 699             }
 700             let shape = shape.offset_left(root_rewrite.len())?;
 701             match &item.rewrite(context, shape) {
 702                 Some(rewrite) => root_rewrite.push_str(rewrite),
 703                 None => break,
 704             }
 705
 706             root_ends_with_block = item.kind.is_block_like(context, &root_rewrite);
 707
 708             self.shared.children = &self.shared.children[1..];
 709             if self.shared.children.is_empty() {
 710                 break;
 711             }
 712         }
 713         self.is_block_like.push(root_ends_with_block);
 714         self.shared.rewrites.push(root_rewrite);
 715         Some(())
 716     }
 717
 718     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape> {
 719         Some(
 720             if self.is_block_like[0] {
 721                 shape.block_indent(0)
 722             } else {
 723                 shape.block_indent(context.config.tab_spaces())
 724             }.with_max_width(context.config),
 725         )
 726     }
 727
 728     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()> {
 729         for item in &self.shared.children[..self.shared.children.len() - 1] {
 730             let rewrite = item.rewrite(context, child_shape)?;
 731             self.is_block_like
 732                 .push(item.kind.is_block_like(context, &rewrite));
 733             self.shared.rewrites.push(rewrite);
 734         }
 735         Some(())
 736     }
 737
 738     fn format_last_child(
 739         &mut self,
 740         context: &RewriteContext,
 741         shape: Shape,
 742         child_shape: Shape,
 743     ) -> Option<()> {
 744         self.shared
 745             .format_last_child(true, context, shape, child_shape)
 746     }
 747
 748     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 749         self.shared
 750             .join_rewrites(context, child_shape, self.is_block_like.iter().cloned())
 751     }
 752
 753     fn pure_root(&mut self) -> Option<String> {
 754         self.shared.pure_root()
 755     }
 756 }
 757
 758 // Format a chain using visual indent.
 759 struct ChainFormatterVisual<'a> {
 760     shared: ChainFormatterShared<'a>,
 761     // The extra offset from the chain's shape to the position of the `.`
 762     offset: usize,
 763 }
 764
 765 impl<'a> ChainFormatterVisual<'a> {
 766     fn new(chain: &'a Chain) -> ChainFormatterVisual<'a> {
 767         ChainFormatterVisual {
 768             shared: ChainFormatterShared::new(chain),
 769             offset: 0,
 770         }
 771     }
 772 }
 773
 774 impl<'a> ChainFormatter for ChainFormatterVisual<'a> {
 775     fn format_root(
 776         &mut self,
 777         parent: &ChainItem,
 778         context: &RewriteContext,
 779         shape: Shape,
 780     ) -> Option<()> {
 781         let parent_shape = shape.visual_indent(0);
 782         let mut root_rewrite = parent.rewrite(context, parent_shape)?;
 783         let multiline = root_rewrite.contains('\n');
 784         self.offset = if multiline {
 785             last_line_width(&root_rewrite).saturating_sub(shape.used_width())
 786         } else {
 787             trimmed_last_line_width(&root_rewrite)
 788         };
 789
 790         if !multiline || parent.kind.is_block_like(context, &root_rewrite) {
 791             let item = &self.shared.children[0];
 792             if let ChainItemKind::Comment(..) = item.kind {
 793                 self.shared.rewrites.push(root_rewrite);
 794                 return Some(());
 795             }
 796             let child_shape = parent_shape
 797                 .visual_indent(self.offset)
 798                 .sub_width(self.offset)?;
 799             let rewrite = item.rewrite(context, child_shape)?;
 800             match wrap_str(rewrite, context.config.max_width(), shape) {
 801                 Some(rewrite) => root_rewrite.push_str(&rewrite),
 802                 None => {
 803                     // We couldn't fit in at the visual indent, try the last
 804                     // indent.
