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
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
//! # Token Streams
//!
//! `TokenStream`s represent syntactic objects before they are converted into ASTs.
//! A `TokenStream` is, roughly speaking, a sequence of [`TokenTree`]s,
//! which are themselves a single [`Token`] or a `Delimited` subsequence of tokens.
//!
//! ## Ownership
//!
//! `TokenStream`s are persistent data structures constructed as ropes with reference
//! counted-children. In general, this means that calling an operation on a `TokenStream`
//! (such as `slice`) produces an entirely new `TokenStream` from the borrowed reference to
//! the original. This essentially coerces `TokenStream`s into "views" of their subparts,
//! and a borrowed `TokenStream` is sufficient to build an owned `TokenStream` without taking
//! ownership of the original.

use crate::ast::{AttrStyle, StmtKind};
use crate::ast_traits::{HasAttrs, HasSpan, HasTokens};
use crate::token::{self, Delimiter, Nonterminal, Token, TokenKind};
use crate::AttrVec;

use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
use rustc_data_structures::sync::{self, Lrc};
use rustc_macros::HashStable_Generic;
use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
use rustc_span::{sym, Span, Symbol, DUMMY_SP};
use smallvec::{smallvec, SmallVec};

use std::borrow::Cow;
use std::{cmp, fmt, iter, mem};

/// When the main Rust parser encounters a syntax-extension invocation, it
/// parses the arguments to the invocation as a token tree. This is a very
/// loose structure, such that all sorts of different AST fragments can
/// be passed to syntax extensions using a uniform type.
///
/// If the syntax extension is an MBE macro, it will attempt to match its
/// LHS token tree against the provided token tree, and if it finds a
/// match, will transcribe the RHS token tree, splicing in any captured
/// `macro_parser::matched_nonterminals` into the `SubstNt`s it finds.
///
/// The RHS of an MBE macro is the only place `SubstNt`s are substituted.
/// Nothing special happens to misnamed or misplaced `SubstNt`s.
#[derive(Debug, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
pub enum TokenTree {
    /// A single token. Should never be `OpenDelim` or `CloseDelim`, because
    /// delimiters are implicitly represented by `Delimited`.
    Token(Token, Spacing),
    /// A delimited sequence of token trees.
    Delimited(DelimSpan, Delimiter, TokenStream),
}

// Ensure all fields of `TokenTree` are `DynSend` and `DynSync`.
#[cfg(parallel_compiler)]
fn _dummy()
where
    Token: sync::DynSend + sync::DynSync,
    Spacing: sync::DynSend + sync::DynSync,
    DelimSpan: sync::DynSend + sync::DynSync,
    Delimiter: sync::DynSend + sync::DynSync,
    TokenStream: sync::DynSend + sync::DynSync,
{
}

impl TokenTree {
    /// Checks if this `TokenTree` is equal to the other, regardless of span information.
    pub fn eq_unspanned(&self, other: &TokenTree) -> bool {
        match (self, other) {
            (TokenTree::Token(token, _), TokenTree::Token(token2, _)) => token.kind == token2.kind,
            (TokenTree::Delimited(_, delim, tts), TokenTree::Delimited(_, delim2, tts2)) => {
                delim == delim2 && tts.eq_unspanned(tts2)
            }
            _ => false,
        }
    }

    /// Retrieves the `TokenTree`'s span.
    pub fn span(&self) -> Span {
        match self {
            TokenTree::Token(token, _) => token.span,
            TokenTree::Delimited(sp, ..) => sp.entire(),
        }
    }

    /// Modify the `TokenTree`'s span in-place.
    pub fn set_span(&mut self, span: Span) {
        match self {
            TokenTree::Token(token, _) => token.span = span,
            TokenTree::Delimited(dspan, ..) => *dspan = DelimSpan::from_single(span),
        }
    }

    /// Create a `TokenTree::Token` with alone spacing.
    pub fn token_alone(kind: TokenKind, span: Span) -> TokenTree {
        TokenTree::Token(Token::new(kind, span), Spacing::Alone)
    }

