rust/syn/generics.rs

// SPDX-License-Identifier: Apache-2.0 OR MIT

use crate::attr::Attribute;
use crate::expr::Expr;
use crate::ident::Ident;
use crate::lifetime::Lifetime;
use crate::path::Path;
use crate::punctuated::{Iter, IterMut, Punctuated};
use crate::token;
use crate::ty::Type;
use proc_macro2::TokenStream;
#[cfg(all(feature = "printing", feature = "extra-traits"))]
use std::fmt::{self, Debug};
#[cfg(all(feature = "printing", feature = "extra-traits"))]
use std::hash::{Hash, Hasher};

ast_struct! {
    /// Lifetimes and type parameters attached to a declaration of a function,
    /// enum, trait, etc.
    ///
    /// This struct represents two distinct optional syntactic elements,
    /// [generic parameters] and [where clause]. In some locations of the
    /// grammar, there may be other tokens in between these two things.
    ///
    /// [generic parameters]: https://doc.rust-lang.org/stable/reference/items/generics.html#generic-parameters
    /// [where clause]: https://doc.rust-lang.org/stable/reference/items/generics.html#where-clauses
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct Generics {
        pub lt_token: Option<Token![<]>,
        pub params: Punctuated<GenericParam, Token![,]>,
        pub gt_token: Option<Token![>]>,
        pub where_clause: Option<WhereClause>,
    }
}

ast_enum_of_structs! {
    /// A generic type parameter, lifetime, or const generic: `T: Into<String>`,
    /// `'a: 'b`, `const LEN: usize`.
    ///
    /// # Syntax tree enum
    ///
    /// This type is a [syntax tree enum].
    ///
    /// [syntax tree enum]: crate::expr::Expr#syntax-tree-enums
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub enum GenericParam {
        /// A lifetime parameter: `'a: 'b + 'c + 'd`.
        Lifetime(LifetimeParam),

        /// A generic type parameter: `T: Into<String>`.
        Type(TypeParam),

        /// A const generic parameter: `const LENGTH: usize`.
        Const(ConstParam),
    }
}

ast_struct! {
    /// A lifetime definition: `'a: 'b + 'c + 'd`.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct LifetimeParam {
        pub attrs: Vec<Attribute>,
        pub lifetime: Lifetime,
        pub colon_token: Option<Token![:]>,
        pub bounds: Punctuated<Lifetime, Token![+]>,
    }
}

ast_struct! {
    /// A generic type parameter: `T: Into<String>`.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct TypeParam {
        pub attrs: Vec<Attribute>,
        pub ident: Ident,
        pub colon_token: Option<Token![:]>,
        pub bounds: Punctuated<TypeParamBound, Token![+]>,
        pub eq_token: Option<Token![=]>,
        pub default: Option<Type>,
    }
}

ast_struct! {
    /// A const generic parameter: `const LENGTH: usize`.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct ConstParam {
        pub attrs: Vec<Attribute>,
        pub const_token: Token![const],
        pub ident: Ident,
        pub colon_token: Token![:],
        pub ty: Type,
        pub eq_token: Option<Token![=]>,
        pub default: Option<Expr>,
    }
}

impl Default for Generics {
    fn default() -> Self {
        Generics {
            lt_token: None,
            params: Punctuated::new(),
            gt_token: None,
            where_clause: None,
        }
    }
}

impl Generics {
    return_impl_trait! {
        /// Iterator over the lifetime parameters in `self.params`.
        pub fn lifetimes(&self) -> impl Iterator<Item = &LifetimeParam> [Lifetimes] {
            Lifetimes(self.params.iter())
        }
    }

    return_impl_trait! {
        /// Iterator over the lifetime parameters in `self.params`.
        pub fn lifetimes_mut(&mut self) -> impl Iterator<Item = &mut LifetimeParam> [LifetimesMut] {
            LifetimesMut(self.params.iter_mut())
        }
    }

    return_impl_trait! {
        /// Iterator over the type parameters in `self.params`.
        pub fn type_params(&self) -> impl Iterator<Item = &TypeParam> [TypeParams] {
            TypeParams(self.params.iter())
        }
    }

    return_impl_trait! {
        /// Iterator over the type parameters in `self.params`.
        pub fn type_params_mut(&mut self) -> impl Iterator<Item = &mut TypeParam> [TypeParamsMut] {
            TypeParamsMut(self.params.iter_mut())
        }
    }

    return_impl_trait! {
        /// Iterator over the constant parameters in `self.params`.
        pub fn const_params(&self) -> impl Iterator<Item = &ConstParam> [ConstParams] {
            ConstParams(self.params.iter())
        }
    }

    return_impl_trait! {
        /// Iterator over the constant parameters in `self.params`.
        pub fn const_params_mut(&mut self) -> impl Iterator<Item = &mut ConstParam> [ConstParamsMut] {
            ConstParamsMut(self.params.iter_mut())
        }
    }

    /// Initializes an empty `where`-clause if there is not one present already.
    pub fn make_where_clause(&mut self) -> &mut WhereClause {
        self.where_clause.get_or_insert_with(|| WhereClause {
            where_token: <Token![where]>::default(),
            predicates: Punctuated::new(),
        })
    }

