Fix panic on directories with only _test.go files

internal/index/scip.go

@@ -206,6 +206,15 @@ func indexVisitPackages(
 			slog.Debug("Visiting package", "path", pkg.PkgPath)
 			visitors.VisitPackageSyntax(opts.ModuleRoot, pkg, pathToDocuments, globalSymbols)
 
+			// A package may have no parsed source files (e.g. a directory
+			// that only contains *_test.go files belonging to an external
+			// "*_test" package). There is nothing to attach package symbol
+			// information or occurrences to, so skip it.
+			if len(pkg.Syntax) == 0 {
+				atomic.AddUint64(&count, 1)
+				continue
+			}
+
 			pkgSymbol, _ := globalSymbols.GetPkgSymbol(pkg)
 
 			symInfo := &scip.SymbolInformation{

internal/testdata/snapshots/input/pr256/README.md

@@ -0,0 +1,13 @@
+# Directories with only `_test.go` files
+
+Reproduction for issue
+[#255](https://github.com/scip-code/scip-go/issues/255).
+
+A directory whose only Go file is a `*_test.go` file belonging to an
+external `*_test` package would cause `scip-go` to panic with
+`index out of range [0] with length 0` because the synthetic regular
+package returned by `packages.Load` had an empty `Syntax` slice.
+
+The fix skips package-symbol attachment for packages with no parsed
+source files; the external `*_test` package still has its own non-empty
+entry and is indexed normally.

internal/testdata/snapshots/input/pr256/go.mod

@@ -0,0 +1,3 @@
+module sg/pr256
+
+go 1.23

internal/testdata/snapshots/input/pr256/pr256_test.go

@@ -0,0 +1,5 @@
+package pr256_test
+
+import "testing"
+
+func TestComment(t *testing.T) {}

internal/testdata/snapshots/output/pr256/pr256_test.go

@@ -0,0 +1,26 @@
+  package pr256_test
+//        ^^^^^^^^^^ definition 0.1.test `sg/pr256_test`/
+//                   kind Package
+//                   display_name pr256_test
+//                   signature_documentation
+//                   > package pr256_test
+
+  import "testing"
+//        ^^^^^^^ reference github.com/golang/go/src go1.22 testing/
+
+//⌄ enclosing_range_start 0.1.test `sg/pr256_test`/TestComment().
+  func TestComment(t *testing.T) {}
+//     ^^^^^^^^^^^ definition 0.1.test `sg/pr256_test`/TestComment().
+//                 kind Function
+//                 display_name TestComment
+//                 signature_documentation
+//                 > func TestComment(t *testing.T)
+//                 ^ definition local 0
+//                   kind Variable
+//                   display_name t
+//                   signature_documentation
+//                   > var t *T
+//                    ^^^^^^^ reference github.com/golang/go/src go1.22 testing/
+//                            ^ reference github.com/golang/go/src go1.22 testing/T#
+//                                ⌃ enclosing_range_end 0.1.test `sg/pr256_test`/TestComment().
+