src/tools/clippy/clippy_lints/src/attrs.rs

   1 //! checks for attributes
   2
   3 use crate::utils::{
   4     first_line_of_span, is_present_in_source, match_panic_def_id, snippet_opt, span_lint, span_lint_and_help,
   5     span_lint_and_sugg, span_lint_and_then, without_block_comments,
   6 };
   7 use if_chain::if_chain;
   8 use rustc_ast::{AttrKind, AttrStyle, Attribute, Lit, LitKind, MetaItemKind, NestedMetaItem};
   9 use rustc_errors::Applicability;
  10 use rustc_hir::{
  11     Block, Expr, ExprKind, ImplItem, ImplItemKind, Item, ItemKind, StmtKind, TraitFn, TraitItem, TraitItemKind,
  12 };
  13 use rustc_lint::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
  14 use rustc_middle::lint::in_external_macro;
  15 use rustc_middle::ty;
  16 use rustc_session::{declare_lint_pass, declare_tool_lint};
  17 use rustc_span::source_map::Span;
  18 use rustc_span::sym;
  19 use rustc_span::symbol::{Symbol, SymbolStr};
  20 use semver::Version;
  21
  22 static UNIX_SYSTEMS: &[&str] = &[
  23     "android",
  24     "dragonfly",
  25     "emscripten",
  26     "freebsd",
  27     "fuchsia",
  28     "haiku",
  29     "illumos",
  30     "ios",
  31     "l4re",
  32     "linux",
  33     "macos",
  34     "netbsd",
  35     "openbsd",
  36     "redox",
  37     "solaris",
  38     "vxworks",
  39 ];
  40
  41 // NOTE: windows is excluded from the list because it's also a valid target family.
  42 static NON_UNIX_SYSTEMS: &[&str] = &["hermit", "none", "wasi"];
  43
  44 declare_clippy_lint! {
  45     /// **What it does:** Checks for items annotated with `#[inline(always)]`,
  46     /// unless the annotated function is empty or simply panics.
  47     ///
  48     /// **Why is this bad?** While there are valid uses of this annotation (and once
  49     /// you know when to use it, by all means `allow` this lint), it's a common
  50     /// newbie-mistake to pepper one's code with it.
  51     ///
  52     /// As a rule of thumb, before slapping `#[inline(always)]` on a function,
  53     /// measure if that additional function call really affects your runtime profile
  54     /// sufficiently to make up for the increase in compile time.
  55     ///
  56     /// **Known problems:** False positives, big time. This lint is meant to be
  57     /// deactivated by everyone doing serious performance work. This means having
  58     /// done the measurement.
  59     ///
  60     /// **Example:**
  61     /// ```ignore
  62     /// #[inline(always)]
  63     /// fn not_quite_hot_code(..) { ... }
  64     /// ```
  65     pub INLINE_ALWAYS,
  66     pedantic,
  67     "use of `#[inline(always)]`"
  68 }
  69
  70 declare_clippy_lint! {
  71     /// **What it does:** Checks for `extern crate` and `use` items annotated with
  72     /// lint attributes.
  73     ///
  74     /// This lint permits `#[allow(unused_imports)]`, `#[allow(deprecated)]`,
  75     /// `#[allow(unreachable_pub)]`, `#[allow(clippy::wildcard_imports)]` and
  76     /// `#[allow(clippy::enum_glob_use)]` on `use` items and `#[allow(unused_imports)]` on
  77     /// `extern crate` items with a `#[macro_use]` attribute.
  78     ///
  79     /// **Why is this bad?** Lint attributes have no effect on crate imports. Most
  80     /// likely a `!` was forgotten.
  81     ///
  82     /// **Known problems:** None.
  83     ///
  84     /// **Example:**
  85     /// ```ignore
  86     /// // Bad
  87     /// #[deny(dead_code)]
  88     /// extern crate foo;
  89     /// #[forbid(dead_code)]
  90     /// use foo::bar;
  91     ///
  92     /// // Ok
  93     /// #[allow(unused_imports)]
  94     /// use foo::baz;
  95     /// #[allow(unused_imports)]
  96     /// #[macro_use]
  97     /// extern crate baz;
  98     /// ```
  99     pub USELESS_ATTRIBUTE,
 100     correctness,
 101     "use of lint attributes on `extern crate` items"
 102 }
 103
 104 declare_clippy_lint! {
 105     /// **What it does:** Checks for `#[deprecated]` annotations with a `since`
 106     /// field that is not a valid semantic version.
