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    //! [![github]](https://github.com/dtolnay/syn)&ensp;[![crates-io]](https://crates.io/crates/syn)&ensp;[![docs-rs]](crate)
    //!
    //! [github]: https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github
    //! [crates-io]: https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust
    //! [docs-rs]: https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logoColor=white&logo=data:image/svg+xml;base64,
    //!
    //! <br>
    //!
    //! Syn is a parsing library for parsing a stream of Rust tokens into a syntax
    //! tree of Rust source code.
    //!
    //! Currently this library is geared toward use in Rust procedural macros, but
    //! contains some APIs that may be useful more generally.
    //!
    //! - **Data structures** — Syn provides a complete syntax tree that can
    //!   represent any valid Rust source code. The syntax tree is rooted at
    //!   [`syn::File`] which represents a full source file, but there are other
    //!   entry points that may be useful to procedural macros including
    //!   [`syn::Item`], [`syn::Expr`] and [`syn::Type`].
    //!
    //! - **Derives** — Of particular interest to derive macros is
    //!   [`syn::DeriveInput`] which is any of the three legal input items to a
    //!   derive macro. An example below shows using this type in a library that can
    //!   derive implementations of a user-defined trait.
    //!
    //! - **Parsing** — Parsing in Syn is built around [parser functions] with the
    //!   signature `fn(ParseStream) -> Result<T>`. Every syntax tree node defined
    //!   by Syn is individually parsable and may be used as a building block for
    //!   custom syntaxes, or you may dream up your own brand new syntax without
    //!   involving any of our syntax tree types.
    //!
    //! - **Location information** — Every token parsed by Syn is associated with a
    //!   `Span` that tracks line and column information back to the source of that
    //!   token. These spans allow a procedural macro to display detailed error
    //!   messages pointing to all the right places in the user's code. There is an
    //!   example of this below.
    //!
    //! - **Feature flags** — Functionality is aggressively feature gated so your
    //!   procedural macros enable only what they need, and do not pay in compile
    //!   time for all the rest.
    //!
    //! [`syn::File`]: File
    //! [`syn::Item`]: Item
    //! [`syn::Expr`]: Expr
    //! [`syn::Type`]: Type
    //! [`syn::DeriveInput`]: DeriveInput
    //! [parser functions]: mod@parse
    //!
    //! <br>
    //!
    //! # Example of a derive macro
    //!
    //! The canonical derive macro using Syn looks like this. We write an ordinary
    //! Rust function tagged with a `proc_macro_derive` attribute and the name of
    //! the trait we are deriving. Any time that derive appears in the user's code,
    //! the Rust compiler passes their data structure as tokens into our macro. We
    //! get to execute arbitrary Rust code to figure out what to do with those
    //! tokens, then hand some tokens back to the compiler to compile into the
    //! user's crate.
    //!
    //! [`TokenStream`]: proc_macro::TokenStream
    //!
    //! ```toml
    //! [dependencies]
    //! syn = "1.0"
    //! quote = "1.0"
    //!
    //! [lib]
    //! proc-macro = true
    //! ```
    //!
    //! ```
    //! # extern crate proc_macro;
    //! #
    //! use proc_macro::TokenStream;
    //! use quote::quote;
    //! use syn::{parse_macro_input, DeriveInput};
    //!
    //! # const IGNORE_TOKENS: &str = stringify! {
    //! #[proc_macro_derive(MyMacro)]
    //! # };
    //! pub fn my_macro(input: TokenStream) -> TokenStream {
    //!     // Parse the input tokens into a syntax tree
    //!     let input = parse_macro_input!(input as DeriveInput);
    //!
    //!     // Build the output, possibly using quasi-quotation
    //!     let expanded = quote! {
    //!         // ...
    //!     };
    //!
    //!     // Hand the output tokens back to the compiler
    //!     TokenStream::from(expanded)
    //! }
    //! ```
    //!
    //! The [`heapsize`] example directory shows a complete working implementation
    //! of a derive macro. It works on any Rust compiler 1.31+. The example derives
    //! a `HeapSize` trait which computes an estimate of the amount of heap memory
    //! owned by a value.
    //!
    //! [`heapsize`]: https://github.com/dtolnay/syn/tree/master/examples/heapsize
    //!
    //! ```
    //! pub trait HeapSize {
    //!     /// Total number of bytes of heap memory owned by `self`.
    //!     fn heap_size_of_children(&self) -> usize;
    //! }
    //! ```
    //!
    //! The derive macro allows users to write `#[derive(HeapSize)]` on data
    //! structures in their program.
    //!
    //! ```
    //! # const IGNORE_TOKENS: &str = stringify! {
    //! #[derive(HeapSize)]
    //! # };
    //! struct Demo<'a, T: ?Sized> {
    //!     a: Box<T>,
    //!     b: u8,
    //!     c: &'a str,
    //!     d: String,
    //! }
    //! ```
    //!
    //! <p><br></p>
    //!
    //! # Spans and error reporting
    //!
    //! The token-based procedural macro API provides great control over where the
    //! compiler's error messages are displayed in user code. Consider the error the
    //! user sees if one of their field types does not implement `HeapSize`.
    //!
    //! ```
    //! # const IGNORE_TOKENS: &str = stringify! {
    //! #[derive(HeapSize)]
    //! # };
    //! struct Broken {
    //!     ok: String,
    //!     bad: std::thread::Thread,
    //! }
    //! ```
    //!
    //! By tracking span information all the way through the expansion of a
    //! procedural macro as shown in the `heapsize` example, token-based macros in
    //! Syn are able to trigger errors that directly pinpoint the source of the
    //! problem.
    //!
    //! ```text
    //! error[E0277]: the trait bound `std::thread::Thread: HeapSize` is not satisfied
    //!  --> src/main.rs:7:5
    //!   |
    //! 7 |     bad: std::thread::Thread,
    //!   |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `HeapSize` is not implemented for `Thread`
    //! ```
    //!
    //! <br>
    //!
    //! # Parsing a custom syntax
    //!
    //! The [`lazy-static`] example directory shows the implementation of a
    //! `functionlike!(...)` procedural macro in which the input tokens are parsed
    //! using Syn's parsing API.
    //!
    //! [`lazy-static`]: https://github.com/dtolnay/syn/tree/master/examples/lazy-static
    //!
    //! The example reimplements the popular `lazy_static` crate from crates.io as a
    //! procedural macro.
    //!
    //! ```
    //! # macro_rules! lazy_static {
    //! #     ($($tt:tt)*) => {}
    //! # }
    //! #
    //! lazy_static! {
    //!     static ref USERNAME: Regex = Regex::new("^[a-z0-9_-]{3,16}$").unwrap();
    //! }
    //! ```
    //!
    //! The implementation shows how to trigger custom warnings and error messages
    //! on the macro input.
    //!
    //! ```text
    //! warning: come on, pick a more creative name
    //!   --> src/main.rs:10:16
    //!    |
    //! 10 |     static ref FOO: String = "lazy_static".to_owned();
    //!    |                ^^^
    //! ```
    //!
    //! <br>
    //!
    //! # Testing
    //!
    //! When testing macros, we often care not just that the macro can be used
    //! successfully but also that when the macro is provided with invalid input it
    //! produces maximally helpful error messages. Consider using the [`trybuild`]
    //! crate to write tests for errors that are emitted by your macro or errors
    //! detected by the Rust compiler in the expanded code following misuse of the
    //! macro. Such tests help avoid regressions from later refactors that
    //! mistakenly make an error no longer trigger or be less helpful than it used
    //! to be.
    //!
    //! [`trybuild`]: https://github.com/dtolnay/trybuild
    //!
    //! <br>
    //!
    //! # Debugging
    //!
    //! When developing a procedural macro it can be helpful to look at what the
    //! generated code looks like. Use `cargo rustc -- -Zunstable-options
    //! --pretty=expanded` or the [`cargo expand`] subcommand.
    //!
    //! [`cargo expand`]: https://github.com/dtolnay/cargo-expand
    //!
    //! To show the expanded code for some crate that uses your procedural macro,
    //! run `cargo expand` from that crate. To show the expanded code for one of
    //! your own test cases, run `cargo expand --test the_test_case` where the last
    //! argument is the name of the test file without the `.rs` extension.
    //!
    //! This write-up by Brandon W Maister discusses debugging in more detail:
    //! [Debugging Rust's new Custom Derive system][debugging].
    //!
    //! [debugging]: https://quodlibetor.github.io/posts/debugging-rusts-new-custom-derive-system/
    //!
    //! <br>
    //!
    //! # Optional features
    //!
    //! Syn puts a lot of functionality behind optional features in order to
    //! optimize compile time for the most common use cases. The following features
    //! are available.
    //!
    //! - **`derive`** *(enabled by default)* — Data structures for representing the
    //!   possible input to a derive macro, including structs and enums and types.
    //! - **`full`** — Data structures for representing the syntax tree of all valid
    //!   Rust source code, including items and expressions.
    //! - **`parsing`** *(enabled by default)* — Ability to parse input tokens into
    //!   a syntax tree node of a chosen type.
    //! - **`printing`** *(enabled by default)* — Ability to print a syntax tree
    //!   node as tokens of Rust source code.
    //! - **`visit`** — Trait for traversing a syntax tree.
    //! - **`visit-mut`** — Trait for traversing and mutating in place a syntax
    //!   tree.
    //! - **`fold`** — Trait for transforming an owned syntax tree.
    //! - **`clone-impls`** *(enabled by default)* — Clone impls for all syntax tree
    //!   types.
    //! - **`extra-traits`** — Debug, Eq, PartialEq, Hash impls for all syntax tree
    //!   types.
    //! - **`proc-macro`** *(enabled by default)* — Runtime dependency on the
    //!   dynamic library libproc_macro from rustc toolchain.

