const_destructure/
lib.rs

1#[doc(hidden)]
2/// Provides access to the inner value of a ManuallyDrop<T>.
3pub const fn __manually_drop_inner_ref<T>(slot: &core::mem::ManuallyDrop<T>) -> &T {
4    // SAFETY: ManuallyDrop<T> and T are guaranteed to have the same layout
5    unsafe { core::mem::transmute(slot) }
6}
7
8#[macro_export]
9macro_rules! const_destructure {
10    // struct
11    (let $S:path { $($field_spec:tt)* } = $value:expr) => {
12        $crate::const_destructure!(@struct ($($field_spec)*) => let $S {} = $value);
13    };
14    // tuple
15    (let ($($var:pat_param),* $(,)?) = $value:expr) => {
16        $crate::const_destructure!(@tuple ($($var),*); (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11) => (); let () = $value);
17    };
18    (@struct ()
19        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
20    ) => {
21        let value = $value;
22        let __destructures_all_fields_and_fields_are_unique = || {
23            let $S { $($field: _),* } = &value;
24        };
25        let value = ::core::mem::ManuallyDrop::new(value);
26        let value = $crate::__manually_drop_inner_ref(&value);
27        // SAFETY: We avoid double free by 1) only reading each field once (since they're unique) and 2) the original is wrapped in ManuallyDrop.
28        $(
29            let $var = unsafe { ::core::ptr::addr_of!(value.$field).read() };
30        )*
31    };
32    (@struct ($next_field:ident: $next_var:pat_param $(,)?)
33        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
34    ) => {
35        $crate::const_destructure!(@struct () => let $S { $($field: $var,)* $next_field: $next_var } = $value);
36    };
37    (@struct ($next_field:ident $(,)?)
38        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
39    ) => {
40        $crate::const_destructure!(@struct () => let $S { $($field: $var,)* $next_field: $next_field } = $value);
41    };
42    (@struct ($next_field:ident: $next_var:pat_param, $($rest:tt)*)
43        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
44    ) => {
45        $crate::const_destructure!(@struct ($($rest)*) => let $S { $($field: $var,)* $next_field: $next_var } = $value);
46    };
47    (@struct ($next_field:ident, $($rest:tt)*)
48        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
49    ) => {
50        $crate::const_destructure!(@struct ($($rest)*) => let $S { $($field: $var,)* $next_field: $next_field } = $value);
51    };
52    (@tuple (); ($($index_rest:tt),*)
53        => ($($ty:tt),*); let ($($index:tt: $var:pat_param),*) = $value:expr
54    ) => {
55        let value: ($($ty,)*) = $value; // asserts correct arity
56        let value = ::core::mem::ManuallyDrop::new(value);
57        let value = $crate::__manually_drop_inner_ref(&value);
58        // SAFETY: We avoid double free by 1) only reading each field once (since they're unique) and 2) the original is wrapped in ManuallyDrop.
59        $(
60            let $var = unsafe { ::core::ptr::addr_of!(value.$index).read() };
61        )*
62    };
63    (@tuple ($var_head:pat_param $(,$var_tail:pat_param)*); ()
64        => ($($ty:tt),*); let ($($index:tt: $var:pat_param),*) = $value:expr
65    ) => {
66        compile_error!("tuple arity is larger than the maximum supported arity 12")
67    };
68    (@tuple ($var_head:pat_param $(,$var_tail:pat_param)*); ($index_head:tt $(,$index_tail:tt)*)
69        => ($($ty:tt),*); let ($($index:tt: $var:pat_param),*) = $value:expr
70    ) => {
71        $crate::const_destructure!(@tuple ($($var_tail),*); ($($index_tail),*) => ($($ty,)* _); let ($($index: $var,)* $index_head: $var_head) = $value);
72    };
73}
const_destructure/lib.rs

const_destructure/
lib.rs
