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
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
use crate::ty::print::{FmtPrinter, Printer};
use crate::ty::subst::{InternalSubsts, Subst};
use crate::ty::{
    self, EarlyBinder, SubstsRef, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitable,
};
use rustc_errors::ErrorGuaranteed;
use rustc_hir::def::Namespace;
use rustc_hir::def_id::{CrateNum, DefId};
use rustc_hir::lang_items::LangItem;
use rustc_macros::HashStable;
use rustc_middle::ty::normalize_erasing_regions::NormalizationError;
use rustc_span::Symbol;

use std::fmt;

/// A monomorphized `InstanceDef`.
///
/// Monomorphization happens on-the-fly and no monomorphized MIR is ever created. Instead, this type
/// simply couples a potentially generic `InstanceDef` with some substs, and codegen and const eval
/// will do all required substitution as they run.
#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, TyEncodable, TyDecodable)]
#[derive(HashStable, Lift, TypeFoldable, TypeVisitable)]
pub struct Instance<'tcx> {
    pub def: InstanceDef<'tcx>,
    pub substs: SubstsRef<'tcx>,
}

#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
#[derive(TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable, Lift)]
pub enum InstanceDef<'tcx> {
    /// A user-defined callable item.
    ///
    /// This includes:
    /// - `fn` items
    /// - closures
    /// - generators
    Item(ty::WithOptConstParam<DefId>),

    /// An intrinsic `fn` item (with `"rust-intrinsic"` or `"platform-intrinsic"` ABI).
    ///
    /// Alongside `Virtual`, this is the only `InstanceDef` that does not have its own callable MIR.
    /// Instead, codegen and const eval "magically" evaluate calls to intrinsics purely in the
    /// caller.
    Intrinsic(DefId),

    /// `<T as Trait>::method` where `method` receives unsizeable `self: Self` (part of the
    /// `unsized_locals` feature).
    ///
    /// The generated shim will take `Self` via `*mut Self` - conceptually this is `&owned Self` -
    /// and dereference the argument to call the original function.
    VTableShim(DefId),

    /// `fn()` pointer where the function itself cannot be turned into a pointer.
    ///
    /// One example is `<dyn Trait as Trait>::fn`, where the shim contains
    /// a virtual call, which codegen supports only via a direct call to the
    /// `<dyn Trait as Trait>::fn` instance (an `InstanceDef::Virtual`).
    ///
    /// Another example is functions annotated with `#[track_caller]`, which
    /// must have their implicit caller location argument populated for a call.
    /// Because this is a required part of the function's ABI but can't be tracked
    /// as a property of the function pointer, we use a single "caller location"
    /// (the definition of the function itself).
    ReifyShim(DefId),

    /// `<fn() as FnTrait>::call_*` (generated `FnTrait` implementation for `fn()` pointers).
    ///
    /// `DefId` is `FnTrait::call_*`.
    FnPtrShim(DefId, Ty<'tcx>),

    /// Dynamic dispatch to `<dyn Trait as Trait>::fn`.
    ///
    /// This `InstanceDef` does not have callable MIR. Calls to `Virtual` instances must be
    /// codegen'd as virtual calls through the vtable.
    ///
    /// If this is reified to a `fn` pointer, a `ReifyShim` is used (see `ReifyShim` above for more
    /// details on that).
    Virtual(DefId, usize),

    /// `<[FnMut closure] as FnOnce>::call_once`.
    ///
    /// The `DefId` is the ID of the `call_once` method in `FnOnce`.
    ClosureOnceShim { call_once: DefId, track_caller: bool },

    /// `core::ptr::drop_in_place::<T>`.
    ///
    /// The `DefId` is for `core::ptr::drop_in_place`.
    /// The `Option<Ty<'tcx>>` is either `Some(T)`, or `None` for empty drop
    /// glue.
    DropGlue(DefId, Option<Ty<'tcx>>),

