1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
//! Main evaluator loop and setting up the initial stack frame.

use std::ffi::{OsStr, OsString};
use std::iter;
use std::panic::{self, AssertUnwindSafe};
use std::path::PathBuf;
use std::task::Poll;
use std::thread;

use crate::concurrency::thread::TlsAllocAction;
use crate::diagnostics::report_leaks;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_hir::def::Namespace;
use rustc_hir::def_id::DefId;
use rustc_middle::ty::{
    self,
    layout::{LayoutCx, LayoutOf},
    Ty, TyCtxt,
};
use rustc_target::spec::abi::Abi;

use rustc_session::config::EntryFnType;

use crate::shims::tls;
use crate::*;

/// 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;

#[derive(Copy, Clone, Debug, PartialEq)]
pub enum AlignmentCheck {
    /// Do not check alignment.
    None,
    /// Check alignment "symbolically", i.e., using only the requested alignment for an allocation and not its real base address.
    Symbolic,
    /// Check alignment on the actual physical integer address.
    Int,
}

#[derive(Copy, Clone, Debug, PartialEq)]
pub enum RejectOpWith {
    /// Isolated op is rejected with an abort of the machine.
    Abort,

    /// If not Abort, miri returns an error for an isolated op.
    /// Following options determine if user should be warned about such error.
    /// Do not print warning about rejected isolated op.
    NoWarning,

    /// Print a warning about rejected isolated op, with backtrace.
    Warning,

    /// Print a warning about rejected isolated op, without backtrace.
    WarningWithoutBacktrace,
}

#[derive(Copy, Clone, Debug, PartialEq)]
pub enum IsolatedOp {
    /// Reject an op requiring communication with the host. By
    /// default, miri rejects the op with an abort. If not, it returns
    /// an error code, and prints a warning about it. Warning levels
    /// are controlled by `RejectOpWith` enum.
    Reject(RejectOpWith),

    /// Execute op requiring communication with the host, i.e. disable isolation.
    Allow,
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum BacktraceStyle {
    /// Prints a terser backtrace which ideally only contains relevant information.
    Short,
    /// Prints a backtrace with all possible information.
    Full,
    /// Prints only the frame that the error occurs in.
    Off,
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum ValidationMode {
    /// Do not perform any kind of validation.
    No,
    /// Validate the interior of the value, but not things behind references.
    Shallow,
    /// Fully recursively validate references.
    Deep,
}

/// 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>,
    /// Whether `core::ptr::Unique` receives special treatment.
    /// If `true` then `Unique` is reborrowed with its own new tag and permission,
    /// otherwise `Unique` is just another raw pointer.
    pub unique_is_unique: bool,
    /// 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,
    /// 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>,
    /// Panic when unsupported functionality is encountered.
    pub panic_on_unsupported: bool,
    /// 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>,
    /// Whether Stacked Borrows and Tree Borrows retagging should recurse into fields of datatypes.
    pub retag_fields: RetagFields,
    /// The location of a shared object file to load when calling external functions
    /// FIXME! consider allowing users to specify paths to multiple files, or to a directory
    pub native_lib: Option<PathBuf>,
    /// 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,
}

impl Default for MiriConfig {
    fn default() -> MiriConfig {
        MiriConfig {
            env: vec![],
            validation: ValidationMode::Shallow,
            borrow_tracker: Some(BorrowTrackerMethod::StackedBorrows),
            unique_is_unique: false,
            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,
            track_outdated_loads: false,
            cmpxchg_weak_failure_rate: 0.8, // 80%
            measureme_out: None,
            panic_on_unsupported: false,
            backtrace_style: BacktraceStyle::Short,
            provenance_mode: ProvenanceMode::Default,
            mute_stdout_stderr: false,
            preemption_rate: 0.01, // 1%
            report_progress: None,
            retag_fields: RetagFields::Yes,
            native_lib: None,
            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,
        }
    }
}

/// The state of the main thread. Implementation detail of `on_main_stack_empty`.
#[derive(Default, Debug)]
enum MainThreadState<'tcx> {
    #[default]
    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 {
            Running => {
                *self = TlsDtors(Default::default());
            }
            TlsDtors(state) =>
                match state.on_stack_empty(this)? {
                    Poll::Pending => {} // just keep going
                    Poll::Ready(()) => {
                        // 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)?;
                // 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)?;
                // Stop interpreter loop.
                throw_machine_stop!(TerminationInfo::Exit { code: exit_code, leak_check: true });
            }
        }
        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: EntryFnType,
    config: &MiriConfig,
) -> InterpResult<'tcx, InterpCx<'tcx, MiriMachine<'tcx>>> {
    let param_env = ty::ParamEnv::reveal_all();
    let layout_cx = LayoutCx { tcx, param_env };
    let mut ecx =
        InterpCx::new(tcx, rustc_span::DUMMY_SP, param_env, MiriMachine::new(config, layout_cx));

    // Some parts of initialization require a full `InterpCx`.
    MiriMachine::late_init(&mut ecx, config, {
        let mut state = MainThreadState::default();
        // Cannot capture anything GC-relevant here.
        Box::new(move |m| state.on_main_stack_empty(m))
    })?;

    // 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. \
            Use `cargo miri setup` to prepare a sysroot that is suitable for Miri."
        );
    }

    // Setup first stack frame.
    let entry_instance = ty::Instance::mono(tcx, entry_id);

    // First argument is constructed later, because it's skipped if the entry function uses #[start].

