repo_description: An interpreter for Rust's mid-level intermediate representation
code: //! Main evaluator loop and setting up the initial stack frame.
  
  use std::ffi::{OsStr, OsString};
  use std::num::NonZeroI32;
  use std::panic::{self, AssertUnwindSafe};
  use std::path::PathBuf;
  use std::rc::Rc;
  use std::task::Poll;
  use std::{iter, thread};
  
  use rustc_abi::ExternAbi;
  use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  use rustc_errors::FatalErrorMarker;
  use rustc_hir::def::Namespace;
  use rustc_hir::def_id::DefId;
  use rustc_middle::ty::layout::{HasTyCtxt, HasTypingEnv, LayoutCx};
  use rustc_middle::ty::{self, Ty, TyCtxt};
  use rustc_session::config::EntryFnType;
  use rustc_target::spec::Os;
  
  use crate::concurrency::GenmcCtx;
  use crate::concurrency::thread::TlsAllocAction;
  use crate::diagnostics::report_leaks;
  use crate::shims::{global_ctor, tls};
  use crate::*;
  
  #[derive(Copy, Clone, Debug)]
  pub enum MiriEntryFnType {
      MiriStart,
      Rustc(EntryFnType),
  }
  
  /// When the main thread would exit, we will yield to any other thread that is ready to execute.
  /// But we must only do that a finite number of times, or a background thread running `loop {}`
  /// will hang the program.
  const MAIN_THREAD_YIELDS_AT_SHUTDOWN: u32 = 256;
  
  /// Configuration needed to spawn a Miri instance.
  #[derive(Clone)]
  pub struct MiriConfig {
      /// The host environment snapshot to use as basis for what is provided to the interpreted program.
      /// (This is still subject to isolation as well as `forwarded_env_vars`.)
      pub env: Vec<(OsString, OsString)>,
      /// Determine if validity checking is enabled.
      pub validation: ValidationMode,
      /// Determines if Stacked Borrows or Tree Borrows is enabled.
      pub borrow_tracker: Option<BorrowTrackerMethod>,
      /// Controls alignment checking.
      pub check_alignment: AlignmentCheck,
      /// Action for an op requiring communication with the host.
      pub isolated_op: IsolatedOp,
      /// Determines if memory leaks should be ignored.
      pub ignore_leaks: bool,
      /// Environment variables that should always be forwarded from the host.
      pub forwarded_env_vars: Vec<String>,
      /// Additional environment variables that should be set in the interpreted program.
      pub set_env_vars: FxHashMap<String, String>,
      /// Command-line arguments passed to the interpreted program.
      pub args: Vec<String>,
      /// The seed to use when non-determinism or randomness are required (e.g. ptr-to-int cast, `getrandom()`).
      pub seed: Option<u64>,
      /// The stacked borrows pointer ids to report about.
      pub tracked_pointer_tags: FxHashSet<BorTag>,
      /// The allocation ids to report about.
      pub tracked_alloc_ids: FxHashSet<AllocId>,
      /// For the tracked alloc ids, also report read/write accesses.
      pub track_alloc_accesses: bool,
      /// Determine if data race detection should be enabled.
      pub data_race_detector: bool,
      /// Determine if weak memory emulation should be enabled. Requires data race detection to be enabled.
      pub weak_memory_emulation: bool,
      /// Determine if we are running in GenMC mode and with which settings. In GenMC mode, Miri will explore multiple concurrent executions of the given program.
      pub genmc_config: Option<GenmcConfig>,
      /// Track when an outdated (weak memory) load happens.
      pub track_outdated_loads: bool,
      /// Rate of spurious failures for compare_exchange_weak atomic operations,
      /// between 0.0 and 1.0, defaulting to 0.8 (80% chance of failure).
      pub cmpxchg_weak_failure_rate: f64,
      /// If `Some`, enable the `measureme` profiler, writing results to a file
      /// with the specified prefix.
      pub measureme_out: Option<String>,
      /// Which style to use for printing backtraces.
      pub backtrace_style: BacktraceStyle,
      /// Which provenance to use for int2ptr casts.
      pub provenance_mode: ProvenanceMode,
      /// Whether to ignore any output by the program. This is helpful when debugging miri
      /// as its messages don't get intermingled with the program messages.
      pub mute_stdout_stderr: bool,
      /// The probability of the active thread being preempted at the end of each basic block.
      pub preemption_rate: f64,
      /// Report the current instruction being executed every N basic blocks.
      pub report_progress: Option<u32>,
      /// The location of the shared object files to load when calling external functions
      pub native_lib: Vec<PathBuf>,
      /// Whether to enable the new native lib tracing system.
      pub native_lib_enable_tracing: bool,
      /// Run a garbage collector for BorTags every N basic blocks.
      pub gc_interval: u32,
      /// The number of CPUs to be reported by miri.
      pub num_cpus: u32,
      /// Requires Miri to emulate pages of a certain size.
      pub page_size: Option<u64>,
      /// Whether to collect a backtrace when each allocation is created, just in case it leaks.
      pub collect_leak_backtraces: bool,
      /// Probability for address reuse.
      pub address_reuse_rate: f64,
      /// Probability for address reuse across threads.
      pub address_reuse_cross_thread_rate: f64,
      /// Round Robin scheduling with no preemption.
      pub fixed_scheduling: bool,
      /// Always prefer the intrinsic fallback body over the native Miri implementation.
      pub force_intrinsic_fallback: bool,
      /// Whether floating-point operations can behave non-deterministically.
      pub float_nondet: bool,
      /// Whether floating-point operations can have a non-deterministic rounding error.
      pub float_rounding_error: FloatRoundingErrorMode,
      /// Whether Miri artificially introduces short reads/writes on file descriptors.
      pub short_fd_operations: bool,
      /// A list of crates that are considered user-relevant.
      pub user_relevant_crates: Vec<String>,
  }
  
