async_codegen/rust/

mod.rs

/*
 * Copyright © 2025 Anand Beh
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

//!
//! Rust syntax elements.
//!
//! Note that no checking exists to make sure the elements are used correctly, i.e. the correct
//! combination of structs. Instead, the library user is expected to have basic knowledge of how
//! Rust syntax is composed, and to combine the structs in this module likewise.
//!
//! Example:
//!
//! ```
//! # use async_codegen::common::{CombinedSeq, NoOpSeq, SingularSeq, Str};
//! # use async_codegen::{Output, Writable};
//! # use async_codegen::rust::{CanHaveAttributes, CfgAttr, Deprecated, FunctionBodyImplement, FunctionDef, FunctionParam, ModPub, MustUse, NoMangle, Parameterized};
//!
//! async fn write_function<O>(output: &mut O) -> Result<(), O::Error> where O: Output {
//!   // For more advanced usage, you can replace Str("") by other Writable implementations
//!   let function_def = FunctionDef {
//!     mods: SingularSeq(ModPub),
//!     name: Str("my_func"),
//!     args: CombinedSeq(
//!      SingularSeq(FunctionParam(Str("var1"), Str("Type"))),
//!      SingularSeq(FunctionParam(Str("var2"), Parameterized(Str("Option"), SingularSeq(Str("bool")))))
//!     ),
//!     return_type: Parameterized(Str("Box"), SingularSeq(Str("str"))),
//!     where_conds: NoOpSeq,
//!     body: FunctionBodyImplement(Str("todo!()"))
//!   };
//!   function_def.write_to(output).await
//!   // Will render as:
//!   /*
//!   pub fn my_func(var1: Type, var2: Option<bool>) -> Box<str> {
//!     todo!()
//!   }
//!    */
//! }
//! ```
//!

use crate::common::{Combined, NoOp, NoOpSeq, Str, SurroundingSeqAccept};
use crate::{Output, SequenceAccept, Writable};
use std::fmt::Debug;

mod syntax;
#[cfg(test)]
mod tests;

/// All possible Rust editions.
/// This is the only type in this module meant to be used as context, and not as a writable itself.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
#[non_exhaustive]
pub enum Edition {
    /// This Rust edition is declared for usability purposes. However, not all [Writable]
    /// implementations are guaranteed to work with it.
    Rust2015,
    Rust2018,
    Rust2021,
    Rust2024,
}

/// Imports a single type so that it can be used later.
/// Renders as `use Type;`. Adds a new line after the semicolon.
#[derive(Clone, Debug)]
pub struct UseType<Type>(pub Type);

/// An attribute enabled conditionally, i.e. `#[cfg_attr(Cond, Attr)]`
#[derive(Clone, Debug)]
pub struct CfgAttr<Cond, Attr>(pub Cond, pub Attr);

/// A cfg attribute. Renders as `cfg(Cond)`.
#[derive(Clone, Debug)]
pub struct Cfg<Cond>(pub Cond);

/// A cfg condition for targeting an OS, OS family, or architecture. For example:
/// ```
/// # use async_codegen::common::{NoOpSeq, SingularSeq, Str};
/// # use async_codegen::context::EmptyContext;
/// # use async_codegen::rust::{FunctionBodyDeclare, Cfg, FunctionDef, Target, CanHaveAttributes};
/// # use async_codegen::util::InMemoryOutput;
/// let function = FunctionDef {
///   mods: NoOpSeq,
///   name: Str("conditional_func"),
///   args: NoOpSeq,
///   return_type: Str("()"),
///   where_conds: NoOpSeq,
///   body: FunctionBodyDeclare
/// }.with_attributes(
///   SingularSeq(Cfg(Target::Os(Str("linux"))))
/// );
/// let string = InMemoryOutput::print_output(EmptyContext, &function);
/// assert_eq!("#[cfg(target_os = \"linux\")]\nfn conditional_func() -> ();\n\n", string);
/// ```
#[derive(Clone, Debug)]
pub enum Target<Value> {
    Os(Value),
    Family(Value),
    Arch(Value),
}

/// The link attribute.
#[derive(Clone, Debug)]
pub struct Link<Arg>(pub Arg);