 107     ///
 108     /// **Why is this bad?** For checking the version of the deprecation, it must be
 109     /// a valid semver. Failing that, the contained information is useless.
 110     ///
 111     /// **Known problems:** None.
 112     ///
 113     /// **Example:**
 114     /// ```rust
 115     /// #[deprecated(since = "forever")]
 116     /// fn something_else() { /* ... */ }
 117     /// ```
 118     pub DEPRECATED_SEMVER,
 119     correctness,
 120     "use of `#[deprecated(since = \"x\")]` where x is not semver"
 121 }
 122
 123 declare_clippy_lint! {
 124     /// **What it does:** Checks for empty lines after outer attributes
 125     ///
 126     /// **Why is this bad?**
 127     /// Most likely the attribute was meant to be an inner attribute using a '!'.
 128     /// If it was meant to be an outer attribute, then the following item
 129     /// should not be separated by empty lines.
 130     ///
 131     /// **Known problems:** Can cause false positives.
 132     ///
 133     /// From the clippy side it's difficult to detect empty lines between an attributes and the
 134     /// following item because empty lines and comments are not part of the AST. The parsing
 135     /// currently works for basic cases but is not perfect.
 136     ///
 137     /// **Example:**
 138     /// ```rust
 139     /// // Good (as inner attribute)
 140     /// #![allow(dead_code)]
 141     ///
 142     /// fn this_is_fine() { }
 143     ///
 144     /// // Bad
 145     /// #[allow(dead_code)]
 146     ///
 147     /// fn not_quite_good_code() { }
 148     ///
 149     /// // Good (as outer attribute)
 150     /// #[allow(dead_code)]
 151     /// fn this_is_fine_too() { }
 152     /// ```
 153     pub EMPTY_LINE_AFTER_OUTER_ATTR,
 154     nursery,
 155     "empty line after outer attribute"
 156 }
 157
 158 declare_clippy_lint! {
 159     /// **What it does:** Checks for `warn`/`deny`/`forbid` attributes targeting the whole clippy::restriction category.
 160     ///
 161     /// **Why is this bad?** Restriction lints sometimes are in contrast with other lints or even go against idiomatic rust.
 162     /// These lints should only be enabled on a lint-by-lint basis and with careful consideration.
 163     ///
 164     /// **Known problems:** None.
 165     ///
 166     /// **Example:**
 167     /// Bad:
 168     /// ```rust
 169     /// #![deny(clippy::restriction)]
 170     /// ```
 171     ///
 172     /// Good:
 173     /// ```rust
 174     /// #![deny(clippy::as_conversions)]
 175     /// ```
 176     pub BLANKET_CLIPPY_RESTRICTION_LINTS,
 177     style,
 178     "enabling the complete restriction group"
 179 }
 180
 181 declare_clippy_lint! {
 182     /// **What it does:** Checks for `#[cfg_attr(rustfmt, rustfmt_skip)]` and suggests to replace it
 183     /// with `#[rustfmt::skip]`.
 184     ///
 185     /// **Why is this bad?** Since tool_attributes ([rust-lang/rust#44690](https://github.com/rust-lang/rust/issues/44690))
 186     /// are stable now, they should be used instead of the old `cfg_attr(rustfmt)` attributes.
 187     ///
 188     /// **Known problems:** This lint doesn't detect crate level inner attributes, because they get
 189     /// processed before the PreExpansionPass lints get executed. See
 190     /// [#3123](https://github.com/rust-lang/rust-clippy/pull/3123#issuecomment-422321765)
 191     ///
 192     /// **Example:**
 193     ///
 194     /// Bad:
 195     /// ```rust
 196     /// #[cfg_attr(rustfmt, rustfmt_skip)]
 197     /// fn main() { }
 198     /// ```
 199     ///
 200     /// Good:
 201     /// ```rust
 202     /// #[rustfmt::skip]
 203     /// fn main() { }
 204     /// ```
 205     pub DEPRECATED_CFG_ATTR,
 206     complexity,
 207     "usage of `cfg_attr(rustfmt)` instead of tool attributes"
 208 }
 209
 210 declare_clippy_lint! {
 211     /// **What it does:** Checks for cfg attributes having operating systems used in target family position.
 212     ///
 213     /// **Why is this bad?** The configuration option will not be recognised and the related item will not be included
 214     /// by the conditional compilation engine.
