src/libsyntax/feature_gate.rs

   1 // Copyright 2013 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 //! Feature gating
  12 //!
  13 //! This module implements the gating necessary for preventing certain compiler
  14 //! features from being used by default. This module will crawl a pre-expanded
  15 //! AST to ensure that there are no features which are used that are not
  16 //! enabled.
  17 //!
  18 //! Features are enabled in programs via the crate-level attributes of
  19 //! `#![feature(...)]` with a comma-separated list of features.
  20 //!
  21 //! For the purpose of future feature-tracking, once code for detection of feature
  22 //! gate usage is added, *do not remove it again* even once the feature
  23 //! becomes stable.
  24
  25 use self::Status::*;
  26 use self::AttributeType::*;
  27
  28 use abi::Abi;
  29 use ast::NodeId;
  30 use ast;
  31 use attr;
  32 use attr::AttrMetaMethods;
  33 use codemap::{CodeMap, Span};
  34 use diagnostic::SpanHandler;
  35 use visit;
  36 use visit::Visitor;
  37 use parse::token::{self, InternedString};
  38
  39 use std::ascii::AsciiExt;
  40
  41 // If you change this list without updating src/doc/reference.md, @cmr will be sad
  42 // Don't ever remove anything from this list; set them to 'Removed'.
  43 // The version numbers here correspond to the version in which the current status
  44 // was set. This is most important for knowing when a particular feature became
  45 // stable (active).
  46 // NB: The featureck.py script parses this information directly out of the source
  47 // so take care when modifying it.
  48 const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
  49     ("globs", "1.0.0", Accepted),
  50     ("macro_rules", "1.0.0", Accepted),
  51     ("struct_variant", "1.0.0", Accepted),
  52     ("asm", "1.0.0", Active),
  53     ("managed_boxes", "1.0.0", Removed),
  54     ("non_ascii_idents", "1.0.0", Active),
  55     ("thread_local", "1.0.0", Active),
  56     ("link_args", "1.0.0", Active),
  57     ("plugin_registrar", "1.0.0", Active),
  58     ("log_syntax", "1.0.0", Active),
  59     ("trace_macros", "1.0.0", Active),
  60     ("concat_idents", "1.0.0", Active),
  61     ("intrinsics", "1.0.0", Active),
  62     ("lang_items", "1.0.0", Active),
  63
  64     ("simd", "1.0.0", Active),
  65     ("default_type_params", "1.0.0", Accepted),
  66     ("quote", "1.0.0", Active),
  67     ("link_llvm_intrinsics", "1.0.0", Active),
  68     ("linkage", "1.0.0", Active),
  69     ("struct_inherit", "1.0.0", Removed),
  70
  71     ("quad_precision_float", "1.0.0", Removed),
  72
  73     ("rustc_diagnostic_macros", "1.0.0", Active),
  74     ("unboxed_closures", "1.0.0", Active),
  75     ("reflect", "1.0.0", Active),
  76     ("import_shadowing", "1.0.0", Removed),
  77     ("advanced_slice_patterns", "1.0.0", Active),
  78     ("tuple_indexing", "1.0.0", Accepted),
  79     ("associated_types", "1.0.0", Accepted),
  80     ("visible_private_types", "1.0.0", Active),
  81     ("slicing_syntax", "1.0.0", Accepted),
  82     ("box_syntax", "1.0.0", Active),
  83     ("on_unimplemented", "1.0.0", Active),
  84     ("simd_ffi", "1.0.0", Active),
  85     ("allocator", "1.0.0", Active),
  86
  87     ("if_let", "1.0.0", Accepted),
  88     ("while_let", "1.0.0", Accepted),
  89
  90     ("plugin", "1.0.0", Active),
  91     ("start", "1.0.0", Active),
  92     ("main", "1.0.0", Active),
  93
  94     ("fundamental", "1.0.0", Active),
  95
  96     // Deprecate after snapshot
  97     // SNAP 5520801
  98     ("unsafe_destructor", "1.0.0", Active),
  99
 100     // A temporary feature gate used to enable parser extensions needed
 101     // to bootstrap fix for #5723.
 102     ("issue_5723_bootstrap", "1.0.0", Accepted),
 103
 104     // A way to temporarily opt out of opt in copy. This will *never* be accepted.
