src/expr.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::Ordering;
  12 use std::borrow::Borrow;
  13
  14 use Indent;
  15 use rewrite::{Rewrite, RewriteContext};
  16 use lists::{write_list, itemize_list, ListFormatting, SeparatorTactic, ListTactic};
  17 use string::{StringFormat, rewrite_string};
  18 use utils::{span_after, extra_offset, first_line_width, last_line_width, wrap_str, binary_search};
  19 use visitor::FmtVisitor;
  20 use config::{StructLitStyle, MultilineStyle};
  21 use comment::{FindUncommented, rewrite_comment, contains_comment};
  22 use types::rewrite_path;
  23 use items::{span_lo_for_arg, span_hi_for_arg};
  24 use chains::rewrite_chain;
  25 use macros::rewrite_macro;
  26
  27 use syntax::{ast, ptr};
  28 use syntax::codemap::{CodeMap, Span, BytePos, mk_sp};
  29 use syntax::visit::Visitor;
  30
  31 impl Rewrite for ast::Expr {
  32     fn rewrite(&self, context: &RewriteContext, width: usize, offset: Indent) -> Option<String> {
  33         match self.node {
  34             ast::Expr_::ExprVec(ref expr_vec) => {
  35                 rewrite_array(expr_vec.iter().map(|e| &**e), self.span, context, width, offset)
  36             }
  37             ast::Expr_::ExprLit(ref l) => {
  38                 match l.node {
  39                     ast::Lit_::LitStr(_, ast::StrStyle::CookedStr) => {
  40                         rewrite_string_lit(context, l.span, width, offset)
  41                     }
  42                     _ => {
  43                         wrap_str(context.snippet(self.span),
  44                                  context.config.max_width,
  45                                  width,
  46                                  offset)
  47                     }
  48                 }
  49             }
  50             ast::Expr_::ExprCall(ref callee, ref args) => {
  51                 let inner_span = mk_sp(callee.span.hi, self.span.hi);
  52                 rewrite_call(context, &**callee, args, inner_span, width, offset)
  53             }
  54             ast::Expr_::ExprParen(ref subexpr) => {
  55                 rewrite_paren(context, subexpr, width, offset)
  56             }
  57             ast::Expr_::ExprBinary(ref op, ref lhs, ref rhs) => {
  58                 rewrite_binary_op(context, op, lhs, rhs, width, offset)
  59             }
  60             ast::Expr_::ExprUnary(ref op, ref subexpr) => {
  61                 rewrite_unary_op(context, op, subexpr, width, offset)
  62             }
  63             ast::Expr_::ExprStruct(ref path, ref fields, ref base) => {
  64                 rewrite_struct_lit(context,
  65                                    path,
  66                                    fields,
  67                                    base.as_ref().map(|e| &**e),
  68                                    self.span,
  69                                    width,
  70                                    offset)
  71             }
  72             ast::Expr_::ExprTup(ref items) => {
  73                 rewrite_tuple_lit(context, items, self.span, width, offset)
  74             }
  75             ast::Expr_::ExprWhile(ref cond, ref block, label) => {
  76                 Loop::new_while(None, cond, block, label).rewrite(context, width, offset)
  77             }
  78             ast::Expr_::ExprWhileLet(ref pat, ref cond, ref block, label) => {
  79                 Loop::new_while(Some(pat), cond, block, label).rewrite(context, width, offset)
  80             }
  81             ast::Expr_::ExprForLoop(ref pat, ref cond, ref block, label) => {
  82                 Loop::new_for(pat, cond, block, label).rewrite(context, width, offset)
  83             }
  84             ast::Expr_::ExprLoop(ref block, label) => {
  85                 Loop::new_loop(block, label).rewrite(context, width, offset)
  86             }
  87             ast::Expr_::ExprBlock(ref block) => {
  88                 block.rewrite(context, width, offset)
  89             }
  90             ast::Expr_::ExprIf(ref cond, ref if_block, ref else_block) => {
  91                 rewrite_if_else(context,
  92                                 cond,
  93                                 if_block,
  94                                 else_block.as_ref().map(|e| &**e),
  95                                 None,
  96                                 width,
  97                                 offset,
  98                                 true)
  99             }
 100             ast::Expr_::ExprIfLet(ref pat, ref cond, ref if_block, ref else_block) => {
 101                 rewrite_if_else(context,
 102                                 cond,
 103                                 if_block,
 104                                 else_block.as_ref().map(|e| &**e),
 105                                 Some(pat),
 106                                 width,
 107                                 offset,
 108                                 true)
 109             }
 110             // We reformat it ourselves because rustc gives us a bad span
 111             // for ranges, see rust#27162
 112             ast::Expr_::ExprRange(ref left, ref right) => {
 113                 rewrite_range(context,
 114                               left.as_ref().map(|e| &**e),
 115                               right.as_ref().map(|e| &**e),
 116                               width,
 117                               offset)
 118             }
 119             ast::Expr_::ExprMatch(ref cond, ref arms, _) => {
 120                 rewrite_match(context, cond, arms, width, offset)
 121             }
 122             ast::Expr_::ExprPath(ref qself, ref path) => {
 123                 rewrite_path(context, qself.as_ref(), path, width, offset)
 124             }
 125             ast::Expr_::ExprAssign(ref lhs, ref rhs) => {
 126                 rewrite_assignment(context, lhs, rhs, None, width, offset)
 127             }
 128             ast::Expr_::ExprAssignOp(ref op, ref lhs, ref rhs) => {
 129                 rewrite_assignment(context, lhs, rhs, Some(op), width, offset)
 130             }
 131             ast::Expr_::ExprAgain(ref opt_ident) => {
 132                 let id_str = match *opt_ident {
 133                     Some(ident) => format!(" {}", ident.node),
 134                     None => String::new(),
 135                 };
 136                 Some(format!("continue{}", id_str))
 137             }
 138             ast::Expr_::ExprBreak(ref opt_ident) => {
 139                 let id_str = match *opt_ident {
 140                     Some(ident) => format!(" {}", ident.node),
 141                     None => String::new(),
 142                 };
 143                 Some(format!("break{}", id_str))
 144             }
 145             ast::Expr_::ExprClosure(capture, ref fn_decl, ref body) => {
 146                 rewrite_closure(capture, fn_decl, body, self.span, context, width, offset)
 147             }
 148             ast::Expr_::ExprField(..) |
 149             ast::Expr_::ExprTupField(..) |
 150             ast::Expr_::ExprMethodCall(..) => {
 151                 rewrite_chain(self, context, width, offset)
 152             }
 153             ast::Expr_::ExprMac(ref mac) => {
 154                 rewrite_macro(mac, context, width, offset)
 155             }
 156             // We do not format these expressions yet, but they should still
 157             // satisfy our width restrictions.
