pgde_derive 0.2.2

A macro library for consuming PostgreSQL row data into structs.
Documentation 
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//! A procedural macro library that enables structs with the [`RowConsumer`] trait to
//! provide a `from_row` implementation. Please refer to pgde for use, examples, and
//! limitations.
extern crate proc_macro;
use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, Data, DeriveInput, Fields, Ident, PathArguments, Type};

/// A macro for deriving a `from_row` implementation onto a struct.
#[proc_macro_derive(RowConsumer)]
pub fn derive_row_consumer(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    let name = input.ident;
    let data = input.data;
    parse_field_setters(&name, &data)
}

/// A function that takes a given [`TokenStream`]'s [`Ident`] and [`Data`] and returns a
/// [`TokenStream`] for implementing a `from_row` from a struct's fields.
fn parse_field_setters(class_name: &Ident, data: &Data) -> TokenStream {
    match *data {
        Data::Struct(ref data) => match data.fields {
            Fields::Named(ref fields) => {
                let field_setters = fields.named.iter().enumerate().map(|(i, f)| {
                    let field_name = &f.ident;
                    let (field_type, field_type_args) = match &f.ty {
                        Type::Path(field_ty) => match field_ty.path.segments.last() {
                            Some(path_segment) => match &path_segment.arguments {
                                PathArguments::AngleBracketed(previous_type_args) => {
                                    let mut new_field_type = f.ty.clone();

                                    match new_field_type {
                                        Type::Path(ref mut field_ty) => match field_ty.path.segments.last_mut() {
                                            Some(v) => {
                                                v.arguments = PathArguments::None;
                                                (new_field_type, Some(previous_type_args))
                                            },
                                            None => (f.ty.clone(), None)
                                        },
                                        _ => (f.ty.clone(), None),
                                    }
                                },
                                _ => (f.ty.clone(), None),
                            },
                            None => (f.ty.clone(), None),
                        },
                        _ => (f.ty.clone(), None),
                    };

                    match field_type_args {
                        Some(v) => quote! {
                            #field_name: match row.try_get::<usize, #field_type::#v>(#i) {
                                Ok(v) => v,
                                Err(_) => {
                                    errors.push(format!("Conversion error occurred for field \"{}\" on class \"{}\"", stringify!(#field_name), stringify!(#class_name)));
                                    #field_type::default()
                                },
                            }
                        },
                        None => quote! {
                            #field_name: match row.try_get::<usize, #field_type>(#i) {
                                Ok(v) => v,
                                Err(_) => {
                                    errors.push(format!("Conversion error occurred for field \"{}\" on class \"{}\"", stringify!(#field_name), stringify!(#class_name)));
                                    #field_type::default()
                                },
                            }
                        },
                    }
                });

                let implementation = quote! {
                    impl pgde::RowConsumer for #class_name {
                        fn from_row(row: Row) -> Result<Self, (Self, Vec<String>)>
                        where
                            Self: Sized,
                        {
                            let mut errors : Vec<String> = Vec::new();

                            let class_instance = Self {
                                #(#field_setters),*
                            };

                            match errors.len() {
                                0 => Ok(class_instance),
                                _ => Err((class_instance, errors)),
                            }
                        }
                    }
                };

                TokenStream::from(implementation)
            }
            Fields::Unnamed(_) | Fields::Unit => panic!(
                "RowConsumer is not supported on unit structs nor structs with unnamed fieds"
            ),
        },
        Data::Enum(_) | Data::Union(_) => panic!("RowConsumer is not supported on enums or unions"),
    }
}