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    //! A support library for macro authors when defining new macros.
    //!
    //! This library, provided by the standard distribution, provides the types
    //! consumed in the interfaces of procedurally defined macro definitions such as
    //! function-like macros `#[proc_macro]`, macro attributes `#[proc_macro_attribute]` and
    //! custom derive attributes`#[proc_macro_derive]`.
    //!
    //! See [the book] for more.
    //!
    //! [the book]: ../book/ch19-06-macros.html#procedural-macros-for-generating-code-from-attributes

    #![stable(feature = "proc_macro_lib", since = "1.15.0")]
    #![deny(missing_docs)]
    #![doc(
        html_root_url = "https://doc.rust-lang.org/nightly/",
        html_playground_url = "https://play.rust-lang.org/",
        issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/",
        test(no_crate_inject, attr(deny(warnings))),
        test(attr(allow(dead_code, deprecated, unused_variables, unused_mut)))
    )]
    #![feature(nll)]
    #![feature(staged_api)]
    #![feature(allow_internal_unstable)]
    #![feature(decl_macro)]
    #![feature(extern_types)]
    #![feature(in_band_lifetimes)]
    #![feature(negative_impls)]
    #![feature(optin_builtin_traits)]
    #![feature(restricted_std)]
    #![feature(rustc_attrs)]
    #![feature(min_specialization)]
    #![recursion_limit = "256"]

    #[unstable(feature = "proc_macro_internals", issue = "27812")]
    #[doc(hidden)]
    pub mod bridge;

    mod diagnostic;

    #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
    pub use diagnostic::{Diagnostic, Level, MultiSpan};

    use std::cmp::Ordering;
    use std::ops::{Bound, RangeBounds};
    use std::path::PathBuf;
    use std::str::FromStr;
    use std::{error, fmt, iter, mem};

    /// Determines whether proc_macro has been made accessible to the currently
    /// running program.
    ///
    /// The proc_macro crate is only intended for use inside the implementation of
    /// procedural macros. All the functions in this crate panic if invoked from
    /// outside of a procedural macro, such as from a build script or unit test or
    /// ordinary Rust binary.
    ///
    /// With consideration for Rust libraries that are designed to support both
    /// macro and non-macro use cases, `proc_macro::is_available()` provides a
    /// non-panicking way to detect whether the infrastructure required to use the
    /// API of proc_macro is presently available. Returns true if invoked from
    /// inside of a procedural macro, false if invoked from any other binary.
    #[unstable(feature = "proc_macro_is_available", issue = "71436")]
    pub fn is_available() -> bool {
        bridge::Bridge::is_available()
    }

    /// The main type provided by this crate, representing an abstract stream of
    /// tokens, or, more specifically, a sequence of token trees.
    /// The type provide interfaces for iterating over those token trees and, conversely,
    /// collecting a number of token trees into one stream.
    ///
    /// This is both the input and output of `#[proc_macro]`, `#[proc_macro_attribute]`
    /// and `#[proc_macro_derive]` definitions.
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    #[derive(Clone)]
    pub struct TokenStream(bridge::client::TokenStream);

    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl !Send for TokenStream {}
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl !Sync for TokenStream {}

    /// Error returned from `TokenStream::from_str`.
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    #[derive(Debug)]
    pub struct LexError {
        _inner: (),
    }

    #[stable(feature = "proc_macro_lexerror_impls", since = "1.44.0")]
    impl fmt::Display for LexError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("cannot parse string into token stream")
        }
    }

    #[stable(feature = "proc_macro_lexerror_impls", since = "1.44.0")]
    impl error::Error for LexError {}

    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl !Send for LexError {}
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl !Sync for LexError {}

    impl TokenStream {
        /// Returns an empty `TokenStream` containing no token trees.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn new() -> TokenStream {
            TokenStream(bridge::client::TokenStream::new())
        }

        /// Checks if this `TokenStream` is empty.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn is_empty(&self) -> bool {
            self.0.is_empty()
        }
    }

    /// Attempts to break the string into tokens and parse those tokens into a token stream.
    /// May fail for a number of reasons, for example, if the string contains unbalanced delimiters
    /// or characters not existing in the language.
    /// All tokens in the parsed stream get `Span::call_site()` spans.
    ///
    /// NOTE: some errors may cause panics instead of returning `LexError`. We reserve the right to
    /// change these errors into `LexError`s later.
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl FromStr for TokenStream {
        type Err = LexError;

        fn from_str(src: &str) -> Result<TokenStream, LexError> {
            Ok(TokenStream(bridge::client::TokenStream::from_str(src)))
        }
    }

    // N.B., the bridge only provides `to_string`, implement `fmt::Display`
    // based on it (the reverse of the usual relationship between the two).
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl ToString for TokenStream {
        fn to_string(&self) -> String {
            self.0.to_string()
        }
    }

