compiler/rustc_parse/src/lib.rs - toolchain/rustc - Git at Google

 //! The main parser interface.

 #![feature(array_windows)]
 #![feature(crate_visibility_modifier)]
 #![feature(if_let_guard)]
 #![cfg_attr(bootstrap, feature(bindings_after_at))]
 #![feature(box_patterns)]
 #![cfg_attr(bootstrap, allow(incomplete_features))] // if_let_guard
 #![recursion_limit = "256"]

 use rustc_ast as ast;
 use rustc_ast::token::{self, Nonterminal, Token, TokenKind};
 use rustc_ast::tokenstream::{self, AttributesData, CanSynthesizeMissingTokens, LazyTokenStream};
 use rustc_ast::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
 use rustc_ast::tokenstream::{Spacing, TokenStream};
 use rustc_ast::AstLike;
 use rustc_ast::Attribute;
 use rustc_ast::{AttrItem, MetaItem};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, Diagnostic, FatalError, Level, PResult};
 use rustc_session::parse::ParseSess;
 use rustc_span::{FileName, SourceFile, Span};

 use std::path::Path;
 use std::str;

 pub const MACRO_ARGUMENTS: Option<&str> = Some("macro arguments");

 #[macro_use]
 pub mod parser;
 use parser::{emit_unclosed_delims, make_unclosed_delims_error, Parser};
 pub mod lexer;
 pub mod validate_attr;

 // A bunch of utility functions of the form `parse_<thing>_from_<source>`
 // where <thing> includes crate, expr, item, stmt, tts, and one that
 // uses a HOF to parse anything, and <source> includes file and
 // `source_str`.

 /// A variant of 'panictry!' that works on a Vec<Diagnostic> instead of a single DiagnosticBuilder.
 macro_rules! panictry_buffer {
     ($handler:expr, $e:expr) => {{
         use rustc_errors::FatalError;
         use std::result::Result::{Err, Ok};
         match $e {
             Ok(e) => e,
             Err(errs) => {
                 for e in errs {
                     $handler.emit_diagnostic(&e);
                 }
                 FatalError.raise()
             }
         }
     }};
 }

 pub fn parse_crate_from_file<'a>(input: &Path, sess: &'a ParseSess) -> PResult<'a, ast::Crate> {
     let mut parser = new_parser_from_file(sess, input, None);
     parser.parse_crate_mod()
 }

 pub fn parse_crate_attrs_from_file<'a>(
     input: &Path,
     sess: &'a ParseSess,
 ) -> PResult<'a, Vec<ast::Attribute>> {
     let mut parser = new_parser_from_file(sess, input, None);
     parser.parse_inner_attributes()
 }

 pub fn parse_crate_from_source_str(
     name: FileName,
     source: String,
     sess: &ParseSess,
 ) -> PResult<'_, ast::Crate> {
     new_parser_from_source_str(sess, name, source).parse_crate_mod()
 }

 pub fn parse_crate_attrs_from_source_str(
     name: FileName,
     source: String,
     sess: &ParseSess,
 ) -> PResult<'_, Vec<ast::Attribute>> {
     new_parser_from_source_str(sess, name, source).parse_inner_attributes()
 }

 pub fn parse_stream_from_source_str(
     name: FileName,
     source: String,
     sess: &ParseSess,
     override_span: Option<Span>,
 ) -> TokenStream {
     let (stream, mut errors) =
         source_file_to_stream(sess, sess.source_map().new_source_file(name, source), override_span);
     emit_unclosed_delims(&mut errors, &sess);
     stream
 }

 /// Creates a new parser from a source string.
 pub fn new_parser_from_source_str(sess: &ParseSess, name: FileName, source: String) -> Parser<'_> {
     panictry_buffer!(&sess.span_diagnostic, maybe_new_parser_from_source_str(sess, name, source))
 }

 /// Creates a new parser from a source string. Returns any buffered errors from lexing the initial
 /// token stream.
 pub fn maybe_new_parser_from_source_str(
     sess: &ParseSess,
     name: FileName,
     source: String,
 ) -> Result<Parser<'_>, Vec<Diagnostic>> {
     maybe_source_file_to_parser(sess, sess.source_map().new_source_file(name, source))
 }