/// The no mangle attribute.
///
/// Requires that the context satisfies [ContextProvides] for [Edition], because in Rust 2024 and
/// beyond, the no-mangle attribute is an unsafe attribute.
#[derive(Clone, Debug)]
pub struct NoMangle;

/// The attribute content for `allow(...)`. The tuple value must be a sequence.
#[derive(Clone, Debug)]
pub struct AllowLints<Lints>(pub Lints);

/// The deprecated attribute. The three variants of this enum correspond to the deprecated
/// attribute's multiple ways of being specified. See:
/// https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-deprecated-attribute
#[derive(Clone, Debug)]
pub enum Deprecated<Msg, Since = NoOp> {
    Basic,
    Message(Msg),
    Full { since: Since, note: Msg },
}

impl Default for Deprecated<NoOp, NoOp> {
    fn default() -> Self {
        Self::Basic
    }
}

impl Deprecated<NoOp, NoOp> {
    pub fn basic() -> Self {
        Self::Basic
    }
}

impl<Msg> Deprecated<Msg> {
    pub fn with_message(msg: Msg) -> Self {
        Self::Message(msg)
    }
}

/// The must_use attribute
#[derive(Clone, Debug)]
pub struct MustUse;

/// The public modifier
#[derive(Clone, Debug)]
pub struct ModPub;

/// The unsafe modifier
#[derive(Clone, Debug)]
pub struct ModUnsafe;

/// The extern modifier, with the ABI selected as the tuple value.
///
/// This struct includes `unsafe`. Since Rust 2024, the unsafe keyword is required for extern
/// functions, and before Rust 2024 it is optional. To make it easy to generate code targeting
/// multiple editions, we unconditionally emit the "unsafe" keyword alongside "extern".
#[derive(Clone, Debug)]
pub struct ModUnsafeExtern<Abi>(pub Abi);

/// A standalone statement. Renders the expression and adds a semicolon and a new line.
#[derive(Clone, Debug)]
pub struct Stmt<Expr>(pub Expr);

/// A let statement. This statement includes the semicolon and a new line.
#[derive(Clone, Debug)]
pub struct LetStmt<Variable, Expr>(pub Variable, pub Expr);

/// An assignation. This statement includes the semicolon and a new line.
#[derive(Clone, Debug)]
pub struct AssignStmt<Variable, Expr>(pub Variable, pub Expr);

/// A return statement. Renders as `return Expr;` with a new line at the end.
#[derive(Clone, Debug)]
pub struct ReturnStmt<Expr>(pub Expr);

/// A let expression.
/// This can be used, for example, as the condition of [IfBlock] in order to create an "if-let" block.
#[derive(Clone, Debug)]
pub struct LetExpr<Pattern, Expr>(pub Pattern, pub Expr);

/// A raw string literal expression, i.e. r#"Content"#. Example:
/// ```
/// # use async_codegen::common::Str;
/// # use async_codegen::context::EmptyContext;
/// # use async_codegen::rust::RawStringLiteral;
/// # use async_codegen::util::InMemoryOutput;
/// let string_lit = RawStringLiteral(Str("hello_world"));
///
/// assert_eq!("r#\"hello_world\"#", InMemoryOutput::print_output(EmptyContext, &string_lit));
/// ```
#[derive(Clone, Debug)]
pub struct RawStringLiteral<Content>(pub Content);

/// An array literal with predefined elements written out.
/// Renders as `[E1, E2, E3, ...]` where EX is in the element sequence.
#[derive(Clone, Debug)]
pub struct ArrayFromElements<Elements>(pub Elements);

/// An item attached to an associated container, via "::".
/// The output will look like `Cont::Item`.
#[derive(Clone, Debug)]
pub struct AssociatedItem<Cont, Item>(pub Cont, pub Item);

/// A question mark following another expression.
#[derive(Clone, Debug)]
pub struct QuestionMarkAfter<Expr>(pub Expr);

/// Uses the `as` expression to perform a qualified trait cast (e.g. ready for a method call).
/// This will render as `<Type as Trait>`.
#[derive(Clone, Debug)]
pub struct TypeAsTrait<Type, Trait>(pub Type, pub Trait);

