src/tools/rustfmt/src/matches.rs

   1 //! Format match expression.
   2
   3 use std::iter::repeat;
   4
   5 use rustc_ast::{ast, ptr};
   6 use rustc_span::{BytePos, Span};
   7
   8 use crate::comment::{combine_strs_with_missing_comments, rewrite_comment};
   9 use crate::config::lists::*;
  10 use crate::config::{Config, ControlBraceStyle, IndentStyle, MatchArmLeadingPipe, Version};
  11 use crate::expr::{
  12     format_expr, is_empty_block, is_simple_block, is_unsafe_block, prefer_next_line, rewrite_cond,
  13     ExprType, RhsTactics,
  14 };
  15 use crate::lists::{itemize_list, write_list, ListFormatting};
  16 use crate::rewrite::{Rewrite, RewriteContext};
  17 use crate::shape::Shape;
  18 use crate::source_map::SpanUtils;
  19 use crate::spanned::Spanned;
  20 use crate::utils::{
  21     contains_skip, extra_offset, first_line_width, inner_attributes, last_line_extendable, mk_sp,
  22     semicolon_for_expr, trimmed_last_line_width, unicode_str_width,
  23 };
  24
  25 /// A simple wrapper type against `ast::Arm`. Used inside `write_list()`.
  26 struct ArmWrapper<'a> {
  27     arm: &'a ast::Arm,
  28     /// `true` if the arm is the last one in match expression. Used to decide on whether we should
  29     /// add trailing comma to the match arm when `config.trailing_comma() == Never`.
  30     is_last: bool,
  31     /// Holds a byte position of `|` at the beginning of the arm pattern, if available.
  32     beginning_vert: Option<BytePos>,
  33 }
  34
  35 impl<'a> ArmWrapper<'a> {
  36     fn new(arm: &'a ast::Arm, is_last: bool, beginning_vert: Option<BytePos>) -> ArmWrapper<'a> {
  37         ArmWrapper {
  38             arm,
  39             is_last,
  40             beginning_vert,
  41         }
  42     }
  43 }
  44
  45 impl<'a> Spanned for ArmWrapper<'a> {
  46     fn span(&self) -> Span {
  47         if let Some(lo) = self.beginning_vert {
  48             let lo = std::cmp::min(lo, self.arm.span().lo());
  49             mk_sp(lo, self.arm.span().hi())
  50         } else {
  51             self.arm.span()
  52         }
  53     }
  54 }
  55
  56 impl<'a> Rewrite for ArmWrapper<'a> {
  57     fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
  58         rewrite_match_arm(
  59             context,
  60             self.arm,
  61             shape,
  62             self.is_last,
  63             self.beginning_vert.is_some(),
  64         )
  65     }
  66 }
  67
  68 pub(crate) fn rewrite_match(
  69     context: &RewriteContext<'_>,
  70     cond: &ast::Expr,
  71     arms: &[ast::Arm],
  72     shape: Shape,
  73     span: Span,
  74     attrs: &[ast::Attribute],
  75 ) -> Option<String> {
  76     // Do not take the rhs overhead from the upper expressions into account
  77     // when rewriting match condition.
  78     let cond_shape = Shape {
  79         width: context.budget(shape.used_width()),
  80         ..shape
  81     };
  82     // 6 = `match `
  83     let cond_shape = match context.config.indent_style() {
  84         IndentStyle::Visual => cond_shape.shrink_left(6)?,
  85         IndentStyle::Block => cond_shape.offset_left(6)?,
  86     };
  87     let cond_str = cond.rewrite(context, cond_shape)?;
  88     let alt_block_sep = &shape.indent.to_string_with_newline(context.config);
  89     let block_sep = match context.config.control_brace_style() {
  90         ControlBraceStyle::AlwaysNextLine => alt_block_sep,
  91         _ if last_line_extendable(&cond_str) => " ",
  92         // 2 = ` {`
  93         _ if cond_str.contains('\n') || cond_str.len() + 2 > cond_shape.width => alt_block_sep,
  94         _ => " ",
  95     };
  96
  97     let nested_indent_str = shape
  98         .indent
  99         .block_indent(context.config)
 100         .to_string(context.config);
 101     // Inner attributes.
