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, 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 handle_post_comment(
 279             post_comment_span: Span,
 280             post_comment_snippet: &str,
 281             prev_span_end: &mut BytePos,
 282             children: &mut Vec<ChainItem>,
 283         ) {
 284             let white_spaces: &[_] = &[' ', '\t'];
 285             if post_comment_snippet
 286                 .trim_matches(white_spaces)
 287                 .starts_with('\n')
 288             {
 289                 // No post comment.
 290                 return;
 291             }
 292             // HACK: Treat `?`s as separators.
 293             let trimmed_snippet = post_comment_snippet.trim_matches('?');
 294             let comment_end = get_comment_end(trimmed_snippet, "?", "", false);
 295             let maybe_post_comment = extract_post_comment(trimmed_snippet, comment_end, "?")
 296                 .and_then(|comment| {
 297                     if comment.is_empty() {
 298                         None
 299                     } else {
 300                         Some((comment, comment_end))
 301                     }
 302                 });
 303
 304             if let Some((post_comment, comment_end)) = maybe_post_comment {
 305                 children.push(ChainItem::comment(
 306                     post_comment_span,
 307                     post_comment,
 308                     CommentPosition::Back,
 309                 ));
 310                 *prev_span_end = *prev_span_end + BytePos(comment_end as u32);
 311             }
 312         }
 313
 314         let parent = rev_children.pop().unwrap();
 315         let mut children = vec![];
 316         let mut prev_span_end = parent.span.hi();
 317         let mut iter = rev_children.into_iter().rev().peekable();
 318         if let Some(first_chain_item) = iter.peek() {
 319             let comment_span = mk_sp(prev_span_end, first_chain_item.span.lo());
 320             let comment_snippet = context.snippet(comment_span);
 321             if !is_tries(comment_snippet.trim()) {
 322                 handle_post_comment(
 323                     comment_span,
 324                     comment_snippet,
 325                     &mut prev_span_end,
 326                     &mut children,
 327                 );
 328             }
 329         }
 330         while let Some(chain_item) = iter.next() {
 331             let comment_snippet = context.snippet(chain_item.span);
 332             // FIXME: Figure out the way to get a correct span when converting `try!` to `?`.
 333             let handle_comment =
 334                 !(context.config.use_try_shorthand() || is_tries(comment_snippet.trim()));
 335
 336             // Pre-comment
 337             if handle_comment {
 338                 let pre_comment_span = mk_sp(prev_span_end, chain_item.span.lo());
 339                 let pre_comment_snippet = context.snippet(pre_comment_span);
 340                 let pre_comment_snippet = pre_comment_snippet.trim().trim_matches('?');
 341                 let (pre_comment, _) = extract_pre_comment(pre_comment_snippet);
 342                 match pre_comment {
 343                     Some(ref comment) if !comment.is_empty() => {
 344                         children.push(ChainItem::comment(
 345                             pre_comment_span,
 346                             comment.to_owned(),
 347                             CommentPosition::Top,
 348                         ));
 349                     }
 350                     _ => (),
 351                 }
 352             }
 353
 354             prev_span_end = chain_item.span.hi();
 355             children.push(chain_item);
 356
 357             // Post-comment
 358             if !handle_comment || iter.peek().is_none() {
 359                 continue;
 360             }
 361
 362             let next_lo = iter.peek().unwrap().span.lo();
 363             let post_comment_span = mk_sp(prev_span_end, next_lo);
 364             let post_comment_snippet = context.snippet(post_comment_span);
 365             handle_post_comment(
 366                 post_comment_span,
 367                 post_comment_snippet,
 368                 &mut prev_span_end,
 369                 &mut children,
 370             );
 371         }
 372
 373         Chain { parent, children }
 374     }
 375
 376     // Returns a Vec of the prefixes of the chain.
 377     // E.g., for input `a.b.c` we return [`a.b.c`, `a.b`, 'a']
 378     fn make_subexpr_list(expr: &ast::Expr, context: &RewriteContext) -> Vec<ast::Expr> {
 379         let mut subexpr_list = vec![expr.clone()];
 380
 381         while let Some(subexpr) = Self::pop_expr_chain(subexpr_list.last().unwrap(), context) {
 382             subexpr_list.push(subexpr.clone());
 383         }
 384
 385         subexpr_list
 386     }
 387
 388     // Returns the expression's subexpression, if it exists. When the subexpr
 389     // is a try! macro, we'll convert it to shorthand when the option is set.