    /// Split a type's generics into the pieces required for impl'ing a trait
    /// for that type.
    ///
    /// ```
    /// # use proc_macro2::{Span, Ident};
    /// # use quote::quote;
    /// #
    /// # let generics: syn::Generics = Default::default();
    /// # let name = Ident::new("MyType", Span::call_site());
    /// #
    /// let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
    /// quote! {
    ///     impl #impl_generics MyTrait for #name #ty_generics #where_clause {
    ///         // ...
    ///     }
    /// }
    /// # ;
    /// ```
    #[cfg(feature = "printing")]
    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    pub fn split_for_impl(&self) -> (ImplGenerics, TypeGenerics, Option<&WhereClause>) {
        (
            ImplGenerics(self),
            TypeGenerics(self),
            self.where_clause.as_ref(),
        )
    }
}

pub struct Lifetimes<'a>(Iter<'a, GenericParam>);

impl<'a> Iterator for Lifetimes<'a> {
    type Item = &'a LifetimeParam;

    fn next(&mut self) -> Option<Self::Item> {
        if let GenericParam::Lifetime(lifetime) = self.0.next()? {
            Some(lifetime)
        } else {
            self.next()
        }
    }
}

pub struct LifetimesMut<'a>(IterMut<'a, GenericParam>);

impl<'a> Iterator for LifetimesMut<'a> {
    type Item = &'a mut LifetimeParam;

    fn next(&mut self) -> Option<Self::Item> {
        if let GenericParam::Lifetime(lifetime) = self.0.next()? {
            Some(lifetime)
        } else {
            self.next()
        }
    }
}

pub struct TypeParams<'a>(Iter<'a, GenericParam>);

impl<'a> Iterator for TypeParams<'a> {
    type Item = &'a TypeParam;

    fn next(&mut self) -> Option<Self::Item> {
        if let GenericParam::Type(type_param) = self.0.next()? {
            Some(type_param)
        } else {
            self.next()
        }
    }
}

pub struct TypeParamsMut<'a>(IterMut<'a, GenericParam>);

impl<'a> Iterator for TypeParamsMut<'a> {
    type Item = &'a mut TypeParam;

    fn next(&mut self) -> Option<Self::Item> {
        if let GenericParam::Type(type_param) = self.0.next()? {
            Some(type_param)
        } else {
            self.next()
        }
    }
}

pub struct ConstParams<'a>(Iter<'a, GenericParam>);

impl<'a> Iterator for ConstParams<'a> {
    type Item = &'a ConstParam;

    fn next(&mut self) -> Option<Self::Item> {
        if let GenericParam::Const(const_param) = self.0.next()? {
            Some(const_param)
        } else {
            self.next()
        }
    }
}

pub struct ConstParamsMut<'a>(IterMut<'a, GenericParam>);

impl<'a> Iterator for ConstParamsMut<'a> {
    type Item = &'a mut ConstParam;

    fn next(&mut self) -> Option<Self::Item> {
        if let GenericParam::Const(const_param) = self.0.next()? {
            Some(const_param)
        } else {
            self.next()
        }
    }
}

/// Returned by `Generics::split_for_impl`.
#[cfg(feature = "printing")]
#[cfg_attr(
    docsrs,
    doc(cfg(all(any(feature = "full", feature = "derive"), feature = "printing")))
)]
pub struct ImplGenerics<'a>(&'a Generics);

/// Returned by `Generics::split_for_impl`.
#[cfg(feature = "printing")]
#[cfg_attr(
    docsrs,
    doc(cfg(all(any(feature = "full", feature = "derive"), feature = "printing")))
)]
pub struct TypeGenerics<'a>(&'a Generics);

/// Returned by `TypeGenerics::as_turbofish`.
#[cfg(feature = "printing")]
#[cfg_attr(
    docsrs,
    doc(cfg(all(any(feature = "full", feature = "derive"), feature = "printing")))
)]
pub struct Turbofish<'a>(&'a Generics);

#[cfg(feature = "printing")]
macro_rules! generics_wrapper_impls {
    ($ty:ident) => {
        #[cfg(feature = "clone-impls")]
        #[cfg_attr(docsrs, doc(cfg(feature = "clone-impls")))]
        impl<'a> Clone for $ty<'a> {
            fn clone(&self) -> Self {
                $ty(self.0)
            }
        }

        #[cfg(feature = "extra-traits")]
        #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
        impl<'a> Debug for $ty<'a> {
            fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter
                    .debug_tuple(stringify!($ty))
                    .field(self.0)
                    .finish()
            }
        }

        #[cfg(feature = "extra-traits")]
        #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
        impl<'a> Eq for $ty<'a> {}

        #[cfg(feature = "extra-traits")]
        #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
        impl<'a> PartialEq for $ty<'a> {
            fn eq(&self, other: &Self) -> bool {
                self.0 == other.0
            }
        }

        #[cfg(feature = "extra-traits")]
        #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
        impl<'a> Hash for $ty<'a> {
            fn hash<H: Hasher>(&self, state: &mut H) {
                self.0.hash(state);
            }
        }
    };
}

#[cfg(feature = "printing")]
generics_wrapper_impls!(ImplGenerics);
#[cfg(feature = "printing")]
generics_wrapper_impls!(TypeGenerics);
#[cfg(feature = "printing")]
generics_wrapper_impls!(Turbofish);

