lsp: emit VS-internal HiddenInEditor tag for unnecessary diagnostics

internal/ls/lsconv/converters.go

@@ -300,9 +300,16 @@ func diagnosticToLSP(ctx context.Context, converters *Converters, diagnostic *as
 
 	var tags []lsproto.DiagnosticTag
 	if len(opts.tagValueSet) > 0 && (diagnostic.ReportsUnnecessary() || diagnostic.ReportsDeprecated()) {
-		tags = make([]lsproto.DiagnosticTag, 0, 2)
+		tags = make([]lsproto.DiagnosticTag, 0, 3)
 		if diagnostic.ReportsUnnecessary() && slices.Contains(opts.tagValueSet, lsproto.DiagnosticTagUnnecessary) {
 			tags = append(tags, lsproto.DiagnosticTagUnnecessary)
+			// Visual Studio's LSP client only renders Unnecessary diagnostics as
+			// faded-out text when they also carry the VS-internal HiddenInEditor
+			// tag; otherwise they appear as a regular squiggle. Emit it so VS
+			// matches the LSP-spec intent of the Unnecessary tag.
+			if opts.visualStudio {
+				tags = append(tags, lsproto.DiagnosticTagVsHiddenInEditor)
+			}
 		}
 		if diagnostic.ReportsDeprecated() && slices.Contains(opts.tagValueSet, lsproto.DiagnosticTagDeprecated) {
 			tags = append(tags, lsproto.DiagnosticTagDeprecated)

internal/lsp/lsproto/_generate/generate.mts

@@ -1112,6 +1112,22 @@ function patchAndPreprocessModel() {
         (reg as any).isRegistrationOnly = !allRequestMethods.has(reg.registrationMethod);
     }
 
+    // Add Visual Studio-specific DiagnosticTag values.
+    // These are VS-internal extensions not part of the LSP spec. They use sentinel
+    // values near int32.MaxValue to avoid collisions with future standard additions.
+    // Servers should only emit them when the client advertises
+    // _vs_supportsVisualStudioExtensions.
+    const diagnosticTagEnum = model.enumerations.find(e => e.name === "DiagnosticTag");
+    if (diagnosticTagEnum) {
+        diagnosticTagEnum.values.push(
+            {
+                name: "VSHiddenInEditor",
+                value: 2147483641, // int32.MaxValue - 6
+                documentation: "Visual Studio-specific tag: the diagnostic is not rendered with a squiggle in the editor. When combined with Unnecessary, Visual Studio renders the code as faded-out text rather than a squiggle.",
+            },
+        );
+    }
+
     // Merge LSPErrorCodes into ErrorCodes and remove LSPErrorCodes
     const errorCodesEnum = model.enumerations.find(e => e.name === "ErrorCodes");
     const lspErrorCodesEnum = model.enumerations.find(e => e.name === "LSPErrorCodes");