codegen/src/main.rs

//! This crate automatically generates the definition of the `Visit`,
//! `VisitMut`, and `Fold` traits in `syn` based on the `syn` source. It
//! discovers structs and enums declared with the `ast_*` macros and generates
//! the functions for those types.
//!
//! It makes a few assumptions about the target crate:
//! 1. All structs which are discovered must be re-exported in the root of the
//!    crate, even if they were declared in a submodule.
//! 2. This code cannot discover submodules which are located in subdirectories
//!    - only submodules located in the same directory.
//! 3. The path to `syn` is hardcoded.

#![recursion_limit = "128"]
#![cfg_attr(
    feature = "cargo-clippy",
    allow(
        needless_pass_by_value,
        redundant_closure,
        write_with_newline,
    )
)]

#[macro_use]
extern crate failure;
extern crate inflections;
extern crate proc_macro2;
#[macro_use]
extern crate quote;
#[macro_use]
extern crate syn;
extern crate rustfmt_nightly as rustfmt;

use failure::{err_msg, Error};
use proc_macro2::{Span, TokenStream};
use quote::ToTokens;
use syn::{Attribute, Data, DataStruct, DeriveInput, Ident, Item};

use std::collections::BTreeMap;
use std::fmt::{self, Debug};
use std::fs::File;
use std::io::{Read, Write};
use std::path::Path;

const SYN_CRATE_ROOT: &str = "../src/lib.rs";

const FOLD_SRC: &str = "../src/gen/fold.rs";
const VISIT_SRC: &str = "../src/gen/visit.rs";
const VISIT_MUT_SRC: &str = "../src/gen/visit_mut.rs";

const IGNORED_MODS: &[&str] = &["fold", "visit", "visit_mut"];

const EXTRA_TYPES: &[&str] = &["Lifetime"];

const TERMINAL_TYPES: &[&str] = &["Span", "Ident"];

fn path_eq(a: &syn::Path, b: &str) -> bool {
    a.leading_colon.is_none()
        && a.segments.len() == 1
        && a.segments[0].ident == b
}

fn get_features(attrs: &[Attribute], mut features: TokenStream) -> TokenStream {
    for attr in attrs {
        if path_eq(&attr.path, "cfg") {
            attr.to_tokens(&mut features);
        }
    }
    features
}

#[derive(Clone)]
pub struct AstItem {
    ast: DeriveInput,
    features: TokenStream,
    // True if this is an ast_enum_of_structs! item with a #full annotation.
    eos_full: bool,
}

impl Debug for AstItem {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("AstItem")
            .field("ast", &self.ast)
            .field("features", &self.features.to_string())
            .finish()
    }
}

// NOTE: BTreeMap is used here instead of HashMap to have deterministic output.
type Lookup = BTreeMap<Ident, AstItem>;

fn load_file<P: AsRef<Path>>(
    name: P,
    features: &TokenStream,
    lookup: &mut Lookup,
) -> Result<(), Error> {
    let name = name.as_ref();
    let parent = name.parent().ok_or_else(|| err_msg("no parent path"))?;

    let mut f = File::open(name)?;
    let mut src = String::new();
    f.read_to_string(&mut src)?;

    // Parse the file
    let file =
        syn::parse_file(&src).map_err(|_| format_err!("failed to parse {}", name.display()))?;

    // Collect all of the interesting AstItems declared in this file or submodules.
    'items: for item in file.items {
        match item {
            Item::Mod(item) => {
                // Don't inspect inline modules.
                if item.content.is_some() {
                    continue;
                }

                // We don't want to try to load the generated rust files and
                // parse them, so we ignore them here.
                for name in IGNORED_MODS {
                    if item.ident == name {
                        continue 'items;
                    }
                }

                // Lookup any #[cfg()] attributes on the module and add them to
                // the feature set.
                //
                // The derive module is weird because it is built with either
                // `full` or `derive` but exported only under `derive`.
                let features = if item.ident == "derive" {
                    quote!(#[cfg(feature = "derive")])
                } else {
                    get_features(&item.attrs, features.clone())
                };

                // Look up the submodule file, and recursively parse it.
                // XXX: Only handles same-directory .rs file submodules.
                let path = parent.join(&format!("{}.rs", item.ident));
                load_file(path, &features, lookup)?;
            }
            Item::Macro(item) => {
                // Lookip any #[cfg()] attributes directly on the macro
                // invocation, and add them to the feature set.
                let features = get_features(&item.attrs, features.clone());

