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 modules 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::RustIntrinsic;
  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     ("unsafe_destructor", "1.0.0", Active),
  62     ("intrinsics", "1.0.0", Active),
  63     ("lang_items", "1.0.0", Active),
  64
  65     ("simd", "1.0.0", Active),
  66     ("default_type_params", "1.0.0", Accepted),
  67     ("quote", "1.0.0", Active),
  68     ("link_llvm_intrinsics", "1.0.0", Active),
  69     ("linkage", "1.0.0", Active),
  70     ("struct_inherit", "1.0.0", Removed),
  71
  72     ("quad_precision_float", "1.0.0", Removed),
  73
  74     ("rustc_diagnostic_macros", "1.0.0", Active),
  75     ("unboxed_closures", "1.0.0", Active),
  76     ("reflect", "1.0.0", Active),
  77     ("import_shadowing", "1.0.0", Removed),
  78     ("advanced_slice_patterns", "1.0.0", Active),
  79     ("tuple_indexing", "1.0.0", Accepted),
  80     ("associated_types", "1.0.0", Accepted),
  81     ("visible_private_types", "1.0.0", Active),
  82     ("slicing_syntax", "1.0.0", Accepted),
  83     ("box_syntax", "1.0.0", Active),
  84     ("on_unimplemented", "1.0.0", Active),
  85     ("simd_ffi", "1.0.0", Active),
  86     ("allocator", "1.0.0", Active),
  87
  88     ("if_let", "1.0.0", Accepted),
  89     ("while_let", "1.0.0", Accepted),
  90
  91     ("plugin", "1.0.0", Active),
  92     ("start", "1.0.0", Active),
  93     ("main", "1.0.0", Active),
  94
  95     // A temporary feature gate used to enable parser extensions needed
  96     // to bootstrap fix for #5723.
  97     ("issue_5723_bootstrap", "1.0.0", Accepted),
  98
  99     // A way to temporarily opt out of opt in copy. This will *never* be accepted.
 100     ("opt_out_copy", "1.0.0", Removed),
 101
 102     // A way to temporarily opt out of the new orphan rules. This will *never* be accepted.
 103     ("old_orphan_check", "1.0.0", Deprecated),
 104
 105     // A way to temporarily opt out of the new impl rules. This will *never* be accepted.
 106     ("old_impl_check", "1.0.0", Deprecated),
 107
 108     // OIBIT specific features
 109     ("optin_builtin_traits", "1.0.0", Active),
 110
 111     // macro reexport needs more discussion and stabilization
 112     ("macro_reexport", "1.0.0", Active),
 113
 114     // These are used to test this portion of the compiler, they don't actually
 115     // mean anything
 116     ("test_accepted_feature", "1.0.0", Accepted),
 117     ("test_removed_feature", "1.0.0", Removed),
 118
 119     // Allows use of #[staged_api]
 120     ("staged_api", "1.0.0", Active),
 121
 122     // Allows using items which are missing stability attributes
 123     ("unmarked_api", "1.0.0", Active),
 124
 125     // Allows using #![no_std]
 126     ("no_std", "1.0.0", Active),
 127
 128     // Allows using `box` in patterns; RFC 469
 129     ("box_patterns", "1.0.0", Active),
 130
 131     // Allows using the unsafe_no_drop_flag attribute (unlikely to
 132     // switch to Accepted; see RFC 320)
 133     ("unsafe_no_drop_flag", "1.0.0", Active),
 134
 135     // Allows the use of custom attributes; RFC 572
 136     ("custom_attribute", "1.0.0", Active),
 137
 138     // Allows the use of #[derive(Anything)] as sugar for
 139     // #[derive_Anything].
 140     ("custom_derive", "1.0.0", Active),
 141
 142     // Allows the use of rustc_* attributes; RFC 572
 143     ("rustc_attrs", "1.0.0", Active),
 144
 145     // Allows the use of `static_assert`
 146     ("static_assert", "1.0.0", Active),
 147
 148     // Allows the use of #[allow_internal_unstable]. This is an
 149     // attribute on macro_rules! and can't use the attribute handling
 150     // below (it has to be checked before expansion possibly makes
 151     // macros disappear).