    // Syn types in rustdoc of other crates get linked to here.
    #![doc(html_root_url = "https://docs.rs/syn/1.0.57")]
    #![cfg_attr(doc_cfg, feature(doc_cfg))]
    // Ignored clippy lints.
    #![allow(
        clippy::doc_markdown,
        clippy::eval_order_dependence,
        clippy::inherent_to_string,
        clippy::large_enum_variant,
        clippy::match_on_vec_items,
        clippy::needless_doctest_main,
        clippy::needless_pass_by_value,
        clippy::never_loop,
        clippy::too_many_arguments,
        clippy::trivially_copy_pass_by_ref,
        clippy::unnecessary_unwrap
    )]
    // Ignored clippy_pedantic lints.
    #![allow(
        clippy::cast_possible_truncation,
        clippy::default_trait_access,
        clippy::empty_enum,
        clippy::expl_impl_clone_on_copy,
        clippy::if_not_else,
        clippy::match_same_arms,
        clippy::missing_errors_doc,
        clippy::module_name_repetitions,
        clippy::must_use_candidate,
        clippy::option_if_let_else,
        clippy::redundant_else,
        clippy::shadow_unrelated,
        clippy::similar_names,
        clippy::single_match_else,
        clippy::too_many_lines,
        clippy::unseparated_literal_suffix,
        clippy::used_underscore_binding,
        clippy::wildcard_imports
    )]