                // Try to parse the AstItem declaration out of the item.
                let tts = &item.mac.tts;
                let found = if path_eq(&item.mac.path, "ast_struct") {
                    syn::parse2::<parsing::AstStruct>(quote!(#tts))
                        .map_err(|_| err_msg("failed to parse ast_struct"))?
                        .0
                } else if path_eq(&item.mac.path, "ast_enum") {
                    syn::parse2::<parsing::AstEnum>(quote!(#tts))
                        .map_err(|_| err_msg("failed to parse ast_enum"))?
                        .0
                } else if path_eq(&item.mac.path, "ast_enum_of_structs") {
                    syn::parse2::<parsing::AstEnumOfStructs>(quote!(#tts))
                        .map_err(|_| err_msg("failed to parse ast_enum_of_structs"))?
                        .0
                } else {
                    continue;
                };

                // Record our features on the parsed AstItems.
                for mut item in found {
                    features.to_tokens(&mut item.features);
                    lookup.insert(item.ast.ident.clone(), item);
                }
            }
            Item::Struct(item) => {
                let ident = item.ident;
                if EXTRA_TYPES.contains(&&ident.to_string()[..]) {
                    lookup.insert(
                        ident.clone(),
                        AstItem {
                            ast: DeriveInput {
                                ident,
                                vis: item.vis,
                                attrs: item.attrs,
                                generics: item.generics,
                                data: Data::Struct(DataStruct {
                                    fields: item.fields,
                                    struct_token: item.struct_token,
                                    semi_token: item.semi_token,
                                }),
                            },
                            features: features.clone(),
                            eos_full: false,
                        },
                    );
                }
            }
            _ => {}
        }
    }
    Ok(())
}

mod parsing {
    use super::AstItem;

    use proc_macro2::TokenStream;
    use syn;
    use syn::parse::{Parse, ParseStream, Result};
    use syn::*;

    fn peek_tag(input: ParseStream, tag: &str) -> bool {
        let ahead = input.fork();
        ahead.parse::<Token![#]>().is_ok() && ahead
            .parse::<Ident>()
            .map(|ident| ident == tag)
            .unwrap_or(false)
    }

    // Parses #full - returns #[cfg(feature = "full")] if it is present, and
    // nothing otherwise.
    fn full(input: ParseStream) -> (TokenStream, bool) {
        if peek_tag(input, "full") {
            input.parse::<Token![#]>().unwrap();
            input.parse::<Ident>().unwrap();
            (quote!(#[cfg(feature = "full")]), true)
        } else {
            (quote!(), false)
        }
    }

    fn skip_manual_extra_traits(input: ParseStream) {
        if peek_tag(input, "manual_extra_traits") {
            input.parse::<Token![#]>().unwrap();
            input.parse::<Ident>().unwrap();
        }
    }

    // Parses a simple AstStruct without the `pub struct` prefix.
    fn ast_struct_inner(input: ParseStream) -> Result<AstItem> {
        let ident: Ident = input.parse()?;
        let (features, eos_full) = full(input);
        skip_manual_extra_traits(input);
        let rest: TokenStream = input.parse()?;
        Ok(AstItem {
            ast: syn::parse2(quote! {
                pub struct #ident #rest
            })?,
            features: features,
            eos_full: eos_full,
        })
    }

    // ast_struct! parsing
    pub struct AstStruct(pub Vec<AstItem>);
    impl Parse for AstStruct {
        fn parse(input: ParseStream) -> Result<Self> {
            input.call(Attribute::parse_outer)?;
            input.parse::<Token![pub]>()?;
            input.parse::<Token![struct]>()?;
            let res = input.call(ast_struct_inner)?;
            Ok(AstStruct(vec![res]))
        }
    }

    fn no_visit(input: ParseStream) -> bool {
        if peek_tag(input, "no_visit") {
            input.parse::<Token![#]>().unwrap();
            input.parse::<Ident>().unwrap();
            true
        } else {
            false
        }
    }

    // ast_enum! parsing
    pub struct AstEnum(pub Vec<AstItem>);
    impl Parse for AstEnum {
        fn parse(input: ParseStream) -> Result<Self> {
            input.call(Attribute::parse_outer)?;
            input.parse::<Token![pub]>()?;
            input.parse::<Token![enum]>()?;
            let ident: Ident = input.parse()?;
            let no_visit = no_visit(input);
            let rest: TokenStream = input.parse()?;
            Ok(AstEnum(if no_visit {
                vec![]
            } else {
                vec![AstItem {
                    ast: syn::parse2(quote! {
                        pub enum #ident #rest
                    })?,
                    features: quote!(),
                    eos_full: false,
                }]
            }))
        }
    }

