lsp: emit VS-internal HiddenInEditor tag for unnecessary diagnostics

The LSP spec says clients are "allowed to render diagnostics [tagged
Unnecessary] faded out instead of having an error squiggle." Visual
Studio's LSP client, however, only renders the faded "unused" appearance
when a diagnostic carries both DiagnosticTag.Unnecessary AND the
VS-internal VSDiagnosticTags.HiddenInEditor tag; with just Unnecessary
the diagnostic shows as a normal squiggle.

When the client advertises _vs_supportsVisualStudioExtensions, also emit
HiddenInEditor (int32.MaxValue - 6 = 2147483641) so unused variables and
similar diagnostics render faded in Visual Studio. Non-VS clients
continue to receive only the standard LSP tags.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: joj

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.VSDiagnosticTagHiddenInEditor)
+			}
 		}
 		if diagnostic.ReportsDeprecated() && slices.Contains(opts.tagValueSet, lsproto.DiagnosticTagDeprecated) {
 			tags = append(tags, lsproto.DiagnosticTagDeprecated)

internal/lsp/lsproto/lsp.go

@@ -312,4 +312,15 @@ func PreferredMarkupKind(formats []MarkupKind) MarkupKind {
 const (
 	CodeActionKindSourceRemoveUnusedImports CodeActionKind = "source.removeUnusedImports"
 	CodeActionKindSourceSortImports         CodeActionKind = "source.sortImports"
+
+	// VSDiagnosticTagHiddenInEditor is a Visual Studio-specific DiagnosticTag
+	// (see Microsoft.VisualStudio.LanguageServer.Protocol.Extensions.VSDiagnosticTags)
+	// indicating the diagnostic should not be rendered with a squiggle in the
+	// editor. When combined with DiagnosticTagUnnecessary, Visual Studio
+	// renders the affected code as faded-out text (the typical "unused
+	// variable" appearance). VS extension tags use sentinel values near
+	// int32.MaxValue to avoid colliding with future standard DiagnosticTag
+	// additions. Only emit when the client advertises
+	// _vs_supportsVisualStudioExtensions.
+	VSDiagnosticTagHiddenInEditor DiagnosticTag = 2147483641 // int32.MaxValue - 6
 )