    #[cfg(all(
        not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
        feature = "proc-macro"
    ))]
    extern crate proc_macro;
    extern crate proc_macro2;
    extern crate unicode_xid;

    #[cfg(feature = "printing")]
    extern crate quote;

    #[macro_use]
    mod macros;

    // Not public API.
    #[cfg(feature = "parsing")]
    #[doc(hidden)]
    #[macro_use]
    pub mod group;

    #[macro_use]
    pub mod token;

    mod ident;
    pub use crate::ident::Ident;

    #[cfg(any(feature = "full", feature = "derive"))]
    mod attr;
    #[cfg(any(feature = "full", feature = "derive"))]
    pub use crate::attr::{
        AttrStyle, Attribute, AttributeArgs, Meta, MetaList, MetaNameValue, NestedMeta,
    };

    mod bigint;

    #[cfg(any(feature = "full", feature = "derive"))]
    mod data;
    #[cfg(any(feature = "full", feature = "derive"))]
    pub use crate::data::{
        Field, Fields, FieldsNamed, FieldsUnnamed, Variant, VisCrate, VisPublic, VisRestricted,
        Visibility,
    };

    #[cfg(any(feature = "full", feature = "derive"))]
    mod expr;
    #[cfg(feature = "full")]
    pub use crate::expr::{
        Arm, FieldValue, GenericMethodArgument, Label, MethodTurbofish, RangeLimits,
    };
    #[cfg(any(feature = "full", feature = "derive"))]
    pub use crate::expr::{
        Expr, ExprArray, ExprAssign, ExprAssignOp, ExprAsync, ExprAwait, ExprBinary, ExprBlock,
        ExprBox, ExprBreak, ExprCall, ExprCast, ExprClosure, ExprContinue, ExprField, ExprForLoop,
        ExprGroup, ExprIf, ExprIndex, ExprLet, ExprLit, ExprLoop, ExprMacro, ExprMatch, ExprMethodCall,
        ExprParen, ExprPath, ExprRange, ExprReference, ExprRepeat, ExprReturn, ExprStruct, ExprTry,
        ExprTryBlock, ExprTuple, ExprType, ExprUnary, ExprUnsafe, ExprWhile, ExprYield, Index, Member,
    };

