3 files changed, 36 insertions, 0 deletions
diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index 8ec06feb5bf..a58caf2667b 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -195,6 +195,9 @@ pub trait Emitter {
 
     fn emit_future_breakage_report(&mut self, _diags: Vec<(FutureBreakage, Diagnostic)>) {}
 
+    /// Emit list of unused externs
+    fn emit_unused_externs(&mut self, _lint_level: &str, _unused_externs: &[&str]) {}
+
     /// Checks if should show explanations about "rustc --explain"
     fn should_show_explain(&self) -> bool {
         true
diff --git a/compiler/rustc_errors/src/json.rs b/compiler/rustc_errors/src/json.rs
index 2bce1ac3c0a..40277006462 100644
--- a/compiler/rustc_errors/src/json.rs
+++ b/compiler/rustc_errors/src/json.rs
@@ -159,6 +159,19 @@ impl Emitter for JsonEmitter {
         }
     }
 
+    fn emit_unused_externs(&mut self, lint_level: &str, unused_externs: &[&str]) {
+        let data = UnusedExterns { lint_level, unused_extern_names: unused_externs };
+        let result = if self.pretty {
+            writeln!(&mut self.dst, "{}", as_pretty_json(&data))
+        } else {
+            writeln!(&mut self.dst, "{}", as_json(&data))
+        }
+        .and_then(|_| self.dst.flush());
+        if let Err(e) = result {
+            panic!("failed to print unused externs: {:?}", e);
+        }
+    }
+
     fn source_map(&self) -> Option<&Lrc<SourceMap>> {
         Some(&self.sm)
     }
@@ -322,6 +335,18 @@ struct FutureIncompatReport {
     future_incompat_report: Vec<FutureBreakageItem>,
 }
 
+// NOTE: Keep this in sync with the equivalent structs in rustdoc's
+// doctest component (as well as cargo).
+// We could unify this struct the one in rustdoc but they have different
+// ownership semantics, so doing so would create wasteful allocations.
+#[derive(Encodable)]
+struct UnusedExterns<'a, 'b, 'c> {
+    /// The severity level of the unused dependencies lint
+    lint_level: &'a str,
+    /// List of unused externs by their names.
+    unused_extern_names: &'b [&'c str],
+}
+
 impl Diagnostic {
     fn from_errors_diagnostic(diag: &crate::Diagnostic, je: &JsonEmitter) -> Diagnostic {
         let sugg = diag.suggestions.iter().map(|sugg| Diagnostic {
diff --git a/compiler/rustc_errors/src/lib.rs b/compiler/rustc_errors/src/lib.rs
index ac7353730ad..0d1f55a6b00 100644
--- a/compiler/rustc_errors/src/lib.rs
+++ b/compiler/rustc_errors/src/lib.rs
@@ -765,6 +765,10 @@ impl Handler {
         self.inner.borrow_mut().emitter.emit_future_breakage_report(diags)
     }
 
+    pub fn emit_unused_externs(&self, lint_level: &str, unused_externs: &[&str]) {
+        self.inner.borrow_mut().emit_unused_externs(lint_level, unused_externs)
+    }
+
     pub fn delay_as_bug(&self, diagnostic: Diagnostic) {
         self.inner.borrow_mut().delay_as_bug(diagnostic)
     }
@@ -839,6 +843,10 @@ impl HandlerInner {
         self.emitter.emit_artifact_notification(path, artifact_type);
     }
 
+    fn emit_unused_externs(&mut self, lint_level: &str, unused_externs: &[&str]) {
+        self.emitter.emit_unused_externs(lint_level, unused_externs);
+    }
+
     fn treat_err_as_bug(&self) -> bool {
         self.flags.treat_err_as_bug.map_or(false, |c| self.err_count() >= c.get())
     }