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
  84 use syntax::source_map::{BytePos, Span};
  85 use syntax::{ast, ptr};
  86
  87 pub fn rewrite_chain(expr: &ast::Expr, context: &RewriteContext, shape: Shape) -> Option<String> {
  88     let chain = Chain::from_ast(expr, context);
  89     debug!("rewrite_chain {:?} {:?}", chain, shape);
  90
  91     // If this is just an expression with some `?`s, then format it trivially and
  92     // return early.
  93     if chain.children.is_empty() {
  94         return chain.parent.rewrite(context, shape);
  95     }
  96
  97     chain.rewrite(context, shape)
  98 }
  99
 100 #[derive(Debug)]
 101 enum CommentPosition {
 102     Back,
 103     Top,
 104 }
 105
 106 // An expression plus trailing `?`s to be formatted together.
 107 #[derive(Debug)]
 108 struct ChainItem {
 109     kind: ChainItemKind,
 110     tries: usize,
 111     span: Span,
 112 }
 113
 114 // FIXME: we can't use a reference here because to convert `try!` to `?` we
 115 // synthesise the AST node. However, I think we could use `Cow` and that
 116 // would remove a lot of cloning.
 117 #[derive(Debug)]
 118 enum ChainItemKind {
 119     Parent(ast::Expr),
 120     MethodCall(
 121         ast::PathSegment,
 122         Vec<ast::GenericArg>,
 123         Vec<ptr::P<ast::Expr>>,
 124     ),
 125     StructField(ast::Ident),
 126     TupleField(ast::Ident, bool),
 127     Comment(String, CommentPosition),
 128 }
 129
 130 impl ChainItemKind {
 131     fn is_block_like(&self, context: &RewriteContext, reps: &str) -> bool {
 132         match self {
 133             ChainItemKind::Parent(ref expr) => is_block_expr(context, expr, reps),
 134             ChainItemKind::MethodCall(..)
 135             | ChainItemKind::StructField(..)
 136             | ChainItemKind::TupleField(..)
 137             | ChainItemKind::Comment(..) => false,
 138         }
 139     }
 140
 141     fn is_tup_field_access(expr: &ast::Expr) -> bool {
 142         match expr.node {
 143             ast::ExprKind::Field(_, ref field) => {
 144                 field.name.to_string().chars().all(|c| c.is_digit(10))
 145             }
 146             _ => false,
 147         }
 148     }
 149
 150     fn from_ast(context: &RewriteContext, expr: &ast::Expr) -> (ChainItemKind, Span) {
 151         let (kind, span) = match expr.node {
 152             ast::ExprKind::MethodCall(ref segment, ref expressions) => {
 153                 let types = if let Some(ref generic_args) = segment.args {
 154                     if let ast::GenericArgs::AngleBracketed(ref data) = **generic_args {
 155                         data.args.clone()
 156                     } else {
 157                         vec![]
 158                     }
 159                 } else {
 160                     vec![]
 161                 };
 162                 let span = mk_sp(expressions[0].span.hi(), expr.span.hi());
 163                 let kind = ChainItemKind::MethodCall(segment.clone(), types, expressions.clone());
 164                 (kind, span)
 165             }
 166             ast::ExprKind::Field(ref nested, field) => {
 167                 let kind = if Self::is_tup_field_access(expr) {
 168                     ChainItemKind::TupleField(field, Self::is_tup_field_access(nested))
 169                 } else {
 170                     ChainItemKind::StructField(field)
 171                 };
 172                 let span = mk_sp(nested.span.hi(), field.span.hi());
 173                 (kind, 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::Comment(ref comment, _) => {
 199                 rewrite_comment(comment, false, shape, context.config)?
