src/tools/rustfmt/src/chains.rs

   1 //! Formatting of chained expressions, i.e., expressions that are chained by
   2 //! dots: struct and enum field access, method calls, and try shorthand (`?`).
   3 //!
   4 //! Instead of walking these subexpressions one-by-one, as is our usual strategy
   5 //! for expression formatting, we collect maximal sequences of these expressions
   6 //! and handle them simultaneously.
   7 //!
   8 //! Whenever possible, the entire chain is put on a single line. If that fails,
   9 //! we put each subexpression on a separate, much like the (default) function
  10 //! argument function argument strategy.
  11 //!
  12 //! Depends on config options: `chain_indent` is the indent to use for
  13 //! blocks in the parent/root/base of the chain (and the rest of the chain's
  14 //! alignment).
  15 //! E.g., `let foo = { aaaa; bbb; ccc }.bar.baz();`, we would layout for the
  16 //! following values of `chain_indent`:
  17 //! Block:
  18 //!
  19 //! ```text
  20 //! let foo = {
  21 //!     aaaa;
  22 //!     bbb;
  23 //!     ccc
  24 //! }.bar
  25 //!     .baz();
  26 //! ```
  27 //!
  28 //! Visual:
  29 //!
  30 //! ```text
  31 //! let foo = {
  32 //!               aaaa;
  33 //!               bbb;
  34 //!               ccc
  35 //!           }
  36 //!           .bar
  37 //!           .baz();
  38 //! ```
  39 //!
  40 //! If the first item in the chain is a block expression, we align the dots with
  41 //! the braces.
  42 //! Block:
  43 //!
  44 //! ```text
  45 //! let a = foo.bar
  46 //!     .baz()
  47 //!     .qux
  48 //! ```
  49 //!
  50 //! Visual:
  51 //!
  52 //! ```text
  53 //! let a = foo.bar
  54 //!            .baz()
  55 //!            .qux
  56 //! ```
  57
  58 use std::borrow::Cow;
  59 use std::cmp::min;
  60
  61 use rustc_ast::{ast, ptr};
  62 use rustc_span::{symbol, BytePos, Span};
  63
  64 use crate::comment::{rewrite_comment, CharClasses, FullCodeCharKind, RichChar};
  65 use crate::config::{IndentStyle, Version};
  66 use crate::expr::rewrite_call;
  67 use crate::lists::extract_pre_comment;
  68 use crate::macros::convert_try_mac;
  69 use crate::rewrite::{Rewrite, RewriteContext};
  70 use crate::shape::Shape;
  71 use crate::source_map::SpanUtils;
  72 use crate::utils::{
  73     self, filtered_str_fits, first_line_width, last_line_extendable, last_line_width, mk_sp,
  74     rewrite_ident, trimmed_last_line_width, wrap_str,
  75 };
  76
  77 /// Provides the original input contents from the span
  78 /// of a chain element with trailing spaces trimmed.
  79 fn format_overflow_style(span: Span, context: &RewriteContext<'_>) -> Option<String> {
  80     context.snippet_provider.span_to_snippet(span).map(|s| {
  81         s.lines()
  82             .map(|l| l.trim_end())
  83             .collect::<Vec<_>>()
  84             .join("\n")
  85     })
  86 }
  87
  88 fn format_chain_item(
  89     item: &ChainItem,
  90     context: &RewriteContext<'_>,
  91     rewrite_shape: Shape,
  92     allow_overflow: bool,
  93 ) -> Option<String> {
  94     if allow_overflow {
  95         item.rewrite(context, rewrite_shape)
  96             .or_else(|| format_overflow_style(item.span, context))
  97     } else {
  98         item.rewrite(context, rewrite_shape)
  99     }
 100 }
 101
 102 fn get_block_child_shape(
 103     prev_ends_with_block: bool,
 104     context: &RewriteContext<'_>,
 105     shape: Shape,
 106 ) -> Shape {
 107     if prev_ends_with_block {
 108         shape.block_indent(0)
 109     } else {
 110         shape.block_indent(context.config.tab_spaces())
 111     }
 112     .with_max_width(context.config)
 113 }
 114
 115 fn get_visual_style_child_shape(
 116     context: &RewriteContext<'_>,
 117     shape: Shape,
 118     offset: usize,
 119     parent_overflowing: bool,
 120 ) -> Option<Shape> {
 121     if !parent_overflowing {
 122         shape
 123             .with_max_width(context.config)
 124             .offset_left(offset)
 125             .map(|s| s.visual_indent(0))
 126     } else {
 127         Some(shape.visual_indent(offset))
 128     }
 129 }
 130
 131 pub(crate) fn rewrite_chain(
 132     expr: &ast::Expr,
 133     context: &RewriteContext<'_>,
 134     shape: Shape,
 135 ) -> Option<String> {
 136     let chain = Chain::from_ast(expr, context);
 137     debug!("rewrite_chain {:?} {:?}", chain, shape);
 138
 139     // If this is just an expression with some `?`s, then format it trivially and
 140     // return early.