    // A single variant of an ast_enum_of_structs!
    struct EosVariant {
        name: Ident,
        member: Option<Path>,
        inner: Option<AstItem>,
    }
    fn eos_variant(input: ParseStream) -> Result<EosVariant> {
        input.call(Attribute::parse_outer)?;
        input.parse::<Token![pub]>()?;
        let variant: Ident = input.parse()?;
        let (member, inner) = if input.peek(token::Paren) {
            let content;
            parenthesized!(content in input);
            if content.fork().call(ast_struct_inner).is_ok() {
                let item = content.call(ast_struct_inner)?;
                (Some(Path::from(item.ast.ident.clone())), Some(item))
            } else {
                let path: Path = content.parse()?;
                (Some(path), None)
            }
        } else {
            (None, None)
        };
        input.parse::<Token![,]>()?;
        Ok(EosVariant {
            name: variant,
            member: member,
            inner: inner,
        })
    }

    // ast_enum_of_structs! parsing
    pub struct AstEnumOfStructs(pub Vec<AstItem>);
    impl Parse for AstEnumOfStructs {
        fn parse(input: ParseStream) -> Result<Self> {
            input.call(Attribute::parse_outer)?;
            input.parse::<Token![pub]>()?;
            input.parse::<Token![enum]>()?;
            let ident: Ident = input.parse()?;

            let content;
            braced!(content in input);
            let mut variants = Vec::new();
            while !content.is_empty() {
                variants.push(content.call(eos_variant)?);
            }

            if let Some(ident) = input.parse::<Option<Ident>>()? {
                assert_eq!(ident, "do_not_generate_to_tokens");
            }

            let enum_item = {
                let variants = variants.iter().map(|v| {
                    let name = v.name.clone();
                    match v.member {
                        Some(ref member) => quote!(#name(#member)),
                        None => quote!(#name),
                    }
                });
                parse_quote! {
                    pub enum #ident {
                        #(#variants),*
                    }
                }
            };
            let mut items = vec![AstItem {
                ast: enum_item,
                features: quote!(),
                eos_full: false,
            }];
            items.extend(variants.into_iter().filter_map(|v| v.inner));
            Ok(AstEnumOfStructs(items))
        }
    }
}

mod codegen {
    use super::{AstItem, Lookup};
    use inflections::Inflect;
    use proc_macro2::{Span, TokenStream};
    use quote::{ToTokens, TokenStreamExt};
    use syn::ext::IdentExt;
    use syn::parse::Parser;
    use syn::punctuated::Punctuated;
    use syn::*;

    #[derive(Default)]
    pub struct State {
        pub visit_trait: TokenStream,
        pub visit_impl: TokenStream,
        pub visit_mut_trait: TokenStream,
        pub visit_mut_impl: TokenStream,
        pub fold_trait: TokenStream,
        pub fold_impl: TokenStream,
    }

    fn under_name(name: Ident) -> Ident {
        Ident::new(&name.to_string().to_snake_case(), Span::call_site())
    }

    enum RelevantType<'a> {
        Box(&'a Type),
        Vec(&'a Type),
        Punctuated(&'a Type),
        Option(&'a Type),
        Tuple(&'a Punctuated<Type, Token![,]>),
        Simple(&'a AstItem),
        TokenPunct(TokenStream),
        TokenKeyword(TokenStream),
        TokenGroup(Ident),
        Pass,
    }

    fn classify<'a>(ty: &'a Type, lookup: &'a Lookup) -> RelevantType<'a> {
        match *ty {
            Type::Path(TypePath {
                qself: None,
                ref path,
            }) => {
                let last = path.segments.last().unwrap().into_value();
                match &last.ident.to_string()[..] {
                    "Box" => RelevantType::Box(first_arg(&last.arguments)),
                    "Vec" => RelevantType::Vec(first_arg(&last.arguments)),
                    "Punctuated" => RelevantType::Punctuated(first_arg(&last.arguments)),
                    "Option" => RelevantType::Option(first_arg(&last.arguments)),
                    "Brace" | "Bracket" | "Paren" | "Group" => {
                        RelevantType::TokenGroup(last.ident.clone())
                    }
                    _ => {
                        if let Some(item) = lookup.get(&last.ident) {
                            RelevantType::Simple(item)
                        } else {
                            RelevantType::Pass
                        }
                    }
                }
            }
            Type::Tuple(TypeTuple { ref elems, .. }) => RelevantType::Tuple(elems),
            Type::Macro(TypeMacro { ref mac })
                if mac.path.segments.last().unwrap().into_value().ident == "Token" =>
            {
                let is_ident = Ident::parse_any.parse2(mac.tts.clone()).is_ok();
                let is_underscore = parse2::<Token![_]>(mac.tts.clone()).is_ok();
                if is_ident && !is_underscore {
                    RelevantType::TokenKeyword(mac.into_token_stream())
                } else {
                    RelevantType::TokenPunct(mac.into_token_stream())
                }
            }
            _ => RelevantType::Pass,
        }
    }

