diff --git a/crates/hir/src/diagnostics.rs b/crates/hir/src/diagnostics.rs
index c18c1c5873..d9ad8db6f7 100644
--- a/crates/hir/src/diagnostics.rs
+++ b/crates/hir/src/diagnostics.rs
@@ -1,6 +1,8 @@
 //! FIXME: write short doc here
 pub use hir_def::diagnostics::{InactiveCode, UnresolvedModule};
-pub use hir_expand::diagnostics::{Diagnostic, DiagnosticSink, DiagnosticSinkBuilder};
+pub use hir_expand::diagnostics::{
+    Diagnostic, DiagnosticCode, DiagnosticSink, DiagnosticSinkBuilder,
+};
 pub use hir_ty::diagnostics::{
     IncorrectCase, MismatchedArgCount, MissingFields, MissingMatchArms, MissingOkInTailExpr,
     NoSuchField,
diff --git a/crates/hir_expand/src/diagnostics.rs b/crates/hir_expand/src/diagnostics.rs
index 78ccc212c8..1043c6aeb5 100644
--- a/crates/hir_expand/src/diagnostics.rs
+++ b/crates/hir_expand/src/diagnostics.rs
@@ -20,7 +20,7 @@ use syntax::SyntaxNodePtr;
 
 use crate::InFile;
 
-#[derive(Copy, Clone, PartialEq)]
+#[derive(Copy, Clone, Debug, PartialEq)]
 pub struct DiagnosticCode(pub &'static str);
 
 impl DiagnosticCode {
diff --git a/crates/ide/src/diagnostics.rs b/crates/ide/src/diagnostics.rs
index 1c7f027632..3df73ed4fa 100644
--- a/crates/ide/src/diagnostics.rs
+++ b/crates/ide/src/diagnostics.rs
@@ -10,7 +10,7 @@ mod field_shorthand;
 use std::cell::RefCell;
 
 use hir::{
-    diagnostics::{Diagnostic as _, DiagnosticSinkBuilder},
+    diagnostics::{Diagnostic as _, DiagnosticCode, DiagnosticSinkBuilder},
     Semantics,
 };
 use ide_db::base_db::SourceDatabase;
@@ -35,15 +35,23 @@ pub struct Diagnostic {
     pub severity: Severity,
     pub fix: Option<Fix>,
     pub unused: bool,
+    pub code: Option<DiagnosticCode>,
 }
 
 impl Diagnostic {
     fn error(range: TextRange, message: String) -> Self {
-        Self { message, range, severity: Severity::Error, fix: None, unused: false }
+        Self { message, range, severity: Severity::Error, fix: None, unused: false, code: None }
     }
 
     fn hint(range: TextRange, message: String) -> Self {
-        Self { message, range, severity: Severity::WeakWarning, fix: None, unused: false }
+        Self {
+            message,
+            range,
+            severity: Severity::WeakWarning,
+            fix: None,
+            unused: false,
+            code: None,
+        }
     }
 
     fn with_fix(self, fix: Option<Fix>) -> Self {
@@ -53,6 +61,10 @@ impl Diagnostic {
     fn with_unused(self, unused: bool) -> Self {
         Self { unused, ..self }
     }
+
+    fn with_code(self, code: Option<DiagnosticCode>) -> Self {
+        Self { code, ..self }
+    }
 }
 
 #[derive(Debug)]
@@ -126,7 +138,8 @@ pub(crate) fn diagnostics(
             // Override severity and mark as unused.
             res.borrow_mut().push(
                 Diagnostic::hint(sema.diagnostics_display_range(d).range, d.message())
-                    .with_unused(true),
+                    .with_unused(true)
+                    .with_code(Some(d.code())),
             );
         })
         // Only collect experimental diagnostics when they're enabled.
@@ -137,8 +150,10 @@ pub(crate) fn diagnostics(
     let mut sink = sink_builder
         // Diagnostics not handled above get no fix and default treatment.
         .build(|d| {
-            res.borrow_mut()
-                .push(Diagnostic::error(sema.diagnostics_display_range(d).range, d.message()));
+            res.borrow_mut().push(
+                Diagnostic::error(sema.diagnostics_display_range(d).range, d.message())
+                    .with_code(Some(d.code())),
+            );
         });
 
     if let Some(m) = sema.to_module_def(file_id) {
@@ -149,11 +164,15 @@ pub(crate) fn diagnostics(
 }
 
 fn diagnostic_with_fix<D: DiagnosticWithFix>(d: &D, sema: &Semantics<RootDatabase>) -> Diagnostic {
-    Diagnostic::error(sema.diagnostics_display_range(d).range, d.message()).with_fix(d.fix(&sema))
+    Diagnostic::error(sema.diagnostics_display_range(d).range, d.message())
+        .with_fix(d.fix(&sema))
+        .with_code(Some(d.code()))
 }
 
 fn warning_with_fix<D: DiagnosticWithFix>(d: &D, sema: &Semantics<RootDatabase>) -> Diagnostic {
-    Diagnostic::hint(sema.diagnostics_display_range(d).range, d.message()).with_fix(d.fix(&sema))
+    Diagnostic::hint(sema.diagnostics_display_range(d).range, d.message())
+        .with_fix(d.fix(&sema))
+        .with_code(Some(d.code()))
 }
 
 fn check_unnecessary_braces_in_use_statement(
@@ -589,6 +608,11 @@ fn test_fn() {
                             },
                         ),
                         unused: false,
+                        code: Some(
+                            DiagnosticCode(
+                                "unresolved-module",
+                            ),
+                        ),
                     },
                 ]
             "#]],
diff --git a/crates/rust-analyzer/src/handlers.rs b/crates/rust-analyzer/src/handlers.rs
index 782797e85f..a1057cc548 100644
--- a/crates/rust-analyzer/src/handlers.rs
+++ b/crates/rust-analyzer/src/handlers.rs
@@ -18,7 +18,7 @@ use lsp_types::{
     CallHierarchyOutgoingCall, CallHierarchyOutgoingCallsParams, CallHierarchyPrepareParams,
     CodeActionKind, CodeLens, Command, CompletionItem, Diagnostic, DiagnosticTag,
     DocumentFormattingParams, DocumentHighlight, DocumentSymbol, FoldingRange, FoldingRangeParams,
-    HoverContents, Location, Position, PrepareRenameResponse, Range, RenameParams,
+    HoverContents, Location, NumberOrString, Position, PrepareRenameResponse, Range, RenameParams,
     SemanticTokensDeltaParams, SemanticTokensFullDeltaResult, SemanticTokensParams,
     SemanticTokensRangeParams, SemanticTokensRangeResult, SemanticTokensResult, SymbolInformation,
     SymbolTag, TextDocumentIdentifier, Url, WorkspaceEdit,
@@ -1127,7 +1127,7 @@ pub(crate) fn publish_diagnostics(
         .map(|d| Diagnostic {
             range: to_proto::range(&line_index, d.range),
             severity: Some(to_proto::diagnostic_severity(d.severity)),
-            code: None,
+            code: d.code.map(|d| d.as_str().to_owned()).map(NumberOrString::String),
             code_description: None,
             source: Some("rust-analyzer".to_string()),
             message: d.message,