 141     if chain.children.is_empty() {
 142         return chain.parent.rewrite(context, shape);
 143     }
 144
 145     chain.rewrite(context, shape)
 146 }
 147
 148 #[derive(Debug)]
 149 enum CommentPosition {
 150     Back,
 151     Top,
 152 }
 153
 154 // An expression plus trailing `?`s to be formatted together.
 155 #[derive(Debug)]
 156 struct ChainItem {
 157     kind: ChainItemKind,
 158     tries: usize,
 159     span: Span,
 160 }
 161
 162 // FIXME: we can't use a reference here because to convert `try!` to `?` we
 163 // synthesise the AST node. However, I think we could use `Cow` and that
 164 // would remove a lot of cloning.
 165 #[derive(Debug)]
 166 enum ChainItemKind {
 167     Parent(ast::Expr),
 168     MethodCall(
 169         ast::PathSegment,
 170         Vec<ast::GenericArg>,
 171         Vec<ptr::P<ast::Expr>>,
 172     ),
 173     StructField(symbol::Ident),
 174     TupleField(symbol::Ident, bool),
 175     Await,
 176     Comment(String, CommentPosition),
 177 }
 178
 179 impl ChainItemKind {
 180     fn is_block_like(&self, context: &RewriteContext<'_>, reps: &str) -> bool {
 181         match self {
 182             ChainItemKind::Parent(ref expr) => utils::is_block_expr(context, expr, reps),
 183             ChainItemKind::MethodCall(..)
 184             | ChainItemKind::StructField(..)
 185             | ChainItemKind::TupleField(..)
 186             | ChainItemKind::Await
 187             | ChainItemKind::Comment(..) => false,
 188         }
 189     }
 190
 191     fn is_tup_field_access(expr: &ast::Expr) -> bool {
 192         match expr.kind {
 193             ast::ExprKind::Field(_, ref field) => {
 194                 field.name.to_string().chars().all(|c| c.is_digit(10))
 195             }
 196             _ => false,
 197         }
 198     }
 199
 200     fn from_ast(context: &RewriteContext<'_>, expr: &ast::Expr) -> (ChainItemKind, Span) {
 201         let (kind, span) = match expr.kind {
 202             ast::ExprKind::MethodCall(ref call) => {
 203                 let types = if let Some(ref generic_args) = call.seg.args {
 204                     if let ast::GenericArgs::AngleBracketed(ref data) = **generic_args {
 205                         data.args
 206                             .iter()
 207                             .filter_map(|x| match x {
 208                                 ast::AngleBracketedArg::Arg(ref generic_arg) => {
 209                                     Some(generic_arg.clone())
 210                                 }
 211                                 _ => None,
 212                             })
 213                             .collect::<Vec<_>>()
 214                     } else {
 215                         vec![]
 216                     }
 217                 } else {
 218                     vec![]
 219                 };
 220                 let span = mk_sp(call.receiver.span.hi(), expr.span.hi());
 221                 let kind = ChainItemKind::MethodCall(call.seg.clone(), types, call.args.clone());
 222                 (kind, span)
 223             }
 224             ast::ExprKind::Field(ref nested, field) => {
 225                 let kind = if Self::is_tup_field_access(expr) {
 226                     ChainItemKind::TupleField(field, Self::is_tup_field_access(nested))
 227                 } else {
 228                     ChainItemKind::StructField(field)
 229                 };
 230                 let span = mk_sp(nested.span.hi(), field.span.hi());
 231                 (kind, span)
 232             }
 233             ast::ExprKind::Await(ref nested) => {
 234                 let span = mk_sp(nested.span.hi(), expr.span.hi());
 235                 (ChainItemKind::Await, span)
 236             }
 237             _ => return (ChainItemKind::Parent(expr.clone()), expr.span),
 238         };
 239
 240         // Remove comments from the span.
