src/vs/workbench/services/configuration/browser/configurationService.ts/WorkspaceService/join
class WorkspaceService:	private async handleWillChangeWorkspaceFolders(changes: IWorkspaceFoldersChangeEvent, fromCache: boolean): Promise<void> {
		const joiners: Promise<void>[] = [];
		this._onWillChangeWorkspaceFolders.fire({
			join(updateWorkspaceTrustStatePromise) {
				joiners.push(updateWorkspaceTrustStatePromise);
			},
			changes,
			fromCache
		});
		try { await Promises.settled(joiners); } catch (error) { /* Ignore */ }
	}