feat(code-action): add quickfix to remove unused let bindings

libnixf/src/Sema/VariableLookup.cpp

@@ -125,6 +125,39 @@ void VariableLookupAnalysis::emitEnvLivenessWarning(
       Diagnostic &D = Diags.emplace_back(Kind, Def->syntax()->range());
       D << Name;
       D.tag(DiagnosticTag::Faded);
+
+      // Add fix for unused let bindings
+      if (Def->source() == Definition::DS_Let) {
+        const Node *LetNode = NewEnv->syntax();
+        if (LetNode && LetNode->kind() == Node::NK_ExprLet) {
+          const auto &Let = static_cast<const ExprLet &>(*LetNode);
+          if (Let.binds()) {
+            for (const auto &BindNode : Let.binds()->bindings()) {
+              // Skip non-Binding nodes (Inherit handled separately)
+              if (BindNode->kind() != Node::NK_Binding)
+                continue;
+
+              const auto &Bind = static_cast<const Binding &>(*BindNode);
+              const auto &PathNames = Bind.path().names();
+              if (PathNames.empty())
+                continue;
+
+              const auto &FirstName = PathNames[0];
+              if (!FirstName)
+                continue;
+
+              // Match by comparing first attrname range with Def->syntax()
+              // range
+              if (D.range().lCur() == FirstName->range().lCur() &&
+                  D.range().rCur() == FirstName->range().rCur()) {
+                D.fix("remove unused binding")
+                    .edit(TextEdit::mkRemoval(Bind.range()));
+                break;
+              }
+            }
+          }
+        }
+      }
     }
   }
 }

nixd/lib/Controller/CodeAction.cpp

@@ -47,6 +47,16 @@ void Controller::onCodeAction(const lspserver::CodeActionParams &Params,
         if (!Range.overlap(DRange))
           continue;
 
+        // Determine if this diagnostic's fixes should be preferred
+        bool IsPreferred = false;
+        switch (D.kind()) {
+        case nixf::Diagnostic::DK_UnusedDefLet:
+          IsPreferred = true;
+          break;
+        default:
+          break;
+        }
+
         // Add fixes.
         for (const nixf::Fix &F : D.fixes()) {
           std::vector<TextEdit> Edits;
@@ -64,6 +74,7 @@ void Controller::onCodeAction(const lspserver::CodeActionParams &Params,
           Actions.emplace_back(CodeAction{
               .title = F.message(),
               .kind = std::string(CodeAction::QUICKFIX_KIND),
+              .isPreferred = IsPreferred,
               .edit = std::move(WE),
           });
         }

nixd/tools/nixd/test/code-action/attr-name/unquote-let-invalid.md

@@ -56,12 +56,15 @@ let "123" = 1; in x
 }
 ```
 
-Invalid identifiers (starting with digit) should return empty result
+Invalid identifiers (starting with digit) should NOT offer unquote action.
+However, a quickfix for unused binding may still be offered.
 
 ```
      CHECK:   "id": 2,
 CHECK-NEXT:   "jsonrpc": "2.0",
-CHECK-NEXT:   "result": []
+CHECK-NEXT:   "result": [
+CHECK-NOT:   "title": "unquote attribute name"
+     CHECK:   "title": "remove unused binding"
 ```
 
 ```json

nixd/tools/nixd/test/code-action/attr-name/unquote-let-keyword.md

@@ -56,12 +56,15 @@ let "true" = 1; in x
 }
 ```
 
-Keywords should return empty result (no unquote action available)
+Keywords should NOT offer unquote action.
+However, a quickfix for unused binding may still be offered.
 
 ```
      CHECK:   "id": 2,
 CHECK-NEXT:   "jsonrpc": "2.0",
-CHECK-NEXT:   "result": []
+CHECK-NEXT:   "result": [
+CHECK-NOT:   "title": "unquote attribute name"
+     CHECK:   "title": "remove unused binding"
 ```
 