 158             _ => wrap_str(context.snippet(self.span), context.config.max_width, width, offset),
 159         }
 160     }
 161 }
 162
 163 pub fn rewrite_array<'a, I>(expr_iter: I,
 164                             span: Span,
 165                             context: &RewriteContext,
 166                             width: usize,
 167                             offset: Indent)
 168                             -> Option<String>
 169     where I: Iterator<Item = &'a ast::Expr>
 170 {
 171     // 2 for brackets;
 172     let max_item_width = try_opt!(width.checked_sub(2));
 173     let items = itemize_list(context.codemap,
 174                              expr_iter,
 175                              "]",
 176                              |item| item.span.lo,
 177                              |item| item.span.hi,
 178                              // 1 = [
 179                              // FIXME(#133): itemize_list doesn't support
 180                              // rewrite failure. This may not be its
 181                              // responsibility, but that of write_list.
 182                              |item| {
 183                                  item.rewrite(context, max_item_width, offset + 1)
 184                                      .unwrap_or_else(|| context.snippet(item.span))
 185                              },
 186                              span_after(span, "[", context.codemap),
 187                              span.hi)
 188                     .collect::<Vec<_>>();
 189
 190     let tactic = if items.iter().any(|li| li.item.len() > 10 || li.is_multiline()) {
 191         ListTactic::HorizontalVertical
 192     } else {
 193         ListTactic::Mixed
 194     };
 195
 196     let fmt = ListFormatting {
 197         tactic: tactic,
 198         separator: ",",
 199         trailing_separator: SeparatorTactic::Never,
 200         indent: offset + 1,
 201         h_width: max_item_width,
 202         v_width: max_item_width,
 203         ends_with_newline: false,
 204         config: context.config,
 205     };
 206     let list_str = try_opt!(write_list(&items, &fmt));
 207
 208     Some(format!("[{}]", list_str))
 209 }
 210
 211 // This functions is pretty messy because of the wrapping and unwrapping of
 212 // expressions into and from blocks. See rust issue #27872.
 213 fn rewrite_closure(capture: ast::CaptureClause,
 214                    fn_decl: &ast::FnDecl,
 215                    body: &ast::Block,
 216                    span: Span,
 217                    context: &RewriteContext,
 218                    width: usize,
 219                    offset: Indent)
 220                    -> Option<String> {
 221     let mover = if capture == ast::CaptureClause::CaptureByValue {
 222         "move "
 223     } else {
 224         ""
 225     };
 226     let offset = offset + mover.len();
 227
 228     // 4 = "|| {".len(), which is overconservative when the closure consists of
 229     // a single expression.
 230     let budget = try_opt!(width.checked_sub(4 + mover.len()));
 231     // 1 = |
 232     let argument_offset = offset + 1;
 233     let ret_str = try_opt!(fn_decl.output.rewrite(context, budget, argument_offset));
 234     // 1 = space between arguments and return type.
 235     let horizontal_budget = budget.checked_sub(ret_str.len() + 1).unwrap_or(0);
 236
 237     let arg_items = itemize_list(context.codemap,
 238                                  fn_decl.inputs.iter(),
 239                                  "|",
 240                                  |arg| span_lo_for_arg(arg),
 241                                  |arg| span_hi_for_arg(arg),
 242                                  |arg| {
 243                                      // FIXME: we should just escalate failure
 244                                      // here, but itemize_list doesn't allow it.
 245                                      arg.rewrite(context, budget, argument_offset)
 246                                         .unwrap_or_else(|| {
 247                                             context.snippet(mk_sp(span_lo_for_arg(arg),
 248                                                                   span_hi_for_arg(arg)))
 249                                         })
 250                                  },
 251                                  span_after(span, "|", context.codemap),
 252                                  body.span.lo);
 253
 254     let fmt = ListFormatting {
 255         tactic: ListTactic::HorizontalVertical,
 256         separator: ",",
 257         trailing_separator: SeparatorTactic::Never,
 258         indent: argument_offset,
 259         h_width: horizontal_budget,
 260         v_width: budget,
 261         ends_with_newline: false,
 262         config: context.config,
 263     };
 264     let list_str = try_opt!(write_list(&arg_items.collect::<Vec<_>>(), &fmt));
 265     let mut prefix = format!("{}|{}|", mover, list_str);
 266
 267     if !ret_str.is_empty() {
 268         if prefix.contains('\n') {
 269             prefix.push('\n');
 270             prefix.push_str(&argument_offset.to_string(context.config));
 271         } else {
 272             prefix.push(' ');
 273         }
 274         prefix.push_str(&ret_str);
 275     }
 276
 277     // Try to format closure body as a single line expression without braces.
 278     if is_simple_block(body, context.codemap) && !prefix.contains('\n') {
 279         let (spacer, closer) = if ret_str.is_empty() {
 280             (" ", "")
 281         } else {
 282             (" { ", " }")
 283         };
 284         let expr = body.expr.as_ref().unwrap();
 285         // All closure bodies are blocks in the eyes of the AST, but we may not
 286         // want to unwrap them when they only contain a single expression.
 287         let inner_expr = match expr.node {
 288             ast::Expr_::ExprBlock(ref inner) if inner.stmts.is_empty() && inner.expr.is_some() &&
 289                                                 inner.rules ==
 290                                                 ast::BlockCheckMode::DefaultBlock => {
 291                 inner.expr.as_ref().unwrap()
 292             }
 293             _ => expr,
 294         };
 295         let extra_offset = extra_offset(&prefix, offset) + spacer.len();
 296         let budget = try_opt!(width.checked_sub(extra_offset + closer.len()));
 297         let rewrite = inner_expr.rewrite(context, budget, offset + extra_offset);
 298
 299         // Checks if rewrite succeeded and fits on a single line.
 300         let accept_rewrite = rewrite.as_ref().map(|result| !result.contains('\n')).unwrap_or(false);
 301
 302         if accept_rewrite {
 303             return Some(format!("{}{}{}{}", prefix, spacer, rewrite.unwrap(), closer));
 304         }
 305     }
 306
 307     // We couldn't format the closure body as a single line expression; fall
 308     // back to block formatting.