    #[cfg(any(feature = "full", feature = "derive"))]
    mod generics;
    #[cfg(any(feature = "full", feature = "derive"))]
    pub use crate::generics::{
        BoundLifetimes, ConstParam, GenericParam, Generics, LifetimeDef, PredicateEq,
        PredicateLifetime, PredicateType, TraitBound, TraitBoundModifier, TypeParam, TypeParamBound,
        WhereClause, WherePredicate,
    };
    #[cfg(all(any(feature = "full", feature = "derive"), feature = "printing"))]
    pub use crate::generics::{ImplGenerics, Turbofish, TypeGenerics};

    #[cfg(feature = "full")]
    mod item;
    #[cfg(feature = "full")]
    pub use crate::item::{
        FnArg, ForeignItem, ForeignItemFn, ForeignItemMacro, ForeignItemStatic, ForeignItemType,
        ImplItem, ImplItemConst, ImplItemMacro, ImplItemMethod, ImplItemType, Item, ItemConst,
        ItemEnum, ItemExternCrate, ItemFn, ItemForeignMod, ItemImpl, ItemMacro, ItemMacro2, ItemMod,
        ItemStatic, ItemStruct, ItemTrait, ItemTraitAlias, ItemType, ItemUnion, ItemUse, Receiver,
        Signature, TraitItem, TraitItemConst, TraitItemMacro, TraitItemMethod, TraitItemType, UseGlob,
        UseGroup, UseName, UsePath, UseRename, UseTree,
    };

    #[cfg(feature = "full")]
    mod file;
    #[cfg(feature = "full")]
    pub use crate::file::File;

    mod lifetime;
    pub use crate::lifetime::Lifetime;

    mod lit;
    pub use crate::lit::{
        Lit, LitBool, LitByte, LitByteStr, LitChar, LitFloat, LitInt, LitStr, StrStyle,
    };

    #[cfg(any(feature = "full", feature = "derive"))]
    mod mac;
    #[cfg(any(feature = "full", feature = "derive"))]
    pub use crate::mac::{Macro, MacroDelimiter};

    #[cfg(any(feature = "full", feature = "derive"))]
    mod derive;
    #[cfg(feature = "derive")]
    pub use crate::derive::{Data, DataEnum, DataStruct, DataUnion, DeriveInput};

    #[cfg(any(feature = "full", feature = "derive"))]
    mod op;
    #[cfg(any(feature = "full", feature = "derive"))]
    pub use crate::op::{BinOp, UnOp};

    #[cfg(feature = "full")]
    mod stmt;
    #[cfg(feature = "full")]
    pub use crate::stmt::{Block, Local, Stmt};

    #[cfg(any(feature = "full", feature = "derive"))]
    mod ty;
    #[cfg(any(feature = "full", feature = "derive"))]
    pub use crate::ty::{
        Abi, BareFnArg, ReturnType, Type, TypeArray, TypeBareFn, TypeGroup, TypeImplTrait, TypeInfer,
        TypeMacro, TypeNever, TypeParen, TypePath, TypePtr, TypeReference, TypeSlice, TypeTraitObject,
        TypeTuple, Variadic,
    };

    #[cfg(feature = "full")]
    mod pat;
    #[cfg(feature = "full")]
    pub use crate::pat::{
        FieldPat, Pat, PatBox, PatIdent, PatLit, PatMacro, PatOr, PatPath, PatRange, PatReference,
        PatRest, PatSlice, PatStruct, PatTuple, PatTupleStruct, PatType, PatWild,
    };

    #[cfg(any(feature = "full", feature = "derive"))]
    mod path;
    #[cfg(any(feature = "full", feature = "derive"))]
    pub use crate::path::{
        AngleBracketedGenericArguments, Binding, Constraint, GenericArgument,
        ParenthesizedGenericArguments, Path, PathArguments, PathSegment, QSelf,
    };

    #[cfg(feature = "parsing")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    pub mod buffer;
    #[cfg(feature = "parsing")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    pub mod ext;
    pub mod punctuated;
    #[cfg(all(any(feature = "full", feature = "derive"), feature = "extra-traits"))]
    mod tt;

    // Not public API except the `parse_quote!` macro.
    #[cfg(feature = "parsing")]
    #[doc(hidden)]
    pub mod parse_quote;

    // Not public API except the `parse_macro_input!` macro.
    #[cfg(all(
        not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
        feature = "parsing",
        feature = "proc-macro"
    ))]
    #[doc(hidden)]
    pub mod parse_macro_input;