    /// Create a `TokenTree::Token` with joint spacing.
    pub fn token_joint(kind: TokenKind, span: Span) -> TokenTree {
        TokenTree::Token(Token::new(kind, span), Spacing::Joint)
    }

    pub fn uninterpolate(&self) -> Cow<'_, TokenTree> {
        match self {
            TokenTree::Token(token, spacing) => match token.uninterpolate() {
                Cow::Owned(token) => Cow::Owned(TokenTree::Token(token, *spacing)),
                Cow::Borrowed(_) => Cow::Borrowed(self),
            },
            _ => Cow::Borrowed(self),
        }
    }
}

impl<CTX> HashStable<CTX> for TokenStream
where
    CTX: crate::HashStableContext,
{
    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
        for sub_tt in self.trees() {
            sub_tt.hash_stable(hcx, hasher);
        }
    }
}

pub trait ToAttrTokenStream: sync::DynSend + sync::DynSync {
    fn to_attr_token_stream(&self) -> AttrTokenStream;
}

impl ToAttrTokenStream for AttrTokenStream {
    fn to_attr_token_stream(&self) -> AttrTokenStream {
        self.clone()
    }
}

/// A lazy version of [`TokenStream`], which defers creation
/// of an actual `TokenStream` until it is needed.
/// `Box` is here only to reduce the structure size.
#[derive(Clone)]
pub struct LazyAttrTokenStream(Lrc<Box<dyn ToAttrTokenStream>>);

impl LazyAttrTokenStream {
    pub fn new(inner: impl ToAttrTokenStream + 'static) -> LazyAttrTokenStream {
        LazyAttrTokenStream(Lrc::new(Box::new(inner)))
    }

    pub fn to_attr_token_stream(&self) -> AttrTokenStream {
        self.0.to_attr_token_stream()
    }
}

impl fmt::Debug for LazyAttrTokenStream {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "LazyAttrTokenStream({:?})", self.to_attr_token_stream())
    }
}

impl<S: Encoder> Encodable<S> for LazyAttrTokenStream {
    fn encode(&self, s: &mut S) {
        // Used by AST json printing.
        Encodable::encode(&self.to_attr_token_stream(), s);
    }
}

impl<D: Decoder> Decodable<D> for LazyAttrTokenStream {
    fn decode(_d: &mut D) -> Self {
        panic!("Attempted to decode LazyAttrTokenStream");
    }
}

impl<CTX> HashStable<CTX> for LazyAttrTokenStream {
    fn hash_stable(&self, _hcx: &mut CTX, _hasher: &mut StableHasher) {
        panic!("Attempted to compute stable hash for LazyAttrTokenStream");
    }
}

/// An `AttrTokenStream` is similar to a `TokenStream`, but with extra
/// information about the tokens for attribute targets. This is used
/// during expansion to perform early cfg-expansion, and to process attributes
/// during proc-macro invocations.
#[derive(Clone, Debug, Default, Encodable, Decodable)]
pub struct AttrTokenStream(pub Lrc<Vec<AttrTokenTree>>);

/// Like `TokenTree`, but for `AttrTokenStream`.
#[derive(Clone, Debug, Encodable, Decodable)]
pub enum AttrTokenTree {
    Token(Token, Spacing),
    Delimited(DelimSpan, Delimiter, AttrTokenStream),
    /// Stores the attributes for an attribute target,
    /// along with the tokens for that attribute target.
    /// See `AttributesData` for more information
    Attributes(AttributesData),
}

impl AttrTokenStream {
    pub fn new(tokens: Vec<AttrTokenTree>) -> AttrTokenStream {
        AttrTokenStream(Lrc::new(tokens))
    }