/// Uses the `as` expression to coerce one type to another.
/// This will render as `<Type1 as Type2>`.
#[derive(Clone, Debug)]
pub struct TypeAsType<Type1, Type2>(pub Type1, pub Type2);

/// Declaration of an extern block, i.e. for FFI.
/// In Rust 2024 and later, the unsafe keyword must be added for extern blocks. Thus, this struct
/// requires that the context satisfies [ContextProvides] for [Edition].
#[derive(Clone, Debug)]
pub struct ExternBlock<Abi, Body> {
    /// The ABI chosen. Must be writable
    pub abi: Abi,
    /// The body of the extern block. Must be writable
    pub body: Body,
}

impl<Abi, Body> CanHaveAttributes for ExternBlock<Abi, Body> {
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self> {
        WithAttributes {
            attr,
            separator: AttributeSeparator::NewLine,
            value: self,
        }
    }
}

/// Declaration of a module block. Renders as `mod Name {Body}`.
#[derive(Clone, Debug)]
pub struct ModBlock<Name, Body> {
    /// The module name
    pub name: Name,
    /// The body. Must be writable
    pub body: Body,
}

/// An if block. The condition and body must both be writable.
#[derive(Clone, Debug)]
pub struct IfBlock<Cond, Body>(pub Cond, pub Body);

/// Represents "else" syntactically. Renders as `Before else After`.
///
/// This struct requires you to specify what comes before and after the else. For example:
/// ```
/// # use async_codegen::common::Str;
/// # use async_codegen::context::EmptyContext;
/// # use async_codegen::rust::{Block, Else, IfBlock};
/// # use async_codegen::util::InMemoryOutput;
///
/// let if_block = IfBlock(Str("true"), Str("log::info(\"Hello\")"));
/// let else_block = Block(Str("panic!()"));
/// let if_else = Else(if_block, else_block);
///
/// let string = InMemoryOutput::print_output(EmptyContext, &if_else);
/// assert_eq!("if true {\nlog::info(\"Hello\")\n} else {\npanic!()\n}", string)
/// ```
#[derive(Clone, Debug)]
pub struct Else<Before, After>(pub Before, pub After);

/// An unlabeled block.
/// This can be used in many contexts, including merely organizing the code.
#[derive(Clone, Debug)]
pub struct Block<Body>(pub Body);

/// Places the expression inside an unsafe block.
/// Adds new lines inside the brackets, wrapping the inner expression.
#[derive(Clone, Debug)]
pub struct UnsafeBlock<Expr>(pub Expr);

/// Writes a closure.
/// Adds new lines inside the brackets, wrapping the inner expression.
#[derive(Clone, Debug)]
pub struct Closure<InputVars, Expr> {
    /// The input variables.
    /// Should be a sequence. They will be comma separated and placed within the pipes.
    /// To use no input variables, use [NoOpSeq].
    pub input_vars: InputVars,
    /// The expression inside the closure block.
    pub inside_block: Expr,
}

/// Performs a call to a function inside code.
#[derive(Clone, Debug)]
pub struct FunctionCall<Recv, Name, Args> {
    /// The function receiver
    pub receiver: Recv,
    /// Whether the function is associated, false if it's a method
    pub is_assoc: bool,
    /// The function name
    pub name: Name,
    /// The arguments. Must be a sequence
    pub args: Args,
}

/// Provides access to the "turbofish" syntax, i.e. `Name::<Args>`.
/// The first tuple value must be writable, and the second must be a sequence.
///
/// Note that if the sequence outputs nothing, this struct will behave as if no args were
/// specified. I.e. `Turbofish(Name, NoOpSeq)` is equivalent to just `Name`.
#[derive(Clone, Debug)]
pub struct Turbofish<Name, Args>(pub Name, pub Args);

