Move public reexports of private extern crates into a separate lint

This is going to be a hard error while all private-in-public errors from rustc_privacy will be reclassified into lints.

Author: petrochenkov

src/librustc/lint/builtin.rs

@@ -130,6 +130,12 @@ declare_lint! {
     "detect private items in public interfaces not caught by the old implementation"
 }
 
+declare_lint! {
+    pub PUB_USE_OF_PRIVATE_EXTERN_CRATE,
+    Deny,
+    "detect public reexports of private extern crates"
+}
+
 declare_lint! {
     pub INVALID_TYPE_PARAM_DEFAULT,
     Deny,
@@ -230,6 +236,7 @@ impl LintPass for HardwiredLints {
             TRIVIAL_CASTS,
             TRIVIAL_NUMERIC_CASTS,
             PRIVATE_IN_PUBLIC,
+            PUB_USE_OF_PRIVATE_EXTERN_CRATE,
             INVALID_TYPE_PARAM_DEFAULT,
             CONST_ERR,
             RENAMED_AND_REMOVED_LINTS,

src/librustc_lint/lib.rs

@@ -184,6 +184,10 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(PRIVATE_IN_PUBLIC),
             reference: "issue #34537 <https://github.com/rust-lang/rust/issues/34537>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(PUB_USE_OF_PRIVATE_EXTERN_CRATE),
+            reference: "issue #34537 <https://github.com/rust-lang/rust/issues/34537>",
+        },
         FutureIncompatibleInfo {
             id: LintId::of(PATTERNS_IN_FNS_WITHOUT_BODY),
             reference: "issue #35203 <https://github.com/rust-lang/rust/issues/35203>",

src/librustc_resolve/resolve_imports.rs

@@ -18,7 +18,7 @@ use {names_to_string, module_to_string};
 use {resolve_error, ResolutionError};
 
 use rustc::ty;
-use rustc::lint::builtin::PRIVATE_IN_PUBLIC;
+use rustc::lint::builtin::PUB_USE_OF_PRIVATE_EXTERN_CRATE;
 use rustc::hir::def_id::DefId;
 use rustc::hir::def::*;
 use rustc::util::nodemap::FxHashMap;
@@ -296,7 +296,8 @@ impl<'a> Resolver<'a> {
     pub fn import(&self, binding: &'a NameBinding<'a>, directive: &'a ImportDirective<'a>)
                   -> &'a NameBinding<'a> {
         let vis = if binding.pseudo_vis().is_at_least(directive.vis.get(), self) ||
-                     !directive.is_glob() && binding.is_extern_crate() { // c.f. `PRIVATE_IN_PUBLIC`
+                     // c.f. `PUB_USE_OF_PRIVATE_EXTERN_CRATE`
+                     !directive.is_glob() && binding.is_extern_crate() {
             directive.vis.get()
         } else {
             binding.pseudo_vis()
@@ -735,7 +736,8 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
                 let msg = format!("extern crate `{}` is private, and cannot be reexported \
                                    (error E0365), consider declaring with `pub`",
                                    ident);
-                self.session.add_lint(PRIVATE_IN_PUBLIC, directive.id, directive.span, msg);
+                self.session.add_lint(PUB_USE_OF_PRIVATE_EXTERN_CRATE,
+                                      directive.id, directive.span, msg);
             } else if ns == TypeNS {
                 struct_span_err!(self.session, directive.span, E0365,
                                  "`{}` is private, and cannot be reexported", ident)

src/test/compile-fail/pub-reexport-priv-extern-crate.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 #![allow(unused)]
-#![deny(private_in_public)]
 
 extern crate core;
 pub use core as reexported_core; //~ ERROR `core` is private, and cannot be reexported