Move doc comment desugaring out of TokenCursor.

`TokenCursor` currently does doc comment desugaring on the fly, if the
`desugar_doc_comment` field is set. This requires also modifying the
token stream on the fly with `replace_prev_and_rewind`.

This commit moves the doc comment desugaring out of `TokenCursor`, by
introducing a new `TokenStream::desugar_doc_comment` method. This
separation of desugaring and iterating makes the code nicer.

compiler/rustc_ast/src/tokenstream.rs

@@ -13,7 +13,7 @@
 //! and a borrowed `TokenStream` is sufficient to build an owned `TokenStream` without taking
 //! ownership of the original.
 
-use crate::ast::StmtKind;
+use crate::ast::{AttrStyle, StmtKind};
 use crate::ast_traits::{HasAttrs, HasSpan, HasTokens};
 use crate::token::{self, Delimiter, Nonterminal, Token, TokenKind};
 use crate::AttrVec;
@@ -22,11 +22,11 @@ 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::{Span, DUMMY_SP};
+use rustc_span::{sym, Span, Symbol, DUMMY_SP};
 use smallvec::{smallvec, SmallVec};
 
 use std::borrow::Cow;
-use std::{fmt, iter, mem};
+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
@@ -566,6 +566,92 @@ impl TokenStream {
     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`

compiler/rustc_parse/src/parser/mod.rs

@@ -24,7 +24,7 @@ use rustc_ast::tokenstream::{TokenStream, TokenTree, TokenTreeCursor};
 use rustc_ast::util::case::Case;
 use rustc_ast::AttrId;
 use rustc_ast::DUMMY_NODE_ID;
-use rustc_ast::{self as ast, AnonConst, AttrStyle, Const, DelimArgs, Extern};
+use rustc_ast::{self as ast, AnonConst, Const, DelimArgs, Extern};
 use rustc_ast::{Async, AttrArgs, AttrArgsEq, Expr, ExprKind, MacDelimiter, Mutability, StrLit};
 use rustc_ast::{HasAttrs, HasTokens, Unsafe, Visibility, VisibilityKind};
 use rustc_ast_pretty::pprust;
@@ -38,7 +38,7 @@ use rustc_session::parse::ParseSess;
 use rustc_span::source_map::{Span, DUMMY_SP};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use std::ops::Range;
-use std::{cmp, mem, slice};
+use std::{mem, slice};
 use thin_vec::ThinVec;
 use tracing::debug;
 
@@ -224,11 +224,6 @@ struct TokenCursor {
     // because it's the outermost token stream which never has delimiters.
     stack: Vec<(TokenTreeCursor, Delimiter, DelimSpan)>,
 
-    // We need to desugar doc comments from `/// foo` form into `#[doc =
-    // r"foo"]` form when parsing declarative macro inputs in `parse_tt`,
-    // because some declarative macros look for `doc` attributes.
-    desugar_doc_comments: bool,
-
     // Counts the number of calls to `{,inlined_}next`.
     num_next_calls: usize,
 
@@ -271,24 +266,12 @@ impl TokenCursor {
             if let Some(tree) = self.tree_cursor.next_ref() {
                 match tree {
                     &TokenTree::Token(ref token, spacing) => {
-                        match (self.desugar_doc_comments, token) {
-                            (
-                                true,
-                                &Token { kind: token::DocComment(_, attr_style, data), span },
-                            ) => {
-                                let desugared = self.desugar(attr_style, data, span);
-                                self.tree_cursor.replace_prev_and_rewind(desugared);
-                                // Continue to get the first token of the desugared doc comment.
-                            }
-                            _ => {
                         debug_assert!(!matches!(
                             token.kind,
                             token::OpenDelim(_) | token::CloseDelim(_)
                         ));
                         return (token.clone(), spacing);
                     }
-                        }
-                    }
                     &TokenTree::Delimited(sp, delim, ref tts) => {
                         let trees = tts.clone().into_trees();
                         self.stack.push((mem::replace(&mut self.tree_cursor, trees), delim, sp));
@@ -311,52 +294,6 @@ impl TokenCursor {
             }
         }
     }
-
-    // Desugar a doc comment into something like `#[doc = r"foo"]`.
-    fn desugar(&mut self, 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]
-        }
-    }
 }
 
 #[derive(Debug, Clone, PartialEq)]
@@ -451,10 +388,14 @@ pub(super) fn token_descr(token: &Token) -> String {
 impl<'a> Parser<'a> {
     pub fn new(
         sess: &'a ParseSess,
-        tokens: TokenStream,
+        mut stream: TokenStream,
         desugar_doc_comments: bool,
         subparser_name: Option<&'static str>,
     ) -> Self {
+        if desugar_doc_comments {
+            stream.desugar_doc_comments();
+        }
+
         let mut parser = Parser {
             sess,
             token: Token::dummy(),
@@ -464,10 +405,9 @@ impl<'a> Parser<'a> {
             restrictions: Restrictions::empty(),
             expected_tokens: Vec::new(),
             token_cursor: TokenCursor {
-                tree_cursor: tokens.into_trees(),
+                tree_cursor: stream.into_trees(),
                 stack: Vec::new(),
                 num_next_calls: 0,
-                desugar_doc_comments,
                 break_last_token: false,
             },
             unmatched_angle_bracket_count: 0,