    /// Prints the token stream as a string that is supposed to be losslessly convertible back
    /// into the same token stream (modulo spans), except for possibly `TokenTree::Group`s
    /// with `Delimiter::None` delimiters and negative numeric literals.
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl fmt::Display for TokenStream {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str(&self.to_string())
        }
    }

    /// Prints token in a form convenient for debugging.
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl fmt::Debug for TokenStream {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("TokenStream ")?;
            f.debug_list().entries(self.clone()).finish()
        }
    }

    #[stable(feature = "proc_macro_token_stream_default", since = "1.45.0")]
    impl Default for TokenStream {
        fn default() -> Self {
            TokenStream::new()
        }
    }

    #[unstable(feature = "proc_macro_quote", issue = "54722")]
    pub use quote::{quote, quote_span};

    /// Creates a token stream containing a single token tree.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl From<TokenTree> for TokenStream {
        fn from(tree: TokenTree) -> TokenStream {
            TokenStream(bridge::client::TokenStream::from_token_tree(match tree {
                TokenTree::Group(tt) => bridge::TokenTree::Group(tt.0),
                TokenTree::Punct(tt) => bridge::TokenTree::Punct(tt.0),
                TokenTree::Ident(tt) => bridge::TokenTree::Ident(tt.0),
                TokenTree::Literal(tt) => bridge::TokenTree::Literal(tt.0),
            }))
        }
    }

    /// Collects a number of token trees into a single stream.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl iter::FromIterator<TokenTree> for TokenStream {
        fn from_iter<I: IntoIterator<Item = TokenTree>>(trees: I) -> Self {
            trees.into_iter().map(TokenStream::from).collect()
        }
    }

    /// A "flattening" operation on token streams, collects token trees
    /// from multiple token streams into a single stream.
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl iter::FromIterator<TokenStream> for TokenStream {
        fn from_iter<I: IntoIterator<Item = TokenStream>>(streams: I) -> Self {
            let mut builder = bridge::client::TokenStreamBuilder::new();
            streams.into_iter().for_each(|stream| builder.push(stream.0));
            TokenStream(builder.build())
        }
    }

    #[stable(feature = "token_stream_extend", since = "1.30.0")]
    impl Extend<TokenTree> for TokenStream {
        fn extend<I: IntoIterator<Item = TokenTree>>(&mut self, trees: I) {
            self.extend(trees.into_iter().map(TokenStream::from));
        }
    }

    #[stable(feature = "token_stream_extend", since = "1.30.0")]
    impl Extend<TokenStream> for TokenStream {
        fn extend<I: IntoIterator<Item = TokenStream>>(&mut self, streams: I) {
            // FIXME(eddyb) Use an optimized implementation if/when possible.
            *self = iter::once(mem::replace(self, Self::new())).chain(streams).collect();
        }
    }

    /// Public implementation details for the `TokenStream` type, such as iterators.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    pub mod token_stream {
        use crate::{bridge, Group, Ident, Literal, Punct, TokenStream, TokenTree};

        /// An iterator over `TokenStream`'s `TokenTree`s.
        /// The iteration is "shallow", e.g., the iterator doesn't recurse into delimited groups,
        /// and returns whole groups as token trees.
        #[derive(Clone)]
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub struct IntoIter(bridge::client::TokenStreamIter);

        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        impl Iterator for IntoIter {
            type Item = TokenTree;

            fn next(&mut self) -> Option<TokenTree> {
                self.0.next().map(|tree| match tree {
                    bridge::TokenTree::Group(tt) => TokenTree::Group(Group(tt)),
                    bridge::TokenTree::Punct(tt) => TokenTree::Punct(Punct(tt)),
                    bridge::TokenTree::Ident(tt) => TokenTree::Ident(Ident(tt)),
                    bridge::TokenTree::Literal(tt) => TokenTree::Literal(Literal(tt)),
                })
            }
        }

        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        impl IntoIterator for TokenStream {
            type Item = TokenTree;
            type IntoIter = IntoIter;

            fn into_iter(self) -> IntoIter {
                IntoIter(self.0.into_iter())
            }
        }
    }

    /// `quote!(..)` accepts arbitrary tokens and expands into a `TokenStream` describing the input.
    /// For example, `quote!(a + b)` will produce a expression, that, when evaluated, constructs
    /// the `TokenStream` `[Ident("a"), Punct('+', Alone), Ident("b")]`.
    ///
    /// Unquoting is done with `$`, and works by taking the single next ident as the unquoted term.
    /// To quote `$` itself, use `$$`.
    #[unstable(feature = "proc_macro_quote", issue = "54722")]
    #[allow_internal_unstable(proc_macro_def_site)]
    #[rustc_builtin_macro]
    pub macro quote($($t:tt)*) {
        /* compiler built-in */
    }