 805                     let rewrite = item.rewrite(context, parent_shape)?;
 806                     root_rewrite.push_str(&rewrite);
 807                     self.offset = 0;
 808                 }
 809             }
 810
 811             self.shared.children = &self.shared.children[1..];
 812         }
 813
 814         self.shared.rewrites.push(root_rewrite);
 815         Some(())
 816     }
 817
 818     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape> {
 819         shape
 820             .with_max_width(context.config)
 821             .offset_left(self.offset)
 822             .map(|s| s.visual_indent(0))
 823     }
 824
 825     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()> {
 826         for item in &self.shared.children[..self.shared.children.len() - 1] {
 827             let rewrite = item.rewrite(context, child_shape)?;
 828             self.shared.rewrites.push(rewrite);
 829         }
 830         Some(())
 831     }
 832
 833     fn format_last_child(
 834         &mut self,
 835         context: &RewriteContext,
 836         shape: Shape,
 837         child_shape: Shape,
 838     ) -> Option<()> {
 839         self.shared
 840             .format_last_child(false, context, shape, child_shape)
 841     }
 842
 843     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 844         self.shared
 845             .join_rewrites(context, child_shape, iter::repeat(false))
 846     }
 847
 848     fn pure_root(&mut self) -> Option<String> {
 849         self.shared.pure_root()
 850     }
 851 }
 852
 853 // States whether an expression's last line exclusively consists of closing
 854 // parens, braces, and brackets in its idiomatic formatting.
 855 fn is_block_expr(context: &RewriteContext, expr: &ast::Expr, repr: &str) -> bool {
 856     match expr.node {
 857         ast::ExprKind::Mac(..)
 858         | ast::ExprKind::Call(..)
 859         | ast::ExprKind::MethodCall(..)
 860         | ast::ExprKind::Struct(..)
 861         | ast::ExprKind::While(..)
 862         | ast::ExprKind::WhileLet(..)
 863         | ast::ExprKind::If(..)
 864         | ast::ExprKind::IfLet(..)
 865         | ast::ExprKind::Block(..)
 866         | ast::ExprKind::Loop(..)
 867         | ast::ExprKind::ForLoop(..)
 868         | ast::ExprKind::Match(..) => repr.contains('\n'),
 869         ast::ExprKind::Paren(ref expr)
 870         | ast::ExprKind::Binary(_, _, ref expr)
 871         | ast::ExprKind::Index(_, ref expr)
 872         | ast::ExprKind::Unary(_, ref expr)
 873         | ast::ExprKind::Closure(_, _, _, _, ref expr, _)
 874         | ast::ExprKind::Try(ref expr)
 875         | ast::ExprKind::Yield(Some(ref expr)) => is_block_expr(context, expr, repr),
 876         // This can only be a string lit
 877         ast::ExprKind::Lit(_) => {
 878             repr.contains('\n') && trimmed_last_line_width(repr) <= context.config.tab_spaces()
 879         }
 880         _ => false,
 881     }
 882 }
 883
 884 /// Remove try operators (`?`s) that appear in the given string. If removing
 885 /// them leaves an empty line, remove that line as well unless it is the first
 886 /// line (we need the first newline for detecting pre/post comment).
 887 fn trim_tries(s: &str) -> String {
 888     let mut result = String::with_capacity(s.len());
 889     let mut line_buffer = String::with_capacity(s.len());
 890     for (kind, rich_char) in CharClasses::new(s.chars()) {
 891         match rich_char.get_char() {
 892             '\n' => {
 893                 if result.is_empty() || !line_buffer.trim().is_empty() {
 894                     result.push_str(&line_buffer);
 895                     result.push('\n')
 896                 }
 897                 line_buffer.clear();
 898             }
 899             '?' if kind == FullCodeCharKind::Normal => continue,
 900             c => line_buffer.push(c),
 901         }
 902     }
 903     if !line_buffer.trim().is_empty() {
 904         result.push_str(&line_buffer);
 905     }
 906     result
 907 }