  impl Default for MiriConfig {
      fn default() -> MiriConfig {
          MiriConfig {
              env: vec![],
              validation: ValidationMode::Shallow,
              borrow_tracker: Some(BorrowTrackerMethod::StackedBorrows),
              check_alignment: AlignmentCheck::Int,
              isolated_op: IsolatedOp::Reject(RejectOpWith::Abort),
              ignore_leaks: false,
              forwarded_env_vars: vec![],
              set_env_vars: FxHashMap::default(),
              args: vec![],
              seed: None,
              tracked_pointer_tags: FxHashSet::default(),
              tracked_alloc_ids: FxHashSet::default(),
              track_alloc_accesses: false,
              data_race_detector: true,
              weak_memory_emulation: true,
              genmc_config: None,
              track_outdated_loads: false,
              cmpxchg_weak_failure_rate: 0.8, // 80%
              measureme_out: None,
              backtrace_style: BacktraceStyle::Short,
              provenance_mode: ProvenanceMode::Default,
              mute_stdout_stderr: false,
              preemption_rate: 0.01, // 1%
              report_progress: None,
              native_lib: vec![],
              native_lib_enable_tracing: false,
              gc_interval: 10_000,
              num_cpus: 1,
              page_size: None,
              collect_leak_backtraces: true,
              address_reuse_rate: 0.5,
              address_reuse_cross_thread_rate: 0.1,
              fixed_scheduling: false,
              force_intrinsic_fallback: false,
              float_nondet: true,
              float_rounding_error: FloatRoundingErrorMode::Random,
              short_fd_operations: true,
              user_relevant_crates: vec![],
          }
      }
  }
  
  /// The state of the main thread. Implementation detail of `on_main_stack_empty`.
  #[derive(Debug)]
  enum MainThreadState<'tcx> {
      GlobalCtors {
          ctor_state: global_ctor::GlobalCtorState<'tcx>,
          /// The main function to call.
          entry_id: DefId,
          entry_type: MiriEntryFnType,
          /// Arguments passed to `main`.
          argc: ImmTy<'tcx>,
          argv: ImmTy<'tcx>,
      },
      Running,
      TlsDtors(tls::TlsDtorsState<'tcx>),
      Yield {
          remaining: u32,
      },
      Done,
  }
  