 152     ("allow_internal_unstable", "1.0.0", Active),
 153 ];
 154 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 155
 156 enum Status {
 157     /// Represents an active feature that is currently being implemented or
 158     /// currently being considered for addition/removal.
 159     Active,
 160
 161     /// Represents a feature gate that is temporarily enabling deprecated behavior.
 162     /// This gate will never be accepted.
 163     Deprecated,
 164
 165     /// Represents a feature which has since been removed (it was once Active)
 166     Removed,
 167
 168     /// This language feature has since been Accepted (it was once Active)
 169     Accepted,
 170 }
 171
 172 // Attributes that have a special meaning to rustc or rustdoc
 173 pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType)] = &[
 174     // Normal attributes
 175
 176     ("warn", Normal),
 177     ("allow", Normal),
 178     ("forbid", Normal),
 179     ("deny", Normal),
 180
 181     ("macro_reexport", Normal),
 182     ("macro_use", Normal),
 183     ("macro_export", Normal),
 184     ("plugin_registrar", Normal),
 185
 186     ("cfg", Normal),
 187     ("cfg_attr", Normal),
 188     ("main", Normal),
 189     ("start", Normal),
 190     ("test", Normal),
 191     ("bench", Normal),
 192     ("simd", Normal),
 193     ("repr", Normal),
 194     ("path", Normal),
 195     ("abi", Normal),
 196     ("unsafe_destructor", 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
 209     ("staged_api", Gated("staged_api",
 210                          "staged_api is for use by rustc only")),
 211     ("plugin", Gated("plugin",
 212                      "compiler plugins are experimental \
 213                       and possibly buggy")),
 214     ("no_std", Gated("no_std",
 215                      "no_std is experimental")),
 216     ("lang", Gated("lang_items",
 217                      "language items are subject to change")),
 218     ("linkage", Gated("linkage",
 219                       "the `linkage` attribute is experimental \
 220                        and not portable across platforms")),
 221     ("thread_local", Gated("thread_local",
 222                             "`#[thread_local]` is an experimental feature, and does not \
 223                              currently handle destructors. There is no corresponding \
 224                              `#[task_local]` mapping to the task model")),
 225
 226     ("rustc_on_unimplemented", Gated("on_unimplemented",
 227                                      "the `#[rustc_on_unimplemented]` attribute \
 228                                       is an experimental feature")),
 229     ("allocator", Gated("allocator",
 230                         "the `#[allocator]` attribute is an experimental feature")),
 231     ("rustc_variance", Gated("rustc_attrs",
 232                              "the `#[rustc_variance]` attribute \
 233                               is an experimental feature")),
 234     ("rustc_error", Gated("rustc_attrs",
 235                           "the `#[rustc_error]` attribute \
 236                            is an experimental feature")),
 237     ("rustc_move_fragments", Gated("rustc_attrs",
 238                                    "the `#[rustc_move_fragments]` attribute \
 239                                     is an experimental feature")),
 240
 241     ("allow_internal_unstable", Gated("allow_internal_unstable",
 242                                       EXPLAIN_ALLOW_INTERNAL_UNSTABLE)),
 243
 244     // FIXME: #14408 whitelist docs since rustdoc looks at them
 245     ("doc", Whitelisted),
 246
 247     // FIXME: #14406 these are processed in trans, which happens after the
 248     // lint pass
 249     ("cold", Whitelisted),
 250     ("export_name", Whitelisted),
 251     ("inline", Whitelisted),
 252     ("link", Whitelisted),
 253     ("link_name", Whitelisted),
 254     ("link_section", Whitelisted),
 255     ("no_builtins", Whitelisted),
 256     ("no_mangle", Whitelisted),
 257     ("no_stack_check", Whitelisted),
 258     ("packed", Whitelisted),