 200             }
 201         };
 202         Some(format!("{}{}", rewrite, "?".repeat(self.tries)))
 203     }
 204 }
 205
 206 impl ChainItem {
 207     fn new(context: &RewriteContext, expr: &ast::Expr, tries: usize) -> ChainItem {
 208         let (kind, span) = ChainItemKind::from_ast(context, expr);
 209         ChainItem { kind, tries, span }
 210     }
 211
 212     fn comment(span: Span, comment: String, pos: CommentPosition) -> ChainItem {
 213         ChainItem {
 214             kind: ChainItemKind::Comment(comment, pos),
 215             tries: 0,
 216             span,
 217         }
 218     }
 219
 220     fn is_comment(&self) -> bool {
 221         match self.kind {
 222             ChainItemKind::Comment(..) => true,
 223             _ => false,
 224         }
 225     }
 226
 227     fn rewrite_method_call(
 228         method_name: ast::Ident,
 229         types: &[ast::GenericArg],
 230         args: &[ptr::P<ast::Expr>],
 231         span: Span,
 232         context: &RewriteContext,
 233         shape: Shape,
 234     ) -> Option<String> {
 235         let type_str = if types.is_empty() {
 236             String::new()
 237         } else {
 238             let type_list = types
 239                 .iter()
 240                 .map(|ty| ty.rewrite(context, shape))
 241                 .collect::<Option<Vec<_>>>()?;
 242
 243             format!("::<{}>", type_list.join(", "))
 244         };
 245         let callee_str = format!(".{}{}", rewrite_ident(context, method_name), type_str);
 246         rewrite_call(context, &callee_str, &args[1..], span, shape)
 247     }
 248 }
 249
 250 #[derive(Debug)]
 251 struct Chain {
 252     parent: ChainItem,
 253     children: Vec<ChainItem>,
 254 }
 255
 256 impl Chain {
 257     fn from_ast(expr: &ast::Expr, context: &RewriteContext) -> Chain {
 258         let subexpr_list = Self::make_subexpr_list(expr, context);
 259
 260         // Un-parse the expression tree into ChainItems
 261         let mut rev_children = vec![];
 262         let mut sub_tries = 0;
 263         for subexpr in &subexpr_list {
 264             match subexpr.node {
 265                 ast::ExprKind::Try(_) => sub_tries += 1,
 266                 _ => {
 267                     rev_children.push(ChainItem::new(context, subexpr, sub_tries));
 268                     sub_tries = 0;
 269                 }
 270             }
 271         }
 272
 273         fn is_tries(s: &str) -> bool {
 274             s.chars().all(|c| c == '?')
 275         }
 276
 277         fn is_post_comment(s: &str) -> bool {
 278             let comment_start_index = s.chars().position(|c| c == '/');
 279             if comment_start_index.is_none() {
 280                 return false;
 281             }
 282
 283             let newline_index = s.chars().position(|c| c == '\n');
 284             if newline_index.is_none() {
 285                 return true;
 286             }
 287
 288             comment_start_index.unwrap() < newline_index.unwrap()
 289         }
 290
 291         fn handle_post_comment(
 292             post_comment_span: Span,
 293             post_comment_snippet: &str,
 294             prev_span_end: &mut BytePos,
 295             children: &mut Vec<ChainItem>,
 296         ) {
 297             let white_spaces: &[_] = &[' ', '\t'];
 298             if post_comment_snippet
 299                 .trim_matches(white_spaces)
 300                 .starts_with('\n')
 301             {
 302                 // No post comment.
