Rollup merge of rust-lang#80763 - petrochenkov:pubusecrate, r=estebank

resolve: Reduce scope of `pub_use_of_private_extern_crate` deprecation lint

This lint was deny-by-default since July 2017, crater showed 7 uses on crates.io back then (rust-lang#42894 (comment)).

Unfortunately, the construction `pub use foo as bar` where `foo` is `extern crate foo;` was used by an older version `bitflags`, so turning it into an error causes too many regressions.
So, this PR reduces the scope of the lint instead of turning it into a hard error, and only turns some more rarely used components of it into errors.

Author: JohnTitor

compiler/rustc_resolve/src/imports.rs

@@ -156,6 +156,21 @@ impl<'a> NameResolution<'a> {
     }
 }
 
+// Reexports of the form `pub use foo as bar;` where `foo` is `extern crate foo;`
+// are permitted for backward-compatibility under a deprecation lint.
+fn pub_use_of_private_extern_crate_hack(import: &Import<'_>, binding: &NameBinding<'_>) -> bool {
+    match (&import.kind, &binding.kind) {
+        (
+            ImportKind::Single { .. },
+            NameBindingKind::Import {
+                import: Import { kind: ImportKind::ExternCrate { .. }, .. },
+                ..
+            },
+        ) => import.vis.get() == ty::Visibility::Public,
+        _ => false,
+    }
+}
+
 impl<'a> Resolver<'a> {
     crate fn resolve_ident_in_module_unadjusted(
         &mut self,
@@ -263,10 +278,7 @@ impl<'a> Resolver<'a> {
                     return Err((Determined, Weak::No));
                 }
             }
-            // `extern crate` are always usable for backwards compatibility, see issue #37020,
-            // remove this together with `PUB_USE_OF_PRIVATE_EXTERN_CRATE`.
-            let usable = this.is_accessible_from(binding.vis, parent_scope.module)
-                || binding.is_extern_crate();
+            let usable = this.is_accessible_from(binding.vis, parent_scope.module);
             if usable { Ok(binding) } else { Err((Determined, Weak::No)) }
         };
 
@@ -309,10 +321,7 @@ impl<'a> Resolver<'a> {
                             }
                         }
 
-                        if !(self.is_accessible_from(binding.vis, parent_scope.module) ||
-                       // Remove this together with `PUB_USE_OF_PRIVATE_EXTERN_CRATE`
-                       (self.last_import_segment && binding.is_extern_crate()))
-                        {
+                        if !self.is_accessible_from(binding.vis, parent_scope.module) {
                             self.privacy_errors.push(PrivacyError {
                                 ident,
                                 binding,
@@ -455,9 +464,8 @@ impl<'a> Resolver<'a> {
         binding: &'a NameBinding<'a>,
         import: &'a Import<'a>,
     ) -> &'a NameBinding<'a> {
-        let vis = if binding.vis.is_at_least(import.vis.get(), self) ||
-                     // cf. `PUB_USE_OF_PRIVATE_EXTERN_CRATE`
-                     !import.is_glob() && binding.is_extern_crate()
+        let vis = if binding.vis.is_at_least(import.vis.get(), self)
+            || pub_use_of_private_extern_crate_hack(import, binding)
         {
             import.vis.get()
         } else {
@@ -1188,7 +1196,7 @@ impl<'a, 'b> ImportResolver<'a, 'b> {
         // All namespaces must be re-exported with extra visibility for an error to occur.
         if !any_successful_reexport {
             let (ns, binding) = reexport_error.unwrap();
-            if ns == TypeNS && binding.is_extern_crate() {
+            if pub_use_of_private_extern_crate_hack(import, binding) {
                 let msg = format!(
                     "extern crate `{}` is private, and cannot be \
                                    re-exported (error E0365), consider declaring with \

src/test/rustdoc/extern-links.rs

@@ -3,7 +3,7 @@
 
 #![crate_name = "foo"]
 
-extern crate extern_links;
+pub extern crate extern_links;
 
 // @!has foo/index.html '//a' 'extern_links'
 #[doc(no_inline)]

src/test/rustdoc/issue-28927.rs

@@ -2,5 +2,5 @@
 // aux-build:issue-28927-1.rs
 // ignore-cross-compile
 
-extern crate issue_28927_1 as inner1;
+pub extern crate issue_28927_1 as inner1;
 pub use inner1 as foo;

src/test/ui/pub/pub-reexport-priv-extern-crate.rs

@@ -1,5 +1,3 @@
-#![allow(unused)]
-
 extern crate core;
 pub use core as reexported_core; //~ ERROR `core` is private, and cannot be re-exported
                                  //~^ WARN this was previously accepted
@@ -9,16 +7,14 @@ mod foo1 {
 }
 
 mod foo2 {
-    use foo1::core; //~ ERROR `core` is private, and cannot be re-exported
-                    //~^ WARN this was previously accepted
+    use foo1::core; //~ ERROR crate import `core` is private
     pub mod bar {
         extern crate core;
     }
 }
 
 mod baz {
-    pub use foo2::bar::core; //~ ERROR `core` is private, and cannot be re-exported
-                             //~^ WARN this was previously accepted
+    pub use foo2::bar::core; //~ ERROR crate import `core` is private
 }
 
 fn main() {}

src/test/ui/pub/pub-reexport-priv-extern-crate.stderr

@@ -1,30 +1,37 @@
-error: extern crate `core` is private, and cannot be re-exported (error E0365), consider declaring with `pub`
-  --> $DIR/pub-reexport-priv-extern-crate.rs:4:9
+error[E0603]: crate import `core` is private
+  --> $DIR/pub-reexport-priv-extern-crate.rs:10:15
    |
-LL | pub use core as reexported_core;
-   |         ^^^^^^^^^^^^^^^^^^^^^^^
+LL |     use foo1::core;
+   |               ^^^^ private crate import
    |
-   = note: `#[deny(pub_use_of_private_extern_crate)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
+note: the crate import `core` is defined here
+  --> $DIR/pub-reexport-priv-extern-crate.rs:6:5
+   |
+LL |     extern crate core;
+   |     ^^^^^^^^^^^^^^^^^^
 
-error: extern crate `core` is private, and cannot be re-exported (error E0365), consider declaring with `pub`
-  --> $DIR/pub-reexport-priv-extern-crate.rs:12:9
+error[E0603]: crate import `core` is private
+  --> $DIR/pub-reexport-priv-extern-crate.rs:17:24
    |
-LL |     use foo1::core;
-   |         ^^^^^^^^^^
+LL |     pub use foo2::bar::core;
+   |                        ^^^^ private crate import
    |
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
+note: the crate import `core` is defined here
+  --> $DIR/pub-reexport-priv-extern-crate.rs:12:9
+   |
+LL |         extern crate core;
+   |         ^^^^^^^^^^^^^^^^^^
 
 error: extern crate `core` is private, and cannot be re-exported (error E0365), consider declaring with `pub`
-  --> $DIR/pub-reexport-priv-extern-crate.rs:20:13
+  --> $DIR/pub-reexport-priv-extern-crate.rs:2:9
    |
-LL |     pub use foo2::bar::core;
-   |             ^^^^^^^^^^^^^^^
+LL | pub use core as reexported_core;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
    |
+   = note: `#[deny(pub_use_of_private_extern_crate)]` on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
 
 error: aborting due to 3 previous errors
 
+For more information about this error, try `rustc --explain E0603`.