% 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
compiler 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
pointers require two lang items, one for allocation
and one for deallocation. A freestanding program that uses the Box
sugar for dynamic allocations via malloc
and free
:
#![feature(lang_items, box_syntax, start, libc, core_intrinsics)]
#![no_std]
use core::intrinsics;
extern crate libc;
#[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;
// Check if `malloc` failed:
if p as usize == 0 {
intrinsics::abort();
}
p
}
#[lang = "exchange_free"]
unsafe fn deallocate(ptr: *mut u8, _size: usize, _align: usize) {
libc::free(ptr as *mut libc::c_void)
}
#[lang = "box_free"]
unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
deallocate(ptr as *mut u8, ::core::mem::size_of_val(&*ptr), ::core::mem::align_of_val(&*ptr));
}
#[start]
fn main(argc: isize, argv: *const *const u8) -> isize {
let x = box 1;
0
}
#[lang = "eh_personality"] extern fn rust_eh_personality() {}
#[lang = "panic_fmt"] extern fn rust_begin_panic() -> ! { unsafe { intrinsics::abort() } }
# #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
# #[no_mangle] pub extern fn rust_eh_register_frames () {}
# #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
Note the use of abort
: 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+
(etc.) operators are all marked with lang items; those specific four areeq
,ord
,deref
, andadd
respectively. - stack unwinding and general failure; the
eh_personality
,eh_unwind_resume
,fail
andfail_bounds_checks
lang items. - the traits in
std::marker
used to indicate types of various kinds; lang itemssend
,sync
andcopy
. - the marker types and variance indicators found in
std::marker
; lang itemscovariant_type
,contravariant_lifetime
, etc.
Lang items are loaded lazily by the compiler; e.g. if one never uses
Box
then there is no need to define functions for exchange_malloc
and exchange_free
. rustc
will emit an error when an item is needed
but not found in the current crate or any that it depends on.