    /// Compiler-generated `<T as Clone>::clone` implementation.
    ///
    /// For all types that automatically implement `Copy`, a trivial `Clone` impl is provided too.
    /// Additionally, arrays, tuples, and closures get a `Clone` shim even if they aren't `Copy`.
    ///
    /// The `DefId` is for `Clone::clone`, the `Ty` is the type `T` with the builtin `Clone` impl.
    CloneShim(DefId, Ty<'tcx>),
}

impl<'tcx> Instance<'tcx> {
    /// Returns the `Ty` corresponding to this `Instance`, with generic substitutions applied and
    /// lifetimes erased, allowing a `ParamEnv` to be specified for use during normalization.
    pub fn ty(&self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> Ty<'tcx> {
        let ty = tcx.type_of(self.def.def_id());
        tcx.subst_and_normalize_erasing_regions(self.substs, param_env, ty)
    }

    /// Finds a crate that contains a monomorphization of this instance that
    /// can be linked to from the local crate. A return value of `None` means
    /// no upstream crate provides such an exported monomorphization.
    ///
    /// This method already takes into account the global `-Zshare-generics`
    /// setting, always returning `None` if `share-generics` is off.
    pub fn upstream_monomorphization(&self, tcx: TyCtxt<'tcx>) -> Option<CrateNum> {
        // If we are not in share generics mode, we don't link to upstream
        // monomorphizations but always instantiate our own internal versions
        // instead.
        if !tcx.sess.opts.share_generics() {
            return None;
        }

        // If this is an item that is defined in the local crate, no upstream
        // crate can know about it/provide a monomorphization.
        if self.def_id().is_local() {
            return None;
        }

        // If this a non-generic instance, it cannot be a shared monomorphization.
        self.substs.non_erasable_generics().next()?;

        match self.def {
            InstanceDef::Item(def) => tcx
                .upstream_monomorphizations_for(def.did)
                .and_then(|monos| monos.get(&self.substs).cloned()),
            InstanceDef::DropGlue(_, Some(_)) => tcx.upstream_drop_glue_for(self.substs),
            _ => None,
        }
    }
}

impl<'tcx> InstanceDef<'tcx> {
    #[inline]
    pub fn def_id(self) -> DefId {
        match self {
            InstanceDef::Item(def) => def.did,
            InstanceDef::VTableShim(def_id)
            | InstanceDef::ReifyShim(def_id)
            | InstanceDef::FnPtrShim(def_id, _)
            | InstanceDef::Virtual(def_id, _)
            | InstanceDef::Intrinsic(def_id)
            | InstanceDef::ClosureOnceShim { call_once: def_id, track_caller: _ }
            | InstanceDef::DropGlue(def_id, _)
            | InstanceDef::CloneShim(def_id, _) => def_id,
        }
    }

    /// Returns the `DefId` of instances which might not require codegen locally.
    pub fn def_id_if_not_guaranteed_local_codegen(self) -> Option<DefId> {
        match self {
            ty::InstanceDef::Item(def) => Some(def.did),
            ty::InstanceDef::DropGlue(def_id, Some(_)) => Some(def_id),
            InstanceDef::VTableShim(..)
            | InstanceDef::ReifyShim(..)
            | InstanceDef::FnPtrShim(..)
            | InstanceDef::Virtual(..)
            | InstanceDef::Intrinsic(..)
            | InstanceDef::ClosureOnceShim { .. }
            | InstanceDef::DropGlue(..)
            | InstanceDef::CloneShim(..) => None,
        }
    }

    #[inline]
    pub fn with_opt_param(self) -> ty::WithOptConstParam<DefId> {
        match self {
            InstanceDef::Item(def) => def,
            InstanceDef::VTableShim(def_id)
            | InstanceDef::ReifyShim(def_id)
            | InstanceDef::FnPtrShim(def_id, _)
            | InstanceDef::Virtual(def_id, _)
            | InstanceDef::Intrinsic(def_id)
            | InstanceDef::ClosureOnceShim { call_once: def_id, track_caller: _ }
            | InstanceDef::DropGlue(def_id, _)
            | InstanceDef::CloneShim(def_id, _) => ty::WithOptConstParam::unknown(def_id),
        }
    }