 215     ///
 216     /// **Known problems:** None.
 217     ///
 218     /// **Example:**
 219     ///
 220     /// Bad:
 221     /// ```rust
 222     /// #[cfg(linux)]
 223     /// fn conditional() { }
 224     /// ```
 225     ///
 226     /// Good:
 227     /// ```rust
 228     /// #[cfg(target_os = "linux")]
 229     /// fn conditional() { }
 230     /// ```
 231     ///
 232     /// Or:
 233     /// ```rust
 234     /// #[cfg(unix)]
 235     /// fn conditional() { }
 236     /// ```
 237     /// Check the [Rust Reference](https://doc.rust-lang.org/reference/conditional-compilation.html#target_os) for more details.
 238     pub MISMATCHED_TARGET_OS,
 239     correctness,
 240     "usage of `cfg(operating_system)` instead of `cfg(target_os = \"operating_system\")`"
 241 }
 242
 243 declare_lint_pass!(Attributes => [
 244     INLINE_ALWAYS,
 245     DEPRECATED_SEMVER,
 246     USELESS_ATTRIBUTE,
 247     BLANKET_CLIPPY_RESTRICTION_LINTS,
 248 ]);
 249
 250 impl<'tcx> LateLintPass<'tcx> for Attributes {
 251     fn check_attribute(&mut self, cx: &LateContext<'tcx>, attr: &'tcx Attribute) {
 252         if let Some(items) = &attr.meta_item_list() {
 253             if let Some(ident) = attr.ident() {
 254                 let ident = &*ident.as_str();
 255                 match ident {
 256                     "allow" | "warn" | "deny" | "forbid" => {
 257                         check_clippy_lint_names(cx, ident, items);
 258                     },
 259                     _ => {},
 260                 }
 261                 if items.is_empty() || !attr.has_name(sym::deprecated) {
 262                     return;
 263                 }
 264                 for item in items {
 265                     if_chain! {
 266                         if let NestedMetaItem::MetaItem(mi) = &item;
 267                         if let MetaItemKind::NameValue(lit) = &mi.kind;
 268                         if mi.has_name(sym::since);
 269                         then {
 270                             check_semver(cx, item.span(), lit);
 271                         }
 272                     }
 273                 }
 274             }
 275         }
 276     }
 277
 278     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx Item<'_>) {
 279         let attrs = cx.tcx.hir().attrs(item.hir_id());
 280         if is_relevant_item(cx, item) {
 281             check_attrs(cx, item.span, item.ident.name, attrs)
 282         }
 283         match item.kind {
 284             ItemKind::ExternCrate(..) | ItemKind::Use(..) => {
 285                 let skip_unused_imports = attrs.iter().any(|attr| attr.has_name(sym::macro_use));
 286
 287                 for attr in attrs {
 288                     if in_external_macro(cx.sess(), attr.span) {
 289                         return;
 290                     }
 291                     if let Some(lint_list) = &attr.meta_item_list() {
 292                         if let Some(ident) = attr.ident() {
 293                             match &*ident.as_str() {
 294                                 "allow" | "warn" | "deny" | "forbid" => {
 295                                     // permit `unused_imports`, `deprecated`, `unreachable_pub`,
 296                                     // `clippy::wildcard_imports`, and `clippy::enum_glob_use` for `use` items
 297                                     // and `unused_imports` for `extern crate` items with `macro_use`
 298                                     for lint in lint_list {
 299                                         match item.kind {
 300                                             ItemKind::Use(..) => {
 301                                                 if is_word(lint, sym!(unused_imports))
 302                                                     || is_word(lint, sym::deprecated)
 303                                                     || is_word(lint, sym!(unreachable_pub))
 304                                                     || is_word(lint, sym!(unused))
 305                                                     || extract_clippy_lint(lint)
 306                                                         .map_or(false, |s| s == "wildcard_imports")
 307                                                     || extract_clippy_lint(lint).map_or(false, |s| s == "enum_glob_use")
 308                                                 {
 309                                                     return;
 310                                                 }
 311                                             },
 312                                             ItemKind::ExternCrate(..) => {
