darling_core/util/
flag.rs

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1use proc_macro2::Span;
2use syn::{spanned::Spanned, Meta};
3
4use crate::{FromMeta, Result};
5
6/// A meta-item that can be present as a word - with no value - or absent.
7///
8/// # Defaulting
9/// Like `Option`, `Flag` does not require `#[darling(default)]` to be optional.
10/// If the caller does not include the property, then an absent `Flag` will be included
11/// in the receiver struct.
12///
13/// # Spans
14/// `Flag` keeps the span where its word was seen.
15/// This enables attaching custom error messages to the word, such as in the case of two
16/// conflicting flags being present.
17///
18/// # Example
19/// ```ignore
20/// #[derive(FromMeta)]
21/// #[darling(and_then = Self::not_both)]
22/// struct Demo {
23///     flag_a: Flag,
24///     flag_b: Flag,
25/// }
26///
27/// impl Demo {
28///     fn not_both(self) -> Result<Self> {
29///         if self.flag_a.is_present() && self.flag_b.is_present() {
30///             Err(Error::custom("Cannot set flag_a and flag_b").with_span(&self.flag_b.span()))
31///         } else {
32///             Ok(self)
33///         }
34///     }
35/// }
36/// ```
37///
38/// The above struct would then produce the following error.
39///
40/// ```ignore
41/// #[example(flag_a, flag_b)]
42/// //                ^^^^^^ Cannot set flag_a and flag_b
43/// ```
44#[derive(Debug, Clone, Copy, Default)]
45pub struct Flag(Option<Span>);
46
47impl Flag {
48    /// Creates a new `Flag` which corresponds to the presence of a value.
49    pub fn present() -> Self {
50        Flag(Some(Span::call_site()))
51    }
52
53    /// Check if the flag is present.
54    pub fn is_present(&self) -> bool {
55        self.0.is_some()
56    }
57
58    #[deprecated(since = "0.14.0", note = "Use Flag::is_present")]
59    pub fn is_some(&self) -> bool {
60        self.is_present()
61    }
62
63    /// Get the span of the flag, or [`Span::call_site`] if the flag was not present.
64    pub fn span(&self) -> Span {
65        self.0.unwrap_or_else(Span::call_site)
66    }
67}
68
69impl FromMeta for Flag {
70    fn from_none() -> Option<Self> {
71        Some(Flag(None))
72    }
73
74    fn from_meta(mi: &syn::Meta) -> Result<Self> {
75        if let Meta::Path(p) = mi {
76            Ok(Flag(Some(p.span())))
77        } else {
78            // The implementation for () will produce an error for all non-path meta items;
79            // call it to make sure the span behaviors and error messages are the same.
80            Err(<()>::from_meta(mi).unwrap_err())
81        }
82    }
83}
84
85impl From<Flag> for bool {
86    fn from(flag: Flag) -> Self {
87        flag.is_present()
88    }
89}
90
91impl From<bool> for Flag {
92    fn from(v: bool) -> Self {
93        if v {
94            Flag::present()
95        } else {
96            Flag(None)
97        }
98    }
99}
darling_core/util/flag.rs

darling_core/util/
flag.rs
