nushell/crates/nu-derive-value/src/from.rs

use convert_case::Casing;
use proc_macro2::TokenStream as TokenStream2;
use quote::{quote, ToTokens};
use syn::{
    spanned::Spanned, Attribute, Data, DataEnum, DataStruct, DeriveInput, Fields, Generics, Ident,
};

use crate::attributes::{self, ContainerAttributes};

#[derive(Debug)]
pub struct FromValue;
type DeriveError = super::error::DeriveError<FromValue>;

/// Inner implementation of the `#[derive(FromValue)]` macro for structs and enums.
///
/// Uses `proc_macro2::TokenStream` for better testing support, unlike `proc_macro::TokenStream`.
///
/// This function directs the `FromValue` trait derivation to the correct implementation based on
/// the input type:
/// - For structs: [`derive_struct_from_value`]
/// - For enums: [`derive_enum_from_value`]
/// - Unions are not supported and will return an error.
pub fn derive_from_value(input: TokenStream2) -> Result<TokenStream2, DeriveError> {
    let input: DeriveInput = syn::parse2(input).map_err(DeriveError::Syn)?;
    match input.data {
        Data::Struct(data_struct) => Ok(derive_struct_from_value(
            input.ident,
            data_struct,
            input.generics,
            input.attrs,
        )?),
        Data::Enum(data_enum) => Ok(derive_enum_from_value(
            input.ident,
            data_enum,
            input.generics,
            input.attrs,
        )?),
        Data::Union(_) => Err(DeriveError::UnsupportedUnions),
    }
}

/// Implements the `#[derive(FromValue)]` macro for structs.
///
/// This function ensures that the helper attribute is not used anywhere, as it is not supported for
/// structs.
/// Other than this, this function provides the impl signature for `FromValue`.
/// The implementation for `FromValue::from_value` is handled by [`struct_from_value`] and the
/// `FromValue::expected_type` is handled by [`struct_expected_type`].
fn derive_struct_from_value(
    ident: Ident,
    data: DataStruct,
    generics: Generics,
    attrs: Vec<Attribute>,
) -> Result<TokenStream2, DeriveError> {
    attributes::deny(&attrs)?;
    attributes::deny_fields(&data.fields)?;
    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
    let from_value_impl = struct_from_value(&data);
    let expected_type_impl = struct_expected_type(&data.fields);
    Ok(quote! {
        #[automatically_derived]
        impl #impl_generics nu_protocol::FromValue for #ident #ty_generics #where_clause {
            #from_value_impl
            #expected_type_impl
        }
    })
}

