src/lists.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 use std::cmp;
  12 use std::iter::Peekable;
  13
  14 use syntax::codemap::{BytePos, CodeMap};
  15
  16 use {Indent, Shape};
  17 use comment::{find_comment_end, rewrite_comment, FindUncommented};
  18 use config::{Config, IndentStyle};
  19 use rewrite::RewriteContext;
  20 use utils::{first_line_width, last_line_width, mk_sp};
  21
  22 #[derive(Eq, PartialEq, Debug, Copy, Clone)]
  23 /// Formatting tactic for lists. This will be cast down to a
  24 /// DefinitiveListTactic depending on the number and length of the items and
  25 /// their comments.
  26 pub enum ListTactic {
  27     // One item per row.
  28     Vertical,
  29     // All items on one row.
  30     Horizontal,
  31     // Try Horizontal layout, if that fails then vertical.
  32     HorizontalVertical,
  33     // HorizontalVertical with a soft limit of n characters.
  34     LimitedHorizontalVertical(usize),
  35     // Pack as many items as possible per row over (possibly) many rows.
  36     Mixed,
  37 }
  38
  39 impl_enum_serialize_and_deserialize!(ListTactic, Vertical, Horizontal, HorizontalVertical, Mixed);
  40
  41 #[derive(Eq, PartialEq, Debug, Copy, Clone)]
  42 pub enum SeparatorTactic {
  43     Always,
  44     Never,
  45     Vertical,
  46 }
  47
  48 impl_enum_serialize_and_deserialize!(SeparatorTactic, Always, Never, Vertical);
  49
  50 impl SeparatorTactic {
  51     pub fn from_bool(b: bool) -> SeparatorTactic {
  52         if b {
  53             SeparatorTactic::Always
  54         } else {
  55             SeparatorTactic::Never
  56         }
  57     }
  58 }
  59
  60 pub struct ListFormatting<'a> {
  61     pub tactic: DefinitiveListTactic,
  62     pub separator: &'a str,
  63     pub trailing_separator: SeparatorTactic,
  64     pub shape: Shape,
  65     // Non-expressions, e.g. items, will have a new line at the end of the list.
  66     // Important for comment styles.
  67     pub ends_with_newline: bool,
  68     // Remove newlines between list elements for expressions.
  69     pub preserve_newline: bool,
  70     pub config: &'a Config,
  71 }
  72
  73 impl AsRef<ListItem> for ListItem {
  74     fn as_ref(&self) -> &ListItem {
  75         self
  76     }
  77 }
  78
  79 pub struct ListItem {
  80     // None for comments mean that they are not present.
  81     pub pre_comment: Option<String>,
  82     // Item should include attributes and doc comments. None indicates a failed
  83     // rewrite.
  84     pub item: Option<String>,
  85     pub post_comment: Option<String>,
  86     // Whether there is extra whitespace before this item.
  87     pub new_lines: bool,
  88 }
  89
  90 impl ListItem {
  91     pub fn inner_as_ref(&self) -> &str {
  92         self.item.as_ref().map_or("", |s| &*s)
  93     }
  94
  95     pub fn is_multiline(&self) -> bool {
  96         self.inner_as_ref().contains('\n') || self.pre_comment.is_some() ||
  97             self.post_comment
  98                 .as_ref()
  99                 .map_or(false, |s| s.contains('\n'))
 100     }
 101
 102     pub fn has_comment(&self) -> bool {
 103         self.pre_comment
 104             .as_ref()
 105             .map_or(false, |comment| comment.starts_with("//")) ||
 106             self.post_comment
 107                 .as_ref()
 108                 .map_or(false, |comment| comment.starts_with("//"))
 109     }
 110
 111     pub fn from_str<S: Into<String>>(s: S) -> ListItem {
 112         ListItem {
 113             pre_comment: None,
 114             item: Some(s.into()),
 115             post_comment: None,
 116             new_lines: false,
 117         }
 118     }
 119 }
 120
 121 #[derive(Eq, PartialEq, Debug, Copy, Clone)]
 122 /// The definitive formatting tactic for lists.
