Language Server: Add Unnecessary diagnostic for <% case %> children (marcoroth#802)

This pull request adds an `Unnecessary` diagnostic for any `children` in
the `ERBCaseNode` and `ERBCaseMatchNode`, since the engine will never
render that content, so it shows up dimmed out in the editor.

<img width="1680" height="620" alt="CleanShot 2025-11-07 at 17 05 11@2x"
src="https://github.com/user-attachments/assets/866b64f4-826e-4e23-b45f-77ed1e43cda9"
/>

<img width="1680" height="620" alt="CleanShot 2025-11-07 at 17 05 04@2x"
src="https://github.com/user-attachments/assets/1ea71980-0dd9-4c61-88a1-910eb98dc45f"
/>

Author: marcoroth

javascript/packages/language-server/src/diagnostics.ts

@@ -1,5 +1,15 @@
 import { TextDocument } from "vscode-languageserver-textdocument"
-import { Connection, Diagnostic } from "vscode-languageserver/node"
+import { Connection, Diagnostic, DiagnosticSeverity, DiagnosticTag } from "vscode-languageserver/node"
+import { Visitor } from "@herb-tools/core"
+
+import type {
+  Node,
+  ERBCaseNode,
+  ERBCaseMatchNode,
+  HTMLTextNode,
+} from "@herb-tools/core"
+
+import { isHTMLTextNode } from "@herb-tools/core"
 
 import { ParserService } from "./parser_service"
 import { LinterService } from "./linter_service"
@@ -36,17 +46,25 @@ export class Diagnostics {
     } else {
       const parseResult = this.parserService.parseDocument(textDocument)
       const lintResult = await this.linterService.lintDocument(textDocument)
+      const unreachableCodeDiagnostics = this.getUnreachableCodeDiagnostics(parseResult.document)
 
       allDiagnostics = [
         ...parseResult.diagnostics,
         ...lintResult.diagnostics,
+        ...unreachableCodeDiagnostics,
       ]
     }
 
     this.diagnostics.set(textDocument, allDiagnostics)
     this.sendDiagnosticsFor(textDocument)
   }
 
+  private getUnreachableCodeDiagnostics(document: Node): Diagnostic[] {
+    const collector = new UnreachableCodeCollector()
+    collector.visit(document)
+    return collector.diagnostics
+  }
+
   async refreshDocument(document: TextDocument) {
     await this.validate(document)
   }
@@ -67,3 +85,48 @@ export class Diagnostics {
     this.diagnostics.delete(textDocument)
   }
 }
+
+export class UnreachableCodeCollector extends Visitor {
+  diagnostics: Diagnostic[] = []
+
+  visitERBCaseNode(node: ERBCaseNode): void {
+    this.checkUnreachableChildren(node.children)
+    this.visitChildNodes(node)
+  }
+
+  visitERBCaseMatchNode(node: ERBCaseMatchNode): void {
+    this.checkUnreachableChildren(node.children)
+    this.visitChildNodes(node)
+  }
+
+  private checkUnreachableChildren(children: Node[]): void {
+    for (const child of children) {
+      if (isHTMLTextNode(child) && child.content.trim() === "") {
+        continue
+      }
+
+      const diagnostic: Diagnostic = {
+        range: {
+          start: {
+            line: this.toZeroBased(child.location.start.line),
+            character: child.location.start.column
+          },
+          end: {
+            line: this.toZeroBased(child.location.end.line),
+            character: child.location.end.column
+          }
+        },
+        message: "Unreachable code: content between case and when/in is never executed",
+        severity: DiagnosticSeverity.Hint,
+        tags: [DiagnosticTag.Unnecessary],
+        source: "Herb Language Server"
+      }
+
+      this.diagnostics.push(diagnostic)
+    }
+  }
+
+  private toZeroBased(line: number): number {
+    return line - 1
+  }
+}