 241         let lo = context.snippet_provider.span_before(span, ".");
 242         (kind, mk_sp(lo, span.hi()))
 243     }
 244 }
 245
 246 impl Rewrite for ChainItem {
 247     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
 248         let shape = shape.sub_width(self.tries)?;
 249         let rewrite = match self.kind {
 250             ChainItemKind::Parent(ref expr) => expr.rewrite(context, shape)?,
 251             ChainItemKind::MethodCall(ref segment, ref types, ref exprs) => {
 252                 Self::rewrite_method_call(segment.ident, types, exprs, self.span, context, shape)?
 253             }
 254             ChainItemKind::StructField(ident) => format!(".{}", rewrite_ident(context, ident)),
 255             ChainItemKind::TupleField(ident, nested) => format!(
 256                 "{}.{}",
 257                 if nested && context.config.version() == Version::One {
 258                     " "
 259                 } else {
 260                     ""
 261                 },
 262                 rewrite_ident(context, ident)
 263             ),
 264             ChainItemKind::Await => ".await".to_owned(),
 265             ChainItemKind::Comment(ref comment, _) => {
 266                 rewrite_comment(comment, false, shape, context.config)?
 267             }
 268         };
 269         Some(format!("{}{}", rewrite, "?".repeat(self.tries)))
 270     }
 271 }
 272
 273 impl ChainItem {
 274     fn new(context: &RewriteContext<'_>, expr: &ast::Expr, tries: usize) -> ChainItem {
 275         let (kind, span) = ChainItemKind::from_ast(context, expr);
 276         ChainItem { kind, tries, span }
 277     }
 278
 279     fn comment(span: Span, comment: String, pos: CommentPosition) -> ChainItem {
 280         ChainItem {
 281             kind: ChainItemKind::Comment(comment, pos),
 282             tries: 0,
 283             span,
 284         }
 285     }
 286
 287     fn is_comment(&self) -> bool {
 288         matches!(self.kind, ChainItemKind::Comment(..))
 289     }
 290
 291     fn rewrite_method_call(
 292         method_name: symbol::Ident,
 293         types: &[ast::GenericArg],
 294         args: &[ptr::P<ast::Expr>],
 295         span: Span,
 296         context: &RewriteContext<'_>,
 297         shape: Shape,
 298     ) -> Option<String> {
 299         let type_str = if types.is_empty() {
 300             String::new()
 301         } else {
 302             let type_list = types
 303                 .iter()
 304                 .map(|ty| ty.rewrite(context, shape))
 305                 .collect::<Option<Vec<_>>>()?;
 306
 307             format!("::<{}>", type_list.join(", "))
 308         };
 309         let callee_str = format!(".{}{}", rewrite_ident(context, method_name), type_str);
 310         rewrite_call(context, &callee_str, &args, span, shape)
 311     }
 312 }
 313
 314 #[derive(Debug)]
 315 struct Chain {
 316     parent: ChainItem,
 317     children: Vec<ChainItem>,
 318 }
 319
 320 impl Chain {
 321     fn from_ast(expr: &ast::Expr, context: &RewriteContext<'_>) -> Chain {
 322         let subexpr_list = Self::make_subexpr_list(expr, context);
 323
 324         // Un-parse the expression tree into ChainItems
 325         let mut rev_children = vec![];
 326         let mut sub_tries = 0;
 327         for subexpr in &subexpr_list {
 328             match subexpr.kind {
 329                 ast::ExprKind::Try(_) => sub_tries += 1,
 330                 _ => {
 331                     rev_children.push(ChainItem::new(context, subexpr, sub_tries));
 332                     sub_tries = 0;
 333                 }
 334             }
 335         }
 336
 337         fn is_tries(s: &str) -> bool {
 338             s.chars().all(|c| c == '?')
 339         }
 340
 341         fn is_post_comment(s: &str) -> bool {
 342             let comment_start_index = s.chars().position(|c| c == '/');
 343             if comment_start_index.is_none() {
 344                 return false;
 345             }
 346
 347             let newline_index = s.chars().position(|c| c == '\n');
 348             if newline_index.is_none() {
 349                 return true;
 350             }
 351
 352             comment_start_index.unwrap() < newline_index.unwrap()
 353         }
 354
 355         fn handle_post_comment(
 356             post_comment_span: Span,
 357             post_comment_snippet: &str,
 358             prev_span_end: &mut BytePos,
 359             children: &mut Vec<ChainItem>,
 360         ) {
 361             let white_spaces: &[_] = &[' ', '\t'];
 362             if post_comment_snippet
 363                 .trim_matches(white_spaces)
 364                 .starts_with('\n')
 365             {
 366                 // No post comment.
