src/librustc/lint.rs

   1 use std::cmp;
   2
   3 use crate::ich::StableHashingContext;
   4 use crate::lint::context::CheckLintNameResult;
   5 use rustc_data_structures::fx::FxHashMap;
   6 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
   7 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, DiagnosticId};
   8 use rustc_hir::HirId;
   9 pub use rustc_session::lint::{builtin, Level, Lint, LintId, LintPass};
  10 use rustc_session::{DiagnosticMessageId, Session};
  11 use rustc_span::hygiene::MacroKind;
  12 use rustc_span::source_map::{DesugaringKind, ExpnKind, MultiSpan};
  13 use rustc_span::symbol::{sym, Symbol};
  14 use rustc_span::Span;
  15 use syntax::ast;
  16 use syntax::attr;
  17 use syntax::print::pprust;
  18 use syntax::sess::feature_err;
  19
  20 use rustc_error_codes::*;
  21
  22 mod context;
  23 pub mod internal;
  24 mod passes;
  25
  26 pub use context::add_elided_lifetime_in_path_suggestion;
  27 pub use context::{EarlyContext, LateContext, LintContext, LintStore};
  28 pub use passes::{EarlyLintPass, EarlyLintPassObject, LateLintPass, LateLintPassObject};
  29
  30 /// How a lint level was set.
  31 #[derive(Clone, Copy, PartialEq, Eq, HashStable)]
  32 pub enum LintSource {
  33     /// Lint is at the default level as declared
  34     /// in rustc or a plugin.
  35     Default,
  36
  37     /// Lint level was set by an attribute.
  38     Node(Symbol, Span, Option<Symbol> /* RFC 2383 reason */),
  39
  40     /// Lint level was set by a command-line flag.
  41     CommandLine(Symbol),
  42 }
  43
  44 pub type LevelSource = (Level, LintSource);
  45
  46 pub struct LintLevelSets {
  47     list: Vec<LintSet>,
  48     lint_cap: Level,
  49 }
  50
  51 enum LintSet {
  52     CommandLine {
  53         // -A,-W,-D flags, a `Symbol` for the flag itself and `Level` for which
  54         // flag.
  55         specs: FxHashMap<LintId, LevelSource>,
  56     },
  57
  58     Node {
  59         specs: FxHashMap<LintId, LevelSource>,
  60         parent: u32,
  61     },
  62 }
  63
  64 impl LintLevelSets {
  65     pub fn new() -> Self {
  66         LintLevelSets { list: Vec::new(), lint_cap: Level::Forbid }
  67     }
  68
  69     pub fn get_lint_level(
  70         &self,
  71         lint: &'static Lint,
  72         idx: u32,
  73         aux: Option<&FxHashMap<LintId, LevelSource>>,
  74         sess: &Session,
  75     ) -> LevelSource {
  76         let (level, mut src) = self.get_lint_id_level(LintId::of(lint), idx, aux);
  77
  78         // If `level` is none then we actually assume the default level for this
  79         // lint.
  80         let mut level = level.unwrap_or_else(|| lint.default_level(sess.edition()));
  81
  82         // If we're about to issue a warning, check at the last minute for any
  83         // directives against the warnings "lint". If, for example, there's an
  84         // `allow(warnings)` in scope then we want to respect that instead.
  85         if level == Level::Warn {
  86             let (warnings_level, warnings_src) =
  87                 self.get_lint_id_level(LintId::of(builtin::WARNINGS), idx, aux);
  88             if let Some(configured_warning_level) = warnings_level {
  89                 if configured_warning_level != Level::Warn {
  90                     level = configured_warning_level;
  91                     src = warnings_src;
  92                 }
  93             }
  94         }
  95
  96         // Ensure that we never exceed the `--cap-lints` argument.
  97         level = cmp::min(level, self.lint_cap);
  98
  99         if let Some(driver_level) = sess.driver_lint_caps.get(&LintId::of(lint)) {
 100             // Ensure that we never exceed driver level.