    // Second argument (argc): length of `config.args`.
    let argc =
        ImmTy::from_int(i64::try_from(config.args.len()).unwrap(), ecx.machine.layouts.isize);
    // Third argument (`argv`): created from `config.args`.
    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 argvs_layout = ecx.layout_of(Ty::new_array(
            tcx,
            Ty::new_imm_ptr(tcx, tcx.types.u8),
            u64::try_from(argvs.len()).unwrap(),
        ))?;
        let argvs_place = ecx.allocate(argvs_layout, MiriMemoryKind::Machine.into())?;
        for (idx, arg) in argvs.into_iter().enumerate() {
            let place = ecx.project_field(&argvs_place, idx)?;
            ecx.write_immediate(arg, &place)?;
        }
        ecx.mark_immutable(&argvs_place);
        // Store `argc` and `argv` for macOS `_NSGetArg{c,v}`.
        {
            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(Ty::new_imm_ptr(tcx, tcx.types.unit))?,
                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`.
        {
            // 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 (idx, &c) in cmd_utf16.iter().enumerate() {
                let place = ecx.project_field(&cmd_place, idx)?;
                ecx.write_scalar(Scalar::from_u16(c), &place)?;
            }
            ecx.mark_immutable(&cmd_place);
        }
        ecx.mplace_to_ref(&argvs_place)?
    };

    // 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 {
        EntryFnType::Main { .. } => {
            let start_id = tcx.lang_items().start_fn().unwrap_or_else(|| {
                tcx.dcx().fatal(
                    "could not find start function. Make sure the entry point is marked with `#[start]`."
                );
            });
            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,
                ty::ParamEnv::reveal_all(),
                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,
                Abi::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),
                StackPopCleanup::Root { cleanup: true },
            )?;
        }
        EntryFnType::Start => {
            ecx.call_function(
                entry_instance,
                Abi::Rust,
                &[argc, argv],
                Some(&ret_place),
                StackPopCleanup::Root { cleanup: true },
            )?;
        }
    }

    Ok(ecx)
}

/// Evaluates the entry function specified by `entry_id`.
/// Returns `Some(return_code)` if program executed completed.
/// Returns `None` if an evaluation error occurred.
#[allow(clippy::needless_lifetimes)]
pub fn eval_entry<'tcx>(
    tcx: TyCtxt<'tcx>,
    entry_id: DefId,
    entry_type: EntryFnType,
    config: MiriConfig,
) -> Option<i64> {
    // Copy setting before we move `config`.
    let ignore_leaks = config.ignore_leaks;

    let mut ecx = match create_ecx(tcx, entry_id, entry_type, &config) {
        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| {
        ecx.handle_ice();
        panic::resume_unwind(panic_payload)
    });
    // `Ok` can never happen.
    #[cfg(not(bootstrap))]
    let Err(res) = res;
    #[cfg(bootstrap)]
    let res = match res {
        Err(res) => res,
        // `Ok` can never happen
        Ok(never) => match never {},
    };

    // Machine cleanup. Only do this if all threads have terminated; threads that are still running
    // might cause Stacked Borrows errors (https://github.com/rust-lang/miri/issues/2396).
    if ecx.have_all_terminated() {
        // Even if all threads have terminated, we have to beware of data races since some threads
        // might not have joined the main thread (https://github.com/rust-lang/miri/issues/2020,
        // https://github.com/rust-lang/miri/issues/2508).
        ecx.allow_data_races_all_threads_done();
        EnvVars::cleanup(&mut ecx).expect("error during env var cleanup");
    }

    // Process the result.
    let (return_code, leak_check) = report_error(&ecx, res)?;
    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");
            return None;
        }
        // Check for memory leaks.
        info!("Additional static roots: {:?}", ecx.machine.static_roots);
        let leaks = ecx.find_leaked_allocations(&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.
            return None;
        }
    }
    Some(return_code)
}

/// 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 arg0 = if let Some(arg0) = args.next() {
            arg0
        } 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('\\').take(nslashes * 2 + 1));
                        cmd.push('"');
                    }
                    Some(c) => {
                        cmd.extend(iter::repeat('\\').take(nslashes));
                        cmd.push(c);
                    }
                    None => {
                        cmd.extend(iter::repeat('\\').take(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""#);
    }
}