 /// Creates a new parser, handling errors as appropriate if the file doesn't exist.
 /// If a span is given, that is used on an error as the source of the problem.
 pub fn new_parser_from_file<'a>(sess: &'a ParseSess, path: &Path, sp: Option<Span>) -> Parser<'a> {
     source_file_to_parser(sess, file_to_source_file(sess, path, sp))
 }

 /// Given a `source_file` and config, returns a parser.
 fn source_file_to_parser(sess: &ParseSess, source_file: Lrc<SourceFile>) -> Parser<'_> {
     panictry_buffer!(&sess.span_diagnostic, maybe_source_file_to_parser(sess, source_file))
 }

 /// Given a `source_file` and config, return a parser. Returns any buffered errors from lexing the
 /// initial token stream.
 fn maybe_source_file_to_parser(
     sess: &ParseSess,
     source_file: Lrc<SourceFile>,
 ) -> Result<Parser<'_>, Vec<Diagnostic>> {
     let end_pos = source_file.end_pos;
     let (stream, unclosed_delims) = maybe_file_to_stream(sess, source_file, None)?;
     let mut parser = stream_to_parser(sess, stream, None);
     parser.unclosed_delims = unclosed_delims;
     if parser.token == token::Eof {
         parser.token.span = Span::new(end_pos, end_pos, parser.token.span.ctxt());
     }

     Ok(parser)
 }

 // Base abstractions

 /// Given a session and a path and an optional span (for error reporting),
 /// add the path to the session's source_map and return the new source_file or
 /// error when a file can't be read.
 fn try_file_to_source_file(
     sess: &ParseSess,
     path: &Path,
     spanopt: Option<Span>,
 ) -> Result<Lrc<SourceFile>, Diagnostic> {
     sess.source_map().load_file(path).map_err(|e| {
         let msg = format!("couldn't read {}: {}", path.display(), e);
         let mut diag = Diagnostic::new(Level::Fatal, &msg);
         if let Some(sp) = spanopt {
             diag.set_span(sp);
         }
         diag
     })
 }

 /// Given a session and a path and an optional span (for error reporting),
 /// adds the path to the session's `source_map` and returns the new `source_file`.
 fn file_to_source_file(sess: &ParseSess, path: &Path, spanopt: Option<Span>) -> Lrc<SourceFile> {
     match try_file_to_source_file(sess, path, spanopt) {
         Ok(source_file) => source_file,
         Err(d) => {
             sess.span_diagnostic.emit_diagnostic(&d);
             FatalError.raise();
         }
     }
 }

 /// Given a `source_file`, produces a sequence of token trees.
 pub fn source_file_to_stream(
     sess: &ParseSess,
     source_file: Lrc<SourceFile>,
     override_span: Option<Span>,
 ) -> (TokenStream, Vec<lexer::UnmatchedBrace>) {
     panictry_buffer!(&sess.span_diagnostic, maybe_file_to_stream(sess, source_file, override_span))
 }

 /// Given a source file, produces a sequence of token trees. Returns any buffered errors from
 /// parsing the token stream.
 pub fn maybe_file_to_stream(
     sess: &ParseSess,
     source_file: Lrc<SourceFile>,
     override_span: Option<Span>,
 ) -> Result<(TokenStream, Vec<lexer::UnmatchedBrace>), Vec<Diagnostic>> {
     let src = source_file.src.as_ref().unwrap_or_else(|| {
         sess.span_diagnostic.bug(&format!(
             "cannot lex `source_file` without source: {}",
             sess.source_map().filename_for_diagnostics(&source_file.name)
         ));
     });

     let (token_trees, unmatched_braces) =
         lexer::parse_token_trees(sess, src.as_str(), source_file.start_pos, override_span);

     match token_trees {
         Ok(stream) => Ok((stream, unmatched_braces)),
         Err(err) => {
             let mut buffer = Vec::with_capacity(1);
             err.buffer(&mut buffer);
             // Not using `emit_unclosed_delims` to use `db.buffer`
             for unmatched in unmatched_braces {
                 if let Some(err) = make_unclosed_delims_error(unmatched, &sess) {
                     err.buffer(&mut buffer);
                 }
             }
             Err(buffer)
         }
     }
 }

 /// Given a stream and the `ParseSess`, produces a parser.
 pub fn stream_to_parser<'a>(
     sess: &'a ParseSess,
     stream: TokenStream,
     subparser_name: Option<&'static str>,
 ) -> Parser<'a> {
     Parser::new(sess, stream, false, subparser_name)
 }