 101             level = cmp::min(*driver_level, level);
 102         }
 103
 104         return (level, src);
 105     }
 106
 107     pub fn get_lint_id_level(
 108         &self,
 109         id: LintId,
 110         mut idx: u32,
 111         aux: Option<&FxHashMap<LintId, LevelSource>>,
 112     ) -> (Option<Level>, LintSource) {
 113         if let Some(specs) = aux {
 114             if let Some(&(level, src)) = specs.get(&id) {
 115                 return (Some(level), src);
 116             }
 117         }
 118         loop {
 119             match self.list[idx as usize] {
 120                 LintSet::CommandLine { ref specs } => {
 121                     if let Some(&(level, src)) = specs.get(&id) {
 122                         return (Some(level), src);
 123                     }
 124                     return (None, LintSource::Default);
 125                 }
 126                 LintSet::Node { ref specs, parent } => {
 127                     if let Some(&(level, src)) = specs.get(&id) {
 128                         return (Some(level), src);
 129                     }
 130                     idx = parent;
 131                 }
 132             }
 133         }
 134     }
 135 }
 136
 137 pub struct LintLevelsBuilder<'a> {
 138     sess: &'a Session,
 139     sets: LintLevelSets,
 140     id_to_set: FxHashMap<HirId, u32>,
 141     cur: u32,
 142     warn_about_weird_lints: bool,
 143 }
 144
 145 pub struct BuilderPush {
 146     prev: u32,
 147     pub changed: bool,
 148 }
 149
 150 impl<'a> LintLevelsBuilder<'a> {
 151     pub fn new(sess: &'a Session, warn_about_weird_lints: bool, store: &LintStore) -> Self {
 152         let mut builder = LintLevelsBuilder {
 153             sess,
 154             sets: LintLevelSets::new(),
 155             cur: 0,
 156             id_to_set: Default::default(),
 157             warn_about_weird_lints,
 158         };
 159         builder.process_command_line(sess, store);
 160         assert_eq!(builder.sets.list.len(), 1);
 161         builder
 162     }
 163
 164     fn process_command_line(&mut self, sess: &Session, store: &LintStore) {
 165         let mut specs = FxHashMap::default();
 166         self.sets.lint_cap = sess.opts.lint_cap.unwrap_or(Level::Forbid);
 167
 168         for &(ref lint_name, level) in &sess.opts.lint_opts {
 169             store.check_lint_name_cmdline(sess, &lint_name, level);
 170
 171             // If the cap is less than this specified level, e.g., if we've got
 172             // `--cap-lints allow` but we've also got `-D foo` then we ignore
 173             // this specification as the lint cap will set it to allow anyway.
 174             let level = cmp::min(level, self.sets.lint_cap);
 175
 176             let lint_flag_val = Symbol::intern(lint_name);
 177             let ids = match store.find_lints(&lint_name) {
 178                 Ok(ids) => ids,
 179                 Err(_) => continue, // errors handled in check_lint_name_cmdline above
 180             };
 181             for id in ids {
 182                 let src = LintSource::CommandLine(lint_flag_val);
 183                 specs.insert(id, (level, src));
 184             }
 185         }
 186
 187         self.sets.list.push(LintSet::CommandLine { specs });
 188     }
 189
 190     /// Pushes a list of AST lint attributes onto this context.
 191     ///
 192     /// This function will return a `BuilderPush` object which should be passed
 193     /// to `pop` when this scope for the attributes provided is exited.
 194     ///
 195     /// This function will perform a number of tasks:
 196     ///
 197     /// * It'll validate all lint-related attributes in `attrs`
 198     /// * It'll mark all lint-related attributes as used
 199     /// * Lint levels will be updated based on the attributes provided
 200     /// * Lint attributes are validated, e.g., a #[forbid] can't be switched to
 201     ///   #[allow]
 202     ///
 203     /// Don't forget to call `pop`!
