85 lines
2.8 KiB
Rust
85 lines
2.8 KiB
Rust
//! 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 a field before returning
|
|
//! 1. Using the `and_then` darling meta-item to post-process the receiver before returning
|
|
|
|
use darling::{FromDeriveInput, FromMeta};
|
|
use syn::parse_str;
|
|
|
|
#[derive(Debug, FromDeriveInput)]
|
|
#[darling(attributes(my_trait), and_then = MyInputReceiver::autocorrect)]
|
|
pub struct MyInputReceiver {
|
|
/// This field must be present and a string or else parsing will panic.
|
|
#[darling(map = MyInputReceiver::make_string_shouty)]
|
|
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 or fail in `and_then`.
|
|
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
|
|
/// `name` field but before initializing `name` with the resulting value. It's
|
|
/// a good place for transforms that are easiest to express on already-built
|
|
/// types.
|
|
fn make_string_shouty(s: String) -> String {
|
|
s.to_uppercase()
|
|
}
|
|
|
|
/// 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) -> darling::Result<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) => (i64::from_meta(&mi)?).unsigned_abs(),
|
|
};
|
|
|
|
Ok(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
|
|
);
|
|
}
|