make pub_use_of_private_extern_crate show up in future breakage reports

Author: RalfJung

compiler/rustc_lint_defs/src/builtin.rs

@@ -1211,7 +1211,7 @@ declare_lint! {
     Deny,
     "detect public re-exports of private extern crates",
     @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps,
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
         reference: "issue #34537 <https://github.com/rust-lang/rust/issues/34537>",
     };
 }

tests/ui/pub/pub-reexport-priv-extern-crate.stderr

@@ -40,3 +40,18 @@ error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0365, E0603.
 For more information about an error, try `rustc --explain E0365`.
+Future incompatibility report: Future breakage diagnostic:
+error[E0365]: extern crate `core` is private and cannot be re-exported
+  --> $DIR/pub-reexport-priv-extern-crate.rs:2:9
+   |
+LL | pub use core as reexported_core;
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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: `#[deny(pub_use_of_private_extern_crate)]` on by default
+help: consider making the `extern crate` item publicly accessible
+   |
+LL | pub extern crate core;
+   | +++
+