 105     ("opt_out_copy", "1.0.0", Removed),
 106
 107     // OIBIT specific features
 108     ("optin_builtin_traits", "1.0.0", Active),
 109
 110     // macro reexport needs more discussion and stabilization
 111     ("macro_reexport", "1.0.0", Active),
 112
 113     // These are used to test this portion of the compiler, they don't actually
 114     // mean anything
 115     ("test_accepted_feature", "1.0.0", Accepted),
 116     ("test_removed_feature", "1.0.0", Removed),
 117
 118     // Allows use of #[staged_api]
 119     ("staged_api", "1.0.0", Active),
 120
 121     // Allows using items which are missing stability attributes
 122     ("unmarked_api", "1.0.0", Active),
 123
 124     // Allows using #![no_std]
 125     ("no_std", "1.0.0", Active),
 126
 127     // Allows using `box` in patterns; RFC 469
 128     ("box_patterns", "1.0.0", Active),
 129
 130     // Allows using the unsafe_no_drop_flag attribute (unlikely to
 131     // switch to Accepted; see RFC 320)
 132     ("unsafe_no_drop_flag", "1.0.0", Active),
 133
 134     // Allows the use of custom attributes; RFC 572
 135     ("custom_attribute", "1.0.0", Active),
 136
 137     // Allows the use of #[derive(Anything)] as sugar for
 138     // #[derive_Anything].
 139     ("custom_derive", "1.0.0", Active),
 140
 141     // Allows the use of rustc_* attributes; RFC 572
 142     ("rustc_attrs", "1.0.0", Active),
 143
 144     // Allows the use of `static_assert`
 145     ("static_assert", "1.0.0", Active),
 146
 147     // Allows the use of #[allow_internal_unstable]. This is an
 148     // attribute on macro_rules! and can't use the attribute handling
 149     // below (it has to be checked before expansion possibly makes
 150     // macros disappear).
 151     ("allow_internal_unstable", "1.0.0", Active),
 152
 153     // #23121. Array patterns have some hazards yet.
 154     ("slice_patterns", "1.0.0", Active),
 155
 156     // Allows use of unary negate on unsigned integers, e.g. -e for e: u8
 157     ("negate_unsigned", "1.0.0", Active),
 158 ];
 159 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 160
 161 enum Status {
 162     /// Represents an active feature that is currently being implemented or
 163     /// currently being considered for addition/removal.
 164     Active,
 165
 166     /// Represents a feature which has since been removed (it was once Active)
 167     Removed,
 168
 169     /// This language feature has since been Accepted (it was once Active)
 170     Accepted,
 171 }
 172
 173 // Attributes that have a special meaning to rustc or rustdoc
 174 pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType)] = &[
 175     // Normal attributes
 176
 177     ("warn", Normal),
 178     ("allow", Normal),
 179     ("forbid", Normal),
 180     ("deny", Normal),
 181
 182     ("macro_reexport", Normal),
 183     ("macro_use", Normal),
 184     ("macro_export", Normal),
 185     ("plugin_registrar", Normal),
 186
 187     ("cfg", Normal),
 188     ("cfg_attr", Normal),
 189     ("main", Normal),
 190     ("start", Normal),
 191     ("test", Normal),
 192     ("bench", Normal),
 193     ("simd", Normal),
 194     ("repr", Normal),
 195     ("path", Normal),
 196     ("abi", Normal),
 197     ("automatically_derived", Normal),
 198     ("no_mangle", Normal),
 199     ("no_link", Normal),
 200     ("derive", Normal),
 201     ("should_panic", Normal),
 202     ("ignore", Normal),
 203     ("no_implicit_prelude", Normal),
 204     ("reexport_test_harness_main", Normal),
 205     ("link_args", Normal),
 206     ("macro_escape", Normal),
 207
 208     ("unsafe_destructor", Gated("unsafe_destructor",
 209                                 "`#[unsafe_destructor]` does nothing anymore")),
 210     ("staged_api", Gated("staged_api",
 211                          "staged_api is for use by rustc only")),
 212     ("plugin", Gated("plugin",
 213                      "compiler plugins are experimental \
 214                       and possibly buggy")),
 215     ("no_std", Gated("no_std",
 216                      "no_std is experimental")),
 217     ("lang", Gated("lang_items",
 218                      "language items are subject to change")),
 219     ("linkage", Gated("linkage",
 220                       "the `linkage` attribute is experimental \
 221                        and not portable across platforms")),