    #[derive(Debug, Eq, PartialEq, Copy, Clone)]
    enum Kind {
        Visit,
        VisitMut,
        Fold,
    }

    enum Operand {
        Borrowed(TokenStream),
        Owned(TokenStream),
    }

    use self::Kind::*;
    use self::Operand::*;

    impl Operand {
        fn tokens(&self) -> &TokenStream {
            match *self {
                Borrowed(ref n) | Owned(ref n) => n,
            }
        }

        fn ref_tokens(&self) -> TokenStream {
            match *self {
                Borrowed(ref n) => n.clone(),
                Owned(ref n) => quote!(&#n),
            }
        }

        fn ref_mut_tokens(&self) -> TokenStream {
            match *self {
                Borrowed(ref n) => n.clone(),
                Owned(ref n) => quote!(&mut #n),
            }
        }

        fn owned_tokens(&self) -> TokenStream {
            match *self {
                Borrowed(ref n) => quote!(*#n),
                Owned(ref n) => n.clone(),
            }
        }
    }

    fn first_arg(params: &PathArguments) -> &Type {
        let data = match *params {
            PathArguments::AngleBracketed(ref data) => data,
            _ => panic!("Expected at least 1 type argument here"),
        };

        match **data
            .args
            .first()
            .expect("Expected at least 1 type argument here")
            .value()
        {
            GenericArgument::Type(ref ty) => ty,
            _ => panic!("Expected at least 1 type argument here"),
        }
    }

    fn simple_visit(item: &AstItem, kind: Kind, name: &Operand) -> TokenStream {
        let ident = under_name(item.ast.ident.clone());

        match kind {
            Visit => {
                let method = Ident::new(&format!("visit_{}", ident), Span::call_site());
                let name = name.ref_tokens();
                quote! {
                    _visitor.#method(#name)
                }
            }
            VisitMut => {
                let method = Ident::new(&format!("visit_{}_mut", ident), Span::call_site());
                let name = name.ref_mut_tokens();
                quote! {
                    _visitor.#method(#name)
                }
            }
            Fold => {
                let method = Ident::new(&format!("fold_{}", ident), Span::call_site());
                let name = name.owned_tokens();
                quote! {
                    _visitor.#method(#name)
                }
            }
        }
    }

    fn box_visit(elem: &Type, lookup: &Lookup, kind: Kind, name: &Operand) -> Option<TokenStream> {
        let name = name.owned_tokens();
        let res = visit(elem, lookup, kind, &Owned(quote!(*#name)))?;
        Some(match kind {
            Fold => quote! {
                Box::new(#res)
            },
            Visit | VisitMut => res,
        })
    }

    fn vec_visit(elem: &Type, lookup: &Lookup, kind: Kind, name: &Operand) -> Option<TokenStream> {
        let operand = match kind {
            Visit | VisitMut => Borrowed(quote!(it)),
            Fold => Owned(quote!(it)),
        };
        let val = visit(elem, lookup, kind, &operand)?;
        Some(match kind {
            Visit => {
                let name = name.ref_tokens();
                quote! {
                    for it in #name {
                        #val
                    }
                }
            }
            VisitMut => {
                let name = name.ref_mut_tokens();
                quote! {
                    for it in #name {
                        #val
                    }
                }
            }
            Fold => {
                let name = name.owned_tokens();
                quote! {
                    FoldHelper::lift(#name, |it| { #val })
                }
            }
        })
    }

    fn punctuated_visit(
        elem: &Type,
        lookup: &Lookup,
        kind: Kind,
        name: &Operand,
    ) -> Option<TokenStream> {
        let operand = match kind {
            Visit | VisitMut => Borrowed(quote!(it)),
            Fold => Owned(quote!(it)),
        };
        let val = visit(elem, lookup, kind, &operand)?;
        Some(match kind {
            Visit => {
                let name = name.ref_tokens();
                quote! {
                    for el in Punctuated::pairs(#name) {
                        let it = el.value();
                        #val
                    }
                }
            }
            VisitMut => {
                let name = name.ref_mut_tokens();
                quote! {
                    for mut el in Punctuated::pairs_mut(#name) {
                        let it = el.value_mut();
                        #val
                    }
                }
            }
            Fold => {
                let name = name.owned_tokens();
                quote! {
                    FoldHelper::lift(#name, |it| { #val })
                }
            }
        })
    }

