1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
use rustc_ast as ast;
use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
use rustc_ast::{attr, AssocConstraint, AssocConstraintKind, NodeId};
use rustc_ast::{token, PatKind, RangeEnd};
use rustc_feature::{AttributeGate, BuiltinAttribute, Features, GateIssue, BUILTIN_ATTRIBUTE_MAP};
use rustc_session::parse::{feature_err, feature_err_issue, feature_warn};
use rustc_session::Session;
use rustc_span::source_map::Spanned;
use rustc_span::symbol::sym;
use rustc_span::Span;
use rustc_target::spec::abi;
use thin_vec::ThinVec;

use crate::errors;

/// The common case.
macro_rules! gate {
    ($visitor:expr, $feature:ident, $span:expr, $explain:expr) => {{
        if !$visitor.features.$feature && !$span.allows_unstable(sym::$feature) {
            #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
            feature_err(&$visitor.sess, sym::$feature, $span, $explain).emit();
        }
    }};
    ($visitor:expr, $feature:ident, $span:expr, $explain:expr, $help:expr) => {{
        if !$visitor.features.$feature && !$span.allows_unstable(sym::$feature) {
            // FIXME: make this translatable
            #[allow(rustc::diagnostic_outside_of_impl)]
            #[allow(rustc::untranslatable_diagnostic)]
            feature_err(&$visitor.sess, sym::$feature, $span, $explain).with_help($help).emit();
        }
    }};
}

/// The unusual case, where the `has_feature` condition is non-standard.
macro_rules! gate_alt {
    ($visitor:expr, $has_feature:expr, $name:expr, $span:expr, $explain:expr) => {{
        if !$has_feature && !$span.allows_unstable($name) {
            #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
            feature_err(&$visitor.sess, $name, $span, $explain).emit();
        }
    }};
}

/// The case involving a multispan.
macro_rules! gate_multi {
    ($visitor:expr, $feature:ident, $spans:expr, $explain:expr) => {{
        if !$visitor.features.$feature {
            let spans: Vec<_> =
                $spans.filter(|span| !span.allows_unstable(sym::$feature)).collect();
            if !spans.is_empty() {
                feature_err(&$visitor.sess, sym::$feature, spans, $explain).emit();
            }
        }
    }};
}

/// The legacy case.
macro_rules! gate_legacy {
    ($visitor:expr, $feature:ident, $span:expr, $explain:expr) => {{
        if !$visitor.features.$feature && !$span.allows_unstable(sym::$feature) {
            feature_warn(&$visitor.sess, sym::$feature, $span, $explain);
        }
    }};
}

pub fn check_attribute(attr: &ast::Attribute, sess: &Session, features: &Features) {
    PostExpansionVisitor { sess, features }.visit_attribute(attr)
}

struct PostExpansionVisitor<'a> {
    sess: &'a Session,

    // `sess` contains a `Features`, but this might not be that one.
    features: &'a Features,
}

impl<'a> PostExpansionVisitor<'a> {
    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
    fn check_abi(&self, abi: ast::StrLit, constness: ast::Const) {
        let ast::StrLit { symbol_unescaped, span, .. } = abi;

        if let ast::Const::Yes(_) = constness {
            match symbol_unescaped {
                // Stable
                sym::Rust | sym::C => {}
                abi => gate!(
                    &self,
                    const_extern_fn,
                    span,
                    format!("`{}` as a `const fn` ABI is unstable", abi)
                ),
            }
        }

        match abi::is_enabled(self.features, span, symbol_unescaped.as_str()) {
            Ok(()) => (),
            Err(abi::AbiDisabled::Unstable { feature, explain }) => {
                feature_err_issue(&self.sess, feature, span, GateIssue::Language, explain).emit();
            }
            Err(abi::AbiDisabled::Unrecognized) => {
                if self.sess.opts.pretty.map_or(true, |ppm| ppm.needs_hir()) {
                    self.sess.dcx().span_delayed_bug(
                        span,
                        format!(
                            "unrecognized ABI not caught in lowering: {}",
                            symbol_unescaped.as_str()
                        ),
                    );
                }
            }
        }
    }

    fn check_extern(&self, ext: ast::Extern, constness: ast::Const) {
        if let ast::Extern::Explicit(abi, _) = ext {
            self.check_abi(abi, constness);
        }
    }