    #[inline]
    pub fn get_attrs(&self, tcx: TyCtxt<'tcx>, attr: Symbol) -> ty::Attributes<'tcx> {
        tcx.get_attrs(self.def_id(), attr)
    }

    /// Returns `true` if the LLVM version of this instance is unconditionally
    /// marked with `inline`. This implies that a copy of this instance is
    /// generated in every codegen unit.
    /// Note that this is only a hint. See the documentation for
    /// `generates_cgu_internal_copy` for more information.
    pub fn requires_inline(&self, tcx: TyCtxt<'tcx>) -> bool {
        use rustc_hir::definitions::DefPathData;
        let def_id = match *self {
            ty::InstanceDef::Item(def) => def.did,
            ty::InstanceDef::DropGlue(_, Some(_)) => return false,
            _ => return true,
        };
        matches!(
            tcx.def_key(def_id).disambiguated_data.data,
            DefPathData::Ctor | DefPathData::ClosureExpr
        )
    }

    /// Returns `true` if the machine code for this instance is instantiated in
    /// each codegen unit that references it.
    /// Note that this is only a hint! The compiler can globally decide to *not*
    /// do this in order to speed up compilation. CGU-internal copies are
    /// only exist to enable inlining. If inlining is not performed (e.g. at
    /// `-Copt-level=0`) then the time for generating them is wasted and it's
    /// better to create a single copy with external linkage.
    pub fn generates_cgu_internal_copy(&self, tcx: TyCtxt<'tcx>) -> bool {
        if self.requires_inline(tcx) {
            return true;
        }
        if let ty::InstanceDef::DropGlue(.., Some(ty)) = *self {
            // Drop glue generally wants to be instantiated at every codegen
            // unit, but without an #[inline] hint. We should make this
            // available to normal end-users.
            if tcx.sess.opts.incremental.is_none() {
                return true;
            }
            // When compiling with incremental, we can generate a *lot* of
            // codegen units. Including drop glue into all of them has a
            // considerable compile time cost.
            //
            // We include enums without destructors to allow, say, optimizing
            // drops of `Option::None` before LTO. We also respect the intent of
            // `#[inline]` on `Drop::drop` implementations.
            return ty.ty_adt_def().map_or(true, |adt_def| {
                adt_def.destructor(tcx).map_or_else(
                    || adt_def.is_enum(),
                    |dtor| tcx.codegen_fn_attrs(dtor.did).requests_inline(),
                )
            });
        }
        tcx.codegen_fn_attrs(self.def_id()).requests_inline()
    }

    pub fn requires_caller_location(&self, tcx: TyCtxt<'_>) -> bool {
        match *self {
            InstanceDef::Item(ty::WithOptConstParam { did: def_id, .. })
            | InstanceDef::Virtual(def_id, _) => {
                tcx.body_codegen_attrs(def_id).flags.contains(CodegenFnAttrFlags::TRACK_CALLER)
            }
            InstanceDef::ClosureOnceShim { call_once: _, track_caller } => track_caller,
            _ => false,
        }
    }