javascript/packages/language-server/test/unreachable_code.test.ts

@@ -0,0 +1,130 @@
+import dedent from "dedent"
+
+import { describe, it, expect, beforeAll } from "vitest"
+import { DiagnosticSeverity, DiagnosticTag } from "vscode-languageserver/node"
+
+import { UnreachableCodeCollector } from "../src/diagnostics"
+import { Herb } from "@herb-tools/node-wasm"
+
+describe("Unreachable Code Diagnostics", () => {
+  beforeAll(async () => {
+    await Herb.load()
+  })
+
+  describe("ERB case statements", () => {
+    it("detects unreachable code between case and when", () => {
+      const content = dedent`
+        <% case abc %>
+          something here that is not renderable
+        <% when String %>
+          actual string
+        <% end %>
+      `
+
+      const parseResult = Herb.parse(content)
+      const collector = new UnreachableCodeCollector()
+      collector.visit(parseResult.value)
+
+      expect(collector.diagnostics.length).toBeGreaterThan(0)
+
+      const diagnostic = collector.diagnostics[0]
+      expect(diagnostic.message).toContain("Unreachable code")
+      expect(diagnostic.severity).toBe(DiagnosticSeverity.Hint)
+      expect(diagnostic.tags).toContain(DiagnosticTag.Unnecessary)
+      expect(diagnostic.source).toBe("Herb Language Server")
+    })
+
+    it("detects unreachable code in case/in statements", () => {
+      const content = dedent`
+        <% case abc %>
+        <% in String %>
+          actual string
+        <% end %>
+      `
+
+      const parseResult = Herb.parse(content)
+      const collector = new UnreachableCodeCollector()
+      collector.visit(parseResult.value)
+
+      expect(collector.diagnostics.length).toBe(0)
+    })
+
+    it("detects unreachable HTML content between case and when", () => {
+      const content = dedent`
+        <% case status %>
+          <div>This will never render</div>
+          <p>Neither will this</p>
+        <% when "active" %>
+          <p>Active</p>
+        <% when "inactive" %>
+          <p>Inactive</p>
+        <% end %>
+      `
+
+      const parseResult = Herb.parse(content)
+      const collector = new UnreachableCodeCollector()
+      collector.visit(parseResult.value)
+
+      expect(collector.diagnostics.length).toBeGreaterThan(0)
+    })
+
+    it("does not report diagnostics for case without unreachable children", () => {
+      const content = dedent`
+        <% case status %>
+        <% when "active" %>
+          <p>Active</p>
+        <% when "inactive" %>
+          <p>Inactive</p>
+        <% else %>
+          <p>Unknown</p>
+        <% end %>
+      `
+
+      const parseResult = Herb.parse(content)
+      const collector = new UnreachableCodeCollector()
+      collector.visit(parseResult.value)
+
+      expect(collector.diagnostics.length).toBe(0)
+    })
+
+    it("detects unreachable code with mixed content", () => {
+      const content = dedent`
+        <% case type %>
+          Some text
+          <%= variable %>
+          <span>HTML</span>
+        <% when :foo %>
+          <p>Foo</p>
+        <% end %>
+      `
+
+      const parseResult = Herb.parse(content)
+      const collector = new UnreachableCodeCollector()
+      collector.visit(parseResult.value)
+
+      expect(collector.diagnostics.length).toBeGreaterThan(0)
+    })
+  })
+
+  describe("nested case statements", () => {
+    it("detects unreachable code in nested case statements", () => {
+      const content = dedent`
+        <% case outer %>
+          unreachable outer
+        <% when "a" %>
+          <% case inner %>
+            unreachable inner
+          <% when "b" %>
+            reachable
+          <% end %>
+        <% end %>
+      `
+
+      const parseResult = Herb.parse(content)
+      const collector = new UnreachableCodeCollector()
+      collector.visit(parseResult.value)
+
+      expect(collector.diagnostics.length).toBeGreaterThanOrEqual(2)
+    })
+  })
+})