 ```json

nixd/tools/nixd/test/code-action/unused-def/all-unused.md

@@ -0,0 +1,78 @@
+# RUN: nixd --lit-test < %s | FileCheck %s
+
+Test `remove unused binding` action when all bindings are unused.
+
+<-- initialize(0)
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":0,
+   "method":"initialize",
+   "params":{
+      "processId":123,
+      "rootPath":"",
+      "capabilities":{
+      },
+      "trace":"off"
+   }
+}
+```
+
+
+<-- textDocument/didOpen
+
+```nix file:///all-unused.nix
+let x = 1; y = 2; in 3
+```
+
+<-- textDocument/codeAction(2)
+
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":2,
+   "method":"textDocument/codeAction",
+   "params":{
+      "textDocument":{
+         "uri":"file:///all-unused.nix"
+      },
+      "range":{
+         "start":{
+            "line": 0,
+            "character": 4
+         },
+         "end":{
+            "line":0,
+            "character": 5
+         }
+      },
+      "context":{
+         "diagnostics":[],
+         "triggerKind":2
+      }
+   }
+}
+```
+
+The action should offer to remove `x = 1;`.
+
+```
+     CHECK: "id": 2,
+     CHECK: "newText": "",
+     CHECK: "range":
+     CHECK:   "end":
+     CHECK:     "character": 10,
+     CHECK:     "line": 0
+     CHECK:   "start":
+     CHECK:     "character": 4,
+     CHECK:     "line": 0
+     CHECK: "isPreferred": true,
+     CHECK: "kind": "quickfix",
+     CHECK: "title": "remove unused binding"
+```
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```

nixd/tools/nixd/test/code-action/unused-def/basic.md

@@ -0,0 +1,78 @@
+# RUN: nixd --lit-test < %s | FileCheck %s
+
+Test basic `remove unused binding` action for unused let binding.
+
+<-- initialize(0)
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":0,
+   "method":"initialize",
+   "params":{
+      "processId":123,
+      "rootPath":"",
+      "capabilities":{
+      },
+      "trace":"off"
+   }
+}
+```
+
+
+<-- textDocument/didOpen
+
+```nix file:///unused-let.nix
+let unused = 1; in 2
+```
+
+<-- textDocument/codeAction(2)
+
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":2,
+   "method":"textDocument/codeAction",
+   "params":{
+      "textDocument":{
+         "uri":"file:///unused-let.nix"
+      },
+      "range":{
+         "start":{
+            "line": 0,
+            "character": 4
+         },
+         "end":{
+            "line":0,
+            "character": 10
+         }
+      },
+      "context":{
+         "diagnostics":[],
+         "triggerKind":2
+      }
+   }
+}
+```
+
+The action should remove `unused = 1;` from the let expression.
+
+```
+     CHECK: "id": 2,
+     CHECK: "newText": "",
+     CHECK: "range":
+     CHECK:   "end":
+     CHECK:     "character": 15,
+     CHECK:     "line": 0
+     CHECK:   "start":
+     CHECK:     "character": 4,
+     CHECK:     "line": 0
+     CHECK: "isPreferred": true,
+     CHECK: "kind": "quickfix",
+     CHECK: "title": "remove unused binding"
+```
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```

nixd/tools/nixd/test/code-action/unused-def/multiline.md

@@ -0,0 +1,79 @@
+# RUN: nixd --lit-test < %s | FileCheck %s
+
+Test `remove unused binding` action with multiline let expression.
+
+<-- initialize(0)
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":0,
+   "method":"initialize",
+   "params":{
+      "processId":123,
+      "rootPath":"",
+      "capabilities":{
+      },
+      "trace":"off"
+   }
+}
+```
+
+
+<-- textDocument/didOpen
+
+```nix file:///multiline.nix
+let
+  unused = 1;
+in
+  2
+```
+
+<-- textDocument/codeAction(2)
+
+
+```json
+{
+   "jsonrpc":"2.0",
+   "id":2,
+   "method":"textDocument/codeAction",
+   "params":{
+      "textDocument":{
+         "uri":"file:///multiline.nix"
+      },
+      "range":{
+         "start":{
+            "line": 1,
+            "character": 2
+         },
+         "end":{
+            "line":1,
+            "character": 8
+         }
+      },
+      "context":{
+         "diagnostics":[],
+         "triggerKind":2
+      }
+   }
+}
+```
+
+The action should remove the entire binding line.
+
+```
+     CHECK: "id": 2,
+     CHECK: "newText": "",
+     CHECK: "range":
+     CHECK:   "end":
+     CHECK:     "line": 1
+     CHECK:   "start":
+     CHECK:     "line": 1
+     CHECK: "isPreferred": true,
+     CHECK: "kind": "quickfix",
+     CHECK: "title": "remove unused binding"
+```
+
+```json
+{"jsonrpc":"2.0","method":"exit"}
+```