darling_core/util/
spanned_value.rs

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1use proc_macro2::Span;
2use std::ops::{Deref, DerefMut};
3use syn::spanned::Spanned;
4
5use crate::{
6    FromDeriveInput, FromField, FromGenericParam, FromGenerics, FromMeta, FromTypeParam,
7    FromVariant, Result,
8};
9
10/// A value and an associated position in source code. The main use case for this is
11/// to preserve position information to emit warnings from proc macros. You can use
12/// a `SpannedValue<T>` as a field in any struct that implements or derives any of
13/// `darling`'s core traits.
14///
15/// To access the underlying value, use the struct's `Deref` implementation.
16///
17/// # Defaulting
18/// This type is meant to be used in conjunction with attribute-extracted options,
19/// but the user may not always explicitly set those options in their source code.
20/// In this case, using `Default::default()` will create an instance which points
21/// to `Span::call_site()`.
22#[derive(Debug, Clone, Copy)]
23pub struct SpannedValue<T> {
24    value: T,
25    span: Span,
26}
27
28impl<T> SpannedValue<T> {
29    pub fn new(value: T, span: Span) -> Self {
30        SpannedValue { value, span }
31    }
32
33    /// Get the source code location referenced by this struct.
34    pub fn span(&self) -> Span {
35        self.span
36    }
37
38    /// Apply a mapping function to a reference to the spanned value.
39    pub fn map_ref<U>(&self, map_fn: impl FnOnce(&T) -> U) -> SpannedValue<U> {
40        SpannedValue::new(map_fn(&self.value), self.span)
41    }
42}
43
44impl<T: Default> Default for SpannedValue<T> {
45    fn default() -> Self {
46        SpannedValue::new(Default::default(), Span::call_site())
47    }
48}
49
50impl<T> Deref for SpannedValue<T> {
51    type Target = T;
52
53    fn deref(&self) -> &T {
54        &self.value
55    }
56}
57
58impl<T> DerefMut for SpannedValue<T> {
59    fn deref_mut(&mut self) -> &mut T {
60        &mut self.value
61    }
62}
63
64impl<T> AsRef<T> for SpannedValue<T> {
65    fn as_ref(&self) -> &T {
66        &self.value
67    }
68}
69
70macro_rules! spanned {
71    ($trayt:ident, $method:ident, $syn:path) => {
72        impl<T: $trayt> $trayt for SpannedValue<T> {
73            fn $method(value: &$syn) -> Result<Self> {
74                Ok(SpannedValue::new(
75                    $trayt::$method(value).map_err(|e| e.with_span(value))?,
76                    value.span(),
77                ))
78            }
79        }
80    };
81}
82
83impl<T: FromMeta> FromMeta for SpannedValue<T> {
84    fn from_meta(item: &syn::Meta) -> Result<Self> {
85        let value = T::from_meta(item).map_err(|e| e.with_span(item))?;
86        let span = match item {
87            // Example: `#[darling(skip)]` as SpannedValue<bool>
88            // should have the span pointing to the word `skip`.
89            syn::Meta::Path(path) => path.span(),
90            // Example: `#[darling(attributes(Value))]` as a SpannedValue<Vec<String>>
91            // should have the span pointing to the list contents.
92            syn::Meta::List(list) => list.tokens.span(),
93            // Example: `#[darling(skip = true)]` as SpannedValue<bool>
94            // should have the span pointing to the word `true`.
95            syn::Meta::NameValue(nv) => nv.value.span(),
96        };
97
98        Ok(Self::new(value, span))
99    }
100}
101
102spanned!(FromGenericParam, from_generic_param, syn::GenericParam);
103spanned!(FromGenerics, from_generics, syn::Generics);
104spanned!(FromTypeParam, from_type_param, syn::TypeParam);
105spanned!(FromDeriveInput, from_derive_input, syn::DeriveInput);
106spanned!(FromField, from_field, syn::Field);
107spanned!(FromVariant, from_variant, syn::Variant);
108
109impl<T: Spanned> From<T> for SpannedValue<T> {
110    fn from(value: T) -> Self {
111        let span = value.span();
112        SpannedValue::new(value, span)
113    }
114}
115
116#[cfg(test)]
117mod tests {
118    use super::*;
119    use proc_macro2::Span;
120
121    /// Make sure that `SpannedValue` can be seamlessly used as its underlying type.
122    #[test]
123    fn deref() {
124        let test = SpannedValue::new("hello", Span::call_site());
125        assert_eq!("hello", test.trim());
126    }
127}
darling_core/util/spanned_value.rs

darling_core/util/
spanned_value.rs
