gopls/semantic: elide zero-length tokens

vscode complains in the Window channel about zero-length semantic
tokens. It may not matter but it seems cleaner to avoid the error
and not send zero-length semantic tokens.

Fixes: golang/go#65254

Change-Id: I470f8052a6288f5bf4b592794cf6cd54d55b5539
Reviewed-on: https://go-review.googlesource.com/c/tools/+/561155
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Peter Weinberger <pjw@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

gopls/internal/golang/semtok.go

@@ -132,6 +132,9 @@ func (tv *tokenVisitor) visit() {
 }
 
 func (tv *tokenVisitor) token(start token.Pos, leng int, typ semtok.TokenType, mods []string) {
+	if leng <= 0 {
+		return // vscode doesn't like 0-length Tokens
+	}
 	if !start.IsValid() {
 		// This is not worth reporting. TODO(pjw): does it still happen?
 		return

gopls/internal/template/implementations.go

@@ -169,6 +169,9 @@ func SemanticTokens(ctx context.Context, snapshot *cache.Snapshot, spn protocol.
 
 	var items []semtok.Token
 	add := func(line, start, len uint32) {
+		if len == 0 {
+			return // vscode doesn't like 0-length Tokens
+		}
 		// TODO(adonovan): don't ignore the rng restriction, if any.
 		items = append(items, semtok.Token{
 			Line:  line,

gopls/internal/test/integration/misc/semantictokens_test.go

@@ -193,3 +193,47 @@ func bar() {}
 		}
 	})
 }
+
+// Make sure no zero-length tokens occur
+func TestSemantic_65254(t *testing.T) {
+	src := `
+-- go.mod --
+module example.com
+	
+go 1.21
+-- main.go --
+package main
+
+/* a comment with an
+
+empty line
+*/
+
+const bad = `
+
+	src += "`foo" + `
+	` + "bar`"
+	want := []fake.SemanticToken{
+		{Token: "package", TokenType: "keyword"},
+		{Token: "main", TokenType: "namespace"},
+		{Token: "/* a comment with an", TokenType: "comment"},
+		// --- Note that the zero length line does not show up
+		{Token: "empty line", TokenType: "comment"},
+		{Token: "*/", TokenType: "comment"},
+		{Token: "const", TokenType: "keyword"},
+		{Token: "bad", TokenType: "variable", Mod: "definition readonly"},
+		{Token: "`foo", TokenType: "string"},
+		// --- Note the zero length line does not show up
+		{Token: "\tbar`", TokenType: "string"},
+	}
+	WithOptions(
+		Modes(Default),
+		Settings{"semanticTokens": true},
+	).Run(t, src, func(t *testing.T, env *Env) {
+		env.OpenFile("main.go")
+		seen := env.SemanticTokensFull("main.go")
+		if x := cmp.Diff(want, seen); x != "" {
+			t.Errorf("Semantic tokens do not match (-want +got):\n%s", x)
+		}
+	})
+}