 123 pub enum DefinitiveListTactic {
 124     Vertical,
 125     Horizontal,
 126     Mixed,
 127 }
 128
 129 impl DefinitiveListTactic {
 130     pub fn ends_with_newline(&self, indent_style: IndentStyle) -> bool {
 131         match indent_style {
 132             IndentStyle::Block => *self != DefinitiveListTactic::Horizontal,
 133             IndentStyle::Visual => false,
 134         }
 135     }
 136 }
 137
 138 pub fn definitive_tactic<I, T>(items: I, tactic: ListTactic, width: usize) -> DefinitiveListTactic
 139 where
 140     I: IntoIterator<Item = T> + Clone,
 141     T: AsRef<ListItem>,
 142 {
 143     let pre_line_comments = items
 144         .clone()
 145         .into_iter()
 146         .any(|item| item.as_ref().has_comment());
 147
 148     let limit = match tactic {
 149         _ if pre_line_comments => return DefinitiveListTactic::Vertical,
 150         ListTactic::Mixed => return DefinitiveListTactic::Mixed,
 151         ListTactic::Horizontal => return DefinitiveListTactic::Horizontal,
 152         ListTactic::Vertical => return DefinitiveListTactic::Vertical,
 153         ListTactic::LimitedHorizontalVertical(limit) => ::std::cmp::min(width, limit),
 154         ListTactic::HorizontalVertical => width,
 155     };
 156
 157     let (sep_count, total_width) = calculate_width(items.clone());
 158     let sep_len = ", ".len(); // FIXME: make more generic?
 159     let total_sep_len = sep_len * sep_count.checked_sub(1).unwrap_or(0);
 160     let real_total = total_width + total_sep_len;
 161
 162     if real_total <= limit && !pre_line_comments &&
 163         !items.into_iter().any(|item| item.as_ref().is_multiline())
 164     {
 165         DefinitiveListTactic::Horizontal
 166     } else {
 167         DefinitiveListTactic::Vertical
 168     }
 169 }
 170
 171 // Format a list of commented items into a string.
 172 // TODO: add unit tests
 173 pub fn write_list<I, T>(items: I, formatting: &ListFormatting) -> Option<String>
 174 where
 175     I: IntoIterator<Item = T> + Clone,
 176     T: AsRef<ListItem>,
 177 {
 178     let tactic = formatting.tactic;
 179     let sep_len = formatting.separator.len();
 180
 181     // Now that we know how we will layout, we can decide for sure if there
 182     // will be a trailing separator.
 183     let mut trailing_separator = needs_trailing_separator(formatting.trailing_separator, tactic);
 184     let mut result = String::new();
 185     let cloned_items = items.clone();
 186     let mut iter = items.into_iter().enumerate().peekable();
 187     let mut item_max_width: Option<usize> = None;
 188
 189     let mut line_len = 0;
 190     let indent_str = &formatting.shape.indent.to_string(formatting.config);
 191     while let Some((i, item)) = iter.next() {
 192         let item = item.as_ref();
 193         let inner_item = try_opt!(item.item.as_ref());
 194         let first = i == 0;
 195         let last = iter.peek().is_none();
 196         let mut separate = !last || trailing_separator;
 197         let item_sep_len = if separate { sep_len } else { 0 };
 198
 199         // Item string may be multi-line. Its length (used for block comment alignment)
 200         // should be only the length of the last line.
 201         let item_last_line = if item.is_multiline() {
 202             inner_item.lines().last().unwrap_or("")
 203         } else {
 204             inner_item.as_ref()
 205         };
 206         let mut item_last_line_width = item_last_line.len() + item_sep_len;
 207         if item_last_line.starts_with(indent_str) {
 208             item_last_line_width -= indent_str.len();
 209         }
 210
 211         match tactic {
 212             DefinitiveListTactic::Horizontal if !first => {
 213                 result.push(' ');
 214             }
 215             DefinitiveListTactic::Vertical if !first => {
 216                 result.push('\n');
 217                 result.push_str(indent_str);
 218             }
 219             DefinitiveListTactic::Mixed => {
 220                 let total_width = total_item_width(item) + item_sep_len;
 221
 222                 // 1 is space between separator and item.
