Fix unknown extension crash in completions

internal/fourslash/tests/completionsPathUnknownExtension_test.go

@@ -0,0 +1,53 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	. "github.com/microsoft/typescript-go/internal/fourslash/tests/util"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestCompletionsPathUnknownExtension(t *testing.T) {
+	t.Parallel()
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+
+	const content = `// @filename: src/some-file.ruhroh
+/* This is just a test file that needs to exist. */
+
+// @filename: package.json
+{
+    "imports": {
+        "#/*": "./src/*"
+    }
+}
+
+// @filename: src/globals.d.ts
+declare module "*.ruhroh";
+
+// @filename: src/a.mts
+import "#//*$*/"
+
+// @filename: tsconfig.json
+{
+    "compilerOptions": {
+        "module": "preserve",
+        "moduleResolution": "bundler",
+        "rootDir": "src"
+    },
+    "include": ["src"]
+}`
+
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+	f.VerifyCompletions(t, "$", &fourslash.CompletionsExpectedList{
+		IsIncomplete: false,
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &[]string{},
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{},
+		},
+	})
+}

internal/ls/string_completions.go

@@ -1939,10 +1939,8 @@ func kindModifiersFromExtension(extension string) lsutil.ScriptElementKindModifi
 		return lsutil.ScriptElementKindModifierCts
 	case tspath.ExtensionTsBuildInfo:
 		panic(fmt.Sprintf("Extension %v is unsupported.", tspath.ExtensionTsBuildInfo))
-	case "":
-		return lsutil.ScriptElementKindModifierNone
 	default:
-		panic(fmt.Sprintf("Unexpected extension: %v", extension))
+		return lsutil.ScriptElementKindModifierNone
 	}
 }