    fn option_visit(
        elem: &Type,
        lookup: &Lookup,
        kind: Kind,
        name: &Operand,
    ) -> Option<TokenStream> {
        let it = match kind {
            Visit | VisitMut => Borrowed(quote!(it)),
            Fold => Owned(quote!(it)),
        };
        let val = visit(elem, lookup, kind, &it)?;
        let name = name.owned_tokens();
        Some(match kind {
            Visit => quote! {
                if let Some(ref it) = #name {
                    #val
                }
            },
            VisitMut => quote! {
                if let Some(ref mut it) = #name {
                    #val
                }
            },
            Fold => quote! {
                (#name).map(|it| { #val })
            },
        })
    }

    fn tuple_visit(
        elems: &Punctuated<Type, Token![,]>,
        lookup: &Lookup,
        kind: Kind,
        name: &Operand,
    ) -> Option<TokenStream> {
        if elems.is_empty() {
            return None;
        }

        let mut code = TokenStream::new();
        for (i, elem) in elems.iter().enumerate() {
            let name = name.tokens();
            let i = Index::from(i);
            let it = Owned(quote!((#name).#i));
            let val = visit(elem, lookup, kind, &it).unwrap_or_else(|| noop_visit(kind, &it));
            code.append_all(val);
            match kind {
                Fold => code.append_all(quote!(,)),
                Visit | VisitMut => code.append_all(quote!(;)),
            }
        }
        Some(match kind {
            Fold => quote! {
                (#code)
            },
            Visit | VisitMut => code,
        })
    }

    fn token_punct_visit(ty: TokenStream, kind: Kind, name: &Operand) -> TokenStream {
        let name = name.tokens();
        match kind {
            Fold => quote! {
                #ty(tokens_helper(_visitor, &#name.spans))
            },
            Visit => quote! {
                tokens_helper(_visitor, &#name.spans)
            },
            VisitMut => quote! {
                tokens_helper(_visitor, &mut #name.spans)
            },
        }
    }

    fn token_keyword_visit(ty: TokenStream, kind: Kind, name: &Operand) -> TokenStream {
        let name = name.tokens();
        match kind {
            Fold => quote! {
                #ty(tokens_helper(_visitor, &#name.span))
            },
            Visit => quote! {
                tokens_helper(_visitor, &#name.span)
            },
            VisitMut => quote! {
                tokens_helper(_visitor, &mut #name.span)
            },
        }
    }

    fn token_group_visit(ty: Ident, kind: Kind, name: &Operand) -> TokenStream {
        let name = name.tokens();
        match kind {
            Fold => quote! {
                #ty(tokens_helper(_visitor, &#name.span))
            },
            Visit => quote! {
                tokens_helper(_visitor, &#name.span)
            },
            VisitMut => quote! {
                tokens_helper(_visitor, &mut #name.span)
            },
        }
    }

    fn noop_visit(kind: Kind, name: &Operand) -> TokenStream {
        match kind {
            Fold => name.owned_tokens(),
            Visit | VisitMut => {
                let name = name.tokens();
                quote! {
                    skip!(#name)
                }
            }
        }
    }

    fn visit(ty: &Type, lookup: &Lookup, kind: Kind, name: &Operand) -> Option<TokenStream> {
        match classify(ty, lookup) {
            RelevantType::Box(elem) => box_visit(elem, lookup, kind, name),
            RelevantType::Vec(elem) => vec_visit(elem, lookup, kind, name),
            RelevantType::Punctuated(elem) => punctuated_visit(elem, lookup, kind, name),
            RelevantType::Option(elem) => option_visit(elem, lookup, kind, name),
            RelevantType::Tuple(elems) => tuple_visit(elems, lookup, kind, name),
            RelevantType::Simple(item) => {
                let mut res = simple_visit(item, kind, name);
                Some(if item.eos_full {
                    quote! {
                        full!(#res)
                    }
                } else {
                    res
                })
            }
            RelevantType::TokenPunct(ty) => Some(token_punct_visit(ty, kind, name)),
            RelevantType::TokenKeyword(ty) => Some(token_keyword_visit(ty, kind, name)),
            RelevantType::TokenGroup(ty) => Some(token_group_visit(ty, kind, name)),
            RelevantType::Pass => None,
        }
    }

