src/ty.rs

use delimited::Delimited;
use super::*;

ast_enum_of_structs! {
    /// The different kinds of types recognized by the compiler
    pub enum Type {
        /// A variable-length array (`[T]`)
        pub Slice(TypeSlice {
            pub bracket_token: token::Bracket,
            pub elem: Box<Type>,
        }),
        /// A fixed length array (`[T; n]`)
        pub Array(TypeArray {
            pub bracket_token: token::Bracket,
            pub elem: Box<Type>,
            pub semi_token: Token![;],
            pub len: Expr,
        }),
        /// A raw pointer (`*const T` or `*mut T`)
        pub Ptr(TypePtr {
            pub star_token: Token![*],
            pub const_token: Option<Token![const]>,
            pub elem: Box<MutType>,
        }),
        /// A reference (`&'a T` or `&'a mut T`)
        pub Reference(TypeReference {
            pub and_token: Token![&],
            pub lifetime: Option<Lifetime>,
            pub elem: Box<MutType>,
        }),
        /// A bare function (e.g. `fn(usize) -> bool`)
        pub BareFn(TypeBareFn {
            pub ty: Box<BareFnType>,
        }),
        /// The never type (`!`)
        pub Never(TypeNever {
            pub bang_token: Token![!],
        }),
        /// A tuple (`(A, B, C, D, ...)`)
        pub Tuple(TypeTuple {
            pub paren_token: token::Paren,
            pub elems: Delimited<Type, Token![,]>,
        }),
        /// A path (`module::module::...::Type`), optionally
        /// "qualified", e.g. `<Vec<T> as SomeTrait>::SomeType`.
        ///
        /// Type arguments are stored in the Path itself
        pub Path(TypePath {
            pub qself: Option<QSelf>,
            pub path: Path,
        }),
        /// A trait object type `Bound1 + Bound2 + Bound3`
        /// where `Bound` is a trait or a lifetime.
        pub TraitObject(TypeTraitObject {
            pub dyn_token: Option<Token![dyn]>,
            pub bounds: Delimited<TypeParamBound, Token![+]>,
        }),
        /// An `impl Bound1 + Bound2 + Bound3` type
        /// where `Bound` is a trait or a lifetime.
        pub ImplTrait(TypeImplTrait {
            pub impl_token: Token![impl],
            pub bounds: Delimited<TypeParamBound, Token![+]>,
        }),
        /// No-op; kept solely so that we can pretty-print faithfully
        pub Paren(TypeParen {
            pub paren_token: token::Paren,
            pub elem: Box<Type>,
        }),
        /// No-op: kept solely so that we can pretty-print faithfully
        pub Group(TypeGroup {
            pub group_token: token::Group,
            pub elem: Box<Type>,
        }),
        /// TypeKind::Infer means the type should be inferred instead of it having been
        /// specified. This can appear anywhere in a type.
        pub Infer(TypeInfer {
            pub underscore_token: Token![_],
        }),
        /// A macro in the type position.
        pub Macro(Macro),
    }
}

ast_struct! {
    pub struct MutType {
        pub mutability: Option<Token![mut]>,
        pub ty: Type,
    }
}

ast_struct! {
    /// A "Path" is essentially Rust's notion of a name.
    ///
    /// It's represented as a sequence of identifiers,
    /// along with a bunch of supporting information.
    ///
    /// E.g. `std::cmp::PartialEq`
    pub struct Path {
        /// A `::foo` path, is relative to the crate root rather than current
        /// module (like paths in an import).
        pub leading_colon: Option<Token![::]>,
        /// The segments in the path: the things separated by `::`.
        pub segments: Delimited<PathSegment, Token![::]>,
    }
}

impl Path {
    pub fn global(&self) -> bool {
        self.leading_colon.is_some()
    }
}