/// Implements `FromValue::from_value` for structs.
///
/// This function constructs the `from_value` function for structs.
/// The implementation is straightforward as most of the heavy lifting is handled by
/// `parse_value_via_fields`, and this function only needs to construct the signature around it.
///
/// For structs with named fields, this constructs a large return type where each field
/// contains the implementation for that specific field.
/// In structs with unnamed fields, a [`VecDeque`](std::collections::VecDeque) is used to load each
/// field one after another, and the result is used to construct the tuple.
/// For unit structs, this only checks if the input value is `Value::Nothing`.
///
/// # Examples
///
/// These examples show what the macro would generate.
///
/// Struct with named fields:
/// ```rust
/// #[derive(IntoValue)]
/// struct Pet {
///     name: String,
///     age: u8,
///     favorite_toy: Option<String>,
/// }
///
/// impl nu_protocol::FromValue for Pet {
///     fn from_value(
///         v: nu_protocol::Value
///     ) -> std::result::Result<Self, nu_protocol::ShellError> {
///         let span = v.span();
///         let mut record = v.into_record()?;
///         std::result::Result::Ok(Pet {
///             name: <String as nu_protocol::FromValue>::from_value(
///                 record
///                     .remove("name")
///                     .ok_or_else(|| nu_protocol::ShellError::CantFindColumn {
///                         col_name: std::string::ToString::to_string("name"),
///                         span: std::option::Option::None,
///                         src_span: span
///                     })?,
///             )?,
///             age: <u8 as nu_protocol::FromValue>::from_value(
///                 record
///                     .remove("age")
///                     .ok_or_else(|| nu_protocol::ShellError::CantFindColumn {
///                         col_name: std::string::ToString::to_string("age"),
///                         span: std::option::Option::None,
///                         src_span: span
///                     })?,
///             )?,
///             favorite_toy: <Option<String> as nu_protocol::FromValue>::from_value(
///                 record
///                     .remove("favorite_toy")
///                     .ok_or_else(|| nu_protocol::ShellError::CantFindColumn {
///                         col_name: std::string::ToString::to_string("favorite_toy"),
///                         span: std::option::Option::None,
///                         src_span: span
///                     })?,
///             )?,          
///         })
///     }
/// }
/// ```
///
/// Struct with unnamed fields:
/// ```rust
/// #[derive(IntoValue)]
/// struct Color(u8, u8, u8);
///
/// impl nu_protocol::FromValue for Color {
///     fn from_value(
///         v: nu_protocol::Value
///     ) -> std::result::Result<Self, nu_protocol::ShellError> {
///         let span = v.span();
///         let list = v.into_list()?;
///         let mut deque: std::collections::VecDeque<_> = std::convert::From::from(list);
///         std::result::Result::Ok(Self(
///             {
///                 <u8 as nu_protocol::FromValue>::from_value(
///                     deque
///                         .pop_front()
///                         .ok_or_else(|| nu_protocol::ShellError::CantFindColumn {
///                             col_name: std::string::ToString::to_string(&0),
///                             span: std::option::Option::None,
///                             src_span: span
///                         })?,
///                 )?
///             },
///             {
///                 <u8 as nu_protocol::FromValue>::from_value(
///                     deque
///                         .pop_front()
///                         .ok_or_else(|| nu_protocol::ShellError::CantFindColumn {
///                             col_name: std::string::ToString::to_string(&1),
///                             span: std::option::Option::None,
///                             src_span: span
///                         })?,
///                 )?
///             },
///             {
///                 <u8 as nu_protocol::FromValue>::from_value(
///                     deque
///                         .pop_front()
///                         .ok_or_else(|| nu_protocol::ShellError::CantFindColumn {
///                             col_name: std::string::ToString::to_string(&2),
///                             span: std::option::Option::None,
///                             src_span: span
///                         })?,
///                 )?
///             }
///         ))
///     }
/// }
/// ```
///
/// Unit struct:
/// ```rust
/// #[derive(IntoValue)]
/// struct Unicorn;
///
/// impl nu_protocol::FromValue for Unicorn {
///     fn from_value(
///         v: nu_protocol::Value
///     ) -> std::result::Result<Self, nu_protocol::ShellError> {
///         match v {
///             nu_protocol::Value::Nothing {..} => Ok(Self),
///             v => std::result::Result::Err(nu_protocol::ShellError::CantConvert {
///                 to_type: std::string::ToString::to_string(&<Self as nu_protocol::FromValue>::expected_type()),
///                 from_type: std::string::ToString::to_string(&v.get_type()),
///                 span: v.span(),
///                 help: std::option::Option::None
///             })
///         }
///     }
/// }
/// ```
fn struct_from_value(data: &DataStruct) -> TokenStream2 {
    let body = parse_value_via_fields(&data.fields, quote!(Self));
    quote! {
        fn from_value(
            v: nu_protocol::Value
        ) -> std::result::Result<Self, nu_protocol::ShellError> {
            #body
        }
    }
}

/// Implements `FromValue::expected_type` for structs.
///
/// This function constructs the `expected_type` function for structs.
/// The type depends on the `fields`: named fields construct a record type with every key and type
/// laid out.
/// Unnamed fields construct a custom type with the name in the format like
/// `list[type0, type1, type2]`.
/// No fields expect the `Type::Nothing`.
///
/// # Examples
///
/// These examples show what the macro would generate.
///
/// Struct with named fields:
/// ```rust
/// #[derive(IntoValue)]
/// struct Pet {
///     name: String,
///     age: u8,
///     favorite_toy: Option<String>,
/// }
///
/// impl nu_protocol::FromValue for Pet {
///     fn expected_type() -> nu_protocol::Type {
///         nu_protocol::Type::Record(
///             std::vec![
///                 (
///                     std::string::ToString::to_string("name"),
///                     <String as nu_protocol::FromValue>::expected_type(),
///                 ),
///                 (
///                     std::string::ToString::to_string("age"),
///                     <u8 as nu_protocol::FromValue>::expected_type(),
///                 ),
///                 (
///                     std::string::ToString::to_string("favorite_toy"),
///                     <Option<String> as nu_protocol::FromValue>::expected_type(),
///                 )
///             ].into_boxed_slice()
///         )
///     }
/// }
/// ```
///
/// Struct with unnamed fields:
/// ```rust
/// #[derive(IntoValue)]
/// struct Color(u8, u8, u8);
///
/// impl nu_protocol::FromValue for Color {
///     fn expected_type() -> nu_protocol::Type {
///         nu_protocol::Type::Custom(
///             std::format!(
///                 "[{}, {}, {}]",
///                 <u8 as nu_protocol::FromValue>::expected_type(),
///                 <u8 as nu_protocol::FromValue>::expected_type(),
///                 <u8 as nu_protocol::FromValue>::expected_type()
///             )
///             .into_boxed_str()
///         )
///     }
/// }
/// ```
///
/// Unit struct:
/// ```rust
/// #[derive(IntoValue)]
/// struct Unicorn;
///
/// impl nu_protocol::FromValue for Color {
///     fn expected_type() -> nu_protocol::Type {
///         nu_protocol::Type::Nothing
///     }
/// }
/// ```
fn struct_expected_type(fields: &Fields) -> TokenStream2 {
    let ty = match fields {
        Fields::Named(fields) => {
            let fields = fields.named.iter().map(|field| {
                let ident = field.ident.as_ref().expect("named has idents");
                let ident_s = ident.to_string();
                let ty = &field.ty;
                quote! {(
                    std::string::ToString::to_string(#ident_s),
                    <#ty as nu_protocol::FromValue>::expected_type(),
                )}
            });
            quote!(nu_protocol::Type::Record(
                std::vec![#(#fields),*].into_boxed_slice()
            ))
        }
        Fields::Unnamed(fields) => {
            let mut iter = fields.unnamed.iter();
            let fields = fields.unnamed.iter().map(|field| {
                let ty = &field.ty;
                quote!(<#ty as nu_protocol::FromValue>::expected_type())
            });
            let mut template = String::new();
            template.push('[');
            if iter.next().is_some() {
                template.push_str("{}")
            }
            iter.for_each(|_| template.push_str(", {}"));
            template.push(']');
            quote! {
                nu_protocol::Type::Custom(
                    std::format!(
                        #template,
                        #(#fields),*
                    )
                    .into_boxed_str()
                )
            }
        }
        Fields::Unit => quote!(nu_protocol::Type::Nothing),
    };