 367                 return;
 368             }
 369             let trimmed_snippet = trim_tries(post_comment_snippet);
 370             if is_post_comment(&trimmed_snippet) {
 371                 children.push(ChainItem::comment(
 372                     post_comment_span,
 373                     trimmed_snippet.trim().to_owned(),
 374                     CommentPosition::Back,
 375                 ));
 376                 *prev_span_end = post_comment_span.hi();
 377             }
 378         }
 379
 380         let parent = rev_children.pop().unwrap();
 381         let mut children = vec![];
 382         let mut prev_span_end = parent.span.hi();
 383         let mut iter = rev_children.into_iter().rev().peekable();
 384         if let Some(first_chain_item) = iter.peek() {
 385             let comment_span = mk_sp(prev_span_end, first_chain_item.span.lo());
 386             let comment_snippet = context.snippet(comment_span);
 387             if !is_tries(comment_snippet.trim()) {
 388                 handle_post_comment(
 389                     comment_span,
 390                     comment_snippet,
 391                     &mut prev_span_end,
 392                     &mut children,
 393                 );
 394             }
 395         }
 396         while let Some(chain_item) = iter.next() {
 397             let comment_snippet = context.snippet(chain_item.span);
 398             // FIXME: Figure out the way to get a correct span when converting `try!` to `?`.
 399             let handle_comment =
 400                 !(context.config.use_try_shorthand() || is_tries(comment_snippet.trim()));
 401
 402             // Pre-comment
 403             if handle_comment {
 404                 let pre_comment_span = mk_sp(prev_span_end, chain_item.span.lo());
 405                 let pre_comment_snippet = trim_tries(context.snippet(pre_comment_span));
 406                 let (pre_comment, _) = extract_pre_comment(&pre_comment_snippet);
 407                 match pre_comment {
 408                     Some(ref comment) if !comment.is_empty() => {
 409                         children.push(ChainItem::comment(
 410                             pre_comment_span,
 411                             comment.to_owned(),
 412                             CommentPosition::Top,
 413                         ));
 414                     }
 415                     _ => (),
 416                 }
 417             }
 418
 419             prev_span_end = chain_item.span.hi();
 420             children.push(chain_item);
 421
 422             // Post-comment
 423             if !handle_comment || iter.peek().is_none() {
 424                 continue;
 425             }
 426
 427             let next_lo = iter.peek().unwrap().span.lo();
 428             let post_comment_span = mk_sp(prev_span_end, next_lo);
 429             let post_comment_snippet = context.snippet(post_comment_span);
 430             handle_post_comment(
 431                 post_comment_span,
 432                 post_comment_snippet,
 433                 &mut prev_span_end,
 434                 &mut children,
 435             );
 436         }
 437
 438         Chain { parent, children }
 439     }
 440
 441     // Returns a Vec of the prefixes of the chain.
 442     // E.g., for input `a.b.c` we return [`a.b.c`, `a.b`, 'a']
 443     fn make_subexpr_list(expr: &ast::Expr, context: &RewriteContext<'_>) -> Vec<ast::Expr> {
 444         let mut subexpr_list = vec![expr.clone()];
 445
 446         while let Some(subexpr) = Self::pop_expr_chain(subexpr_list.last().unwrap(), context) {
 447             subexpr_list.push(subexpr.clone());
 448         }
 449
 450         subexpr_list
 451     }
 452
 453     // Returns the expression's subexpression, if it exists. When the subexpr
 454     // is a try! macro, we'll convert it to shorthand when the option is set.