 309     let body_rewrite = body.expr
 310                            .as_ref()
 311                            .and_then(|body_expr| {
 312                                if let ast::Expr_::ExprBlock(ref inner) = body_expr.node {
 313                                    Some(inner.rewrite(&context, 2, Indent::empty()))
 314                                } else {
 315                                    None
 316                                }
 317                            })
 318                            .unwrap_or_else(|| body.rewrite(&context, 2, Indent::empty()));
 319
 320     Some(format!("{} {}", prefix, try_opt!(body_rewrite)))
 321 }
 322
 323 impl Rewrite for ast::Block {
 324     fn rewrite(&self, context: &RewriteContext, width: usize, offset: Indent) -> Option<String> {
 325         let user_str = context.snippet(self.span);
 326         if user_str == "{}" && width >= 2 {
 327             return Some(user_str);
 328         }
 329
 330         let mut visitor = FmtVisitor::from_codemap(context.codemap, context.config);
 331         visitor.block_indent = context.block_indent + context.overflow_indent;
 332
 333         let prefix = match self.rules {
 334             ast::BlockCheckMode::PushUnsafeBlock(..) |
 335             ast::BlockCheckMode::UnsafeBlock(..) => {
 336                 let snippet = context.snippet(self.span);
 337                 let open_pos = try_opt!(snippet.find_uncommented("{"));
 338                 visitor.last_pos = self.span.lo + BytePos(open_pos as u32);
 339
 340                 // Extract comment between unsafe and block start.
 341                 let trimmed = &snippet[6..open_pos].trim();
 342
 343                 let prefix = if !trimmed.is_empty() {
 344                     // 9 = "unsafe  {".len(), 7 = "unsafe ".len()
 345                     let budget = try_opt!(width.checked_sub(9));
 346                     format!("unsafe {} ",
 347                             rewrite_comment(trimmed, true, budget, offset + 7, context.config))
 348                 } else {
 349                     "unsafe ".to_owned()
 350                 };
 351
 352                 if is_simple_block(self, context.codemap) && prefix.len() < width {
 353                     let body = self.expr
 354                                    .as_ref()
 355                                    .unwrap()
 356                                    .rewrite(context, width - prefix.len(), offset);
 357                     if let Some(ref expr_str) = body {
 358                         let result = format!("{}{{ {} }}", prefix, expr_str);
 359                         if result.len() <= width && !result.contains('\n') {
 360                             return Some(result);
 361                         }
 362                     }
 363                 }
 364
 365                 prefix
 366             }
 367             ast::BlockCheckMode::PopUnsafeBlock(..) |
 368             ast::BlockCheckMode::DefaultBlock => {
 369                 visitor.last_pos = self.span.lo;
 370
 371                 String::new()
 372             }
 373         };
 374
 375         visitor.visit_block(self);
 376
 377         // Push text between last block item and end of block
 378         let snippet = visitor.snippet(mk_sp(visitor.last_pos, self.span.hi));
 379         visitor.buffer.push_str(&snippet);
 380
 381         Some(format!("{}{}", prefix, visitor.buffer))
 382     }
 383 }
 384
 385 // FIXME(#18): implement pattern formatting
 386 impl Rewrite for ast::Pat {
 387     fn rewrite(&self, context: &RewriteContext, _: usize, _: Indent) -> Option<String> {
 388         Some(context.snippet(self.span))
 389     }
 390 }
 391
 392 // Abstraction over for, while and loop expressions
 393 struct Loop<'a> {
 394     cond: Option<&'a ast::Expr>,
 395     block: &'a ast::Block,
 396     label: Option<ast::Ident>,
 397     pat: Option<&'a ast::Pat>,
 398     keyword: &'a str,
 399     matcher: &'a str,
 400     connector: &'a str,
 401 }
 402
 403 impl<'a> Loop<'a> {
 404     fn new_loop(block: &'a ast::Block, label: Option<ast::Ident>) -> Loop<'a> {
 405         Loop {
 406             cond: None,
 407             block: block,
 408             label: label,
 409             pat: None,
 410             keyword: "loop",
 411             matcher: "",
 412             connector: "",
 413         }
 414     }
 415
 416     fn new_while(pat: Option<&'a ast::Pat>,
 417                  cond: &'a ast::Expr,
 418                  block: &'a ast::Block,
 419                  label: Option<ast::Ident>)
 420                  -> Loop<'a> {
 421         Loop {
 422             cond: Some(cond),
 423             block: block,
 424             label: label,
 425             pat: pat,
 426             keyword: "while ",
 427             matcher: match pat {
 428                 Some(..) => "let ",
 429                 None => "",
 430             },
 431             connector: " =",
 432         }
 433     }
 434
 435     fn new_for(pat: &'a ast::Pat,
 436                cond: &'a ast::Expr,
 437                block: &'a ast::Block,
 438                label: Option<ast::Ident>)
 439                -> Loop<'a> {
 440         Loop {
 441             cond: Some(cond),
 442             block: block,
 443             label: label,
 444             pat: Some(pat),
 445             keyword: "for ",
 446             matcher: "",
 447             connector: " in",
 448         }
 449     }
 450 }
 451
 452 impl<'a> Rewrite for Loop<'a> {
 453     fn rewrite(&self, context: &RewriteContext, width: usize, offset: Indent) -> Option<String> {
 454         let label_string = rewrite_label(self.label);
 455         // 2 = " {".len()
 456         let inner_width = try_opt!(width.checked_sub(self.keyword.len() + 2 + label_string.len()));
 457         let inner_offset = offset + self.keyword.len() + label_string.len();
 458
 459         let pat_expr_string = match self.cond {
 460             Some(cond) => try_opt!(rewrite_pat_expr(context,
 461                                                     self.pat,
 462                                                     cond,
 463                                                     self.matcher,
 464                                                     self.connector,
 465                                                     inner_width,
 466                                                     inner_offset)),
 467             None => String::new(),
 468         };
 469
 470         // FIXME: this drops any comment between "loop" and the block.
 471         self.block
 472             .rewrite(context, width, offset)
 473             .map(|result| format!("{}{}{} {}", label_string, self.keyword, pat_expr_string, result))
 474     }
 475 }
 476
 477 fn rewrite_label(label: Option<ast::Ident>) -> String {
 478     match label {
 479         Some(ident) => format!("{}: ", ident),
 480         None => "".to_owned(),
 481     }
 482 }
 483
 484 // FIXME: this doesn't play well with line breaks
 485 fn rewrite_range(context: &RewriteContext,
 486                  left: Option<&ast::Expr>,
 487                  right: Option<&ast::Expr>,
 488                  width: usize,
 489                  offset: Indent)
 490                  -> Option<String> {
 491     let left_string = match left {
 492         Some(expr) => {
 493             // 2 = ..
