Source code
Revision control
Copy as Markdown
Other Tools
// The use of fields in debug print commands does not count as "used",
// which causes the fields to trigger an unwanted dead code warning.
#![allow(dead_code)]
//! This example shows how to do struct and field parsing using darling.
use darling::{ast, FromDeriveInput, FromField, FromMeta};
use proc_macro2::TokenStream;
use quote::{quote, ToTokens};
use syn::parse_str;
/// A speaking volume. Deriving `FromMeta` will cause this to be usable
/// as a string value for a meta-item key.
#[derive(Debug, Clone, Copy, FromMeta)]
#[darling(default)]
enum Volume {
Normal,
Whisper,
Shout,
}
impl Default for Volume {
fn default() -> Self {
Volume::Normal
}
}
/// Support parsing from a full derive input. Unlike FromMeta, this isn't
/// composable; each darling-dependent crate should have its own struct to handle
/// when its trait is derived.
#[derive(Debug, FromDeriveInput)]
// This line says that we want to process all attributes declared with `my_trait`,
// and that darling should panic if this receiver is given an enum.
#[darling(attributes(my_trait), supports(struct_any))]
struct MyInputReceiver {
/// The struct ident.
ident: syn::Ident,
/// The type's generics. You'll need these any time your trait is expected
/// to work with types that declare generics.
generics: syn::Generics,
/// Receives the body of the struct or enum. We don't care about
/// struct fields because we previously told darling we only accept structs.
data: ast::Data<(), MyFieldReceiver>,
/// The Input Receiver demands a volume, so use `Volume::Normal` if the
/// caller doesn't provide one.
#[darling(default)]
volume: Volume,
}
impl ToTokens for MyInputReceiver {
fn to_tokens(&self, tokens: &mut TokenStream) {
let MyInputReceiver {
ref ident,
ref generics,
ref data,
volume,
} = *self;
let (imp, ty, wher) = generics.split_for_impl();
let fields = data
.as_ref()
.take_struct()
.expect("Should never be enum")
.fields;
// Generate the format string which shows each field and its name
let fmt_string = fields
.iter()
.enumerate()
.map(|(i, f)| {
// We have to preformat the ident in this case so we can fall back
// to the field index for unnamed fields. It's not easy to read,
// unfortunately.
format!(
"{} = {{}}",
f.ident
.as_ref()
.map(|v| format!("{}", v))
.unwrap_or_else(|| format!("{}", i))
)
})
.collect::<Vec<_>>()
.join(", ");
// Generate the actual values to fill the format string.
let field_list = fields
.into_iter()
.enumerate()
.map(|(i, f)| {
let field_volume = f.volume.unwrap_or(volume);
// This works with named or indexed fields, so we'll fall back to the index so we can
// write the output as a key-value pair.
let field_ident = f.ident
.as_ref()
.map(|v| quote!(#v))
.unwrap_or_else(|| {
let i = syn::Index::from(i);
quote!(#i)
});
match field_volume {
Volume::Normal => quote!(self.#field_ident),
Volume::Shout => {
quote!(::std::string::ToString::to_string(&self.#field_ident).to_uppercase())
}
Volume::Whisper => {
quote!(::std::string::ToString::to_string(&self.#field_ident).to_lowercase())
}
}
})
.collect::<Vec<_>>();
tokens.extend(quote! {
impl #imp Speak for #ident #ty #wher {
fn speak(&self, writer: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
write!(writer, #fmt_string, #(#field_list),*)
}
}
});
}
}
#[derive(Debug, FromField)]
#[darling(attributes(my_trait))]
struct MyFieldReceiver {
/// Get the ident of the field. For fields in tuple or newtype structs or
/// enum bodies, this can be `None`.
ident: Option<syn::Ident>,
/// This magic field name pulls the type from the input.
ty: syn::Type,
/// We declare this as an `Option` so that during tokenization we can write
/// `field.volume.unwrap_or(derive_input.volume)` to facilitate field-level
/// overrides of struct-level settings.
///
/// Because this field is an `Option`, we don't need to include `#[darling(default)]`
volume: Option<Volume>,
}
fn main() {
let input = r#"#[derive(MyTrait)]
#[my_trait(volume = "shout")]
pub struct Foo {
#[my_trait(volume = "whisper")]
bar: bool,
baz: i64,
}"#;
let parsed = parse_str(input).unwrap();
let receiver = MyInputReceiver::from_derive_input(&parsed).unwrap();
let tokens = quote!(#receiver);
println!(
r#"
INPUT:
{}
PARSED AS:
{:?}
EMITS:
{}
"#,
input, receiver, tokens
);
}