#[cfg(feature = "printing")]
#[cfg_attr(feature = "extra-traits", derive(Debug, Eq, PartialEq, Hash))]
#[cfg_attr(feature = "clone-impls", derive(Clone))]
pub struct PathTokens<'a>(pub &'a Option<QSelf>, pub &'a Path);

impl<T> From<T> for Path
where
    T: Into<PathSegment>,
{
    fn from(segment: T) -> Self {
        Path {
            leading_colon: None,
            segments: vec![(segment.into(), None)].into(),
        }
    }
}

ast_struct! {
    /// A segment of a path: an identifier, an optional lifetime, and a set of types.
    ///
    /// E.g. `std`, `String` or `Box<T>`
    pub struct PathSegment {
        /// The identifier portion of this path segment.
        pub ident: Ident,
        /// Type/lifetime arguments attached to this path. They come in
        /// two flavors: `Path<A,B,C>` and `Path(A,B) -> C`. Note that
        /// this is more than just simple syntactic sugar; the use of
        /// parens affects the region binding rules, so we preserve the
        /// distinction.
        pub arguments: PathArguments,
    }
}

impl<T> From<T> for PathSegment
where
    T: Into<Ident>,
{
    fn from(ident: T) -> Self {
        PathSegment {
            ident: ident.into(),
            arguments: PathArguments::None,
        }
    }
}

ast_enum! {
    /// Arguments of a path segment.
    ///
    /// E.g. `<A, B>` as in `Foo<A, B>` or `(A, B)` as in `Foo(A, B)`
    pub enum PathArguments {
        None,
        /// The `<'a, A, B, C>` in `foo::bar::baz::<'a, A, B, C>`
        AngleBracketed(AngleBracketedGenericArguments),
        /// The `(A, B)` and `C` in `Foo(A, B) -> C`
        Parenthesized(ParenthesizedGenericArguments),
    }
}

impl Default for PathArguments {
    fn default() -> Self {
        PathArguments::None
    }
}

impl PathArguments {
    pub fn is_empty(&self) -> bool {
        match *self {
            PathArguments::None => true,
            PathArguments::AngleBracketed(ref bracketed) => bracketed.args.is_empty(),
            PathArguments::Parenthesized(_) => false,
        }
    }
}

ast_enum! {
    /// A individual generic argument, like `'a`, `T`, or `Item=T`.
    pub enum GenericArgument {
        /// The lifetime parameters for this path segment.
        Lifetime(Lifetime),
        /// The type parameters for this path segment, if present.
        Type(Type),
        /// Bindings (equality constraints) on associated types, if present.
        ///
        /// E.g., `Foo<A=Bar>`.
        TypeBinding(TypeBinding),
        /// Const expression. Must be inside of a block.
        ///
        /// NOTE: Identity expressions are represented as Type arguments, as
        /// they are indistinguishable syntactically.
        Const(Expr),
    }
}

ast_struct! {
    /// A path like `Foo<'a, T>`
    pub struct AngleBracketedGenericArguments {
        pub colon2_token: Option<Token![::]>,
        pub lt_token: Token![<],
        pub args: Delimited<GenericArgument, Token![,]>,
        pub gt_token: Token![>],
    }
}

ast_struct! {
    /// Bind a type to an associated type: `A=Foo`.
    pub struct TypeBinding {
        pub ident: Ident,
        pub eq_token: Token![=],
        pub ty: Type,
    }
}

ast_struct! {
    /// A path like `Foo(A,B) -> C`
    pub struct ParenthesizedGenericArguments {
        pub paren_token: token::Paren,
        /// `(A, B)`
        pub inputs: Delimited<Type, Token![,]>,
        /// `C`
        pub output: ReturnType,
    }
}

ast_struct! {
    pub struct PolyTraitRef {
        /// The `for<'a>` in `for<'a> Foo<&'a T>`
        pub bound_lifetimes: Option<BoundLifetimes>,
        /// The `Foo<&'a T>` in `for<'a> Foo<&'a T>`
        pub trait_ref: Path,
    }
}