    pub fn generate(state: &mut State, lookup: &Lookup, s: &AstItem) {
        let features = &s.features;
        let under_name = under_name(s.ast.ident.clone());
        let ty = &s.ast.ident;
        let visit_fn = Ident::new(&format!("visit_{}", under_name), Span::call_site());
        let visit_mut_fn = Ident::new(&format!("visit_{}_mut", under_name), Span::call_site());
        let fold_fn = Ident::new(&format!("fold_{}", under_name), Span::call_site());

        let mut visit_impl = TokenStream::new();
        let mut visit_mut_impl = TokenStream::new();
        let mut fold_impl = TokenStream::new();

        match s.ast.data {
            Data::Enum(ref e) => {
                let mut visit_variants = TokenStream::new();
                let mut visit_mut_variants = TokenStream::new();
                let mut fold_variants = TokenStream::new();

                for variant in &e.variants {
                    let variant_ident = &variant.ident;

                    match variant.fields {
                        Fields::Named(..) => panic!("Doesn't support enum struct variants"),
                        Fields::Unnamed(ref fields) => {
                            let mut bind_visit_fields = TokenStream::new();
                            let mut bind_visit_mut_fields = TokenStream::new();
                            let mut bind_fold_fields = TokenStream::new();

                            let mut visit_fields = TokenStream::new();
                            let mut visit_mut_fields = TokenStream::new();
                            let mut fold_fields = TokenStream::new();

                            for (idx, field) in fields.unnamed.iter().enumerate() {
                                let name = format!("_binding_{}", idx);
                                let binding = Ident::new(&name, Span::call_site());

                                bind_visit_fields.append_all(quote! {
                                    ref #binding,
                                });
                                bind_visit_mut_fields.append_all(quote! {
                                    ref mut #binding,
                                });
                                bind_fold_fields.append_all(quote! {
                                    #binding,
                                });

                                let borrowed_binding = Borrowed(quote!(#binding));
                                let owned_binding = Owned(quote!(#binding));

                                visit_fields.append_all(
                                    visit(&field.ty, lookup, Visit, &borrowed_binding)
                                        .unwrap_or_else(|| noop_visit(Visit, &borrowed_binding)),
                                );
                                visit_mut_fields.append_all(
                                    visit(&field.ty, lookup, VisitMut, &borrowed_binding)
                                        .unwrap_or_else(|| noop_visit(VisitMut, &borrowed_binding)),
                                );
                                fold_fields.append_all(
                                    visit(&field.ty, lookup, Fold, &owned_binding)
                                        .unwrap_or_else(|| noop_visit(Fold, &owned_binding)),
                                );

                                visit_fields.append_all(quote!(;));
                                visit_mut_fields.append_all(quote!(;));
                                fold_fields.append_all(quote!(,));
                            }

                            visit_variants.append_all(quote! {
                                #ty::#variant_ident(#bind_visit_fields) => {
                                    #visit_fields
                                }
                            });

                            visit_mut_variants.append_all(quote! {
                                #ty::#variant_ident(#bind_visit_mut_fields) => {
                                    #visit_mut_fields
                                }
                            });

                            fold_variants.append_all(quote! {
                                #ty::#variant_ident(#bind_fold_fields) => {
                                    #ty::#variant_ident(
                                        #fold_fields
                                    )
                                }
                            });
                        }
                        Fields::Unit => {
                            visit_variants.append_all(quote! {
                                #ty::#variant_ident => {}
                            });
                            visit_mut_variants.append_all(quote! {
                                #ty::#variant_ident => {}
                            });
                            fold_variants.append_all(quote! {
                                #ty::#variant_ident => {
                                    #ty::#variant_ident
                                }
                            });
                        }
                    }
                }

                visit_impl.append_all(quote! {
                    match *_i {
                        #visit_variants
                    }
                });

                visit_mut_impl.append_all(quote! {
                    match *_i {
                        #visit_mut_variants
                    }
                });

                fold_impl.append_all(quote! {
                    match _i {
                        #fold_variants
                    }
                });
            }
            Data::Struct(ref v) => {
                let mut fold_fields = TokenStream::new();

