Make missing_doc lint check typedefs

Closes #19543

Author: sfackler

src/libcore/fmt/mod.rs

@@ -34,6 +34,7 @@ mod float;
 pub mod rt;
 
 #[experimental = "core and I/O reconciliation may alter this definition"]
+/// The type returned by formatter methods.
 pub type Result = result::Result<(), Error>;
 
 /// The error type which is returned from formatting a message into a stream.

src/librustc/lint/builtin.rs

@@ -1430,6 +1430,7 @@ impl LintPass for MissingDoc {
             ast::ItemEnum(..) => "an enum",
             ast::ItemStruct(..) => "a struct",
             ast::ItemTrait(..) => "a trait",
+            ast::ItemTy(..) => "a type alias",
             _ => return
         };
         self.check_missing_docs_attrs(cx, Some(it.id), it.attrs.as_slice(),

src/test/compile-fail/lint-missing-doc.rs

@@ -17,6 +17,9 @@
 //! Some garbage docs for the crate here
 #![doc="More garbage"]
 
+type Typedef = String;
+pub type PubTypedef = String; //~ ERROR: missing documentation
+
 struct Foo {
     a: int,
     b: int,