/// Accesses a struct field by name. Can be used for named fields or tuple fields.
/// Renders as `Owner.Field`. For example:
/// ```
/// # use async_codegen::common::Str;
/// # use async_codegen::context::EmptyContext;
/// # use async_codegen::rust::{FieldAccess, LetStmt, RefOf};
/// # use async_codegen::util::InMemoryOutput;
/// let string = InMemoryOutput::print_output(EmptyContext, &LetStmt(
///   Str("borrowed_field"), RefOf(FieldAccess(Str("my_var"), Str("my_field")))
/// ));
/// assert_eq!(string, "let borrowed_field = &my_var.my_field;\n");
/// ```
#[derive(Clone, Debug)]
pub struct FieldAccess<Owner, Field>(pub Owner, pub Field);

/// A function declaration
#[derive(Clone, Debug)]
pub struct FunctionDef<Mods, Name, Args, Return, Where, Body> {
    /// The modifiers. Must be a sequence.
    pub mods: Mods,
    /// The function name. Type variables can be declared here via [Parameterized]
    pub name: Name,
    /// The arguments. Must be a sequence
    pub args: Args,
    /// The return type, i.e. after the `->` arrow
    pub return_type: Return,
    /// The "where" conditions. Must be a sequence. Set to [NoOp] to disable.
    /// Will render as `where C1, C2, C3, ...` where CX is a value in the sequence.
    pub where_conds: Where,
    /// The function body.
    /// To only declare the function, this must be `;` so use [FunctionBodyDeclare]
    /// To implement the function, use [FunctionBodyImplement]
    pub body: Body,
}

impl<Mods, Name, Args, Return, Where, Body> CanHaveAttributes
    for FunctionDef<Mods, Name, Args, Return, Where, Body>
{
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self> {
        WithAttributes {
            attr,
            separator: AttributeSeparator::NewLine,
            value: self,
        }
    }
}

/// Declares a function body. This is equivalent to just a semicolon.
#[derive(Clone, Debug)]
pub struct FunctionBodyDeclare;

/// Implements a function body. Places the contents inside brackets
#[derive(Clone, Debug)]
pub struct FunctionBodyImplement<Inner>(pub Inner);

/// A function pointer. Can be used for `fn`, `Fn`, `FnMut`, and `FnOnce`.
///
/// Example:
/// ```
/// # use async_codegen::common::{SingularSeq, Str};
/// # use async_codegen::context::EmptyContext;
/// # use async_codegen::rust::{FunctionPtr, FunctionPtrKind};
/// # use async_codegen::util::InMemoryOutput;
/// let function_ptr = FunctionPtr {
///   kind: FunctionPtrKind::FnMut,
///   args: SingularSeq(Str("String")),
///   return_type: Str("bool")
/// };
/// let string = InMemoryOutput::print_output(EmptyContext, &function_ptr);
/// assert_eq!("FnMut(String) -> bool", string);
/// ```
#[derive(Clone, Debug)]
pub struct FunctionPtr<Args, Return> {
    /// The function pointer kind
    pub kind: FunctionPtrKind,
    /// The arguments. Must be a sequence
    pub args: Args,
    /// The return type, i.e. after the `->` arrow
    pub return_type: Return,
}

/// The kind of function type
#[derive(Clone, Debug)]
pub enum FunctionPtrKind {
    /// An `fn` pointer. E.g. `fn(String) -> bool`.
    FnPtr,
    /// Represents [Fn]
    Fn,
    /// Represents [FnMut]
    FnMut,
    /// Represents [FnOnce]
    FnOnce,
}

/// Renders as `Type=Value`. Intended to be used as a type argument, to specify associated types.
#[derive(Clone, Debug)]
pub struct AssociatedTypeEquals<Type, Value>(pub Type, pub Value);

/// Adds a "dyn " before a type expression.
#[derive(Clone, Debug)]
pub struct DynOf<Type>(pub Type);

/// Adds a "&" before a type expression
#[derive(Clone, Debug)]
pub struct RefOf<Expr>(pub Expr);

/// Dereferences an expression using `*`
#[derive(Clone, Debug)]
pub struct Dereference<Expr>(pub Expr);

/// Surrounds an expression with parentheses, rendering as `(Expr)`.
/// Even if the expression is empty, the parentheses will still be rendered.
#[derive(Clone, Debug)]
pub struct ParenthesesAround<Expr>(pub Expr);

