darling_core/util/
spanned_value.rs

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    /// Gets the inner value, consuming `self` in the process.
44    pub fn into_inner(self) -> T {
45        self.value
46    }
47}
48
49impl<T: Default> Default for SpannedValue<T> {
50    fn default() -> Self {
51        SpannedValue::new(Default::default(), Span::call_site())
52    }
53}
54
55impl<T> Deref for SpannedValue<T> {
56    type Target = T;
57
58    fn deref(&self) -> &T {
59        &self.value
60    }
61}
62
63impl<T> DerefMut for SpannedValue<T> {
64    fn deref_mut(&mut self) -> &mut T {
65        &mut self.value
66    }
67}
68
69impl<T> AsRef<T> for SpannedValue<T> {
70    fn as_ref(&self) -> &T {
71        &self.value
72    }
73}
74
75macro_rules! spanned {
76    ($trayt:ident, $method:ident, $syn:path) => {
77        impl<T: $trayt> $trayt for SpannedValue<T> {
78            fn $method(value: &$syn) -> Result<Self> {
79                Ok(SpannedValue::new(
80                    $trayt::$method(value).map_err(|e| e.with_span(value))?,
81                    value.span(),
82                ))
83            }
84        }
85    };
86}
87
88impl<T: FromMeta> FromMeta for SpannedValue<T> {
89    fn from_meta(item: &syn::Meta) -> Result<Self> {
90        let value = T::from_meta(item).map_err(|e| e.with_span(item))?;
91        let span = match item {
92            // Example: `#[darling(skip)]` as SpannedValue<bool>
93            // should have the span pointing to the word `skip`.
94            syn::Meta::Path(path) => path.span(),
95            // Example: `#[darling(attributes(Value))]` as a SpannedValue<Vec<String>>
96            // should have the span pointing to the list contents.
97            syn::Meta::List(list) => list.tokens.span(),
98            // Example: `#[darling(skip = true)]` as SpannedValue<bool>
99            // should have the span pointing to the word `true`.
100            syn::Meta::NameValue(nv) => nv.value.span(),
101        };
102
103        Ok(Self::new(value, span))
104    }
105
106    fn from_nested_meta(item: &crate::ast::NestedMeta) -> Result<Self> {
107        T::from_nested_meta(item)
108            .map(|value| Self::new(value, item.span()))
109            .map_err(|e| e.with_span(item))
110    }
111
112    fn from_value(literal: &syn::Lit) -> Result<Self> {
113        T::from_value(literal)
114            .map(|value| Self::new(value, literal.span()))
115            .map_err(|e| e.with_span(literal))
116    }
117
118    fn from_expr(expr: &syn::Expr) -> Result<Self> {
119        T::from_expr(expr)
120            .map(|value| Self::new(value, expr.span()))
121            .map_err(|e| e.with_span(expr))
122    }
123}
124
125spanned!(FromGenericParam, from_generic_param, syn::GenericParam);
126spanned!(FromGenerics, from_generics, syn::Generics);
127spanned!(FromTypeParam, from_type_param, syn::TypeParam);
128spanned!(FromDeriveInput, from_derive_input, syn::DeriveInput);
129spanned!(FromField, from_field, syn::Field);
130spanned!(FromVariant, from_variant, syn::Variant);
131
132impl<T: Spanned> From<T> for SpannedValue<T> {
133    fn from(value: T) -> Self {
134        let span = value.span();
135        SpannedValue::new(value, span)
136    }
137}
138
139#[cfg(test)]
140mod tests {
141    use super::*;
142    use proc_macro2::Span;
143
144    /// Make sure that `SpannedValue` can be seamlessly used as its underlying type.
145    #[test]
146    fn deref() {
147        let test = SpannedValue::new("hello", Span::call_site());
148        assert_eq!("hello", test.trim());
149    }
150}
darling_core/util/spanned_value.rs

darling_core/util/
spanned_value.rs