 222     ("thread_local", Gated("thread_local",
 223                             "`#[thread_local]` is an experimental feature, and does not \
 224                              currently handle destructors. There is no corresponding \
 225                              `#[task_local]` mapping to the task model")),
 226
 227     ("rustc_on_unimplemented", Gated("on_unimplemented",
 228                                      "the `#[rustc_on_unimplemented]` attribute \
 229                                       is an experimental feature")),
 230     ("allocator", Gated("allocator",
 231                         "the `#[allocator]` attribute is an experimental feature")),
 232     ("rustc_variance", Gated("rustc_attrs",
 233                              "the `#[rustc_variance]` attribute \
 234                               is an experimental feature")),
 235     ("rustc_error", Gated("rustc_attrs",
 236                           "the `#[rustc_error]` attribute \
 237                            is an experimental feature")),
 238     ("rustc_move_fragments", Gated("rustc_attrs",
 239                                    "the `#[rustc_move_fragments]` attribute \
 240                                     is an experimental feature")),
 241
 242     ("allow_internal_unstable", Gated("allow_internal_unstable",
 243                                       EXPLAIN_ALLOW_INTERNAL_UNSTABLE)),
 244
 245     ("fundamental", Gated("fundamental",
 246                           "the `#[fundamental]` attribute \
 247                            is an experimental feature")),
 248
 249     // FIXME: #14408 whitelist docs since rustdoc looks at them
 250     ("doc", Whitelisted),
 251
 252     // FIXME: #14406 these are processed in trans, which happens after the
 253     // lint pass
 254     ("cold", Whitelisted),
 255     ("export_name", Whitelisted),
 256     ("inline", Whitelisted),
 257     ("link", Whitelisted),
 258     ("link_name", Whitelisted),
 259     ("link_section", Whitelisted),
 260     ("no_builtins", Whitelisted),
 261     ("no_mangle", Whitelisted),
 262     ("no_stack_check", Whitelisted),
 263     ("packed", Whitelisted),
 264     ("static_assert", Gated("static_assert",
 265                             "`#[static_assert]` is an experimental feature, and has a poor API")),
 266     ("no_debug", Whitelisted),
 267     ("omit_gdb_pretty_printer_section", Whitelisted),
 268     ("unsafe_no_drop_flag", Gated("unsafe_no_drop_flag",
 269                                   "unsafe_no_drop_flag has unstable semantics \
 270                                    and may be removed in the future")),
 271
 272     // used in resolve
 273     ("prelude_import", Whitelisted),
 274
 275     // FIXME: #14407 these are only looked at on-demand so we can't
 276     // guarantee they'll have already been checked
 277     ("deprecated", Whitelisted),
 278     ("must_use", Whitelisted),
 279     ("stable", Whitelisted),
 280     ("unstable", Whitelisted),
 281
 282     ("rustc_paren_sugar", Gated("unboxed_closures",
 283                                 "unboxed_closures are still evolving")),
 284     ("rustc_reflect_like", Gated("reflect",
 285                                  "defining reflective traits is still evolving")),
 286
 287     // Crate level attributes
 288     ("crate_name", CrateLevel),
 289     ("crate_type", CrateLevel),
 290     ("crate_id", CrateLevel),
 291     ("feature", CrateLevel),
 292     ("no_start", CrateLevel),
 293     ("no_main", CrateLevel),
 294     ("no_builtins", CrateLevel),
 295     ("recursion_limit", CrateLevel),
 296 ];
 297
 298 #[derive(PartialEq, Copy, Clone, Debug)]
 299 pub enum AttributeType {
 300     /// Normal, builtin attribute that is consumed
 301     /// by the compiler before the unused_attribute check
 302     Normal,
 303
 304     /// Builtin attribute that may not be consumed by the compiler
 305     /// before the unused_attribute check. These attributes
 306     /// will be ignored by the unused_attribute lint
 307     Whitelisted,
 308
 309     /// Is gated by a given feature gate and reason
 310     /// These get whitelisted too
 311     Gated(&'static str, &'static str),
 312
 313     /// Builtin attribute that is only allowed at the crate level
 314     CrateLevel,
 315 }
 316
 317 /// A set of features to be used by later passes.