/// Adds an "impl " before a type expression
pub struct ImplOf<Type>(pub Type);

/// Adds a reference with a lifetime before a type expression, i.e. `&'<lifetime> <type>`
#[derive(Clone, Debug)]
pub struct LifetimedRefOf<'l, Type>(pub &'l str, pub Type);

/// Uses the `&raw const` syntax to get a pointer from another pointer. For example:
/// ```
/// # use async_codegen::common::Str;
/// # use async_codegen::context::EmptyContext;
/// # use async_codegen::rust::{FieldAccess, LetStmt, RawConstOf};
/// # use async_codegen::util::InMemoryOutput;
/// let pointer_var = Str("ptr");
/// let let_stmt = LetStmt(pointer_var, RawConstOf(FieldAccess(Str("packed"), Str("field1"))));
/// let string = InMemoryOutput::print_output(EmptyContext, &let_stmt);
/// assert_eq!("let ptr = &raw const packed.field1;\n", string);
/// ```
#[derive(Clone, Debug)]
pub struct RawConstOf<Expr>(pub Expr);

/// Uses the `&raw must` syntax to get a pointer from another pointer. For example:
/// ```
/// # use async_codegen::common::Str;
/// # use async_codegen::context::EmptyContext;
/// # use async_codegen::rust::{FieldAccess, LetStmt, RawMutOf};
/// # use async_codegen::util::InMemoryOutput;
/// let pointer_var = Str("ptr");
/// let let_stmt = LetStmt(pointer_var, RawMutOf(FieldAccess(Str("packed"), Str("field1"))));
/// let string = InMemoryOutput::print_output(EmptyContext, &let_stmt);
/// assert_eq!("let ptr = &raw mut packed.field1;\n", string);
/// ```
#[derive(Clone, Debug)]
pub struct RawMutOf<Expr>(pub Expr);

/// Declares an associated type, rendering as `type VarName = Value;`.
/// Adds new lines before and after.
#[derive(Clone, Debug)]
pub struct AssociatedTypeDef<VarName, Value>(pub VarName, pub Value);

/// The declaration of a trait
#[derive(Clone, Debug)]
pub struct TraitDef<Mods, Name, TypeVars, SuperTraits, Body> {
    /// The trait modifiers, e.g. visibility. Must be a sequence.
    pub mods: Mods,
    /// The name of the trait
    pub name: Name,
    /// The type variables. Must be a sequence
    pub type_variables: TypeVars,
    /// The super traits. Must be a sequence
    pub super_traits: SuperTraits,
    /// The trait definition's body. Use [NoOp] if none exists.
    pub body: Body,
}

impl<Mods, Name, TypeVars, SuperTraits, Body> CanHaveAttributes
    for TraitDef<Mods, Name, TypeVars, SuperTraits, Body>
{
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self> {
        WithAttributes {
            attr,
            separator: AttributeSeparator::NewLine,
            value: self,
        }
    }
}

/// The implementation declaration for a trait, applying to a certain receiver.
#[derive(Clone, Debug)]
pub struct TraitImpl<Mods, TypeVars, Trait, Recv, Where, Body> {
    /// The modifiers on the `impl` block.
    /// Set to [NoOpSeq] for none, or use `SingularSeq(ModUnsafe)` to generate an `unsafe impl`.
    pub mods: Mods,
    /// The type variables to use for the impl block itself. All type variables that appear later
    /// on the trait or the receiver must be declared here, per Rust language rules.
    ///
    /// This field must be a sequence.
    pub type_variables: TypeVars,
    /// The trait being implemented
    pub the_trait: Trait,
    /// The receiver for which it is implemented
    pub receiver: Recv,
    /// The "where" conditions. Must be a sequence. Set to [NoOpSeq] to disable.
    /// Will render as `where C1, C2, C3, ...` where CX is a value in the sequence.
    pub where_conds: Where,
    /// The body. Use [NoOp] if none exists.
    pub body: Body,
}

impl<Mods, TypeVars, Trait, Recv, Where, Body> CanHaveAttributes
    for TraitImpl<Mods, TypeVars, Trait, Recv, Where, Body>
{
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self> {
        WithAttributes {
            attr,
            separator: AttributeSeparator::NewLine,
            value: self,
        }
    }
}