 303                 return;
 304             }
 305             let trimmed_snippet = trim_tries(post_comment_snippet);
 306             if is_post_comment(&trimmed_snippet) {
 307                 children.push(ChainItem::comment(
 308                     post_comment_span,
 309                     trimmed_snippet.trim().to_owned(),
 310                     CommentPosition::Back,
 311                 ));
 312                 *prev_span_end = post_comment_span.hi();
 313             }
 314         }
 315
 316         let parent = rev_children.pop().unwrap();
 317         let mut children = vec![];
 318         let mut prev_span_end = parent.span.hi();
 319         let mut iter = rev_children.into_iter().rev().peekable();
 320         if let Some(first_chain_item) = iter.peek() {
 321             let comment_span = mk_sp(prev_span_end, first_chain_item.span.lo());
 322             let comment_snippet = context.snippet(comment_span);
 323             if !is_tries(comment_snippet.trim()) {
 324                 handle_post_comment(
 325                     comment_span,
 326                     comment_snippet,
 327                     &mut prev_span_end,
 328                     &mut children,
 329                 );
 330             }
 331         }
 332         while let Some(chain_item) = iter.next() {
 333             let comment_snippet = context.snippet(chain_item.span);
 334             // FIXME: Figure out the way to get a correct span when converting `try!` to `?`.
 335             let handle_comment =
 336                 !(context.config.use_try_shorthand() || is_tries(comment_snippet.trim()));
 337
 338             // Pre-comment
 339             if handle_comment {
 340                 let pre_comment_span = mk_sp(prev_span_end, chain_item.span.lo());
 341                 let pre_comment_snippet = trim_tries(context.snippet(pre_comment_span));
 342                 let (pre_comment, _) = extract_pre_comment(&pre_comment_snippet);
 343                 match pre_comment {
 344                     Some(ref comment) if !comment.is_empty() => {
 345                         children.push(ChainItem::comment(
 346                             pre_comment_span,
 347                             comment.to_owned(),
 348                             CommentPosition::Top,
 349                         ));
 350                     }
 351                     _ => (),
 352                 }
 353             }
 354
 355             prev_span_end = chain_item.span.hi();
 356             children.push(chain_item);
 357
 358             // Post-comment
 359             if !handle_comment || iter.peek().is_none() {
 360                 continue;
 361             }
 362
 363             let next_lo = iter.peek().unwrap().span.lo();
 364             let post_comment_span = mk_sp(prev_span_end, next_lo);
 365             let post_comment_snippet = context.snippet(post_comment_span);
 366             handle_post_comment(
 367                 post_comment_span,
 368                 post_comment_snippet,
 369                 &mut prev_span_end,
 370                 &mut children,
 371             );
 372         }
 373
 374         Chain { parent, children }
 375     }
 376
 377     // Returns a Vec of the prefixes of the chain.
 378     // E.g., for input `a.b.c` we return [`a.b.c`, `a.b`, 'a']
 379     fn make_subexpr_list(expr: &ast::Expr, context: &RewriteContext) -> Vec<ast::Expr> {
 380         let mut subexpr_list = vec![expr.clone()];
 381
 382         while let Some(subexpr) = Self::pop_expr_chain(subexpr_list.last().unwrap(), context) {
 383             subexpr_list.push(subexpr.clone());
 384         }
 385
 386         subexpr_list
 387     }
 388
 389     // Returns the expression's subexpression, if it exists. When the subexpr
 390     // is a try! macro, we'll convert it to shorthand when the option is set.
 391     fn pop_expr_chain(expr: &ast::Expr, context: &RewriteContext) -> Option<ast::Expr> {
 392         match expr.node {
 393             ast::ExprKind::MethodCall(_, ref expressions) => {
 394                 Some(Self::convert_try(&expressions[0], context))
 395             }
 396             ast::ExprKind::Field(ref subexpr, _) | ast::ExprKind::Try(ref subexpr) => {
 397                 Some(Self::convert_try(subexpr, context))
 398             }
 399             _ => None,
 400         }
 401     }
 402
 403     fn convert_try(expr: &ast::Expr, context: &RewriteContext) -> ast::Expr {
 404         match expr.node {
 405             ast::ExprKind::Mac(ref mac) if context.config.use_try_shorthand() => {
 406                 if let Some(subexpr) = convert_try_mac(mac, context) {
 407                     subexpr
 408                 } else {
 409                     expr.clone()
 410                 }
 411             }
 412             _ => expr.clone(),
 413         }
 414     }
 415 }
 416
 417 impl Rewrite for Chain {
 418     fn rewrite(&self, context: &RewriteContext, shape: Shape) -> Option<String> {
 419         debug!("rewrite chain {:?} {:?}", self, shape);
 420
 421         let mut formatter = match context.config.indent_style() {
 422             IndentStyle::Block => Box::new(ChainFormatterBlock::new(self)) as Box<ChainFormatter>,
 423             IndentStyle::Visual => Box::new(ChainFormatterVisual::new(self)) as Box<ChainFormatter>,
 424         };
 425
 426         formatter.format_root(&self.parent, context, shape)?;
 427         if let Some(result) = formatter.pure_root() {
 428             return wrap_str(result, context.config.max_width(), shape);
 429         }
 430
 431         // Decide how to layout the rest of the chain.