#[cfg(feature = "printing")]
impl<'a> TypeGenerics<'a> {
    /// Turn a type's generics like `<X, Y>` into a turbofish like `::<X, Y>`.
    pub fn as_turbofish(&self) -> Turbofish<'a> {
        Turbofish(self.0)
    }
}

ast_struct! {
    /// A set of bound lifetimes: `for<'a, 'b, 'c>`.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct BoundLifetimes {
        pub for_token: Token![for],
        pub lt_token: Token![<],
        pub lifetimes: Punctuated<GenericParam, Token![,]>,
        pub gt_token: Token![>],
    }
}

impl Default for BoundLifetimes {
    fn default() -> Self {
        BoundLifetimes {
            for_token: Default::default(),
            lt_token: Default::default(),
            lifetimes: Punctuated::new(),
            gt_token: Default::default(),
        }
    }
}

impl LifetimeParam {
    pub fn new(lifetime: Lifetime) -> Self {
        LifetimeParam {
            attrs: Vec::new(),
            lifetime,
            colon_token: None,
            bounds: Punctuated::new(),
        }
    }
}

impl From<Ident> for TypeParam {
    fn from(ident: Ident) -> Self {
        TypeParam {
            attrs: vec![],
            ident,
            colon_token: None,
            bounds: Punctuated::new(),
            eq_token: None,
            default: None,
        }
    }
}

ast_enum_of_structs! {
    /// A trait or lifetime used as a bound on a type parameter.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    #[non_exhaustive]
    pub enum TypeParamBound {
        Trait(TraitBound),
        Lifetime(Lifetime),
        PreciseCapture(PreciseCapture),
        Verbatim(TokenStream),
    }
}

ast_struct! {
    /// A trait used as a bound on a type parameter.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct TraitBound {
        pub paren_token: Option<token::Paren>,
        pub modifier: TraitBoundModifier,
        /// The `for<'a>` in `for<'a> Foo<&'a T>`
        pub lifetimes: Option<BoundLifetimes>,
        /// The `Foo<&'a T>` in `for<'a> Foo<&'a T>`
        pub path: Path,
    }
}

ast_enum! {
    /// A modifier on a trait bound, currently only used for the `?` in
    /// `?Sized`.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub enum TraitBoundModifier {
        None,
        Maybe(Token![?]),
    }
}

ast_struct! {
    /// Precise capturing bound: the 'use&lt;&hellip;&gt;' in `impl Trait +
    /// use<'a, T>`.
    #[cfg_attr(docsrs, doc(cfg(feature = "full")))]
    pub struct PreciseCapture #full {
        pub use_token: Token![use],
        pub lt_token: Token![<],
        pub params: Punctuated<CapturedParam, Token![,]>,
        pub gt_token: Token![>],
    }
}

#[cfg(feature = "full")]
ast_enum! {
    /// Single parameter in a precise capturing bound.
    #[cfg_attr(docsrs, doc(cfg(feature = "full")))]
    #[non_exhaustive]
    pub enum CapturedParam {
        /// A lifetime parameter in precise capturing bound: `fn f<'a>() -> impl
        /// Trait + use<'a>`.
        Lifetime(Lifetime),
        /// A type parameter or const generic parameter in precise capturing
        /// bound: `fn f<T>() -> impl Trait + use<T>` or `fn f<const K: T>() ->
        /// impl Trait + use<K>`.
        Ident(Ident),
    }
}

ast_struct! {
    /// A `where` clause in a definition: `where T: Deserialize<'de>, D:
    /// 'static`.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct WhereClause {
        pub where_token: Token![where],
        pub predicates: Punctuated<WherePredicate, Token![,]>,
    }
}

ast_enum_of_structs! {
    /// A single predicate in a `where` clause: `T: Deserialize<'de>`.
    ///
    /// # Syntax tree enum
    ///
    /// This type is a [syntax tree enum].
    ///
    /// [syntax tree enum]: crate::expr::Expr#syntax-tree-enums
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    #[non_exhaustive]
    pub enum WherePredicate {
        /// A lifetime predicate in a `where` clause: `'a: 'b + 'c`.
        Lifetime(PredicateLifetime),

        /// A type predicate in a `where` clause: `for<'c> Foo<'c>: Trait<'c>`.
        Type(PredicateType),
    }
}

ast_struct! {
    /// A lifetime predicate in a `where` clause: `'a: 'b + 'c`.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct PredicateLifetime {
        pub lifetime: Lifetime,
        pub colon_token: Token![:],
        pub bounds: Punctuated<Lifetime, Token![+]>,
    }
}

ast_struct! {
    /// A type predicate in a `where` clause: `for<'c> Foo<'c>: Trait<'c>`.
    #[cfg_attr(docsrs, doc(cfg(any(feature = "full", feature = "derive"))))]
    pub struct PredicateType {
        /// Any lifetimes from a `for` binding
        pub lifetimes: Option<BoundLifetimes>,
        /// The type being bounded
        pub bounded_ty: Type,
        pub colon_token: Token![:],
        /// Trait and lifetime bounds (`Clone+Send+'static`)
        pub bounds: Punctuated<TypeParamBound, Token![+]>,
    }
}