 455     fn pop_expr_chain(expr: &ast::Expr, context: &RewriteContext<'_>) -> Option<ast::Expr> {
 456         match expr.kind {
 457             ast::ExprKind::MethodCall(ref call) => Some(Self::convert_try(&call.receiver, context)),
 458             ast::ExprKind::Field(ref subexpr, _)
 459             | ast::ExprKind::Try(ref subexpr)
 460             | ast::ExprKind::Await(ref subexpr) => Some(Self::convert_try(subexpr, context)),
 461             _ => None,
 462         }
 463     }
 464
 465     fn convert_try(expr: &ast::Expr, context: &RewriteContext<'_>) -> ast::Expr {
 466         match expr.kind {
 467             ast::ExprKind::MacCall(ref mac) if context.config.use_try_shorthand() => {
 468                 if let Some(subexpr) = convert_try_mac(mac, context) {
 469                     subexpr
 470                 } else {
 471                     expr.clone()
 472                 }
 473             }
 474             _ => expr.clone(),
 475         }
 476     }
 477 }
 478
 479 impl Rewrite for Chain {
 480     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
 481         debug!("rewrite chain {:?} {:?}", self, shape);
 482
 483         let mut formatter = match context.config.indent_style() {
 484             IndentStyle::Block => {
 485                 Box::new(ChainFormatterBlock::new(self)) as Box<dyn ChainFormatter>
 486             }
 487             IndentStyle::Visual => {
 488                 Box::new(ChainFormatterVisual::new(self)) as Box<dyn ChainFormatter>
 489             }
 490         };
 491
 492         formatter.format_root(&self.parent, context, shape)?;
 493         if let Some(result) = formatter.pure_root() {
 494             return wrap_str(result, context.config.max_width(), shape);
 495         }
 496
 497         // Decide how to layout the rest of the chain.
 498         let child_shape = formatter.child_shape(context, shape)?;
 499
 500         formatter.format_children(context, child_shape)?;
 501         formatter.format_last_child(context, shape, child_shape)?;
 502
 503         let result = formatter.join_rewrites(context, child_shape)?;
 504         wrap_str(result, context.config.max_width(), shape)
 505     }
 506 }
 507
 508 // There are a few types for formatting chains. This is because there is a lot
 509 // in common between formatting with block vs visual indent, but they are
 510 // different enough that branching on the indent all over the place gets ugly.
 511 // Anything that can format a chain is a ChainFormatter.
 512 trait ChainFormatter {
 513     // Parent is the first item in the chain, e.g., `foo` in `foo.bar.baz()`.
 514     // Root is the parent plus any other chain items placed on the first line to
 515     // avoid an orphan. E.g.,
 516     // ```text
 517     // foo.bar
 518     //     .baz()
 519     // ```
 520     // If `bar` were not part of the root, then foo would be orphaned and 'float'.
 521     fn format_root(
 522         &mut self,
 523         parent: &ChainItem,
 524         context: &RewriteContext<'_>,
 525         shape: Shape,
 526     ) -> Option<()>;
 527     fn child_shape(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<Shape>;
 528     fn format_children(&mut self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<()>;
 529     fn format_last_child(
 530         &mut self,
 531         context: &RewriteContext<'_>,
 532         shape: Shape,
 533         child_shape: Shape,
 534     ) -> Option<()>;
 535     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String>;
 536     // Returns `Some` if the chain is only a root, None otherwise.
 537     fn pure_root(&mut self) -> Option<String>;
 538 }
 539
 540 // Data and behaviour that is shared by both chain formatters. The concrete
 541 // formatters can delegate much behaviour to `ChainFormatterShared`.
 542 struct ChainFormatterShared<'a> {
 543     // The current working set of child items.
 544     children: &'a [ChainItem],
 545     // The current rewrites of items (includes trailing `?`s, but not any way to
 546     // connect the rewrites together).
 547     rewrites: Vec<String>,
 548     // Whether the chain can fit on one line.
 549     fits_single_line: bool,
 550     // The number of children in the chain. This is not equal to `self.children.len()`
 551     // because `self.children` will change size as we process the chain.