 494             let max_width = try_opt!(width.checked_sub(2));
 495             try_opt!(expr.rewrite(context, max_width, offset))
 496         }
 497         None => String::new(),
 498     };
 499
 500     let right_string = match right {
 501         Some(expr) => {
 502             let max_width = try_opt!(width.checked_sub(left_string.len() + 2));
 503             try_opt!(expr.rewrite(context, max_width, offset + 2 + left_string.len()))
 504         }
 505         None => String::new(),
 506     };
 507
 508     Some(format!("{}..{}", left_string, right_string))
 509 }
 510
 511 // Rewrites if-else blocks. If let Some(_) = pat, the expression is
 512 // treated as an if-let-else expression.
 513 fn rewrite_if_else(context: &RewriteContext,
 514                    cond: &ast::Expr,
 515                    if_block: &ast::Block,
 516                    else_block_opt: Option<&ast::Expr>,
 517                    pat: Option<&ast::Pat>,
 518                    width: usize,
 519                    offset: Indent,
 520                    allow_single_line: bool)
 521                    -> Option<String> {
 522     // 3 = "if ", 2 = " {"
 523     let pat_expr_string = try_opt!(rewrite_pat_expr(context,
 524                                                     pat,
 525                                                     cond,
 526                                                     "let ",
 527                                                     " =",
 528                                                     try_opt!(width.checked_sub(3 + 2)),
 529                                                     offset + 3));
 530
 531     // Try to format if-else on single line.
 532     if allow_single_line && context.config.single_line_if_else {
 533         let trial = single_line_if_else(context, &pat_expr_string, if_block, else_block_opt, width);
 534
 535         if trial.is_some() {
 536             return trial;
 537         }
 538     }
 539
 540     let if_block_string = try_opt!(if_block.rewrite(context, width, offset));
 541     let mut result = format!("if {} {}", pat_expr_string, if_block_string);
 542
 543     if let Some(else_block) = else_block_opt {
 544         let rewrite = match else_block.node {
 545             // If the else expression is another if-else expression, prevent it
 546             // from being formatted on a single line.
 547             ast::Expr_::ExprIfLet(ref pat, ref cond, ref if_block, ref else_block) => {
 548                 rewrite_if_else(context,
 549                                 cond,
 550                                 if_block,
 551                                 else_block.as_ref().map(|e| &**e),
 552                                 Some(pat),
 553                                 width,
 554                                 offset,
 555                                 false)
 556             }
 557             ast::Expr_::ExprIf(ref cond, ref if_block, ref else_block) => {
 558                 rewrite_if_else(context,
 559                                 cond,
 560                                 if_block,
 561                                 else_block.as_ref().map(|e| &**e),
 562                                 None,
 563                                 width,
 564                                 offset,
 565                                 false)
 566             }
 567             _ => else_block.rewrite(context, width, offset),
 568         };
 569
 570         result.push_str(" else ");
 571         result.push_str(&&try_opt!(rewrite));
 572     }
 573
 574     Some(result)
 575 }
 576
 577 fn single_line_if_else(context: &RewriteContext,
 578                        pat_expr_str: &str,
 579                        if_node: &ast::Block,
 580                        else_block_opt: Option<&ast::Expr>,
 581                        width: usize)
 582                        -> Option<String> {
 583     let else_block = try_opt!(else_block_opt);
 584     let fixed_cost = "if  {  } else {  }".len();
 585
 586     if let ast::ExprBlock(ref else_node) = else_block.node {
 587         if !is_simple_block(if_node, context.codemap) ||
 588            !is_simple_block(else_node, context.codemap) || pat_expr_str.contains('\n') {
 589             return None;
 590         }
 591
 592         let new_width = try_opt!(width.checked_sub(pat_expr_str.len() + fixed_cost));
 593         let if_expr = if_node.expr.as_ref().unwrap();
 594         let if_str = try_opt!(if_expr.rewrite(context, new_width, Indent::empty()));
 595
 596         let new_width = try_opt!(new_width.checked_sub(if_str.len()));
 597         let else_expr = else_node.expr.as_ref().unwrap();
 598         let else_str = try_opt!(else_expr.rewrite(context, new_width, Indent::empty()));
 599
 600         // FIXME: this check shouldn't be necessary. Rewrites should either fail
 601         // or wrap to a newline when the object does not fit the width.
 602         let fits_line = fixed_cost + pat_expr_str.len() + if_str.len() + else_str.len() <= width;
 603
 604         if fits_line && !if_str.contains('\n') && !else_str.contains('\n') {
 605             return Some(format!("if {} {{ {} }} else {{ {} }}", pat_expr_str, if_str, else_str));
 606         }
 607     }
 608
 609     None
 610 }
 611
 612 // Checks that a block contains no statements, an expression and no comments.
 613 fn is_simple_block(block: &ast::Block, codemap: &CodeMap) -> bool {
 614     if !block.stmts.is_empty() || block.expr.is_none() {
 615         return false;
 616     }
 617
 618     let snippet = codemap.span_to_snippet(block.span).unwrap();
 619
 620     !contains_comment(&snippet)
 621 }
 622
 623 fn rewrite_match(context: &RewriteContext,
 624                  cond: &ast::Expr,
 625                  arms: &[ast::Arm],
 626                  width: usize,
 627                  offset: Indent)
 628                  -> Option<String> {
 629     if arms.is_empty() {
 630         return None;
 631     }
 632
 633     // `match `cond` {`
 634     let cond_budget = try_opt!(width.checked_sub(8));
 635     let cond_str = try_opt!(cond.rewrite(context, cond_budget, offset + 6));
 636     let mut result = format!("match {} {{", cond_str);
 637
 638     let nested_context = context.nested_context();
 639     let arm_indent = nested_context.block_indent + context.overflow_indent;
 640     let arm_indent_str = arm_indent.to_string(context.config);
 641
 642     let open_brace_pos = span_after(mk_sp(cond.span.hi, arm_start_pos(&arms[0])),
 643                                     "{",
 644                                     context.codemap);
 645
 646     for (i, arm) in arms.iter().enumerate() {
 647         // Make sure we get the stuff between arms.
 648         let missed_str = if i == 0 {
 649             context.snippet(mk_sp(open_brace_pos + BytePos(1), arm_start_pos(arm)))
 650         } else {
 651             context.snippet(mk_sp(arm_end_pos(&arms[i-1]), arm_start_pos(arm)))
 652         };
 653         let missed_str = match missed_str.find_uncommented(",") {
 654             Some(n) => &missed_str[n+1..],
 655             None => &missed_str[..],
 656         };
 657         // first = first non-whitespace byte index.