 390     fn pop_expr_chain(expr: &ast::Expr, context: &RewriteContext) -> Option<ast::Expr> {
 391         match expr.node {
 392             ast::ExprKind::MethodCall(_, ref expressions) => {
 393                 Some(Self::convert_try(&expressions[0], context))
 394             }
 395             ast::ExprKind::Field(ref subexpr, _) | ast::ExprKind::Try(ref subexpr) => {
 396                 Some(Self::convert_try(subexpr, context))
 397             }
 398             _ => None,
 399         }
 400     }
 401
 402     fn convert_try(expr: &ast::Expr, context: &RewriteContext) -> ast::Expr {
 403         match expr.node {
 404             ast::ExprKind::Mac(ref mac) if context.config.use_try_shorthand() => {
 405                 if let Some(subexpr) = convert_try_mac(mac, context) {
 406                     subexpr
 407                 } else {
 408                     expr.clone()
 409                 }
 410             }
 411             _ => expr.clone(),
 412         }
 413     }
 414 }
 415
 416 impl Rewrite for Chain {
 417     fn rewrite(&self, context: &RewriteContext, shape: Shape) -> Option<String> {
 418         debug!("rewrite chain {:?} {:?}", self, shape);
 419
 420         let mut formatter = match context.config.indent_style() {
 421             IndentStyle::Block => Box::new(ChainFormatterBlock::new(self)) as Box<ChainFormatter>,
 422             IndentStyle::Visual => Box::new(ChainFormatterVisual::new(self)) as Box<ChainFormatter>,
 423         };
 424
 425         formatter.format_root(&self.parent, context, shape)?;
 426         if let Some(result) = formatter.pure_root() {
 427             return wrap_str(result, context.config.max_width(), shape);
 428         }
 429
 430         // Decide how to layout the rest of the chain.
 431         let child_shape = formatter.child_shape(context, shape)?;
 432
 433         formatter.format_children(context, child_shape)?;
 434         formatter.format_last_child(context, shape, child_shape)?;
 435
 436         let result = formatter.join_rewrites(context, child_shape)?;
 437         wrap_str(result, context.config.max_width(), shape)
 438     }
 439 }
 440
 441 // There are a few types for formatting chains. This is because there is a lot
 442 // in common between formatting with block vs visual indent, but they are
 443 // different enough that branching on the indent all over the place gets ugly.
 444 // Anything that can format a chain is a ChainFormatter.
 445 trait ChainFormatter {
 446     // Parent is the first item in the chain, e.g., `foo` in `foo.bar.baz()`.
 447     // Root is the parent plus any other chain items placed on the first line to
 448     // avoid an orphan. E.g.,
 449     // ```
 450     // foo.bar
 451     //     .baz()
 452     // ```
 453     // If `bar` were not part of the root, then foo would be orphaned and 'float'.
 454     fn format_root(
 455         &mut self,
 456         parent: &ChainItem,
 457         context: &RewriteContext,
 458         shape: Shape,
 459     ) -> Option<()>;
 460     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape>;
 461     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()>;
 462     fn format_last_child(
 463         &mut self,
 464         context: &RewriteContext,
 465         shape: Shape,
 466         child_shape: Shape,
 467     ) -> Option<()>;
 468     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String>;
 469     // Returns `Some` if the chain is only a root, None otherwise.
 470     fn pure_root(&mut self) -> Option<String>;
 471 }
 472
 473 // Data and behaviour that is shared by both chain formatters. The concrete
 474 // formatters can delegate much behaviour to `ChainFormatterShared`.
 475 struct ChainFormatterShared<'a> {
 476     // The current working set of child items.
 477     children: &'a [ChainItem],
 478     // The current rewrites of items (includes trailing `?`s, but not any way to
 479     // connect the rewrites together).
 480     rewrites: Vec<String>,
 481     // Whether the chain can fit on one line.
 482     fits_single_line: bool,
 483     // The number of children in the chain. This is not equal to `self.children.len()`
 484     // because `self.children` will change size as we process the chain.