 204     pub fn push(&mut self, attrs: &[ast::Attribute], store: &LintStore) -> BuilderPush {
 205         let mut specs = FxHashMap::default();
 206         let sess = self.sess;
 207         let bad_attr = |span| struct_span_err!(sess, span, E0452, "malformed lint attribute input");
 208         for attr in attrs {
 209             let level = match Level::from_symbol(attr.name_or_empty()) {
 210                 None => continue,
 211                 Some(lvl) => lvl,
 212             };
 213
 214             let meta = unwrap_or!(attr.meta(), continue);
 215             attr::mark_used(attr);
 216
 217             let mut metas = unwrap_or!(meta.meta_item_list(), continue);
 218
 219             if metas.is_empty() {
 220                 // FIXME (#55112): issue unused-attributes lint for `#[level()]`
 221                 continue;
 222             }
 223
 224             // Before processing the lint names, look for a reason (RFC 2383)
 225             // at the end.
 226             let mut reason = None;
 227             let tail_li = &metas[metas.len() - 1];
 228             if let Some(item) = tail_li.meta_item() {
 229                 match item.kind {
 230                     ast::MetaItemKind::Word => {} // actual lint names handled later
 231                     ast::MetaItemKind::NameValue(ref name_value) => {
 232                         if item.path == sym::reason {
 233                             // found reason, reslice meta list to exclude it
 234                             metas = &metas[0..metas.len() - 1];
 235                             // FIXME (#55112): issue unused-attributes lint if we thereby
 236                             // don't have any lint names (`#[level(reason = "foo")]`)
 237                             if let ast::LitKind::Str(rationale, _) = name_value.kind {
 238                                 if !self.sess.features_untracked().lint_reasons {
 239                                     feature_err(
 240                                         &self.sess.parse_sess,
 241                                         sym::lint_reasons,
 242                                         item.span,
 243                                         "lint reasons are experimental",
 244                                     )
 245                                     .emit();
 246                                 }
 247                                 reason = Some(rationale);
 248                             } else {
 249                                 bad_attr(name_value.span)
 250                                     .span_label(name_value.span, "reason must be a string literal")
 251                                     .emit();
 252                             }
 253                         } else {
 254                             bad_attr(item.span)
 255                                 .span_label(item.span, "bad attribute argument")
 256                                 .emit();
 257                         }
 258                     }
 259                     ast::MetaItemKind::List(_) => {
 260                         bad_attr(item.span).span_label(item.span, "bad attribute argument").emit();
 261                     }
 262                 }
 263             }
 264
 265             for li in metas {
 266                 let meta_item = match li.meta_item() {
 267                     Some(meta_item) if meta_item.is_word() => meta_item,
 268                     _ => {
 269                         let sp = li.span();
 270                         let mut err = bad_attr(sp);
 271                         let mut add_label = true;
 272                         if let Some(item) = li.meta_item() {
 273                             if let ast::MetaItemKind::NameValue(_) = item.kind {
 274                                 if item.path == sym::reason {
 275                                     err.span_label(sp, "reason in lint attribute must come last");
 276                                     add_label = false;
 277                                 }
 278                             }
 279                         }
 280                         if add_label {
 281                             err.span_label(sp, "bad attribute argument");
 282                         }
 283                         err.emit();
 284                         continue;
 285                     }
 286                 };
 287                 let tool_name = if meta_item.path.segments.len() > 1 {
 288                     let tool_ident = meta_item.path.segments[0].ident;
 289                     if !attr::is_known_lint_tool(tool_ident) {
 290                         struct_span_err!(
 291                             sess,
 292                             tool_ident.span,
 293                             E0710,
 294                             "an unknown tool name found in scoped lint: `{}`",
 295                             pprust::path_to_string(&meta_item.path),
 296                         )
 297                         .emit();
 298                         continue;
 299                     }
 300
 301                     Some(tool_ident.name)
 302                 } else {