    #[cfg(all(feature = "parsing", feature = "printing"))]
    #[cfg_attr(doc_cfg, doc(cfg(all(feature = "parsing", feature = "printing"))))]
    pub mod spanned;

    #[cfg(all(feature = "parsing", feature = "full"))]
    mod whitespace;

    mod gen {
        /// Syntax tree traversal to walk a shared borrow of a syntax tree.
        ///
        /// Each method of the [`Visit`] trait is a hook that can be overridden to
        /// customize the behavior when visiting the corresponding type of node. By
        /// default, every method recursively visits the substructure of the input
        /// by invoking the right visitor method of each of its fields.
        ///
        /// [`Visit`]: visit::Visit
        ///
        /// ```
        /// # use syn::{Attribute, BinOp, Expr, ExprBinary};
        /// #
        /// pub trait Visit<'ast> {
        ///     /* ... */
        ///
        ///     fn visit_expr_binary(&mut self, node: &'ast ExprBinary) {
        ///         visit_expr_binary(self, node);
        ///     }
        ///
        ///     /* ... */
        ///     # fn visit_attribute(&mut self, node: &'ast Attribute);
        ///     # fn visit_expr(&mut self, node: &'ast Expr);
        ///     # fn visit_bin_op(&mut self, node: &'ast BinOp);
        /// }
        ///
        /// pub fn visit_expr_binary<'ast, V>(v: &mut V, node: &'ast ExprBinary)
        /// where
        ///     V: Visit<'ast> + ?Sized,
        /// {
        ///     for attr in &node.attrs {
        ///         v.visit_attribute(attr);
        ///     }
        ///     v.visit_expr(&*node.left);
        ///     v.visit_bin_op(&node.op);
        ///     v.visit_expr(&*node.right);
        /// }
        ///
        /// /* ... */
        /// ```
        ///
        /// *This module is available only if Syn is built with the `"visit"` feature.*
        ///
        /// <br>
        ///
        /// # Example
        ///
        /// This visitor will print the name of every freestanding function in the
        /// syntax tree, including nested functions.
        ///
        /// ```
        /// // [dependencies]
        /// // quote = "1.0"
        /// // syn = { version = "1.0", features = ["full", "visit"] }
        ///
        /// use quote::quote;
        /// use syn::visit::{self, Visit};
        /// use syn::{File, ItemFn};
        ///
        /// struct FnVisitor;
        ///
        /// impl<'ast> Visit<'ast> for FnVisitor {
        ///     fn visit_item_fn(&mut self, node: &'ast ItemFn) {
        ///         println!("Function with name={}", node.sig.ident);
        ///
        ///         // Delegate to the default impl to visit any nested functions.
        ///         visit::visit_item_fn(self, node);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let code = quote! {
        ///         pub fn f() {
        ///             fn g() {}
        ///         }
        ///     };
        ///
        ///     let syntax_tree: File = syn::parse2(code).unwrap();
        ///     FnVisitor.visit_file(&syntax_tree);
        /// }
        /// ```
        ///
        /// The `'ast` lifetime on the input references means that the syntax tree
        /// outlives the complete recursive visit call, so the visitor is allowed to
        /// hold on to references into the syntax tree.
        ///
        /// ```
        /// use quote::quote;
        /// use syn::visit::{self, Visit};
        /// use syn::{File, ItemFn};
        ///
        /// struct FnVisitor<'ast> {
        ///     functions: Vec<&'ast ItemFn>,
        /// }
        ///
        /// impl<'ast> Visit<'ast> for FnVisitor<'ast> {
        ///     fn visit_item_fn(&mut self, node: &'ast ItemFn) {
        ///         self.functions.push(node);
        ///         visit::visit_item_fn(self, node);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let code = quote! {
        ///         pub fn f() {
        ///             fn g() {}
        ///         }
        ///     };
        ///
        ///     let syntax_tree: File = syn::parse2(code).unwrap();
        ///     let mut visitor = FnVisitor { functions: Vec::new() };
        ///     visitor.visit_file(&syntax_tree);
        ///     for f in visitor.functions {
        ///         println!("Function with name={}", f.sig.ident);
        ///     }
        /// }
        /// ```
        #[cfg(feature = "visit")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "visit")))]
        #[rustfmt::skip]
        pub mod visit;