    /// Converts this `AttrTokenStream` to a plain `TokenStream`.
    /// During conversion, `AttrTokenTree::Attributes` get 'flattened'
    /// back to a `TokenStream` of the form `outer_attr attr_target`.
    /// If there are inner attributes, they are inserted into the proper
    /// place in the attribute target tokens.
    pub fn to_tokenstream(&self) -> TokenStream {
        let trees: Vec<_> = self
            .0
            .iter()
            .flat_map(|tree| match &tree {
                AttrTokenTree::Token(inner, spacing) => {
                    smallvec![TokenTree::Token(inner.clone(), *spacing)].into_iter()
                }
                AttrTokenTree::Delimited(span, delim, stream) => {
                    smallvec![TokenTree::Delimited(*span, *delim, stream.to_tokenstream()),]
                        .into_iter()
                }
                AttrTokenTree::Attributes(data) => {
                    let idx = data
                        .attrs
                        .partition_point(|attr| matches!(attr.style, crate::AttrStyle::Outer));
                    let (outer_attrs, inner_attrs) = data.attrs.split_at(idx);

                    let mut target_tokens: Vec<_> = data
                        .tokens
                        .to_attr_token_stream()
                        .to_tokenstream()
                        .0
                        .iter()
                        .cloned()
                        .collect();
                    if !inner_attrs.is_empty() {
                        let mut found = false;
                        // Check the last two trees (to account for a trailing semi)
                        for tree in target_tokens.iter_mut().rev().take(2) {
                            if let TokenTree::Delimited(span, delim, delim_tokens) = tree {
                                // Inner attributes are only supported on extern blocks, functions,
                                // impls, and modules. All of these have their inner attributes
                                // placed at the beginning of the rightmost outermost braced group:
                                // e.g. fn foo() { #![my_attr} }
                                //
                                // Therefore, we can insert them back into the right location
                                // without needing to do any extra position tracking.
                                //
                                // Note: Outline modules are an exception - they can
                                // have attributes like `#![my_attr]` at the start of a file.
                                // Support for custom attributes in this position is not
                                // properly implemented - we always synthesize fake tokens,
                                // so we never reach this code.

                                let mut stream = TokenStream::default();
                                for inner_attr in inner_attrs {
                                    stream.push_stream(inner_attr.tokens());
                                }
                                stream.push_stream(delim_tokens.clone());
                                *tree = TokenTree::Delimited(*span, *delim, stream);
                                found = true;
                                break;
                            }
                        }

                        assert!(
                            found,
                            "Failed to find trailing delimited group in: {target_tokens:?}"
                        );
                    }
                    let mut flat: SmallVec<[_; 1]> =
                        SmallVec::with_capacity(target_tokens.len() + outer_attrs.len());
                    for attr in outer_attrs {
                        flat.extend(attr.tokens().0.iter().cloned());
                    }
                    flat.extend(target_tokens);
                    flat.into_iter()
                }
            })
            .collect();
        TokenStream::new(trees)
    }
}

/// Stores the tokens for an attribute target, along
/// with its attributes.
///
/// This is constructed during parsing when we need to capture
/// tokens.
///
/// For example, `#[cfg(FALSE)] struct Foo {}` would
/// have an `attrs` field containing the `#[cfg(FALSE)]` attr,
/// and a `tokens` field storing the (unparsed) tokens `struct Foo {}`
#[derive(Clone, Debug, Encodable, Decodable)]
pub struct AttributesData {
    /// Attributes, both outer and inner.
    /// These are stored in the original order that they were parsed in.
    pub attrs: AttrVec,
    /// The underlying tokens for the attribute target that `attrs`
    /// are applied to
    pub tokens: LazyAttrTokenStream,
}

/// A `TokenStream` is an abstract sequence of tokens, organized into [`TokenTree`]s.
///
/// The goal is for procedural macros to work with `TokenStream`s and `TokenTree`s
/// instead of a representation of the abstract syntax tree.
/// Today's `TokenTree`s can still contain AST via `token::Interpolated` for
/// backwards compatibility.
#[derive(Clone, Debug, Default, Encodable, Decodable)]
pub struct TokenStream(pub(crate) Lrc<Vec<TokenTree>>);

/// Similar to `proc_macro::Spacing`, but for tokens.
///
/// Note that all `ast::TokenTree::Token` instances have a `Spacing`, but when
/// we convert to `proc_macro::TokenTree` for proc macros only `Punct`
/// `TokenTree`s have a `proc_macro::Spacing`.
#[derive(Clone, Copy, Debug, PartialEq, Encodable, Decodable, HashStable_Generic)]
pub enum Spacing {
    /// The token is not immediately followed by an operator token (as
    /// determined by `Token::is_op`). E.g. a `+` token is `Alone` in `+ =`,
    /// `+/*foo*/=`, `+ident`, and `+()`.
    Alone,