    #[unstable(feature = "proc_macro_internals", issue = "27812")]
    #[doc(hidden)]
    mod quote;

    /// A region of source code, along with macro expansion information.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    #[derive(Copy, Clone)]
    pub struct Span(bridge::client::Span);

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl !Send for Span {}
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl !Sync for Span {}

    macro_rules! diagnostic_method {
        ($name:ident, $level:expr) => {
            /// Creates a new `Diagnostic` with the given `message` at the span
            /// `self`.
            #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
            pub fn $name<T: Into<String>>(self, message: T) -> Diagnostic {
                Diagnostic::spanned(self, $level, message)
            }
        };
    }

    impl Span {
        /// A span that resolves at the macro definition site.
        #[unstable(feature = "proc_macro_def_site", issue = "54724")]
        pub fn def_site() -> Span {
            Span(bridge::client::Span::def_site())
        }

        /// The span of the invocation of the current procedural macro.
        /// Identifiers created with this span will be resolved as if they were written
        /// directly at the macro call location (call-site hygiene) and other code
        /// at the macro call site will be able to refer to them as well.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn call_site() -> Span {
            Span(bridge::client::Span::call_site())
        }

        /// A span that represents `macro_rules` hygiene, and sometimes resolves at the macro
        /// definition site (local variables, labels, `$crate`) and sometimes at the macro
        /// call site (everything else).
        /// The span location is taken from the call-site.
        #[stable(feature = "proc_macro_mixed_site", since = "1.45.0")]
        pub fn mixed_site() -> Span {
            Span(bridge::client::Span::mixed_site())
        }

        /// The original source file into which this span points.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn source_file(&self) -> SourceFile {
            SourceFile(self.0.source_file())
        }

        /// The `Span` for the tokens in the previous macro expansion from which
        /// `self` was generated from, if any.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn parent(&self) -> Option<Span> {
            self.0.parent().map(Span)
        }

        /// The span for the origin source code that `self` was generated from. If
        /// this `Span` wasn't generated from other macro expansions then the return
        /// value is the same as `*self`.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn source(&self) -> Span {
            Span(self.0.source())
        }

        /// Gets the starting line/column in the source file for this span.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn start(&self) -> LineColumn {
            self.0.start()
        }

        /// Gets the ending line/column in the source file for this span.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn end(&self) -> LineColumn {
            self.0.end()
        }

        /// Creates a new span encompassing `self` and `other`.
        ///
        /// Returns `None` if `self` and `other` are from different files.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn join(&self, other: Span) -> Option<Span> {
            self.0.join(other.0).map(Span)
        }

        /// Creates a new span with the same line/column information as `self` but
        /// that resolves symbols as though it were at `other`.
        #[stable(feature = "proc_macro_span_resolved_at", since = "1.45.0")]
        pub fn resolved_at(&self, other: Span) -> Span {
            Span(self.0.resolved_at(other.0))
        }

        /// Creates a new span with the same name resolution behavior as `self` but
        /// with the line/column information of `other`.
        #[stable(feature = "proc_macro_span_located_at", since = "1.45.0")]
        pub fn located_at(&self, other: Span) -> Span {
            other.resolved_at(*self)
        }

        /// Compares to spans to see if they're equal.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn eq(&self, other: &Span) -> bool {
            self.0 == other.0
        }

        /// Returns the source text behind a span. This preserves the original source
        /// code, including spaces and comments. It only returns a result if the span
        /// corresponds to real source code.
        ///
        /// Note: The observable result of a macro should only rely on the tokens and
        /// not on this source text. The result of this function is a best effort to
        /// be used for diagnostics only.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn source_text(&self) -> Option<String> {
            self.0.source_text()
        }

        diagnostic_method!(error, Level::Error);
        diagnostic_method!(warning, Level::Warning);
        diagnostic_method!(note, Level::Note);
        diagnostic_method!(help, Level::Help);
    }

    /// Prints a span in a form convenient for debugging.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Debug for Span {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            self.0.fmt(f)
        }
    }

    /// A line-column pair representing the start or end of a `Span`.
    #[unstable(feature = "proc_macro_span", issue = "54725")]
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    pub struct LineColumn {
        /// The 1-indexed line in the source file on which the span starts or ends (inclusive).
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub line: usize,
        /// The 0-indexed column (in UTF-8 characters) in the source file on which
        /// the span starts or ends (inclusive).
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub column: usize,
    }

    #[unstable(feature = "proc_macro_span", issue = "54725")]
    impl !Send for LineColumn {}
    #[unstable(feature = "proc_macro_span", issue = "54725")]
    impl !Sync for LineColumn {}