  impl<'tcx> MainThreadState<'tcx> {
      fn on_main_stack_empty(
          &mut self,
          this: &mut MiriInterpCx<'tcx>,
      ) -> InterpResult<'tcx, Poll<()>> {
          use MainThreadState::*;
          match self {
              GlobalCtors { ctor_state, entry_id, entry_type, argc, argv } => {
                  match ctor_state.on_stack_empty(this)? {
                      Poll::Pending => {} // just keep going
                      Poll::Ready(()) => {
                          call_main(this, *entry_id, *entry_type, argc.clone(), argv.clone())?;
                          *self = Running;
                      }
                  }
              }
              Running => {
                  *self = TlsDtors(Default::default());
              }
              TlsDtors(state) =>
                  match state.on_stack_empty(this)? {
                      Poll::Pending => {} // just keep going
                      Poll::Ready(()) => {
                          if this.machine.data_race.as_genmc_ref().is_some() {
                              // In GenMC mode, we don't yield at the end of the main thread.
                              // Instead, the `GenmcCtx` will ensure that unfinished threads get a chance to run at this point.
                              *self = Done;
                          } else {
                              // Give background threads a chance to finish by yielding the main thread a
                              // couple of times -- but only if we would also preempt threads randomly.
                              if this.machine.preemption_rate > 0.0 {
                                  // There is a non-zero chance they will yield back to us often enough to
                                  // make Miri terminate eventually.
                                  *self = Yield { remaining: MAIN_THREAD_YIELDS_AT_SHUTDOWN };
                              } else {
                                  // The other threads did not get preempted, so no need to yield back to
                                  // them.
                                  *self = Done;
                              }
                          }
                      }
                  },
              Yield { remaining } =>
                  match remaining.checked_sub(1) {
                      None => *self = Done,
                      Some(new_remaining) => {
                          *remaining = new_remaining;
                          this.yield_active_thread();
                      }
                  },
              Done => {
                  // Figure out exit code.
                  let ret_place = this.machine.main_fn_ret_place.clone().unwrap();
                  let exit_code = this.read_target_isize(&ret_place)?;
                  // Rust uses `isize` but the underlying type of an exit code is `i32`.
                  // Do a saturating cast.
                  let exit_code = i32::try_from(exit_code).unwrap_or(if exit_code >= 0 {
                      i32::MAX
                  } else {
                      i32::MIN
                  });
                  // Deal with our thread-local memory. We do *not* want to actually free it, instead we consider TLS
                  // to be like a global `static`, so that all memory reached by it is considered to "not leak".
                  this.terminate_active_thread(TlsAllocAction::Leak)?;
  
                  // In GenMC mode, we do not immediately stop execution on main thread exit.
                  if let Some(genmc_ctx) = this.machine.data_race.as_genmc_ref() {
                      // If there's no error, execution will continue (on another thread).
                      genmc_ctx.handle_exit(
                          ThreadId::MAIN_THREAD,
                          exit_code,
                          crate::concurrency::ExitType::MainThreadFinish,
                      )?;
                  } else {
                      // Stop interpreter loop.
                      throw_machine_stop!(TerminationInfo::Exit {
                          code: exit_code,
                          leak_check: true
                      });
                  }
              }
          }
          interp_ok(Poll::Pending)
      }
  }
  
  /// Returns a freshly created `InterpCx`.
  /// Public because this is also used by `priroda`.
  pub fn create_ecx<'tcx>(
      tcx: TyCtxt<'tcx>,
      entry_id: DefId,
      entry_type: MiriEntryFnType,
      config: &MiriConfig,
      genmc_ctx: Option<Rc<GenmcCtx>>,
  ) -> InterpResult<'tcx, InterpCx<'tcx, MiriMachine<'tcx>>> {
      let typing_env = ty::TypingEnv::fully_monomorphized();
      let layout_cx = LayoutCx::new(tcx, typing_env);
      let mut ecx = InterpCx::new(
          tcx,
          rustc_span::DUMMY_SP,
          typing_env,
          MiriMachine::new(config, layout_cx, genmc_ctx),
      );
  
      // Make sure we have MIR. We check MIR for some stable monomorphic function in libcore.
      let sentinel =
          helpers::try_resolve_path(tcx, &["core", "ascii", "escape_default"], Namespace::ValueNS);
      if !matches!(sentinel, Some(s) if tcx.is_mir_available(s.def.def_id())) {
          tcx.dcx().fatal(
              "the current sysroot was built without `-Zalways-encode-mir`, or libcore seems missing.\n\
              Note that directly invoking the `miri` binary is not supported; please use `cargo miri` instead."
          );
      }
  