#[cfg(feature = "parsing")]
pub(crate) mod parsing {
    use crate::attr::Attribute;
    #[cfg(feature = "full")]
    use crate::error;
    use crate::error::{Error, Result};
    use crate::ext::IdentExt as _;
    use crate::generics::{
        BoundLifetimes, ConstParam, GenericParam, Generics, LifetimeParam, PredicateLifetime,
        PredicateType, TraitBound, TraitBoundModifier, TypeParam, TypeParamBound, WhereClause,
        WherePredicate,
    };
    #[cfg(feature = "full")]
    use crate::generics::{CapturedParam, PreciseCapture};
    use crate::ident::Ident;
    use crate::lifetime::Lifetime;
    use crate::parse::{Parse, ParseStream};
    use crate::path::{self, ParenthesizedGenericArguments, Path, PathArguments};
    use crate::punctuated::Punctuated;
    use crate::token;
    use crate::ty::Type;
    use crate::verbatim;

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for Generics {
        fn parse(input: ParseStream) -> Result<Self> {
            if !input.peek(Token![<]) {
                return Ok(Generics::default());
            }

            let lt_token: Token![<] = input.parse()?;

            let mut params = Punctuated::new();
            loop {
                if input.peek(Token![>]) {
                    break;
                }

                let attrs = input.call(Attribute::parse_outer)?;
                let lookahead = input.lookahead1();
                if lookahead.peek(Lifetime) {
                    params.push_value(GenericParam::Lifetime(LifetimeParam {
                        attrs,
                        ..input.parse()?
                    }));
                } else if lookahead.peek(Ident) {
                    params.push_value(GenericParam::Type(TypeParam {
                        attrs,
                        ..input.parse()?
                    }));
                } else if lookahead.peek(Token![const]) {
                    params.push_value(GenericParam::Const(ConstParam {
                        attrs,
                        ..input.parse()?
                    }));
                } else if input.peek(Token![_]) {
                    params.push_value(GenericParam::Type(TypeParam {
                        attrs,
                        ident: input.call(Ident::parse_any)?,
                        colon_token: None,
                        bounds: Punctuated::new(),
                        eq_token: None,
                        default: None,
                    }));
                } else {
                    return Err(lookahead.error());
                }

                if input.peek(Token![>]) {
                    break;
                }
                let punct = input.parse()?;
                params.push_punct(punct);
            }

            let gt_token: Token![>] = input.parse()?;

            Ok(Generics {
                lt_token: Some(lt_token),
                params,
                gt_token: Some(gt_token),
                where_clause: None,
            })
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for GenericParam {
        fn parse(input: ParseStream) -> Result<Self> {
            let attrs = input.call(Attribute::parse_outer)?;

            let lookahead = input.lookahead1();
            if lookahead.peek(Ident) {
                Ok(GenericParam::Type(TypeParam {
                    attrs,
                    ..input.parse()?
                }))
            } else if lookahead.peek(Lifetime) {
                Ok(GenericParam::Lifetime(LifetimeParam {
                    attrs,
                    ..input.parse()?
                }))
            } else if lookahead.peek(Token![const]) {
                Ok(GenericParam::Const(ConstParam {
                    attrs,
                    ..input.parse()?
                }))
            } else {
                Err(lookahead.error())
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LifetimeParam {
        fn parse(input: ParseStream) -> Result<Self> {
            let has_colon;
            Ok(LifetimeParam {
                attrs: input.call(Attribute::parse_outer)?,
                lifetime: input.parse()?,
                colon_token: {
                    if input.peek(Token![:]) {
                        has_colon = true;
                        Some(input.parse()?)
                    } else {
                        has_colon = false;
                        None
                    }
                },
                bounds: {
                    let mut bounds = Punctuated::new();
                    if has_colon {
                        loop {
                            if input.peek(Token![,]) || input.peek(Token![>]) {
                                break;
                            }
                            let value = input.parse()?;
                            bounds.push_value(value);
                            if !input.peek(Token![+]) {
                                break;
                            }
                            let punct = input.parse()?;
                            bounds.push_punct(punct);
                        }
                    }
                    bounds
                },
            })
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for BoundLifetimes {
        fn parse(input: ParseStream) -> Result<Self> {
            Ok(BoundLifetimes {
                for_token: input.parse()?,
                lt_token: input.parse()?,
                lifetimes: {
                    let mut lifetimes = Punctuated::new();
                    while !input.peek(Token![>]) {
                        lifetimes.push_value(input.parse()?);
                        if input.peek(Token![>]) {
                            break;
                        }
                        lifetimes.push_punct(input.parse()?);
                    }
                    lifetimes
                },
                gt_token: input.parse()?,
            })
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for Option<BoundLifetimes> {
        fn parse(input: ParseStream) -> Result<Self> {
            if input.peek(Token![for]) {
                input.parse().map(Some)
            } else {
                Ok(None)
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for TypeParam {
        fn parse(input: ParseStream) -> Result<Self> {
            let attrs = input.call(Attribute::parse_outer)?;
            let ident: Ident = input.parse()?;
            let colon_token: Option<Token![:]> = input.parse()?;

            let mut bounds = Punctuated::new();
            if colon_token.is_some() {
                loop {
                    if input.peek(Token![,]) || input.peek(Token![>]) || input.peek(Token![=]) {
                        break;
                    }
                    bounds.push_value({
                        let allow_precise_capture = false;
                        let allow_const = true;
                        TypeParamBound::parse_single(input, allow_precise_capture, allow_const)?
                    });
                    if !input.peek(Token![+]) {
                        break;
                    }
                    let punct: Token![+] = input.parse()?;
                    bounds.push_punct(punct);
                }
            }

            let eq_token: Option<Token![=]> = input.parse()?;
            let default = if eq_token.is_some() {
                Some(input.parse::<Type>()?)
            } else {
                None
            };

