Macro stackbox

macro_rules! stackbox {
    (
        // Same as `StackBox::new_in`, except for it allowing an unsized
        // coercion to take place.
        $place:expr,
        $value:expr $(,)?
    ) => { ... };
    (
        // Create a new `mut` `StackBox` without mentioning the backing _slot_:
        // `let mut <binding> = stackbox!($expr);`
        // Examples:
        //   - `stackbox!(let mut new_var = <expr>);`
        //   - `stackbox!(let mut new_var: StackBox<[_]> = <array expr>);`
        let mut $var:ident $(: $T:ty)? = $expr:expr
    ) => { ... };
    (
        // Create a new `StackBox` without mentioning the backing _slot_:
        // `let <binding> = stackbox!($expr);`
        // Examples:
        //   - `stackbox!(let new_var = <expr>);`
        //   - `stackbox!(let new_var: StackBox<[_]> = <array expr>);`
        let $var:ident $(: $T:ty)? = $expr:expr
    ) => { ... };
    (
        // Internal-ish: assign the result of a `stackbox!($expr)` to "some place"
        // where "some place" may be a new `let` binding or an actual assignment.
        //
        // No need to explicitly mention the backing _slot_ either.
        // Examples:
        //   - `let var: Ty; stackbox!(<expr> => var);`
        $expr:expr => $($binding:tt)*
    ) => { ... };
    (
        // Shorthand for `stackbox!(let mut $var = $var)`
        let mut $var:ident
    ) => { ... };
    (
        // Shorthand for `stackbox!(let $var = $var)`
        let $var:ident
    ) => { ... };
    (
        // To be used as a temporary fed to a function parameter, or as a
        // `[::with_locals::with]` "return" value.
        //
        // Examples:
        //   - `fun(stackbox!(value))`
        $expr:expr
    ) => { ... };
}

Convenience macro for more ergonomic StackBox constructions.