 318 pub struct Features {
 319     pub unboxed_closures: bool,
 320     pub rustc_diagnostic_macros: bool,
 321     pub visible_private_types: bool,
 322     pub allow_quote: bool,
 323     pub allow_asm: bool,
 324     pub allow_log_syntax: bool,
 325     pub allow_concat_idents: bool,
 326     pub allow_trace_macros: bool,
 327     pub allow_internal_unstable: bool,
 328     pub allow_custom_derive: bool,
 329     pub simd_ffi: bool,
 330     pub unmarked_api: bool,
 331     pub negate_unsigned: bool,
 332     /// spans of #![feature] attrs for stable language features. for error reporting
 333     pub declared_stable_lang_features: Vec<Span>,
 334     /// #![feature] attrs for non-language (library) features
 335     pub declared_lib_features: Vec<(InternedString, Span)>
 336 }
 337
 338 impl Features {
 339     pub fn new() -> Features {
 340         Features {
 341             unboxed_closures: false,
 342             rustc_diagnostic_macros: false,
 343             visible_private_types: false,
 344             allow_quote: false,
 345             allow_asm: false,
 346             allow_log_syntax: false,
 347             allow_concat_idents: false,
 348             allow_trace_macros: false,
 349             allow_internal_unstable: false,
 350             allow_custom_derive: false,
 351             simd_ffi: false,
 352             unmarked_api: false,
 353             negate_unsigned: false,
 354             declared_stable_lang_features: Vec::new(),
 355             declared_lib_features: Vec::new()
 356         }
 357     }
 358 }
 359
 360 struct Context<'a> {
 361     features: Vec<&'static str>,
 362     span_handler: &'a SpanHandler,
 363     cm: &'a CodeMap,
 364 }
 365
 366 impl<'a> Context<'a> {
 367     fn gate_feature(&self, feature: &str, span: Span, explain: &str) {
 368         let has_feature = self.has_feature(feature);
 369         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", feature, span, has_feature);
 370         if !has_feature {
 371             emit_feature_err(self.span_handler, feature, span, explain);
 372         }
 373     }
 374     fn has_feature(&self, feature: &str) -> bool {
 375         self.features.iter().any(|&n| n == feature)
 376     }
 377
 378     fn check_attribute(&self, attr: &ast::Attribute) {
 379         debug!("check_attribute(attr = {:?})", attr);
 380         let name = &*attr.name();
 381         for &(n, ty) in KNOWN_ATTRIBUTES {
 382             if n == name {
 383                 if let Gated(gate, desc) = ty {
 384                     self.gate_feature(gate, attr.span, desc);
 385                 }
 386                 debug!("check_attribute: {:?} is known, {:?}", name, ty);
 387                 return;
 388             }
 389         }
 390         if name.starts_with("rustc_") {
 391             self.gate_feature("rustc_attrs", attr.span,
 392                               "unless otherwise specified, attributes \
 393                                with the prefix `rustc_` \
 394                                are reserved for internal compiler diagnostics");
 395         } else if name.starts_with("derive_") {
 396             self.gate_feature("custom_derive", attr.span,
 397                               "attributes of the form `#[derive_*]` are reserved
 398                                for the compiler");
 399         } else {
 400             self.gate_feature("custom_attribute", attr.span,
 401                        &format!("The attribute `{}` is currently \
 402                                 unknown to the compiler and \
 403                                 may have meaning \
 404                                 added to it in the future",
 405                                 name));
 406         }
 407     }
 408 }
 409
 410 pub fn emit_feature_err(diag: &SpanHandler, feature: &str, span: Span, explain: &str) {
 411     diag.span_err(span, explain);
 412
 413     // #23973: do not suggest `#![feature(...)]` if we are in beta/stable
 414     if option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some() { return; }
 415     diag.fileline_help(span, &format!("add #![feature({})] to the \
 416                                    crate attributes to enable",
 417                                   feature));
 418 }
 419
 420 pub fn emit_feature_warn(diag: &SpanHandler, feature: &str, span: Span, explain: &str) {
 421     diag.span_warn(span, explain);
 422
 423     // #23973: do not suggest `#![feature(...)]` if we are in beta/stable
 424     if option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some() { return; }
 425     if diag.handler.can_emit_warnings {
 426         diag.fileline_help(span, &format!("add #![feature({})] to the \
 427                                        crate attributes to silence this warning",