 /// Runs the given subparser `f` on the tokens of the given `attr`'s item.
 pub fn parse_in<'a, T>(
     sess: &'a ParseSess,
     tts: TokenStream,
     name: &'static str,
     mut f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
 ) -> PResult<'a, T> {
     let mut parser = Parser::new(sess, tts, false, Some(name));
     let result = f(&mut parser)?;
     if parser.token != token::Eof {
         parser.unexpected()?;
     }
     Ok(result)
 }

 // NOTE(Centril): The following probably shouldn't be here but it acknowledges the
 // fact that architecturally, we are using parsing (read on below to understand why).

 pub fn nt_to_tokenstream(
     nt: &Nonterminal,
     sess: &ParseSess,
     synthesize_tokens: CanSynthesizeMissingTokens,
 ) -> TokenStream {
     // A `Nonterminal` is often a parsed AST item. At this point we now
     // need to convert the parsed AST to an actual token stream, e.g.
     // un-parse it basically.
     //
     // Unfortunately there's not really a great way to do that in a
     // guaranteed lossless fashion right now. The fallback here is to just
     // stringify the AST node and reparse it, but this loses all span
     // information.
     //
     // As a result, some AST nodes are annotated with the token stream they
     // came from. Here we attempt to extract these lossless token streams
     // before we fall back to the stringification.

     let convert_tokens =
         |tokens: Option<&LazyTokenStream>| Some(tokens?.create_token_stream().to_tokenstream());

     let tokens = match *nt {
         Nonterminal::NtItem(ref item) => prepend_attrs(&item.attrs, item.tokens.as_ref()),
         Nonterminal::NtBlock(ref block) => convert_tokens(block.tokens.as_ref()),
         Nonterminal::NtStmt(ref stmt) if let ast::StmtKind::Empty = stmt.kind => {
             let tokens = AttrAnnotatedTokenStream::new(vec![(
                 tokenstream::AttrAnnotatedTokenTree::Token(Token::new(
                     TokenKind::Semi,
                     stmt.span,
                 )),
                 Spacing::Alone,
             )]);
             prepend_attrs(&stmt.attrs(), Some(&LazyTokenStream::new(tokens)))
         }
         Nonterminal::NtStmt(ref stmt) => prepend_attrs(&stmt.attrs(), stmt.tokens()),
         Nonterminal::NtPat(ref pat) => convert_tokens(pat.tokens.as_ref()),
         Nonterminal::NtTy(ref ty) => convert_tokens(ty.tokens.as_ref()),
         Nonterminal::NtIdent(ident, is_raw) => {
             Some(tokenstream::TokenTree::token(token::Ident(ident.name, is_raw), ident.span).into())
         }
         Nonterminal::NtLifetime(ident) => {
             Some(tokenstream::TokenTree::token(token::Lifetime(ident.name), ident.span).into())
         }
         Nonterminal::NtMeta(ref attr) => convert_tokens(attr.tokens.as_ref()),
         Nonterminal::NtPath(ref path) => convert_tokens(path.tokens.as_ref()),
         Nonterminal::NtVis(ref vis) => convert_tokens(vis.tokens.as_ref()),
         Nonterminal::NtTT(ref tt) => Some(tt.clone().into()),
         Nonterminal::NtExpr(ref expr) | Nonterminal::NtLiteral(ref expr) => {
             prepend_attrs(&expr.attrs, expr.tokens.as_ref())
         }
     };

     if let Some(tokens) = tokens {
         return tokens;
     } else if matches!(synthesize_tokens, CanSynthesizeMissingTokens::Yes) {
         return fake_token_stream(sess, nt);
     } else {
         panic!(
             "Missing tokens for nt {:?} at {:?}: {:?}",
             nt,
             nt.span(),
             pprust::nonterminal_to_string(nt)
         );
     }
 }

 fn prepend_attrs(attrs: &[Attribute], tokens: Option<&LazyTokenStream>) -> Option<TokenStream> {
     let tokens = tokens?;
     if attrs.is_empty() {
         return Some(tokens.create_token_stream().to_tokenstream());
     }
     let attr_data = AttributesData { attrs: attrs.to_vec().into(), tokens: tokens.clone() };
     let wrapped = AttrAnnotatedTokenStream::new(vec![(
         AttrAnnotatedTokenTree::Attributes(attr_data),
         Spacing::Alone,
     )]);
     Some(wrapped.to_tokenstream())
 }