    /// Feature gate `impl Trait` inside `type Alias = $type_expr;`.
    fn check_impl_trait(&self, ty: &ast::Ty, in_associated_ty: bool) {
        struct ImplTraitVisitor<'a> {
            vis: &'a PostExpansionVisitor<'a>,
            in_associated_ty: bool,
        }
        impl Visitor<'_> for ImplTraitVisitor<'_> {
            fn visit_ty(&mut self, ty: &ast::Ty) {
                if let ast::TyKind::ImplTrait(..) = ty.kind {
                    if self.in_associated_ty {
                        gate!(
                            &self.vis,
                            impl_trait_in_assoc_type,
                            ty.span,
                            "`impl Trait` in associated types is unstable"
                        );
                    } else {
                        gate!(
                            &self.vis,
                            type_alias_impl_trait,
                            ty.span,
                            "`impl Trait` in type aliases is unstable"
                        );
                    }
                }
                visit::walk_ty(self, ty);
            }
        }
        ImplTraitVisitor { vis: self, in_associated_ty }.visit_ty(ty);
    }

    fn check_late_bound_lifetime_defs(&self, params: &[ast::GenericParam]) {
        // Check only lifetime parameters are present and that the
        // generic parameters that are present have no bounds.
        let non_lt_param_spans = params.iter().filter_map(|param| match param.kind {
            ast::GenericParamKind::Lifetime { .. } => None,
            _ => Some(param.ident.span),
        });
        gate_multi!(
            &self,
            non_lifetime_binders,
            non_lt_param_spans,
            crate::fluent_generated::ast_passes_forbidden_non_lifetime_param
        );

        for param in params {
            if !param.bounds.is_empty() {
                let spans: Vec<_> = param.bounds.iter().map(|b| b.span()).collect();
                self.sess.dcx().emit_err(errors::ForbiddenBound { spans });
            }
        }
    }
}

impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
    fn visit_attribute(&mut self, attr: &ast::Attribute) {
        let attr_info = attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name));
        // Check feature gates for built-in attributes.
        if let Some(BuiltinAttribute {
            gate: AttributeGate::Gated(_, name, descr, has_feature),
            ..
        }) = attr_info
        {
            gate_alt!(self, has_feature(self.features), *name, attr.span, *descr);
        }
        // Check unstable flavors of the `#[doc]` attribute.
        if attr.has_name(sym::doc) {
            for nested_meta in attr.meta_item_list().unwrap_or_default() {
                macro_rules! gate_doc { ($($s:literal { $($name:ident => $feature:ident)* })*) => {
                    $($(if nested_meta.has_name(sym::$name) {
                        let msg = concat!("`#[doc(", stringify!($name), ")]` is ", $s);
                        gate!(self, $feature, attr.span, msg);
                    })*)*
                }}

                gate_doc!(
                    "experimental" {
                        cfg => doc_cfg
                        cfg_hide => doc_cfg_hide
                        masked => doc_masked
                        notable_trait => doc_notable_trait
                    }
                    "meant for internal use only" {
                        keyword => rustdoc_internals
                        fake_variadic => rustdoc_internals
                    }
                );
            }
        }

        // Emit errors for non-staged-api crates.
        if !self.features.staged_api {
            if attr.has_name(sym::unstable)
                || attr.has_name(sym::stable)
                || attr.has_name(sym::rustc_const_unstable)
                || attr.has_name(sym::rustc_const_stable)
                || attr.has_name(sym::rustc_default_body_unstable)
            {
                self.sess.dcx().emit_err(errors::StabilityOutsideStd { span: attr.span });
            }
        }
    }

    fn visit_item(&mut self, i: &'a ast::Item) {
        match &i.kind {
            ast::ItemKind::ForeignMod(foreign_module) => {
                if let Some(abi) = foreign_module.abi {
                    self.check_abi(abi, ast::Const::No);
                }
            }