 658         let first = missed_str.find(|c: char| !c.is_whitespace()).unwrap_or(missed_str.len());
 659         if missed_str[..first].chars().filter(|c| c == &'\n').count() >= 2 {
 660             // There were multiple line breaks which got trimmed to nothing
 661             // that means there should be some vertical white space. Lets
 662             // replace that with just one blank line.
 663             result.push('\n');
 664         }
 665         let missed_str = missed_str.trim();
 666         if !missed_str.is_empty() {
 667             result.push('\n');
 668             result.push_str(&arm_indent_str);
 669             result.push_str(missed_str);
 670         }
 671         result.push('\n');
 672         result.push_str(&arm_indent_str);
 673
 674         let arm_str = arm.rewrite(&nested_context,
 675                                   context.config.max_width - arm_indent.width(),
 676                                   arm_indent);
 677         if let Some(ref arm_str) = arm_str {
 678             result.push_str(arm_str);
 679         } else {
 680             // We couldn't format the arm, just reproduce the source.
 681             let snippet = context.snippet(mk_sp(arm_start_pos(arm), arm_end_pos(arm)));
 682             result.push_str(&snippet);
 683         }
 684     }
 685
 686     // We'll miss any comments etc. between the last arm and the end of the
 687     // match expression, but meh.
 688
 689     result.push('\n');
 690     result.push_str(&(context.block_indent + context.overflow_indent).to_string(context.config));
 691     result.push('}');
 692     Some(result)
 693 }
 694
 695 fn arm_start_pos(arm: &ast::Arm) -> BytePos {
 696     let &ast::Arm { ref attrs, ref pats, .. } = arm;
 697     if !attrs.is_empty() {
 698         return attrs[0].span.lo
 699     }
 700
 701     pats[0].span.lo
 702 }
 703
 704 fn arm_end_pos(arm: &ast::Arm) -> BytePos {
 705     arm.body.span.hi
 706 }
 707
 708 // Match arms.
 709 impl Rewrite for ast::Arm {
 710     fn rewrite(&self, context: &RewriteContext, width: usize, offset: Indent) -> Option<String> {
 711         let &ast::Arm { ref attrs, ref pats, ref guard, ref body } = self;
 712         let indent_str = offset.to_string(context.config);
 713
 714         // FIXME this is all a bit grotty, would be nice to abstract out the
 715         // treatment of attributes.
 716         let attr_str = if !attrs.is_empty() {
 717             // We only use this visitor for the attributes, should we use it for
 718             // more?
 719             let mut attr_visitor = FmtVisitor::from_codemap(context.codemap, context.config);
 720             attr_visitor.block_indent = context.block_indent;
 721             attr_visitor.last_pos = attrs[0].span.lo;
 722             if attr_visitor.visit_attrs(attrs) {
 723                 // Attributes included a skip instruction.
 724                 let snippet = context.snippet(mk_sp(attrs[0].span.lo, body.span.hi));
 725                 return Some(snippet);
 726             }
 727             attr_visitor.format_missing(pats[0].span.lo);
 728             attr_visitor.buffer.to_string()
 729         } else {
 730             String::new()
 731         };
 732
 733         // Patterns
 734         // 5 = ` => {`
 735         let pat_budget = try_opt!(width.checked_sub(5));
 736         let pat_strs = try_opt!(pats.iter()
 737                                     .map(|p| {
 738                                         p.rewrite(context,
 739                                                   pat_budget,
 740                                                   offset.block_indent(context.config))
 741                                     })
 742                                     .collect::<Option<Vec<_>>>());
 743
 744         let mut total_width = pat_strs.iter().fold(0, |a, p| a + p.len());
 745         // Add ` | `.len().
 746         total_width += (pat_strs.len() - 1) * 3;
 747
 748         let mut vertical = total_width > pat_budget || pat_strs.iter().any(|p| p.contains('\n'));
 749         if !vertical && context.config.take_source_hints {
 750             // If the patterns were previously stacked, keep them stacked.
 751             let pat_span = mk_sp(pats[0].span.lo, pats[pats.len() - 1].span.hi);
 752             let pat_str = context.snippet(pat_span);
 753             vertical = pat_str.find('\n').is_some();
 754         }
 755
 756         let pats_width = if vertical {
 757             pat_strs[pat_strs.len() - 1].len()
 758         } else {
 759             total_width
 760         };
 761
 762         let mut pats_str = String::new();
 763         for p in pat_strs {
 764             if !pats_str.is_empty() {
 765                 if vertical {
 766                     pats_str.push_str(" |\n");
 767                     pats_str.push_str(&indent_str);
 768                 } else {
 769                     pats_str.push_str(" | ");
 770                 }
 771             }
 772             pats_str.push_str(&p);
 773         }
 774
 775         let guard_str = try_opt!(rewrite_guard(context, guard, width, offset, pats_width));
 776
 777         let pats_str = format!("{}{}", pats_str, guard_str);
 778         // Where the next text can start.
 779         let mut line_start = last_line_width(&pats_str);
 780         if pats_str.find('\n').is_none() {
 781             line_start += offset.width();
 782         }
 783
 784         let mut line_indent = offset + pats_width;
 785         if vertical {
 786             line_indent = line_indent.block_indent(context.config);
 787         }
 788
 789         let comma = if let ast::ExprBlock(_) = body.node {
 790             ""
 791         } else {
 792             ","
 793         };
 794
 795         // Let's try and get the arm body on the same line as the condition.
 796         // 4 = ` => `.len()
 797         if context.config.max_width > line_start + comma.len() + 4 {
 798             let budget = context.config.max_width - line_start - comma.len() - 4;
 799             if let Some(ref body_str) = body.rewrite(context, budget, line_indent + 4) {
 800                 if first_line_width(body_str) <= budget {
 801                     return Some(format!("{}{} => {}{}",
 802                                         attr_str.trim_left(),
 803                                         pats_str,
 804                                         body_str,
 805                                         comma));
 806                 }
 807             }
 808         }
 809
 810         // We have to push the body to the next line.
 811         if comma.is_empty() {
 812             // We're trying to fit a block in, but it still failed, give up.
 813             return None;
 814         }
 815
 816         let body_budget = try_opt!(width.checked_sub(context.config.tab_spaces));
 817         let body_str = try_opt!(body.rewrite(context, body_budget, context.block_indent));
 818         Some(format!("{}{} =>\n{}{},",
 819                      attr_str.trim_left(),
 820                      pats_str,
 821                      offset.block_indent(context.config).to_string(context.config),
 822                      body_str))
 823     }
 824 }
 825
 826 // The `if ...` guard on a match arm.
 827 fn rewrite_guard(context: &RewriteContext,
 828                  guard: &Option<ptr::P<ast::Expr>>,
 829                  width: usize,
 830                  offset: Indent,
 831                  // The amount of space used up on this line for the pattern in
 832                  // the arm (excludes offset).