 303                     None
 304                 };
 305                 let name = meta_item.path.segments.last().expect("empty lint name").ident.name;
 306                 match store.check_lint_name(&name.as_str(), tool_name) {
 307                     CheckLintNameResult::Ok(ids) => {
 308                         let src = LintSource::Node(name, li.span(), reason);
 309                         for id in ids {
 310                             specs.insert(*id, (level, src));
 311                         }
 312                     }
 313
 314                     CheckLintNameResult::Tool(result) => {
 315                         match result {
 316                             Ok(ids) => {
 317                                 let complete_name = &format!("{}::{}", tool_name.unwrap(), name);
 318                                 let src = LintSource::Node(
 319                                     Symbol::intern(complete_name),
 320                                     li.span(),
 321                                     reason,
 322                                 );
 323                                 for id in ids {
 324                                     specs.insert(*id, (level, src));
 325                                 }
 326                             }
 327                             Err((Some(ids), new_lint_name)) => {
 328                                 let lint = builtin::RENAMED_AND_REMOVED_LINTS;
 329                                 let (lvl, src) =
 330                                     self.sets.get_lint_level(lint, self.cur, Some(&specs), &sess);
 331                                 let msg = format!(
 332                                     "lint name `{}` is deprecated \
 333                                      and may not have an effect in the future. \
 334                                      Also `cfg_attr(cargo-clippy)` won't be necessary anymore",
 335                                     name
 336                                 );
 337                                 struct_lint_level(
 338                                     self.sess,
 339                                     lint,
 340                                     lvl,
 341                                     src,
 342                                     Some(li.span().into()),
 343                                     &msg,
 344                                 )
 345                                 .span_suggestion(
 346                                     li.span(),
 347                                     "change it to",
 348                                     new_lint_name.to_string(),
 349                                     Applicability::MachineApplicable,
 350                                 )
 351                                 .emit();
 352
 353                                 let src = LintSource::Node(
 354                                     Symbol::intern(&new_lint_name),
 355                                     li.span(),
 356                                     reason,
 357                                 );
 358                                 for id in ids {
 359                                     specs.insert(*id, (level, src));
 360                                 }
 361                             }
 362                             Err((None, _)) => {
 363                                 // If Tool(Err(None, _)) is returned, then either the lint does not
 364                                 // exist in the tool or the code was not compiled with the tool and
 365                                 // therefore the lint was never added to the `LintStore`. To detect
 366                                 // this is the responsibility of the lint tool.
 367                             }
 368                         }
 369                     }
 370
 371                     _ if !self.warn_about_weird_lints => {}
 372
 373                     CheckLintNameResult::Warning(msg, renamed) => {
 374                         let lint = builtin::RENAMED_AND_REMOVED_LINTS;
 375                         let (level, src) =
 376                             self.sets.get_lint_level(lint, self.cur, Some(&specs), &sess);
 377                         let mut err = struct_lint_level(
 378                             self.sess,
 379                             lint,
 380                             level,
 381                             src,
 382                             Some(li.span().into()),
 383                             &msg,
 384                         );
 385                         if let Some(new_name) = renamed {
 386                             err.span_suggestion(
 387                                 li.span(),
 388                                 "use the new name",
 389                                 new_name,
 390                                 Applicability::MachineApplicable,
 391                             );
 392                         }
 393                         err.emit();
 394                     }
 395                     CheckLintNameResult::NoLint(suggestion) => {
 396                         let lint = builtin::UNKNOWN_LINTS;
 397                         let (level, src) =