            ast::ItemKind::Fn(..) => {
                if attr::contains_name(&i.attrs, sym::start) {
                    gate!(
                        &self,
                        start,
                        i.span,
                        "`#[start]` functions are experimental and their signature may change \
                         over time"
                    );
                }
            }

            ast::ItemKind::Struct(..) => {
                for attr in attr::filter_by_name(&i.attrs, sym::repr) {
                    for item in attr.meta_item_list().unwrap_or_else(ThinVec::new) {
                        if item.has_name(sym::simd) {
                            gate!(
                                &self,
                                repr_simd,
                                attr.span,
                                "SIMD types are experimental and possibly buggy"
                            );
                        }
                    }
                }
            }

            ast::ItemKind::Impl(box ast::Impl { polarity, defaultness, of_trait, .. }) => {
                if let &ast::ImplPolarity::Negative(span) = polarity {
                    gate!(
                        &self,
                        negative_impls,
                        span.to(of_trait.as_ref().map_or(span, |t| t.path.span)),
                        "negative trait bounds are not yet fully implemented; \
                         use marker types for now"
                    );
                }

                if let ast::Defaultness::Default(_) = defaultness {
                    gate!(&self, specialization, i.span, "specialization is unstable");
                }
            }

            ast::ItemKind::Trait(box ast::Trait { is_auto: ast::IsAuto::Yes, .. }) => {
                gate!(
                    &self,
                    auto_traits,
                    i.span,
                    "auto traits are experimental and possibly buggy"
                );
            }

            ast::ItemKind::TraitAlias(..) => {
                gate!(&self, trait_alias, i.span, "trait aliases are experimental");
            }

            ast::ItemKind::MacroDef(ast::MacroDef { macro_rules: false, .. }) => {
                let msg = "`macro` is experimental";
                gate!(&self, decl_macro, i.span, msg);
            }

            ast::ItemKind::TyAlias(box ast::TyAlias { ty: Some(ty), .. }) => {
                self.check_impl_trait(ty, false)
            }

            _ => {}
        }

        visit::walk_item(self, i);
    }

    fn visit_foreign_item(&mut self, i: &'a ast::ForeignItem) {
        match i.kind {
            ast::ForeignItemKind::Fn(..) | ast::ForeignItemKind::Static(..) => {
                let link_name = attr::first_attr_value_str_by_name(&i.attrs, sym::link_name);
                let links_to_llvm = link_name.is_some_and(|val| val.as_str().starts_with("llvm."));
                if links_to_llvm {
                    gate!(
                        &self,
                        link_llvm_intrinsics,
                        i.span,
                        "linking to LLVM intrinsics is experimental"
                    );
                }
            }
            ast::ForeignItemKind::TyAlias(..) => {
                gate!(&self, extern_types, i.span, "extern types are experimental");
            }
            ast::ForeignItemKind::MacCall(..) => {}
        }

        visit::walk_foreign_item(self, i)
    }

    fn visit_ty(&mut self, ty: &'a ast::Ty) {
        match &ty.kind {
            ast::TyKind::BareFn(bare_fn_ty) => {
                // Function pointers cannot be `const`
                self.check_extern(bare_fn_ty.ext, ast::Const::No);
                self.check_late_bound_lifetime_defs(&bare_fn_ty.generic_params);
            }
            ast::TyKind::Never => {
                gate!(&self, never_type, ty.span, "the `!` type is experimental");
            }
            _ => {}
        }
        visit::walk_ty(self, ty)
    }

    fn visit_generics(&mut self, g: &'a ast::Generics) {
        for predicate in &g.where_clause.predicates {
            match predicate {
                ast::WherePredicate::BoundPredicate(bound_pred) => {
                    // A type binding, eg `for<'c> Foo: Send+Clone+'c`
                    self.check_late_bound_lifetime_defs(&bound_pred.bound_generic_params);
                }
                _ => {}
            }
        }
        visit::walk_generics(self, g);
    }

    fn visit_fn_ret_ty(&mut self, ret_ty: &'a ast::FnRetTy) {
        if let ast::FnRetTy::Ty(output_ty) = ret_ty {
            if let ast::TyKind::Never = output_ty.kind {
                // Do nothing.
            } else {
                self.visit_ty(output_ty)
            }
        }
    }