 259     ("static_assert", Gated("static_assert",
 260                             "`#[static_assert]` is an experimental feature, and has a poor API")),
 261     ("no_debug", Whitelisted),
 262     ("omit_gdb_pretty_printer_section", Whitelisted),
 263     ("unsafe_no_drop_flag", Gated("unsafe_no_drop_flag",
 264                                   "unsafe_no_drop_flag has unstable semantics \
 265                                    and may be removed in the future")),
 266
 267     // used in resolve
 268     ("prelude_import", Whitelisted),
 269
 270     // FIXME: #14407 these are only looked at on-demand so we can't
 271     // guarantee they'll have already been checked
 272     ("deprecated", Whitelisted),
 273     ("must_use", Whitelisted),
 274     ("stable", Whitelisted),
 275     ("unstable", Whitelisted),
 276
 277     // FIXME: #19470 this shouldn't be needed forever
 278     ("old_orphan_check", Whitelisted),
 279     ("old_impl_check", Whitelisted),
 280
 281     ("rustc_paren_sugar", Gated("unboxed_closures",
 282                                 "unboxed_closures are still evolving")),
 283     ("rustc_reflect_like", Gated("reflect",
 284                                  "defining reflective traits is still evolving")),
 285
 286     // Crate level attributes
 287     ("crate_name", CrateLevel),
 288     ("crate_type", CrateLevel),
 289     ("crate_id", CrateLevel),
 290     ("feature", CrateLevel),
 291     ("no_start", CrateLevel),
 292     ("no_main", CrateLevel),
 293     ("no_builtins", CrateLevel),
 294     ("recursion_limit", CrateLevel),
 295 ];
 296
 297 #[derive(PartialEq, Copy, Debug)]
 298 pub enum AttributeType {
 299     /// Normal, builtin attribute that is consumed
 300     /// by the compiler before the unused_attribute check
 301     Normal,
 302
 303     /// Builtin attribute that may not be consumed by the compiler
 304     /// before the unused_attribute check. These attributes
 305     /// will be ignored by the unused_attribute lint
 306     Whitelisted,
 307
 308     /// Is gated by a given feature gate and reason
 309     /// These get whitelisted too
 310     Gated(&'static str, &'static str),
 311
 312     /// Builtin attribute that is only allowed at the crate level
 313     CrateLevel,
 314 }
 315
 316 /// A set of features to be used by later passes.
 317 pub struct Features {
 318     pub unboxed_closures: bool,
 319     pub rustc_diagnostic_macros: bool,
 320     pub visible_private_types: bool,
 321     pub allow_quote: bool,
 322     pub allow_asm: bool,
 323     pub allow_log_syntax: bool,
 324     pub allow_concat_idents: bool,
 325     pub allow_trace_macros: bool,
 326     pub allow_internal_unstable: bool,
 327     pub allow_custom_derive: bool,
 328     pub old_orphan_check: bool,
 329     pub simd_ffi: bool,
 330     pub unmarked_api: bool,
 331     /// spans of #![feature] attrs for stable language features. for error reporting
 332     pub declared_stable_lang_features: Vec<Span>,
 333     /// #![feature] attrs for non-language (library) features
 334     pub declared_lib_features: Vec<(InternedString, Span)>
 335 }
 336
 337 impl Features {
 338     pub fn new() -> Features {
 339         Features {
 340             unboxed_closures: false,
 341             rustc_diagnostic_macros: false,
 342             visible_private_types: false,
 343             allow_quote: false,
 344             allow_asm: false,
 345             allow_log_syntax: false,
 346             allow_concat_idents: false,
 347             allow_trace_macros: false,
 348             allow_internal_unstable: false,
 349             allow_custom_derive: false,
 350             old_orphan_check: false,
 351             simd_ffi: false,
 352             unmarked_api: false,
 353             declared_stable_lang_features: Vec::new(),
 354             declared_lib_features: Vec::new()
 355         }
 356     }
 357 }
 358
 359 struct Context<'a> {
 360     features: Vec<&'static str>,
 361     span_handler: &'a SpanHandler,
 362     cm: &'a CodeMap,
 363 }
 364
 365 impl<'a> Context<'a> {