    /// The token is immediately followed by an operator token. E.g. a `+`
    /// token is `Joint` in `+=` and `++`.
    Joint,
}

impl TokenStream {
    /// Given a `TokenStream` with a `Stream` of only two arguments, return a new `TokenStream`
    /// separating the two arguments with a comma for diagnostic suggestions.
    pub fn add_comma(&self) -> Option<(TokenStream, Span)> {
        // Used to suggest if a user writes `foo!(a b);`
        let mut suggestion = None;
        let mut iter = self.0.iter().enumerate().peekable();
        while let Some((pos, ts)) = iter.next() {
            if let Some((_, next)) = iter.peek() {
                let sp = match (&ts, &next) {
                    (_, TokenTree::Token(Token { kind: token::Comma, .. }, _)) => continue,
                    (
                        TokenTree::Token(token_left, Spacing::Alone),
                        TokenTree::Token(token_right, _),
                    ) if ((token_left.is_ident() && !token_left.is_reserved_ident())
                        || token_left.is_lit())
                        && ((token_right.is_ident() && !token_right.is_reserved_ident())
                            || token_right.is_lit()) =>
                    {
                        token_left.span
                    }
                    (TokenTree::Delimited(sp, ..), _) => sp.entire(),
                    _ => continue,
                };
                let sp = sp.shrink_to_hi();
                let comma = TokenTree::token_alone(token::Comma, sp);
                suggestion = Some((pos, comma, sp));
            }
        }
        if let Some((pos, comma, sp)) = suggestion {
            let mut new_stream = Vec::with_capacity(self.0.len() + 1);
            let parts = self.0.split_at(pos + 1);
            new_stream.extend_from_slice(parts.0);
            new_stream.push(comma);
            new_stream.extend_from_slice(parts.1);
            return Some((TokenStream::new(new_stream), sp));
        }
        None
    }
}

impl FromIterator<TokenTree> for TokenStream {
    fn from_iter<I: IntoIterator<Item = TokenTree>>(iter: I) -> Self {
        TokenStream::new(iter.into_iter().collect::<Vec<TokenTree>>())
    }
}

impl Eq for TokenStream {}

impl PartialEq<TokenStream> for TokenStream {
    fn eq(&self, other: &TokenStream) -> bool {
        self.trees().eq(other.trees())
    }
}

impl TokenStream {
    pub fn new(streams: Vec<TokenTree>) -> TokenStream {
        TokenStream(Lrc::new(streams))
    }

    pub fn is_empty(&self) -> bool {
        self.0.is_empty()
    }

    pub fn len(&self) -> usize {
        self.0.len()
    }

    pub fn trees(&self) -> RefTokenTreeCursor<'_> {
        RefTokenTreeCursor::new(self)
    }

    pub fn into_trees(self) -> TokenTreeCursor {
        TokenTreeCursor::new(self)
    }

    /// Compares two `TokenStream`s, checking equality without regarding span information.
    pub fn eq_unspanned(&self, other: &TokenStream) -> bool {
        let mut t1 = self.trees();
        let mut t2 = other.trees();
        for (t1, t2) in iter::zip(&mut t1, &mut t2) {
            if !t1.eq_unspanned(t2) {
                return false;
            }
        }
        t1.next().is_none() && t2.next().is_none()
    }

    /// Applies the supplied function to each `TokenTree` and its index in `self`, returning a new `TokenStream`
    ///
    /// It is equivalent to `TokenStream::new(self.trees().cloned().enumerate().map(|(i, tt)| f(i, tt)).collect())`.
    pub fn map_enumerated_owned(
        mut self,
        mut f: impl FnMut(usize, TokenTree) -> TokenTree,
    ) -> TokenStream {
        let owned = Lrc::make_mut(&mut self.0); // clone if necessary
        // rely on vec's in-place optimizations to avoid another allocation
        *owned = mem::take(owned).into_iter().enumerate().map(|(i, tree)| f(i, tree)).collect();
        self
    }

