const-destructure 0.1.1

Destructuring in const contexts on stable Rust.
Documentation 
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#[doc(hidden)]
/// Provides access to the inner value of a ManuallyDrop<T>.
pub const fn __manually_drop_inner_ref<T>(slot: &core::mem::ManuallyDrop<T>) -> &T {
    // SAFETY: ManuallyDrop<T> and T are guaranteed to have the same layout
    unsafe { core::mem::transmute(slot) }
}

#[macro_export]
macro_rules! const_destructure {
    // struct
    (let $S:path { $($field_spec:tt)* } = $value:expr) => {
        $crate::const_destructure!(@struct ($($field_spec)*) => let $S {} = $value);
    };
    // tuple
    (let ($($var:pat_param),* $(,)?) = $value:expr) => {
        $crate::const_destructure!(@tuple ($($var),*); (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11) => (); let () = $value);
    };
    (@struct ()
        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
    ) => {
        let value = $value;
        let __destructures_all_fields_and_fields_are_unique = || {
            let $S { $($field: _),* } = &value;
        };
        let value = ::core::mem::ManuallyDrop::new(value);
        let value = $crate::__manually_drop_inner_ref(&value);
        // SAFETY: We avoid double free by 1) only reading each field once (since they're unique) and 2) the original is wrapped in ManuallyDrop.
        $(
            let $var = unsafe { ::core::ptr::addr_of!(value.$field).read() };
        )*
    };
    (@struct ($next_field:ident: $next_var:pat_param $(,)?)
        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
    ) => {
        $crate::const_destructure!(@struct () => let $S { $($field: $var,)* $next_field: $next_var } = $value);
    };
    (@struct ($next_field:ident $(,)?)
        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
    ) => {
        $crate::const_destructure!(@struct () => let $S { $($field: $var,)* $next_field: $next_field } = $value);
    };
    (@struct ($next_field:ident: $next_var:pat_param, $($rest:tt)*)
        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
    ) => {
        $crate::const_destructure!(@struct ($($rest)*) => let $S { $($field: $var,)* $next_field: $next_var } = $value);
    };
    (@struct ($next_field:ident, $($rest:tt)*)
        => let $S:path { $($field:ident: $var:pat_param),* } = $value:expr
    ) => {
        $crate::const_destructure!(@struct ($($rest)*) => let $S { $($field: $var,)* $next_field: $next_field } = $value);
    };
    (@tuple (); ($($index_rest:tt),*)
        => ($($ty:tt),*); let ($($index:tt: $var:pat_param),*) = $value:expr
    ) => {
        let value: ($($ty,)*) = $value; // asserts correct arity
        let value = ::core::mem::ManuallyDrop::new(value);
        let value = $crate::__manually_drop_inner_ref(&value);
        // SAFETY: We avoid double free by 1) only reading each field once (since they're unique) and 2) the original is wrapped in ManuallyDrop.
        $(
            let $var = unsafe { ::core::ptr::addr_of!(value.$index).read() };
        )*
    };
    (@tuple ($var_head:pat_param $(,$var_tail:pat_param)*); ()
        => ($($ty:tt),*); let ($($index:tt: $var:pat_param),*) = $value:expr
    ) => {
        compile_error!("tuple arity is larger than the maximum supported arity 12")
    };
    (@tuple ($var_head:pat_param $(,$var_tail:pat_param)*); ($index_head:tt $(,$index_tail:tt)*)
        => ($($ty:tt),*); let ($($index:tt: $var:pat_param),*) = $value:expr
    ) => {
        $crate::const_destructure!(@tuple ($($var_tail),*); ($($index_tail),*) => ($($ty,)* _); let ($($index: $var,)* $index_head: $var_head) = $value);
    };
}