            Ok(TypeParam {
                attrs,
                ident,
                colon_token,
                bounds,
                eq_token,
                default,
            })
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for TypeParamBound {
        fn parse(input: ParseStream) -> Result<Self> {
            let allow_precise_capture = true;
            let allow_const = true;
            Self::parse_single(input, allow_precise_capture, allow_const)
        }
    }

    impl TypeParamBound {
        pub(crate) fn parse_single(
            input: ParseStream,
            #[cfg_attr(not(feature = "full"), allow(unused_variables))] allow_precise_capture: bool,
            allow_const: bool,
        ) -> Result<Self> {
            if input.peek(Lifetime) {
                return input.parse().map(TypeParamBound::Lifetime);
            }

            #[cfg(feature = "full")]
            {
                if input.peek(Token![use]) {
                    let precise_capture: PreciseCapture = input.parse()?;
                    return if allow_precise_capture {
                        Ok(TypeParamBound::PreciseCapture(precise_capture))
                    } else {
                        let msg = "`use<...>` precise capturing syntax is not allowed here";
                        Err(error::new2(
                            precise_capture.use_token.span,
                            precise_capture.gt_token.span,
                            msg,
                        ))
                    };
                }
            }

            let begin = input.fork();

            let content;
            let (paren_token, content) = if input.peek(token::Paren) {
                (Some(parenthesized!(content in input)), &content)
            } else {
                (None, input)
            };

            if let Some(mut bound) = TraitBound::do_parse(content, allow_const)? {
                bound.paren_token = paren_token;
                Ok(TypeParamBound::Trait(bound))
            } else {
                Ok(TypeParamBound::Verbatim(verbatim::between(&begin, input)))
            }
        }

        pub(crate) fn parse_multiple(
            input: ParseStream,
            allow_plus: bool,
            allow_precise_capture: bool,
            allow_const: bool,
        ) -> Result<Punctuated<Self, Token![+]>> {
            let mut bounds = Punctuated::new();
            loop {
                let bound = Self::parse_single(input, allow_precise_capture, allow_const)?;
                bounds.push_value(bound);
                if !(allow_plus && input.peek(Token![+])) {
                    break;
                }
                bounds.push_punct(input.parse()?);
                if !(input.peek(Ident::peek_any)
                    || input.peek(Token![::])
                    || input.peek(Token![?])
                    || input.peek(Lifetime)
                    || input.peek(token::Paren)
                    || (allow_const && (input.peek(token::Bracket) || input.peek(Token![const]))))
                {
                    break;
                }
            }
            Ok(bounds)
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for TraitBound {
        fn parse(input: ParseStream) -> Result<Self> {
            let allow_const = false;
            Self::do_parse(input, allow_const).map(Option::unwrap)
        }
    }

    impl TraitBound {
        fn do_parse(input: ParseStream, allow_const: bool) -> Result<Option<Self>> {
            let mut lifetimes: Option<BoundLifetimes> = input.parse()?;

            let is_conditionally_const = cfg!(feature = "full") && input.peek(token::Bracket);
            let is_unconditionally_const = cfg!(feature = "full") && input.peek(Token![const]);
            if is_conditionally_const {
                let conditionally_const;
                let bracket_token = bracketed!(conditionally_const in input);
                conditionally_const.parse::<Token![const]>()?;
                if !allow_const {
                    let msg = "`[const]` is not allowed here";
                    return Err(Error::new(bracket_token.span.join(), msg));
                }
            } else if is_unconditionally_const {
                let const_token: Token![const] = input.parse()?;
                if !allow_const {
                    let msg = "`const` is not allowed here";
                    return Err(Error::new(const_token.span, msg));
                }
            }

            let modifier: TraitBoundModifier = input.parse()?;
            if lifetimes.is_none() && matches!(modifier, TraitBoundModifier::Maybe(_)) {
                lifetimes = input.parse()?;
            }

            let mut path: Path = input.parse()?;
            if path.segments.last().unwrap().arguments.is_empty()
                && (input.peek(token::Paren) || input.peek(Token![::]) && input.peek3(token::Paren))
            {
                input.parse::<Option<Token![::]>>()?;
                let args: ParenthesizedGenericArguments = input.parse()?;
                let parenthesized = PathArguments::Parenthesized(args);
                path.segments.last_mut().unwrap().arguments = parenthesized;
            }

            if lifetimes.is_some() {
                match modifier {
                    TraitBoundModifier::None => {}
                    TraitBoundModifier::Maybe(maybe) => {
                        let msg = "`for<...>` binder not allowed with `?` trait polarity modifier";
                        return Err(Error::new(maybe.span, msg));
                    }
                }
            }

            if is_conditionally_const || is_unconditionally_const {
                Ok(None)
            } else {
                Ok(Some(TraitBound {
                    paren_token: None,
                    modifier,
                    lifetimes,
                    path,
                }))
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for TraitBoundModifier {
        fn parse(input: ParseStream) -> Result<Self> {
            if input.peek(Token![?]) {
                input.parse().map(TraitBoundModifier::Maybe)
            } else {
                Ok(TraitBoundModifier::None)
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for ConstParam {
        fn parse(input: ParseStream) -> Result<Self> {
            let mut default = None;
            Ok(ConstParam {
                attrs: input.call(Attribute::parse_outer)?,
                const_token: input.parse()?,
                ident: input.parse()?,
                colon_token: input.parse()?,
                ty: input.parse()?,
                eq_token: {
                    if input.peek(Token![=]) {
                        let eq_token = input.parse()?;
                        default = Some(path::parsing::const_argument(input)?);
                        Some(eq_token)
                    } else {
                        None
                    }
                },
                default,
            })
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for WhereClause {
        fn parse(input: ParseStream) -> Result<Self> {
            let where_token: Token![where] = input.parse()?;

            if choose_generics_over_qpath(input) {
                return Err(input
                    .error("generic parameters on `where` clauses are reserved for future use"));
            }