 366     fn gate_feature(&self, feature: &str, span: Span, explain: &str) {
 367         let has_feature = self.has_feature(feature);
 368         debug!("gate_feature(feature = {:?}, span = {:?}); has? {}", feature, span, has_feature);
 369         if !has_feature {
 370             emit_feature_err(self.span_handler, feature, span, explain);
 371         }
 372     }
 373     fn has_feature(&self, feature: &str) -> bool {
 374         self.features.iter().any(|&n| n == feature)
 375     }
 376
 377     fn check_attribute(&self, attr: &ast::Attribute) {
 378         debug!("check_attribute(attr = {:?})", attr);
 379         let name = &*attr.name();
 380         for &(n, ty) in KNOWN_ATTRIBUTES {
 381             if n == name {
 382                 if let Gated(gate, desc) = ty {
 383                     self.gate_feature(gate, attr.span, desc);
 384                 }
 385                 debug!("check_attribute: {:?} is known, {:?}", name, ty);
 386                 return;
 387             }
 388         }
 389         if name.starts_with("rustc_") {
 390             self.gate_feature("rustc_attrs", attr.span,
 391                               "unless otherwise specified, attributes \
 392                                with the prefix `rustc_` \
 393                                are reserved for internal compiler diagnostics");
 394         } else if name.starts_with("derive_") {
 395             self.gate_feature("custom_derive", attr.span,
 396                               "attributes of the form `#[derive_*]` are reserved
 397                                for the compiler");
 398         } else {
 399             self.gate_feature("custom_attribute", attr.span,
 400                        &format!("The attribute `{}` is currently \
 401                                 unknown to the the compiler and \
 402                                 may have meaning \
 403                                 added to it in the future",
 404                                 name));
 405         }
 406     }
 407 }
 408
 409 pub fn emit_feature_err(diag: &SpanHandler, feature: &str, span: Span, explain: &str) {
 410     diag.span_err(span, explain);
 411     diag.fileline_help(span, &format!("add #![feature({})] to the \
 412                                    crate attributes to enable",
 413                                   feature));
 414 }
 415
 416 pub fn emit_feature_warn(diag: &SpanHandler, feature: &str, span: Span, explain: &str) {
 417     diag.span_warn(span, explain);
 418     if diag.handler.can_emit_warnings {
 419         diag.fileline_help(span, &format!("add #![feature({})] to the \
 420                                        crate attributes to silence this warning",
 421                                       feature));
 422     }
 423 }
 424
 425 pub const EXPLAIN_ASM: &'static str =
 426     "inline assembly is not stable enough for use and is subject to change";
 427
 428 pub const EXPLAIN_LOG_SYNTAX: &'static str =
 429     "`log_syntax!` is not stable enough for use and is subject to change";
 430
 431 pub const EXPLAIN_CONCAT_IDENTS: &'static str =
 432     "`concat_idents` is not stable enough for use and is subject to change";
 433
 434 pub const EXPLAIN_TRACE_MACROS: &'static str =
 435     "`trace_macros` is not stable enough for use and is subject to change";
 436 pub const EXPLAIN_ALLOW_INTERNAL_UNSTABLE: &'static str =
 437     "allow_internal_unstable side-steps feature gating and stability checks";
 438
 439 pub const EXPLAIN_CUSTOM_DERIVE: &'static str =
 440     "`#[derive]` for custom traits is not stable enough for use and is subject to change";
 441
 442 struct MacroVisitor<'a> {
 443     context: &'a Context<'a>
 444 }
 445
 446 impl<'a, 'v> Visitor<'v> for MacroVisitor<'a> {
 447     fn visit_mac(&mut self, mac: &ast::Mac) {
 448         let ast::MacInvocTT(ref path, _, _) = mac.node;
 449         let id = path.segments.last().unwrap().identifier;
 450
 451         // Issue 22234: If you add a new case here, make sure to also
 452         // add code to catch the macro during or after expansion.