/// The declaration of a struct.
#[derive(Clone, Debug)]
pub struct StructDef<Mods, Name, Elements> {
    /// The struct modifiers. Must be a sequence.
    pub mods: Mods,
    /// The kind of the struct.
    ///
    /// It is suggested to use either a [NamedTuple] or [StructCall]. A semicolon will be
    /// automatically added afterward, as is needed for tuple structs, and this semicolon will not
    /// affect structs with named fields.
    pub kind: StructKind<Name, Elements>,
}

impl<Mods, Name, Elements> CanHaveAttributes for StructDef<Mods, Name, Elements> {
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self> {
        WithAttributes {
            attr,
            separator: AttributeSeparator::NewLine,
            value: self,
        }
    }
}

/// Completes the struct definition as either a named tuple or a struct with named fields.
#[derive(Clone, Debug)]
pub enum StructKind<Name, Elements> {
    /// A named tuple. This will function similarly to [NamedTuple], except a semicolon will
    /// be added afterward.
    ///
    /// `Name` must be writable, and `Elements` must be a writable sequence for the tuple arguments.
    Tuple(Name, Elements),
    /// A struct with named fields. This will function similarly to [StructCall].
    ///
    /// `Name` must be writable, and `Elements` must be writable sequence for the struct fields.
    NamedFields(Name, Elements),
}

/// The construction or deconstruction of a struct.
///
/// When rendered, will use the format `Name { Body }`. Spaces will be added automatically.
///
/// This should **not** be used for tuple structs, for that see [NamedTuple].
#[derive(Clone, Debug)]
pub struct StructCall<Name, Body> {
    /// The struct name. Must be writable.
    ///
    /// If you are declaring a struct for the first time, you can use [Parameterized] in order
    /// to declare type variables.
    pub name: Name,
    /// The body. Must be writable.
    ///
    /// It is suggested to use [StructFields] for multiple fields, or [DeclareField] for just one.
    pub body: Body,
}

/// Named struct fields. This will place every field on a new line with a comma afterward.
/// It is recommended that the sequence should pass [DeclareField].
///
/// If you have a single field, you can skip using a sequence and just use [DeclareField] directly.
pub struct StructFields<Fields>(pub Fields);

/// Declares a single field within a struct. Renders as `Name: Value`.
///
/// Does not add attributes. If you want to use attributes for declaration purposes, you can use
/// [CanHaveAttributes::with_attributes] on this field.
pub struct DeclareField<Name, Value>(pub Name, pub Value);

impl<Name, Value> CanHaveAttributes for DeclareField<Name, Value> {
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self> {
        WithAttributes {
            attr,
            separator: AttributeSeparator::NewLine,
            value: self,
        }
    }
}

/// A named tuple type.
///
/// Renders as `Name(A1, A2, A3, ...)` where AX is part of the argument sequence.
/// If no arguments exist, will render only as `Name` (i.e., a unit struct).
pub struct NamedTuple<Name, Args> {
    pub name: Name,
    pub args: Args,
}

/// An anonymous tuple type. This struct's tuple value must be a sequence.
///
/// Renders as `(A1, A2, A3, ...)` where AX is part of the argument sequence.
#[derive(Clone, Debug)]
pub struct AnonTuple<Args>(pub Args);

/// The unit type, i.e. `()`
pub type UnitType = AnonTuple<NoOpSeq>;

impl AnonTuple<NoOpSeq> {
    /// Creates
    pub fn unit() -> Self {
        Self(NoOpSeq)
    }
}