    #[unstable(feature = "proc_macro_span", issue = "54725")]
    impl Ord for LineColumn {
        fn cmp(&self, other: &Self) -> Ordering {
            self.line.cmp(&other.line).then(self.column.cmp(&other.column))
        }
    }

    #[unstable(feature = "proc_macro_span", issue = "54725")]
    impl PartialOrd for LineColumn {
        fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
            Some(self.cmp(other))
        }
    }

    /// The source file of a given `Span`.
    #[unstable(feature = "proc_macro_span", issue = "54725")]
    #[derive(Clone)]
    pub struct SourceFile(bridge::client::SourceFile);

    impl SourceFile {
        /// Gets the path to this source file.
        ///
        /// ### Note
        /// If the code span associated with this `SourceFile` was generated by an external macro, this
        /// macro, this may not be an actual path on the filesystem. Use [`is_real`] to check.
        ///
        /// Also note that even if `is_real` returns `true`, if `--remap-path-prefix` was passed on
        /// the command line, the path as given may not actually be valid.
        ///
        /// [`is_real`]: Self::is_real
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn path(&self) -> PathBuf {
            PathBuf::from(self.0.path())
        }

        /// Returns `true` if this source file is a real source file, and not generated by an external
        /// macro's expansion.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn is_real(&self) -> bool {
            // This is a hack until intercrate spans are implemented and we can have real source files
            // for spans generated in external macros.
            // https://github.com/rust-lang/rust/pull/43604#issuecomment-333334368
            self.0.is_real()
        }
    }

    #[unstable(feature = "proc_macro_span", issue = "54725")]
    impl fmt::Debug for SourceFile {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("SourceFile")
                .field("path", &self.path())
                .field("is_real", &self.is_real())
                .finish()
        }
    }

    #[unstable(feature = "proc_macro_span", issue = "54725")]
    impl PartialEq for SourceFile {
        fn eq(&self, other: &Self) -> bool {
            self.0.eq(&other.0)
        }
    }

    #[unstable(feature = "proc_macro_span", issue = "54725")]
    impl Eq for SourceFile {}

    /// A single token or a delimited sequence of token trees (e.g., `[1, (), ..]`).
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    #[derive(Clone)]
    pub enum TokenTree {
        /// A token stream surrounded by bracket delimiters.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Group(#[stable(feature = "proc_macro_lib2", since = "1.29.0")] Group),
        /// An identifier.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Ident(#[stable(feature = "proc_macro_lib2", since = "1.29.0")] Ident),
        /// A single punctuation character (`+`, `,`, `$`, etc.).
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Punct(#[stable(feature = "proc_macro_lib2", since = "1.29.0")] Punct),
        /// A literal character (`'a'`), string (`"hello"`), number (`2.3`), etc.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Literal(#[stable(feature = "proc_macro_lib2", since = "1.29.0")] Literal),
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl !Send for TokenTree {}
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl !Sync for TokenTree {}

    impl TokenTree {
        /// Returns the span of this tree, delegating to the `span` method of
        /// the contained token or a delimited stream.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn span(&self) -> Span {
            match *self {
                TokenTree::Group(ref t) => t.span(),
                TokenTree::Ident(ref t) => t.span(),
                TokenTree::Punct(ref t) => t.span(),
                TokenTree::Literal(ref t) => t.span(),
            }
        }

        /// Configures the span for *only this token*.
        ///
        /// Note that if this token is a `Group` then this method will not configure
        /// the span of each of the internal tokens, this will simply delegate to
        /// the `set_span` method of each variant.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn set_span(&mut self, span: Span) {
            match *self {
                TokenTree::Group(ref mut t) => t.set_span(span),
                TokenTree::Ident(ref mut t) => t.set_span(span),
                TokenTree::Punct(ref mut t) => t.set_span(span),
                TokenTree::Literal(ref mut t) => t.set_span(span),
            }
        }
    }

    /// Prints token tree in a form convenient for debugging.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Debug for TokenTree {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            // Each of these has the name in the struct type in the derived debug,
            // so don't bother with an extra layer of indirection
            match *self {
                TokenTree::Group(ref tt) => tt.fmt(f),
                TokenTree::Ident(ref tt) => tt.fmt(f),
                TokenTree::Punct(ref tt) => tt.fmt(f),
                TokenTree::Literal(ref tt) => tt.fmt(f),
            }
        }
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl From<Group> for TokenTree {
        fn from(g: Group) -> TokenTree {
            TokenTree::Group(g)
        }
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl From<Ident> for TokenTree {
        fn from(g: Ident) -> TokenTree {
            TokenTree::Ident(g)
        }
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl From<Punct> for TokenTree {
        fn from(g: Punct) -> TokenTree {
            TokenTree::Punct(g)
        }
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl From<Literal> for TokenTree {
        fn from(g: Literal) -> TokenTree {
            TokenTree::Literal(g)
        }
    }