            Ok(WhereClause {
                where_token,
                predicates: {
                    let mut predicates = Punctuated::new();
                    loop {
                        if input.is_empty()
                            || input.peek(token::Brace)
                            || input.peek(Token![,])
                            || input.peek(Token![;])
                            || input.peek(Token![:]) && !input.peek(Token![::])
                            || input.peek(Token![=])
                        {
                            break;
                        }
                        let value = input.parse()?;
                        predicates.push_value(value);
                        if !input.peek(Token![,]) {
                            break;
                        }
                        let punct = input.parse()?;
                        predicates.push_punct(punct);
                    }
                    predicates
                },
            })
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for Option<WhereClause> {
        fn parse(input: ParseStream) -> Result<Self> {
            if input.peek(Token![where]) {
                input.parse().map(Some)
            } else {
                Ok(None)
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for WherePredicate {
        fn parse(input: ParseStream) -> Result<Self> {
            if input.peek(Lifetime) && input.peek2(Token![:]) {
                Ok(WherePredicate::Lifetime(PredicateLifetime {
                    lifetime: input.parse()?,
                    colon_token: input.parse()?,
                    bounds: {
                        let mut bounds = Punctuated::new();
                        loop {
                            if input.is_empty()
                                || input.peek(token::Brace)
                                || input.peek(Token![,])
                                || input.peek(Token![;])
                                || input.peek(Token![:])
                                || input.peek(Token![=])
                            {
                                break;
                            }
                            let value = input.parse()?;
                            bounds.push_value(value);
                            if !input.peek(Token![+]) {
                                break;
                            }
                            let punct = input.parse()?;
                            bounds.push_punct(punct);
                        }
                        bounds
                    },
                }))
            } else {
                Ok(WherePredicate::Type(PredicateType {
                    lifetimes: input.parse()?,
                    bounded_ty: input.parse()?,
                    colon_token: input.parse()?,
                    bounds: {
                        let mut bounds = Punctuated::new();
                        loop {
                            if input.is_empty()
                                || input.peek(token::Brace)
                                || input.peek(Token![,])
                                || input.peek(Token![;])
                                || input.peek(Token![:]) && !input.peek(Token![::])
                                || input.peek(Token![=])
                            {
                                break;
                            }
                            bounds.push_value({
                                let allow_precise_capture = false;
                                let allow_const = true;
                                TypeParamBound::parse_single(
                                    input,
                                    allow_precise_capture,
                                    allow_const,
                                )?
                            });
                            if !input.peek(Token![+]) {
                                break;
                            }
                            let punct = input.parse()?;
                            bounds.push_punct(punct);
                        }
                        bounds
                    },
                }))
            }
        }
    }

    #[cfg(feature = "full")]
    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for PreciseCapture {
        fn parse(input: ParseStream) -> Result<Self> {
            let use_token: Token![use] = input.parse()?;
            let lt_token: Token![<] = input.parse()?;
            let mut params = Punctuated::new();
            loop {
                let lookahead = input.lookahead1();
                params.push_value(
                    if lookahead.peek(Lifetime) || lookahead.peek(Ident) || input.peek(Token![Self])
                    {
                        input.parse::<CapturedParam>()?
                    } else if lookahead.peek(Token![>]) {
                        break;
                    } else {
                        return Err(lookahead.error());
                    },
                );
                let lookahead = input.lookahead1();
                params.push_punct(if lookahead.peek(Token![,]) {
                    input.parse::<Token![,]>()?
                } else if lookahead.peek(Token![>]) {
                    break;
                } else {
                    return Err(lookahead.error());
                });
            }
            let gt_token: Token![>] = input.parse()?;
            Ok(PreciseCapture {
                use_token,
                lt_token,
                params,
                gt_token,
            })
        }
    }

    #[cfg(feature = "full")]
    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for CapturedParam {
        fn parse(input: ParseStream) -> Result<Self> {
            let lookahead = input.lookahead1();
            if lookahead.peek(Lifetime) {
                input.parse().map(CapturedParam::Lifetime)
            } else if lookahead.peek(Ident) || input.peek(Token![Self]) {
                input.call(Ident::parse_any).map(CapturedParam::Ident)
            } else {
                Err(lookahead.error())
            }
        }
    }

