Update missing-docs lint to check associated type declarations

[breaking-change]

Fixes #20648

Author: ipetkov

src/librustc/lint/builtin.rs

@@ -1577,6 +1577,14 @@ impl LintPass for MissingDoc {
                                      tm.span, "a type method");
     }
 
+    fn check_trait_method(&mut self, cx: &Context, it: &ast::TraitItem) {
+        if let ast::TraitItem::TypeTraitItem(ref ty) = *it {
+            let assoc_ty = &ty.ty_param;
+            self.check_missing_docs_attrs(cx, Some(assoc_ty.id), &ty.attrs,
+                                          assoc_ty.span, "an associated type");
+        }
+    }
+
     fn check_struct_field(&mut self, cx: &Context, sf: &ast::StructField) {
         if let ast::NamedField(_, vis) = sf.node.kind {
             if vis == ast::Public || self.in_variant {

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

@@ -68,6 +68,19 @@ pub trait D {
     fn dummy(&self) { }
 }
 
+/// dox
+pub trait E {
+    type AssociatedType; //~ ERROR: missing documentation
+    type AssociatedTypeDef = Self; //~ ERROR: missing documentation
+
+    /// dox
+    type DocumentedType;
+    /// dox
+    type DocumentedTypeDef = Self;
+    /// dox
+    fn dummy(&self) {}
+}
+
 impl Foo {
     pub fn foo() {}
     fn bar() {}