Rollup merge of #127656 - RalfJung:pub_use_of_private_extern_crate, r=petrochenkov

make pub_use_of_private_extern_crate show up in cargo's future breakage reports

This has been a lint for many years.

However, turns out that outright removing it right now would lead to [tons of crater regressions](#127656 (comment)) due to crates depending on an ancient version of `bitflags`. So for now this PR just makes this future-compat lint show up in cargo's reports, so people are warned when they use a dependency that is affected by this.

r? `@petrochenkov`

Author: matthiaskrgr

compiler/rustc_lint_defs/src/builtin.rs

@@ -1203,16 +1203,16 @@ declare_lint! {
     /// This was historically allowed, but is not the intended behavior
     /// according to the visibility rules. This is a [future-incompatible]
     /// lint to transition this to a hard error in the future. See [issue
-    /// #34537] for more details.
+    /// #127909] for more details.
     ///
-    /// [issue #34537]: https://github.com/rust-lang/rust/issues/34537
+    /// [issue #127909]: https://github.com/rust-lang/rust/issues/127909
     /// [future-incompatible]: ../index.md#future-incompatible-lints
     pub PUB_USE_OF_PRIVATE_EXTERN_CRATE,
     Deny,
     "detect public re-exports of private extern crates",
     @future_incompatible = FutureIncompatibleInfo {
-        reason: FutureIncompatibilityReason::FutureReleaseErrorDontReportInDeps,
-        reference: "issue #34537 <https://github.com/rust-lang/rust/issues/34537>",
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
+        reference: "issue #127909 <https://github.com/rust-lang/rust/issues/127909>",
     };
 }
 

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

@@ -29,7 +29,7 @@ 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: for more information, see issue #127909 <https://github.com/rust-lang/rust/issues/127909>
    = note: `#[deny(pub_use_of_private_extern_crate)]` on by default
 help: consider making the `extern crate` item publicly accessible
    |
@@ -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 #127909 <https://github.com/rust-lang/rust/issues/127909>
+   = 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;
+   | +++
+