Make --json unused-extern deny/forbid level messages cause exit with error status

Closes: https://github.com/rust-lang/rust/issues/96068

compiler/rustc_errors/src/lib.rs

@@ -970,7 +970,13 @@ pub fn emit_future_breakage_report(&self, diags: Vec<Diagnostic>) {
     }
 
     pub fn emit_unused_externs(&self, lint_level: &str, unused_externs: &[&str]) {
-        self.inner.borrow_mut().emit_unused_externs(lint_level, unused_externs)
+        let mut inner = self.inner.borrow_mut();
+
+        if lint_level == "deny" || lint_level == "forbid" {
+            inner.bump_err_count();
+        }
+
+        inner.emit_unused_externs(lint_level, unused_externs)
     }
 
     pub fn update_unstable_expectation_id(

src/test/ui/unused-crate-deps/deny-cmdline-json.rs

@@ -1,4 +1,4 @@
-// Check for unused crate dep, no path
+// Check for unused crate dep, json event, deny, expect compile failure
 
 // edition:2018
 // compile-flags: -Dunused-crate-dependencies  -Zunstable-options --json unused-externs --error-format=json

src/test/ui/unused-crate-deps/deny-cmdline.rs

@@ -1,4 +1,4 @@
-// Check for unused crate dep, no path
+// Check for unused crate dep, deny, expect failure
 
 // edition:2018
 // compile-flags: -Dunused-crate-dependencies

src/test/ui/unused-crate-deps/warn-cmdline-json.rs

@@ -1,4 +1,4 @@
-// Check for unused crate dep, no path
+// Check for unused crate dep, warn, json event, expect pass
 
 // edition:2018
 // check-pass