    /// Returns `true` when the MIR body associated with this instance should be monomorphized
    /// by its users (e.g. codegen or miri) by substituting the `substs` from `Instance` (see
    /// `Instance::substs_for_mir_body`).
    ///
    /// Otherwise, returns `false` only for some kinds of shims where the construction of the MIR
    /// body should perform necessary substitutions.
    pub fn has_polymorphic_mir_body(&self) -> bool {
        match *self {
            InstanceDef::CloneShim(..)
            | InstanceDef::FnPtrShim(..)
            | InstanceDef::DropGlue(_, Some(_)) => false,
            InstanceDef::ClosureOnceShim { .. }
            | InstanceDef::DropGlue(..)
            | InstanceDef::Item(_)
            | InstanceDef::Intrinsic(..)
            | InstanceDef::ReifyShim(..)
            | InstanceDef::Virtual(..)
            | InstanceDef::VTableShim(..) => true,
        }
    }
}

impl<'tcx> fmt::Display for Instance<'tcx> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        ty::tls::with(|tcx| {
            let substs = tcx.lift(self.substs).expect("could not lift for printing");
            let s = FmtPrinter::new(tcx, Namespace::ValueNS)
                .print_def_path(self.def_id(), substs)?
                .into_buffer();
            f.write_str(&s)
        })?;

        match self.def {
            InstanceDef::Item(_) => Ok(()),
            InstanceDef::VTableShim(_) => write!(f, " - shim(vtable)"),
            InstanceDef::ReifyShim(_) => write!(f, " - shim(reify)"),
            InstanceDef::Intrinsic(_) => write!(f, " - intrinsic"),
            InstanceDef::Virtual(_, num) => write!(f, " - virtual#{}", num),
            InstanceDef::FnPtrShim(_, ty) => write!(f, " - shim({})", ty),
            InstanceDef::ClosureOnceShim { .. } => write!(f, " - shim"),
            InstanceDef::DropGlue(_, None) => write!(f, " - shim(None)"),
            InstanceDef::DropGlue(_, Some(ty)) => write!(f, " - shim(Some({}))", ty),
            InstanceDef::CloneShim(_, ty) => write!(f, " - shim({})", ty),
        }
    }
}

impl<'tcx> Instance<'tcx> {
    pub fn new(def_id: DefId, substs: SubstsRef<'tcx>) -> Instance<'tcx> {
        assert!(
            !substs.has_escaping_bound_vars(),
            "substs of instance {:?} not normalized for codegen: {:?}",
            def_id,
            substs
        );
        Instance { def: InstanceDef::Item(ty::WithOptConstParam::unknown(def_id)), substs }
    }

    pub fn mono(tcx: TyCtxt<'tcx>, def_id: DefId) -> Instance<'tcx> {
        let substs = InternalSubsts::for_item(tcx, def_id, |param, _| match param.kind {
            ty::GenericParamDefKind::Lifetime => tcx.lifetimes.re_erased.into(),
            ty::GenericParamDefKind::Type { .. } => {
                bug!("Instance::mono: {:?} has type parameters", def_id)
            }
            ty::GenericParamDefKind::Const { .. } => {
                bug!("Instance::mono: {:?} has const parameters", def_id)
            }
        });

        Instance::new(def_id, substs)
    }

    #[inline]
    pub fn def_id(&self) -> DefId {
        self.def.def_id()
    }

    /// Resolves a `(def_id, substs)` pair to an (optional) instance -- most commonly,
    /// this is used to find the precise code that will run for a trait method invocation,
    /// if known.
    ///
    /// Returns `Ok(None)` if we cannot resolve `Instance` to a specific instance.
    /// For example, in a context like this,
    ///
    /// ```ignore (illustrative)
    /// fn foo<T: Debug>(t: T) { ... }
    /// ```
    ///
    /// trying to resolve `Debug::fmt` applied to `T` will yield `Ok(None)`, because we do not
    /// know what code ought to run. (Note that this setting is also affected by the
    /// `RevealMode` in the parameter environment.)
    ///
    /// Presuming that coherence and type-check have succeeded, if this method is invoked
    /// in a monomorphic context (i.e., like during codegen), then it is guaranteed to return
    /// `Ok(Some(instance))`.
    ///
    /// Returns `Err(ErrorGuaranteed)` when the `Instance` resolution process
    /// couldn't complete due to errors elsewhere - this is distinct
    /// from `Ok(None)` to avoid misleading diagnostics when an error
    /// has already been/will be emitted, for the original cause
    pub fn resolve(
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        def_id: DefId,
        substs: SubstsRef<'tcx>,
    ) -> Result<Option<Instance<'tcx>>, ErrorGuaranteed> {
        Instance::resolve_opt_const_arg(
            tcx,
            param_env,
            ty::WithOptConstParam::unknown(def_id),
            substs,
        )
    }