 398                             self.sets.get_lint_level(lint, self.cur, Some(&specs), self.sess);
 399                         let msg = format!("unknown lint: `{}`", name);
 400                         let mut db = struct_lint_level(
 401                             self.sess,
 402                             lint,
 403                             level,
 404                             src,
 405                             Some(li.span().into()),
 406                             &msg,
 407                         );
 408
 409                         if let Some(suggestion) = suggestion {
 410                             db.span_suggestion(
 411                                 li.span(),
 412                                 "did you mean",
 413                                 suggestion.to_string(),
 414                                 Applicability::MachineApplicable,
 415                             );
 416                         }
 417
 418                         db.emit();
 419                     }
 420                 }
 421             }
 422         }
 423
 424         for (id, &(level, ref src)) in specs.iter() {
 425             if level == Level::Forbid {
 426                 continue;
 427             }
 428             let forbid_src = match self.sets.get_lint_id_level(*id, self.cur, None) {
 429                 (Some(Level::Forbid), src) => src,
 430                 _ => continue,
 431             };
 432             let forbidden_lint_name = match forbid_src {
 433                 LintSource::Default => id.to_string(),
 434                 LintSource::Node(name, _, _) => name.to_string(),
 435                 LintSource::CommandLine(name) => name.to_string(),
 436             };
 437             let (lint_attr_name, lint_attr_span) = match *src {
 438                 LintSource::Node(name, span, _) => (name, span),
 439                 _ => continue,
 440             };
 441             let mut diag_builder = struct_span_err!(
 442                 self.sess,
 443                 lint_attr_span,
 444                 E0453,
 445                 "{}({}) overruled by outer forbid({})",
 446                 level.as_str(),
 447                 lint_attr_name,
 448                 forbidden_lint_name
 449             );
 450             diag_builder.span_label(lint_attr_span, "overruled by previous forbid");
 451             match forbid_src {
 452                 LintSource::Default => {}
 453                 LintSource::Node(_, forbid_source_span, reason) => {
 454                     diag_builder.span_label(forbid_source_span, "`forbid` level set here");
 455                     if let Some(rationale) = reason {
 456                         diag_builder.note(&rationale.as_str());
 457                     }
 458                 }
 459                 LintSource::CommandLine(_) => {
 460                     diag_builder.note("`forbid` lint level was set on command line");
 461                 }
 462             }
 463             diag_builder.emit();
 464             // don't set a separate error for every lint in the group
 465             break;
 466         }
 467
 468         let prev = self.cur;
 469         if specs.len() > 0 {
 470             self.cur = self.sets.list.len() as u32;
 471             self.sets.list.push(LintSet::Node { specs: specs, parent: prev });
 472         }
 473
 474         BuilderPush { prev: prev, changed: prev != self.cur }
 475     }
 476
 477     /// Called after `push` when the scope of a set of attributes are exited.
 478     pub fn pop(&mut self, push: BuilderPush) {
 479         self.cur = push.prev;
 480     }
 481
 482     /// Used to emit a lint-related diagnostic based on the current state of
 483     /// this lint context.
 484     pub fn struct_lint(
 485         &self,
 486         lint: &'static Lint,
 487         span: Option<MultiSpan>,
 488         msg: &str,
 489     ) -> DiagnosticBuilder<'a> {
 490         let (level, src) = self.sets.get_lint_level(lint, self.cur, None, self.sess);
 491         struct_lint_level(self.sess, lint, level, src, span, msg)
 492     }
 493
 494     /// Registers the ID provided with the current set of lints stored in
 495     /// this context.
 496     pub fn register_id(&mut self, id: HirId) {
 497         self.id_to_set.insert(id, self.cur);
 498     }
 499
 500     pub fn build(self) -> LintLevelSets {
 501         self.sets
 502     }
 503
 504     pub fn build_map(self) -> LintLevelMap {
 505         LintLevelMap { sets: self.sets, id_to_set: self.id_to_set }
 506     }
 507 }
 508
 509 pub struct LintLevelMap {
 510     sets: LintLevelSets,
 511     id_to_set: FxHashMap<HirId, u32>,
 512 }
 513
 514 impl LintLevelMap {
 515     /// If the `id` was previously registered with `register_id` when building
 516     /// this `LintLevelMap` this returns the corresponding lint level and source
 517     /// of the lint level for the lint provided.
 518     ///
 519     /// If the `id` was not previously registered, returns `None`. If `None` is
 520     /// returned then the parent of `id` should be acquired and this function
 521     /// should be called again.
