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 the
Boxsugar for dynamic allocations via
mallocand
` 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 {} }
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:
==
`==,
`, <
`<, dereferencing (
*) and
`) and +
`+(etc.) operators are all marked with lang items; those specific four are
eq,
`, ord
`ord,
`,
deref
`deref, and
`, and add
`add` respectively.eh_personality
, fail
`failand
`
and fail_bounds_checks
lang items.std::marker
used to indicate types of
various kinds; lang items send
`send,
`, sync
`syncand
` and copy
`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
`Boxthen there is no need to define functions for
exchange_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.