ast_struct! {
    /// The explicit Self type in a "qualified path". The actual
    /// path, including the trait and the associated item, is stored
    /// separately. `position` represents the index of the associated
    /// item qualified with this Self type.
    ///
    /// ```text
    /// <Vec<T> as a::b::Trait>::AssociatedItem
    ///  ^~~~~~    ~~~~~~~~~~~~~~^
    ///  ty        position = 3
    ///
    /// <Vec<T>>::AssociatedItem
    ///  ^~~~~~   ^
    ///  ty       position = 0
    /// ```
    pub struct QSelf {
        pub lt_token: Token![<],
        pub ty: Box<Type>,
        pub position: usize,
        pub as_token: Option<Token![as]>,
        pub gt_token: Token![>],
    }
}

ast_struct! {
    pub struct BareFnType {
        pub unsafety: Option<Token![unsafe]>,
        pub abi: Option<Abi>,
        pub fn_token: Token![fn],
        pub lifetimes: Option<BoundLifetimes>,
        pub paren_token: token::Paren,
        pub inputs: Delimited<BareFnArg, Token![,]>,
        pub variadic: Option<Token![...]>,
        pub output: ReturnType,
    }
}

ast_struct! {
    pub struct Abi {
        pub extern_token: Token![extern],
        pub kind: AbiKind,
    }
}

ast_enum! {
    pub enum AbiKind {
        Named(Lit),
        Default,
    }
}

ast_struct! {
    /// An argument in a function type.
    ///
    /// E.g. `bar: usize` as in `fn foo(bar: usize)`
    pub struct BareFnArg {
        pub name: Option<(BareFnArgName, Token![:])>,
        pub ty: Type,
    }
}

ast_enum! {
    /// Names of arguments in the `BareFnArg` structure
    pub enum BareFnArgName {
        /// Argument with the provided name
        Named(Ident),
        /// Argument matched with `_`
        Wild(Token![_]),
    }
}

ast_enum! {
    pub enum ReturnType {
        /// Return type is not specified.
        ///
        /// Functions default to `()` and
        /// closures default to inference. Span points to where return
        /// type would be inserted.
        Default,
        /// Everything else
        Type(Token![->], Box<Type>),
    }
}

#[cfg(feature = "parsing")]
pub mod parsing {
    use super::*;
    use synom::Synom;

    impl Synom for Type {
        named!(parse -> Self, call!(ambig_ty, true));

        fn description() -> Option<&'static str> {
            Some("type")
        }
    }

    impl Type {
        /// In some positions, types may not contain the `+` character, to
        /// disambiguate them. For example in the expression `1 as T`, T may not
        /// contain a `+` character.
        ///
        /// This parser does not allow a `+`, while the default parser does.
        named!(pub without_plus -> Self, call!(ambig_ty, false));
    }

    named!(ambig_ty(allow_plus: bool) -> Type, alt!(
        syn!(TypeGroup) => { Type::Group }
        |
        // must be before TypeTuple
        syn!(TypeParen) => { Type::Paren }
        |
        // must be before TypePath
        syn!(Macro) => { Type::Macro }
        |
        // must be before TypeTraitObject
        call!(TypePath::parse, allow_plus) => { Type::Path }
        |
        syn!(TypeSlice) => { Type::Slice }
        |
        syn!(TypeArray) => { Type::Array }
        |
        syn!(TypePtr) => { Type::Ptr }
        |
        syn!(TypeReference) => { Type::Reference }
        |
        syn!(TypeBareFn) => { Type::BareFn }
        |
        syn!(TypeNever) => { Type::Never }
        |
        syn!(TypeTuple) => { Type::Tuple }
        |
        // Don't try parsing more than one trait bound if we aren't allowing it
        call!(TypeTraitObject::parse, allow_plus) => { Type::TraitObject }
        |
        syn!(TypeImplTrait) => { Type::ImplTrait }
        |
        syn!(TypeInfer) => { Type::Infer }
    ));

    impl Synom for TypeSlice {
        named!(parse -> Self, map!(
            brackets!(syn!(Type)),
            |(ty, b)| TypeSlice {
                elem: Box::new(ty),
                bracket_token: b,
            }
        ));
    }