    // This should be kept up to date with `resolve`.
    pub fn resolve_opt_const_arg(
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        def: ty::WithOptConstParam<DefId>,
        substs: SubstsRef<'tcx>,
    ) -> Result<Option<Instance<'tcx>>, ErrorGuaranteed> {
        // All regions in the result of this query are erased, so it's
        // fine to erase all of the input regions.

        // HACK(eddyb) erase regions in `substs` first, so that `param_env.and(...)`
        // below is more likely to ignore the bounds in scope (e.g. if the only
        // generic parameters mentioned by `substs` were lifetime ones).
        let substs = tcx.erase_regions(substs);

        // FIXME(eddyb) should this always use `param_env.with_reveal_all()`?
        if let Some((did, param_did)) = def.as_const_arg() {
            tcx.resolve_instance_of_const_arg(
                tcx.erase_regions(param_env.and((did, param_did, substs))),
            )
        } else {
            tcx.resolve_instance(tcx.erase_regions(param_env.and((def.did, substs))))
        }
    }

    pub fn resolve_for_fn_ptr(
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        def_id: DefId,
        substs: SubstsRef<'tcx>,
    ) -> Option<Instance<'tcx>> {
        debug!("resolve(def_id={:?}, substs={:?})", def_id, substs);
        // Use either `resolve_closure` or `resolve_for_vtable`
        assert!(!tcx.is_closure(def_id), "Called `resolve_for_fn_ptr` on closure: {:?}", def_id);
        Instance::resolve(tcx, param_env, def_id, substs).ok().flatten().map(|mut resolved| {
            match resolved.def {
                InstanceDef::Item(def) if resolved.def.requires_caller_location(tcx) => {
                    debug!(" => fn pointer created for function with #[track_caller]");
                    resolved.def = InstanceDef::ReifyShim(def.did);
                }
                InstanceDef::Virtual(def_id, _) => {
                    debug!(" => fn pointer created for virtual call");
                    resolved.def = InstanceDef::ReifyShim(def_id);
                }
                _ => {}
            }

            resolved
        })
    }

    pub fn resolve_for_vtable(
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        def_id: DefId,
        substs: SubstsRef<'tcx>,
    ) -> Option<Instance<'tcx>> {
        debug!("resolve_for_vtable(def_id={:?}, substs={:?})", def_id, substs);
        let fn_sig = tcx.fn_sig(def_id);
        let is_vtable_shim = !fn_sig.inputs().skip_binder().is_empty()
            && fn_sig.input(0).skip_binder().is_param(0)
            && tcx.generics_of(def_id).has_self;
        if is_vtable_shim {
            debug!(" => associated item with unsizeable self: Self");
            Some(Instance { def: InstanceDef::VTableShim(def_id), substs })
        } else {
            Instance::resolve(tcx, param_env, def_id, substs).ok().flatten().map(|mut resolved| {
                match resolved.def {
                    InstanceDef::Item(def) => {
                        // We need to generate a shim when we cannot guarantee that
                        // the caller of a trait object method will be aware of
                        // `#[track_caller]` - this ensures that the caller
                        // and callee ABI will always match.
                        //
                        // The shim is generated when all of these conditions are met:
                        //
                        // 1) The underlying method expects a caller location parameter
                        // in the ABI
                        if resolved.def.requires_caller_location(tcx)
                            // 2) The caller location parameter comes from having `#[track_caller]`
                            // on the implementation, and *not* on the trait method.
                            && !tcx.should_inherit_track_caller(def.did)
                            // If the method implementation comes from the trait definition itself
                            // (e.g. `trait Foo { #[track_caller] my_fn() { /* impl */ } }`),
                            // then we don't need to generate a shim. This check is needed because
                            // `should_inherit_track_caller` returns `false` if our method
                            // implementation comes from the trait block, and not an impl block
                            && !matches!(
                                tcx.opt_associated_item(def.did),
                                Some(ty::AssocItem {
                                    container: ty::AssocItemContainer::TraitContainer,
                                    ..
                                })
                            )
                        {
                            if tcx.is_closure(def.did) {
                                debug!(" => vtable fn pointer created for closure with #[track_caller]: {:?} for method {:?} {:?}",
                                       def.did, def_id, substs);