 223                 if line_len > 0 && line_len + 1 + total_width > formatting.shape.width {
 224                     result.push('\n');
 225                     result.push_str(indent_str);
 226                     line_len = 0;
 227                     if tactic == DefinitiveListTactic::Mixed && formatting.ends_with_newline {
 228                         if last {
 229                             separate = true;
 230                         } else {
 231                             trailing_separator = true;
 232                         }
 233                     }
 234                 }
 235
 236                 if line_len > 0 {
 237                     result.push(' ');
 238                     line_len += 1;
 239                 }
 240
 241                 line_len += total_width;
 242             }
 243             _ => {}
 244         }
 245
 246         // Pre-comments
 247         if let Some(ref comment) = item.pre_comment {
 248             // Block style in non-vertical mode.
 249             let block_mode = tactic != DefinitiveListTactic::Vertical;
 250             // Width restriction is only relevant in vertical mode.
 251             let comment = try_opt!(rewrite_comment(
 252                 comment,
 253                 block_mode,
 254                 formatting.shape,
 255                 formatting.config,
 256             ));
 257             result.push_str(&comment);
 258
 259             if tactic == DefinitiveListTactic::Vertical {
 260                 result.push('\n');
 261                 result.push_str(indent_str);
 262             } else {
 263                 result.push(' ');
 264             }
 265             item_max_width = None;
 266         }
 267
 268         result.push_str(&inner_item[..]);
 269
 270         // Post-comments
 271         if tactic != DefinitiveListTactic::Vertical && item.post_comment.is_some() {
 272             let comment = item.post_comment.as_ref().unwrap();
 273             let formatted_comment = try_opt!(rewrite_comment(
 274                 comment,
 275                 true,
 276                 Shape::legacy(formatting.shape.width, Indent::empty()),
 277                 formatting.config,
 278             ));
 279
 280             result.push(' ');
 281             result.push_str(&formatted_comment);
 282         }
 283
 284         if separate {
 285             result.push_str(formatting.separator);
 286         }
 287
 288         if tactic == DefinitiveListTactic::Vertical && item.post_comment.is_some() {
 289             let comment = item.post_comment.as_ref().unwrap();
 290             let overhead = last_line_width(&result) + first_line_width(comment.trim());
 291
 292             let rewrite_post_comment = |item_max_width: &mut Option<usize>| {
 293                 if item_max_width.is_none() && !last && !inner_item.contains('\n') {
 294                     *item_max_width = Some(max_width_of_item_with_post_comment(
 295                         &cloned_items,
 296                         i,
 297                         overhead,
 298                         formatting.config.max_width(),
 299                     ));
 300                 }
 301                 let overhead = if let &mut Some(max_width) = item_max_width {
 302                     max_width + 2
 303                 } else {
 304                     // 1 = space between item and comment.
 305                     item_last_line_width + 1
 306                 };
 307                 let width = formatting.shape.width.checked_sub(overhead).unwrap_or(1);
 308                 let offset = formatting.shape.indent + overhead;
 309                 let comment_shape = Shape::legacy(width, offset);
 310
 311                 // Use block-style only for the last item or multiline comments.
 312                 let block_style = !formatting.ends_with_newline && last ||
 313                     comment.trim().contains('\n') ||
 314                     comment.trim().len() > width;
 315
 316                 rewrite_comment(comment, block_style, comment_shape, formatting.config)
 317             };
 318
 319             let mut formatted_comment = try_opt!(rewrite_post_comment(&mut item_max_width));
 320
 321             if !formatted_comment.starts_with('\n') {
 322                 let mut comment_alignment =
 323                     post_comment_alignment(item_max_width, inner_item.len());
 324                 if first_line_width(&formatted_comment) + last_line_width(&result) +
 325                     comment_alignment + 1 > formatting.config.max_width()
 326                 {
 327                     item_max_width = None;
 328                     formatted_comment = try_opt!(rewrite_post_comment(&mut item_max_width));
 329                     comment_alignment = post_comment_alignment(item_max_width, inner_item.len());
 330                 }
 331                 for _ in 0..(comment_alignment + 1) {
 332                     result.push(' ');
 333                 }
 334                 // An additional space for the missing trailing comma