 552     child_count: usize,
 553     // Whether elements are allowed to overflow past the max_width limit
 554     allow_overflow: bool,
 555 }
 556
 557 impl<'a> ChainFormatterShared<'a> {
 558     fn new(chain: &'a Chain) -> ChainFormatterShared<'a> {
 559         ChainFormatterShared {
 560             children: &chain.children,
 561             rewrites: Vec::with_capacity(chain.children.len() + 1),
 562             fits_single_line: false,
 563             child_count: chain.children.len(),
 564             // TODO(calebcartwright)
 565             allow_overflow: false,
 566         }
 567     }
 568
 569     fn pure_root(&mut self) -> Option<String> {
 570         if self.children.is_empty() {
 571             assert_eq!(self.rewrites.len(), 1);
 572             Some(self.rewrites.pop().unwrap())
 573         } else {
 574             None
 575         }
 576     }
 577
 578     fn format_children(&mut self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<()> {
 579         for item in &self.children[..self.children.len() - 1] {
 580             let rewrite = format_chain_item(item, context, child_shape, self.allow_overflow)?;
 581             self.rewrites.push(rewrite);
 582         }
 583         Some(())
 584     }
 585
 586     // Rewrite the last child. The last child of a chain requires special treatment. We need to
 587     // know whether 'overflowing' the last child make a better formatting:
 588     //
 589     // A chain with overflowing the last child:
 590     // ```text
 591     // parent.child1.child2.last_child(
 592     //     a,
 593     //     b,
 594     //     c,
 595     // )
 596     // ```
 597     //
 598     // A chain without overflowing the last child (in vertical layout):
 599     // ```text
 600     // parent
 601     //     .child1
 602     //     .child2
 603     //     .last_child(a, b, c)
 604     // ```
 605     //
 606     // In particular, overflowing is effective when the last child is a method with a multi-lined
 607     // block-like argument (e.g., closure):
 608     // ```text
 609     // parent.child1.child2.last_child(|a, b, c| {
 610     //     let x = foo(a, b, c);
 611     //     let y = bar(a, b, c);
 612     //
 613     //     // ...
 614     //
 615     //     result
 616     // })
 617     // ```
 618     fn format_last_child(
 619         &mut self,
 620         may_extend: bool,
 621         context: &RewriteContext<'_>,
 622         shape: Shape,
 623         child_shape: Shape,
 624     ) -> Option<()> {
 625         let last = self.children.last()?;
 626         let extendable = may_extend && last_line_extendable(&self.rewrites[0]);
 627         let prev_last_line_width = last_line_width(&self.rewrites[0]);
 628
 629         // Total of all items excluding the last.
 630         let almost_total = if extendable {
 631             prev_last_line_width
 632         } else {
 633             self.rewrites
 634                 .iter()
 635                 .map(|rw| utils::unicode_str_width(rw))
 636                 .sum()
 637         } + last.tries;
 638         let one_line_budget = if self.child_count == 1 {
 639             shape.width
 640         } else {
 641             min(shape.width, context.config.chain_width())
 642         }
 643         .saturating_sub(almost_total);
 644
 645         let all_in_one_line = !self.children.iter().any(ChainItem::is_comment)
 646             && self.rewrites.iter().all(|s| !s.contains('\n'))
 647             && one_line_budget > 0;
 648         let last_shape = if all_in_one_line {
 649             shape.sub_width(last.tries)?
 650         } else if extendable {
 651             child_shape.sub_width(last.tries)?
 652         } else {
 653             child_shape.sub_width(shape.rhs_overhead(context.config) + last.tries)?
 654         };
 655
 656         let mut last_subexpr_str = None;
 657         if all_in_one_line || extendable {
 658             // First we try to 'overflow' the last child and see if it looks better than using
 659             // vertical layout.
 660             let one_line_shape = if context.use_block_indent() {
 661                 last_shape.offset_left(almost_total)
 662             } else {
 663                 last_shape
 664                     .visual_indent(almost_total)
 665                     .sub_width(almost_total)
 666             };
 667
 668             if let Some(one_line_shape) = one_line_shape {
 669                 if let Some(rw) = last.rewrite(context, one_line_shape) {
 670                     // We allow overflowing here only if both of the following conditions match:
 671                     // 1. The entire chain fits in a single line except the last child.
 672                     // 2. `last_child_str.lines().count() >= 5`.
 673                     let line_count = rw.lines().count();
 674                     let could_fit_single_line = first_line_width(&rw) <= one_line_budget;
 675                     if could_fit_single_line && line_count >= 5 {
 676                         last_subexpr_str = Some(rw);
 677                         self.fits_single_line = all_in_one_line;
 678                     } else {
 679                         // We could not know whether overflowing is better than using vertical
 680                         // layout, just by looking at the overflowed rewrite. Now we rewrite the
 681                         // last child on its own line, and compare two rewrites to choose which is
 682                         // better.