 485     child_count: usize,
 486 }
 487
 488 impl<'a> ChainFormatterShared<'a> {
 489     fn new(chain: &'a Chain) -> ChainFormatterShared<'a> {
 490         ChainFormatterShared {
 491             children: &chain.children,
 492             rewrites: Vec::with_capacity(chain.children.len() + 1),
 493             fits_single_line: false,
 494             child_count: chain.children.len(),
 495         }
 496     }
 497
 498     fn pure_root(&mut self) -> Option<String> {
 499         if self.children.is_empty() {
 500             assert_eq!(self.rewrites.len(), 1);
 501             Some(self.rewrites.pop().unwrap())
 502         } else {
 503             None
 504         }
 505     }
 506
 507     // Rewrite the last child. The last child of a chain requires special treatment. We need to
 508     // know whether 'overflowing' the last child make a better formatting:
 509     //
 510     // A chain with overflowing the last child:
 511     // ```
 512     // parent.child1.child2.last_child(
 513     //     a,
 514     //     b,
 515     //     c,
 516     // )
 517     // ```
 518     //
 519     // A chain without overflowing the last child (in vertical layout):
 520     // ```
 521     // parent
 522     //     .child1
 523     //     .child2
 524     //     .last_child(a, b, c)
 525     // ```
 526     //
 527     // In particular, overflowing is effective when the last child is a method with a multi-lined
 528     // block-like argument (e.g. closure):
 529     // ```
 530     // parent.child1.child2.last_child(|a, b, c| {
 531     //     let x = foo(a, b, c);
 532     //     let y = bar(a, b, c);
 533     //
 534     //     // ...
 535     //
 536     //     result
 537     // })
 538     // ```
 539     fn format_last_child(
 540         &mut self,
 541         may_extend: bool,
 542         context: &RewriteContext,
 543         shape: Shape,
 544         child_shape: Shape,
 545     ) -> Option<()> {
 546         let last = self.children.last()?;
 547         let extendable = may_extend && last_line_extendable(&self.rewrites[0]);
 548         let prev_last_line_width = last_line_width(&self.rewrites[0]);
 549
 550         // Total of all items excluding the last.
 551         let almost_total = if extendable {
 552             prev_last_line_width
 553         } else {
 554             self.rewrites.iter().fold(0, |a, b| a + b.len())
 555         } + last.tries;
 556         let one_line_budget = if self.child_count == 1 {
 557             shape.width
 558         } else {
 559             min(shape.width, context.config.width_heuristics().chain_width)
 560         }.saturating_sub(almost_total);
 561
 562         let all_in_one_line = !self.children.iter().any(ChainItem::is_comment)
 563             && self.rewrites.iter().all(|s| !s.contains('\n'))
 564             && one_line_budget > 0;
 565         let last_shape = if all_in_one_line {
 566             shape.sub_width(last.tries)?
 567         } else if extendable {
 568             child_shape.sub_width(last.tries)?
 569         } else {
 570             child_shape.sub_width(shape.rhs_overhead(context.config) + last.tries)?
 571         };
 572
 573         let mut last_subexpr_str = None;
 574         if all_in_one_line || extendable {
 575             // First we try to 'overflow' the last child and see if it looks better than using
 576             // vertical layout.
 577             if let Some(one_line_shape) = last_shape.offset_left(almost_total) {
 578                 if let Some(rw) = last.rewrite(context, one_line_shape) {
 579                     // We allow overflowing here only if both of the following conditions match:
 580                     // 1. The entire chain fits in a single line except the last child.
 581                     // 2. `last_child_str.lines().count() >= 5`.
 582                     let line_count = rw.lines().count();
 583                     let could_fit_single_line = first_line_width(&rw) <= one_line_budget;
 584                     if could_fit_single_line && line_count >= 5 {
 585                         last_subexpr_str = Some(rw);
 586                         self.fits_single_line = all_in_one_line;
 587                     } else {
 588                         // We could not know whether overflowing is better than using vertical
 589                         // layout, just by looking at the overflowed rewrite. Now we rewrite the
 590                         // last child on its own line, and compare two rewrites to choose which is
 591                         // better.
 592                         let last_shape = child_shape
 593                             .sub_width(shape.rhs_overhead(context.config) + last.tries)?;
 594                         match last.rewrite(context, last_shape) {
 595                             Some(ref new_rw) if !could_fit_single_line => {
 596                                 last_subexpr_str = Some(new_rw.clone());
 597                             }
 598                             Some(ref new_rw) if new_rw.lines().count() >= line_count => {
 599                                 last_subexpr_str = Some(rw);
 600                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 601                             }
 602                             new_rw @ Some(..) => {
 603                                 last_subexpr_str = new_rw;
 604                             }
 605                             _ => {
 606                                 last_subexpr_str = Some(rw);
 607                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 608                             }
 609                         }
 610                     }
 611                 }
 612             }
 613         }
 614
 615         last_subexpr_str = last_subexpr_str.or_else(|| last.rewrite(context, last_shape));
 616         self.rewrites.push(last_subexpr_str?);
 617         Some(())
 618     }
 619
 620     fn join_rewrites(
 621         &self,
 622         context: &RewriteContext,
 623         child_shape: Shape,
 624         block_like_iter: impl Iterator<Item = bool>,
 625     ) -> Option<String> {
 626         let connector = if self.fits_single_line {
 627             // Yay, we can put everything on one line.