    /// Create a token stream containing a single token with alone spacing.
    pub fn token_alone(kind: TokenKind, span: Span) -> TokenStream {
        TokenStream::new(vec![TokenTree::token_alone(kind, span)])
    }

    /// Create a token stream containing a single token with joint spacing.
    pub fn token_joint(kind: TokenKind, span: Span) -> TokenStream {
        TokenStream::new(vec![TokenTree::token_joint(kind, span)])
    }

    /// Create a token stream containing a single `Delimited`.
    pub fn delimited(span: DelimSpan, delim: Delimiter, tts: TokenStream) -> TokenStream {
        TokenStream::new(vec![TokenTree::Delimited(span, delim, tts)])
    }

    pub fn from_ast(node: &(impl HasAttrs + HasSpan + HasTokens + fmt::Debug)) -> TokenStream {
        let Some(tokens) = node.tokens() else {
            panic!("missing tokens for node at {:?}: {:?}", node.span(), node);
        };
        let attrs = node.attrs();
        let attr_stream = if attrs.is_empty() {
            tokens.to_attr_token_stream()
        } else {
            let attr_data =
                AttributesData { attrs: attrs.iter().cloned().collect(), tokens: tokens.clone() };
            AttrTokenStream::new(vec![AttrTokenTree::Attributes(attr_data)])
        };
        attr_stream.to_tokenstream()
    }

    pub fn from_nonterminal_ast(nt: &Nonterminal) -> TokenStream {
        match nt {
            Nonterminal::NtIdent(ident, is_raw) => {
                TokenStream::token_alone(token::Ident(ident.name, *is_raw), ident.span)
            }
            Nonterminal::NtLifetime(ident) => {
                TokenStream::token_alone(token::Lifetime(ident.name), ident.span)
            }
            Nonterminal::NtItem(item) => TokenStream::from_ast(item),
            Nonterminal::NtBlock(block) => TokenStream::from_ast(block),
            Nonterminal::NtStmt(stmt) if let StmtKind::Empty = stmt.kind => {
                // FIXME: Properly collect tokens for empty statements.
                TokenStream::token_alone(token::Semi, stmt.span)
            }
            Nonterminal::NtStmt(stmt) => TokenStream::from_ast(stmt),
            Nonterminal::NtPat(pat) => TokenStream::from_ast(pat),
            Nonterminal::NtTy(ty) => TokenStream::from_ast(ty),
            Nonterminal::NtMeta(attr) => TokenStream::from_ast(attr),
            Nonterminal::NtPath(path) => TokenStream::from_ast(path),
            Nonterminal::NtVis(vis) => TokenStream::from_ast(vis),
            Nonterminal::NtExpr(expr) | Nonterminal::NtLiteral(expr) => TokenStream::from_ast(expr),
        }
    }

    fn flatten_token(token: &Token, spacing: Spacing) -> TokenTree {
        match &token.kind {
            token::Interpolated(nt) if let token::NtIdent(ident, is_raw) = **nt => {
                TokenTree::Token(Token::new(token::Ident(ident.name, is_raw), ident.span), spacing)
            }
            token::Interpolated(nt) => TokenTree::Delimited(
                DelimSpan::from_single(token.span),
                Delimiter::Invisible,
                TokenStream::from_nonterminal_ast(nt).flattened(),
            ),
            _ => TokenTree::Token(token.clone(), spacing),
        }
    }

    fn flatten_token_tree(tree: &TokenTree) -> TokenTree {
        match tree {
            TokenTree::Token(token, spacing) => TokenStream::flatten_token(token, *spacing),
            TokenTree::Delimited(span, delim, tts) => {
                TokenTree::Delimited(*span, *delim, tts.flattened())
            }
        }
    }