    // N.B., the bridge only provides `to_string`, implement `fmt::Display`
    // based on it (the reverse of the usual relationship between the two).
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl ToString for TokenTree {
        fn to_string(&self) -> String {
            match *self {
                TokenTree::Group(ref t) => t.to_string(),
                TokenTree::Ident(ref t) => t.to_string(),
                TokenTree::Punct(ref t) => t.to_string(),
                TokenTree::Literal(ref t) => t.to_string(),
            }
        }
    }

    /// Prints the token tree as a string that is supposed to be losslessly convertible back
    /// into the same token tree (modulo spans), except for possibly `TokenTree::Group`s
    /// with `Delimiter::None` delimiters and negative numeric literals.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Display for TokenTree {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str(&self.to_string())
        }
    }

    /// A delimited token stream.
    ///
    /// A `Group` internally contains a `TokenStream` which is surrounded by `Delimiter`s.
    #[derive(Clone)]
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    pub struct Group(bridge::client::Group);

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl !Send for Group {}
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl !Sync for Group {}

    /// Describes how a sequence of token trees is delimited.
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    pub enum Delimiter {
        /// `( ... )`
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Parenthesis,
        /// `{ ... }`
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Brace,
        /// `[ ... ]`
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Bracket,
        /// `Ø ... Ø`
        /// An implicit delimiter, that may, for example, appear around tokens coming from a
        /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
        /// `$var * 3` where `$var` is `1 + 2`.
        /// Implicit delimiters may not survive roundtrip of a token stream through a string.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        None,
    }

    impl Group {
        /// Creates a new `Group` with the given delimiter and token stream.
        ///
        /// This constructor will set the span for this group to
        /// `Span::call_site()`. To change the span you can use the `set_span`
        /// method below.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn new(delimiter: Delimiter, stream: TokenStream) -> Group {
            Group(bridge::client::Group::new(delimiter, stream.0))
        }

        /// Returns the delimiter of this `Group`
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn delimiter(&self) -> Delimiter {
            self.0.delimiter()
        }

        /// Returns the `TokenStream` of tokens that are delimited in this `Group`.
        ///
        /// Note that the returned token stream does not include the delimiter
        /// returned above.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn stream(&self) -> TokenStream {
            TokenStream(self.0.stream())
        }

        /// Returns the span for the delimiters of this token stream, spanning the
        /// entire `Group`.
        ///
        /// ```text
        /// pub fn span(&self) -> Span {
        ///            ^^^^^^^
        /// ```
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn span(&self) -> Span {
            Span(self.0.span())
        }

        /// Returns the span pointing to the opening delimiter of this group.
        ///
        /// ```text
        /// pub fn span_open(&self) -> Span {
        ///                 ^
        /// ```
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn span_open(&self) -> Span {
            Span(self.0.span_open())
        }

        /// Returns the span pointing to the closing delimiter of this group.
        ///
        /// ```text
        /// pub fn span_close(&self) -> Span {
        ///                        ^
        /// ```
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn span_close(&self) -> Span {
            Span(self.0.span_close())
        }

        /// Configures the span for this `Group`'s delimiters, but not its internal
        /// tokens.
        ///
        /// This method will **not** set the span of all the internal tokens spanned
        /// by this group, but rather it will only set the span of the delimiter
        /// tokens at the level of the `Group`.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn set_span(&mut self, span: Span) {
            self.0.set_span(span.0);
        }
    }

    // N.B., the bridge only provides `to_string`, implement `fmt::Display`
    // based on it (the reverse of the usual relationship between the two).
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl ToString for Group {
        fn to_string(&self) -> String {
            TokenStream::from(TokenTree::from(self.clone())).to_string()
        }
    }

    /// Prints the group as a string that should be losslessly convertible back
    /// into the same group (modulo spans), except for possibly `TokenTree::Group`s
    /// with `Delimiter::None` delimiters.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Display for Group {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str(&self.to_string())
        }
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Debug for Group {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Group")
                .field("delimiter", &self.delimiter())
                .field("stream", &self.stream())
                .field("span", &self.span())
                .finish()
        }
    }

    /// An `Punct` is an single punctuation character like `+`, `-` or `#`.
    ///
    /// Multi-character operators like `+=` are represented as two instances of `Punct` with different
    /// forms of `Spacing` returned.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    #[derive(Clone)]
    pub struct Punct(bridge::client::Punct);

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl !Send for Punct {}
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl !Sync for Punct {}