        /// Syntax tree traversal to mutate an exclusive borrow of a syntax tree in
        /// place.
        ///
        /// Each method of the [`VisitMut`] trait is a hook that can be overridden
        /// to customize the behavior when mutating the corresponding type of node.
        /// By default, every method recursively visits the substructure of the
        /// input by invoking the right visitor method of each of its fields.
        ///
        /// [`VisitMut`]: visit_mut::VisitMut
        ///
        /// ```
        /// # use syn::{Attribute, BinOp, Expr, ExprBinary};
        /// #
        /// pub trait VisitMut {
        ///     /* ... */
        ///
        ///     fn visit_expr_binary_mut(&mut self, node: &mut ExprBinary) {
        ///         visit_expr_binary_mut(self, node);
        ///     }
        ///
        ///     /* ... */
        ///     # fn visit_attribute_mut(&mut self, node: &mut Attribute);
        ///     # fn visit_expr_mut(&mut self, node: &mut Expr);
        ///     # fn visit_bin_op_mut(&mut self, node: &mut BinOp);
        /// }
        ///
        /// pub fn visit_expr_binary_mut<V>(v: &mut V, node: &mut ExprBinary)
        /// where
        ///     V: VisitMut + ?Sized,
        /// {
        ///     for attr in &mut node.attrs {
        ///         v.visit_attribute_mut(attr);
        ///     }
        ///     v.visit_expr_mut(&mut *node.left);
        ///     v.visit_bin_op_mut(&mut node.op);
        ///     v.visit_expr_mut(&mut *node.right);
        /// }
        ///
        /// /* ... */
        /// ```
        ///
        /// *This module is available only if Syn is built with the `"visit-mut"`
        /// feature.*
        ///
        /// <br>
        ///
        /// # Example
        ///
        /// This mut visitor replace occurrences of u256 suffixed integer literals
        /// like `999u256` with a macro invocation `bigint::u256!(999)`.
        ///
        /// ```
        /// // [dependencies]
        /// // quote = "1.0"
        /// // syn = { version = "1.0", features = ["full", "visit-mut"] }
        ///
        /// use quote::quote;
        /// use syn::visit_mut::{self, VisitMut};
        /// use syn::{parse_quote, Expr, File, Lit, LitInt};
        ///
        /// struct BigintReplace;
        ///
        /// impl VisitMut for BigintReplace {
        ///     fn visit_expr_mut(&mut self, node: &mut Expr) {
        ///         if let Expr::Lit(expr) = &node {
        ///             if let Lit::Int(int) = &expr.lit {
        ///                 if int.suffix() == "u256" {
        ///                     let digits = int.base10_digits();
        ///                     let unsuffixed: LitInt = syn::parse_str(digits).unwrap();
        ///                     *node = parse_quote!(bigint::u256!(#unsuffixed));
        ///                     return;
        ///                 }
        ///             }
        ///         }
        ///
        ///         // Delegate to the default impl to visit nested expressions.
        ///         visit_mut::visit_expr_mut(self, node);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let code = quote! {
        ///         fn main() {
        ///             let _ = 999u256;
        ///         }
        ///     };
        ///
        ///     let mut syntax_tree: File = syn::parse2(code).unwrap();
        ///     BigintReplace.visit_file_mut(&mut syntax_tree);
        ///     println!("{}", quote!(#syntax_tree));
        /// }
        /// ```
        #[cfg(feature = "visit-mut")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "visit-mut")))]
        #[rustfmt::skip]
        pub mod visit_mut;

