Add module-level docs to rustc_middle::ty

Author: pierwill

compiler/rustc_middle/src/ty/mod.rs

@@ -1,3 +1,14 @@
+//! Defines how the compiler represents types internally.
+//!
+//! Two important entities in this module are:
+//!
+//! - [`rustc_middle::ty::Ty`], used to represent the semantics of a type.
+//! - [`rustc_middle::ty::TyCtxt`], the central data structure in the compiler.
+//!
+//! For more information, see ["The `ty` module: representing types"] in the ructc-dev-guide.
+//!
+//! ["The `ty` module: representing types"]: https://rustc-dev-guide.rust-lang.org/ty.html
+
 // ignore-tidy-filelength
 pub use self::fold::{TypeFoldable, TypeFolder, TypeVisitor};
 pub use self::AssocItemContainer::*;