Macro emplace

macro_rules! emplace {
    ($P:ident <- $S:ident { $($fields:tt)* }) => { ... };
    (@INIT [$pret:ident]) => { ... };
    (@INIT [$pret:ident] $field:ident <- $F:ident ( $($args:expr),* $(,)? ), $($rest:tt)*) => { ... };
    (@INIT [$pret:ident] $field:ident : $expr:expr, $($rest:tt)*) => { ... };
    (@PAT $S:ident { $( $field:ident $(:)? $(<-)? $_:expr, )* }) => { ... };
}

Initializes a struct, field by field, in-place on the heap.

Examples

A translated version of the code from the Rust-for-Linux talk at CTCFT (Nov 2021).

struct SharedState {
    state_changed: CondVar,
    inner: Mutex<usize>,
}

fn try_new() -> Result<Ref<Self>> {
    let ret = emplace!(UniqueRef <- SharedState {
        state_changed <- CondVar("SharedState::state_changed"),
        inner <- Mutex(0, "SharedState::inner"),
    })?;

    Ok(ret.into())
}

This assumes Emplace is implemented for Mutex and CondVar and would require a small tweak to the macro to support fallible allocation (try_emplace?).