Add some comments.

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1,3 +1,7 @@
+//! AST pretty printing.
+//!
+//! Note that HIR pretty printing is layered on top of this crate.
+
 mod expr;
 mod item;
 
@@ -256,6 +260,7 @@ impl std::ops::DerefMut for State<'_> {
     }
 }
 
+/// This trait is used for both AST and HIR pretty-printing.
 pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::DerefMut {
     fn comments(&mut self) -> &mut Option<Comments<'a>>;
     fn ann_post(&mut self, ident: Ident);

compiler/rustc_hir_pretty/src/lib.rs

@@ -1,3 +1,6 @@
+//! HIR pretty-printing is layered on top of AST pretty-printing. A number of
+//! the definitions in this file have equivalents in `rustc_ast_pretty`.
+
 #![recursion_limit = "256"]
 #![deny(rustc::untranslatable_diagnostic)]
 #![deny(rustc::diagnostic_outside_of_impl)]