 102     let inner_attrs = &inner_attributes(attrs);
 103     let inner_attrs_str = if inner_attrs.is_empty() {
 104         String::new()
 105     } else {
 106         inner_attrs
 107             .rewrite(context, shape)
 108             .map(|s| format!("{}{}\n", nested_indent_str, s))?
 109     };
 110
 111     let open_brace_pos = if inner_attrs.is_empty() {
 112         let hi = if arms.is_empty() {
 113             span.hi()
 114         } else {
 115             arms[0].span().lo()
 116         };
 117         context
 118             .snippet_provider
 119             .span_after(mk_sp(cond.span.hi(), hi), "{")
 120     } else {
 121         inner_attrs[inner_attrs.len() - 1].span.hi()
 122     };
 123
 124     if arms.is_empty() {
 125         let snippet = context.snippet(mk_sp(open_brace_pos, span.hi() - BytePos(1)));
 126         if snippet.trim().is_empty() {
 127             Some(format!("match {cond_str} {{}}"))
 128         } else {
 129             // Empty match with comments or inner attributes? We are not going to bother, sorry ;)
 130             Some(context.snippet(span).to_owned())
 131         }
 132     } else {
 133         let span_after_cond = mk_sp(cond.span.hi(), span.hi());
 134         Some(format!(
 135             "match {}{}{{\n{}{}{}\n{}}}",
 136             cond_str,
 137             block_sep,
 138             inner_attrs_str,
 139             nested_indent_str,
 140             rewrite_match_arms(context, arms, shape, span_after_cond, open_brace_pos)?,
 141             shape.indent.to_string(context.config),
 142         ))
 143     }
 144 }
 145
 146 fn arm_comma(config: &Config, body: &ast::Expr, is_last: bool) -> &'static str {
 147     if is_last && config.trailing_comma() == SeparatorTactic::Never {
 148         ""
 149     } else if config.match_block_trailing_comma() {
 150         ","
 151     } else if let ast::ExprKind::Block(ref block, _) = body.kind {
 152         if let ast::BlockCheckMode::Default = block.rules {
 153             ""
 154         } else {
 155             ","
 156         }
 157     } else {
 158         ","
 159     }
 160 }
 161
 162 /// Collect a byte position of the beginning `|` for each arm, if available.
 163 fn collect_beginning_verts(
 164     context: &RewriteContext<'_>,
 165     arms: &[ast::Arm],
 166 ) -> Vec<Option<BytePos>> {
 167     arms.iter()
 168         .map(|a| {
 169             context
 170                 .snippet(a.pat.span)
 171                 .starts_with('|')
 172                 .then(|| a.pat.span().lo())
 173         })
 174         .collect()
 175 }
 176
 177 fn rewrite_match_arms(
 178     context: &RewriteContext<'_>,
 179     arms: &[ast::Arm],
 180     shape: Shape,
 181     span: Span,
 182     open_brace_pos: BytePos,
 183 ) -> Option<String> {
 184     let arm_shape = shape
 185         .block_indent(context.config.tab_spaces())
 186         .with_max_width(context.config);
 187
 188     let arm_len = arms.len();
 189     let is_last_iter = repeat(false)
 190         .take(arm_len.saturating_sub(1))
 191         .chain(repeat(true));
 192     let beginning_verts = collect_beginning_verts(context, arms);
 193     let items = itemize_list(
 194         context.snippet_provider,
 195         arms.iter()
 196             .zip(is_last_iter)
 197             .zip(beginning_verts.into_iter())
 198             .map(|((arm, is_last), beginning_vert)| ArmWrapper::new(arm, is_last, beginning_vert)),
 199         "}",
 200         "|",
 201         |arm| arm.span().lo(),
 202         |arm| arm.span().hi(),
 203         |arm| arm.rewrite(context, arm_shape),
 204         open_brace_pos,
 205         span.hi(),
 206         false,
 207     );
 208     let arms_vec: Vec<_> = items.collect();
 209     // We will add/remove commas inside `arm.rewrite()`, and hence no separator here.