 453         //
 454         // We still keep this MacroVisitor (rather than *solely*
 455         // relying on catching cases during or after expansion) to
 456         // catch uses of these macros within conditionally-compiled
 457         // code, e.g. `#[cfg]`-guarded functions.
 458
 459         if id == token::str_to_ident("asm") {
 460             self.context.gate_feature("asm", path.span, EXPLAIN_ASM);
 461         }
 462
 463         else if id == token::str_to_ident("log_syntax") {
 464             self.context.gate_feature("log_syntax", path.span, EXPLAIN_LOG_SYNTAX);
 465         }
 466
 467         else if id == token::str_to_ident("trace_macros") {
 468             self.context.gate_feature("trace_macros", path.span, EXPLAIN_TRACE_MACROS);
 469         }
 470
 471         else if id == token::str_to_ident("concat_idents") {
 472             self.context.gate_feature("concat_idents", path.span, EXPLAIN_CONCAT_IDENTS);
 473         }
 474     }
 475
 476     fn visit_attribute(&mut self, attr: &'v ast::Attribute) {
 477         self.context.check_attribute(attr);
 478     }
 479 }
 480
 481 struct PostExpansionVisitor<'a> {
 482     context: &'a Context<'a>
 483 }
 484
 485 impl<'a> PostExpansionVisitor<'a> {
 486     fn gate_feature(&self, feature: &str, span: Span, explain: &str) {
 487         if !self.context.cm.span_allows_unstable(span) {
 488             self.context.gate_feature(feature, span, explain)
 489         }
 490     }
 491 }
 492
 493 impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
 494     fn visit_attribute(&mut self, attr: &ast::Attribute) {
 495         if !self.context.cm.span_allows_unstable(attr.span) {
 496             self.context.check_attribute(attr);
 497         }
 498     }
 499
 500     fn visit_name(&mut self, sp: Span, name: ast::Name) {
 501         if !token::get_name(name).is_ascii() {
 502             self.gate_feature("non_ascii_idents", sp,
 503                               "non-ascii idents are not fully supported.");
 504         }
 505     }
 506
 507     fn visit_item(&mut self, i: &ast::Item) {
 508         match i.node {
 509             ast::ItemExternCrate(_) => {
 510                 if attr::contains_name(&i.attrs[..], "macro_reexport") {
 511                     self.gate_feature("macro_reexport", i.span,
 512                                       "macros reexports are experimental \
 513                                        and possibly buggy");
 514                 }
 515             }
 516
 517             ast::ItemForeignMod(ref foreign_module) => {
 518                 if attr::contains_name(&i.attrs[..], "link_args") {
 519                     self.gate_feature("link_args", i.span,
 520                                       "the `link_args` attribute is not portable \
 521                                        across platforms, it is recommended to \
 522                                        use `#[link(name = \"foo\")]` instead")
 523                 }
 524                 if foreign_module.abi == RustIntrinsic {
 525                     self.gate_feature("intrinsics",
 526                                       i.span,
 527                                       "intrinsics are subject to change")
 528                 }
 529             }
 530
 531             ast::ItemFn(..) => {
 532                 if attr::contains_name(&i.attrs[..], "plugin_registrar") {
 533                     self.gate_feature("plugin_registrar", i.span,
 534                                       "compiler plugins are experimental and possibly buggy");
 535                 }
 536                 if attr::contains_name(&i.attrs[..], "start") {
 537                     self.gate_feature("start", i.span,
 538                                       "a #[start] function is an experimental \
 539                                        feature whose signature may change \
 540                                        over time");
 541                 }
 542                 if attr::contains_name(&i.attrs[..], "main") {
 543                     self.gate_feature("main", i.span,
 544                                       "declaration of a nonstandard #[main] \
 545                                        function may change over time, for now \
 546                                        a top-level `fn main()` is required");
 547                 }
 548             }
 549
 550             ast::ItemStruct(..) => {
 551                 if attr::contains_name(&i.attrs[..], "simd") {
 552                     self.gate_feature("simd", i.span,
 553                                       "SIMD types are experimental and possibly buggy");
 554                 }
 555             }
 556