    fn visit_generic_args(&mut self, args: &'a ast::GenericArgs) {
        // This check needs to happen here because the never type can be returned from a function,
        // but cannot be used in any other context. If this check was in `visit_fn_ret_ty`, it
        // include both functions and generics like `impl Fn() -> !`.
        if let ast::GenericArgs::Parenthesized(generic_args) = args
            && let ast::FnRetTy::Ty(ref ty) = generic_args.output
            && matches!(ty.kind, ast::TyKind::Never)
        {
            gate!(&self, never_type, ty.span, "the `!` type is experimental");
        }
        visit::walk_generic_args(self, args);
    }

    fn visit_expr(&mut self, e: &'a ast::Expr) {
        match e.kind {
            ast::ExprKind::TryBlock(_) => {
                gate!(&self, try_blocks, e.span, "`try` expression is experimental");
            }
            ast::ExprKind::Lit(token::Lit { kind: token::LitKind::Float, suffix, .. }) => {
                match suffix {
                    Some(sym::f16) => {
                        gate!(&self, f16, e.span, "the type `f16` is unstable")
                    }
                    Some(sym::f128) => {
                        gate!(&self, f128, e.span, "the type `f128` is unstable")
                    }
                    _ => (),
                }
            }
            _ => {}
        }
        visit::walk_expr(self, e)
    }

    fn visit_pat(&mut self, pattern: &'a ast::Pat) {
        match &pattern.kind {
            PatKind::Slice(pats) => {
                for pat in pats {
                    let inner_pat = match &pat.kind {
                        PatKind::Ident(.., Some(pat)) => pat,
                        _ => pat,
                    };
                    if let PatKind::Range(Some(_), None, Spanned { .. }) = inner_pat.kind {
                        gate!(
                            &self,
                            half_open_range_patterns_in_slices,
                            pat.span,
                            "`X..` patterns in slices are experimental"
                        );
                    }
                }
            }
            PatKind::Box(..) => {
                gate!(&self, box_patterns, pattern.span, "box pattern syntax is experimental");
            }
            PatKind::Range(_, Some(_), Spanned { node: RangeEnd::Excluded, .. }) => {
                gate!(
                    &self,
                    exclusive_range_pattern,
                    pattern.span,
                    "exclusive range pattern syntax is experimental",
                    "use an inclusive range pattern, like N..=M"
                );
            }
            _ => {}
        }
        visit::walk_pat(self, pattern)
    }

    fn visit_poly_trait_ref(&mut self, t: &'a ast::PolyTraitRef) {
        self.check_late_bound_lifetime_defs(&t.bound_generic_params);
        visit::walk_poly_trait_ref(self, t);
    }

    fn visit_fn(&mut self, fn_kind: FnKind<'a>, span: Span, _: NodeId) {
        if let Some(header) = fn_kind.header() {
            // Stability of const fn methods are covered in `visit_assoc_item` below.
            self.check_extern(header.ext, header.constness);
        }

        if let FnKind::Closure(ast::ClosureBinder::For { generic_params, .. }, ..) = fn_kind {
            self.check_late_bound_lifetime_defs(generic_params);
        }

        if fn_kind.ctxt() != Some(FnCtxt::Foreign) && fn_kind.decl().c_variadic() {
            gate!(&self, c_variadic, span, "C-variadic functions are unstable");
        }

        visit::walk_fn(self, fn_kind)
    }

    fn visit_assoc_constraint(&mut self, constraint: &'a AssocConstraint) {
        if let AssocConstraintKind::Bound { .. } = constraint.kind {
            if let Some(ast::GenericArgs::Parenthesized(args)) = constraint.gen_args.as_ref()
                && args.inputs.is_empty()
                && matches!(args.output, ast::FnRetTy::Default(..))
            {
                gate!(
                    &self,
                    return_type_notation,
                    constraint.span,
                    "return type notation is experimental"
                );
            }
        }
        visit::walk_assoc_constraint(self, constraint)
    }