/// Adds attributes to ANY item.
///
/// The first tuple value must be a sequence. The second must be a writable value. This struct
/// is typically constructed via [CanHaveAttributes::with_attributes].
///
/// Rust attributes can be put in many places, so this enables you to add attributes to any
/// writable item. For example, adding attributes to function parameters can be done like so:
///
/// ```rust
/// # use async_codegen::common::{SingularSeq, Str};
/// # use async_codegen::context::EmptyContext;
/// # use async_codegen::rust::{Cfg, FunctionParam, MustUse, Target, WithAttributes, CanHaveAttributes};
/// # use async_codegen::util::InMemoryOutput;
///
/// let function_param = FunctionParam(Str("conditional_param"), Str("Fd")).with_attributes(
///   SingularSeq(Cfg(Target::Os(Str("linux"))))
/// );
/// let string = InMemoryOutput::print_output(EmptyContext, &function_param);
/// assert_eq!("#[cfg(target_os = \"linux\")] conditional_param: Fd", string);
/// ```
#[derive(Clone, Debug)]
pub struct WithAttributes<Attr, Value> {
    /// The attributes. Must be a sequence.
    pub attr: Attr,
    /// The separator between each attribute
    pub separator: AttributeSeparator,
    /// The value
    pub value: Value,
}

#[derive(Copy, Clone, Debug)]
pub enum AttributeSeparator {
    Space,
    NewLine,
}

/// A writable that can have attributes attached to it
pub trait CanHaveAttributes: Sized {
    /// Adds attributes to this writable
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self>;
}

/// Defines an enum.
///
/// In order to use or refer to an enum, you can use [AssociatedItem] together with [NamedTuple]
/// or [StructCall].
pub struct EnumDef<Mods, Name, Entries> {
    /// The modifiers on the type. Must be a sequence.
    pub mods: Mods,
    /// The name of the enum
    pub name: Name,
    /// The enum entries. Must be a sequence, each entry will be written on a new line with a comma
    ///
    /// As for the entries themselves, it is suggested to use [NamedTuple] or [StructCall]
    /// depending on which kind of enum entry you want to create.
    pub entries: Entries,
}

impl<Mods, Name, Entries> CanHaveAttributes for EnumDef<Mods, Name, Entries> {
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self> {
        WithAttributes {
            attr,
            separator: AttributeSeparator::NewLine,
            value: self,
        }
    }
}

/// A type argument-parameterized expression. Used in relation to parameterized names and their
/// arguments. Examples: `function_name<args>`, `TypeName<'lifetime, args>`, `MyType<Assoc=Value>`.
///
/// If no type args exist, [NoOpSeq] should be used. In any case, the second tuple value of this
/// struct must be a sequence.
#[derive(Clone, Debug)]
pub struct Parameterized<Name, TypeArgs>(pub Name, pub TypeArgs);

/// A type variable with a sequence of bounds.
/// Will render as `TypeVar: B1 + B2 + ...`
#[derive(Clone, Debug)]
pub struct BoundedTypeVar<TypeVar, Bounds>(pub TypeVar, pub Bounds);

/// A standalone lifetime, intended to be used as a type argument or variable
#[derive(Clone, Debug)]
pub struct Lifetime<'l>(pub &'l str);

/// Renders an individual function parameter, `Name: Type`
#[derive(Clone, Debug)]
pub struct FunctionParam<Name, Type>(pub Name, pub Type);

impl<Name, Type> CanHaveAttributes for FunctionParam<Name, Type> {
    fn with_attributes<Attr>(self, attr: Attr) -> WithAttributes<Attr, Self> {
        WithAttributes {
            attr,
            separator: AttributeSeparator::Space,
            value: self,
        }
    }
}

/// A sequence acceptor that writes attributes. Every attribute will be surrounded with "#[]"
#[derive(Debug)]
pub struct AttributesAccept<'o, O, Sep> {
    inner: SurroundingSeqAccept<'o, O, Str<&'static str>, Combined<Str<&'static str>, Sep>>,
}

impl<'o, O, Sep> AttributesAccept<'o, O, Sep> {
    pub fn with_separator(output: &'o mut O, separator: Sep) -> Self {
        Self {
            inner: SurroundingSeqAccept::new(output, Str("#["), Combined(Str("]"), separator)),
        }
    }
}

impl<'o, O, Sep> SequenceAccept<O> for AttributesAccept<'o, O, Sep>
where
    O: Output,
    Sep: Writable<O>,
{
    async fn accept<W: Writable<O>>(&mut self, writable: &W) -> Result<(), O::Error> {
        self.inner.accept(writable).await
    }
}