        /// Syntax tree traversal to transform the nodes of an owned syntax tree.
        ///
        /// Each method of the [`Fold`] trait is a hook that can be overridden to
        /// customize the behavior when transforming the corresponding type of node.
        /// By default, every method recursively visits the substructure of the
        /// input by invoking the right visitor method of each of its fields.
        ///
        /// [`Fold`]: fold::Fold
        ///
        /// ```
        /// # use syn::{Attribute, BinOp, Expr, ExprBinary};
        /// #
        /// pub trait Fold {
        ///     /* ... */
        ///
        ///     fn fold_expr_binary(&mut self, node: ExprBinary) -> ExprBinary {
        ///         fold_expr_binary(self, node)
        ///     }
        ///
        ///     /* ... */
        ///     # fn fold_attribute(&mut self, node: Attribute) -> Attribute;
        ///     # fn fold_expr(&mut self, node: Expr) -> Expr;
        ///     # fn fold_bin_op(&mut self, node: BinOp) -> BinOp;
        /// }
        ///
        /// pub fn fold_expr_binary<V>(v: &mut V, node: ExprBinary) -> ExprBinary
        /// where
        ///     V: Fold + ?Sized,
        /// {
        ///     ExprBinary {
        ///         attrs: node
        ///             .attrs
        ///             .into_iter()
        ///             .map(|attr| v.fold_attribute(attr))
        ///             .collect(),
        ///         left: Box::new(v.fold_expr(*node.left)),
        ///         op: v.fold_bin_op(node.op),
        ///         right: Box::new(v.fold_expr(*node.right)),
        ///     }
        /// }
        ///
        /// /* ... */
        /// ```
        ///
        /// *This module is available only if Syn is built with the `"fold"` feature.*
        ///
        /// <br>
        ///
        /// # Example
        ///
        /// This fold inserts parentheses to fully parenthesizes any expression.
        ///
        /// ```
        /// // [dependencies]
        /// // quote = "1.0"
        /// // syn = { version = "1.0", features = ["fold", "full"] }
        ///
        /// use quote::quote;
        /// use syn::fold::{fold_expr, Fold};
        /// use syn::{token, Expr, ExprParen};
        ///
        /// struct ParenthesizeEveryExpr;
        ///
        /// impl Fold for ParenthesizeEveryExpr {
        ///     fn fold_expr(&mut self, expr: Expr) -> Expr {
        ///         Expr::Paren(ExprParen {
        ///             attrs: Vec::new(),
        ///             expr: Box::new(fold_expr(self, expr)),
        ///             paren_token: token::Paren::default(),
        ///         })
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let code = quote! { a() + b(1) * c.d };
        ///     let expr: Expr = syn::parse2(code).unwrap();
        ///     let parenthesized = ParenthesizeEveryExpr.fold_expr(expr);
        ///     println!("{}", quote!(#parenthesized));
        ///
        ///     // Output: (((a)()) + (((b)((1))) * ((c).d)))
        /// }
        /// ```
        #[cfg(feature = "fold")]
        #[cfg_attr(doc_cfg, doc(cfg(feature = "fold")))]
        #[rustfmt::skip]
        pub mod fold;

        #[cfg(feature = "clone-impls")]
        #[rustfmt::skip]
        mod clone;

        #[cfg(feature = "extra-traits")]
        #[rustfmt::skip]
        mod eq;

        #[cfg(feature = "extra-traits")]
        #[rustfmt::skip]
        mod hash;

        #[cfg(feature = "extra-traits")]
        #[rustfmt::skip]
        mod debug;

        #[cfg(any(feature = "full", feature = "derive"))]
        #[path = "../gen_helper.rs"]
        mod helper;
    }
    pub use crate::gen::*;

    // Not public API.
    #[doc(hidden)]
    pub mod export;

    mod custom_keyword;
    mod custom_punctuation;
    mod sealed;
    mod span;
    mod thread;

    #[cfg(feature = "parsing")]
    mod lookahead;

    #[cfg(feature = "parsing")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    pub mod parse;

    #[cfg(feature = "full")]
    mod reserved;

    #[cfg(all(any(feature = "full", feature = "derive"), feature = "parsing"))]
    mod verbatim;

    #[cfg(all(any(feature = "full", feature = "derive"), feature = "printing"))]
    mod print;

    ////////////////////////////////////////////////////////////////////////////////

    #[allow(dead_code, non_camel_case_types)]
    struct private;

    // https://github.com/rust-lang/rust/issues/62830
    #[cfg(feature = "parsing")]
    mod rustdoc_workaround {
        pub use crate::parse::{self as parse_module};
    }

    ////////////////////////////////////////////////////////////////////////////////

    mod error;
    pub use crate::error::{Error, Result};

    /// Parse tokens of source code into the chosen syntax tree node.
    ///
    /// This is preferred over parsing a string because tokens are able to preserve
    /// information about where in the user's code they were originally written (the
    /// "span" of the token), possibly allowing the compiler to produce better error
    /// messages.
    ///
    /// This function parses a `proc_macro::TokenStream` which is the type used for
    /// interop with the compiler in a procedural macro. To parse a
    /// `proc_macro2::TokenStream`, use [`syn::parse2`] instead.
    ///
    /// [`syn::parse2`]: parse2
    ///
    /// *This function is available only if Syn is built with both the `"parsing"` and
    /// `"proc-macro"` features.*
    ///
    /// # Examples
    ///
    /// ```
    /// # extern crate proc_macro;
    /// #
    /// use proc_macro::TokenStream;
    /// use quote::quote;
    /// use syn::DeriveInput;
    ///
    /// # const IGNORE_TOKENS: &str = stringify! {
    /// #[proc_macro_derive(MyMacro)]
    /// # };
    /// pub fn my_macro(input: TokenStream) -> TokenStream {
    ///     // Parse the tokens into a syntax tree
    ///     let ast: DeriveInput = syn::parse(input).unwrap();
    ///
    ///     // Build the output, possibly using quasi-quotation
    ///     let expanded = quote! {
    ///         /* ... */
    ///     };
    ///
    ///     // Convert into a token stream and return it
    ///     expanded.into()
    /// }
    /// ```
    #[cfg(all(
        not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
        feature = "parsing",
        feature = "proc-macro"
    ))]
    #[cfg_attr(doc_cfg, doc(cfg(all(feature = "parsing", feature = "proc-macro"))))]
    pub fn parse<T: parse::Parse>(tokens: proc_macro::TokenStream) -> Result<T> {
        parse::Parser::parse(T::parse, tokens)
    }