    fn visit_assoc_item(&mut self, i: &'a ast::AssocItem, ctxt: AssocCtxt) {
        let is_fn = match &i.kind {
            ast::AssocItemKind::Fn(_) => true,
            ast::AssocItemKind::Type(box ast::TyAlias { ty, .. }) => {
                if let (Some(_), AssocCtxt::Trait) = (ty, ctxt) {
                    gate!(
                        &self,
                        associated_type_defaults,
                        i.span,
                        "associated type defaults are unstable"
                    );
                }
                if let Some(ty) = ty {
                    self.check_impl_trait(ty, true);
                }
                false
            }
            _ => false,
        };
        if let ast::Defaultness::Default(_) = i.kind.defaultness() {
            // Limit `min_specialization` to only specializing functions.
            gate_alt!(
                &self,
                self.features.specialization || (is_fn && self.features.min_specialization),
                sym::specialization,
                i.span,
                "specialization is unstable"
            );
        }
        visit::walk_assoc_item(self, i, ctxt)
    }
}

pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
    maybe_stage_features(sess, features, krate);
    check_incompatible_features(sess, features);
    let mut visitor = PostExpansionVisitor { sess, features };

    let spans = sess.psess.gated_spans.spans.borrow();
    macro_rules! gate_all {
        ($gate:ident, $msg:literal) => {
            if let Some(spans) = spans.get(&sym::$gate) {
                for span in spans {
                    gate!(&visitor, $gate, *span, $msg);
                }
            }
        };
        ($gate:ident, $msg:literal, $help:literal) => {
            if let Some(spans) = spans.get(&sym::$gate) {
                for span in spans {
                    gate!(&visitor, $gate, *span, $msg, $help);
                }
            }
        };
    }
    gate_all!(
        if_let_guard,
        "`if let` guards are experimental",
        "you can write `if matches!(<expr>, <pattern>)` instead of `if let <pattern> = <expr>`"
    );
    gate_all!(let_chains, "`let` expressions in this position are unstable");
    gate_all!(
        async_closure,
        "async closures are unstable",
        "to use an async block, remove the `||`: `async {`"
    );
    gate_all!(async_for_loop, "`for await` loops are experimental");
    gate_all!(
        closure_lifetime_binder,
        "`for<...>` binders for closures are experimental",
        "consider removing `for<...>`"
    );
    gate_all!(more_qualified_paths, "usage of qualified paths in this context is experimental");
    for &span in spans.get(&sym::yield_expr).iter().copied().flatten() {
        if !span.at_least_rust_2024() {
            gate!(&visitor, coroutines, span, "yield syntax is experimental");
        }
    }
    gate_all!(gen_blocks, "gen blocks are experimental");
    gate_all!(raw_ref_op, "raw address of syntax is experimental");
    gate_all!(const_trait_impl, "const trait impls are experimental");
    gate_all!(
        half_open_range_patterns_in_slices,
        "half-open range patterns in slices are unstable"
    );
    gate_all!(inline_const, "inline-const is experimental");
    gate_all!(inline_const_pat, "inline-const in pattern position is experimental");
    gate_all!(associated_const_equality, "associated const equality is incomplete");
    gate_all!(yeet_expr, "`do yeet` expression is experimental");
    gate_all!(dyn_star, "`dyn*` trait objects are experimental");
    gate_all!(const_closures, "const closures are experimental");
    gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
    gate_all!(explicit_tail_calls, "`become` expression is experimental");
    gate_all!(generic_const_items, "generic const items are experimental");
    gate_all!(unnamed_fields, "unnamed fields are not yet fully implemented");
    gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
    gate_all!(postfix_match, "postfix match is experimental");

    if !visitor.features.never_patterns {
        if let Some(spans) = spans.get(&sym::never_patterns) {
            for &span in spans {
                if span.allows_unstable(sym::never_patterns) {
                    continue;
                }
                let sm = sess.source_map();
                // We gate two types of spans: the span of a `!` pattern, and the span of a
                // match arm without a body. For the latter we want to give the user a normal
                // error.
                if let Ok(snippet) = sm.span_to_snippet(span)
                    && snippet == "!"
                {
                    #[allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
                    feature_err(sess, sym::never_patterns, span, "`!` patterns are experimental")
                        .emit();
                } else {
                    let suggestion = span.shrink_to_hi();
                    sess.dcx().emit_err(errors::MatchArmWithNoBody { span, suggestion });
                }
            }
        }
    }

    if !visitor.features.negative_bounds {
        for &span in spans.get(&sym::negative_bounds).iter().copied().flatten() {
            sess.dcx().emit_err(errors::NegativeBoundUnsupported { span });
        }
    }

