darling_core/util/
over_ride.rs

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1use std::fmt;
2
3use syn::Lit;
4
5use crate::ast::NestedMeta;
6use crate::{FromMeta, Result};
7
8use self::Override::*;
9
10/// A value which can inherit a default value or have an explicit value specified.
11///
12/// # Usage
13/// This type is meant for attributes like `default` in `darling`, which can take the following forms:
14///
15/// * `#[darling(default)]`
16/// * `#[darling(default="path::to::fn")]`
17///
18/// In a struct collecting input for this attribute, that would be written as:
19///
20/// ```rust,ignore
21/// use darling::{util::Override, FromField};
22/// #[derive(FromField)]
23/// #[darling(attributes(darling))]
24/// pub struct Options {
25///    default: Option<Override<syn::Path>>,
26/// }
27///
28/// impl Options {
29///     fn hydrate(self) -> Option<syn::Path> {
30///         self.default.map(|ov| ov.unwrap_or(syn::parse_path("::Default::default").unwrap()))
31///     }
32/// }
33/// ```
34///
35/// The `word` format (with no associated value), would produce `Override::Inherit`, while a list
36/// or value format would produce `Override::Explicit`.
37#[derive(Debug, Clone, PartialEq, Eq)]
38pub enum Override<T> {
39    /// Inherit the eventual value from an external source.
40    Inherit,
41
42    /// Explicitly set the value.
43    Explicit(T),
44}
45
46impl<T> Override<T> {
47    /// Converts from `Override<T>` to `Override<&T>`.
48    ///
49    /// Produces a new `Override`, containing a reference into the original, leaving the original in place.
50    pub fn as_ref(&self) -> Override<&T> {
51        match *self {
52            Inherit => Inherit,
53            Explicit(ref val) => Explicit(val),
54        }
55    }
56
57    /// Converts from `Override<T>` to `Override<&mut T>`.
58    ///
59    /// Produces a new `Override`, containing a mutable reference into the original.
60    pub fn as_mut(&mut self) -> Override<&mut T> {
61        match *self {
62            Inherit => Inherit,
63            Explicit(ref mut val) => Explicit(val),
64        }
65    }
66
67    /// Returns `true` if the override is an `Explicit` value.
68    pub fn is_explicit(&self) -> bool {
69        match *self {
70            Inherit => false,
71            Explicit(_) => true,
72        }
73    }
74
75    /// Converts from `Override<T>` to `Option<T>`.
76    pub fn explicit(self) -> Option<T> {
77        match self {
78            Inherit => None,
79            Explicit(val) => Some(val),
80        }
81    }
82
83    /// Unwraps an override, yielding the content of an `Explicit`. Otherwise, it returns `optb`.
84    pub fn unwrap_or(self, optb: T) -> T {
85        match self {
86            Inherit => optb,
87            Explicit(val) => val,
88        }
89    }
90
91    /// Unwraps an override, yielding the content of an `Explicit`. Otherwise, it calls `op`.
92    pub fn unwrap_or_else<F>(self, op: F) -> T
93    where
94        F: FnOnce() -> T,
95    {
96        match self {
97            Inherit => op(),
98            Explicit(val) => val,
99        }
100    }
101}
102
103impl<T: Default> Override<T> {
104    /// Returns the contained value or the default value of `T`.
105    pub fn unwrap_or_default(self) -> T {
106        match self {
107            Inherit => Default::default(),
108            Explicit(val) => val,
109        }
110    }
111}
112
113impl<T> Default for Override<T> {
114    fn default() -> Self {
115        Inherit
116    }
117}
118
119impl<T> From<Option<T>> for Override<T> {
120    fn from(v: Option<T>) -> Self {
121        match v {
122            None => Inherit,
123            Some(val) => Explicit(val),
124        }
125    }
126}
127
128impl<T: fmt::Display> fmt::Display for Override<T> {
129    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
130        match *self {
131            Inherit => write!(f, "Inherit"),
132            Explicit(ref val) => write!(f, "Explicit `{}`", val),
133        }
134    }
135}
136
137/// Parses a `Meta`. A bare word will produce `Override::Inherit`, while
138/// any value will be forwarded to `T::from_meta`.
139impl<T: FromMeta> FromMeta for Override<T> {
140    fn from_word() -> Result<Self> {
141        Ok(Inherit)
142    }
143
144    fn from_list(items: &[NestedMeta]) -> Result<Self> {
145        Ok(Explicit(FromMeta::from_list(items)?))
146    }
147
148    fn from_value(lit: &Lit) -> Result<Self> {
149        Ok(Explicit(FromMeta::from_value(lit)?))
150    }
151}
darling_core/util/over_ride.rs

darling_core/util/
over_ride.rs