    /// Parse a proc-macro2 token stream into the chosen syntax tree node.
    ///
    /// This function parses a `proc_macro2::TokenStream` which is commonly useful
    /// when the input comes from a node of the Syn syntax tree, for example the
    /// body tokens of a [`Macro`] node. When in a procedural macro parsing the
    /// `proc_macro::TokenStream` provided by the compiler, use [`syn::parse`]
    /// instead.
    ///
    /// [`syn::parse`]: parse()
    ///
    /// *This function is available only if Syn is built with the `"parsing"` feature.*
    #[cfg(feature = "parsing")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    pub fn parse2<T: parse::Parse>(tokens: proc_macro2::TokenStream) -> Result<T> {
        parse::Parser::parse2(T::parse, tokens)
    }

    /// Parse a string of Rust code into the chosen syntax tree node.
    ///
    /// *This function is available only if Syn is built with the `"parsing"` feature.*
    ///
    /// # Hygiene
    ///
    /// Every span in the resulting syntax tree will be set to resolve at the macro
    /// call site.
    ///
    /// # Examples
    ///
    /// ```
    /// use syn::{Expr, Result};
    ///
    /// fn run() -> Result<()> {
    ///     let code = "assert_eq!(u8::max_value(), 255)";
    ///     let expr = syn::parse_str::<Expr>(code)?;
    ///     println!("{:#?}", expr);
    ///     Ok(())
    /// }
    /// #
    /// # run().unwrap();
    /// ```
    #[cfg(feature = "parsing")]
    #[cfg_attr(doc_cfg, doc(cfg(feature = "parsing")))]
    pub fn parse_str<T: parse::Parse>(s: &str) -> Result<T> {
        parse::Parser::parse_str(T::parse, s)
    }

    // FIXME the name parse_file makes it sound like you might pass in a path to a
    // file, rather than the content.
    /// Parse the content of a file of Rust code.
    ///
    /// This is different from `syn::parse_str::<File>(content)` in two ways:
    ///
    /// - It discards a leading byte order mark `\u{FEFF}` if the file has one.
    /// - It preserves the shebang line of the file, such as `#!/usr/bin/env rustx`.
    ///
    /// If present, either of these would be an error using `from_str`.
    ///
    /// *This function is available only if Syn is built with the `"parsing"` and
    /// `"full"` features.*
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::error::Error;
    /// use std::fs::File;
    /// use std::io::Read;
    ///
    /// fn run() -> Result<(), Box<Error>> {
    ///     let mut file = File::open("path/to/code.rs")?;
    ///     let mut content = String::new();
    ///     file.read_to_string(&mut content)?;
    ///
    ///     let ast = syn::parse_file(&content)?;
    ///     if let Some(shebang) = ast.shebang {
    ///         println!("{}", shebang);
    ///     }
    ///     println!("{} items", ast.items.len());
    ///
    ///     Ok(())
    /// }
    /// #
    /// # run().unwrap();
    /// ```
    #[cfg(all(feature = "parsing", feature = "full"))]
    #[cfg_attr(doc_cfg, doc(cfg(all(feature = "parsing", feature = "full"))))]
    pub fn parse_file(mut content: &str) -> Result<File> {
        // Strip the BOM if it is present
        const BOM: &str = "\u{feff}";
        if content.starts_with(BOM) {
            content = &content[BOM.len()..];
        }

        let mut shebang = None;
        if content.starts_with("#!") {
            let rest = whitespace::skip(&content[2..]);
            if !rest.starts_with('[') {
                if let Some(idx) = content.find('\n') {
                    shebang = Some(content[..idx].to_string());
                    content = &content[idx..];
                } else {
                    shebang = Some(content.to_string());
                    content = "";
                }
            }
        }

        let mut file: File = parse_str(content)?;
        file.shebang = shebang;
        Ok(file)
    }