      // Compute argc and argv from `config.args`.
      let argc =
          ImmTy::from_int(i64::try_from(config.args.len()).unwrap(), ecx.machine.layouts.isize);
      let argv = {
          // Put each argument in memory, collect pointers.
          let mut argvs = Vec::<Immediate<Provenance>>::with_capacity(config.args.len());
          for arg in config.args.iter() {
              // Make space for `0` terminator.
              let size = u64::try_from(arg.len()).unwrap().strict_add(1);
              let arg_type = Ty::new_array(tcx, tcx.types.u8, size);
              let arg_place =
                  ecx.allocate(ecx.layout_of(arg_type)?, MiriMemoryKind::Machine.into())?;
              ecx.write_os_str_to_c_str(OsStr::new(arg), arg_place.ptr(), size)?;
              ecx.mark_immutable(&arg_place);
              argvs.push(arg_place.to_ref(&ecx));
          }
          // Make an array with all these pointers, in the Miri memory.
          let u8_ptr_type = Ty::new_imm_ptr(tcx, tcx.types.u8);
          let u8_ptr_ptr_type = Ty::new_imm_ptr(tcx, u8_ptr_type);
          let argvs_layout =
              ecx.layout_of(Ty::new_array(tcx, u8_ptr_type, u64::try_from(argvs.len()).unwrap()))?;
          let argvs_place = ecx.allocate(argvs_layout, MiriMemoryKind::Machine.into())?;
          for (arg, idx) in argvs.into_iter().zip(0..) {
              let place = ecx.project_index(&argvs_place, idx)?;
              ecx.write_immediate(arg, &place)?;
          }
          ecx.mark_immutable(&argvs_place);
          // Store `argc` and `argv` for macOS `_NSGetArg{c,v}`, and for the GC to see them.
          {
              let argc_place =
                  ecx.allocate(ecx.machine.layouts.isize, MiriMemoryKind::Machine.into())?;
              ecx.write_immediate(*argc, &argc_place)?;
              ecx.mark_immutable(&argc_place);
              ecx.machine.argc = Some(argc_place.ptr());
  
              let argv_place =
                  ecx.allocate(ecx.layout_of(u8_ptr_ptr_type)?, MiriMemoryKind::Machine.into())?;
              ecx.write_pointer(argvs_place.ptr(), &argv_place)?;
              ecx.mark_immutable(&argv_place);
              ecx.machine.argv = Some(argv_place.ptr());
          }
          // Store command line as UTF-16 for Windows `GetCommandLineW`.
          if tcx.sess.target.os == Os::Windows {
              // Construct a command string with all the arguments.
              let cmd_utf16: Vec<u16> = args_to_utf16_command_string(config.args.iter());
  
              let cmd_type =
                  Ty::new_array(tcx, tcx.types.u16, u64::try_from(cmd_utf16.len()).unwrap());
              let cmd_place =
                  ecx.allocate(ecx.layout_of(cmd_type)?, MiriMemoryKind::Machine.into())?;
              ecx.machine.cmd_line = Some(cmd_place.ptr());
              // Store the UTF-16 string. We just allocated so we know the bounds are fine.
              for (&c, idx) in cmd_utf16.iter().zip(0..) {
                  let place = ecx.project_index(&cmd_place, idx)?;
                  ecx.write_scalar(Scalar::from_u16(c), &place)?;
              }
              ecx.mark_immutable(&cmd_place);
          }
          let imm = argvs_place.to_ref(&ecx);
          let layout = ecx.layout_of(u8_ptr_ptr_type)?;
          ImmTy::from_immediate(imm, layout)
      };
  
      // Some parts of initialization require a full `InterpCx`.
      MiriMachine::late_init(&mut ecx, config, {
          let mut main_thread_state = MainThreadState::GlobalCtors {
              entry_id,
              entry_type,
              argc,
              argv,
              ctor_state: global_ctor::GlobalCtorState::default(),
          };
  
          // Cannot capture anything GC-relevant here.
          // `argc` and `argv` *are* GC_relevant, but they also get stored in `machine.argc` and
          // `machine.argv` so we are good.
          Box::new(move |m| main_thread_state.on_main_stack_empty(m))
      })?;
  
      interp_ok(ecx)
  }
  
  // Call the entry function.
  fn call_main<'tcx>(
      ecx: &mut MiriInterpCx<'tcx>,
      entry_id: DefId,
      entry_type: MiriEntryFnType,
      argc: ImmTy<'tcx>,
      argv: ImmTy<'tcx>,
  ) -> InterpResult<'tcx, ()> {
      let tcx = ecx.tcx();
  
      // Setup first stack frame.
      let entry_instance = ty::Instance::mono(tcx, entry_id);
  
      // Return place (in static memory so that it does not count as leak).
      let ret_place = ecx.allocate(ecx.machine.layouts.isize, MiriMemoryKind::Machine.into())?;
      ecx.machine.main_fn_ret_place = Some(ret_place.clone());
  