 522     pub fn level_and_source(
 523         &self,
 524         lint: &'static Lint,
 525         id: HirId,
 526         session: &Session,
 527     ) -> Option<LevelSource> {
 528         self.id_to_set.get(&id).map(|idx| self.sets.get_lint_level(lint, *idx, None, session))
 529     }
 530 }
 531
 532 impl<'a> HashStable<StableHashingContext<'a>> for LintLevelMap {
 533     #[inline]
 534     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
 535         let LintLevelMap { ref sets, ref id_to_set } = *self;
 536
 537         id_to_set.hash_stable(hcx, hasher);
 538
 539         let LintLevelSets { ref list, lint_cap } = *sets;
 540
 541         lint_cap.hash_stable(hcx, hasher);
 542
 543         hcx.while_hashing_spans(true, |hcx| {
 544             list.len().hash_stable(hcx, hasher);
 545
 546             // We are working under the assumption here that the list of
 547             // lint-sets is built in a deterministic order.
 548             for lint_set in list {
 549                 ::std::mem::discriminant(lint_set).hash_stable(hcx, hasher);
 550
 551                 match *lint_set {
 552                     LintSet::CommandLine { ref specs } => {
 553                         specs.hash_stable(hcx, hasher);
 554                     }
 555                     LintSet::Node { ref specs, parent } => {
 556                         specs.hash_stable(hcx, hasher);
 557                         parent.hash_stable(hcx, hasher);
 558                     }
 559                 }
 560             }
 561         })
 562     }
 563 }
 564
 565 pub fn struct_lint_level<'a>(
 566     sess: &'a Session,
 567     lint: &'static Lint,
 568     level: Level,
 569     src: LintSource,
 570     span: Option<MultiSpan>,
 571     msg: &str,
 572 ) -> DiagnosticBuilder<'a> {
 573     let mut err = match (level, span) {
 574         (Level::Allow, _) => return sess.diagnostic().struct_dummy(),
 575         (Level::Warn, Some(span)) => sess.struct_span_warn(span, msg),
 576         (Level::Warn, None) => sess.struct_warn(msg),
 577         (Level::Deny, Some(span)) | (Level::Forbid, Some(span)) => sess.struct_span_err(span, msg),
 578         (Level::Deny, None) | (Level::Forbid, None) => sess.struct_err(msg),
 579     };
 580
 581     // Check for future incompatibility lints and issue a stronger warning.
 582     let lint_id = LintId::of(lint);
 583     let future_incompatible = lint.future_incompatible;
 584
 585     // If this code originates in a foreign macro, aka something that this crate
 586     // did not itself author, then it's likely that there's nothing this crate
 587     // can do about it. We probably want to skip the lint entirely.
 588     if err.span.primary_spans().iter().any(|s| in_external_macro(sess, *s)) {
 589         // Any suggestions made here are likely to be incorrect, so anything we
 590         // emit shouldn't be automatically fixed by rustfix.
 591         err.allow_suggestions(false);
 592
 593         // If this is a future incompatible lint it'll become a hard error, so
 594         // we have to emit *something*. Also allow lints to whitelist themselves
 595         // on a case-by-case basis for emission in a foreign macro.
 596         if future_incompatible.is_none() && !lint.report_in_external_macro {
 597             err.cancel();
 598             // Don't continue further, since we don't want to have
 599             // `diag_span_note_once` called for a diagnostic that isn't emitted.