 210     let fmt = ListFormatting::new(arm_shape, context.config)
 211         .separator("")
 212         .preserve_newline(true);
 213
 214     write_list(&arms_vec, &fmt)
 215 }
 216
 217 fn rewrite_match_arm(
 218     context: &RewriteContext<'_>,
 219     arm: &ast::Arm,
 220     shape: Shape,
 221     is_last: bool,
 222     has_leading_pipe: bool,
 223 ) -> Option<String> {
 224     let (missing_span, attrs_str) = if !arm.attrs.is_empty() {
 225         if contains_skip(&arm.attrs) {
 226             let (_, body) = flatten_arm_body(context, &arm.body, None);
 227             // `arm.span()` does not include trailing comma, add it manually.
 228             return Some(format!(
 229                 "{}{}",
 230                 context.snippet(arm.span()),
 231                 arm_comma(context.config, body, is_last),
 232             ));
 233         }
 234         let missing_span = mk_sp(arm.attrs[arm.attrs.len() - 1].span.hi(), arm.pat.span.lo());
 235         (missing_span, arm.attrs.rewrite(context, shape)?)
 236     } else {
 237         (mk_sp(arm.span().lo(), arm.span().lo()), String::new())
 238     };
 239
 240     // Leading pipe offset
 241     // 2 = `| `
 242     let (pipe_offset, pipe_str) = match context.config.match_arm_leading_pipes() {
 243         MatchArmLeadingPipe::Never => (0, ""),
 244         MatchArmLeadingPipe::Preserve if !has_leading_pipe => (0, ""),
 245         MatchArmLeadingPipe::Preserve | MatchArmLeadingPipe::Always => (2, "| "),
 246     };
 247
 248     // Patterns
 249     let pat_shape = match &arm.body.kind {
 250         ast::ExprKind::Block(_, Some(label)) => {
 251             // Some block with a label ` => 'label: {`
 252             // 7 = ` => : {`
 253             let label_len = label.ident.as_str().len();
 254             shape.sub_width(7 + label_len)?.offset_left(pipe_offset)?
 255         }
 256         _ => {
 257             // 5 = ` => {`
 258             shape.sub_width(5)?.offset_left(pipe_offset)?
 259         }
 260     };
 261     let pats_str = arm.pat.rewrite(context, pat_shape)?;
 262
 263     // Guard
 264     let block_like_pat = trimmed_last_line_width(&pats_str) <= context.config.tab_spaces();
 265     let new_line_guard = pats_str.contains('\n') && !block_like_pat;
 266     let guard_str = rewrite_guard(
 267         context,
 268         &arm.guard,
 269         shape,
 270         trimmed_last_line_width(&pats_str),
 271         new_line_guard,
 272     )?;
 273
 274     let lhs_str = combine_strs_with_missing_comments(
 275         context,
 276         &attrs_str,
 277         &format!("{pipe_str}{pats_str}{guard_str}"),
 278         missing_span,
 279         shape,
 280         false,
 281     )?;
 282
 283     let arrow_span = mk_sp(arm.pat.span.hi(), arm.body.span().lo());
 284     rewrite_match_body(
 285         context,
 286         &arm.body,
 287         &lhs_str,
 288         shape,
 289         guard_str.contains('\n'),
 290         arrow_span,
 291         is_last,
 292     )
 293 }
 294
 295 fn stmt_is_expr_mac(stmt: &ast::Stmt) -> bool {
 296     if let ast::StmtKind::Expr(expr) = &stmt.kind {
 297         if let ast::ExprKind::MacCall(_) = &expr.kind {
 298             return true;
 299         }
 300     }
 301     false
 302 }
 303
 304 fn block_can_be_flattened<'a>(
 305     context: &RewriteContext<'_>,
 306     expr: &'a ast::Expr,
 307 ) -> Option<&'a ast::Block> {
 308     match expr.kind {
 309         ast::ExprKind::Block(ref block, label)
 310             if label.is_none()
 311                 && !is_unsafe_block(block)
 312                 && !context.inside_macro()
 313                 && is_simple_block(context, block, Some(&expr.attrs))