    #[must_use]
    pub fn flattened(&self) -> TokenStream {
        fn can_skip(stream: &TokenStream) -> bool {
            stream.trees().all(|tree| match tree {
                TokenTree::Token(token, _) => !matches!(token.kind, token::Interpolated(_)),
                TokenTree::Delimited(_, _, inner) => can_skip(inner),
            })
        }

        if can_skip(self) {
            return self.clone();
        }

        self.trees().map(|tree| TokenStream::flatten_token_tree(tree)).collect()
    }

    // If `vec` is not empty, try to glue `tt` onto its last token. The return
    // value indicates if gluing took place.
    fn try_glue_to_last(vec: &mut Vec<TokenTree>, tt: &TokenTree) -> bool {
        if let Some(TokenTree::Token(last_tok, Spacing::Joint)) = vec.last()
            && let TokenTree::Token(tok, spacing) = tt
            && let Some(glued_tok) = last_tok.glue(tok)
        {
            // ...then overwrite the last token tree in `vec` with the
            // glued token, and skip the first token tree from `stream`.
            *vec.last_mut().unwrap() = TokenTree::Token(glued_tok, *spacing);
            true
        } else {
            false
        }
    }

    /// Push `tt` onto the end of the stream, possibly gluing it to the last
    /// token. Uses `make_mut` to maximize efficiency.
    pub fn push_tree(&mut self, tt: TokenTree) {
        let vec_mut = Lrc::make_mut(&mut self.0);

        if Self::try_glue_to_last(vec_mut, &tt) {
            // nothing else to do
        } else {
            vec_mut.push(tt);
        }
    }

    /// Push `stream` onto the end of the stream, possibly gluing the first
    /// token tree to the last token. (No other token trees will be glued.)
    /// Uses `make_mut` to maximize efficiency.
    pub fn push_stream(&mut self, stream: TokenStream) {
        let vec_mut = Lrc::make_mut(&mut self.0);

        let stream_iter = stream.0.iter().cloned();

        if let Some(first) = stream.0.first() && Self::try_glue_to_last(vec_mut, first) {
            // Now skip the first token tree from `stream`.
            vec_mut.extend(stream_iter.skip(1));
        } else {
            // Append all of `stream`.
            vec_mut.extend(stream_iter);
        }
    }

    pub fn chunks(&self, chunk_size: usize) -> core::slice::Chunks<'_, TokenTree> {
        self.0.chunks(chunk_size)
    }

    /// Desugar doc comments like `/// foo` in the stream into `#[doc =
    /// r"foo"]`. Modifies the `TokenStream` via `Lrc::make_mut`, but as little
    /// as possible.
    pub fn desugar_doc_comments(&mut self) {
        if let Some(desugared_stream) = desugar_inner(self.clone()) {
            *self = desugared_stream;
        }

        // The return value is `None` if nothing in `stream` changed.
        fn desugar_inner(mut stream: TokenStream) -> Option<TokenStream> {
            let mut i = 0;
            let mut modified = false;
            while let Some(tt) = stream.0.get(i) {
                match tt {
                    &TokenTree::Token(
                        Token { kind: token::DocComment(_, attr_style, data), span },
                        _spacing,
                    ) => {
                        let desugared = desugared_tts(attr_style, data, span);
                        let desugared_len = desugared.len();
                        Lrc::make_mut(&mut stream.0).splice(i..i + 1, desugared);
                        modified = true;
                        i += desugared_len;
                    }

                    &TokenTree::Token(..) => i += 1,

                    &TokenTree::Delimited(sp, delim, ref delim_stream) => {
                        if let Some(desugared_delim_stream) = desugar_inner(delim_stream.clone()) {
                            let new_tt = TokenTree::Delimited(sp, delim, desugared_delim_stream);
                            Lrc::make_mut(&mut stream.0)[i] = new_tt;
                            modified = true;
                        }
                        i += 1;
                    }
                }
            }
            if modified { Some(stream) } else { None }
        }

        fn desugared_tts(attr_style: AttrStyle, data: Symbol, span: Span) -> Vec<TokenTree> {
            // Searches for the occurrences of `"#*` and returns the minimum number of `#`s
            // required to wrap the text. E.g.
            // - `abc d` is wrapped as `r"abc d"` (num_of_hashes = 0)
            // - `abc "d"` is wrapped as `r#"abc "d""#` (num_of_hashes = 1)
            // - `abc "##d##"` is wrapped as `r###"abc ##"d"##"###` (num_of_hashes = 3)
            let mut num_of_hashes = 0;
            let mut count = 0;
            for ch in data.as_str().chars() {
                count = match ch {
                    '"' => 1,
                    '#' if count > 0 => count + 1,
                    _ => 0,
                };
                num_of_hashes = cmp::max(num_of_hashes, count);
            }