 pub fn fake_token_stream(sess: &ParseSess, nt: &Nonterminal) -> TokenStream {
     let source = pprust::nonterminal_to_string(nt);
     let filename = FileName::macro_expansion_source_code(&source);
     parse_stream_from_source_str(filename, source, sess, Some(nt.span()))
 }

 pub fn parse_cfg_attr(
     attr: &Attribute,
     parse_sess: &ParseSess,
 ) -> Option<(MetaItem, Vec<(AttrItem, Span)>)> {
     match attr.get_normal_item().args {
         ast::MacArgs::Delimited(dspan, delim, ref tts) if !tts.is_empty() => {
             let msg = "wrong `cfg_attr` delimiters";
             crate::validate_attr::check_meta_bad_delim(parse_sess, dspan, delim, msg);
             match parse_in(parse_sess, tts.clone(), "`cfg_attr` input", |p| p.parse_cfg_attr()) {
                 Ok(r) => return Some(r),
                 Err(mut e) => {
                     e.help(&format!("the valid syntax is `{}`", CFG_ATTR_GRAMMAR_HELP))
                         .note(CFG_ATTR_NOTE_REF)
                         .emit();
                 }
             }
         }
         _ => error_malformed_cfg_attr_missing(attr.span, parse_sess),
     }
     None
 }

 const CFG_ATTR_GRAMMAR_HELP: &str = "#[cfg_attr(condition, attribute, other_attribute, ...)]";
 const CFG_ATTR_NOTE_REF: &str = "for more information, visit \
     <https://doc.rust-lang.org/reference/conditional-compilation.html\
     #the-cfg_attr-attribute>";

 fn error_malformed_cfg_attr_missing(span: Span, parse_sess: &ParseSess) {
     parse_sess
         .span_diagnostic
         .struct_span_err(span, "malformed `cfg_attr` attribute input")
         .span_suggestion(
             span,
             "missing condition and attribute",
             CFG_ATTR_GRAMMAR_HELP.to_string(),
             Applicability::HasPlaceholders,
         )
         .note(CFG_ATTR_NOTE_REF)
         .emit();
 }
	//! The main parser interface.

	#![feature(array_windows)]
	#![feature(crate_visibility_modifier)]
	#![feature(if_let_guard)]
	#![cfg_attr(bootstrap, feature(bindings_after_at))]
	#![feature(box_patterns)]
	#![cfg_attr(bootstrap, allow(incomplete_features))] // if_let_guard
	#![recursion_limit = "256"]

	use rustc_ast as ast;
	use rustc_ast::token::{self, Nonterminal, Token, TokenKind};
	use rustc_ast::tokenstream::{self, AttributesData, CanSynthesizeMissingTokens, LazyTokenStream};
	use rustc_ast::tokenstream::{AttrAnnotatedTokenStream, AttrAnnotatedTokenTree};
	use rustc_ast::tokenstream::{Spacing, TokenStream};
	use rustc_ast::AstLike;
	use rustc_ast::Attribute;
	use rustc_ast::{AttrItem, MetaItem};
	use rustc_ast_pretty::pprust;
	use rustc_data_structures::sync::Lrc;
	use rustc_errors::{Applicability, Diagnostic, FatalError, Level, PResult};
	use rustc_session::parse::ParseSess;
	use rustc_span::{FileName, SourceFile, Span};

	use std::path::Path;
	use std::str;

	pub const MACRO_ARGUMENTS: Option<&str> = Some("macro arguments");

	#[macro_use]
	pub mod parser;
	use parser::{emit_unclosed_delims, make_unclosed_delims_error, Parser};
	pub mod lexer;
	pub mod validate_attr;

	// A bunch of utility functions of the form `parse_<thing>_from_<source>`
	// where <thing> includes crate, expr, item, stmt, tts, and one that
	// uses a HOF to parse anything, and <source> includes file and
	// `source_str`.

