Lang items

Note: lang items are often provided by crates in the Rust distribution, and lang items themselves have an unstable interface. It is recommended to use officially distributed crates instead of defining your own lang items.

The rustc`rustccompiler has certain pluggable operations, that is, functionality that isn't hard-coded into the language, but is implemented in libraries, with a special marker to tell the compiler it exists. The marker is the attribute#[lang = "..."]and there are various different values of...`, i.e. various different 'lang items'.

For example, Box`Boxpointers require two lang items, one for allocation and one for deallocation. A freestanding program that uses theBoxsugar for dynamic allocations viamallocand` and free`free`:

#![feature(lang_items, box_syntax, start, no_std, libc)] #![no_std] extern crate libc; extern { fn abort() -> !; } #[lang = "owned_box"] pub struct Box<T>(*mut T); #[lang = "exchange_malloc"] unsafe fn allocate(size: usize, _align: usize) -> *mut u8 { let p = libc::malloc(size as libc::size_t) as *mut u8; // malloc failed if p as usize == 0 { abort(); } p } #[lang = "exchange_free"] unsafe fn deallocate(ptr: *mut u8, _size: usize, _align: usize) { libc::free(ptr as *mut libc::c_void) } #[start] fn main(argc: isize, argv: *const *const u8) -> isize { let x = box 1; 0 } #[lang = "stack_exhausted"] extern fn stack_exhausted() {} #[lang = "eh_personality"] extern fn eh_personality() {} #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} } #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}

#![feature(lang_items, box_syntax, start, no_std, libc)]
#![no_std]

extern crate libc;

extern {
    fn abort() -> !;
}

#[lang = "owned_box"]
pub struct Box<T>(*mut T);

#[lang = "exchange_malloc"]
unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
    let p = libc::malloc(size as libc::size_t) as *mut u8;

    // malloc failed
    if p as usize == 0 {
        abort();
    }

    p
}
#[lang = "exchange_free"]
unsafe fn deallocate(ptr: *mut u8, _size: usize, _align: usize) {
    libc::free(ptr as *mut libc::c_void)
}

#[start]
fn main(argc: isize, argv: *const *const u8) -> isize {
    let x = box 1;

    0
}

#[lang = "stack_exhausted"] extern fn stack_exhausted() {}
#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }

Note the use of abort`abort: the`: the exchange_malloc lang item is assumed to return a valid pointer, and so needs to do the check internally.

Other features provided by lang items include:

• overloadable operators via traits: the traits corresponding to the ==`==,`, <`<, dereferencing (*) and`) and +`+(etc.) operators are all marked with lang items; those specific four areeq,`, ord`ord,`, deref`deref, and`, and add`add` respectively.
• stack unwinding and general failure; the eh_personality, fail`failand` and fail_bounds_checks lang items.
• the traits in std::marker used to indicate types of various kinds; lang items send`send,`, sync`syncand` and copy`copy`.
• the marker types and variance indicators found in std::marker; lang items covariant_type, contravariant_lifetime, etc.

Lang items are loaded lazily by the compiler; e.g. if one never uses Box`Boxthen there is no need to define functions forexchange_mallocand` and exchange_free. rustc`rustc` will emit an error when an item is needed but not found in the current crate or any that it depends on.