 833                  pattern_width: usize)
 834                  -> Option<String> {
 835     if let &Some(ref guard) = guard {
 836         // First try to fit the guard string on the same line as the pattern.
 837         // 4 = ` if `, 5 = ` => {`
 838         let overhead = pattern_width + 4 + 5;
 839         if overhead < width {
 840             let cond_str = guard.rewrite(context, width - overhead, offset + pattern_width + 4);
 841             if let Some(cond_str) = cond_str {
 842                 return Some(format!(" if {}", cond_str));
 843             }
 844         }
 845
 846         // Not enough space to put the guard after the pattern, try a newline.
 847         let overhead = context.config.tab_spaces + 4 + 5;
 848         if overhead < width {
 849             let cond_str = guard.rewrite(context,
 850                                          width - overhead,
 851                                          offset.block_indent(context.config));
 852             if let Some(cond_str) = cond_str {
 853                 return Some(format!("\n{}if {}",
 854                                     offset.block_indent(context.config).to_string(context.config),
 855                                     cond_str));
 856             }
 857         }
 858
 859         None
 860     } else {
 861         Some(String::new())
 862     }
 863 }
 864
 865 fn rewrite_pat_expr(context: &RewriteContext,
 866                     pat: Option<&ast::Pat>,
 867                     expr: &ast::Expr,
 868                     matcher: &str,
 869                     // Connecting piece between pattern and expression,
 870                     // *without* trailing space.
 871                     connector: &str,
 872                     width: usize,
 873                     offset: Indent)
 874                     -> Option<String> {
 875     let pat_offset = offset + matcher.len();
 876     let mut result = match pat {
 877         Some(pat) => {
 878             let pat_budget = try_opt!(width.checked_sub(connector.len() + matcher.len()));
 879             let pat_string = try_opt!(pat.rewrite(context, pat_budget, pat_offset));
 880             format!("{}{}{}", matcher, pat_string, connector)
 881         }
 882         None => String::new(),
 883     };
 884
 885     // Consider only the last line of the pat string.
 886     let extra_offset = extra_offset(&result, offset);
 887
 888     // The expression may (partionally) fit on the current line.
 889     if width > extra_offset + 1 {
 890         let spacer = if pat.is_some() {
 891             " "
 892         } else {
 893             ""
 894         };
 895
 896         let expr_rewrite = expr.rewrite(context,
 897                                         width - extra_offset - spacer.len(),
 898                                         offset + extra_offset + spacer.len());
 899
 900         if let Some(expr_string) = expr_rewrite {
 901             result.push_str(spacer);
 902             result.push_str(&expr_string);
 903             return Some(result);
 904         }
 905     }
 906
 907     // The expression won't fit on the current line, jump to next.
 908     result.push('\n');
 909     result.push_str(&pat_offset.to_string(context.config));
 910
 911     let expr_rewrite = expr.rewrite(context,
 912                                     context.config.max_width - pat_offset.width(),
 913                                     pat_offset);
 914     result.push_str(&&try_opt!(expr_rewrite));
 915
 916     Some(result)
 917 }
 918
 919 fn rewrite_string_lit(context: &RewriteContext,
 920                       span: Span,
 921                       width: usize,
 922                       offset: Indent)
 923                       -> Option<String> {
 924     if !context.config.format_strings {
 925         return Some(context.snippet(span));
 926     }
 927
 928     let fmt = StringFormat {
 929         opener: "\"",
 930         closer: "\"",
 931         line_start: " ",
 932         line_end: "\\",
 933         width: width,
 934         offset: offset,
 935         trim_end: false,
 936         config: context.config,
 937     };
 938
 939     let string_lit = context.snippet(span);
 940     let str_lit = &string_lit[1..string_lit.len() - 1]; // Remove the quote characters.
 941
 942     Some(rewrite_string(str_lit, &fmt))
 943 }
 944
 945 pub fn rewrite_call<R>(context: &RewriteContext,
 946                        callee: &R,
 947                        args: &[ptr::P<ast::Expr>],
 948                        span: Span,
 949                        width: usize,
 950                        offset: Indent)
 951                        -> Option<String>
 952     where R: Rewrite
 953 {
 954     let closure = |callee_max_width| {
 955         rewrite_call_inner(context, callee, callee_max_width, args, span, width, offset)
 956     };
 957
 958     // 2 is for parens
 959     let max_width = try_opt!(width.checked_sub(2));
 960     binary_search(1, max_width, closure)
 961 }
 962
 963 fn rewrite_call_inner<R>(context: &RewriteContext,
 964                          callee: &R,
 965                          max_callee_width: usize,
 966                          args: &[ptr::P<ast::Expr>],
 967                          span: Span,
 968                          width: usize,
 969                          offset: Indent)
 970                          -> Result<String, Ordering>
 971     where R: Rewrite
 972 {
 973     let callee = callee.borrow();
 974     // FIXME using byte lens instead of char lens (and probably all over the
 975     // place too)
 976     let callee_str = match callee.rewrite(context, max_callee_width, offset) {
 977         Some(string) => {
 978             if !string.contains('\n') && string.len() > max_callee_width {
 979                 panic!("{:?} {}", string, max_callee_width);
 980             } else {
 981                 string
 982             }
 983         }
 984         None => return Err(Ordering::Greater),
 985     };
 986
 987     let span_lo = span_after(span, "(", context.codemap);
 988     let span = mk_sp(span_lo, span.hi);
 989
 990     let extra_offset = extra_offset(&callee_str, offset);
 991     // 2 is for parens.
 992     let remaining_width = match width.checked_sub(extra_offset + 2) {
 993         Some(str) => str,
 994         None => return Err(Ordering::Greater),
 995     };
 996     let offset = offset + extra_offset + 1;
 997     let block_indent = if args.len() == 1 {
 998         context.block_indent
 999     } else {
1000         offset
1001     };
1002     let inner_context = &RewriteContext { block_indent: block_indent, ..*context };
1003
1004     let items = itemize_list(context.codemap,
1005                              args.iter(),
1006                              ")",
1007                              |item| item.span.lo,
1008                              |item| item.span.hi,
1009                              // Take old span when rewrite fails.