      // Call start function.
      match entry_type {
          MiriEntryFnType::Rustc(EntryFnType::Main { .. }) => {
              let start_id = tcx.lang_items().start_fn().unwrap_or_else(|| {
                  tcx.dcx().fatal("could not find start lang item");
              });
              let main_ret_ty = tcx.fn_sig(entry_id).no_bound_vars().unwrap().output();
              let main_ret_ty = main_ret_ty.no_bound_vars().unwrap();
              let start_instance = ty::Instance::try_resolve(
                  tcx,
                  ecx.typing_env(),
                  start_id,
                  tcx.mk_args(&[ty::GenericArg::from(main_ret_ty)]),
              )
              .unwrap()
              .unwrap();
  
              let main_ptr = ecx.fn_ptr(FnVal::Instance(entry_instance));
  
              // Always using DEFAULT is okay since we don't support signals in Miri anyway.
              // (This means we are effectively ignoring `-Zon-broken-pipe`.)
              let sigpipe = rustc_session::config::sigpipe::DEFAULT;
  
              ecx.call_function(
                  start_instance,
                  ExternAbi::Rust,
                  &[
                      ImmTy::from_scalar(
                          Scalar::from_pointer(main_ptr, ecx),
                          // FIXME use a proper fn ptr type
                          ecx.machine.layouts.const_raw_ptr,
                      ),
                      argc,
                      argv,
                      ImmTy::from_uint(sigpipe, ecx.machine.layouts.u8),
                  ],
                  Some(&ret_place),
                  ReturnContinuation::Stop { cleanup: true },
              )?;
          }
          MiriEntryFnType::MiriStart => {
              ecx.call_function(
                  entry_instance,
                  ExternAbi::Rust,
                  &[argc, argv],
                  Some(&ret_place),
                  ReturnContinuation::Stop { cleanup: true },
              )?;
          }
      }
  
      interp_ok(())
  }
  
  /// Evaluates the entry function specified by `entry_id`.
  /// Returns `Some(return_code)` if program execution completed.
  /// Returns `None` if an evaluation error occurred.
  pub fn eval_entry<'tcx>(
      tcx: TyCtxt<'tcx>,
      entry_id: DefId,
      entry_type: MiriEntryFnType,
      config: &MiriConfig,
      genmc_ctx: Option<Rc<GenmcCtx>>,
  ) -> Result<(), NonZeroI32> {
      // Copy setting before we move `config`.
      let ignore_leaks = config.ignore_leaks;
  
      let mut ecx = match create_ecx(tcx, entry_id, entry_type, config, genmc_ctx).report_err() {
          Ok(v) => v,
          Err(err) => {
              let (kind, backtrace) = err.into_parts();
              backtrace.print_backtrace();
              panic!("Miri initialization error: {kind:?}")
          }
      };
  
      // Perform the main execution.
      let res: thread::Result<InterpResult<'_, !>> =
          panic::catch_unwind(AssertUnwindSafe(|| ecx.run_threads()));
      let res = res.unwrap_or_else(|panic_payload| {
          // rustc "handles" some errors by unwinding with FatalErrorMarker
          // (after emitting suitable diagnostics), so do not treat those as ICEs.
          if !panic_payload.is::<FatalErrorMarker>() {
              ecx.handle_ice();
          }
          panic::resume_unwind(panic_payload)
      });
      // Obtain the result of the execution. This is always an `Err`, but that doesn't necessarily
      // indicate an error.
      let Err(res) = res.report_err();
  
      // Error reporting: if we survive all checks, we return the exit code the program gave us.
      'miri_error: {
          // Show diagnostic, if any.
          let Some((return_code, leak_check)) = report_result(&ecx, res) else {
              break 'miri_error;
          };
  
          // If we get here there was no fatal error -- yet.
          // Possibly check for memory leaks.
          if leak_check && !ignore_leaks {
              // Check for thread leaks.
              if !ecx.have_all_terminated() {
                  tcx.dcx()
                      .err("the main thread terminated without waiting for all remaining threads");
                  tcx.dcx().note("set `MIRIFLAGS=-Zmiri-ignore-leaks` to disable this check");
                  break 'miri_error;
              }
              // Check for memory leaks.
              info!("Additional static roots: {:?}", ecx.machine.static_roots);
              let leaks = ecx.take_leaked_allocations(|ecx| &ecx.machine.static_roots);
              if !leaks.is_empty() {
                  report_leaks(&ecx, leaks);
                  tcx.dcx().note("set `MIRIFLAGS=-Zmiri-ignore-leaks` to disable this check");
                  // Ignore the provided return code - let the reported error
                  // determine the return code.
                  break 'miri_error;
              }
          }
  
