1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147

use std::fmt;

use syn::{Lit, NestedMeta};

use crate::{FromMeta, Result};

use self::Override::*;

/// A value which can inherit a default value or have an explicit value specified.
///
/// # Usage
/// This type is meant for attributes like `default` in `darling`, which can take the following forms:
///
/// * `#[darling(default)]`
/// * `#[darling(default="path::to::fn")]`
///
/// In a struct collecting input for this attribute, that would be written as:
///
/// ```rust,ignore
/// use darling::{util::Override, FromField};
/// #[derive(FromField)]
/// #[darling(attributes(darling))]
/// pub struct Options {
///    default: Option<Override<syn::Path>>,
/// }
///
/// impl Options {
///     fn hydrate(self) -> Option<syn::Path> {
///         self.default.map(|ov| ov.unwrap_or(syn::parse_path("::Default::default").unwrap()))
///     }
/// }
/// ```
///
/// The `word` format (with no associated value), would produce `Override::Inherit`, while a list
/// or value format would produce `Override::Explicit`.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Override<T> {
    /// Inherit the eventual value from an external source.
    Inherit,

    /// Explicitly set the value.
    Explicit(T),
}

impl<T> Override<T> {
    /// Converts from `Override<T>` to `Override<&T>`.
    ///
    /// Produces a new `Override`, containing a reference into the original, leaving the original in place.
    pub fn as_ref(&self) -> Override<&T> {
        match *self {
            Inherit => Inherit,
            Explicit(ref val) => Explicit(val),
        }
    }

    /// Converts from `Override<T>` to `Override<&mut T>`.
    ///
    /// Produces a new `Override`, containing a mutable reference into the original.
    pub fn as_mut(&mut self) -> Override<&mut T> {
        match *self {
            Inherit => Inherit,
            Explicit(ref mut val) => Explicit(val),
        }
    }

    /// Returns `true` if the override is an `Explicit` value.
    pub fn is_explicit(&self) -> bool {
        match *self {
            Inherit => false,
            Explicit(_) => true,
        }
    }

    /// Converts from `Override<T>` to `Option<T>`.
    pub fn explicit(self) -> Option<T> {
        match self {
            Inherit => None,
            Explicit(val) => Some(val),
        }
    }

    /// Unwraps an override, yielding the content of an `Explicit`. Otherwise, it returns `optb`.
    pub fn unwrap_or(self, optb: T) -> T {
        match self {
            Inherit => optb,
            Explicit(val) => val,
        }
    }

    /// Unwraps an override, yielding the content of an `Explicit`. Otherwise, it calls `op`.
    pub fn unwrap_or_else<F>(self, op: F) -> T
    where
        F: FnOnce() -> T,
    {
        match self {
            Inherit => op(),
            Explicit(val) => val,
        }
    }
}

impl<T: Default> Override<T> {
    /// Returns the contained value or the default value of `T`.
    pub fn unwrap_or_default(self) -> T {
        self.unwrap_or_else(Default::default)
    }
}

impl<T> Default for Override<T> {
    fn default() -> Self {
        Inherit
    }
}

impl<T> From<Option<T>> for Override<T> {
    fn from(v: Option<T>) -> Self {
        match v {
            None => Inherit,
            Some(val) => Explicit(val),
        }
    }
}

impl<T: fmt::Display> fmt::Display for Override<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            Inherit => write!(f, "Inherit"),
            Explicit(ref val) => write!(f, "Explicit `{}`", val),
        }
    }
}

/// Parses a `Meta`. A bare word will produce `Override::Inherit`, while
/// any value will be forwarded to `T::from_meta`.
impl<T: FromMeta> FromMeta for Override<T> {
    fn from_word() -> Result<Self> {
        Ok(Inherit)
    }

    fn from_list(items: &[NestedMeta]) -> Result<Self> {
        Ok(Explicit(FromMeta::from_list(items)?))
    }

    fn from_value(lit: &Lit) -> Result<Self> {
        Ok(Explicit(FromMeta::from_value(lit)?))
    }
}