1010                              |item| {
1011                                  item.rewrite(&inner_context, remaining_width, offset)
1012                                      .unwrap_or(context.snippet(item.span))
1013                              },
1014                              span.lo,
1015                              span.hi);
1016
1017     let fmt = ListFormatting::for_fn(remaining_width, offset, context.config);
1018     let list_str = match write_list(&items.collect::<Vec<_>>(), &fmt) {
1019         Some(str) => str,
1020         None => return Err(Ordering::Less),
1021     };
1022
1023     Ok(format!("{}({})", callee_str, list_str))
1024 }
1025
1026 fn rewrite_paren(context: &RewriteContext,
1027                  subexpr: &ast::Expr,
1028                  width: usize,
1029                  offset: Indent)
1030                  -> Option<String> {
1031     debug!("rewrite_paren, width: {}, offset: {:?}", width, offset);
1032     // 1 is for opening paren, 2 is for opening+closing, we want to keep the closing
1033     // paren on the same line as the subexpr.
1034     let subexpr_str = subexpr.rewrite(context, try_opt!(width.checked_sub(2)), offset + 1);
1035     debug!("rewrite_paren, subexpr_str: `{:?}`", subexpr_str);
1036     subexpr_str.map(|s| format!("({})", s))
1037 }
1038
1039 fn rewrite_struct_lit<'a>(context: &RewriteContext,
1040                           path: &ast::Path,
1041                           fields: &'a [ast::Field],
1042                           base: Option<&'a ast::Expr>,
1043                           span: Span,
1044                           width: usize,
1045                           offset: Indent)
1046                           -> Option<String> {
1047     debug!("rewrite_struct_lit: width {}, offset {:?}", width, offset);
1048     assert!(!fields.is_empty() || base.is_some());
1049
1050     enum StructLitField<'a> {
1051         Regular(&'a ast::Field),
1052         Base(&'a ast::Expr),
1053     }
1054
1055     // 2 = " {".len()
1056     let path_budget = try_opt!(width.checked_sub(2));
1057     let path_str = try_opt!(path.rewrite(context, path_budget, offset));
1058
1059     // Foo { a: Foo } - indent is +3, width is -5.
1060     let h_budget = try_opt!(width.checked_sub(path_str.len() + 5));
1061     let (indent, v_budget) = match context.config.struct_lit_style {
1062         StructLitStyle::Visual => {
1063             (offset + path_str.len() + 3, h_budget)
1064         }
1065         StructLitStyle::Block => {
1066             // If we are all on one line, then we'll ignore the indent, and we
1067             // have a smaller budget.
1068             let indent = context.block_indent.block_indent(context.config);
1069             let v_budget = context.config.max_width.checked_sub(indent.width()).unwrap_or(0);
1070             (indent, v_budget)
1071         }
1072     };
1073
1074     let field_iter = fields.into_iter()
1075                            .map(StructLitField::Regular)
1076                            .chain(base.into_iter().map(StructLitField::Base));
1077
1078     let inner_context = &RewriteContext { block_indent: indent, ..*context };
1079
1080     let items = itemize_list(context.codemap,
1081                              field_iter,
1082                              "}",
1083                              |item| {
1084                                  match *item {
1085                                      StructLitField::Regular(ref field) => field.span.lo,
1086                                      StructLitField::Base(ref expr) => {
1087                                          let last_field_hi = fields.last()
1088                                                                    .map_or(span.lo,
1089                                                                            |field| field.span.hi);
1090                                          let snippet = context.snippet(mk_sp(last_field_hi,
1091                                                                              expr.span.lo));
1092                                          let pos = snippet.find_uncommented("..").unwrap();
1093                                          last_field_hi + BytePos(pos as u32)
1094                                      }
1095                                  }
1096                              },
1097                              |item| {
1098                                  match *item {
1099                                      StructLitField::Regular(ref field) => field.span.hi,
1100                                      StructLitField::Base(ref expr) => expr.span.hi,
1101                                  }
1102                              },
1103                              |item| {
1104                                  match *item {
1105                                      StructLitField::Regular(ref field) => {
1106                                          rewrite_field(inner_context, &field, h_budget, indent)
1107                                              .unwrap_or(context.snippet(field.span))
1108                                      }
1109                                      StructLitField::Base(ref expr) => {
1110                                          // 2 = ..
1111                                          expr.rewrite(inner_context, h_budget - 2, indent + 2)
1112                                              .map(|s| format!("..{}", s))
1113                                              .unwrap_or(context.snippet(expr.span))
1114                                      }
1115                                  }
1116                              },
1117                              span_after(span, "{", context.codemap),
1118                              span.hi);
1119
1120     let fmt = ListFormatting {
1121         tactic: match (context.config.struct_lit_style, fields.len()) {
1122             (StructLitStyle::Visual, 1) => ListTactic::HorizontalVertical,
1123             _ => context.config.struct_lit_multiline_style.to_list_tactic(),
1124         },
1125         separator: ",",
1126         trailing_separator: if base.is_some() {
1127             SeparatorTactic::Never
1128         } else {
1129             context.config.struct_lit_trailing_comma
1130         },
1131         indent: indent,
1132         h_width: h_budget,
1133         v_width: v_budget,
1134         ends_with_newline: false,
1135         config: context.config,
1136     };
1137     let fields_str = try_opt!(write_list(&items.collect::<Vec<_>>(), &fmt));
1138
1139     let format_on_newline = || {
1140         let inner_indent = context.block_indent
1141                                   .block_indent(context.config)
1142                                   .to_string(context.config);
1143         let outer_indent = context.block_indent.to_string(context.config);
1144         Some(format!("{} {{\n{}{}\n{}}}", path_str, inner_indent, fields_str, outer_indent))
1145     };
1146
1147     match (context.config.struct_lit_style, context.config.struct_lit_multiline_style) {
1148         (StructLitStyle::Block, _) if fields_str.contains('\n') => format_on_newline(),
1149         (StructLitStyle::Block, MultilineStyle::ForceMulti) => format_on_newline(),
1150         _ => Some(format!("{} {{ {} }}", path_str, fields_str)),
1151     }
1152
1153     // FIXME if context.config.struct_lit_style == Visual, but we run out
1154     // of space, we should fall back to BlockIndent.