    impl Synom for TypeArray {
        named!(parse -> Self, map!(
            brackets!(do_parse!(
                elem: syn!(Type) >>
                    semi: punct!(;) >>
                    len: syn!(Expr) >>
                    (elem, semi, len)
            )),
            |((elem, semi, len), brackets)| {
                TypeArray {
                    elem: Box::new(elem),
                    len: len,
                    bracket_token: brackets,
                    semi_token: semi,
                }
            }
        ));
    }

    impl Synom for TypePtr {
        named!(parse -> Self, do_parse!(
            star: punct!(*) >>
            mutability: alt!(
                keyword!(const) => { |c| (None, Some(c)) }
                |
                keyword!(mut) => { |m| (Some(m), None) }
            ) >>
            target: call!(Type::without_plus) >>
            (TypePtr {
                const_token: mutability.1,
                star_token: star,
                elem: Box::new(MutType {
                    ty: target,
                    mutability: mutability.0,
                }),
            })
        ));
    }

    impl Synom for TypeReference {
        named!(parse -> Self, do_parse!(
            amp: punct!(&) >>
            life: option!(syn!(Lifetime)) >>
            mutability: option!(keyword!(mut)) >>
            // & binds tighter than +, so we don't allow + here.
            target: call!(Type::without_plus) >>
            (TypeReference {
                lifetime: life,
                elem: Box::new(MutType {
                    ty: target,
                    mutability: mutability,
                }),
                and_token: amp,
            })
        ));
    }

    impl Synom for TypeBareFn {
        named!(parse -> Self, do_parse!(
            lifetimes: option!(syn!(BoundLifetimes)) >>
            unsafety: option!(keyword!(unsafe)) >>
            abi: option!(syn!(Abi)) >>
            fn_: keyword!(fn) >>
            parens: parens!(do_parse!(
                inputs: call!(Delimited::parse_terminated) >>
                variadic: option!(cond_reduce!(inputs.is_empty() || inputs.trailing_delim(),
                                                punct!(...))) >>
                (inputs, variadic)
            )) >>
            output: syn!(ReturnType) >>
            (TypeBareFn {
                ty: Box::new(BareFnType {
                    unsafety: unsafety,
                    abi: abi,
                    lifetimes: lifetimes,
                    output: output,
                    variadic: (parens.0).1,
                    fn_token: fn_,
                    paren_token: parens.1,
                    inputs: (parens.0).0,
                }),
            })
        ));
    }

    impl Synom for TypeNever {
        named!(parse -> Self, map!(
            punct!(!),
            |b| TypeNever { bang_token: b }
        ));
    }

    impl Synom for TypeInfer {
        named!(parse -> Self, map!(
            punct!(_),
            |u| TypeInfer { underscore_token: u }
        ));
    }

    impl Synom for TypeTuple {
        named!(parse -> Self, do_parse!(
            data: parens!(call!(Delimited::parse_terminated)) >>
            (TypeTuple {
                elems: data.0,
                paren_token: data.1,
            })
        ));
    }

    impl TypePath {
        named!(parse(allow_plus: bool) -> Self, do_parse!(
            qpath: qpath >>
            parenthesized: cond!(
                qpath.1.segments.last().unwrap().item().arguments.is_empty(),
                option!(syn!(ParenthesizedGenericArguments))
            ) >>
            cond!(allow_plus, not!(peek!(punct!(+)))) >>
            ({
                let (qself, mut path) = qpath;
                if let Some(Some(parenthesized)) = parenthesized {
                    let parenthesized = PathArguments::Parenthesized(parenthesized);
                    path.segments.last_mut().unwrap().item_mut().arguments = parenthesized;
                }
                TypePath { qself: qself, path: path }
            })
        ));
    }

