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|
//! This example demonstrates techniques for performing custom error handling
//! in a derive-input receiver.
//!
//! 1. Using `darling::Result` as a carrier to preserve the error for later display
//! 1. Using `Result<T, syn::Meta>` to attempt a recovery in imperative code
//! 1. Using the `map` darling meta-item to post-process the receiver before returning.
#[macro_use]
extern crate darling;
extern crate syn;
use darling::{FromDeriveInput, FromMeta};
use syn::parse_str;
#[derive(Debug, FromDeriveInput)]
#[darling(attributes(my_trait), map = "MyInputReceiver::autocorrect")]
pub struct MyInputReceiver {
/// This field must be present and a string or else parsing will panic.
name: String,
/// If this field fails to parse, the struct can still be built; the field
/// will contain the error. The consuming struct can then decide if this
/// blocks code generation. If so, panic. Otherwise, recover and proceed.
frequency: darling::Result<i64>,
/// If this field fails to parse, the struct can still be built; the field
/// will contain an `Err` with the original `syn::Meta`. This can be used
/// for alternate parsing attempts before panicking.
amplitude: Result<u64, syn::Meta>,
}
impl MyInputReceiver {
/// This function will be called by `darling` _after_ it's finished parsing the
/// input but before returning to the caller. This is a good place to initialize
/// skipped fields or to perform corrections that don't lend themselves to being
/// done elsewhere.
fn autocorrect(self) -> Self {
let Self {
name,
frequency,
amplitude,
} = self;
// Amplitude doesn't have a sign, so if we received a negative number then
// we'll go ahead and make it positive.
let amplitude = match amplitude {
Ok(amp) => amp,
Err(mi) => {
let val: i64 = if let Ok(v) = FromMeta::from_meta(&mi) {
v
} else {
panic!(format!("amplitude should have been an integer"))
};
val.abs() as u64
}
};
Self {
name,
frequency,
amplitude: Ok(amplitude),
}
}
}
fn main() {
let input = r#"#[derive(MyTrait)]
#[my_trait(name="Jon", amplitude = "-1", frequency = "1")]
pub struct Foo;"#;
let parsed = parse_str(input).unwrap();
let receiver = MyInputReceiver::from_derive_input(&parsed).unwrap();
println!(
r#"
INPUT:
{}
PARSED AS:
{:?}
"#,
input, receiver
);
}
|