    pub(crate) fn choose_generics_over_qpath(input: ParseStream) -> bool {
        // Rust syntax has an ambiguity between generic parameters and qualified
        // paths. In `impl <T> :: Thing<T, U> {}` this may either be a generic
        // inherent impl `impl<T> ::Thing<T, U>` or a non-generic inherent impl
        // for an associated type `impl <T>::Thing<T, U>`.
        //
        // After `<` the following continuations can only begin generics, not a
        // qualified path:
        //
        //     `<` `>`                  - empty generic parameters
        //     `<` `#`                  - generic parameters with attribute
        //     `<` LIFETIME `>`         - single lifetime parameter
        //     `<` (LIFETIME|IDENT) `,` - first generic parameter in a list
        //     `<` (LIFETIME|IDENT) `:` - generic parameter with bounds
        //     `<` (LIFETIME|IDENT) `=` - generic parameter with a default
        //     `<` const                - generic const parameter
        //
        // The only truly ambiguous case is:
        //
        //     `<` IDENT `>` `::` IDENT ...
        //
        // which we disambiguate in favor of generics because this is almost
        // always the expected one in the context of real-world code.
        input.peek(Token![<])
            && (input.peek2(Token![>])
                || input.peek2(Token![#])
                || (input.peek2(Lifetime) || input.peek2(Ident))
                    && (input.peek3(Token![>])
                        || input.peek3(Token![,])
                        || input.peek3(Token![:]) && !input.peek3(Token![::])
                        || input.peek3(Token![=]))
                || input.peek2(Token![const]))
    }

    #[cfg(feature = "full")]
    pub(crate) fn choose_generics_over_qpath_after_keyword(input: ParseStream) -> bool {
        let input = input.fork();
        input.call(Ident::parse_any).unwrap(); // `impl` or `for` or `where`
        choose_generics_over_qpath(&input)
    }
}

#[cfg(feature = "printing")]
pub(crate) mod printing {
    use crate::attr::FilterAttrs;
    #[cfg(feature = "full")]
    use crate::expr;
    use crate::expr::Expr;
    #[cfg(feature = "full")]
    use crate::fixup::FixupContext;
    use crate::generics::{
        BoundLifetimes, ConstParam, GenericParam, Generics, ImplGenerics, LifetimeParam,
        PredicateLifetime, PredicateType, TraitBound, TraitBoundModifier, Turbofish, TypeGenerics,
        TypeParam, WhereClause,
    };
    #[cfg(feature = "full")]
    use crate::generics::{CapturedParam, PreciseCapture};
    use crate::print::TokensOrDefault;
    use crate::token;
    use proc_macro2::TokenStream;
    use quote::{ToTokens, TokenStreamExt};

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for Generics {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            if self.params.is_empty() {
                return;
            }

            TokensOrDefault(&self.lt_token).to_tokens(tokens);

            // Print lifetimes before types and consts, regardless of their
            // order in self.params.
            let mut trailing_or_empty = true;
            for param in self.params.pairs() {
                if let GenericParam::Lifetime(_) = **param.value() {
                    param.to_tokens(tokens);
                    trailing_or_empty = param.punct().is_some();
                }
            }
            for param in self.params.pairs() {
                match param.value() {
                    GenericParam::Type(_) | GenericParam::Const(_) => {
                        if !trailing_or_empty {
                            <Token![,]>::default().to_tokens(tokens);
                            trailing_or_empty = true;
                        }
                        param.to_tokens(tokens);
                    }
                    GenericParam::Lifetime(_) => {}
                }
            }

            TokensOrDefault(&self.gt_token).to_tokens(tokens);
        }
    }

    impl<'a> ToTokens for ImplGenerics<'a> {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            if self.0.params.is_empty() {
                return;
            }

            TokensOrDefault(&self.0.lt_token).to_tokens(tokens);

            // Print lifetimes before types and consts, regardless of their
            // order in self.params.
            let mut trailing_or_empty = true;
            for param in self.0.params.pairs() {
                if let GenericParam::Lifetime(_) = **param.value() {
                    param.to_tokens(tokens);
                    trailing_or_empty = param.punct().is_some();
                }
            }
            for param in self.0.params.pairs() {
                if let GenericParam::Lifetime(_) = **param.value() {
                    continue;
                }
                if !trailing_or_empty {
                    <Token![,]>::default().to_tokens(tokens);
                    trailing_or_empty = true;
                }
                match param.value() {
                    GenericParam::Lifetime(_) => unreachable!(),
                    GenericParam::Type(param) => {
                        // Leave off the type parameter defaults
                        tokens.append_all(param.attrs.outer());
                        param.ident.to_tokens(tokens);
                        if !param.bounds.is_empty() {
                            TokensOrDefault(&param.colon_token).to_tokens(tokens);
                            param.bounds.to_tokens(tokens);
                        }
                    }
                    GenericParam::Const(param) => {
                        // Leave off the const parameter defaults
                        tokens.append_all(param.attrs.outer());
                        param.const_token.to_tokens(tokens);
                        param.ident.to_tokens(tokens);
                        param.colon_token.to_tokens(tokens);
                        param.ty.to_tokens(tokens);
                    }
                }
                param.punct().to_tokens(tokens);
            }

            TokensOrDefault(&self.0.gt_token).to_tokens(tokens);
        }
    }

    impl<'a> ToTokens for TypeGenerics<'a> {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            if self.0.params.is_empty() {
                return;
            }

            TokensOrDefault(&self.0.lt_token).to_tokens(tokens);

