interop blog post 1

rust1.rs

/// Allocate space for a mappings string of the given size (in bytes).
///
/// It is the JS callers responsibility to initialize the resulting buffer by
/// copying the JS `String` holding the source map's "mappings" into it (encoded
/// in ascii).
#[no_mangle]
pub extern "C" fn allocate_mappings(size: usize) -> *mut u8 {
    // Make sure that we don't lose any bytes from size in the remainder.
    let size_in_units_of_usize = (size + mem::size_of::<usize>() - 1) / mem::size_of::<usize>();

    // Make room for two additional `usize`s: we'll stuff capacity and length in
    // there.
    let mut vec: Vec<usize> = Vec::with_capacity(size_in_units_of_usize + 2);

    // And do the stuffing.
    let capacity = vec.capacity();
    vec.push(capacity);
    vec.push(size);

    // Leak the vec's elements and get a pointer to them.
    let ptr = vec.as_mut_ptr();
    debug_assert!(!ptr.is_null());
    mem::forget(vec);

    // Advance the pointer past our stuffed data and return it to JS, so that JS
    // can write the mappings string into it.
    let ptr = ptr.wrapping_offset(2) as *mut u8;
    assert_pointer_is_word_aligned(ptr);
    ptr
}