1 //! This example demonstrates techniques for performing custom error handling
2 //! in a derive-input receiver.
3 //!
4 //! 1. Using `darling::Result` as a carrier to preserve the error for later display
5 //! 1. Using `Result<T, syn::Meta>` to attempt a recovery in imperative code
6 //! 1. Using the `map` darling meta-item to post-process a field before returning
7 //! 1. Using the `and_then` darling meta-item to post-process the receiver before returning
8
9 use darling::{FromDeriveInput, FromMeta};
10 use syn::parse_str;
11
12 #[derive(Debug, FromDeriveInput)]
13 #[darling(attributes(my_trait), and_then = MyInputReceiver::autocorrect)]
14 pub struct MyInputReceiver {
15 /// This field must be present and a string or else parsing will panic.
16 #[darling(map = MyInputReceiver::make_string_shouty)]
17 name: String,
18
19 /// If this field fails to parse, the struct can still be built; the field
20 /// will contain the error. The consuming struct can then decide if this
21 /// blocks code generation. If so, panic or fail in `and_then`.
22 frequency: darling::Result<i64>,
23
24 /// If this field fails to parse, the struct can still be built; the field
25 /// will contain an `Err` with the original `syn::Meta`. This can be used
26 /// for alternate parsing attempts before panicking.
27 amplitude: Result<u64, syn::Meta>,
28 }
29
30 impl MyInputReceiver {
31 /// This function will be called by `darling` _after_ it's finished parsing the
32 /// `name` field but before initializing `name` with the resulting value. It's
33 /// a good place for transforms that are easiest to express on already-built
34 /// types.
make_string_shouty(s: String) -> String35 fn make_string_shouty(s: String) -> String {
36 s.to_uppercase()
37 }
38
39 /// This function will be called by `darling` _after_ it's finished parsing the
40 /// input but before returning to the caller. This is a good place to initialize
41 /// skipped fields or to perform corrections that don't lend themselves to being
42 /// done elsewhere.
autocorrect(self) -> darling::Result<Self>43 fn autocorrect(self) -> darling::Result<Self> {
44 let Self {
45 name,
46 frequency,
47 amplitude,
48 } = self;
49
50 // Amplitude doesn't have a sign, so if we received a negative number then
51 // we'll go ahead and make it positive.
52 let amplitude = match amplitude {
53 Ok(amp) => amp,
54 Err(mi) => (i64::from_meta(&mi)?).unsigned_abs(),
55 };
56
57 Ok(Self {
58 name,
59 frequency,
60 amplitude: Ok(amplitude),
61 })
62 }
63 }
64
main()65 fn main() {
66 let input = r#"#[derive(MyTrait)]
67 #[my_trait(name="Jon", amplitude = "-1", frequency = 1)]
68 pub struct Foo;"#;
69
70 let parsed = parse_str(input).unwrap();
71 let receiver = MyInputReceiver::from_derive_input(&parsed).unwrap();
72
73 println!(
74 r#"
75 INPUT:
76
77 {}
78
79 PARSED AS:
80
81 {:?}
82 "#,
83 input, receiver
84 );
85 }
86