            // Print lifetimes before types and consts, regardless of their
            // order in self.params.
            let mut trailing_or_empty = true;
            for param in self.0.params.pairs() {
                if let GenericParam::Lifetime(def) = *param.value() {
                    // Leave off the lifetime bounds and attributes
                    def.lifetime.to_tokens(tokens);
                    param.punct().to_tokens(tokens);
                    trailing_or_empty = param.punct().is_some();
                }
            }
            for param in self.0.params.pairs() {
                if let GenericParam::Lifetime(_) = **param.value() {
                    continue;
                }
                if !trailing_or_empty {
                    <Token![,]>::default().to_tokens(tokens);
                    trailing_or_empty = true;
                }
                match param.value() {
                    GenericParam::Lifetime(_) => unreachable!(),
                    GenericParam::Type(param) => {
                        // Leave off the type parameter defaults
                        param.ident.to_tokens(tokens);
                    }
                    GenericParam::Const(param) => {
                        // Leave off the const parameter defaults
                        param.ident.to_tokens(tokens);
                    }
                }
                param.punct().to_tokens(tokens);
            }

            TokensOrDefault(&self.0.gt_token).to_tokens(tokens);
        }
    }

    impl<'a> ToTokens for Turbofish<'a> {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            if !self.0.params.is_empty() {
                <Token![::]>::default().to_tokens(tokens);
                TypeGenerics(self.0).to_tokens(tokens);
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for BoundLifetimes {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.for_token.to_tokens(tokens);
            self.lt_token.to_tokens(tokens);
            self.lifetimes.to_tokens(tokens);
            self.gt_token.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LifetimeParam {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            tokens.append_all(self.attrs.outer());
            self.lifetime.to_tokens(tokens);
            if !self.bounds.is_empty() {
                TokensOrDefault(&self.colon_token).to_tokens(tokens);
                self.bounds.to_tokens(tokens);
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for TypeParam {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            tokens.append_all(self.attrs.outer());
            self.ident.to_tokens(tokens);
            if !self.bounds.is_empty() {
                TokensOrDefault(&self.colon_token).to_tokens(tokens);
                self.bounds.to_tokens(tokens);
            }
            if let Some(default) = &self.default {
                TokensOrDefault(&self.eq_token).to_tokens(tokens);
                default.to_tokens(tokens);
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for TraitBound {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            let to_tokens = |tokens: &mut TokenStream| {
                self.modifier.to_tokens(tokens);
                self.lifetimes.to_tokens(tokens);
                self.path.to_tokens(tokens);
            };
            match &self.paren_token {
                Some(paren) => paren.surround(tokens, to_tokens),
                None => to_tokens(tokens),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for TraitBoundModifier {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            match self {
                TraitBoundModifier::None => {}
                TraitBoundModifier::Maybe(t) => t.to_tokens(tokens),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for ConstParam {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            tokens.append_all(self.attrs.outer());
            self.const_token.to_tokens(tokens);
            self.ident.to_tokens(tokens);
            self.colon_token.to_tokens(tokens);
            self.ty.to_tokens(tokens);
            if let Some(default) = &self.default {
                TokensOrDefault(&self.eq_token).to_tokens(tokens);
                print_const_argument(default, tokens);
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for WhereClause {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            if !self.predicates.is_empty() {
                self.where_token.to_tokens(tokens);
                self.predicates.to_tokens(tokens);
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for PredicateLifetime {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.lifetime.to_tokens(tokens);
            self.colon_token.to_tokens(tokens);
            self.bounds.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for PredicateType {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.lifetimes.to_tokens(tokens);
            self.bounded_ty.to_tokens(tokens);
            self.colon_token.to_tokens(tokens);
            self.bounds.to_tokens(tokens);
        }
    }

    #[cfg(feature = "full")]
    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for PreciseCapture {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.use_token.to_tokens(tokens);
            self.lt_token.to_tokens(tokens);

            // Print lifetimes before types and consts, regardless of their
            // order in self.params.
            let mut trailing_or_empty = true;
            for param in self.params.pairs() {
                if let CapturedParam::Lifetime(_) = **param.value() {
                    param.to_tokens(tokens);
                    trailing_or_empty = param.punct().is_some();
                }
            }
            for param in self.params.pairs() {
                if let CapturedParam::Ident(_) = **param.value() {
                    if !trailing_or_empty {
                        <Token![,]>::default().to_tokens(tokens);
                        trailing_or_empty = true;
                    }
                    param.to_tokens(tokens);
                }
            }

            self.gt_token.to_tokens(tokens);
        }
    }

    #[cfg(feature = "full")]
    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for CapturedParam {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            match self {
                CapturedParam::Lifetime(lifetime) => lifetime.to_tokens(tokens),
                CapturedParam::Ident(ident) => ident.to_tokens(tokens),
            }
        }
    }

    pub(crate) fn print_const_argument(expr: &Expr, tokens: &mut TokenStream) {
        match expr {
            Expr::Lit(expr) => expr.to_tokens(tokens),

            Expr::Path(expr)
                if expr.attrs.is_empty()
                    && expr.qself.is_none()
                    && expr.path.get_ident().is_some() =>
            {
                expr.to_tokens(tokens);
            }

            #[cfg(feature = "full")]
            Expr::Block(expr) => expr.to_tokens(tokens),

            #[cfg(not(feature = "full"))]
            Expr::Verbatim(expr) => expr.to_tokens(tokens),

            // ERROR CORRECTION: Add braces to make sure that the
            // generated code is valid.
            _ => token::Brace::default().surround(tokens, |tokens| {
                #[cfg(feature = "full")]
                expr::printing::print_expr(expr, tokens, FixupContext::new_stmt());

                #[cfg(not(feature = "full"))]
                expr.to_tokens(tokens);
            }),
        }
    }
}