 314                 && !stmt_is_expr_mac(&block.stmts[0]) =>
 315         {
 316             Some(&*block)
 317         }
 318         _ => None,
 319     }
 320 }
 321
 322 // (extend, body)
 323 // @extend: true if the arm body can be put next to `=>`
 324 // @body: flattened body, if the body is block with a single expression
 325 fn flatten_arm_body<'a>(
 326     context: &'a RewriteContext<'_>,
 327     body: &'a ast::Expr,
 328     opt_shape: Option<Shape>,
 329 ) -> (bool, &'a ast::Expr) {
 330     let can_extend =
 331         |expr| !context.config.force_multiline_blocks() && can_flatten_block_around_this(expr);
 332
 333     if let Some(block) = block_can_be_flattened(context, body) {
 334         if let ast::StmtKind::Expr(ref expr) = block.stmts[0].kind {
 335             if let ast::ExprKind::Block(..) = expr.kind {
 336                 if expr.attrs.is_empty() {
 337                     flatten_arm_body(context, expr, None)
 338                 } else {
 339                     (true, body)
 340                 }
 341             } else {
 342                 let cond_becomes_muti_line = opt_shape
 343                     .and_then(|shape| rewrite_cond(context, expr, shape))
 344                     .map_or(false, |cond| cond.contains('\n'));
 345                 if cond_becomes_muti_line {
 346                     (false, &*body)
 347                 } else {
 348                     (can_extend(expr), &*expr)
 349                 }
 350             }
 351         } else {
 352             (false, &*body)
 353         }
 354     } else {
 355         (can_extend(body), &*body)
 356     }
 357 }
 358
 359 fn rewrite_match_body(
 360     context: &RewriteContext<'_>,
 361     body: &ptr::P<ast::Expr>,
 362     pats_str: &str,
 363     shape: Shape,
 364     has_guard: bool,
 365     arrow_span: Span,
 366     is_last: bool,
 367 ) -> Option<String> {
 368     let (extend, body) = flatten_arm_body(
 369         context,
 370         body,
 371         shape.offset_left(extra_offset(pats_str, shape) + 4),
 372     );
 373     let (is_block, is_empty_block) = if let ast::ExprKind::Block(ref block, _) = body.kind {
 374         (true, is_empty_block(context, block, Some(&body.attrs)))
 375     } else {
 376         (false, false)
 377     };
 378
 379     let comma = arm_comma(context.config, body, is_last);
 380     let alt_block_sep = &shape.indent.to_string_with_newline(context.config);
 381
 382     let combine_orig_body = |body_str: &str| {
 383         let block_sep = match context.config.control_brace_style() {
 384             ControlBraceStyle::AlwaysNextLine if is_block => alt_block_sep,
 385             _ => " ",
 386         };
 387
 388         Some(format!("{} =>{}{}{}", pats_str, block_sep, body_str, comma))
 389     };
 390
 391     let next_line_indent = if !is_block || is_empty_block {
 392         shape.indent.block_indent(context.config)
 393     } else {
 394         shape.indent
 395     };
 396
 397     let forbid_same_line =
 398         (has_guard && pats_str.contains('\n') && !is_empty_block) || !body.attrs.is_empty();
 399
 400     // Look for comments between `=>` and the start of the body.
 401     let arrow_comment = {
 402         let arrow_snippet = context.snippet(arrow_span).trim();
 403         // search for the arrow starting from the end of the snippet since there may be a match
 404         // expression within the guard
 405         let arrow_index = arrow_snippet.rfind("=>").unwrap();
 406         // 2 = `=>`
 407         let comment_str = arrow_snippet[arrow_index + 2..].trim();
 408         if comment_str.is_empty() {
 409             String::new()
 410         } else {
 411             rewrite_comment(comment_str, false, shape, context.config)?
