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//! Example showing potentially-nested meta item parsing with `darling::util::Override`.
//!
//! Based on https://stackoverflow.com/q/68046070/86381 by https://github.com/peterjoel
// 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)]
use std::borrow::Cow;
use darling::{util::Override, FromDeriveInput, FromMeta};
use syn::{Ident, Path};
#[derive(Debug, FromDeriveInput)]
#[darling(attributes(myderive))]
struct MyDeriveInput {
ident: Ident,
/// We can infer the right "table" behavior for this derive, but we want the caller to be
/// explicit that they're expecting the inference behavior to avoid cluttering some hypothetical
/// database. Therefore this field is required, but can take word form or key-value form.
///
/// To make this field optional, we could add `#[darling(default)]`, or we could
/// wrap it in `Option` if the presence or absence of the word makes a difference.
table: Override<Table>,
}
impl MyDeriveInput {
fn table(&self) -> Cow<'_, Table> {
match &self.table {
Override::Explicit(value) => Cow::Borrowed(value),
Override::Inherit => Cow::Owned(Table {
name: self.ident.to_string(),
value: None,
}),
}
}
}
#[derive(Debug, Clone, FromMeta)]
struct Table {
name: String,
value: Option<Path>,
}
fn from_str(s: &str) -> darling::Result<MyDeriveInput> {
FromDeriveInput::from_derive_input(&syn::parse_str(s)?)
}
fn main() {
let missing = from_str(
r#"
#[derive(MyTrait)]
struct Foo(u64);
"#,
)
.unwrap_err();
let short_form = from_str(
r#"
#[derive(MyTrait)]
#[myderive(table)]
struct Foo(u64);
"#,
)
.unwrap();
let long_form = from_str(
r#"
#[derive(MyTrait)]
#[myderive(table(name = "Custom"))]
struct Foo(u64);
"#,
)
.unwrap();
println!("Error when missing: {}", missing);
println!("Short form: {:?}", short_form.table());
println!("Long form: {:?}", long_form.table());
}
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