 313                                                 if is_word(lint, sym!(unused_imports)) && skip_unused_imports {
 314                                                     return;
 315                                                 }
 316                                                 if is_word(lint, sym!(unused_extern_crates)) {
 317                                                     return;
 318                                                 }
 319                                             },
 320                                             _ => {},
 321                                         }
 322                                     }
 323                                     let line_span = first_line_of_span(cx, attr.span);
 324
 325                                     if let Some(mut sugg) = snippet_opt(cx, line_span) {
 326                                         if sugg.contains("#[") {
 327                                             span_lint_and_then(
 328                                                 cx,
 329                                                 USELESS_ATTRIBUTE,
 330                                                 line_span,
 331                                                 "useless lint attribute",
 332                                                 |diag| {
 333                                                     sugg = sugg.replacen("#[", "#![", 1);
 334                                                     diag.span_suggestion(
 335                                                         line_span,
 336                                                         "if you just forgot a `!`, use",
 337                                                         sugg,
 338                                                         Applicability::MaybeIncorrect,
 339                                                     );
 340                                                 },
 341                                             );
 342                                         }
 343                                     }
 344                                 },
 345                                 _ => {},
 346                             }
 347                         }
 348                     }
 349                 }
 350             },
 351             _ => {},
 352         }
 353     }
 354
 355     fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx ImplItem<'_>) {
 356         if is_relevant_impl(cx, item) {
 357             check_attrs(cx, item.span, item.ident.name, cx.tcx.hir().attrs(item.hir_id()))
 358         }
 359     }
 360
 361     fn check_trait_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx TraitItem<'_>) {
 362         if is_relevant_trait(cx, item) {
 363             check_attrs(cx, item.span, item.ident.name, cx.tcx.hir().attrs(item.hir_id()))
 364         }
 365     }
 366 }
 367
 368 /// Returns the lint name if it is clippy lint.
 369 fn extract_clippy_lint(lint: &NestedMetaItem) -> Option<SymbolStr> {
 370     if_chain! {
 371         if let Some(meta_item) = lint.meta_item();
 372         if meta_item.path.segments.len() > 1;
 373         if let tool_name = meta_item.path.segments[0].ident;
 374         if tool_name.name == sym::clippy;
 375         let lint_name = meta_item.path.segments.last().unwrap().ident.name;
 376         then {
 377             return Some(lint_name.as_str());
 378         }
 379     }
 380     None
 381 }
 382
 383 fn check_clippy_lint_names(cx: &LateContext<'_>, ident: &str, items: &[NestedMetaItem]) {
 384     for lint in items {
 385         if let Some(lint_name) = extract_clippy_lint(lint) {
 386             if lint_name == "restriction" && ident != "allow" {
 387                 span_lint_and_help(
 388                     cx,
 389                     BLANKET_CLIPPY_RESTRICTION_LINTS,
 390                     lint.span(),
 391                     "restriction lints are not meant to be all enabled",
 392                     None,
 393                     "try enabling only the lints you really need",
 394                 );
 395             }
 396         }
 397     }
 398 }
 399
 400 fn is_relevant_item(cx: &LateContext<'_>, item: &Item<'_>) -> bool {
 401     if let ItemKind::Fn(_, _, eid) = item.kind {
 402         is_relevant_expr(cx, cx.tcx.typeck_body(eid), &cx.tcx.hir().body(eid).value)
 403     } else {
 404         true
 405     }
 406 }
 407
 408 fn is_relevant_impl(cx: &LateContext<'_>, item: &ImplItem<'_>) -> bool {
 409     match item.kind {
 410         ImplItemKind::Fn(_, eid) => is_relevant_expr(cx, cx.tcx.typeck_body(eid), &cx.tcx.hir().body(eid).value),
 411         _ => false,
 412     }