 412         }
 413     };
 414
 415     let combine_next_line_body = |body_str: &str| {
 416         let nested_indent_str = next_line_indent.to_string_with_newline(context.config);
 417
 418         if is_block {
 419             let mut result = pats_str.to_owned();
 420             result.push_str(" =>");
 421             if !arrow_comment.is_empty() {
 422                 result.push_str(&nested_indent_str);
 423                 result.push_str(&arrow_comment);
 424             }
 425             result.push_str(&nested_indent_str);
 426             result.push_str(body_str);
 427             result.push_str(comma);
 428             return Some(result);
 429         }
 430
 431         let indent_str = shape.indent.to_string_with_newline(context.config);
 432         let (body_prefix, body_suffix) =
 433             if context.config.match_arm_blocks() && !context.inside_macro() {
 434                 let comma = if context.config.match_block_trailing_comma() {
 435                     ","
 436                 } else {
 437                     ""
 438                 };
 439                 let semicolon = if context.config.version() == Version::One {
 440                     ""
 441                 } else {
 442                     if semicolon_for_expr(context, body) {
 443                         ";"
 444                     } else {
 445                         ""
 446                     }
 447                 };
 448                 ("{", format!("{}{}}}{}", semicolon, indent_str, comma))
 449             } else {
 450                 ("", String::from(","))
 451             };
 452
 453         let block_sep = match context.config.control_brace_style() {
 454             ControlBraceStyle::AlwaysNextLine => format!("{}{}", alt_block_sep, body_prefix),
 455             _ if body_prefix.is_empty() => "".to_owned(),
 456             _ if forbid_same_line || !arrow_comment.is_empty() => {
 457                 format!("{}{}", alt_block_sep, body_prefix)
 458             }
 459             _ => format!(" {}", body_prefix),
 460         } + &nested_indent_str;
 461
 462         let mut result = pats_str.to_owned();
 463         result.push_str(" =>");
 464         if !arrow_comment.is_empty() {
 465             result.push_str(&indent_str);
 466             result.push_str(&arrow_comment);
 467         }
 468         result.push_str(&block_sep);
 469         result.push_str(body_str);
 470         result.push_str(&body_suffix);
 471         Some(result)
 472     };
 473
 474     // Let's try and get the arm body on the same line as the condition.
 475     // 4 = ` => `.len()
 476     let orig_body_shape = shape
 477         .offset_left(extra_offset(pats_str, shape) + 4)
 478         .and_then(|shape| shape.sub_width(comma.len()));
 479     let orig_body = if forbid_same_line || !arrow_comment.is_empty() {
 480         None
 481     } else if let Some(body_shape) = orig_body_shape {
 482         let rewrite = nop_block_collapse(
 483             format_expr(body, ExprType::Statement, context, body_shape),
 484             body_shape.width,
 485         );
 486
 487         match rewrite {
 488             Some(ref body_str)
 489                 if is_block
 490                     || (!body_str.contains('\n')
 491                         && unicode_str_width(body_str) <= body_shape.width) =>
 492             {
 493                 return combine_orig_body(body_str);
 494             }
 495             _ => rewrite,
 496         }
 497     } else {
 498         None
 499     };
 500     let orig_budget = orig_body_shape.map_or(0, |shape| shape.width);
 501
 502     // Try putting body on the next line and see if it looks better.
 503     let next_line_body_shape = Shape::indented(next_line_indent, context.config);
 504     let next_line_body = nop_block_collapse(
 505         format_expr(body, ExprType::Statement, context, next_line_body_shape),
 506         next_line_body_shape.width,
 507     );
 508     match (orig_body, next_line_body) {
 509         (Some(ref orig_str), Some(ref next_line_str))
 510             if prefer_next_line(orig_str, next_line_str, RhsTactics::Default) =>
 511         {
 512             combine_next_line_body(next_line_str)
 513         }
 514         (Some(ref orig_str), _) if extend && first_line_width(orig_str) <= orig_budget => {
 515             combine_orig_body(orig_str)
 516         }
 517         (Some(ref orig_str), Some(ref next_line_str)) if orig_str.contains('\n') => {
 518             combine_next_line_body(next_line_str)
 519         }
 520         (None, Some(ref next_line_str)) => combine_next_line_body(next_line_str),
 521         (None, None) => None,
 522         (Some(ref orig_str), _) => combine_orig_body(orig_str),
 523     }
 524 }
 525
 526 // The `if ...` guard on a match arm.