 335                 if last && item_max_width.is_some() && !separate {
 336                     result.push(' ');
 337                 }
 338             }
 339             if formatted_comment.contains('\n') {
 340                 item_max_width = None;
 341             }
 342             result.push_str(&formatted_comment);
 343         } else {
 344             item_max_width = None;
 345         }
 346
 347         if formatting.preserve_newline && !last && tactic == DefinitiveListTactic::Vertical &&
 348             item.new_lines
 349         {
 350             item_max_width = None;
 351             result.push('\n');
 352         }
 353     }
 354
 355     Some(result)
 356 }
 357
 358 fn max_width_of_item_with_post_comment<I, T>(
 359     items: &I,
 360     i: usize,
 361     overhead: usize,
 362     max_budget: usize,
 363 ) -> usize
 364 where
 365     I: IntoIterator<Item = T> + Clone,
 366     T: AsRef<ListItem>,
 367 {
 368     let mut max_width = 0;
 369     let mut first = true;
 370     for item in items.clone().into_iter().skip(i) {
 371         let item = item.as_ref();
 372         let inner_item_width = item.inner_as_ref().len();
 373         if !first &&
 374             (item.is_multiline() || !item.post_comment.is_some() ||
 375                 inner_item_width + overhead > max_budget)
 376         {
 377             return max_width;
 378         }
 379         if max_width < inner_item_width {
 380             max_width = inner_item_width;
 381         }
 382         if item.new_lines {
 383             return max_width;
 384         }
 385         first = false;
 386     }
 387     max_width
 388 }
 389
 390 fn post_comment_alignment(item_max_width: Option<usize>, inner_item_len: usize) -> usize {
 391     item_max_width
 392         .and_then(|max_line_width| max_line_width.checked_sub(inner_item_len))
 393         .unwrap_or(0)
 394 }
 395
 396 pub struct ListItems<'a, I, F1, F2, F3>
 397 where
 398     I: Iterator,
 399 {
 400     codemap: &'a CodeMap,
 401     inner: Peekable<I>,
 402     get_lo: F1,
 403     get_hi: F2,
 404     get_item_string: F3,
 405     prev_span_end: BytePos,
 406     next_span_start: BytePos,
 407     terminator: &'a str,
 408 }
 409
 410 impl<'a, T, I, F1, F2, F3> Iterator for ListItems<'a, I, F1, F2, F3>
 411 where
 412     I: Iterator<Item = T>,
 413     F1: Fn(&T) -> BytePos,
 414     F2: Fn(&T) -> BytePos,
 415     F3: Fn(&T) -> Option<String>,
 416 {
 417     type Item = ListItem;
 418
 419     fn next(&mut self) -> Option<Self::Item> {
 420         let white_space: &[_] = &[' ', '\t'];
 421
 422         self.inner.next().map(|item| {
 423             let mut new_lines = false;
 424             // Pre-comment
 425             let pre_snippet = self.codemap
 426                 .span_to_snippet(mk_sp(self.prev_span_end, (self.get_lo)(&item)))
 427                 .unwrap();
 428             let trimmed_pre_snippet = pre_snippet.trim();
 429             let has_pre_comment =
 430                 trimmed_pre_snippet.contains("//") || trimmed_pre_snippet.contains("/*");
 431             let pre_comment = if has_pre_comment {
 432                 Some(trimmed_pre_snippet.to_owned())
 433             } else {
 434                 None
 435             };
 436
 437             // Post-comment
 438             let next_start = match self.inner.peek() {
 439                 Some(next_item) => (self.get_lo)(next_item),
 440                 None => self.next_span_start,
 441             };
 442             let post_snippet = self.codemap
 443                 .span_to_snippet(mk_sp((self.get_hi)(&item), next_start))
 444                 .unwrap();
 445
 446             let comment_end = match self.inner.peek() {
 447                 Some(..) => {
 448                     let mut block_open_index = post_snippet.find("/*");
 449                     // check if it realy is a block comment (and not //*)
 450                     if let Some(i) = block_open_index {
 451                         if i > 0 && &post_snippet[i - 1..i] == "/" {
 452                             block_open_index = None;
 453                         }
 454                     }
 455                     let newline_index = post_snippet.find('\n');
 456                     let separator_index = post_snippet.find_uncommented(",").unwrap();
 457
 458                     match (block_open_index, newline_index) {
 459                         // Separator before comment, with the next item on same line.
 460                         // Comment belongs to next item.
 461                         (Some(i), None) if i > separator_index => separator_index + 1,
 462                         // Block-style post-comment before the separator.
 463                         (Some(i), None) => cmp::max(
 464                             find_comment_end(&post_snippet[i..]).unwrap() + i,
 465                             separator_index + 1,
 466                         ),
 467                         // Block-style post-comment. Either before or after the separator.