    quote! {
        fn expected_type() -> nu_protocol::Type {
            #ty
        }
    }
}

/// Implements the `#[derive(FromValue)]` macro for enums.
///
/// This function constructs the implementation of the `FromValue` trait for enums.
/// It is designed to be on the same level as [`derive_struct_from_value`], even though this
/// function only provides the impl signature for `FromValue`.
/// The actual implementation for `FromValue::from_value` is handled by [`enum_from_value`].
///
/// Since variants are difficult to type with the current type system, this function uses the
/// default implementation for `expected_type`.
fn derive_enum_from_value(
    ident: Ident,
    data: DataEnum,
    generics: Generics,
    attrs: Vec<Attribute>,
) -> Result<TokenStream2, DeriveError> {
    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
    let from_value_impl = enum_from_value(&data, &attrs)?;
    Ok(quote! {
        #[automatically_derived]
        impl #impl_generics nu_protocol::FromValue for #ident #ty_generics #where_clause {
            #from_value_impl
        }
    })
}

/// Implements `FromValue::from_value` for enums.
///
/// This function constructs the `from_value` implementation for enums.
/// It only accepts enums with unit variants, as it is currently unclear how other types of enums
/// should be represented via a `Value`.
/// This function checks that every field is a unit variant and constructs a match statement over
/// all possible variants.
/// The input value is expected to be a `Value::String` containing the name of the variant formatted
/// as defined by the `#[nu_value(rename_all = "...")]` attribute.
/// If no attribute is given, [`convert_case::Case::Snake`] is expected.
///
/// If no matching variant is found, `ShellError::CantConvert` is returned.
///
/// This is how such a derived implementation looks:
/// ```rust
/// #[derive(IntoValue)]
/// enum Weather {
///     Sunny,
///     Cloudy,
///     Raining
/// }
///
/// impl nu_protocol::IntoValue for Weather {
///     fn into_value(self, span: nu_protocol::Span) -> nu_protocol::Value {
///         let span = v.span();
///         let ty = v.get_type();
///
///         let s = v.into_string()?;
///         match s.as_str() {
///             "sunny" => std::result::Ok(Self::Sunny),
///             "cloudy" => std::result::Ok(Self::Cloudy),
///             "raining" => std::result::Ok(Self::Raining),
///             _ => std::result::Result::Err(nu_protocol::ShellError::CantConvert {
///                 to_type: std::string::ToString::to_string(
///                     &<Self as nu_protocol::FromValue>::expected_type()
///                 ),
///                 from_type: std::string::ToString::to_string(&ty),
///                 span: span,help: std::option::Option::None,
///             }),
///         }
///     }
/// }
/// ```
fn enum_from_value(data: &DataEnum, attrs: &[Attribute]) -> Result<TokenStream2, DeriveError> {
    let container_attrs = ContainerAttributes::parse_attrs(attrs.iter())?;
    let arms: Vec<TokenStream2> = data
        .variants
        .iter()
        .map(|variant| {
            attributes::deny(&variant.attrs)?;
            let ident = &variant.ident;
            let ident_s = format!("{ident}")
                .as_str()
                .to_case(container_attrs.rename_all);
            match &variant.fields {
                Fields::Named(fields) => Err(DeriveError::UnsupportedEnums {
                    fields_span: fields.span(),
                }),
                Fields::Unnamed(fields) => Err(DeriveError::UnsupportedEnums {
                    fields_span: fields.span(),
                }),
                Fields::Unit => Ok(quote!(#ident_s => std::result::Result::Ok(Self::#ident))),
            }
        })
        .collect::<Result<_, _>>()?;