    named!(pub qpath -> (Option<QSelf>, Path), alt!(
        map!(syn!(Path), |p| (None, p))
        |
        do_parse!(
            lt: punct!(<) >>
            this: syn!(Type) >>
            path: option!(tuple!(keyword!(as), syn!(Path))) >>
            gt: punct!(>) >>
            colon2: punct!(::) >>
            rest: call!(Delimited::parse_separated_nonempty) >>
            ({
                let (pos, as_, path) = match path {
                    Some((as_, mut path)) => {
                        let pos = path.segments.len();
                        if !path.segments.is_empty() && !path.segments.trailing_delim() {
                            path.segments.push_trailing(colon2);
                        }
                        for item in rest {
                            path.segments.push(item);
                        }
                        (pos, Some(as_), path)
                    }
                    None => {
                        (0, None, Path {
                            leading_colon: Some(colon2),
                            segments: rest,
                        })
                    }
                };
                (Some(QSelf {
                    lt_token: lt,
                    ty: Box::new(this),
                    position: pos,
                    as_token: as_,
                    gt_token: gt,
                }), path)
            })
        )
        |
        map!(keyword!(self), |s| (None, s.into()))
    ));

    impl Synom for ParenthesizedGenericArguments {
        named!(parse -> Self, do_parse!(
            data: parens!(call!(Delimited::parse_terminated)) >>
            output: syn!(ReturnType) >>
            (ParenthesizedGenericArguments {
                paren_token: data.1,
                inputs: data.0,
                output: output,
            })
        ));
    }

    impl Synom for ReturnType {
        named!(parse -> Self, alt!(
            do_parse!(
                arrow: punct!(->) >>
                ty: syn!(Type) >>
                (ReturnType::Type(arrow, Box::new(ty)))
            )
            |
            epsilon!() => { |_| ReturnType::Default }
        ));
    }

    impl TypeTraitObject {
        // Only allow multiple trait references if allow_plus is true.
        named!(parse(allow_plus: bool) -> Self, do_parse!(
            dyn_token: option!(keyword!(dyn)) >>
            bounds: alt!(
                cond_reduce!(allow_plus, call!(Delimited::parse_terminated_nonempty))
                |
                syn!(TypeParamBound) => { |x| vec![x].into() }
            ) >>
            (TypeTraitObject {
                dyn_token: dyn_token,
                bounds: bounds,
            })
        ));
    }

    impl Synom for TypeImplTrait {
        named!(parse -> Self, do_parse!(
            impl_: keyword!(impl) >>
            // NOTE: rust-lang/rust#34511 includes discussion about whether or
            // not + should be allowed in ImplTrait directly without ().
            elem: call!(Delimited::parse_terminated_nonempty) >>
            (TypeImplTrait {
                impl_token: impl_,
                bounds: elem,
            })
        ));
    }

    impl Synom for TypeGroup {
        named!(parse -> Self, do_parse!(
            data: grouped!(syn!(Type)) >>
            (TypeGroup {
                group_token: data.1,
                elem: Box::new(data.0),
            })
        ));
    }

    impl Synom for TypeParen {
        named!(parse -> Self, do_parse!(
            data: parens!(syn!(Type)) >>
            (TypeParen {
                paren_token: data.1,
                elem: Box::new(data.0),
            })
        ));
    }

    impl Synom for Path {
        named!(parse -> Self, do_parse!(
            colon: option!(punct!(::)) >>
            segments: call!(Delimited::<PathSegment, Token![::]>::parse_separated_nonempty) >>
            cond_reduce!(segments.first().map_or(true, |seg| seg.item().ident != "dyn"), epsilon!()) >>
            (Path {
                leading_colon: colon,
                segments: segments,
            })
        ));

        fn description() -> Option<&'static str> {
            Some("path")
        }
    }

    #[cfg(not(feature = "full"))]
    impl Synom for GenericArgument {
        named!(parse -> Self, alt!(
            call!(ty_no_eq_after) => { GenericArgument::Type }
            |
            syn!(Lifetime) => { GenericArgument::Lifetime }
            |
            syn!(TypeBinding) => { GenericArgument::TypeBinding }
        ));
    }