	/// A variant of 'panictry!' that works on a Vec<Diagnostic> instead of a single DiagnosticBuilder.
	macro_rules! panictry_buffer {
	($handler:expr, $e:expr) => {{
	use rustc_errors::FatalError;
	use std::result::Result::{Err, Ok};
	match $e {
	Ok(e) => e,
	Err(errs) => {
	for e in errs {
	$handler.emit_diagnostic(&e);
	}
	FatalError.raise()
	}
	}
	}};
	}

	pub fn parse_crate_from_file<'a>(input: &Path, sess: &'a ParseSess) -> PResult<'a, ast::Crate> {
	let mut parser = new_parser_from_file(sess, input, None);
	parser.parse_crate_mod()
	}

	pub fn parse_crate_attrs_from_file<'a>(
	input: &Path,
	sess: &'a ParseSess,
	) -> PResult<'a, Vec<ast::Attribute>> {
	let mut parser = new_parser_from_file(sess, input, None);
	parser.parse_inner_attributes()
	}

	pub fn parse_crate_from_source_str(
	name: FileName,
	source: String,
	sess: &ParseSess,
	) -> PResult<'_, ast::Crate> {
	new_parser_from_source_str(sess, name, source).parse_crate_mod()
	}

	pub fn parse_crate_attrs_from_source_str(
	name: FileName,
	source: String,
	sess: &ParseSess,
	) -> PResult<'_, Vec<ast::Attribute>> {
	new_parser_from_source_str(sess, name, source).parse_inner_attributes()
	}

	pub fn parse_stream_from_source_str(
	name: FileName,
	source: String,
	sess: &ParseSess,
	override_span: Option<Span>,
	) -> TokenStream {
	let (stream, mut errors) =
	source_file_to_stream(sess, sess.source_map().new_source_file(name, source), override_span);
	emit_unclosed_delims(&mut errors, &sess);
	stream
	}

	/// Creates a new parser from a source string.
	pub fn new_parser_from_source_str(sess: &ParseSess, name: FileName, source: String) -> Parser<'_> {
	panictry_buffer!(&sess.span_diagnostic, maybe_new_parser_from_source_str(sess, name, source))
	}

	/// Creates a new parser from a source string. Returns any buffered errors from lexing the initial
	/// token stream.
	pub fn maybe_new_parser_from_source_str(
	sess: &ParseSess,
	name: FileName,
	source: String,
	) -> Result<Parser<'_>, Vec<Diagnostic>> {
	maybe_source_file_to_parser(sess, sess.source_map().new_source_file(name, source))
	}

	/// Creates a new parser, handling errors as appropriate if the file doesn't exist.
	/// If a span is given, that is used on an error as the source of the problem.
	pub fn new_parser_from_file<'a>(sess: &'a ParseSess, path: &Path, sp: Option<Span>) -> Parser<'a> {
	source_file_to_parser(sess, file_to_source_file(sess, path, sp))
	}

	/// Given a `source_file` and config, returns a parser.
	fn source_file_to_parser(sess: &ParseSess, source_file: Lrc<SourceFile>) -> Parser<'_> {
	panictry_buffer!(&sess.span_diagnostic, maybe_source_file_to_parser(sess, source_file))
	}

	/// Given a `source_file` and config, return a parser. Returns any buffered errors from lexing the
	/// initial token stream.
	fn maybe_source_file_to_parser(
	sess: &ParseSess,
	source_file: Lrc<SourceFile>,
	) -> Result<Parser<'_>, Vec<Diagnostic>> {
	let end_pos = source_file.end_pos;
	let (stream, unclosed_delims) = maybe_file_to_stream(sess, source_file, None)?;
	let mut parser = stream_to_parser(sess, stream, None);
	parser.unclosed_delims = unclosed_delims;
	if parser.token == token::Eof {
	parser.token.span = Span::new(end_pos, end_pos, parser.token.span.ctxt());
	}

	Ok(parser)
	}

	// Base abstractions

	/// Given a session and a path and an optional span (for error reporting),
	/// add the path to the session's source_map and return the new source_file or
	/// error when a file can't be read.
	fn try_file_to_source_file(
	sess: &ParseSess,
	path: &Path,
	spanopt: Option<Span>,
	) -> Result<Lrc<SourceFile>, Diagnostic> {
	sess.source_map().load_file(path).map_err(\|e\| {
	let msg = format!("couldn't read {}: {}", path.display(), e);
	let mut diag = Diagnostic::new(Level::Fatal, &msg);
	if let Some(sp) = spanopt {
	diag.set_span(sp);
	}
	diag
	})
	}

