Document lang items (#1119)

* Document lang items

* Apply suggestions from code review

Co-authored-by: Camelid <camelidcamel@gmail.com>

* Add an example of retrieving lang items

* Add two missing words

* Fix line lengths

Co-authored-by: Mikail Bagishov <bagishov.mikail@yandex.ru>
Co-authored-by: Camelid <camelidcamel@gmail.com>

src/SUMMARY.md

@@ -81,6 +81,7 @@
     - [Panic Implementation](./panic-implementation.md)
     - [AST Validation](./ast-validation.md)
     - [Feature Gate Checking](./feature-gate-ck.md)
+    - [Lang Items](./lang-items.md)
 - [The HIR (High-level IR)](./hir.md)
     - [Lowering AST to HIR](./lowering.md)
     - [Debugging](./hir-debugging.md)

src/lang-items.md

@@ -0,0 +1,67 @@
+# 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:
+
+```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 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`.
+
+[`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