 600             return err;
 601         }
 602     }
 603
 604     let name = lint.name_lower();
 605     match src {
 606         LintSource::Default => {
 607             sess.diag_note_once(
 608                 &mut err,
 609                 DiagnosticMessageId::from(lint),
 610                 &format!("`#[{}({})]` on by default", level.as_str(), name),
 611             );
 612         }
 613         LintSource::CommandLine(lint_flag_val) => {
 614             let flag = match level {
 615                 Level::Warn => "-W",
 616                 Level::Deny => "-D",
 617                 Level::Forbid => "-F",
 618                 Level::Allow => panic!(),
 619             };
 620             let hyphen_case_lint_name = name.replace("_", "-");
 621             if lint_flag_val.as_str() == name {
 622                 sess.diag_note_once(
 623                     &mut err,
 624                     DiagnosticMessageId::from(lint),
 625                     &format!(
 626                         "requested on the command line with `{} {}`",
 627                         flag, hyphen_case_lint_name
 628                     ),
 629                 );
 630             } else {
 631                 let hyphen_case_flag_val = lint_flag_val.as_str().replace("_", "-");
 632                 sess.diag_note_once(
 633                     &mut err,
 634                     DiagnosticMessageId::from(lint),
 635                     &format!(
 636                         "`{} {}` implied by `{} {}`",
 637                         flag, hyphen_case_lint_name, flag, hyphen_case_flag_val
 638                     ),
 639                 );
 640             }
 641         }
 642         LintSource::Node(lint_attr_name, src, reason) => {
 643             if let Some(rationale) = reason {
 644                 err.note(&rationale.as_str());
 645             }
 646             sess.diag_span_note_once(
 647                 &mut err,
 648                 DiagnosticMessageId::from(lint),
 649                 src,
 650                 "lint level defined here",
 651             );
 652             if lint_attr_name.as_str() != name {
 653                 let level_str = level.as_str();
 654                 sess.diag_note_once(
 655                     &mut err,
 656                     DiagnosticMessageId::from(lint),
 657                     &format!(
 658                         "`#[{}({})]` implied by `#[{}({})]`",
 659                         level_str, name, level_str, lint_attr_name
 660                     ),
 661                 );
 662             }
 663         }
 664     }
 665
 666     err.code(DiagnosticId::Lint(name));
 667
 668     if let Some(future_incompatible) = future_incompatible {
 669         const STANDARD_MESSAGE: &str = "this was previously accepted by the compiler but is being phased out; \
 670              it will become a hard error";
 671
 672         let explanation = if lint_id == LintId::of(builtin::UNSTABLE_NAME_COLLISIONS) {
 673             "once this method is added to the standard library, \
 674              the ambiguity may cause an error or change in behavior!"
 675                 .to_owned()
 676         } else if lint_id == LintId::of(builtin::MUTABLE_BORROW_RESERVATION_CONFLICT) {
 677             "this borrowing pattern was not meant to be accepted, \
 678              and may become a hard error in the future"
 679                 .to_owned()
 680         } else if let Some(edition) = future_incompatible.edition {
 681             format!("{} in the {} edition!", STANDARD_MESSAGE, edition)
 682         } else {
 683             format!("{} in a future release!", STANDARD_MESSAGE)
 684         };
 685         let citation = format!("for more information, see {}", future_incompatible.reference);
 686         err.warn(&explanation);
 687         err.note(&citation);
 688     }
 689
 690     return err;
 691 }
 692
 693 /// Returns whether `span` originates in a foreign crate's external macro.
 694 ///
 695 /// This is used to test whether a lint should not even begin to figure out whether it should
 696 /// be reported on the current node.
 697 pub fn in_external_macro(sess: &Session, span: Span) -> bool {
 698     let expn_data = span.ctxt().outer_expn_data();
 699     match expn_data.kind {
 700         ExpnKind::Root | ExpnKind::Desugaring(DesugaringKind::ForLoop) => false,
 701         ExpnKind::AstPass(_) | ExpnKind::Desugaring(_) => true, // well, it's "external"
 702         ExpnKind::Macro(MacroKind::Bang, _) => {
 703             if expn_data.def_site.is_dummy() {
 704                 // Dummy span for the `def_site` means it's an external macro.
 705                 return true;
 706             }
 707             match sess.source_map().span_to_snippet(expn_data.def_site) {
 708                 Ok(code) => !code.starts_with("macro_rules"),
 709                 // No snippet means external macro or compiler-builtin expansion.
 710                 Err(_) => true,
 711             }
 712         }
 713         ExpnKind::Macro(..) => true, // definitely a plugin
 714     }
 715 }