src/lang-items.md

# 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 overridden 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:

```rust
// 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 library, 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`.

[`tcx.lang_items()`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html#method.lang_items

## 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.

[`rustc_hir::LangItem`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/lang_items/enum.LangItem.html