devela/code/util/
maybe.rs

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1// devela::code::util::maybe
2//
3//! [`maybe!`] macro helper.
4//
5// MAYBE add: Debug, Display, PartialEq, PartialOrd, Drop
6
7/// Helper for using optionally implemented traits, like `Default` or `Clone`.
8///
9/// The first boolean argument says whether `$T` implements the given trait.
10///
11/// # Examples
12/// ```
13/// # use devela::{assert_eq_all, maybe, NonZeroU8};
14/// assert_eq![maybe![default:true, u8], Some(0)];
15/// assert_eq![maybe![default:true, &str], Some("")];
16/// assert_eq![maybe![default:false, u8], None];
17/// assert_eq![maybe![default:false, NonZeroU8], None];
18///
19/// let s1 = String::from("string1");
20/// let s2 = maybe![clone:true, String, &s1].expect("cloned");
21/// let s3 = maybe![clone:true, String, &s1].expect("cloned");
22/// assert_eq_all![&s1, &s2, &s3];
23/// ```
24/// ```compile_fail
25/// # use devela::{maybe, NonZeroU8};
26/// let _ = maybe![default:true, NonZeroU8];
27/// ```
28#[macro_export]
29#[cfg_attr(cargo_primary_package, doc(hidden))]
30macro_rules! maybe {
31    ( // Returns either Some(<$T>::default()) or `None`.
32      default: $implements_default:stmt, $T:ty ) => {{
33        /* didactic notes */
34
35        // // For this to compile $T always has to implement Default:
36        //
37        // $implements_default.then(|| Self::Value::$C_name(<$T>::default())),
38
39        // // WAIT attributes on expressions are experimental
40        // // https://github.com/rust-lang/rust/issues/15701
41        //
42        // #[crate::compile($implements_default)]
43        // { Some(Self::Value::$C_name(<$T>::default())) }
44        // #[crate::compile(not($implements_default))]
45        // { None }
46
47        // // WAIT: custom attributes can't be applied to statements
48        // // https://github.com/rust-lang/rust/issues/54727
49        //
50        // #[crate::compile($implements_default)]
51        // let res = Some(Self::Value::$C_name(<$T>::default()));
52        // #[crate::compile(not($implements_default))]
53        // let res = None;
54        // res
55
56        // The only solution for now:
57        #[$crate::compile($implements_default)]
58        fn maybe_default<T: Default>() -> Option<T> {
59            Some(T::default())
60        }
61        #[$crate::compile(not($implements_default))]
62        fn maybe_default<T>() -> Option<T> {
63            None
64        }
65        maybe_default::<$T>()
66    }};
67    (
68      // Returns either Some(<$value: $T>.clone()) or `None`.
69      clone: $implements_clone:stmt, $T:ty, $value:expr ) => {{
70        #[$crate::compile($implements_clone)]
71        fn maybe_clone<T: Clone>(value: &T) -> Option<T> {
72            Some(value.clone())
73        }
74        #[$crate::compile(not($implements_clone))]
75        fn maybe_clone<T>(_value: &T) -> Option<T> {
76            None
77        }
78        maybe_clone::<$T>($value)
79    }};
80}
81#[doc(inline)]
82pub use maybe;
devela/code/util/maybe.rs

devela/code/util/
maybe.rs