    #[cfg(feature = "full")]
    impl Synom for GenericArgument {
        named!(parse -> Self, alt!(
            call!(ty_no_eq_after) => { GenericArgument::Type }
            |
            syn!(Lifetime) => { GenericArgument::Lifetime }
            |
            syn!(TypeBinding) => { GenericArgument::TypeBinding }
            |
            syn!(ExprLit) => { |l| GenericArgument::Const(Expr::Lit(l).into()) }
            |
            syn!(ExprBlock) => { |b| GenericArgument::Const(Expr::Block(b).into()) }
        ));
    }

    impl Synom for AngleBracketedGenericArguments {
        named!(parse -> Self, do_parse!(
            colon2: option!(punct!(::)) >>
            lt: punct!(<) >>
            args: call!(Delimited::parse_terminated) >>
            gt: punct!(>) >>
            (AngleBracketedGenericArguments {
                colon2_token: colon2,
                lt_token: lt,
                args: args,
                gt_token: gt,
            })
        ));
    }

    impl Synom for PathSegment {
        named!(parse -> Self, alt!(
            do_parse!(
                ident: syn!(Ident) >>
                arguments: syn!(AngleBracketedGenericArguments) >>
                (PathSegment {
                    ident: ident,
                    arguments: PathArguments::AngleBracketed(arguments),
                })
            )
            |
            mod_style_path_segment
        ));
    }

    named!(pub ty_no_eq_after -> Type, terminated!(syn!(Type), not!(punct!(=))));

    impl Path {
        named!(pub parse_mod_style -> Self, do_parse!(
            colon: option!(punct!(::)) >>
            segments: call!(Delimited::parse_separated_nonempty_with,
                            mod_style_path_segment) >>
            (Path {
                leading_colon: colon,
                segments: segments,
            })
        ));
    }

    named!(mod_style_path_segment -> PathSegment, alt!(
        syn!(Ident) => { Into::into }
        |
        keyword!(super) => { Into::into }
        |
        keyword!(self) => { Into::into }
        |
        keyword!(Self) => { Into::into }
        |
        keyword!(crate) => { Into::into }
    ));

    impl Synom for TypeBinding {
        named!(parse -> Self, do_parse!(
            id: syn!(Ident) >>
            eq: punct!(=) >>
            ty: syn!(Type) >>
            (TypeBinding {
                ident: id,
                eq_token: eq,
                ty: ty,
            })
        ));
    }

    impl Synom for PolyTraitRef {
        named!(parse -> Self, do_parse!(
            bound_lifetimes: option!(syn!(BoundLifetimes)) >>
            trait_ref: syn!(Path) >>
            parenthesized: option!(cond_reduce!(
                trait_ref.segments.get(trait_ref.segments.len() - 1).item().arguments.is_empty(),
                syn!(ParenthesizedGenericArguments)
            )) >>
            ({
                let mut trait_ref = trait_ref;
                if let Some(parenthesized) = parenthesized {
                    let parenthesized = PathArguments::Parenthesized(parenthesized);
                    let len = trait_ref.segments.len();
                    trait_ref.segments.get_mut(len - 1).item_mut().arguments = parenthesized;
                }
                PolyTraitRef {
                    bound_lifetimes: bound_lifetimes,
                    trait_ref: trait_ref,
                }
            })
        ));
    }

    impl Synom for BareFnArg {
        named!(parse -> Self, do_parse!(
            name: option!(do_parse!(
                name: syn!(BareFnArgName) >>
                not!(punct!(::)) >>
                colon: punct!(:) >>
                (name, colon)
            )) >>
            ty: syn!(Type) >>
            (BareFnArg {
                name: name,
                ty: ty,
            })
        ));
    }

    impl Synom for BareFnArgName {
        named!(parse -> Self, alt!(
            map!(syn!(Ident), BareFnArgName::Named)
            |
            map!(punct!(_), BareFnArgName::Wild)
        ));
    }

    impl Synom for Abi {
        named!(parse -> Self, do_parse!(
            extern_: keyword!(extern) >>
            // TODO: this parses all literals, not just strings
            name: option!(syn!(Lit)) >>
            (Abi {
                extern_token: extern_,
                kind: match name {
                    Some(name) => AbiKind::Named(name),
                    None => AbiKind::Default,
                },
            })
        ));
    }
}