                                // Create a shim for the `FnOnce/FnMut/Fn` method we are calling
                                // - unlike functions, invoking a closure always goes through a
                                // trait.
                                resolved = Instance { def: InstanceDef::ReifyShim(def_id), substs };
                            } else {
                                debug!(
                                    " => vtable fn pointer created for function with #[track_caller]: {:?}", def.did
                                );
                                resolved.def = InstanceDef::ReifyShim(def.did);
                            }
                        }
                    }
                    InstanceDef::Virtual(def_id, _) => {
                        debug!(" => vtable fn pointer created for virtual call");
                        resolved.def = InstanceDef::ReifyShim(def_id);
                    }
                    _ => {}
                }

                resolved
            })
        }
    }

    pub fn resolve_closure(
        tcx: TyCtxt<'tcx>,
        def_id: DefId,
        substs: ty::SubstsRef<'tcx>,
        requested_kind: ty::ClosureKind,
    ) -> Option<Instance<'tcx>> {
        let actual_kind = substs.as_closure().kind();

        match needs_fn_once_adapter_shim(actual_kind, requested_kind) {
            Ok(true) => Instance::fn_once_adapter_instance(tcx, def_id, substs),
            _ => Some(Instance::new(def_id, substs)),
        }
    }

    pub fn resolve_drop_in_place(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> ty::Instance<'tcx> {
        let def_id = tcx.require_lang_item(LangItem::DropInPlace, None);
        let substs = tcx.intern_substs(&[ty.into()]);
        Instance::resolve(tcx, ty::ParamEnv::reveal_all(), def_id, substs).unwrap().unwrap()
    }

    pub fn fn_once_adapter_instance(
        tcx: TyCtxt<'tcx>,
        closure_did: DefId,
        substs: ty::SubstsRef<'tcx>,
    ) -> Option<Instance<'tcx>> {
        debug!("fn_once_adapter_shim({:?}, {:?})", closure_did, substs);
        let fn_once = tcx.require_lang_item(LangItem::FnOnce, None);
        let call_once = tcx
            .associated_items(fn_once)
            .in_definition_order()
            .find(|it| it.kind == ty::AssocKind::Fn)
            .unwrap()
            .def_id;
        let track_caller =
            tcx.codegen_fn_attrs(closure_did).flags.contains(CodegenFnAttrFlags::TRACK_CALLER);
        let def = ty::InstanceDef::ClosureOnceShim { call_once, track_caller };

        let self_ty = tcx.mk_closure(closure_did, substs);

        let sig = substs.as_closure().sig();
        let sig =
            tcx.try_normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), sig).ok()?;
        assert_eq!(sig.inputs().len(), 1);
        let substs = tcx.mk_substs_trait(self_ty, &[sig.inputs()[0].into()]);

        debug!("fn_once_adapter_shim: self_ty={:?} sig={:?}", self_ty, sig);
        Some(Instance { def, substs })
    }