                for (idx, field) in v.fields.iter().enumerate() {
                    let id = match field.ident {
                        Some(ref ident) => Member::Named(ident.clone()),
                        None => Member::Unnamed(Index::from(idx)),
                    };
                    let ref_toks = Owned(quote!(_i.#id));
                    let visit_field = visit(&field.ty, lookup, Visit, &ref_toks)
                        .unwrap_or_else(|| noop_visit(Visit, &ref_toks));
                    visit_impl.append_all(quote! {
                        #visit_field;
                    });
                    let visit_mut_field = visit(&field.ty, lookup, VisitMut, &ref_toks)
                        .unwrap_or_else(|| noop_visit(VisitMut, &ref_toks));
                    visit_mut_impl.append_all(quote! {
                        #visit_mut_field;
                    });
                    let fold = visit(&field.ty, lookup, Fold, &ref_toks)
                        .unwrap_or_else(|| noop_visit(Fold, &ref_toks));
                    if let Some(ref name) = field.ident {
                        fold_fields.append_all(quote! {
                            #name: #fold,
                        });
                    } else {
                        fold_fields.append_all(quote! {
                            #fold,
                        });
                    }
                }

                match v.fields {
                    Fields::Named(..) | Fields::Unnamed(..) => fold_impl.append_all(quote! {
                        #ty {
                            #fold_fields
                        }
                    }),
                    Fields::Unit => {
                        if ty == "Ident" {
                            fold_impl.append_all(quote! {
                                let mut _i = _i;
                                let span = _visitor.fold_span(_i.span());
                                _i.set_span(span);
                            });
                        }
                        fold_impl.append_all(quote! {
                            _i
                        });
                    }
                };
            }
            Data::Union(..) => panic!("Union not supported"),
        }

        let mut include_fold_impl = true;
        if let Data::Struct(ref data) = s.ast.data {
            if let Fields::Named(ref fields) = data.fields {
                if fields
                    .named
                    .iter()
                    .any(|field| field.vis == Visibility::Inherited)
                {
                    // Discard the generated impl if there are private fields.
                    // These have to be handwritten.
                    include_fold_impl = false;
                }
            }
        }

        state.visit_trait.append_all(quote! {
            #features
            fn #visit_fn(&mut self, i: &'ast #ty) {
                #visit_fn(self, i)
            }
        });

        state.visit_impl.append_all(quote! {
            #features
            pub fn #visit_fn<'ast, V: Visit<'ast> + ?Sized>(
                _visitor: &mut V, _i: &'ast #ty
            ) {
                #visit_impl
            }
        });

        state.visit_mut_trait.append_all(quote! {
            #features
            fn #visit_mut_fn(&mut self, i: &mut #ty) {
                #visit_mut_fn(self, i)
            }
        });

        state.visit_mut_impl.append_all(quote! {
            #features
            pub fn #visit_mut_fn<V: VisitMut + ?Sized>(
                _visitor: &mut V, _i: &mut #ty
            ) {
                #visit_mut_impl
            }
        });

        state.fold_trait.append_all(quote! {
            #features
            fn #fold_fn(&mut self, i: #ty) -> #ty {
                #fold_fn(self, i)
            }
        });

        if include_fold_impl {
            state.fold_impl.append_all(quote! {
                #features
                pub fn #fold_fn<V: Fold + ?Sized>(
                    _visitor: &mut V, _i: #ty
                ) -> #ty {
                    #fold_impl
                }
            });
        }
    }
}

fn write_file(path: &str, content: TokenStream) {
    let mut file = File::create(path).unwrap();
    write!(
        file,
        "// THIS FILE IS AUTOMATICALLY GENERATED; DO NOT EDIT\n\n"
    ).unwrap();
    let mut config = rustfmt::Config::default();
    config.set().emit_mode(rustfmt::EmitMode::Stdout);
    config.set().verbose(rustfmt::Verbosity::Quiet);
    config.set().format_macro_matchers(true);
    let mut session = rustfmt::Session::new(config, Some(&mut file));
    session
        .format(rustfmt::Input::Text(content.to_string()))
        .unwrap();
}

fn main() {
    let mut lookup = BTreeMap::new();
    load_file(SYN_CRATE_ROOT, &quote!(), &mut lookup).unwrap();

    // Load in any terminal types
    for &tt in TERMINAL_TYPES {
        use syn::*;
        lookup.insert(
            Ident::new(&tt, Span::call_site()),
            AstItem {
                ast: DeriveInput {
                    ident: Ident::new(tt, Span::call_site()),
                    vis: Visibility::Public(VisPublic {
                        pub_token: <Token![pub]>::default(),
                    }),
                    attrs: vec![],
                    generics: Generics::default(),
                    data: Data::Struct(DataStruct {
                        fields: Fields::Unit,
                        struct_token: <Token![struct]>::default(),
                        semi_token: None,
                    }),
                },
                features: TokenStream::new(),
                eos_full: false,
            },
        );
    }

    let mut state = codegen::State::default();
    for s in lookup.values() {
        codegen::generate(&mut state, &lookup, s);
    }

    let full_macro = quote! {
        #[cfg(feature = "full")]
        macro_rules! full {
            ($e:expr) => {
                $e
            };
        }