#[cfg(feature = "printing")]
mod printing {
    use super::*;
    use quote::{ToTokens, Tokens};

    impl ToTokens for TypeSlice {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.bracket_token.surround(tokens, |tokens| {
                self.elem.to_tokens(tokens);
            });
        }
    }

    impl ToTokens for TypeArray {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.bracket_token.surround(tokens, |tokens| {
                self.elem.to_tokens(tokens);
                self.semi_token.to_tokens(tokens);
                self.len.to_tokens(tokens);
            });
        }
    }

    impl ToTokens for TypePtr {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.star_token.to_tokens(tokens);
            match self.elem.mutability {
                Some(ref tok) => tok.to_tokens(tokens),
                None => {
                    TokensOrDefault(&self.const_token).to_tokens(tokens);
                }
            }
            self.elem.ty.to_tokens(tokens);
        }
    }

    impl ToTokens for TypeReference {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.and_token.to_tokens(tokens);
            self.lifetime.to_tokens(tokens);
            self.elem.mutability.to_tokens(tokens);
            self.elem.ty.to_tokens(tokens);
        }
    }

    impl ToTokens for TypeBareFn {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.ty.to_tokens(tokens)
        }
    }

    impl ToTokens for TypeNever {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.bang_token.to_tokens(tokens);
        }
    }

    impl ToTokens for TypeTuple {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.paren_token.surround(tokens, |tokens| {
                self.elems.to_tokens(tokens);
            })
        }
    }

    impl ToTokens for TypePath {
        fn to_tokens(&self, tokens: &mut Tokens) {
            PathTokens(&self.qself, &self.path).to_tokens(tokens);
        }
    }

    impl<'a> ToTokens for PathTokens<'a> {
        fn to_tokens(&self, tokens: &mut Tokens) {
            let qself = match *self.0 {
                Some(ref qself) => qself,
                None => return self.1.to_tokens(tokens),
            };
            qself.lt_token.to_tokens(tokens);
            qself.ty.to_tokens(tokens);

            // XXX: Gross.
            let pos = if qself.position > 0 && qself.position >= self.1.segments.len() {
                self.1.segments.len() - 1
            } else {
                qself.position
            };
            let mut segments = self.1.segments.iter();
            if pos > 0 {
                TokensOrDefault(&qself.as_token).to_tokens(tokens);
                self.1.leading_colon.to_tokens(tokens);
                for (i, segment) in (&mut segments).take(pos).enumerate() {
                    if i + 1 == pos {
                        segment.item().to_tokens(tokens);
                        qself.gt_token.to_tokens(tokens);
                        segment.delimiter().to_tokens(tokens);
                    } else {
                        segment.to_tokens(tokens);
                    }
                }
            } else {
                qself.gt_token.to_tokens(tokens);
                self.1.leading_colon.to_tokens(tokens);
            }
            for segment in segments {
                segment.to_tokens(tokens);
            }
        }
    }

    impl ToTokens for TypeTraitObject {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.dyn_token.to_tokens(tokens);
            self.bounds.to_tokens(tokens);
        }
    }

    impl ToTokens for TypeImplTrait {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.impl_token.to_tokens(tokens);
            self.bounds.to_tokens(tokens);
        }
    }

    impl ToTokens for TypeGroup {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.group_token.surround(tokens, |tokens| {
                self.elem.to_tokens(tokens);
            });
        }
    }

    impl ToTokens for TypeParen {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.paren_token.surround(tokens, |tokens| {
                self.elem.to_tokens(tokens);
            });
        }
    }

    impl ToTokens for TypeInfer {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.underscore_token.to_tokens(tokens);
        }
    }

    impl ToTokens for Path {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.leading_colon.to_tokens(tokens);
            self.segments.to_tokens(tokens);
        }
    }

    impl ToTokens for PathSegment {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.ident.to_tokens(tokens);
            self.arguments.to_tokens(tokens);
        }
    }

