Implement compatibility lint for legacy constructor visibilities

src/librustc/lint/builtin.rs

@@ -223,6 +223,12 @@
     "detects names that resolve to ambiguous glob imports with RFC 1560"
 }
 
+declare_lint! {
+    pub LEGACY_CONSTRUCTOR_VISIBILITY,
+    Deny,
+    "detects use of struct constructors that would be invisible with new visibility rules"
+}
+
 declare_lint! {
     pub DEPRECATED,
     Warn,
@@ -271,6 +277,7 @@ fn get_lints(&self) -> LintArray {
             EXTRA_REQUIREMENT_IN_IMPL,
             LEGACY_DIRECTORY_OWNERSHIP,
             LEGACY_IMPORTS,
+            LEGACY_CONSTRUCTOR_VISIBILITY,
             DEPRECATED
         )
     }

src/librustc_lint/lib.rs

@@ -240,6 +240,10 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
             id: LintId::of(LEGACY_IMPORTS),
             reference: "issue #38260 <https://github.com/rust-lang/rust/issues/38260>",
         },
+        FutureIncompatibleInfo {
+            id: LintId::of(LEGACY_CONSTRUCTOR_VISIBILITY),
+            reference: "issue #39207 <https://github.com/rust-lang/rust/issues/39207>",
+        },
         ]);
 
     // Register renamed and removed lints

src/librustc_resolve/build_reduced_graph.rs

@@ -345,6 +345,9 @@ fn build_reduced_graph_for_item(&mut self, item: &Item, expansion: Mark) {
                     let ctor_def = Def::StructCtor(self.definitions.local_def_id(struct_def.id()),
                                                    CtorKind::from_ast(struct_def));
                     self.define(parent, ident, ValueNS, (ctor_def, ctor_vis, sp, expansion));
+                    if !ctor_vis.is_at_least(vis, &*self) {
+                        self.legacy_ctor_visibilities.insert(def.def_id(), (ctor_def, ctor_vis));
+                    }
                 }
             }
 

src/librustc_resolve/lib.rs

@@ -45,7 +45,7 @@
 use rustc::hir::def_id::{CrateNum, CRATE_DEF_INDEX, LOCAL_CRATE, DefId};
 use rustc::ty;
 use rustc::hir::{Freevar, FreevarMap, TraitCandidate, TraitMap, GlobMap};
-use rustc::util::nodemap::{NodeMap, NodeSet, FxHashMap, FxHashSet};
+use rustc::util::nodemap::{NodeMap, NodeSet, FxHashMap, FxHashSet, DefIdMap};
 
 use syntax::ext::hygiene::{Mark, SyntaxContext};
 use syntax::ast::{self, Name, NodeId, Ident, SpannedIdent, FloatTy, IntTy, UintTy};
@@ -1131,6 +1131,9 @@ pub struct Resolver<'a> {
     warned_proc_macros: FxHashSet<Name>,
 
     potentially_unused_imports: Vec<&'a ImportDirective<'a>>,
+
+    // Auxiliary map used only for reporting `legacy_constructor_visibility` lint.
+    legacy_ctor_visibilities: DefIdMap<(Def, ty::Visibility)>,
 }
 
 pub struct ResolverArenas<'a> {
@@ -1310,6 +1313,7 @@ pub fn new(session: &'a Session,
             proc_macro_enabled: features.proc_macro,
             warned_proc_macros: FxHashSet(),
             potentially_unused_imports: Vec::new(),
+            legacy_ctor_visibilities: DefIdMap(),
         }
     }
 
@@ -2235,7 +2239,23 @@ fn smart_resolve_path_fragment(&mut self,
                 if is_expected(resolution.base_def) || resolution.base_def == Def::Err {
                     resolution
                 } else {
-                    report_errors(self, Some(resolution.base_def))
+                    // Add a temporary hack to smooth the transition to new struct ctor
+                    // visibility rules. See #38932 for more details.
+                    let mut res = None;
+                    if let Def::Struct(def_id) = resolution.base_def {
+                        if let Some((ctor_def, ctor_vis))
+                                = self.legacy_ctor_visibilities.get(&def_id).cloned() {
+                            if is_expected(ctor_def) && self.is_accessible(ctor_vis) {
+                                let lint = lint::builtin::LEGACY_CONSTRUCTOR_VISIBILITY;
+                                self.session.add_lint(lint, id, span,
+                                    "private struct constructors are not usable through \
+                                     reexports in outer modules".to_string());
+                                res = Some(PathResolution::new(ctor_def));
+                            }
+                        }
+                    }
+
+                    res.unwrap_or_else(|| report_errors(self, Some(resolution.base_def)))
                 }
             }
             Some(resolution) if source.defer_to_typeck() => {

src/test/compile-fail/privacy/legacy-ctor-visibility.rs

+// Copyright 2017 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.
+
+#![allow(unused)]
+
+use m::S;
+
+mod m {
+    pub struct S(u8);
+
+    mod n {
+        use S;
+        fn f() {
+            S(10);
+            //~^ ERROR private struct constructors are not usable through reexports in outer modules
+            //~| WARN this was previously accepted
+        }
+    }
+}
+
+fn main() {}