    Ok(quote! {
        fn from_value(
            v: nu_protocol::Value
        ) -> std::result::Result<Self, nu_protocol::ShellError> {
            let span = v.span();
            let ty = v.get_type();

            let s = v.into_string()?;
            match s.as_str() {
                #(#arms,)*
                _ => std::result::Result::Err(nu_protocol::ShellError::CantConvert {
                    to_type: std::string::ToString::to_string(
                        &<Self as nu_protocol::FromValue>::expected_type()
                    ),
                    from_type: std::string::ToString::to_string(&ty),
                    span: span,
                    help: std::option::Option::None,
                }),
            }
        }
    })
}

/// Parses a `Value` into self.
///
/// This function handles the actual parsing of a `Value` into self.
/// It takes two parameters: `fields` and `self_ident`.
/// The `fields` parameter determines the expected type of `Value`: named fields expect a
/// `Value::Record`, unnamed fields expect a `Value::List`, and a unit expects `Value::Nothing`.
///
/// For named fields, the `fields` parameter indicates which field in the record corresponds to
/// which struct field.
/// For both named and unnamed fields, it also helps cast the type into a `FromValue` type.
/// This approach maintains
/// [hygiene](https://doc.rust-lang.org/reference/macros-by-example.html#hygiene).
///
/// The `self_ident` parameter is used to describe the identifier of the returned value.
/// For structs, `Self` is usually sufficient, but for enums, `Self::Variant` may be needed in the
/// future.
///
/// This function is more complex than the equivalent for `IntoValue` due to error handling
/// requirements.
/// For missing fields, `ShellError::CantFindColumn` is used, and for unit structs,
/// `ShellError::CantConvert` is used.
/// The implementation avoids local variables for fields to prevent accidental shadowing, ensuring
/// that poorly named fields don't cause issues.
/// While this style is not typically recommended in handwritten Rust, it is acceptable for code
/// generation.
fn parse_value_via_fields(fields: &Fields, self_ident: impl ToTokens) -> TokenStream2 {
    match fields {
        Fields::Named(fields) => {
            let fields = fields.named.iter().map(|field| {
                // TODO: handle missing fields for Options as None
                let ident = field.ident.as_ref().expect("named has idents");
                let ident_s = ident.to_string();
                let ty = &field.ty;
                quote! {
                    #ident: <#ty as nu_protocol::FromValue>::from_value(
                        record
                            .remove(#ident_s)
                            .ok_or_else(|| nu_protocol::ShellError::CantFindColumn {
                                col_name: std::string::ToString::to_string(#ident_s),
                                span: std::option::Option::None,
                                src_span: span
                            })?,
                    )?
                }
            });
            quote! {
                let span = v.span();
                let mut record = v.into_record()?;
                std::result::Result::Ok(#self_ident {#(#fields),*})
            }
        }
        Fields::Unnamed(fields) => {
            let fields = fields.unnamed.iter().enumerate().map(|(i, field)| {
                let ty = &field.ty;
                quote! {{
                    <#ty as nu_protocol::FromValue>::from_value(
                        deque
                            .pop_front()
                            .ok_or_else(|| nu_protocol::ShellError::CantFindColumn {
                                col_name: std::string::ToString::to_string(&#i),
                                span: std::option::Option::None,
                                src_span: span
                            })?,
                    )?
                }}
            });
            quote! {
                let span = v.span();
                let list = v.into_list()?;
                let mut deque: std::collections::VecDeque<_> = std::convert::From::from(list);
                std::result::Result::Ok(#self_ident(#(#fields),*))
            }
        }
        Fields::Unit => quote! {
            match v {
                nu_protocol::Value::Nothing {..} => Ok(#self_ident),
                v => std::result::Result::Err(nu_protocol::ShellError::CantConvert {
                    to_type: std::string::ToString::to_string(&<Self as nu_protocol::FromValue>::expected_type()),
                    from_type: std::string::ToString::to_string(&v.get_type()),
                    span: v.span(),
                    help: std::option::Option::None
                })
            }
        },
    }
}