Document lang items

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 +1,41 @@
+# 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
+  `==`, `<`, dereferencing (`*`) and `+` (etc.) operators are all
+  marked with lang items; those specific four are `eq`, `ord`,
+  `deref`, and `add` respectively.
+- stack unwinding and general failure; 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 `libcore`, but if you're trying to build an
+executable with `#![no_std]`, you'll run into the need for 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