    impl ToTokens for PathArguments {
        fn to_tokens(&self, tokens: &mut Tokens) {
            match *self {
                PathArguments::None => {}
                PathArguments::AngleBracketed(ref arguments) => {
                    arguments.to_tokens(tokens);
                }
                PathArguments::Parenthesized(ref arguments) => {
                    arguments.to_tokens(tokens);
                }
            }
        }
    }

    impl ToTokens for GenericArgument {
        #[cfg_attr(feature = "cargo-clippy", allow(match_same_arms))]
        fn to_tokens(&self, tokens: &mut Tokens) {
            match *self {
                GenericArgument::Lifetime(ref lt) => lt.to_tokens(tokens),
                GenericArgument::Type(ref ty) => ty.to_tokens(tokens),
                GenericArgument::TypeBinding(ref tb) => tb.to_tokens(tokens),
                GenericArgument::Const(ref e) => match *e {
                    Expr::Lit(_) => e.to_tokens(tokens),

                    // NOTE: We should probably support parsing blocks with only
                    // expressions in them without the full feature for const
                    // generics.
                    #[cfg(feature = "full")]
                    Expr::Block(_) => e.to_tokens(tokens),

                    // ERROR CORRECTION: Add braces to make sure that the
                    // generated code is valid.
                    _ => token::Brace::default().surround(tokens, |tokens| {
                        e.to_tokens(tokens);
                    }),
                },
            }
        }
    }

    impl ToTokens for AngleBracketedGenericArguments {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.colon2_token.to_tokens(tokens);
            self.lt_token.to_tokens(tokens);
            self.args.to_tokens(tokens);
            self.gt_token.to_tokens(tokens);
        }
    }

    impl ToTokens for TypeBinding {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.ident.to_tokens(tokens);
            self.eq_token.to_tokens(tokens);
            self.ty.to_tokens(tokens);
        }
    }

    impl ToTokens for ParenthesizedGenericArguments {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.paren_token.surround(tokens, |tokens| {
                self.inputs.to_tokens(tokens);
            });
            self.output.to_tokens(tokens);
        }
    }

    impl ToTokens for ReturnType {
        fn to_tokens(&self, tokens: &mut Tokens) {
            match *self {
                ReturnType::Default => {}
                ReturnType::Type(ref arrow, ref ty) => {
                    arrow.to_tokens(tokens);
                    ty.to_tokens(tokens);
                }
            }
        }
    }

    impl ToTokens for PolyTraitRef {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.bound_lifetimes.to_tokens(tokens);
            self.trait_ref.to_tokens(tokens);
        }
    }

    impl ToTokens for BareFnType {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.lifetimes.to_tokens(tokens);
            self.unsafety.to_tokens(tokens);
            self.abi.to_tokens(tokens);
            self.fn_token.to_tokens(tokens);
            self.paren_token.surround(tokens, |tokens| {
                self.inputs.to_tokens(tokens);
                if let Some(ref variadic) = self.variadic {
                    if !self.inputs.empty_or_trailing() {
                        let span = variadic.0[0];
                        <Token![,]>::new(span).to_tokens(tokens);
                    }
                    variadic.to_tokens(tokens);
                }
            });
            self.output.to_tokens(tokens);
        }
    }

    impl ToTokens for BareFnArg {
        fn to_tokens(&self, tokens: &mut Tokens) {
            if let Some((ref name, ref colon)) = self.name {
                name.to_tokens(tokens);
                colon.to_tokens(tokens);
            }
            self.ty.to_tokens(tokens);
        }
    }

    impl ToTokens for BareFnArgName {
        fn to_tokens(&self, tokens: &mut Tokens) {
            match *self {
                BareFnArgName::Named(ref t) => t.to_tokens(tokens),
                BareFnArgName::Wild(ref t) => t.to_tokens(tokens),
            }
        }
    }

    impl ToTokens for Abi {
        fn to_tokens(&self, tokens: &mut Tokens) {
            self.extern_token.to_tokens(tokens);
            match self.kind {
                AbiKind::Named(ref named) => named.to_tokens(tokens),
                AbiKind::Default => {}
            }
        }
    }
}