          // The interpreter has not reported an error.
          // (There could still be errors in the session if there are other interpreters.)
          return match NonZeroI32::new(return_code) {
              None => Ok(()),
              Some(return_code) => Err(return_code),
          };
      }
  
      // The interpreter reported an error.
      assert!(tcx.dcx().has_errors().is_some());
      Err(NonZeroI32::new(rustc_driver::EXIT_FAILURE).unwrap())
  }
  
  /// Turns an array of arguments into a Windows command line string.
  ///
  /// The string will be UTF-16 encoded and NUL terminated.
  ///
  /// Panics if the zeroth argument contains the `"` character because doublequotes
  /// in `argv[0]` cannot be encoded using the standard command line parsing rules.
  ///
  /// Further reading:
  /// * [Parsing C++ command-line arguments](https://docs.microsoft.com/en-us/cpp/cpp/main-function-command-line-args?view=msvc-160#parsing-c-command-line-arguments)
  /// * [The C/C++ Parameter Parsing Rules](https://daviddeley.com/autohotkey/parameters/parameters.htm#WINCRULES)
  fn args_to_utf16_command_string<I, T>(mut args: I) -> Vec<u16>
  where
      I: Iterator<Item = T>,
      T: AsRef<str>,
  {
      // Parse argv[0]. Slashes aren't escaped. Literal double quotes are not allowed.
      let mut cmd = {
          let Some(arg0) = args.next() else {
              return vec![0];
          };
          let arg0 = arg0.as_ref();
          if arg0.contains('"') {
              panic!("argv[0] cannot contain a doublequote (\") character");
          } else {
              // Always surround argv[0] with quotes.
              let mut s = String::new();
              s.push('"');
              s.push_str(arg0);
              s.push('"');
              s
          }
      };
  
      // Build the other arguments.
      for arg in args {
          let arg = arg.as_ref();
          cmd.push(' ');
          if arg.is_empty() {
              cmd.push_str("\"\"");
          } else if !arg.bytes().any(|c| matches!(c, b'"' | b'\t' | b' ')) {
              // No quote, tab, or space -- no escaping required.
              cmd.push_str(arg);
          } else {
              // Spaces and tabs are escaped by surrounding them in quotes.
              // Quotes are themselves escaped by using backslashes when in a
              // quoted block.
              // Backslashes only need to be escaped when one or more are directly
              // followed by a quote. Otherwise they are taken literally.
  
              cmd.push('"');
              let mut chars = arg.chars().peekable();
              loop {
                  let mut nslashes = 0;
                  while let Some(&'\\') = chars.peek() {
                      chars.next();
                      nslashes += 1;
                  }
  
                  match chars.next() {
                      Some('"') => {
                          cmd.extend(iter::repeat_n('\\', nslashes * 2 + 1));
                          cmd.push('"');
                      }
                      Some(c) => {
                          cmd.extend(iter::repeat_n('\\', nslashes));
                          cmd.push(c);
                      }
                      None => {
                          cmd.extend(iter::repeat_n('\\', nslashes * 2));
                          break;
                      }
                  }
              }
              cmd.push('"');
          }
      }
  
      if cmd.contains('\0') {
          panic!("interior null in command line arguments");
      }
      cmd.encode_utf16().chain(iter::once(0)).collect()
  }
  
  #[cfg(test)]
  mod tests {
      use super::*;
      #[test]
      #[should_panic(expected = "argv[0] cannot contain a doublequote (\") character")]
      fn windows_argv0_panic_on_quote() {
          args_to_utf16_command_string(["\""].iter());
      }
      #[test]
      fn windows_argv0_no_escape() {
          // Ensure that a trailing backslash in argv[0] is not escaped.
          let cmd = String::from_utf16_lossy(&args_to_utf16_command_string(
              [r"C:\Program Files\", "arg1", "arg 2", "arg \" 3"].iter(),
          ));
          assert_eq!(cmd.trim_end_matches('\0'), r#""C:\Program Files\" arg1 "arg 2" "arg \" 3""#);
      }
  }
  