	/// Given a session and a path and an optional span (for error reporting),
	/// adds the path to the session's `source_map` and returns the new `source_file`.
	fn file_to_source_file(sess: &ParseSess, path: &Path, spanopt: Option<Span>) -> Lrc<SourceFile> {
	match try_file_to_source_file(sess, path, spanopt) {
	Ok(source_file) => source_file,
	Err(d) => {
	sess.span_diagnostic.emit_diagnostic(&d);
	FatalError.raise();
	}
	}
	}

	/// Given a `source_file`, produces a sequence of token trees.
	pub fn source_file_to_stream(
	sess: &ParseSess,
	source_file: Lrc<SourceFile>,
	override_span: Option<Span>,
	) -> (TokenStream, Vec<lexer::UnmatchedBrace>) {
	panictry_buffer!(&sess.span_diagnostic, maybe_file_to_stream(sess, source_file, override_span))
	}

	/// Given a source file, produces a sequence of token trees. Returns any buffered errors from
	/// parsing the token stream.
	pub fn maybe_file_to_stream(
	sess: &ParseSess,
	source_file: Lrc<SourceFile>,
	override_span: Option<Span>,
	) -> Result<(TokenStream, Vec<lexer::UnmatchedBrace>), Vec<Diagnostic>> {
	let src = source_file.src.as_ref().unwrap_or_else(\|\| {
	sess.span_diagnostic.bug(&format!(
	"cannot lex `source_file` without source: {}",
	sess.source_map().filename_for_diagnostics(&source_file.name)
	));
	});

	let (token_trees, unmatched_braces) =
	lexer::parse_token_trees(sess, src.as_str(), source_file.start_pos, override_span);

	match token_trees {
	Ok(stream) => Ok((stream, unmatched_braces)),
	Err(err) => {
	let mut buffer = Vec::with_capacity(1);
	err.buffer(&mut buffer);
	// Not using `emit_unclosed_delims` to use `db.buffer`
	for unmatched in unmatched_braces {
	if let Some(err) = make_unclosed_delims_error(unmatched, &sess) {
	err.buffer(&mut buffer);
	}
	}
	Err(buffer)
	}
	}
	}

	/// Given a stream and the `ParseSess`, produces a parser.
	pub fn stream_to_parser<'a>(
	sess: &'a ParseSess,
	stream: TokenStream,
	subparser_name: Option<&'static str>,
	) -> Parser<'a> {
	Parser::new(sess, stream, false, subparser_name)
	}

	/// Runs the given subparser `f` on the tokens of the given `attr`'s item.
	pub fn parse_in<'a, T>(
	sess: &'a ParseSess,
	tts: TokenStream,
	name: &'static str,
	mut f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
	) -> PResult<'a, T> {
	let mut parser = Parser::new(sess, tts, false, Some(name));
	let result = f(&mut parser)?;
	if parser.token != token::Eof {
	parser.unexpected()?;
	}
	Ok(result)
	}

	// NOTE(Centril): The following probably shouldn't be here but it acknowledges the
	// fact that architecturally, we are using parsing (read on below to understand why).

	pub fn nt_to_tokenstream(
	nt: &Nonterminal,
	sess: &ParseSess,
	synthesize_tokens: CanSynthesizeMissingTokens,
	) -> TokenStream {
	// A `Nonterminal` is often a parsed AST item. At this point we now
	// need to convert the parsed AST to an actual token stream, e.g.
	// un-parse it basically.
	//
	// Unfortunately there's not really a great way to do that in a
	// guaranteed lossless fashion right now. The fallback here is to just
	// stringify the AST node and reparse it, but this loses all span
	// information.
	//
	// As a result, some AST nodes are annotated with the token stream they
	// came from. Here we attempt to extract these lossless token streams
	// before we fall back to the stringification.

	let convert_tokens =
	\|tokens: Option<&LazyTokenStream>\| Some(tokens?.create_token_stream().to_tokenstream());