 683                         let last_shape = child_shape
 684                             .sub_width(shape.rhs_overhead(context.config) + last.tries)?;
 685                         match last.rewrite(context, last_shape) {
 686                             Some(ref new_rw) if !could_fit_single_line => {
 687                                 last_subexpr_str = Some(new_rw.clone());
 688                             }
 689                             Some(ref new_rw) if new_rw.lines().count() >= line_count => {
 690                                 last_subexpr_str = Some(rw);
 691                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 692                             }
 693                             new_rw @ Some(..) => {
 694                                 last_subexpr_str = new_rw;
 695                             }
 696                             _ => {
 697                                 last_subexpr_str = Some(rw);
 698                                 self.fits_single_line = could_fit_single_line && all_in_one_line;
 699                             }
 700                         }
 701                     }
 702                 }
 703             }
 704         }
 705
 706         let last_shape = if context.use_block_indent() {
 707             last_shape
 708         } else {
 709             child_shape.sub_width(shape.rhs_overhead(context.config) + last.tries)?
 710         };
 711
 712         last_subexpr_str = last_subexpr_str.or_else(|| last.rewrite(context, last_shape));
 713         self.rewrites.push(last_subexpr_str?);
 714         Some(())
 715     }
 716
 717     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String> {
 718         let connector = if self.fits_single_line {
 719             // Yay, we can put everything on one line.
 720             Cow::from("")
 721         } else {
 722             // Use new lines.
 723             if context.force_one_line_chain.get() {
 724                 return None;
 725             }
 726             child_shape.to_string_with_newline(context.config)
 727         };
 728
 729         let mut rewrite_iter = self.rewrites.iter();
 730         let mut result = rewrite_iter.next().unwrap().clone();
 731         let children_iter = self.children.iter();
 732         let iter = rewrite_iter.zip(children_iter);
 733
 734         for (rewrite, chain_item) in iter {
 735             match chain_item.kind {
 736                 ChainItemKind::Comment(_, CommentPosition::Back) => result.push(' '),
 737                 ChainItemKind::Comment(_, CommentPosition::Top) => result.push_str(&connector),
 738                 _ => result.push_str(&connector),
 739             }
 740             result.push_str(rewrite);
 741         }
 742
 743         Some(result)
 744     }
 745 }
 746
 747 // Formats a chain using block indent.
 748 struct ChainFormatterBlock<'a> {
 749     shared: ChainFormatterShared<'a>,
 750     root_ends_with_block: bool,
 751 }
 752
 753 impl<'a> ChainFormatterBlock<'a> {
 754     fn new(chain: &'a Chain) -> ChainFormatterBlock<'a> {
 755         ChainFormatterBlock {
 756             shared: ChainFormatterShared::new(chain),
 757             root_ends_with_block: false,
 758         }
 759     }
 760 }
 761
 762 impl<'a> ChainFormatter for ChainFormatterBlock<'a> {
 763     fn format_root(
 764         &mut self,
 765         parent: &ChainItem,
 766         context: &RewriteContext<'_>,
 767         shape: Shape,
 768     ) -> Option<()> {
 769         let mut root_rewrite: String = parent.rewrite(context, shape)?;
 770
 771         let mut root_ends_with_block = parent.kind.is_block_like(context, &root_rewrite);
 772         let tab_width = context.config.tab_spaces().saturating_sub(shape.offset);
 773
 774         while root_rewrite.len() <= tab_width && !root_rewrite.contains('\n') {
 775             let item = &self.shared.children[0];
 776             if let ChainItemKind::Comment(..) = item.kind {
 777                 break;
 778             }
 779             let shape = shape.offset_left(root_rewrite.len())?;
 780             match &item.rewrite(context, shape) {
 781                 Some(rewrite) => root_rewrite.push_str(rewrite),
 782                 None => break,
 783             }
 784
 785             root_ends_with_block = last_line_extendable(&root_rewrite);
 786
 787             self.shared.children = &self.shared.children[1..];
 788             if self.shared.children.is_empty() {
 789                 break;
 790             }
 791         }
 792         self.shared.rewrites.push(root_rewrite);
 793         self.root_ends_with_block = root_ends_with_block;
 794         Some(())
 795     }
 796
 797     fn child_shape(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<Shape> {
 798         let block_end = self.root_ends_with_block;
 799         Some(get_block_child_shape(block_end, context, shape))
 800     }
 801
 802     fn format_children(&mut self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<()> {
 803         self.shared.format_children(context, child_shape)
 804     }
 805
 806     fn format_last_child(
 807         &mut self,
 808         context: &RewriteContext<'_>,
 809         shape: Shape,
 810         child_shape: Shape,
 811     ) -> Option<()> {
 812         self.shared
 813             .format_last_child(true, context, shape, child_shape)
 814     }
 815
 816     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String> {
 817         self.shared.join_rewrites(context, child_shape)
 818     }
 819
 820     fn pure_root(&mut self) -> Option<String> {
 821         self.shared.pure_root()
 822     }
 823 }
 824
 825 // Format a chain using visual indent.