 468                         (Some(i), Some(j)) if i < j => cmp::max(
 469                             find_comment_end(&post_snippet[i..]).unwrap() + i,
 470                             separator_index + 1,
 471                         ),
 472                         // Potential *single* line comment.
 473                         (_, Some(j)) if j > separator_index => j + 1,
 474                         _ => post_snippet.len(),
 475                     }
 476                 }
 477                 None => post_snippet
 478                     .find_uncommented(self.terminator)
 479                     .unwrap_or(post_snippet.len()),
 480             };
 481
 482             if !post_snippet.is_empty() && comment_end > 0 {
 483                 // Account for extra whitespace between items. This is fiddly
 484                 // because of the way we divide pre- and post- comments.
 485
 486                 // Everything from the separator to the next item.
 487                 let test_snippet = &post_snippet[comment_end - 1..];
 488                 let first_newline = test_snippet.find('\n').unwrap_or(test_snippet.len());
 489                 // From the end of the first line of comments.
 490                 let test_snippet = &test_snippet[first_newline..];
 491                 let first = test_snippet
 492                     .find(|c: char| !c.is_whitespace())
 493                     .unwrap_or(test_snippet.len());
 494                 // From the end of the first line of comments to the next non-whitespace char.
 495                 let test_snippet = &test_snippet[..first];
 496
 497                 if test_snippet.chars().filter(|c| c == &'\n').count() > 1 {
 498                     // There were multiple line breaks which got trimmed to nothing.
 499                     new_lines = true;
 500                 }
 501             }
 502
 503             // Cleanup post-comment: strip separators and whitespace.
 504             self.prev_span_end = (self.get_hi)(&item) + BytePos(comment_end as u32);
 505             let post_snippet = post_snippet[..comment_end].trim();
 506
 507             let post_snippet_trimmed = if post_snippet.starts_with(',') {
 508                 post_snippet[1..].trim_matches(white_space)
 509             } else if post_snippet.ends_with(',') {
 510                 post_snippet[..(post_snippet.len() - 1)].trim_matches(white_space)
 511             } else {
 512                 post_snippet
 513             };
 514
 515             let post_comment = if !post_snippet_trimmed.is_empty() {
 516                 Some(post_snippet_trimmed.to_owned())
 517             } else {
 518                 None
 519             };
 520
 521             ListItem {
 522                 pre_comment: pre_comment,
 523                 item: (self.get_item_string)(&item),
 524                 post_comment: post_comment,
 525                 new_lines: new_lines,
 526             }
 527         })
 528     }
 529 }
 530
 531 // Creates an iterator over a list's items with associated comments.
 532 pub fn itemize_list<'a, T, I, F1, F2, F3>(
 533     codemap: &'a CodeMap,
 534     inner: I,
 535     terminator: &'a str,
 536     get_lo: F1,
 537     get_hi: F2,
 538     get_item_string: F3,
 539     prev_span_end: BytePos,
 540     next_span_start: BytePos,
 541 ) -> ListItems<'a, I, F1, F2, F3>
 542 where
 543     I: Iterator<Item = T>,
 544     F1: Fn(&T) -> BytePos,
 545     F2: Fn(&T) -> BytePos,
 546     F3: Fn(&T) -> Option<String>,
 547 {
 548     ListItems {
 549         codemap: codemap,
 550         inner: inner.peekable(),
 551         get_lo: get_lo,
 552         get_hi: get_hi,
 553         get_item_string: get_item_string,
 554         prev_span_end: prev_span_end,
 555         next_span_start: next_span_start,
 556         terminator: terminator,
 557     }
 558 }
 559
 560 fn needs_trailing_separator(
 561     separator_tactic: SeparatorTactic,
 562     list_tactic: DefinitiveListTactic,
 563 ) -> bool {
 564     match separator_tactic {
 565         SeparatorTactic::Always => true,
 566         SeparatorTactic::Vertical => list_tactic == DefinitiveListTactic::Vertical,
 567         SeparatorTactic::Never => false,
 568     }
 569 }
 570
 571 /// Returns the count and total width of the list items.