 428                                       feature));
 429     }
 430 }
 431
 432 pub const EXPLAIN_ASM: &'static str =
 433     "inline assembly is not stable enough for use and is subject to change";
 434
 435 pub const EXPLAIN_LOG_SYNTAX: &'static str =
 436     "`log_syntax!` is not stable enough for use and is subject to change";
 437
 438 pub const EXPLAIN_CONCAT_IDENTS: &'static str =
 439     "`concat_idents` is not stable enough for use and is subject to change";
 440
 441 pub const EXPLAIN_TRACE_MACROS: &'static str =
 442     "`trace_macros` is not stable enough for use and is subject to change";
 443 pub const EXPLAIN_ALLOW_INTERNAL_UNSTABLE: &'static str =
 444     "allow_internal_unstable side-steps feature gating and stability checks";
 445
 446 pub const EXPLAIN_CUSTOM_DERIVE: &'static str =
 447     "`#[derive]` for custom traits is not stable enough for use and is subject to change";
 448
 449 struct MacroVisitor<'a> {
 450     context: &'a Context<'a>
 451 }
 452
 453 impl<'a, 'v> Visitor<'v> for MacroVisitor<'a> {
 454     fn visit_mac(&mut self, mac: &ast::Mac) {
 455         let ast::MacInvocTT(ref path, _, _) = mac.node;
 456         let id = path.segments.last().unwrap().identifier;
 457
 458         // Issue 22234: If you add a new case here, make sure to also
 459         // add code to catch the macro during or after expansion.
 460         //
 461         // We still keep this MacroVisitor (rather than *solely*
 462         // relying on catching cases during or after expansion) to
 463         // catch uses of these macros within conditionally-compiled
 464         // code, e.g. `#[cfg]`-guarded functions.
 465
 466         if id == token::str_to_ident("asm") {
 467             self.context.gate_feature("asm", path.span, EXPLAIN_ASM);
 468         }
 469
 470         else if id == token::str_to_ident("log_syntax") {
 471             self.context.gate_feature("log_syntax", path.span, EXPLAIN_LOG_SYNTAX);
 472         }
 473
 474         else if id == token::str_to_ident("trace_macros") {
 475             self.context.gate_feature("trace_macros", path.span, EXPLAIN_TRACE_MACROS);
 476         }
 477
 478         else if id == token::str_to_ident("concat_idents") {
 479             self.context.gate_feature("concat_idents", path.span, EXPLAIN_CONCAT_IDENTS);
 480         }
 481     }
 482
 483     fn visit_attribute(&mut self, attr: &'v ast::Attribute) {
 484         self.context.check_attribute(attr);
 485     }
 486 }
 487
 488 struct PostExpansionVisitor<'a> {
 489     context: &'a Context<'a>
 490 }
 491
 492 impl<'a> PostExpansionVisitor<'a> {
 493     fn gate_feature(&self, feature: &str, span: Span, explain: &str) {
 494         if !self.context.cm.span_allows_unstable(span) {
 495             self.context.gate_feature(feature, span, explain)
 496         }
 497     }
 498 }
 499
 500 impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
 501     fn visit_attribute(&mut self, attr: &ast::Attribute) {
 502         if !self.context.cm.span_allows_unstable(attr.span) {
 503             self.context.check_attribute(attr);
 504         }
 505     }
 506
 507     fn visit_name(&mut self, sp: Span, name: ast::Name) {
 508         if !token::get_name(name).is_ascii() {
 509             self.gate_feature("non_ascii_idents", sp,
 510                               "non-ascii idents are not fully supported.");
 511         }
 512     }
 513
 514     fn visit_item(&mut self, i: &ast::Item) {
 515         match i.node {
 516             ast::ItemExternCrate(_) => {
 517                 if attr::contains_name(&i.attrs[..], "macro_reexport") {
 518                     self.gate_feature("macro_reexport", i.span,
 519                                       "macros reexports are experimental \
 520                                        and possibly buggy");
 521                 }
 522             }
 523
 524             ast::ItemForeignMod(ref foreign_module) => {
 525                 if attr::contains_name(&i.attrs[..], "link_args") {
 526                     self.gate_feature("link_args", i.span,
 527                                       "the `link_args` attribute is not portable \
 528                                        across platforms, it is recommended to \
 529                                        use `#[link(name = \"foo\")]` instead")
 530                 }
 531                 if foreign_module.abi == Abi::RustIntrinsic {
 532                     self.gate_feature("intrinsics",
 533                                       i.span,
 534                                       "intrinsics are subject to change")
 535                 }
 536             }
 537
 538             ast::ItemFn(..) => {
 539                 if attr::contains_name(&i.attrs[..], "plugin_registrar") {
 540                     self.gate_feature("plugin_registrar", i.span,
