Allow dynamic files without external project and also use file names starting with ^ as dynamic file

Fixes #21204

src/harness/unittests/tsserverProjectSystem.ts

@@ -2846,6 +2846,45 @@ namespace ts.projectSystem {
             const options = project.getCompilerOptions();
             assert.equal(options.outDir, "C:/a/b", "");
         });
+
+        it("dynamic file without external project", () => {
+            const file: FileOrFolder = {
+                path: "^walkThroughSnippet:/Users/UserName/projects/someProject/out/someFile#1.js",
+                content: "var x = 10;"
+            };
+            const host = createServerHost([libFile], { useCaseSensitiveFileNames: true });
+            const projectService = createProjectService(host);
+            projectService.setCompilerOptionsForInferredProjects({
+                module: ModuleKind.CommonJS,
+                allowJs: true,
+                allowSyntheticDefaultImports: true,
+                allowNonTsExtensions: true
+            });
+            projectService.openClientFile(file.path, "var x = 10;");
+
+            projectService.checkNumberOfProjects({ inferredProjects: 1 });
+            const project = projectService.inferredProjects[0];
+            checkProjectRootFiles(project, [file.path]);
+            checkProjectActualFiles(project, [file.path, libFile.path]);
+
+            assert.strictEqual(projectService.getDefaultProjectForFile(server.toNormalizedPath(file.path), /*ensureProject*/ true), project);
+            const indexOfX = file.content.indexOf("x");
+            assert.deepEqual(project.getLanguageService(/*ensureSynchronized*/ true).getQuickInfoAtPosition(file.path, indexOfX), {
+                kind: ScriptElementKind.variableElement,
+                kindModifiers: "",
+                textSpan: { start: indexOfX, length: 1 },
+                displayParts: [
+                    { text: "var", kind: "keyword" },
+                    { text: " ", kind: "space" },
+                    { text: "x", kind: "localName" },
+                    { text: ":", kind: "punctuation" },
+                    { text: " ", kind: "space" },
+                    { text: "number", kind: "keyword" }
+                ],
+                documentation: [],
+                tags: []
+            });
+        });
     });
 
     describe("tsserverProjectSystem Proper errors", () => {

src/server/editorServices.ts

@@ -898,7 +898,7 @@ namespace ts.server {
 
             const project = this.getOrCreateInferredProjectForProjectRootPathIfEnabled(info, projectRootPath) ||
                 this.getOrCreateSingleInferredProjectIfEnabled() ||
-                this.createInferredProject(getDirectoryPath(info.path));
+                this.createInferredProject(info.isDynamic ? this.currentDirectory : getDirectoryPath(info.path));
 
             project.addRoot(info);
             project.updateGraph();
@@ -1655,7 +1655,7 @@ namespace ts.server {
         }
 
         private getOrCreateInferredProjectForProjectRootPathIfEnabled(info: ScriptInfo, projectRootPath: NormalizedPath | undefined): InferredProject | undefined {
-            if (!this.useInferredProjectPerProjectRoot) {
+            if (info.isDynamic || !this.useInferredProjectPerProjectRoot) {
                 return undefined;
             }
 

src/server/scriptInfo.ts

@@ -196,7 +196,7 @@ namespace ts.server {
 
     /*@internal*/
     export function isDynamicFileName(fileName: NormalizedPath) {
-        return getBaseFileName(fileName)[0] === "^";
+        return fileName[0] === "^" || getBaseFileName(fileName)[0] === "^";
     }
 
     export class ScriptInfo {