 628             Cow::from("")
 629         } else {
 630             // Use new lines.
 631             if *context.force_one_line_chain.borrow() {
 632                 return None;
 633             }
 634             child_shape.to_string_with_newline(context.config)
 635         };
 636
 637         let mut rewrite_iter = self.rewrites.iter();
 638         let mut result = rewrite_iter.next().unwrap().clone();
 639         let children_iter = self.children.iter();
 640         let iter = rewrite_iter.zip(block_like_iter).zip(children_iter);
 641
 642         for ((rewrite, prev_is_block_like), chain_item) in iter {
 643             match chain_item.kind {
 644                 ChainItemKind::Comment(_, CommentPosition::Back) => result.push(' '),
 645                 ChainItemKind::Comment(_, CommentPosition::Top) => result.push_str(&connector),
 646                 _ => {
 647                     if !prev_is_block_like {
 648                         result.push_str(&connector);
 649                     }
 650                 }
 651             }
 652             result.push_str(&rewrite);
 653         }
 654
 655         Some(result)
 656     }
 657 }
 658
 659 // Formats a chain using block indent.
 660 struct ChainFormatterBlock<'a> {
 661     shared: ChainFormatterShared<'a>,
 662     // For each rewrite, whether the corresponding item is block-like.
 663     is_block_like: Vec<bool>,
 664 }
 665
 666 impl<'a> ChainFormatterBlock<'a> {
 667     fn new(chain: &'a Chain) -> ChainFormatterBlock<'a> {
 668         ChainFormatterBlock {
 669             shared: ChainFormatterShared::new(chain),
 670             is_block_like: Vec::with_capacity(chain.children.len() + 1),
 671         }
 672     }
 673 }
 674
 675 impl<'a> ChainFormatter for ChainFormatterBlock<'a> {
 676     fn format_root(
 677         &mut self,
 678         parent: &ChainItem,
 679         context: &RewriteContext,
 680         shape: Shape,
 681     ) -> Option<()> {
 682         let mut root_rewrite: String = parent.rewrite(context, shape)?;
 683
 684         let mut root_ends_with_block = parent.kind.is_block_like(context, &root_rewrite);
 685         let tab_width = context.config.tab_spaces().saturating_sub(shape.offset);
 686
 687         while root_rewrite.len() <= tab_width && !root_rewrite.contains('\n') {
 688             let item = &self.shared.children[0];
 689             if let ChainItemKind::Comment(..) = item.kind {
 690                 break;
 691             }
 692             let shape = shape.offset_left(root_rewrite.len())?;
 693             match &item.rewrite(context, shape) {
 694                 Some(rewrite) => root_rewrite.push_str(rewrite),
 695                 None => break,
 696             }
 697
 698             root_ends_with_block = item.kind.is_block_like(context, &root_rewrite);
 699
 700             self.shared.children = &self.shared.children[1..];
 701             if self.shared.children.is_empty() {
 702                 break;
 703             }
 704         }
 705         self.is_block_like.push(root_ends_with_block);
 706         self.shared.rewrites.push(root_rewrite);
 707         Some(())
 708     }
 709
 710     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape> {
 711         Some(
 712             if self.is_block_like[0] {
 713                 shape.block_indent(0)
 714             } else {
 715                 shape.block_indent(context.config.tab_spaces())
 716             }.with_max_width(context.config),
 717         )
 718     }
 719
 720     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()> {
 721         for item in &self.shared.children[..self.shared.children.len() - 1] {
 722             let rewrite = item.rewrite(context, child_shape)?;
 723             self.is_block_like
 724                 .push(item.kind.is_block_like(context, &rewrite));
 725             self.shared.rewrites.push(rewrite);
 726         }
 727         Some(())
 728     }
 729
 730     fn format_last_child(
 731         &mut self,
 732         context: &RewriteContext,
 733         shape: Shape,
 734         child_shape: Shape,
 735     ) -> Option<()> {
 736         self.shared
 737             .format_last_child(true, context, shape, child_shape)
 738     }
 739
 740     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 741         self.shared
 742             .join_rewrites(context, child_shape, self.is_block_like.iter().cloned())
 743     }
 744
 745     fn pure_root(&mut self) -> Option<String> {
 746         self.shared.pure_root()
 747     }
 748 }
 749
 750 // Format a chain using visual indent.