    /// Whether an `Punct` is followed immediately by another `Punct` or
    /// followed by another token or whitespace.
    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    pub enum Spacing {
        /// e.g., `+` is `Alone` in `+ =`, `+ident` or `+()`.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Alone,
        /// e.g., `+` is `Joint` in `+=` or `'#`.
        /// Additionally, single quote `'` can join with identifiers to form lifetimes `'ident`.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        Joint,
    }

    impl Punct {
        /// Creates a new `Punct` from the given character and spacing.
        /// The `ch` argument must be a valid punctuation character permitted by the language,
        /// otherwise the function will panic.
        ///
        /// The returned `Punct` will have the default span of `Span::call_site()`
        /// which can be further configured with the `set_span` method below.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn new(ch: char, spacing: Spacing) -> Punct {
            Punct(bridge::client::Punct::new(ch, spacing))
        }

        /// Returns the value of this punctuation character as `char`.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn as_char(&self) -> char {
            self.0.as_char()
        }

        /// Returns the spacing of this punctuation character, indicating whether it's immediately
        /// followed by another `Punct` in the token stream, so they can potentially be combined into
        /// a multi-character operator (`Joint`), or it's followed by some other token or whitespace
        /// (`Alone`) so the operator has certainly ended.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn spacing(&self) -> Spacing {
            self.0.spacing()
        }

        /// Returns the span for this punctuation character.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn span(&self) -> Span {
            Span(self.0.span())
        }

        /// Configure the span for this punctuation character.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn set_span(&mut self, span: Span) {
            self.0 = self.0.with_span(span.0);
        }
    }

    // N.B., the bridge only provides `to_string`, implement `fmt::Display`
    // based on it (the reverse of the usual relationship between the two).
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl ToString for Punct {
        fn to_string(&self) -> String {
            TokenStream::from(TokenTree::from(self.clone())).to_string()
        }
    }

    /// Prints the punctuation character as a string that should be losslessly convertible
    /// back into the same character.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Display for Punct {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str(&self.to_string())
        }
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Debug for Punct {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Punct")
                .field("ch", &self.as_char())
                .field("spacing", &self.spacing())
                .field("span", &self.span())
                .finish()
        }
    }

    /// An identifier (`ident`).
    #[derive(Clone)]
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    pub struct Ident(bridge::client::Ident);

    impl Ident {
        /// Creates a new `Ident` with the given `string` as well as the specified
        /// `span`.
        /// The `string` argument must be a valid identifier permitted by the
        /// language (including keywords, e.g. `self` or `fn`). Otherwise, the function will panic.
        ///
        /// Note that `span`, currently in rustc, configures the hygiene information
        /// for this identifier.
        ///
        /// As of this time `Span::call_site()` explicitly opts-in to "call-site" hygiene
        /// meaning that identifiers created with this span will be resolved as if they were written
        /// directly at the location of the macro call, and other code at the macro call site will be
        /// able to refer to them as well.
        ///
        /// Later spans like `Span::def_site()` will allow to opt-in to "definition-site" hygiene
        /// meaning that identifiers created with this span will be resolved at the location of the
        /// macro definition and other code at the macro call site will not be able to refer to them.
        ///
        /// Due to the current importance of hygiene this constructor, unlike other
        /// tokens, requires a `Span` to be specified at construction.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn new(string: &str, span: Span) -> Ident {
            Ident(bridge::client::Ident::new(string, span.0, false))
        }

        /// Same as `Ident::new`, but creates a raw identifier (`r#ident`).
        /// The `string` argument be a valid identifier permitted by the language
        /// (including keywords, e.g. `fn`). Keywords which are usable in path segments
        /// (e.g. `self`, `super`) are not supported, and will cause a panic.
        #[stable(feature = "proc_macro_raw_ident", since = "1.47.0")]
        pub fn new_raw(string: &str, span: Span) -> Ident {
            Ident(bridge::client::Ident::new(string, span.0, true))
        }

        /// Returns the span of this `Ident`, encompassing the entire string returned
        /// by `as_str`.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn span(&self) -> Span {
            Span(self.0.span())
        }

        /// Configures the span of this `Ident`, possibly changing its hygiene context.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn set_span(&mut self, span: Span) {
            self.0 = self.0.with_span(span.0);
        }
    }

    // N.B., the bridge only provides `to_string`, implement `fmt::Display`
    // based on it (the reverse of the usual relationship between the two).
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl ToString for Ident {
        fn to_string(&self) -> String {
            TokenStream::from(TokenTree::from(self.clone())).to_string()
        }
    }

    /// Prints the identifier as a string that should be losslessly convertible
    /// back into the same identifier.
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Display for Ident {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str(&self.to_string())
        }
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Debug for Ident {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Ident")
                .field("ident", &self.to_string())
                .field("span", &self.span())
                .finish()
        }
    }