 413 }
 414
 415 fn is_relevant_trait(cx: &LateContext<'_>, item: &TraitItem<'_>) -> bool {
 416     match item.kind {
 417         TraitItemKind::Fn(_, TraitFn::Required(_)) => true,
 418         TraitItemKind::Fn(_, TraitFn::Provided(eid)) => {
 419             is_relevant_expr(cx, cx.tcx.typeck_body(eid), &cx.tcx.hir().body(eid).value)
 420         },
 421         _ => false,
 422     }
 423 }
 424
 425 fn is_relevant_block(cx: &LateContext<'_>, typeck_results: &ty::TypeckResults<'_>, block: &Block<'_>) -> bool {
 426     block.stmts.first().map_or(
 427         block
 428             .expr
 429             .as_ref()
 430             .map_or(false, |e| is_relevant_expr(cx, typeck_results, e)),
 431         |stmt| match &stmt.kind {
 432             StmtKind::Local(_) => true,
 433             StmtKind::Expr(expr) | StmtKind::Semi(expr) => is_relevant_expr(cx, typeck_results, expr),
 434             _ => false,
 435         },
 436     )
 437 }
 438
 439 fn is_relevant_expr(cx: &LateContext<'_>, typeck_results: &ty::TypeckResults<'_>, expr: &Expr<'_>) -> bool {
 440     match &expr.kind {
 441         ExprKind::Block(block, _) => is_relevant_block(cx, typeck_results, block),
 442         ExprKind::Ret(Some(e)) => is_relevant_expr(cx, typeck_results, e),
 443         ExprKind::Ret(None) | ExprKind::Break(_, None) => false,
 444         ExprKind::Call(path_expr, _) => {
 445             if let ExprKind::Path(qpath) = &path_expr.kind {
 446                 typeck_results
 447                     .qpath_res(qpath, path_expr.hir_id)
 448                     .opt_def_id()
 449                     .map_or(true, |fun_id| !match_panic_def_id(cx, fun_id))
 450             } else {
 451                 true
 452             }
 453         },
 454         _ => true,
 455     }
 456 }
 457
 458 fn check_attrs(cx: &LateContext<'_>, span: Span, name: Symbol, attrs: &[Attribute]) {
 459     if span.from_expansion() {
 460         return;
 461     }
 462
 463     for attr in attrs {
 464         if let Some(values) = attr.meta_item_list() {
 465             if values.len() != 1 || !attr.has_name(sym::inline) {
 466                 continue;
 467             }
 468             if is_word(&values[0], sym::always) {
 469                 span_lint(
 470                     cx,
 471                     INLINE_ALWAYS,
 472                     attr.span,
 473                     &format!(
 474                         "you have declared `#[inline(always)]` on `{}`. This is usually a bad idea",
 475                         name
 476                     ),
 477                 );
 478             }
 479         }
 480     }
 481 }
 482
 483 fn check_semver(cx: &LateContext<'_>, span: Span, lit: &Lit) {
 484     if let LitKind::Str(is, _) = lit.kind {
 485         if Version::parse(&is.as_str()).is_ok() {
 486             return;
 487         }
 488     }
 489     span_lint(
 490         cx,
 491         DEPRECATED_SEMVER,
 492         span,
 493         "the since field must contain a semver-compliant version",
 494     );
 495 }
 496
 497 fn is_word(nmi: &NestedMetaItem, expected: Symbol) -> bool {
 498     if let NestedMetaItem::MetaItem(mi) = &nmi {
 499         mi.is_word() && mi.has_name(expected)
 500     } else {
 501         false
 502     }
 503 }
 504
 505 declare_lint_pass!(EarlyAttributes => [
 506     DEPRECATED_CFG_ATTR,
 507     MISMATCHED_TARGET_OS,
 508     EMPTY_LINE_AFTER_OUTER_ATTR,
 509 ]);
 510
 511 impl EarlyLintPass for EarlyAttributes {
 512     fn check_item(&mut self, cx: &EarlyContext<'_>, item: &rustc_ast::Item) {
 513         check_empty_line_after_outer_attr(cx, item);
 514     }
 515
 516     fn check_attribute(&mut self, cx: &EarlyContext<'_>, attr: &Attribute) {
 517         check_deprecated_cfg_attr(cx, attr);
 518         check_mismatched_target_os(cx, attr);
 519     }
 520 }
 521
 522 fn check_empty_line_after_outer_attr(cx: &EarlyContext<'_>, item: &rustc_ast::Item) {
 523     for attr in &item.attrs {
 524         let attr_item = if let AttrKind::Normal(ref attr, _) = attr.kind {
 525             attr
 526         } else {
 527             return;
 528         };
 529
 530         if attr.style == AttrStyle::Outer {
 531             if attr_item.args.inner_tokens().is_empty() || !is_present_in_source(cx, attr.span) {
 532                 return;
 533             }
 534
 535             let begin_of_attr_to_item = Span::new(attr.span.lo(), item.span.lo(), item.span.ctxt());
 536             let end_of_attr_to_item = Span::new(attr.span.hi(), item.span.lo(), item.span.ctxt());
 537
 538             if let Some(snippet) = snippet_opt(cx, end_of_attr_to_item) {