 432         let child_shape = formatter.child_shape(context, shape)?;
 433
 434         formatter.format_children(context, child_shape)?;
 435         formatter.format_last_child(context, shape, child_shape)?;
 436
 437         let result = formatter.join_rewrites(context, child_shape)?;
 438         wrap_str(result, context.config.max_width(), shape)
 439     }
 440 }
 441
 442 // There are a few types for formatting chains. This is because there is a lot
 443 // in common between formatting with block vs visual indent, but they are
 444 // different enough that branching on the indent all over the place gets ugly.
 445 // Anything that can format a chain is a ChainFormatter.
 446 trait ChainFormatter {
 447     // Parent is the first item in the chain, e.g., `foo` in `foo.bar.baz()`.
 448     // Root is the parent plus any other chain items placed on the first line to
 449     // avoid an orphan. E.g.,
 450     // ```
 451     // foo.bar
 452     //     .baz()
 453     // ```
 454     // If `bar` were not part of the root, then foo would be orphaned and 'float'.
 455     fn format_root(
 456         &mut self,
 457         parent: &ChainItem,
 458         context: &RewriteContext,
 459         shape: Shape,
 460     ) -> Option<()>;
 461     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape>;
 462     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()>;
 463     fn format_last_child(
 464         &mut self,
 465         context: &RewriteContext,
 466         shape: Shape,
 467         child_shape: Shape,
 468     ) -> Option<()>;
 469     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String>;
 470     // Returns `Some` if the chain is only a root, None otherwise.
 471     fn pure_root(&mut self) -> Option<String>;
 472 }
 473
 474 // Data and behaviour that is shared by both chain formatters. The concrete
 475 // formatters can delegate much behaviour to `ChainFormatterShared`.
 476 struct ChainFormatterShared<'a> {
 477     // The current working set of child items.
 478     children: &'a [ChainItem],
 479     // The current rewrites of items (includes trailing `?`s, but not any way to
 480     // connect the rewrites together).
 481     rewrites: Vec<String>,
 482     // Whether the chain can fit on one line.
 483     fits_single_line: bool,
 484     // The number of children in the chain. This is not equal to `self.children.len()`
 485     // because `self.children` will change size as we process the chain.
 486     child_count: usize,
 487 }
 488
 489 impl<'a> ChainFormatterShared<'a> {
 490     fn new(chain: &'a Chain) -> ChainFormatterShared<'a> {
 491         ChainFormatterShared {
 492             children: &chain.children,
 493             rewrites: Vec::with_capacity(chain.children.len() + 1),
 494             fits_single_line: false,
 495             child_count: chain.children.len(),
 496         }
 497     }
 498
 499     fn pure_root(&mut self) -> Option<String> {
 500         if self.children.is_empty() {
 501             assert_eq!(self.rewrites.len(), 1);
 502             Some(self.rewrites.pop().unwrap())
 503         } else {
 504             None
 505         }
 506     }
 507
 508     // Rewrite the last child. The last child of a chain requires special treatment. We need to
 509     // know whether 'overflowing' the last child make a better formatting:
 510     //
 511     // A chain with overflowing the last child:
 512     // ```
 513     // parent.child1.child2.last_child(
 514     //     a,
 515     //     b,
 516     //     c,
 517     // )
 518     // ```
 519     //
 520     // A chain without overflowing the last child (in vertical layout):
 521     // ```
 522     // parent
 523     //     .child1
 524     //     .child2
 525     //     .last_child(a, b, c)
 526     // ```
 527     //
 528     // In particular, overflowing is effective when the last child is a method with a multi-lined
 529     // block-like argument (e.g. closure):
 530     // ```
 531     // parent.child1.child2.last_child(|a, b, c| {
 532     //     let x = foo(a, b, c);
 533     //     let y = bar(a, b, c);
 534     //
 535     //     // ...
