2 files changed, 52 insertions, 0 deletions

diff --git a/crates/ide/src/diagnostics.rs b/crates/ide/src/diagnostics.rs
index 3307e240b..1fbb7131d 100644
--- a/crates/ide/src/diagnostics.rs
+++ b/crates/ide/src/diagnostics.rs
@@ -5,6 +5,7 @@
 //! original files. So we need to map the ranges.
 
 mod unresolved_module;
+mod unresolved_extern_crate;
 mod missing_fields;
 
 mod fixes;
@@ -229,8 +230,10 @@ pub(crate) fn diagnostics(
 
     let ctx = DiagnosticsContext { config, sema, resolve };
     for diag in diags {
+        #[rustfmt::skip]
         let d = match diag {
             AnyDiagnostic::UnresolvedModule(d) => unresolved_module::unresolved_module(&ctx, &d),
+            AnyDiagnostic::UnresolvedExternCrate(d) => unresolved_extern_crate::unresolved_extern_crate(&ctx, &d),
             AnyDiagnostic::MissingFields(d) => missing_fields::missing_fields(&ctx, &d),
         };
         if let Some(code) = d.code {
diff --git a/crates/ide/src/diagnostics/unresolved_extern_crate.rs b/crates/ide/src/diagnostics/unresolved_extern_crate.rs
new file mode 100644
index 000000000..2ea79c2ee
--- /dev/null
+++ b/crates/ide/src/diagnostics/unresolved_extern_crate.rs
@@ -0,0 +1,49 @@
+use crate::diagnostics::{Diagnostic, DiagnosticsContext};
+
+// Diagnostic: unresolved-extern-crate
+//
+// This diagnostic is triggered if rust-analyzer is unable to discover referred extern crate.
+pub(super) fn unresolved_extern_crate(
+    ctx: &DiagnosticsContext<'_>,
+    d: &hir::UnresolvedExternCrate,
+) -> Diagnostic {
+    Diagnostic::new(
+        "unresolved-extern-crate",
+        "unresolved extern crate",
+        ctx.sema.diagnostics_display_range(d.decl.clone().map(|it| it.into())).range,
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::diagnostics::tests::check_diagnostics;
+
+    #[test]
+    fn unresolved_extern_crate() {
+        check_diagnostics(
+            r#"
+//- /main.rs crate:main deps:core
+extern crate core;
+  extern crate doesnotexist;
+//^^^^^^^^^^^^^^^^^^^^^^^^^^ unresolved extern crate
+//- /lib.rs crate:core
+"#,
+        );
+    }
+
+    #[test]
+    fn extern_crate_self_as() {
+        cov_mark::check!(extern_crate_self_as);
+        check_diagnostics(
+            r#"
+//- /lib.rs
+  extern crate doesnotexist;
+//^^^^^^^^^^^^^^^^^^^^^^^^^^ unresolved extern crate
+// Should not error.
+extern crate self as foo;
+struct Foo;
+use foo::Foo as Bar;
+"#,
+        );
+    }
+}