 557             ast::ItemDefaultImpl(..) => {
 558                 self.gate_feature("optin_builtin_traits",
 559                                   i.span,
 560                                   "default trait implementations are experimental \
 561                                    and possibly buggy");
 562             }
 563
 564             ast::ItemImpl(_, polarity, _, _, _, _) => {
 565                 match polarity {
 566                     ast::ImplPolarity::Negative => {
 567                         self.gate_feature("optin_builtin_traits",
 568                                           i.span,
 569                                           "negative trait bounds are not yet fully implemented; \
 570                                           use marker types for now");
 571                     },
 572                     _ => {}
 573                 }
 574
 575                 if attr::contains_name(&i.attrs,
 576                                        "unsafe_destructor") {
 577                     self.gate_feature("unsafe_destructor",
 578                                       i.span,
 579                                       "`#[unsafe_destructor]` allows too \
 580                                        many unsafe patterns and may be \
 581                                        removed in the future");
 582                 }
 583
 584                 if attr::contains_name(&i.attrs[..],
 585                                        "old_orphan_check") {
 586                     self.gate_feature(
 587                         "old_orphan_check",
 588                         i.span,
 589                         "the new orphan check rules will eventually be strictly enforced");
 590                 }
 591
 592                 if attr::contains_name(&i.attrs[..],
 593                                        "old_impl_check") {
 594                     self.gate_feature("old_impl_check",
 595                                       i.span,
 596                                       "`#[old_impl_check]` will be removed in the future");
 597                 }
 598             }
 599
 600             _ => {}
 601         }
 602
 603         visit::walk_item(self, i);
 604     }
 605
 606     fn visit_foreign_item(&mut self, i: &ast::ForeignItem) {
 607         let links_to_llvm = match attr::first_attr_value_str_by_name(&i.attrs,
 608                                                                      "link_name") {
 609             Some(val) => val.starts_with("llvm."),
 610             _ => false
 611         };
 612         if links_to_llvm {
 613             self.gate_feature("link_llvm_intrinsics", i.span,
 614                               "linking to LLVM intrinsics is experimental");
 615         }
 616
 617         visit::walk_foreign_item(self, i)
 618     }
 619
 620     fn visit_expr(&mut self, e: &ast::Expr) {
 621         match e.node {
 622             ast::ExprBox(..) | ast::ExprUnary(ast::UnOp::UnUniq, _) => {
 623                 self.gate_feature("box_syntax",
 624                                   e.span,
 625                                   "box expression syntax is experimental; \
 626                                    you can call `Box::new` instead.");
 627             }
 628             _ => {}
 629         }
 630         visit::walk_expr(self, e);
 631     }
 632
 633     fn visit_pat(&mut self, pattern: &ast::Pat) {
 634         match pattern.node {
 635             ast::PatVec(_, Some(_), ref last) if !last.is_empty() => {
 636                 self.gate_feature("advanced_slice_patterns",
 637                                   pattern.span,
 638                                   "multiple-element slice matches anywhere \
 639                                    but at the end of a slice (e.g. \
 640                                    `[0, ..xs, 0]` are experimental")
 641             }
 642             ast::PatBox(..) => {
 643                 self.gate_feature("box_patterns",
 644                                   pattern.span,
 645                                   "box pattern syntax is experimental");
 646             }
 647             _ => {}
 648         }
 649         visit::walk_pat(self, pattern)
 650     }
 651
 652     fn visit_fn(&mut self,
 653                 fn_kind: visit::FnKind<'v>,
 654                 fn_decl: &'v ast::FnDecl,
 655                 block: &'v ast::Block,
 656                 span: Span,
 657                 _node_id: NodeId) {
 658         match fn_kind {
 659             visit::FkItemFn(_, _, _, abi) if abi == RustIntrinsic => {
 660                 self.gate_feature("intrinsics",
 661                                   span,
 662                                   "intrinsics are subject to change")