 541                                       "compiler plugins are experimental and possibly buggy");
 542                 }
 543                 if attr::contains_name(&i.attrs[..], "start") {
 544                     self.gate_feature("start", i.span,
 545                                       "a #[start] function is an experimental \
 546                                        feature whose signature may change \
 547                                        over time");
 548                 }
 549                 if attr::contains_name(&i.attrs[..], "main") {
 550                     self.gate_feature("main", i.span,
 551                                       "declaration of a nonstandard #[main] \
 552                                        function may change over time, for now \
 553                                        a top-level `fn main()` is required");
 554                 }
 555             }
 556
 557             ast::ItemStruct(..) => {
 558                 if attr::contains_name(&i.attrs[..], "simd") {
 559                     self.gate_feature("simd", i.span,
 560                                       "SIMD types are experimental and possibly buggy");
 561                 }
 562             }
 563
 564             ast::ItemDefaultImpl(..) => {
 565                 self.gate_feature("optin_builtin_traits",
 566                                   i.span,
 567                                   "default trait implementations are experimental \
 568                                    and possibly buggy");
 569             }
 570
 571             ast::ItemImpl(_, polarity, _, _, _, _) => {
 572                 match polarity {
 573                     ast::ImplPolarity::Negative => {
 574                         self.gate_feature("optin_builtin_traits",
 575                                           i.span,
 576                                           "negative trait bounds are not yet fully implemented; \
 577                                           use marker types for now");
 578                     },
 579                     _ => {}
 580                 }
 581             }
 582
 583             _ => {}
 584         }
 585
 586         visit::walk_item(self, i);
 587     }
 588
 589     fn visit_foreign_item(&mut self, i: &ast::ForeignItem) {
 590         let links_to_llvm = match attr::first_attr_value_str_by_name(&i.attrs,
 591                                                                      "link_name") {
 592             Some(val) => val.starts_with("llvm."),
 593             _ => false
 594         };
 595         if links_to_llvm {
 596             self.gate_feature("link_llvm_intrinsics", i.span,
 597                               "linking to LLVM intrinsics is experimental");
 598         }
 599
 600         visit::walk_foreign_item(self, i)
 601     }
 602
 603     fn visit_expr(&mut self, e: &ast::Expr) {
 604         match e.node {
 605             ast::ExprBox(..) | ast::ExprUnary(ast::UnOp::UnUniq, _) => {
 606                 self.gate_feature("box_syntax",
 607                                   e.span,
 608                                   "box expression syntax is experimental; \
 609                                    you can call `Box::new` instead.");
 610             }
 611             _ => {}
 612         }
 613         visit::walk_expr(self, e);
 614     }
 615
 616     fn visit_pat(&mut self, pattern: &ast::Pat) {
 617         match pattern.node {
 618             ast::PatVec(_, Some(_), ref last) if !last.is_empty() => {
 619                 self.gate_feature("advanced_slice_patterns",
 620                                   pattern.span,
 621                                   "multiple-element slice matches anywhere \
 622                                    but at the end of a slice (e.g. \
 623                                    `[0, ..xs, 0]` are experimental")
 624             }
 625             ast::PatVec(..) => {
 626                 self.gate_feature("slice_patterns",
 627                                   pattern.span,
 628                                   "slice pattern syntax is experimental");
 629             }
 630             ast::PatBox(..) => {
 631                 self.gate_feature("box_patterns",
 632                                   pattern.span,
 633                                   "box pattern syntax is experimental");
 634             }
 635             _ => {}
 636         }
 637         visit::walk_pat(self, pattern)
 638     }
 639
 640     fn visit_fn(&mut self,
 641                 fn_kind: visit::FnKind<'v>,
 642                 fn_decl: &'v ast::FnDecl,
 643                 block: &'v ast::Block,
 644                 span: Span,
 645                 _node_id: NodeId) {
 646         match fn_kind {
 647             visit::FkItemFn(_, _, _, abi, _) if abi == Abi::RustIntrinsic => {