        #[cfg(all(feature = "derive", not(feature = "full")))]
        macro_rules! full {
            ($e:expr) => {
                unreachable!()
            };
        }
    };

    let skip_macro = quote! {
        #[cfg(any(feature = "full", feature = "derive"))]
        macro_rules! skip {
            ($($tt:tt)*) => {};
        }
    };

    let fold_trait = state.fold_trait;
    let fold_impl = state.fold_impl;
    write_file(
        FOLD_SRC,
        quote! {
            // Unreachable code is generated sometimes without the full feature.
            #![allow(unreachable_code)]
            #![cfg_attr(feature = "cargo-clippy", allow(needless_pass_by_value))]

            use *;
            #[cfg(any(feature = "full", feature = "derive"))]
            use token::{Brace, Bracket, Paren, Group};
            use proc_macro2::Span;
            #[cfg(any(feature = "full", feature = "derive"))]
            use gen::helper::fold::*;

            #full_macro

            /// Syntax tree traversal to transform the nodes of an owned syntax tree.
            ///
            /// See the [module documentation] for details.
            ///
            /// [module documentation]: index.html
            ///
            /// *This trait is available if Syn is built with the `"fold"` feature.*
            pub trait Fold {
                #fold_trait
            }

            #[cfg(any(feature = "full", feature = "derive"))]
            macro_rules! fold_span_only {
                ($f:ident : $t:ident) => {
                    pub fn $f<V: Fold + ?Sized>(_visitor: &mut V, mut _i: $t) -> $t {
                        let span = _visitor.fold_span(_i.span());
                        _i.set_span(span);
                        _i
                    }
                }
            }

            #[cfg(any(feature = "full", feature = "derive"))]
            fold_span_only!(fold_lit_byte: LitByte);
            #[cfg(any(feature = "full", feature = "derive"))]
            fold_span_only!(fold_lit_byte_str: LitByteStr);
            #[cfg(any(feature = "full", feature = "derive"))]
            fold_span_only!(fold_lit_char: LitChar);
            #[cfg(any(feature = "full", feature = "derive"))]
            fold_span_only!(fold_lit_float: LitFloat);
            #[cfg(any(feature = "full", feature = "derive"))]
            fold_span_only!(fold_lit_int: LitInt);
            #[cfg(any(feature = "full", feature = "derive"))]
            fold_span_only!(fold_lit_str: LitStr);

            #fold_impl
        },
    );

    let visit_trait = state.visit_trait;
    let visit_impl = state.visit_impl;
    write_file(
        VISIT_SRC,
        quote! {
            #![cfg_attr(feature = "cargo-clippy", allow(match_same_arms))]

            use *;
            #[cfg(any(feature = "full", feature = "derive"))]
            use punctuated::Punctuated;
            use proc_macro2::Span;
            #[cfg(any(feature = "full", feature = "derive"))]
            use gen::helper::visit::*;

            #full_macro
            #skip_macro

            /// Syntax tree traversal to walk a shared borrow of a syntax tree.
            ///
            /// See the [module documentation] for details.
            ///
            /// [module documentation]: index.html
            ///
            /// *This trait is available if Syn is built with the `"visit"` feature.*
            pub trait Visit<'ast> {
                #visit_trait
            }

            #visit_impl
        },
    );

    let visit_mut_trait = state.visit_mut_trait;
    let visit_mut_impl = state.visit_mut_impl;
    write_file(
        VISIT_MUT_SRC,
        quote! {
            #![cfg_attr(feature = "cargo-clippy", allow(match_same_arms))]

            use *;
            #[cfg(any(feature = "full", feature = "derive"))]
            use punctuated::Punctuated;
            use proc_macro2::Span;
            #[cfg(any(feature = "full", feature = "derive"))]
            use gen::helper::visit_mut::*;

            #full_macro
            #skip_macro

            /// Syntax tree traversal to mutate an exclusive borrow of a syntax tree in
            /// place.
            ///
            /// See the [module documentation] for details.
            ///
            /// [module documentation]: index.html
            ///
            /// *This trait is available if Syn is built with the `"visit-mut"` feature.*
            pub trait VisitMut {
                #visit_mut_trait
            }

            #visit_mut_impl
        },
    );
}