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use proc_macro2::TokenStream;
use quote::quote;
use syn::parse::Error;
use syn::spanned::Spanned;
use syn::DeriveInput;
use crate::default_attr::{ConversionStrategy, DefaultAttr};
use crate::util::find_only;
pub fn impl_my_derive(input: &DeriveInput) -> Result<TokenStream, Error> {
let name = &input.ident;
let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
let (default_expr, doc) = match input.data {
syn::Data::Struct(ref body) => {
let (body_assignment, doc) = default_body_tt(&body.fields)?;
(
quote! {
#name #body_assignment
},
format!("Return `{}{}`", name, doc),
)
}
syn::Data::Enum(ref body) => {
let default_variant = find_only(body.variants.iter(), |variant| {
if let Some(meta) = DefaultAttr::find_in_attributes(&variant.attrs)? {
if meta.code.is_none() {
Ok(true)
} else {
Err(Error::new(
meta.code.span(),
"Attribute #[default] on variants should have no value",
))
}
} else {
Ok(false)
}
})?
.ok_or_else(|| Error::new(input.span(), "No default variant"))?;
let default_variant_name = &default_variant.ident;
let (body_assignment, doc) = default_body_tt(&default_variant.fields)?;
(
quote! {
#name :: #default_variant_name #body_assignment
},
format!("Return `{}::{}{}`", name, default_variant_name, doc),
)
}
syn::Data::Union(_) => {
panic!()
}
};
Ok(quote! {
#[automatically_derived]
impl #impl_generics Default for #name #ty_generics #where_clause {
#[doc = #doc]
fn default() -> Self {
#default_expr
}
}
})
}
/// Return a token-tree for the default "body" - the part after the name that contains the values.
/// That is, the `{ ... }` part for structs, the `(...)` part for tuples, and nothing for units.
fn default_body_tt(body: &syn::Fields) -> Result<(TokenStream, String), Error> {
let mut doc = String::new();
use std::fmt::Write;
let body_tt = match body {
syn::Fields::Named(ref fields) => {
doc.push_str(" {");
let result = {
let field_assignments = fields
.named
.iter()
.map(|field| {
let field_name = field.ident.as_ref();
let (default_value, default_doc) = field_default_expr_and_doc(field)?;
write!(
&mut doc,
"\n {}: {},",
field_name.expect("field value in struct is empty"),
default_doc
)
.unwrap();
// let default_value = default_value.into_token_stream();
Ok(quote! { #field_name : #default_value })
})
.collect::<Result<Vec<_>, Error>>()?;
quote! {
{
#( #field_assignments ),*
}
}
};
if doc.ends_with(',') {
doc.pop();
doc.push('\n');
};
doc.push('}');
result
}
syn::Fields::Unnamed(ref fields) => {
doc.push('(');
let result = {
let field_assignments = fields
.unnamed
.iter()
.map(|field| {
let (default_value, default_doc) = field_default_expr_and_doc(field)?;
write!(&mut doc, "{}, ", default_doc).unwrap();
Ok(default_value)
})
.collect::<Result<Vec<TokenStream>, Error>>()?;
quote! {
(
#( #field_assignments ),*
)
}
};
if doc.ends_with(", ") {
doc.pop();
doc.pop();
};
doc.push(')');
result
}
&syn::Fields::Unit => quote! {},
};
Ok((body_tt, doc))
}
/// Return a default expression for a field based on it's `#[default = "..."]` attribute. Panic
/// if there is more than one, of if there is a `#[default]` attribute without value.
fn field_default_expr_and_doc(field: &syn::Field) -> Result<(TokenStream, String), Error> {
if let Some(default_attr) = DefaultAttr::find_in_attributes(&field.attrs)? {
let conversion_strategy = default_attr.conversion_strategy();
let field_value = default_attr.code.ok_or_else(|| {
Error::new(field.span(), "Expected #[default = ...] or #[default(...)]")
})?;
let field_value = match conversion_strategy {
ConversionStrategy::NoConversion => field_value,
ConversionStrategy::Into => quote!((#field_value).into()),
};
let field_doc = format!("{}", field_value);
Ok((field_value, field_doc))
} else {
Ok((
quote! {
Default::default()
},
"Default::default()".to_owned(),
))
}
}