    /// Depending on the kind of `InstanceDef`, the MIR body associated with an
    /// instance is expressed in terms of the generic parameters of `self.def_id()`, and in other
    /// cases the MIR body is expressed in terms of the types found in the substitution array.
    /// In the former case, we want to substitute those generic types and replace them with the
    /// values from the substs when monomorphizing the function body. But in the latter case, we
    /// don't want to do that substitution, since it has already been done effectively.
    ///
    /// This function returns `Some(substs)` in the former case and `None` otherwise -- i.e., if
    /// this function returns `None`, then the MIR body does not require substitution during
    /// codegen.
    fn substs_for_mir_body(&self) -> Option<SubstsRef<'tcx>> {
        if self.def.has_polymorphic_mir_body() { Some(self.substs) } else { None }
    }

    pub fn subst_mir<T>(&self, tcx: TyCtxt<'tcx>, v: &T) -> T
    where
        T: TypeFoldable<'tcx> + Copy,
    {
        if let Some(substs) = self.substs_for_mir_body() {
            EarlyBinder(*v).subst(tcx, substs)
        } else {
            *v
        }
    }

    #[inline(always)]
    pub fn subst_mir_and_normalize_erasing_regions<T>(
        &self,
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        v: T,
    ) -> T
    where
        T: TypeFoldable<'tcx> + Clone,
    {
        if let Some(substs) = self.substs_for_mir_body() {
            tcx.subst_and_normalize_erasing_regions(substs, param_env, v)
        } else {
            tcx.normalize_erasing_regions(param_env, v)
        }
    }

    #[inline(always)]
    pub fn try_subst_mir_and_normalize_erasing_regions<T>(
        &self,
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        v: T,
    ) -> Result<T, NormalizationError<'tcx>>
    where
        T: TypeFoldable<'tcx> + Clone,
    {
        if let Some(substs) = self.substs_for_mir_body() {
            tcx.try_subst_and_normalize_erasing_regions(substs, param_env, v)
        } else {
            tcx.try_normalize_erasing_regions(param_env, v)
        }
    }

    /// Returns a new `Instance` where generic parameters in `instance.substs` are replaced by
    /// identity parameters if they are determined to be unused in `instance.def`.
    pub fn polymorphize(self, tcx: TyCtxt<'tcx>) -> Self {
        debug!("polymorphize: running polymorphization analysis");
        if !tcx.sess.opts.unstable_opts.polymorphize {
            return self;
        }

        let polymorphized_substs = polymorphize(tcx, self.def, self.substs);
        debug!("polymorphize: self={:?} polymorphized_substs={:?}", self, polymorphized_substs);
        Self { def: self.def, substs: polymorphized_substs }
    }
}

fn polymorphize<'tcx>(
    tcx: TyCtxt<'tcx>,
    instance: ty::InstanceDef<'tcx>,
    substs: SubstsRef<'tcx>,
) -> SubstsRef<'tcx> {
    debug!("polymorphize({:?}, {:?})", instance, substs);
    let unused = tcx.unused_generic_params(instance);
    debug!("polymorphize: unused={:?}", unused);

    // If this is a closure or generator then we need to handle the case where another closure
    // from the function is captured as an upvar and hasn't been polymorphized. In this case,
    // the unpolymorphized upvar closure would result in a polymorphized closure producing
    // multiple mono items (and eventually symbol clashes).
    let def_id = instance.def_id();
    let upvars_ty = if tcx.is_closure(def_id) {
        Some(substs.as_closure().tupled_upvars_ty())
    } else if tcx.type_of(def_id).is_generator() {
        Some(substs.as_generator().tupled_upvars_ty())
    } else {
        None
    };
    let has_upvars = upvars_ty.map_or(false, |ty| !ty.tuple_fields().is_empty());
    debug!("polymorphize: upvars_ty={:?} has_upvars={:?}", upvars_ty, has_upvars);

    struct PolymorphizationFolder<'tcx> {
        tcx: TyCtxt<'tcx>,
    }

    impl<'tcx> ty::TypeFolder<'tcx> for PolymorphizationFolder<'tcx> {
        fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
            self.tcx
        }

        fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
            debug!("fold_ty: ty={:?}", ty);
            match *ty.kind() {
                ty::Closure(def_id, substs) => {
                    let polymorphized_substs = polymorphize(
                        self.tcx,
                        ty::InstanceDef::Item(ty::WithOptConstParam::unknown(def_id)),
                        substs,
                    );
                    if substs == polymorphized_substs {
                        ty
                    } else {
                        self.tcx.mk_closure(def_id, polymorphized_substs)
                    }
                }
                ty::Generator(def_id, substs, movability) => {
                    let polymorphized_substs = polymorphize(
                        self.tcx,
                        ty::InstanceDef::Item(ty::WithOptConstParam::unknown(def_id)),
                        substs,
                    );
                    if substs == polymorphized_substs {
                        ty
                    } else {
                        self.tcx.mk_generator(def_id, polymorphized_substs, movability)
                    }
                }
                _ => ty.super_fold_with(self),
            }
        }
    }

    InternalSubsts::for_item(tcx, def_id, |param, _| {
        let is_unused = unused.contains(param.index).unwrap_or(false);
        debug!("polymorphize: param={:?} is_unused={:?}", param, is_unused);
        match param.kind {
            // Upvar case: If parameter is a type parameter..
            ty::GenericParamDefKind::Type { .. } if
                // ..and has upvars..
                has_upvars &&
                // ..and this param has the same type as the tupled upvars..
                upvars_ty == Some(substs[param.index as usize].expect_ty()) => {
                    // ..then double-check that polymorphization marked it used..
                    debug_assert!(!is_unused);
                    // ..and polymorphize any closures/generators captured as upvars.
                    let upvars_ty = upvars_ty.unwrap();
                    let polymorphized_upvars_ty = upvars_ty.fold_with(
                        &mut PolymorphizationFolder { tcx });
                    debug!("polymorphize: polymorphized_upvars_ty={:?}", polymorphized_upvars_ty);
                    ty::GenericArg::from(polymorphized_upvars_ty)
                },

            // Simple case: If parameter is a const or type parameter..
            ty::GenericParamDefKind::Const { .. } | ty::GenericParamDefKind::Type { .. } if
                // ..and is within range and unused..
                unused.contains(param.index).unwrap_or(false) =>
                    // ..then use the identity for this parameter.
                    tcx.mk_param_from_def(param),

            // Otherwise, use the parameter as before.
            _ => substs[param.index as usize],
        }
    })
}

fn needs_fn_once_adapter_shim(
    actual_closure_kind: ty::ClosureKind,
    trait_closure_kind: ty::ClosureKind,
) -> Result<bool, ()> {
    match (actual_closure_kind, trait_closure_kind) {
        (ty::ClosureKind::Fn, ty::ClosureKind::Fn)
        | (ty::ClosureKind::FnMut, ty::ClosureKind::FnMut)
        | (ty::ClosureKind::FnOnce, ty::ClosureKind::FnOnce) => {
            // No adapter needed.
            Ok(false)
        }
        (ty::ClosureKind::Fn, ty::ClosureKind::FnMut) => {
            // The closure fn `llfn` is a `fn(&self, ...)`.  We want a
            // `fn(&mut self, ...)`. In fact, at codegen time, these are
            // basically the same thing, so we can just return llfn.
            Ok(false)
        }
        (ty::ClosureKind::Fn | ty::ClosureKind::FnMut, ty::ClosureKind::FnOnce) => {
            // The closure fn `llfn` is a `fn(&self, ...)` or `fn(&mut
            // self, ...)`.  We want a `fn(self, ...)`. We can produce
            // this by doing something like:
            //
            //     fn call_once(self, ...) { call_mut(&self, ...) }
            //     fn call_once(mut self, ...) { call_mut(&mut self, ...) }
            //
            // These are both the same at codegen time.
            Ok(true)
        }
        (ty::ClosureKind::FnMut | ty::ClosureKind::FnOnce, _) => Err(()),
    }
}