 648                 self.gate_feature("intrinsics",
 649                                   span,
 650                                   "intrinsics are subject to change")
 651             }
 652             visit::FkItemFn(_, _, _, abi, _) |
 653             visit::FkMethod(_, &ast::MethodSig { abi, .. }, _) if abi == Abi::RustCall => {
 654                 self.gate_feature("unboxed_closures",
 655                                   span,
 656                                   "rust-call ABI is subject to change")
 657             }
 658             _ => {}
 659         }
 660         visit::walk_fn(self, fn_kind, fn_decl, block, span);
 661     }
 662 }
 663
 664 fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
 665                         krate: &ast::Crate,
 666                         check: F)
 667                        -> Features
 668     where F: FnOnce(&mut Context, &ast::Crate)
 669 {
 670     let mut cx = Context {
 671         features: Vec::new(),
 672         span_handler: span_handler,
 673         cm: cm,
 674     };
 675
 676     let mut accepted_features = Vec::new();
 677     let mut unknown_features = Vec::new();
 678
 679     for attr in &krate.attrs {
 680         if !attr.check_name("feature") {
 681             continue
 682         }
 683
 684         match attr.meta_item_list() {
 685             None => {
 686                 span_handler.span_err(attr.span, "malformed feature attribute, \
 687                                                   expected #![feature(...)]");
 688             }
 689             Some(list) => {
 690                 for mi in list {
 691                     let name = match mi.node {
 692                         ast::MetaWord(ref word) => (*word).clone(),
 693                         _ => {
 694                             span_handler.span_err(mi.span,
 695                                                   "malformed feature, expected just \
 696                                                    one word");
 697                             continue
 698                         }
 699                     };
 700                     match KNOWN_FEATURES.iter()
 701                                         .find(|& &(n, _, _)| name == n) {
 702                         Some(&(name, _, Active)) => {
 703                             cx.features.push(name);
 704                         }
 705                         Some(&(_, _, Removed)) => {
 706                             span_handler.span_err(mi.span, "feature has been removed");
 707                         }
 708                         Some(&(_, _, Accepted)) => {
 709                             accepted_features.push(mi.span);
 710                         }
 711                         None => {
 712                             unknown_features.push((name, mi.span));
 713                         }
 714                     }
 715                 }
 716             }
 717         }
 718     }
 719
 720     check(&mut cx, krate);
 721
 722     // FIXME (pnkfelix): Before adding the 99th entry below, change it
 723     // to a single-pass (instead of N calls to `.has_feature`).
 724
 725     Features {
 726         unboxed_closures: cx.has_feature("unboxed_closures"),
 727         rustc_diagnostic_macros: cx.has_feature("rustc_diagnostic_macros"),
 728         visible_private_types: cx.has_feature("visible_private_types"),
 729         allow_quote: cx.has_feature("quote"),
 730         allow_asm: cx.has_feature("asm"),
 731         allow_log_syntax: cx.has_feature("log_syntax"),
 732         allow_concat_idents: cx.has_feature("concat_idents"),
 733         allow_trace_macros: cx.has_feature("trace_macros"),
 734         allow_internal_unstable: cx.has_feature("allow_internal_unstable"),
 735         allow_custom_derive: cx.has_feature("custom_derive"),
 736         simd_ffi: cx.has_feature("simd_ffi"),
 737         unmarked_api: cx.has_feature("unmarked_api"),
 738         negate_unsigned: cx.has_feature("negate_unsigned"),
 739         declared_stable_lang_features: accepted_features,
 740         declared_lib_features: unknown_features
 741     }
 742 }
 743
 744 pub fn check_crate_macros(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast::Crate)
 745 -> Features {
 746     check_crate_inner(cm, span_handler, krate,
 747                       |ctx, krate| visit::walk_crate(&mut MacroVisitor { context: ctx }, krate))
 748 }
 749
 750 pub fn check_crate(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast::Crate)
 751                    -> Features
 752 {
 753     check_crate_inner(cm, span_handler, krate,
 754                       |ctx, krate| visit::walk_crate(&mut PostExpansionVisitor { context: ctx },
 755                                                      krate))
 756 }