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() } }Run
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:
==, <, dereferencing (*) and + (etc.) operators are all
marked with lang items; those specific four are eq, ord,
deref, and add respectively.eh_personality,
eh_unwind_resume, fail and fail_bounds_checks lang items.std::marker used to indicate types of
various kinds; lang items send, sync and copy.std::marker; lang items covariant_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.