constructor-lite 0.3.0

// constructor-lite - Generate minimal constructors for structs
// Copyright (C) 2023 d-k-bo
// SPDX-License-Identifier: MIT

//! This crate provides the `ConstructorLite` derive macro for generating
//! minimal constructors for a struct from its fields.
//!
//! It is primarily designed for structs where deriving [`Default`] is not
//! possible because some fields don't implement it.
//!
//! By default, an associated function `new()` is generated, which expects every
//! field that is not `Option<T>` as an argument.
//!
//! - To add an optional field to expected arguments of the constructor function,
//!   it can be marked with `#[constructor(required)]`.
//! - To remove a non-optional field that implements `Default` from the constructor
//!   function, it can be marked with `#[constructor(default)]`.
//! - To change the name of the generated function, the struct can be marked with e. g.
//!   `#[constructor(name = "function_name")]`.
//! - By default, the generated function has the same visibility as the struct.
//!   To override this behaviour, the struct can be marked with e. g.
//!   `#[constructor(visibility = "pub(super)")]`.
//!
//! For more advanced uses you might prefer using
//! [`derive-new`](https://lib.rs/crates/derive-new) or
//! [`derive_builder`](https://lib.rs/crates/derive_builder) instead.
//!
//! # Example
//!
//! ```rust
//! use constructor_lite::ConstructorLite;
//!
//! #[derive(Debug, PartialEq, ConstructorLite)]
//! struct Movie {
//!     title: String,
//!     year: Option<u16>,
//! }
//!
//! assert_eq!(
//!     Movie::new("Star Wars".to_owned()),
//!     Movie { title: "Star Wars".to_owned(), year: None },
//! )
//! ```

use darling::{ast::Data, util::Flag, Error, FromDeriveInput, FromField};
use proc_macro2::Span;
use quote::quote;
use syn::{parse_macro_input, GenericParam, Generics, Ident, Path, Type, Visibility};

#[derive(Debug, FromField)]
#[darling(attributes(constructor), and_then = "Self::not_both")]
struct Field {
    ident: Option<Ident>,
    ty: Type,

    required: Flag,
    default: Flag,
}
impl Field {
    fn not_both(self) -> darling::Result<Self> {
        if self.required.is_present() && self.default.is_present() {
            Err(
                Error::custom("Field cannot use `required` and `default`at the same time.")
                    .with_span(&self.default.span()),
            )
        } else {
            Ok(self)
        }
    }
}

#[derive(Debug, FromDeriveInput)]
#[darling(attributes(constructor), supports(struct_named))]
struct ConstructorLite {
    vis: Visibility,
    ident: Ident,
    generics: Generics,
    data: Data<(), Field>,

    visibility: Option<Visibility>,
    name: Option<Ident>,
}
impl ConstructorLite {
    fn constructor(&self) -> darling::Result<proc_macro::TokenStream> {
        let Self {
            vis,
            ident,
            generics,
            data,
            visibility,
            name,
        } = self;

        let Data::Struct(fields) = data else {
            return Err(Error::custom("ConstructorLite supports only structs."));
        };

        let mut arguments = Vec::new();
        let mut required_field_idents = Vec::new();
        let mut optional_field_idents = Vec::new();

        for Field {
            ident,
            ty,
            required,
            default,
        } in fields.iter()
        {
            if required.is_present() {
                arguments.push(quote!(#ident: #ty));
                required_field_idents.push(ident);
                continue;
            }
            if default.is_present() {
                optional_field_idents.push(ident);
                continue;
            }

            if let Type::Path(ty) = &ty {
                if path_is_option(&ty.path) {
                    optional_field_idents.push(ident);
                } else {
                    arguments.push(quote!(#ident: #ty));
                    required_field_idents.push(ident);
                }
            }
        }

        let mut generics_definitions = generics.clone();
        generics_definitions.where_clause = None;
        for param in &mut generics_definitions.params {
            match param {
                GenericParam::Lifetime(..) => {}
                GenericParam::Type(param) => {
                    param.default = None;
                }
                GenericParam::Const(param) => {
                    param.default = None;
                }
            }
        }

        let mut generics = generics.clone();
        for param in &mut generics.params {
            match param {
                GenericParam::Lifetime(param) => {
                    param.bounds.clear();
                }
                GenericParam::Type(param) => {
                    param.bounds.clear();
                    param.default = None;
                }
                GenericParam::Const(param) => {
                    param.default = None;
                }
            }
        }
        let where_clause = generics.where_clause.take();

        let vis = visibility.as_ref().unwrap_or(vis);
        let name: Ident = name
            .clone()
            .unwrap_or_else(|| Ident::new("new", Span::call_site()));

        let constructor = quote!(
            impl #generics_definitions #ident #generics #where_clause {
                #vis fn #name ( #( #arguments ),* ) -> Self {
                    Self {
                        #(
                            #required_field_idents,
                        )*
                        #(
                            #optional_field_idents: Default::default(),
                        )*
                    }
                }
            }
        );

        Ok(constructor.into())
    }
}

fn path_is_option(path: &Path) -> bool {
    // Option<T>
    if path.leading_colon.is_none()
        && path.segments.len() == 1
        && path.segments.first().unwrap().ident == "Option"
    {
        return true;
    }

    let mut segments = path.segments.iter();

    // core::option::Option<T>
    // ::core::option::Option<T>
    // std::option::Option<T>
    // ::std::option::Option<T>
    segments
        .next()
        .map(|seg| seg.ident == "core" || seg.ident == "std")
        .unwrap_or(false)
        && segments
            .next()
            .map(|seg| seg.ident == "option")
            .unwrap_or(false)
        && segments
            .next()
            .map(|seg| seg.ident == "Option")
            .unwrap_or(false)
}

/// Generate a constructor for the required fields of the struct.
///
/// See the [`constructor-lite`](crate) crate documentation for more details.
#[proc_macro_derive(ConstructorLite, attributes(constructor))]
pub fn constructor_lite_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    ConstructorLite::from_derive_input(&parse_macro_input!(input as syn::DeriveInput))
        .and_then(|constructor| constructor.constructor())
        .unwrap_or_else(|e| e.write_errors().into())
}