    // All uses of `gate_all_legacy_dont_use!` below this point were added in #65742,
    // and subsequently disabled (with the non-early gating readded).
    // We emit an early future-incompatible warning for these.
    // New syntax gates should go above here to get a hard error gate.
    macro_rules! gate_all_legacy_dont_use {
        ($gate:ident, $msg:literal) => {
            for span in spans.get(&sym::$gate).unwrap_or(&vec![]) {
                gate_legacy!(&visitor, $gate, *span, $msg);
            }
        };
    }

    gate_all_legacy_dont_use!(box_patterns, "box pattern syntax is experimental");
    gate_all_legacy_dont_use!(trait_alias, "trait aliases are experimental");
    // Despite being a new feature, `where T: Trait<Assoc(): Sized>`, which is RTN syntax now,
    // used to be gated under associated_type_bounds, which are right above, so RTN needs to
    // be too.
    gate_all_legacy_dont_use!(return_type_notation, "return type notation is experimental");
    gate_all_legacy_dont_use!(decl_macro, "`macro` is experimental");
    gate_all_legacy_dont_use!(
        exclusive_range_pattern,
        "exclusive range pattern syntax is experimental"
    );
    gate_all_legacy_dont_use!(try_blocks, "`try` blocks are unstable");
    gate_all_legacy_dont_use!(auto_traits, "`auto` traits are unstable");

    visit::walk_crate(&mut visitor, krate);
}

fn maybe_stage_features(sess: &Session, features: &Features, krate: &ast::Crate) {
    // checks if `#![feature]` has been used to enable any lang feature
    // does not check the same for lib features unless there's at least one
    // declared lang feature
    if !sess.opts.unstable_features.is_nightly_build() {
        let lang_features = &features.declared_lang_features;
        if lang_features.len() == 0 {
            return;
        }
        for attr in krate.attrs.iter().filter(|attr| attr.has_name(sym::feature)) {
            let mut err = errors::FeatureOnNonNightly {
                span: attr.span,
                channel: option_env!("CFG_RELEASE_CHANNEL").unwrap_or("(unknown)"),
                stable_features: vec![],
                sugg: None,
            };

            let mut all_stable = true;
            for ident in
                attr.meta_item_list().into_iter().flatten().flat_map(|nested| nested.ident())
            {
                let name = ident.name;
                let stable_since = lang_features
                    .iter()
                    .flat_map(|&(feature, _, since)| if feature == name { since } else { None })
                    .next();
                if let Some(since) = stable_since {
                    err.stable_features.push(errors::StableFeature { name, since });
                } else {
                    all_stable = false;
                }
            }
            if all_stable {
                err.sugg = Some(attr.span);
            }
            sess.dcx().emit_err(err);
        }
    }
}

fn check_incompatible_features(sess: &Session, features: &Features) {
    let declared_features = features
        .declared_lang_features
        .iter()
        .copied()
        .map(|(name, span, _)| (name, span))
        .chain(features.declared_lib_features.iter().copied());

    for (f1, f2) in rustc_feature::INCOMPATIBLE_FEATURES
        .iter()
        .filter(|&&(f1, f2)| features.active(f1) && features.active(f2))
    {
        if let Some((f1_name, f1_span)) = declared_features.clone().find(|(name, _)| name == f1) {
            if let Some((f2_name, f2_span)) = declared_features.clone().find(|(name, _)| name == f2)
            {
                let spans = vec![f1_span, f2_span];
                sess.dcx().emit_err(errors::IncompatibleFeatures {
                    spans,
                    f1: f1_name,
                    f2: f2_name,
                });
            }
        }
    }
}