 663             }
 664             _ => {}
 665         }
 666         visit::walk_fn(self, fn_kind, fn_decl, block, span);
 667     }
 668 }
 669
 670 fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
 671                         krate: &ast::Crate,
 672                         check: F)
 673                        -> Features
 674     where F: FnOnce(&mut Context, &ast::Crate)
 675 {
 676     let mut cx = Context {
 677         features: Vec::new(),
 678         span_handler: span_handler,
 679         cm: cm,
 680     };
 681
 682     let mut accepted_features = Vec::new();
 683     let mut unknown_features = Vec::new();
 684
 685     for attr in &krate.attrs {
 686         if !attr.check_name("feature") {
 687             continue
 688         }
 689
 690         match attr.meta_item_list() {
 691             None => {
 692                 span_handler.span_err(attr.span, "malformed feature attribute, \
 693                                                   expected #![feature(...)]");
 694             }
 695             Some(list) => {
 696                 for mi in list {
 697                     let name = match mi.node {
 698                         ast::MetaWord(ref word) => (*word).clone(),
 699                         _ => {
 700                             span_handler.span_err(mi.span,
 701                                                   "malformed feature, expected just \
 702                                                    one word");
 703                             continue
 704                         }
 705                     };
 706                     match KNOWN_FEATURES.iter()
 707                                         .find(|& &(n, _, _)| name == n) {
 708                         Some(&(name, _, Active)) => {
 709                             cx.features.push(name);
 710                         }
 711                         Some(&(name, _, Deprecated)) => {
 712                             cx.features.push(name);
 713                             span_handler.span_warn(
 714                                 mi.span,
 715                                 "feature is deprecated and will only be available \
 716                                  for a limited time, please rewrite code that relies on it");
 717                         }
 718                         Some(&(_, _, Removed)) => {
 719                             span_handler.span_err(mi.span, "feature has been removed");
 720                         }
 721                         Some(&(_, _, Accepted)) => {
 722                             accepted_features.push(mi.span);
 723                         }
 724                         None => {
 725                             unknown_features.push((name, mi.span));
 726                         }
 727                     }
 728                 }
 729             }
 730         }
 731     }
 732
 733     check(&mut cx, krate);
 734
 735     // FIXME (pnkfelix): Before adding the 99th entry below, change it
 736     // to a single-pass (instead of N calls to `.has_feature`).
 737
 738     Features {
 739         unboxed_closures: cx.has_feature("unboxed_closures"),
 740         rustc_diagnostic_macros: cx.has_feature("rustc_diagnostic_macros"),
 741         visible_private_types: cx.has_feature("visible_private_types"),
 742         allow_quote: cx.has_feature("quote"),
 743         allow_asm: cx.has_feature("asm"),
 744         allow_log_syntax: cx.has_feature("log_syntax"),
 745         allow_concat_idents: cx.has_feature("concat_idents"),
 746         allow_trace_macros: cx.has_feature("trace_macros"),
 747         allow_internal_unstable: cx.has_feature("allow_internal_unstable"),
 748         allow_custom_derive: cx.has_feature("custom_derive"),
 749         old_orphan_check: cx.has_feature("old_orphan_check"),
 750         simd_ffi: cx.has_feature("simd_ffi"),
 751         unmarked_api: cx.has_feature("unmarked_api"),
 752         declared_stable_lang_features: accepted_features,
 753         declared_lib_features: unknown_features
 754     }
 755 }
 756
 757 pub fn check_crate_macros(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast::Crate)
 758 -> Features {
 759     check_crate_inner(cm, span_handler, krate,
 760                       |ctx, krate| visit::walk_crate(&mut MacroVisitor { context: ctx }, krate))
 761 }
 762
 763 pub fn check_crate(cm: &CodeMap, span_handler: &SpanHandler, krate: &ast::Crate)
 764                    -> Features
 765 {
 766     check_crate_inner(cm, span_handler, krate,
 767                       |ctx, krate| visit::walk_crate(&mut PostExpansionVisitor { context: ctx },
 768                                                      krate))
 769 }