            // `/// foo` becomes `doc = r"foo"`.
            let delim_span = DelimSpan::from_single(span);
            let body = TokenTree::Delimited(
                delim_span,
                Delimiter::Bracket,
                [
                    TokenTree::token_alone(token::Ident(sym::doc, false), span),
                    TokenTree::token_alone(token::Eq, span),
                    TokenTree::token_alone(
                        TokenKind::lit(token::StrRaw(num_of_hashes), data, None),
                        span,
                    ),
                ]
                .into_iter()
                .collect::<TokenStream>(),
            );

            if attr_style == AttrStyle::Inner {
                vec![
                    TokenTree::token_alone(token::Pound, span),
                    TokenTree::token_alone(token::Not, span),
                    body,
                ]
            } else {
                vec![TokenTree::token_alone(token::Pound, span), body]
            }
        }
    }
}

/// By-reference iterator over a [`TokenStream`], that produces `&TokenTree`
/// items.
#[derive(Clone)]
pub struct RefTokenTreeCursor<'t> {
    stream: &'t TokenStream,
    index: usize,
}

impl<'t> RefTokenTreeCursor<'t> {
    fn new(stream: &'t TokenStream) -> Self {
        RefTokenTreeCursor { stream, index: 0 }
    }

    pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
        self.stream.0.get(self.index + n)
    }
}

impl<'t> Iterator for RefTokenTreeCursor<'t> {
    type Item = &'t TokenTree;

    fn next(&mut self) -> Option<&'t TokenTree> {
        self.stream.0.get(self.index).map(|tree| {
            self.index += 1;
            tree
        })
    }
}

/// Owning by-value iterator over a [`TokenStream`], that produces `&TokenTree`
/// items.
///
/// Doesn't impl `Iterator` because Rust doesn't permit an owning iterator to
/// return `&T` from `next`; the need for an explicit lifetime in the `Item`
/// associated type gets in the way. Instead, use `next_ref` (which doesn't
/// involve associated types) for getting individual elements, or
/// `RefTokenTreeCursor` if you really want an `Iterator`, e.g. in a `for`
/// loop.
#[derive(Clone)]
pub struct TokenTreeCursor {
    pub stream: TokenStream,
    index: usize,
}

impl TokenTreeCursor {
    fn new(stream: TokenStream) -> Self {
        TokenTreeCursor { stream, index: 0 }
    }

    #[inline]
    pub fn next_ref(&mut self) -> Option<&TokenTree> {
        self.stream.0.get(self.index).map(|tree| {
            self.index += 1;
            tree
        })
    }

    pub fn look_ahead(&self, n: usize) -> Option<&TokenTree> {
        self.stream.0.get(self.index + n)
    }
}

#[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable, HashStable_Generic)]
pub struct DelimSpan {
    pub open: Span,
    pub close: Span,
}

impl DelimSpan {
    pub fn from_single(sp: Span) -> Self {
        DelimSpan { open: sp, close: sp }
    }

    pub fn from_pair(open: Span, close: Span) -> Self {
        DelimSpan { open, close }
    }

    pub fn dummy() -> Self {
        Self::from_single(DUMMY_SP)
    }

    pub fn entire(self) -> Span {
        self.open.with_hi(self.close.hi())
    }
}

// Some types are used a lot. Make sure they don't unintentionally get bigger.
#[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
mod size_asserts {
    use super::*;
    use rustc_data_structures::static_assert_size;
    // tidy-alphabetical-start
    static_assert_size!(AttrTokenStream, 8);
    static_assert_size!(AttrTokenTree, 32);
    static_assert_size!(LazyAttrTokenStream, 8);
    static_assert_size!(TokenStream, 8);
    static_assert_size!(TokenTree, 32);
    // tidy-alphabetical-end
}