Lang items

The 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'.

Many such lang items can be implemented only in one sensible way, such as add (trait core::ops::Add) or future_trait (trait core::future::Future). Others can be overriden to achieve some specific goals; for example, you can control your binary's entrypoint.

Features provided by lang items include:

  • overloadable operators via traits: the traits corresponding to the ==, <, dereference (*), +, etc. operators are all marked with lang items; those specific four are eq, ord, deref, and add respectively.
  • panicking and stack unwinding; the eh_personality, panic and panic_bounds_checks lang items.
  • the traits in std::marker used to indicate properties of types used by the compiler; lang items send, sync and copy.
  • the special marker types used for variance indicators found in core::marker; lang item phantom_data.

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 box_free. rustc will emit an error when an item is needed but not found in the current crate or any that it depends on.

Most lang items are defined by the core library, but if you're trying to build an executable with #![no_std], you'll still need to define a few lang items that are usually provided by std.

Retrieving a language item

You can retrieve lang items by calling tcx.lang_items().

Here's a small example of retrieving the trait Sized {} language item:


#![allow(unused)]
fn main() {
// Note that in case of `#![no_core]`, the trait is not available.
if let Some(sized_trait_def_id) = tcx.lang_items().sized_trait() {
    // do something with `sized_trait_def_id`
}
}

Note that sized_trait() returns an Option, not the DefId itself. That's because language items are defined in the standard libray, so if someone compiles with #![no_core] (or for some lang items, #![no_std]), the lang item may not be present. You can either:

  • Give a hard error if the lang item is necessary to continue (don't panic, since this can happen in user code).
  • Proceed with limited functionality, by just omitting whatever you were going to do with the DefId.

List of all language items

You can find language items in the following places:

  • An exhaustive reference in the compiler documentation: rustc_hir::LangItem
  • An auto-generated list with source locations by using ripgrep: rg '#\[.*lang =' library/

Note that language items are explicitly unstable and may change in any new release.