diff --git a/src/librustc/lib.rs b/src/librustc/lib.rs
index bdbb1a98ac5..4ac4e3a5a9f 100644
--- a/src/librustc/lib.rs
+++ b/src/librustc/lib.rs
@@ -114,13 +114,7 @@ pub mod metadata;
 
 pub mod driver;
 
-pub mod plugin {
-    pub use self::registry::Registry;
-
-    pub mod registry;
-    pub mod load;
-    pub mod build;
-}
+pub mod plugin;
 
 pub mod util {
     pub mod common;
diff --git a/src/librustc/plugin/build.rs b/src/librustc/plugin/build.rs
index a27632882fc..ad35c4efe11 100644
--- a/src/librustc/plugin/build.rs
+++ b/src/librustc/plugin/build.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! Used by `rustc` when compiling a plugin crate.
+
 use syntax::ast;
 use syntax::attr;
 use syntax::codemap::Span;
@@ -36,7 +38,6 @@ impl Visitor<()> for RegistrarFinder {
 }
 
 /// Find the function marked with `#[plugin_registrar]`, if any.
-/// Used while compiling a crate which defines a registrar.
 pub fn find_plugin_registrar(diagnostic: &diagnostic::SpanHandler,
                              krate: &ast::Crate) -> Option<ast::NodeId> {
     let mut finder = RegistrarFinder { registrars: Vec::new() };
diff --git a/src/librustc/plugin/load.rs b/src/librustc/plugin/load.rs
index c80f8f96c4b..ba50a15a82b 100644
--- a/src/librustc/plugin/load.rs
+++ b/src/librustc/plugin/load.rs
@@ -8,9 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use plugin::registry::PluginRegistrarFun;
+//! Used by `rustc` when loading a plugin.
+
 use driver::session::Session;
 use metadata::creader::PluginMetadataReader;
+use plugin::registry::Registry;
 
 use std::mem;
 use std::os;
@@ -22,14 +24,25 @@ use syntax::visit::Visitor;
 use syntax::ext::expand::ExportedMacros;
 use syntax::attr::AttrMetaMethods;
 
+/// Plugin-related crate metadata.
 pub struct PluginMetadata {
+    /// Source code of macros exported by the crate.
     pub macros: Vec<String>,
+    /// Path to the shared library file.
     pub lib: Option<Path>,
+    /// Symbol name of the plugin registrar function.
     pub registrar_symbol: Option<String>,
 }
 
+/// Pointer to a registrar function.
+pub type PluginRegistrarFun =
+    fn(&mut Registry);
+
+/// Information about loaded plugins.
 pub struct Plugins {
+    /// Source code of exported macros.
     pub macros: Vec<ExportedMacros>,
+    /// Registrars, as function pointers.
     pub registrars: Vec<PluginRegistrarFun>,
 }
 
@@ -52,6 +65,7 @@ impl<'a> PluginLoader<'a> {
     }
 }
 
+/// Read plugin metadata and dynamically load registrar functions.
 pub fn load_plugins(sess: &Session, krate: &ast::Crate) -> Plugins {
     let mut loader = PluginLoader::new(sess);
     visit::walk_crate(&mut loader, krate, ());
diff --git a/src/librustc/plugin/mod.rs b/src/librustc/plugin/mod.rs
new file mode 100644
index 00000000000..fa70ffc7392
--- /dev/null
+++ b/src/librustc/plugin/mod.rs
@@ -0,0 +1,64 @@
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+/*!
+ * Infrastructure for compiler plugins.
+ *
+ * Plugins are Rust libraries which extend the behavior of `rustc`
+ * in various ways.
+ *
+ * Plugin authors will use the `Registry` type re-exported by
+ * this module, along with its methods.  The rest of the module
+ * is for use by `rustc` itself.
+ *
+ * To define a plugin, build a dylib crate with a
+ * `#[plugin_registrar]` function:
+ *
+ * ```rust,ignore
+ * #![crate_id = "myplugin"]
+ * #![crate_type = "dylib"]
+ * #![feature(plugin_registrar)]
+ *
+ * extern crate rustc;
+ *
+ * use rustc::plugin::Registry;
+ *
+ * #[plugin_registrar]
+ * pub fn plugin_registrar(reg: &mut Registry) {
+ *     reg.register_macro("mymacro", expand_mymacro);
+ * }
+ *
+ * fn expand_mymacro(...) {  // details elided
+ * ```
+ *
+ * WARNING: We currently don't check that the registrar function
+ * has the appropriate type!
+ *
+ * To use a plugin while compiling another crate:
+ *
+ * ```rust
+ * #![feature(phase)]
+ *
+ * #[phase(plugin)]
+ * extern crate myplugin;
+ * ```
+ *
+ * If you also need the plugin crate available at runtime, use
+ * `phase(plugin, link)`.
+ *
+ * See `src/test/auxiliary/macro_crate_test.rs` and `src/libfourcc`
+ * for examples of syntax extension plugins.
+ */
+
+pub use self::registry::Registry;
+
+pub mod registry;
+pub mod load;
+pub mod build;
diff --git a/src/librustc/plugin/registry.rs b/src/librustc/plugin/registry.rs
index 6402b116536..f6e37822325 100644
--- a/src/librustc/plugin/registry.rs
+++ b/src/librustc/plugin/registry.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+//! Used by plugin crates to tell `rustc` about the plugins they provide.
+
 use syntax::ext::base::{SyntaxExtension, NamedSyntaxExtension, NormalTT};
 use syntax::ext::base::{IdentTT, ItemDecorator, ItemModifier, BasicMacroExpander};
 use syntax::ext::base::{MacroExpanderFn};
@@ -15,6 +17,14 @@ use syntax::codemap::Span;
 use syntax::parse::token;
 use syntax::ast;
 
+/// Structure used to register plugins.
+///
+/// A plugin registrar function takes an `&mut Registry` and should call
+/// methods to register its plugins.
+///
+/// This struct has public fields and other methods for use by `rustc`
+/// itself. They are not documented here, and plugin authors should
+/// not use them.
 pub struct Registry {
     #[doc(hidden)]
     pub krate_span: Span,
@@ -23,9 +33,6 @@ pub struct Registry {
     pub syntax_exts: Vec<NamedSyntaxExtension>,
 }
 
-pub type PluginRegistrarFun =
-    fn(&mut Registry);
-
 impl Registry {
     #[doc(hidden)]
     pub fn new(krate: &ast::Crate) -> Registry {
@@ -35,6 +42,9 @@ impl Registry {
         }
     }
 
+    /// Register a syntax extension of any kind.
+    ///
+    /// This is the most general hook into `libsyntax`'s expansion behavior.
     pub fn register_syntax_extension(&mut self, name: ast::Name, extension: SyntaxExtension) {
         self.syntax_exts.push((name, match extension {
             NormalTT(ext, _) => NormalTT(ext, Some(self.krate_span)),
@@ -44,6 +54,11 @@ impl Registry {
         }));
     }
 
+    /// Register a macro of the usual kind.
+    ///
+    /// This is a convenience wrapper for `register_syntax_extension`.
+    /// It builds for you a `NormalTT` with a `BasicMacroExpander`,
+    /// and also takes care of interning the macro's name.
     pub fn register_macro(&mut self, name: &str, expander: MacroExpanderFn) {
         self.register_syntax_extension(
             token::intern(name),