    /// A literal string (`"hello"`), byte string (`b"hello"`),
    /// character (`'a'`), byte character (`b'a'`), an integer or floating point number
    /// with or without a suffix (`1`, `1u8`, `2.3`, `2.3f32`).
    /// Boolean literals like `true` and `false` do not belong here, they are `Ident`s.
    #[derive(Clone)]
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    pub struct Literal(bridge::client::Literal);

    macro_rules! suffixed_int_literals {
        ($($name:ident => $kind:ident,)*) => ($(
            /// Creates a new suffixed integer literal with the specified value.
            ///
            /// This function will create an integer like `1u32` where the integer
            /// value specified is the first part of the token and the integral is
            /// also suffixed at the end.
            /// Literals created from negative numbers may not survive round-trips through
            /// `TokenStream` or strings and may be broken into two tokens (`-` and positive literal).
            ///
            /// Literals created through this method have the `Span::call_site()`
            /// span by default, which can be configured with the `set_span` method
            /// below.
            #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
            pub fn $name(n: $kind) -> Literal {
                Literal(bridge::client::Literal::typed_integer(&n.to_string(), stringify!($kind)))
            }
        )*)
    }

    macro_rules! unsuffixed_int_literals {
        ($($name:ident => $kind:ident,)*) => ($(
            /// Creates a new unsuffixed integer literal with the specified value.
            ///
            /// This function will create an integer like `1` where the integer
            /// value specified is the first part of the token. No suffix is
            /// specified on this token, meaning that invocations like
            /// `Literal::i8_unsuffixed(1)` are equivalent to
            /// `Literal::u32_unsuffixed(1)`.
            /// Literals created from negative numbers may not survive rountrips through
            /// `TokenStream` or strings and may be broken into two tokens (`-` and positive literal).
            ///
            /// Literals created through this method have the `Span::call_site()`
            /// span by default, which can be configured with the `set_span` method
            /// below.
            #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
            pub fn $name(n: $kind) -> Literal {
                Literal(bridge::client::Literal::integer(&n.to_string()))
            }
        )*)
    }

    impl Literal {
        suffixed_int_literals! {
            u8_suffixed => u8,
            u16_suffixed => u16,
            u32_suffixed => u32,
            u64_suffixed => u64,
            u128_suffixed => u128,
            usize_suffixed => usize,
            i8_suffixed => i8,
            i16_suffixed => i16,
            i32_suffixed => i32,
            i64_suffixed => i64,
            i128_suffixed => i128,
            isize_suffixed => isize,
        }

        unsuffixed_int_literals! {
            u8_unsuffixed => u8,
            u16_unsuffixed => u16,
            u32_unsuffixed => u32,
            u64_unsuffixed => u64,
            u128_unsuffixed => u128,
            usize_unsuffixed => usize,
            i8_unsuffixed => i8,
            i16_unsuffixed => i16,
            i32_unsuffixed => i32,
            i64_unsuffixed => i64,
            i128_unsuffixed => i128,
            isize_unsuffixed => isize,
        }

        /// Creates a new unsuffixed floating-point literal.
        ///
        /// This constructor is similar to those like `Literal::i8_unsuffixed` where
        /// the float's value is emitted directly into the token but no suffix is
        /// used, so it may be inferred to be a `f64` later in the compiler.
        /// Literals created from negative numbers may not survive rountrips through
        /// `TokenStream` or strings and may be broken into two tokens (`-` and positive literal).
        ///
        /// # Panics
        ///
        /// This function requires that the specified float is finite, for
        /// example if it is infinity or NaN this function will panic.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn f32_unsuffixed(n: f32) -> Literal {
            if !n.is_finite() {
                panic!("Invalid float literal {}", n);
            }
            Literal(bridge::client::Literal::float(&n.to_string()))
        }

        /// Creates a new suffixed floating-point literal.
        ///
        /// This constructor will create a literal like `1.0f32` where the value
        /// specified is the preceding part of the token and `f32` is the suffix of
        /// the token. This token will always be inferred to be an `f32` in the
        /// compiler.
        /// Literals created from negative numbers may not survive rountrips through
        /// `TokenStream` or strings and may be broken into two tokens (`-` and positive literal).
        ///
        /// # Panics
        ///
        /// This function requires that the specified float is finite, for
        /// example if it is infinity or NaN this function will panic.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn f32_suffixed(n: f32) -> Literal {
            if !n.is_finite() {
                panic!("Invalid float literal {}", n);
            }
            Literal(bridge::client::Literal::f32(&n.to_string()))
        }