 826 struct ChainFormatterVisual<'a> {
 827     shared: ChainFormatterShared<'a>,
 828     // The extra offset from the chain's shape to the position of the `.`
 829     offset: usize,
 830 }
 831
 832 impl<'a> ChainFormatterVisual<'a> {
 833     fn new(chain: &'a Chain) -> ChainFormatterVisual<'a> {
 834         ChainFormatterVisual {
 835             shared: ChainFormatterShared::new(chain),
 836             offset: 0,
 837         }
 838     }
 839 }
 840
 841 impl<'a> ChainFormatter for ChainFormatterVisual<'a> {
 842     fn format_root(
 843         &mut self,
 844         parent: &ChainItem,
 845         context: &RewriteContext<'_>,
 846         shape: Shape,
 847     ) -> Option<()> {
 848         let parent_shape = shape.visual_indent(0);
 849         let mut root_rewrite = parent.rewrite(context, parent_shape)?;
 850         let multiline = root_rewrite.contains('\n');
 851         self.offset = if multiline {
 852             last_line_width(&root_rewrite).saturating_sub(shape.used_width())
 853         } else {
 854             trimmed_last_line_width(&root_rewrite)
 855         };
 856
 857         if !multiline || parent.kind.is_block_like(context, &root_rewrite) {
 858             let item = &self.shared.children[0];
 859             if let ChainItemKind::Comment(..) = item.kind {
 860                 self.shared.rewrites.push(root_rewrite);
 861                 return Some(());
 862             }
 863             let child_shape = parent_shape
 864                 .visual_indent(self.offset)
 865                 .sub_width(self.offset)?;
 866             let rewrite = item.rewrite(context, child_shape)?;
 867             if filtered_str_fits(&rewrite, context.config.max_width(), shape) {
 868                 root_rewrite.push_str(&rewrite);
 869             } else {
 870                 // We couldn't fit in at the visual indent, try the last
 871                 // indent.
 872                 let rewrite = item.rewrite(context, parent_shape)?;
 873                 root_rewrite.push_str(&rewrite);
 874                 self.offset = 0;
 875             }
 876
 877             self.shared.children = &self.shared.children[1..];
 878         }
 879
 880         self.shared.rewrites.push(root_rewrite);
 881         Some(())
 882     }
 883
 884     fn child_shape(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<Shape> {
 885         get_visual_style_child_shape(
 886             context,
 887             shape,
 888             self.offset,
 889             // TODO(calebcartwright): self.shared.permissibly_overflowing_parent,
 890             false,
 891         )
 892     }
 893
 894     fn format_children(&mut self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<()> {
 895         self.shared.format_children(context, child_shape)
 896     }
 897
 898     fn format_last_child(
 899         &mut self,
 900         context: &RewriteContext<'_>,
 901         shape: Shape,
 902         child_shape: Shape,
 903     ) -> Option<()> {
 904         self.shared
 905             .format_last_child(false, context, shape, child_shape)
 906     }
 907
 908     fn join_rewrites(&self, context: &RewriteContext<'_>, child_shape: Shape) -> Option<String> {
 909         self.shared.join_rewrites(context, child_shape)
 910     }
 911
 912     fn pure_root(&mut self) -> Option<String> {
 913         self.shared.pure_root()
 914     }
 915 }
 916
 917 /// Removes try operators (`?`s) that appear in the given string. If removing
 918 /// them leaves an empty line, remove that line as well unless it is the first
 919 /// line (we need the first newline for detecting pre/post comment).
 920 fn trim_tries(s: &str) -> String {
 921     let mut result = String::with_capacity(s.len());
 922     let mut line_buffer = String::with_capacity(s.len());
 923     for (kind, rich_char) in CharClasses::new(s.chars()) {
 924         match rich_char.get_char() {
 925             '\n' => {
 926                 if result.is_empty() || !line_buffer.trim().is_empty() {
 927                     result.push_str(&line_buffer);
 928                     result.push('\n')
 929                 }
 930                 line_buffer.clear();
 931             }
 932             '?' if kind == FullCodeCharKind::Normal => continue,
 933             c => line_buffer.push(c),
 934         }
 935     }
 936     if !line_buffer.trim().is_empty() {
 937         result.push_str(&line_buffer);
 938     }
 939     result
 940 }