 536     //
 537     //     result
 538     // })
 539     // ```
 540     fn format_last_child(
 541         &mut self,
 542         may_extend: bool,
 543         context: &RewriteContext,
 544         shape: Shape,
 545         child_shape: Shape,
 546     ) -> Option<()> {
 547         let last = self.children.last()?;
 548         let extendable = may_extend && last_line_extendable(&self.rewrites[0]);
 549         let prev_last_line_width = last_line_width(&self.rewrites[0]);
 550
 551         // Total of all items excluding the last.
 552         let almost_total = if extendable {
 553             prev_last_line_width
 554         } else {
 555             self.rewrites.iter().fold(0, |a, b| a + b.len())
 556         } + last.tries;
 557         let one_line_budget = if self.child_count == 1 {
 558             shape.width
 559         } else {
 560             min(shape.width, context.config.width_heuristics().chain_width)
 561         }
 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(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 629         let connector = if self.fits_single_line {
 630             // Yay, we can put everything on one line.
 631             Cow::from("")
 632         } else {
 633             // Use new lines.
 634             if *context.force_one_line_chain.borrow() {
 635                 return None;
 636             }
 637             child_shape.to_string_with_newline(context.config)
 638         };
 639
 640         let mut rewrite_iter = self.rewrites.iter();
 641         let mut result = rewrite_iter.next().unwrap().clone();
 642         let children_iter = self.children.iter();
 643         let iter = rewrite_iter.zip(children_iter);
 644
 645         for (rewrite, chain_item) in iter {
 646             match chain_item.kind {
 647                 ChainItemKind::Comment(_, CommentPosition::Back) => result.push(' '),
 648                 ChainItemKind::Comment(_, CommentPosition::Top) => result.push_str(&connector),
 649                 _ => result.push_str(&connector),
 650             }
 651             result.push_str(&rewrite);
 652         }
 653
 654         Some(result)
 655     }
 656 }
 657
 658 // Formats a chain using block indent.
 659 struct ChainFormatterBlock<'a> {
 660     shared: ChainFormatterShared<'a>,
 661     root_ends_with_block: bool,
 662 }
 663
 664 impl<'a> ChainFormatterBlock<'a> {
 665     fn new(chain: &'a Chain) -> ChainFormatterBlock<'a> {
 666         ChainFormatterBlock {
 667             shared: ChainFormatterShared::new(chain),
 668             root_ends_with_block: false,
 669         }
 670     }
 671 }
 672
 673 impl<'a> ChainFormatter for ChainFormatterBlock<'a> {
 674     fn format_root(
 675         &mut self,
 676         parent: &ChainItem,
 677         context: &RewriteContext,
 678         shape: Shape,
 679     ) -> Option<()> {
 680         let mut root_rewrite: String = parent.rewrite(context, shape)?;
 681
 682         let mut root_ends_with_block = parent.kind.is_block_like(context, &root_rewrite);
 683         let tab_width = context.config.tab_spaces().saturating_sub(shape.offset);
 684
 685         while root_rewrite.len() <= tab_width && !root_rewrite.contains('\n') {
 686             let item = &self.shared.children[0];
 687             if let ChainItemKind::Comment(..) = item.kind {
 688                 break;
 689             }
 690             let shape = shape.offset_left(root_rewrite.len())?;
 691             match &item.rewrite(context, shape) {
 692                 Some(rewrite) => root_rewrite.push_str(rewrite),
 693                 None => break,
 694             }
 695