1155 }
1156
1157 fn rewrite_field(context: &RewriteContext,
1158                  field: &ast::Field,
1159                  width: usize,
1160                  offset: Indent)
1161                  -> Option<String> {
1162     let name = &field.ident.node.to_string();
1163     let overhead = name.len() + 2;
1164     let expr = field.expr
1165                     .rewrite(context, try_opt!(width.checked_sub(overhead)), offset + overhead);
1166     expr.map(|s| format!("{}: {}", name, s))
1167 }
1168
1169 fn rewrite_tuple_lit(context: &RewriteContext,
1170                      items: &[ptr::P<ast::Expr>],
1171                      span: Span,
1172                      width: usize,
1173                      offset: Indent)
1174                      -> Option<String> {
1175     debug!("rewrite_tuple_lit: width: {}, offset: {:?}", width, offset);
1176     let indent = offset + 1;
1177     // In case of length 1, need a trailing comma
1178     if items.len() == 1 {
1179         // 3 = "(" + ",)"
1180         let budget = try_opt!(width.checked_sub(3));
1181         return items[0].rewrite(context, budget, indent).map(|s| format!("({},)", s));
1182     }
1183
1184     let items = itemize_list(context.codemap,
1185                              items.iter(),
1186                              ")",
1187                              |item| item.span.lo,
1188                              |item| item.span.hi,
1189                              |item| {
1190                                  let inner_width = context.config.max_width - indent.width() - 1;
1191                                  item.rewrite(context, inner_width, indent)
1192                                      .unwrap_or(context.snippet(item.span))
1193                              },
1194                              span.lo + BytePos(1), // Remove parens
1195                              span.hi - BytePos(1));
1196
1197     let budget = try_opt!(width.checked_sub(2));
1198     let fmt = ListFormatting::for_fn(budget, indent, context.config);
1199     let list_str = try_opt!(write_list(&items.collect::<Vec<_>>(), &fmt));
1200
1201     Some(format!("({})", list_str))
1202 }
1203
1204 fn rewrite_binary_op(context: &RewriteContext,
1205                      op: &ast::BinOp,
1206                      lhs: &ast::Expr,
1207                      rhs: &ast::Expr,
1208                      width: usize,
1209                      offset: Indent)
1210                      -> Option<String> {
1211     // FIXME: format comments between operands and operator
1212
1213     let operator_str = context.snippet(op.span);
1214
1215     // Get "full width" rhs and see if it fits on the current line. This
1216     // usually works fairly well since it tends to place operands of
1217     // operations with high precendence close together.
1218     let rhs_result = try_opt!(rhs.rewrite(context, width, offset));
1219
1220     // Second condition is needed in case of line break not caused by a
1221     // shortage of space, but by end-of-line comments, for example.
1222     // Note that this is non-conservative, but its just to see if it's even
1223     // worth trying to put everything on one line.
1224     if rhs_result.len() + 2 + operator_str.len() < width && !rhs_result.contains('\n') {
1225         // 1 = space between lhs expr and operator
1226         if let Some(mut result) = lhs.rewrite(context, width - 1 - operator_str.len(), offset) {
1227             result.push(' ');
1228             result.push_str(&operator_str);
1229             result.push(' ');
1230
1231             let remaining_width = width.checked_sub(last_line_width(&result)).unwrap_or(0);
1232
1233             if rhs_result.len() <= remaining_width {
1234                 result.push_str(&rhs_result);
1235                 return Some(result);
1236             }
1237
1238             if let Some(rhs_result) = rhs.rewrite(context, remaining_width, offset + result.len()) {
1239                 if rhs_result.len() <= remaining_width {
1240                     result.push_str(&rhs_result);
1241                     return Some(result);
1242                 }
1243             }
1244         }
1245     }
1246
1247     // We have to use multiple lines.
1248
1249     // Re-evaluate the lhs because we have more space now:
1250     let budget = try_opt!(context.config
1251                                  .max_width
1252                                  .checked_sub(offset.width() + 1 + operator_str.len()));
1253     Some(format!("{} {}\n{}{}",
1254                  try_opt!(lhs.rewrite(context, budget, offset)),
1255                  operator_str,
1256                  offset.to_string(context.config),
1257                  rhs_result))
1258 }
1259
1260 fn rewrite_unary_op(context: &RewriteContext,
1261                     op: &ast::UnOp,
1262                     expr: &ast::Expr,
1263                     width: usize,
1264                     offset: Indent)
1265                     -> Option<String> {
1266     // For some reason, an UnOp is not spanned like BinOp!
1267     let operator_str = match *op {
1268         ast::UnOp::UnUniq => "box ",
1269         ast::UnOp::UnDeref => "*",
1270         ast::UnOp::UnNot => "!",
1271         ast::UnOp::UnNeg => "-",
1272     };
1273     let operator_len = operator_str.len();
1274
1275     expr.rewrite(context, try_opt!(width.checked_sub(operator_len)), offset + operator_len)
1276         .map(|r| format!("{}{}", operator_str, r))
1277 }
1278
1279 fn rewrite_assignment(context: &RewriteContext,
1280                       lhs: &ast::Expr,
1281                       rhs: &ast::Expr,
1282                       op: Option<&ast::BinOp>,
1283                       width: usize,
1284                       offset: Indent)
1285                       -> Option<String> {
1286     let operator_str = match op {
1287         Some(op) => context.snippet(op.span),
1288         None => "=".to_owned(),
1289     };
1290
1291     // 1 = space between lhs and operator.
1292     let max_width = try_opt!(width.checked_sub(operator_str.len() + 1));
1293     let lhs_str = format!("{} {}", try_opt!(lhs.rewrite(context, max_width, offset)), operator_str);
1294
1295     rewrite_assign_rhs(&context, lhs_str, rhs, width, offset)
1296 }
1297
1298 // The left hand side must contain everything up to, and including, the
1299 // assignment operator.
1300 pub fn rewrite_assign_rhs<S: Into<String>>(context: &RewriteContext,
1301                                            lhs: S,
1302                                            ex: &ast::Expr,
1303                                            width: usize,
1304                                            offset: Indent)
1305                                            -> Option<String> {
1306     let mut result = lhs.into();
1307
1308     // 1 = space between operator and rhs.
1309     let max_width = try_opt!(width.checked_sub(result.len() + 1));
1310     let rhs = ex.rewrite(&context, max_width, offset + result.len() + 1);
1311
1312     match rhs {
1313         Some(new_str) => {
1314             result.push(' ');
1315             result.push_str(&new_str)
1316         }
1317         None => {
1318             // Expression did not fit on the same line as the identifier. Retry
1319             // on the next line.
1320             let new_offset = offset.block_indent(context.config);
1321             result.push_str(&format!("\n{}", new_offset.to_string(context.config)));
1322
1323             // FIXME: we probably should related max_width to width instead of config.max_width
1324             // where is the 1 coming from anyway?
1325             let max_width = try_opt!(context.config.max_width.checked_sub(new_offset.width() + 1));
1326             let rhs_indent = Indent::new(context.config.tab_spaces, 0);
1327             let overflow_context = context.overflow_context(rhs_indent);
1328             let rhs = ex.rewrite(&overflow_context, max_width, new_offset);
1329
1330             result.push_str(&&try_opt!(rhs));
1331         }
1332     }
1333
1334     Some(result)
1335 }