 751 struct ChainFormatterVisual<'a> {
 752     shared: ChainFormatterShared<'a>,
 753     // The extra offset from the chain's shape to the position of the `.`
 754     offset: usize,
 755 }
 756
 757 impl<'a> ChainFormatterVisual<'a> {
 758     fn new(chain: &'a Chain) -> ChainFormatterVisual<'a> {
 759         ChainFormatterVisual {
 760             shared: ChainFormatterShared::new(chain),
 761             offset: 0,
 762         }
 763     }
 764 }
 765
 766 impl<'a> ChainFormatter for ChainFormatterVisual<'a> {
 767     fn format_root(
 768         &mut self,
 769         parent: &ChainItem,
 770         context: &RewriteContext,
 771         shape: Shape,
 772     ) -> Option<()> {
 773         let parent_shape = shape.visual_indent(0);
 774         let mut root_rewrite = parent.rewrite(context, parent_shape)?;
 775         let multiline = root_rewrite.contains('\n');
 776         self.offset = if multiline {
 777             last_line_width(&root_rewrite).saturating_sub(shape.used_width())
 778         } else {
 779             trimmed_last_line_width(&root_rewrite)
 780         };
 781
 782         if !multiline || parent.kind.is_block_like(context, &root_rewrite) {
 783             let item = &self.shared.children[0];
 784             if let ChainItemKind::Comment(..) = item.kind {
 785                 self.shared.rewrites.push(root_rewrite);
 786                 return Some(());
 787             }
 788             let child_shape = parent_shape
 789                 .visual_indent(self.offset)
 790                 .sub_width(self.offset)?;
 791             let rewrite = item.rewrite(context, child_shape)?;
 792             match wrap_str(rewrite, context.config.max_width(), shape) {
 793                 Some(rewrite) => root_rewrite.push_str(&rewrite),
 794                 None => {
 795                     // We couldn't fit in at the visual indent, try the last
 796                     // indent.
 797                     let rewrite = item.rewrite(context, parent_shape)?;
 798                     root_rewrite.push_str(&rewrite);
 799                     self.offset = 0;
 800                 }
 801             }
 802
 803             self.shared.children = &self.shared.children[1..];
 804         }
 805
 806         self.shared.rewrites.push(root_rewrite);
 807         Some(())
 808     }
 809
 810     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape> {
 811         shape
 812             .with_max_width(context.config)
 813             .offset_left(self.offset)
 814             .map(|s| s.visual_indent(0))
 815     }
 816
 817     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()> {
 818         for item in &self.shared.children[..self.shared.children.len() - 1] {
 819             let rewrite = item.rewrite(context, child_shape)?;
 820             self.shared.rewrites.push(rewrite);
 821         }
 822         Some(())
 823     }
 824
 825     fn format_last_child(
 826         &mut self,
 827         context: &RewriteContext,
 828         shape: Shape,
 829         child_shape: Shape,
 830     ) -> Option<()> {
 831         self.shared
 832             .format_last_child(false, context, shape, child_shape)
 833     }
 834
 835     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 836         self.shared
 837             .join_rewrites(context, child_shape, iter::repeat(false))
 838     }
 839
 840     fn pure_root(&mut self) -> Option<String> {
 841         self.shared.pure_root()
 842     }
 843 }
 844
 845 // States whether an expression's last line exclusively consists of closing
 846 // parens, braces, and brackets in its idiomatic formatting.
 847 fn is_block_expr(context: &RewriteContext, expr: &ast::Expr, repr: &str) -> bool {
 848     match expr.node {
 849         ast::ExprKind::Mac(..)
 850         | ast::ExprKind::Call(..)
 851         | ast::ExprKind::MethodCall(..)
 852         | ast::ExprKind::Struct(..)
 853         | ast::ExprKind::While(..)
 854         | ast::ExprKind::WhileLet(..)
 855         | ast::ExprKind::If(..)
 856         | ast::ExprKind::IfLet(..)
 857         | ast::ExprKind::Block(..)
 858         | ast::ExprKind::Loop(..)
 859         | ast::ExprKind::ForLoop(..)
 860         | ast::ExprKind::Match(..) => repr.contains('\n'),
 861         ast::ExprKind::Paren(ref expr)
 862         | ast::ExprKind::Binary(_, _, ref expr)
 863         | ast::ExprKind::Index(_, ref expr)
 864         | ast::ExprKind::Unary(_, ref expr)
 865         | ast::ExprKind::Closure(_, _, _, _, ref expr, _)
 866         | ast::ExprKind::Try(ref expr)
 867         | ast::ExprKind::Yield(Some(ref expr)) => is_block_expr(context, expr, repr),
 868         // This can only be a string lit
 869         ast::ExprKind::Lit(_) => {
 870             repr.contains('\n') && trimmed_last_line_width(repr) <= context.config.tab_spaces()
 871         }
 872         _ => false,
 873     }
 874 }