 696             root_ends_with_block = last_line_extendable(&root_rewrite);
 697
 698             self.shared.children = &self.shared.children[1..];
 699             if self.shared.children.is_empty() {
 700                 break;
 701             }
 702         }
 703         self.shared.rewrites.push(root_rewrite);
 704         self.root_ends_with_block = root_ends_with_block;
 705         Some(())
 706     }
 707
 708     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape> {
 709         Some(
 710             if self.root_ends_with_block {
 711                 shape.block_indent(0)
 712             } else {
 713                 shape.block_indent(context.config.tab_spaces())
 714             }
 715             .with_max_width(context.config),
 716         )
 717     }
 718
 719     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()> {
 720         for item in &self.shared.children[..self.shared.children.len() - 1] {
 721             let rewrite = item.rewrite(context, child_shape)?;
 722             self.shared.rewrites.push(rewrite);
 723         }
 724         Some(())
 725     }
 726
 727     fn format_last_child(
 728         &mut self,
 729         context: &RewriteContext,
 730         shape: Shape,
 731         child_shape: Shape,
 732     ) -> Option<()> {
 733         self.shared
 734             .format_last_child(true, context, shape, child_shape)
 735     }
 736
 737     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 738         self.shared.join_rewrites(context, child_shape)
 739     }
 740
 741     fn pure_root(&mut self) -> Option<String> {
 742         self.shared.pure_root()
 743     }
 744 }
 745
 746 // Format a chain using visual indent.
 747 struct ChainFormatterVisual<'a> {
 748     shared: ChainFormatterShared<'a>,
 749     // The extra offset from the chain's shape to the position of the `.`
 750     offset: usize,
 751 }
 752
 753 impl<'a> ChainFormatterVisual<'a> {
 754     fn new(chain: &'a Chain) -> ChainFormatterVisual<'a> {
 755         ChainFormatterVisual {
 756             shared: ChainFormatterShared::new(chain),
 757             offset: 0,
 758         }
 759     }
 760 }
 761
 762 impl<'a> ChainFormatter for ChainFormatterVisual<'a> {
 763     fn format_root(
 764         &mut self,
 765         parent: &ChainItem,
 766         context: &RewriteContext,
 767         shape: Shape,
 768     ) -> Option<()> {
 769         let parent_shape = shape.visual_indent(0);
 770         let mut root_rewrite = parent.rewrite(context, parent_shape)?;
 771         let multiline = root_rewrite.contains('\n');
 772         self.offset = if multiline {
 773             last_line_width(&root_rewrite).saturating_sub(shape.used_width())
 774         } else {
 775             trimmed_last_line_width(&root_rewrite)
 776         };
 777
 778         if !multiline || parent.kind.is_block_like(context, &root_rewrite) {
 779             let item = &self.shared.children[0];
 780             if let ChainItemKind::Comment(..) = item.kind {
 781                 self.shared.rewrites.push(root_rewrite);
 782                 return Some(());
 783             }
 784             let child_shape = parent_shape
 785                 .visual_indent(self.offset)
 786                 .sub_width(self.offset)?;
 787             let rewrite = item.rewrite(context, child_shape)?;
 788             match wrap_str(rewrite, context.config.max_width(), shape) {
 789                 Some(rewrite) => root_rewrite.push_str(&rewrite),
 790                 None => {
 791                     // We couldn't fit in at the visual indent, try the last
 792                     // indent.
