Linting public reexport of private dependencies

compiler/rustc_resolve/messages.ftl

@@ -160,6 +160,8 @@ resolve_forward_declared_generic_param =
 resolve_found_an_item_configured_out =
     found an item that was configured out
 
+resolve_from_private_dep_in_public_reexport =
+    {$kind} `{$descr}` from private dependency '{$krate}' is reexported
 resolve_generic_arguments_in_macro_path =
     generic arguments in macro path
 

compiler/rustc_resolve/src/imports.rs

@@ -10,6 +10,7 @@ use rustc_errors::codes::*;
 use rustc_errors::{Applicability, MultiSpan, pluralize, struct_span_code_err};
 use rustc_hir::def::{self, DefKind, PartialRes};
 use rustc_hir::def_id::DefId;
+use rustc_macros::Diagnostic;
 use rustc_middle::metadata::{ModChild, Reexport};
 use rustc_middle::{span_bug, ty};
 use rustc_session::lint::BuiltinLintDiag;
@@ -40,6 +41,14 @@ use crate::{
 
 type Res = def::Res<NodeId>;
 
+#[derive(Diagnostic)]
+#[diag(resolve_from_private_dep_in_public_reexport)]
+struct FromPrivateDependencyInPublicInterface<'a> {
+    pub kind: &'a str,
+    pub descr: &'a str,
+    pub krate: Symbol,
+}
+
 /// Contains data for specific kinds of imports.
 #[derive(Clone)]
 pub(crate) enum ImportKind<'ra> {
@@ -546,6 +555,41 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             self.finalize_resolutions_in(*module);
         }
 
+        for (_module_def_id, module_children) in
+            self.tcx.with_stable_hashing_context(|hcx| self.module_children.to_sorted(&hcx, false))
+        {
+            for import in module_children {
+                if !import.vis.is_public() {
+                    continue;
+                }
+
+                let def::Res::<_>::Def(reexported_kind, reexported_def_id) = import.res else {
+                    continue;
+                };
+
+                if let DefKind::Ctor(..) = reexported_kind {
+                    continue;
+                }
+
+                if reexported_def_id.is_local() {
+                    continue;
+                }
+
+                if !self.tcx.is_private_dep(reexported_def_id.krate) {
+                    continue;
+                }
+
+                self.dcx()
+                    .create_warn(FromPrivateDependencyInPublicInterface {
+                        kind: import.res.descr(),
+                        descr: import.ident.as_str(),
+                        krate: self.tcx.crate_name(reexported_def_id.krate),
+                    })
+                    .with_span(import.ident.span)
+                    .emit();
+            }
+        }
+
         let mut seen_spans = FxHashSet::default();
         let mut errors = vec![];
         let mut prev_root_id: NodeId = NodeId::ZERO;

library/std/Cargo.toml

@@ -23,7 +23,7 @@ unwind = { path = "../unwind" }
 hashbrown = { version = "0.15", default-features = false, features = [
     'rustc-dep-of-std',
 ] }
-std_detect = { path = "../stdarch/crates/std_detect", default-features = false, features = [
+std_detect = { path = "../stdarch/crates/std_detect", public = true, default-features = false, features = [
     'rustc-dep-of-std',
 ] }
 

tests/ui/privacy/pub-priv-dep/pub-priv1.rs

@@ -9,10 +9,10 @@
 #![deny(exported_private_dependencies)]
 
 // This crate is a private dependency
-// FIXME: This should trigger.
 pub extern crate priv_dep;
+//~^ WARN crate `priv_dep` from private dependency 'priv_dep' is reexported
 // This crate is a public dependency
-extern crate pub_dep;
+pub extern crate pub_dep;
 // This crate is a private dependency
 extern crate pm;
 
@@ -91,16 +91,16 @@ pub struct AllowedPrivType {
     pub allowed: OtherType,
 }
 
-// FIXME: This should trigger.
 pub use priv_dep::m;
-// FIXME: This should trigger.
+//~^ WARN macro `m` from private dependency 'priv_dep' is reexported
 pub use pm::fn_like;
-// FIXME: This should trigger.
+//~^ WARN macro `fn_like` from private dependency 'pm' is reexported
 pub use pm::PmDerive;
-// FIXME: This should trigger.
+//~^ WARN macro `PmDerive` from private dependency 'pm' is reexported
 pub use pm::pm_attr;
+//~^ WARN macro `pm_attr` from private dependency 'pm' is reexported
 
-// FIXME: This should trigger.
 pub use priv_dep::E::V1;
+//~^ WARN variant `V1` from private dependency 'priv_dep' is reexported
 
 fn main() {}

tests/ui/privacy/pub-priv-dep/pub-priv1.stderr

@@ -1,3 +1,39 @@
+warning: crate `priv_dep` from private dependency 'priv_dep' is reexported
+  --> $DIR/pub-priv1.rs:12:18
+   |
+LL | pub extern crate priv_dep;
+   |                  ^^^^^^^^
+
+warning: macro `m` from private dependency 'priv_dep' is reexported
+  --> $DIR/pub-priv1.rs:94:19
+   |
+LL | pub use priv_dep::m;
+   |                   ^
+
+warning: macro `fn_like` from private dependency 'pm' is reexported
+  --> $DIR/pub-priv1.rs:96:13
+   |
+LL | pub use pm::fn_like;
+   |             ^^^^^^^
+
+warning: derive macro `PmDerive` from private dependency 'pm' is reexported
+  --> $DIR/pub-priv1.rs:98:13
+   |
+LL | pub use pm::PmDerive;
+   |             ^^^^^^^^
+
+warning: attribute macro `pm_attr` from private dependency 'pm' is reexported
+  --> $DIR/pub-priv1.rs:100:13
+   |
+LL | pub use pm::pm_attr;
+   |             ^^^^^^^
+
+warning: variant `V1` from private dependency 'priv_dep' is reexported
+  --> $DIR/pub-priv1.rs:103:22
+   |
+LL | pub use priv_dep::E::V1;
+   |                      ^^
+
 error: type `OtherType` from private dependency 'priv_dep' in public interface
   --> $DIR/pub-priv1.rs:29:5
    |
@@ -90,5 +126,5 @@ LL | impl PubTraitOnPrivate for OtherType {}
    |
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 14 previous errors
+error: aborting due to 14 previous errors; 6 warnings emitted