        /// Creates a new unsuffixed floating-point literal.
        ///
        /// This constructor is similar to those like `Literal::i8_unsuffixed` where
        /// the float's value is emitted directly into the token but no suffix is
        /// used, so it may be inferred to be a `f64` later in the compiler.
        /// Literals created from negative numbers may not survive rountrips through
        /// `TokenStream` or strings and may be broken into two tokens (`-` and positive literal).
        ///
        /// # Panics
        ///
        /// This function requires that the specified float is finite, for
        /// example if it is infinity or NaN this function will panic.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn f64_unsuffixed(n: f64) -> Literal {
            if !n.is_finite() {
                panic!("Invalid float literal {}", n);
            }
            Literal(bridge::client::Literal::float(&n.to_string()))
        }

        /// Creates a new suffixed floating-point literal.
        ///
        /// This constructor will create a literal like `1.0f64` where the value
        /// specified is the preceding part of the token and `f64` is the suffix of
        /// the token. This token will always be inferred to be an `f64` in the
        /// compiler.
        /// Literals created from negative numbers may not survive rountrips through
        /// `TokenStream` or strings and may be broken into two tokens (`-` and positive literal).
        ///
        /// # Panics
        ///
        /// This function requires that the specified float is finite, for
        /// example if it is infinity or NaN this function will panic.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn f64_suffixed(n: f64) -> Literal {
            if !n.is_finite() {
                panic!("Invalid float literal {}", n);
            }
            Literal(bridge::client::Literal::f64(&n.to_string()))
        }

        /// String literal.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn string(string: &str) -> Literal {
            Literal(bridge::client::Literal::string(string))
        }

        /// Character literal.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn character(ch: char) -> Literal {
            Literal(bridge::client::Literal::character(ch))
        }

        /// Byte string literal.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn byte_string(bytes: &[u8]) -> Literal {
            Literal(bridge::client::Literal::byte_string(bytes))
        }

        /// Returns the span encompassing this literal.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn span(&self) -> Span {
            Span(self.0.span())
        }

        /// Configures the span associated for this literal.
        #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
        pub fn set_span(&mut self, span: Span) {
            self.0.set_span(span.0);
        }

        /// Returns a `Span` that is a subset of `self.span()` containing only the
        /// source bytes in range `range`. Returns `None` if the would-be trimmed
        /// span is outside the bounds of `self`.
        // FIXME(SergioBenitez): check that the byte range starts and ends at a
        // UTF-8 boundary of the source. otherwise, it's likely that a panic will
        // occur elsewhere when the source text is printed.
        // FIXME(SergioBenitez): there is no way for the user to know what
        // `self.span()` actually maps to, so this method can currently only be
        // called blindly. For example, `to_string()` for the character 'c' returns
        // "'\u{63}'"; there is no way for the user to know whether the source text
        // was 'c' or whether it was '\u{63}'.
        #[unstable(feature = "proc_macro_span", issue = "54725")]
        pub fn subspan<R: RangeBounds<usize>>(&self, range: R) -> Option<Span> {
            // HACK(eddyb) something akin to `Option::cloned`, but for `Bound<&T>`.
            fn cloned_bound<T: Clone>(bound: Bound<&T>) -> Bound<T> {
                match bound {
                    Bound::Included(x) => Bound::Included(x.clone()),
                    Bound::Excluded(x) => Bound::Excluded(x.clone()),
                    Bound::Unbounded => Bound::Unbounded,
                }
            }

            self.0.subspan(cloned_bound(range.start_bound()), cloned_bound(range.end_bound())).map(Span)
        }
    }

    // N.B., the bridge only provides `to_string`, implement `fmt::Display`
    // based on it (the reverse of the usual relationship between the two).
    #[stable(feature = "proc_macro_lib", since = "1.15.0")]
    impl ToString for Literal {
        fn to_string(&self) -> String {
            TokenStream::from(TokenTree::from(self.clone())).to_string()
        }
    }

    /// Prints the literal as a string that should be losslessly convertible
    /// back into the same literal (except for possible rounding for floating point literals).
    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Display for Literal {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str(&self.to_string())
        }
    }

    #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
    impl fmt::Debug for Literal {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            self.0.fmt(f)
        }
    }

    /// Tracked access to environment variables.
    #[unstable(feature = "proc_macro_tracked_env", issue = "74690")]
    pub mod tracked_env {
        use std::env::{self, VarError};
        use std::ffi::OsStr;

        /// Retrieve an environment variable and add it to build dependency info.
        /// Build system executing the compiler will know that the variable was accessed during
        /// compilation, and will be able to rerun the build when the value of that variable changes.
        /// Besides the dependency tracking this function should be equivalent to `env::var` from the
        /// standard library, except that the argument must be UTF-8.
        #[unstable(feature = "proc_macro_tracked_env", issue = "74690")]
        pub fn var<K: AsRef<OsStr> + AsRef<str>>(key: K) -> Result<String, VarError> {
            let key: &str = key.as_ref();
            let value = env::var(key);
            crate::bridge::client::FreeFunctions::track_env_var(key, value.as_deref().ok());
            value
        }
    }