internal/template: return available semantic tokens even on template error

Fixes golang/go#50801

Change-Id: Ic5c541d6244bd56b90bee204be7c2b2fdfa90264
Reviewed-on: https://go-review.googlesource.com/c/tools/+/381014
Run-TryBot: Peter Weinberger <pjw@google.com>
Trust: Peter Weinberger <pjw@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

gopls/internal/regtest/template/template_test.go

@@ -33,6 +33,7 @@ Hello {{}} <-- missing body
 		EditorConfig{
 			Settings: map[string]interface{}{
 				"templateExtensions": []string{"tmpl"},
+				"semanticTokens":     true,
 			},
 		},
 	).Run(t, files, func(t *testing.T, env *Env) {
@@ -46,6 +47,17 @@ Hello {{}} <-- missing body
 		if d[0].Source != "template" {
 			t.Errorf("expected Source 'template', got %q", d[0].Source)
 		}
+		// issue 50801 (even broken templates could return some semantic tokens)
+		var p protocol.SemanticTokensParams
+		p.TextDocument.URI = env.Sandbox.Workdir.URI("hello.tmpl")
+		toks, err := env.Editor.Server.SemanticTokensFull(env.Ctx, &p)
+		if err != nil {
+			t.Errorf("semantic token failed: %v", err)
+		}
+		if toks == nil || len(toks.Data) == 0 {
+			t.Errorf("got no semantic tokens")
+		}
+
 		env.WriteWorkspaceFile("hello.tmpl", "{{range .Planets}}\nHello {{.}}\n{{end}}")
 		env.Await(EmptyDiagnostics("hello.tmpl"))
 	})

internal/lsp/fake/editor.go

@@ -304,6 +304,14 @@ func (e *Editor) initialize(ctx context.Context, workspaceFolders []string) erro
 	}
 
 	params.Capabilities.TextDocument.Completion.CompletionItem.SnippetSupport = true
+	params.Capabilities.TextDocument.SemanticTokens.Requests.Full = true
+	// copied from lsp/semantic.go to avoid import cycle in tests
+	params.Capabilities.TextDocument.SemanticTokens.TokenTypes = []string{
+		"namespace", "type", "class", "enum", "interface",
+		"struct", "typeParameter", "parameter", "variable", "property", "enumMember",
+		"event", "function", "method", "macro", "keyword", "modifier", "comment",
+		"string", "number", "regexp", "operator",
+	}
 
 	// This is a bit of a hack, since the fake editor doesn't actually support
 	// watching changed files that match a specific glob pattern. However, the

internal/lsp/template/implementations.go

@@ -157,9 +157,7 @@ func SemanticTokens(ctx context.Context, snapshot source.Snapshot, spn span.URI,
 		return nil, err
 	}
 	p := parseBuffer(buf)
-	if p.ParseErr != nil {
-		return nil, p.ParseErr
-	}
+
 	for _, t := range p.Tokens() {
 		if t.Multiline {
 			la, ca := p.LineCol(t.Start)