 539                 let lines = snippet.split('\n').collect::<Vec<_>>();
 540                 let lines = without_block_comments(lines);
 541
 542                 if lines.iter().filter(|l| l.trim().is_empty()).count() > 2 {
 543                     span_lint(
 544                         cx,
 545                         EMPTY_LINE_AFTER_OUTER_ATTR,
 546                         begin_of_attr_to_item,
 547                         "found an empty line after an outer attribute. \
 548                         Perhaps you forgot to add a `!` to make it an inner attribute?",
 549                     );
 550                 }
 551             }
 552         }
 553     }
 554 }
 555
 556 fn check_deprecated_cfg_attr(cx: &EarlyContext<'_>, attr: &Attribute) {
 557     if_chain! {
 558         // check cfg_attr
 559         if attr.has_name(sym::cfg_attr);
 560         if let Some(items) = attr.meta_item_list();
 561         if items.len() == 2;
 562         // check for `rustfmt`
 563         if let Some(feature_item) = items[0].meta_item();
 564         if feature_item.has_name(sym::rustfmt);
 565         // check for `rustfmt_skip` and `rustfmt::skip`
 566         if let Some(skip_item) = &items[1].meta_item();
 567         if skip_item.has_name(sym!(rustfmt_skip)) ||
 568             skip_item.path.segments.last().expect("empty path in attribute").ident.name == sym!(skip);
 569         // Only lint outer attributes, because custom inner attributes are unstable
 570         // Tracking issue: https://github.com/rust-lang/rust/issues/54726
 571         if let AttrStyle::Outer = attr.style;
 572         then {
 573             span_lint_and_sugg(
 574                 cx,
 575                 DEPRECATED_CFG_ATTR,
 576                 attr.span,
 577                 "`cfg_attr` is deprecated for rustfmt and got replaced by tool attributes",
 578                 "use",
 579                 "#[rustfmt::skip]".to_string(),
 580                 Applicability::MachineApplicable,
 581             );
 582         }
 583     }
 584 }
 585
 586 fn check_mismatched_target_os(cx: &EarlyContext<'_>, attr: &Attribute) {
 587     fn find_os(name: &str) -> Option<&'static str> {
 588         UNIX_SYSTEMS
 589             .iter()
 590             .chain(NON_UNIX_SYSTEMS.iter())
 591             .find(|&&os| os == name)
 592             .copied()
 593     }
 594
 595     fn is_unix(name: &str) -> bool {
 596         UNIX_SYSTEMS.iter().any(|&os| os == name)
 597     }
 598
 599     fn find_mismatched_target_os(items: &[NestedMetaItem]) -> Vec<(&str, Span)> {
 600         let mut mismatched = Vec::new();
 601
 602         for item in items {
 603             if let NestedMetaItem::MetaItem(meta) = item {
 604                 match &meta.kind {
 605                     MetaItemKind::List(list) => {
 606                         mismatched.extend(find_mismatched_target_os(&list));
 607                     },
 608                     MetaItemKind::Word => {
 609                         if_chain! {
 610                             if let Some(ident) = meta.ident();
 611                             if let Some(os) = find_os(&*ident.name.as_str());
 612                             then {
 613                                 mismatched.push((os, ident.span));
 614                             }
 615                         }
 616                     },
 617                     _ => {},
 618                 }
 619             }
 620         }
 621
 622         mismatched
 623     }
 624
 625     if_chain! {
 626         if attr.has_name(sym::cfg);
 627         if let Some(list) = attr.meta_item_list();
 628         let mismatched = find_mismatched_target_os(&list);
 629         if !mismatched.is_empty();
 630         then {
 631             let mess = "operating system used in target family position";
 632
 633             span_lint_and_then(cx, MISMATCHED_TARGET_OS, attr.span, &mess, |diag| {
 634                 // Avoid showing the unix suggestion multiple times in case
 635                 // we have more than one mismatch for unix-like systems
 636                 let mut unix_suggested = false;
 637
 638                 for (os, span) in mismatched {
 639                     let sugg = format!("target_os = \"{}\"", os);
 640                     diag.span_suggestion(span, "try", sugg, Applicability::MaybeIncorrect);
 641
 642                     if !unix_suggested && is_unix(os) {
 643                         diag.help("did you mean `unix`?");
 644                         unix_suggested = true;
 645                     }
 646                 }
 647             });
 648         }
 649     }
 650 }