 793                     let rewrite = item.rewrite(context, parent_shape)?;
 794                     root_rewrite.push_str(&rewrite);
 795                     self.offset = 0;
 796                 }
 797             }
 798
 799             self.shared.children = &self.shared.children[1..];
 800         }
 801
 802         self.shared.rewrites.push(root_rewrite);
 803         Some(())
 804     }
 805
 806     fn child_shape(&self, context: &RewriteContext, shape: Shape) -> Option<Shape> {
 807         shape
 808             .with_max_width(context.config)
 809             .offset_left(self.offset)
 810             .map(|s| s.visual_indent(0))
 811     }
 812
 813     fn format_children(&mut self, context: &RewriteContext, child_shape: Shape) -> Option<()> {
 814         for item in &self.shared.children[..self.shared.children.len() - 1] {
 815             let rewrite = item.rewrite(context, child_shape)?;
 816             self.shared.rewrites.push(rewrite);
 817         }
 818         Some(())
 819     }
 820
 821     fn format_last_child(
 822         &mut self,
 823         context: &RewriteContext,
 824         shape: Shape,
 825         child_shape: Shape,
 826     ) -> Option<()> {
 827         self.shared
 828             .format_last_child(false, context, shape, child_shape)
 829     }
 830
 831     fn join_rewrites(&self, context: &RewriteContext, child_shape: Shape) -> Option<String> {
 832         self.shared.join_rewrites(context, child_shape)
 833     }
 834
 835     fn pure_root(&mut self) -> Option<String> {
 836         self.shared.pure_root()
 837     }
 838 }
 839
 840 // States whether an expression's last line exclusively consists of closing
 841 // parens, braces, and brackets in its idiomatic formatting.
 842 fn is_block_expr(context: &RewriteContext, expr: &ast::Expr, repr: &str) -> bool {
 843     match expr.node {
 844         ast::ExprKind::Mac(..)
 845         | ast::ExprKind::Call(..)
 846         | ast::ExprKind::MethodCall(..)
 847         | ast::ExprKind::Struct(..)
 848         | ast::ExprKind::While(..)
 849         | ast::ExprKind::WhileLet(..)
 850         | ast::ExprKind::If(..)
 851         | ast::ExprKind::IfLet(..)
 852         | ast::ExprKind::Block(..)
 853         | ast::ExprKind::Loop(..)
 854         | ast::ExprKind::ForLoop(..)
 855         | ast::ExprKind::Match(..) => repr.contains('\n'),
 856         ast::ExprKind::Paren(ref expr)
 857         | ast::ExprKind::Binary(_, _, ref expr)
 858         | ast::ExprKind::Index(_, ref expr)
 859         | ast::ExprKind::Unary(_, ref expr)
 860         | ast::ExprKind::Closure(_, _, _, _, ref expr, _)
 861         | ast::ExprKind::Try(ref expr)
 862         | ast::ExprKind::Yield(Some(ref expr)) => is_block_expr(context, expr, repr),
 863         // This can only be a string lit
 864         ast::ExprKind::Lit(_) => {
 865             repr.contains('\n') && trimmed_last_line_width(repr) <= context.config.tab_spaces()
 866         }
 867         _ => false,
 868     }
 869 }
 870
 871 /// Remove try operators (`?`s) that appear in the given string. If removing
 872 /// them leaves an empty line, remove that line as well unless it is the first
 873 /// line (we need the first newline for detecting pre/post comment).
 874 fn trim_tries(s: &str) -> String {
 875     let mut result = String::with_capacity(s.len());
 876     let mut line_buffer = String::with_capacity(s.len());
 877     for (kind, rich_char) in CharClasses::new(s.chars()) {
 878         match rich_char.get_char() {
 879             '\n' => {
 880                 if result.is_empty() || !line_buffer.trim().is_empty() {
 881                     result.push_str(&line_buffer);
 882                     result.push('\n')
 883                 }
 884                 line_buffer.clear();
 885             }
 886             '?' if kind == FullCodeCharKind::Normal => continue,
 887             c => line_buffer.push(c),
 888         }
 889     }
 890     if !line_buffer.trim().is_empty() {
 891         result.push_str(&line_buffer);
 892     }
 893     result
 894 }