 572 fn calculate_width<I, T>(items: I) -> (usize, usize)
 573 where
 574     I: IntoIterator<Item = T>,
 575     T: AsRef<ListItem>,
 576 {
 577     items
 578         .into_iter()
 579         .map(|item| total_item_width(item.as_ref()))
 580         .fold((0, 0), |acc, l| (acc.0 + 1, acc.1 + l))
 581 }
 582
 583 fn total_item_width(item: &ListItem) -> usize {
 584     comment_len(item.pre_comment.as_ref().map(|x| &(*x)[..])) +
 585         comment_len(item.post_comment.as_ref().map(|x| &(*x)[..])) +
 586         item.item.as_ref().map_or(0, |str| str.len())
 587 }
 588
 589 fn comment_len(comment: Option<&str>) -> usize {
 590     match comment {
 591         Some(s) => {
 592             let text_len = s.trim().len();
 593             if text_len > 0 {
 594                 // We'll put " /*" before and " */" after inline comments.
 595                 text_len + 6
 596             } else {
 597                 text_len
 598             }
 599         }
 600         None => 0,
 601     }
 602 }
 603
 604 // Compute horizontal and vertical shapes for a struct-lit-like thing.
 605 pub fn struct_lit_shape(
 606     shape: Shape,
 607     context: &RewriteContext,
 608     prefix_width: usize,
 609     suffix_width: usize,
 610 ) -> Option<(Option<Shape>, Shape)> {
 611     let v_shape = match context.config.struct_lit_style() {
 612         IndentStyle::Visual => try_opt!(
 613             try_opt!(shape.visual_indent(0).shrink_left(prefix_width)).sub_width(suffix_width)
 614         ),
 615         IndentStyle::Block => {
 616             let shape = shape.block_indent(context.config.tab_spaces());
 617             Shape {
 618                 width: try_opt!(context.config.max_width().checked_sub(shape.indent.width())),
 619                 ..shape
 620             }
 621         }
 622     };
 623     let shape_width = shape.width.checked_sub(prefix_width + suffix_width);
 624     if let Some(w) = shape_width {
 625         let shape_width = cmp::min(w, context.config.struct_lit_width());
 626         Some((Some(Shape::legacy(shape_width, shape.indent)), v_shape))
 627     } else {
 628         Some((None, v_shape))
 629     }
 630 }
 631
 632 // Compute the tactic for the internals of a struct-lit-like thing.
 633 pub fn struct_lit_tactic(
 634     h_shape: Option<Shape>,
 635     context: &RewriteContext,
 636     items: &[ListItem],
 637 ) -> DefinitiveListTactic {
 638     if let Some(h_shape) = h_shape {
 639         let prelim_tactic = match (context.config.struct_lit_style(), items.len()) {
 640             (IndentStyle::Visual, 1) => ListTactic::HorizontalVertical,
 641             _ => context.config.struct_lit_multiline_style().to_list_tactic(),
 642         };
 643         definitive_tactic(items, prelim_tactic, h_shape.width)
 644     } else {
 645         DefinitiveListTactic::Vertical
 646     }
 647 }
 648
 649 // Given a tactic and possible shapes for horizontal and vertical layout,
 650 // come up with the actual shape to use.
 651 pub fn shape_for_tactic(
 652     tactic: DefinitiveListTactic,
 653     h_shape: Option<Shape>,
 654     v_shape: Shape,
 655 ) -> Shape {
 656     match tactic {
 657         DefinitiveListTactic::Horizontal => h_shape.unwrap(),
 658         _ => v_shape,
 659     }
 660 }
 661
 662 // Create a ListFormatting object for formatting the internals of a
 663 // struct-lit-like thing, that is a series of fields.
 664 pub fn struct_lit_formatting<'a>(
 665     shape: Shape,
 666     tactic: DefinitiveListTactic,
 667     context: &'a RewriteContext,
 668     force_no_trailing_comma: bool,
 669 ) -> ListFormatting<'a> {
 670     let ends_with_newline = context.config.struct_lit_style() != IndentStyle::Visual &&
 671         tactic == DefinitiveListTactic::Vertical;
 672     ListFormatting {
 673         tactic: tactic,
 674         separator: ",",
 675         trailing_separator: if force_no_trailing_comma {
 676             SeparatorTactic::Never
 677         } else {
 678             context.config.trailing_comma()
 679         },
 680         shape: shape,
 681         ends_with_newline: ends_with_newline,
 682         preserve_newline: true,
 683         config: context.config,
 684     }
 685 }