 527 fn rewrite_guard(
 528     context: &RewriteContext<'_>,
 529     guard: &Option<ptr::P<ast::Expr>>,
 530     shape: Shape,
 531     // The amount of space used up on this line for the pattern in
 532     // the arm (excludes offset).
 533     pattern_width: usize,
 534     multiline_pattern: bool,
 535 ) -> Option<String> {
 536     if let Some(ref guard) = *guard {
 537         // First try to fit the guard string on the same line as the pattern.
 538         // 4 = ` if `, 5 = ` => {`
 539         let cond_shape = shape
 540             .offset_left(pattern_width + 4)
 541             .and_then(|s| s.sub_width(5));
 542         if !multiline_pattern {
 543             if let Some(cond_shape) = cond_shape {
 544                 if let Some(cond_str) = guard.rewrite(context, cond_shape) {
 545                     if !cond_str.contains('\n') || pattern_width <= context.config.tab_spaces() {
 546                         return Some(format!(" if {cond_str}"));
 547                     }
 548                 }
 549             }
 550         }
 551
 552         // Not enough space to put the guard after the pattern, try a newline.
 553         // 3 = `if `, 5 = ` => {`
 554         let cond_shape = Shape::indented(shape.indent.block_indent(context.config), context.config)
 555             .offset_left(3)
 556             .and_then(|s| s.sub_width(5));
 557         if let Some(cond_shape) = cond_shape {
 558             if let Some(cond_str) = guard.rewrite(context, cond_shape) {
 559                 return Some(format!(
 560                     "{}if {}",
 561                     cond_shape.indent.to_string_with_newline(context.config),
 562                     cond_str
 563                 ));
 564             }
 565         }
 566
 567         None
 568     } else {
 569         Some(String::new())
 570     }
 571 }
 572
 573 fn nop_block_collapse(block_str: Option<String>, budget: usize) -> Option<String> {
 574     debug!("nop_block_collapse {:?} {}", block_str, budget);
 575     block_str.map(|block_str| {
 576         if block_str.starts_with('{')
 577             && budget >= 2
 578             && (block_str[1..].find(|c: char| !c.is_whitespace()).unwrap() == block_str.len() - 2)
 579         {
 580             String::from("{}")
 581         } else {
 582             block_str
 583         }
 584     })
 585 }
 586
 587 fn can_flatten_block_around_this(body: &ast::Expr) -> bool {
 588     match body.kind {
 589         // We do not allow `if` to stay on the same line, since we could easily mistake
 590         // `pat => if cond { ... }` and `pat if cond => { ... }`.
 591         ast::ExprKind::If(..) => false,
 592         // We do not allow collapsing a block around expression with condition
 593         // to avoid it being cluttered with match arm.
 594         ast::ExprKind::ForLoop(..) | ast::ExprKind::While(..) => false,
 595         ast::ExprKind::Loop(..)
 596         | ast::ExprKind::Match(..)
 597         | ast::ExprKind::Block(..)
 598         | ast::ExprKind::Closure(..)
 599         | ast::ExprKind::Array(..)
 600         | ast::ExprKind::Call(..)
 601         | ast::ExprKind::MethodCall(..)
 602         | ast::ExprKind::MacCall(..)
 603         | ast::ExprKind::Struct(..)
 604         | ast::ExprKind::Tup(..) => true,
 605         ast::ExprKind::AddrOf(_, _, ref expr)
 606         | ast::ExprKind::Try(ref expr)
 607         | ast::ExprKind::Unary(_, ref expr)
 608         | ast::ExprKind::Index(ref expr, _, _)
 609         | ast::ExprKind::Cast(ref expr, _) => can_flatten_block_around_this(expr),
 610         _ => false,
 611     }
 612 }