lint: handle private traits better

Due to a long-standing conservative approach to trait exports, all traits are
considered exported. However, the missing_docs lint uses the export map to
determine if something is public and ought to have documentation. This commit
modifies the lint to check if traits are private before emitting the warning.

Closes #11592

src/librustc_lint/builtin.rs

@@ -39,7 +39,7 @@ use util::ppaux::{ty_to_string};
 use util::nodemap::{FnvHashMap, NodeSet};
 use lint::{Level, Context, LintPass, LintArray, Lint};
 
-use std::collections::BitSet;
+use std::collections::{HashSet, BitSet};
 use std::collections::hash_map::Entry::{Occupied, Vacant};
 use std::num::SignedInt;
 use std::{cmp, slice};
@@ -1437,6 +1437,9 @@ pub struct MissingDoc {
     /// Stack of whether #[doc(hidden)] is set
     /// at each level which has lint attributes.
     doc_hidden_stack: Vec<bool>,
+
+    /// Private traits or trait items that leaked through. Don't check their methods.
+    private_traits: HashSet<ast::NodeId>,
 }
 
 impl MissingDoc {
@@ -1445,6 +1448,7 @@ impl MissingDoc {
             struct_def_stack: vec!(),
             in_variant: false,
             doc_hidden_stack: vec!(false),
+            private_traits: HashSet::new(),
         }
     }
 
@@ -1531,18 +1535,46 @@ impl LintPass for MissingDoc {
             ast::ItemMod(..) => "a module",
             ast::ItemEnum(..) => "an enum",
             ast::ItemStruct(..) => "a struct",
-            ast::ItemTrait(..) => "a trait",
+            ast::ItemTrait(_, _, _, ref items) => {
+                // Issue #11592, traits are always considered exported, even when private.
+                if it.vis == ast::Visibility::Inherited {
+                    self.private_traits.insert(it.id);
+                    for itm in items {
+                        self.private_traits.insert(itm.id);
+                    }
+                    return
+                }
+                "a trait"
+            },
             ast::ItemTy(..) => "a type alias",
+            ast::ItemImpl(_, _, _, Some(ref trait_ref), _, ref impl_items) => {
+                // If the trait is private, add the impl items to private_traits so they don't get
+                // reported for missing docs.
+                let real_trait = ty::trait_ref_to_def_id(cx.tcx, trait_ref);
+                match cx.tcx.map.find(real_trait.node) {
+                    Some(ast_map::NodeItem(item)) => if item.vis == ast::Visibility::Inherited {
+                        for itm in impl_items {
+                            self.private_traits.insert(itm.id);
+                        }
+                    },
+                    _ => { }
+                }
+                return
+            },
             _ => return
         };
+
         self.check_missing_docs_attrs(cx, Some(it.id), &it.attrs, it.span, desc);
     }
 
     fn check_trait_item(&mut self, cx: &Context, trait_item: &ast::TraitItem) {
+        if self.private_traits.contains(&trait_item.id) { return }
+
         let desc = match trait_item.node {
             ast::MethodTraitItem(..) => "a trait method",
             ast::TypeTraitItem(..) => "an associated type"
         };
+
         self.check_missing_docs_attrs(cx, Some(trait_item.id),
                                       &trait_item.attrs,
                                       trait_item.span, desc);

src/test/run-pass/issue-11592.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 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.
+
+//! Ensure the private trait Bar isn't complained about.
+
+#![deny(missing_docs)]
+
+mod foo {
+    trait Bar { fn bar(&self) { } }
+    impl Bar for i8 { fn bar(&self) { } }
+}
+
+fn main() { }