	let tokens = match *nt {
	Nonterminal::NtItem(ref item) => prepend_attrs(&item.attrs, item.tokens.as_ref()),
	Nonterminal::NtBlock(ref block) => convert_tokens(block.tokens.as_ref()),
	Nonterminal::NtStmt(ref stmt) if let ast::StmtKind::Empty = stmt.kind => {
	let tokens = AttrAnnotatedTokenStream::new(vec![(
	tokenstream::AttrAnnotatedTokenTree::Token(Token::new(
	TokenKind::Semi,
	stmt.span,
	)),
	Spacing::Alone,
	)]);
	prepend_attrs(&stmt.attrs(), Some(&LazyTokenStream::new(tokens)))
	}
	Nonterminal::NtStmt(ref stmt) => prepend_attrs(&stmt.attrs(), stmt.tokens()),
	Nonterminal::NtPat(ref pat) => convert_tokens(pat.tokens.as_ref()),
	Nonterminal::NtTy(ref ty) => convert_tokens(ty.tokens.as_ref()),
	Nonterminal::NtIdent(ident, is_raw) => {
	Some(tokenstream::TokenTree::token(token::Ident(ident.name, is_raw), ident.span).into())
	}
	Nonterminal::NtLifetime(ident) => {
	Some(tokenstream::TokenTree::token(token::Lifetime(ident.name), ident.span).into())
	}
	Nonterminal::NtMeta(ref attr) => convert_tokens(attr.tokens.as_ref()),
	Nonterminal::NtPath(ref path) => convert_tokens(path.tokens.as_ref()),
	Nonterminal::NtVis(ref vis) => convert_tokens(vis.tokens.as_ref()),
	Nonterminal::NtTT(ref tt) => Some(tt.clone().into()),
	Nonterminal::NtExpr(ref expr) \| Nonterminal::NtLiteral(ref expr) => {
	prepend_attrs(&expr.attrs, expr.tokens.as_ref())
	}
	};

	if let Some(tokens) = tokens {
	return tokens;
	} else if matches!(synthesize_tokens, CanSynthesizeMissingTokens::Yes) {
	return fake_token_stream(sess, nt);
	} else {
	panic!(
	"Missing tokens for nt {:?} at {:?}: {:?}",
	nt,
	nt.span(),
	pprust::nonterminal_to_string(nt)
	);
	}
	}

	fn prepend_attrs(attrs: &[Attribute], tokens: Option<&LazyTokenStream>) -> Option<TokenStream> {
	let tokens = tokens?;
	if attrs.is_empty() {
	return Some(tokens.create_token_stream().to_tokenstream());
	}
	let attr_data = AttributesData { attrs: attrs.to_vec().into(), tokens: tokens.clone() };
	let wrapped = AttrAnnotatedTokenStream::new(vec![(
	AttrAnnotatedTokenTree::Attributes(attr_data),
	Spacing::Alone,
	)]);
	Some(wrapped.to_tokenstream())
	}

	pub fn fake_token_stream(sess: &ParseSess, nt: &Nonterminal) -> TokenStream {
	let source = pprust::nonterminal_to_string(nt);
	let filename = FileName::macro_expansion_source_code(&source);
	parse_stream_from_source_str(filename, source, sess, Some(nt.span()))
	}

	pub fn parse_cfg_attr(
	attr: &Attribute,
	parse_sess: &ParseSess,
	) -> Option<(MetaItem, Vec<(AttrItem, Span)>)> {
	match attr.get_normal_item().args {
	ast::MacArgs::Delimited(dspan, delim, ref tts) if !tts.is_empty() => {
	let msg = "wrong `cfg_attr` delimiters";
	crate::validate_attr::check_meta_bad_delim(parse_sess, dspan, delim, msg);
	match parse_in(parse_sess, tts.clone(), "`cfg_attr` input", \|p\| p.parse_cfg_attr()) {
	Ok(r) => return Some(r),
	Err(mut e) => {
	e.help(&format!("the valid syntax is `{}`", CFG_ATTR_GRAMMAR_HELP))
	.note(CFG_ATTR_NOTE_REF)
	.emit();
	}
	}
	}
	_ => error_malformed_cfg_attr_missing(attr.span, parse_sess),
	}
	None
	}

	const CFG_ATTR_GRAMMAR_HELP: &str = "#[cfg_attr(condition, attribute, other_attribute, ...)]";
	const CFG_ATTR_NOTE_REF: &str = "for more information, visit \
	<https://doc.rust-lang.org/reference/conditional-compilation.html\
	#the-cfg_attr-attribute>";

	fn error_malformed_cfg_attr_missing(span: Span, parse_sess: &ParseSess) {
	parse_sess
	.span_diagnostic
	.struct_span_err(span, "malformed `cfg_attr` attribute input")
	.span_suggestion(
	span,
	"missing condition and attribute",
	CFG_ATTR_GRAMMAR_HELP.to_string(),
	Applicability::HasPlaceholders,
	)
	.note(CFG_ATTR_NOTE_REF)
	.emit();
	}