charon_lib/transform/normalize/
partial_monomorphization.rs

1//! This module implements partial monomorphization, which allows specializing generic items on
2//! some specific instanciation patterns. This is used by Aeneas to avoid nested mutable borrows:
3//! we transform `Iter<'a, &'b mut T>` to `{Iter::<_, &mut U>}<'a, 'b, T>`, where
4//! ```ignore
5//! struct {Iter::<'a, &'b mut U>}<'a, 'b, U> {
6//!   // the field of `Iter` but instantiated with `T -> &'b mut U`.
7//! }
8//! ```
9//!
10//! Note: We may need to partial-mono the same item multiple times: `Foo::<&mut A, B>`, `Foo::<A,
11//! &mut B>`. Note also that partial-mono is infectious: `Foo<Bar<&mut A>>` generates `Bar::<&mut
12//! A>` then `Foo::<Bar::<&mut A>>``.
13use std::collections::{HashMap, HashSet, VecDeque};
14use std::fmt::Display;
15use std::mem;
16
17use derive_generic_visitor::Visitor;
18use index_vec::Idx;
19
20use crate::ast::types_utils::TyVisitable;
21use crate::ast::visitor::{VisitWithBinderDepth, VisitorWithBinderDepth};
22use crate::formatter::IntoFormatter;
23use crate::ids::IndexVec;
24use crate::options::MonomorphizeMut;
25use crate::pretty::FmtWithCtx;
26use crate::register_error;
27use crate::transform::ctx::TransformPass;
28use crate::{transform::TransformCtx, ullbc_ast::*};
29
30type MutabilityShape = Binder<GenericArgs>;
31
32/// See the docs of `MutabilityShapeBuilder::compute_shape`.
33#[derive(Visitor)]
34struct MutabilityShapeBuilder<'pm, 'ctx> {
35    pm: &'pm PartialMonomorphizer<'ctx>,
36    /// The parameters that will constitute the final binder.
37    params: GenericParams,
38    /// The arguments to pass to the final binder to recover the input arguments.
39    extracted: GenericArgs,
40    /// Current depth under which we're visiting.
41    binder_depth: DeBruijnId,
42}
43
44impl<'pm, 'ctx> MutabilityShapeBuilder<'pm, 'ctx> {
45    /// Compute the mutability "shape" of a set of generic arguments by factoring out the minimal
46    /// amount of information that still allows reconstructing the original arguments while keeping the
47    /// "shape arguments" free of mutable borrows.
48    ///
49    /// For example, for input:
50    ///   <u32, &'a mut &'b A, Option::<&'a mut bool>>
51    /// we want to build:
52    ///   binder<'a, 'b, A, B, C> <A, &'a mut B, Option::<&'b mut C>>
53    /// from which we can recover the original arguments by instantiating it with:
54    ///   <'a, 'a, u32, &'b A, bool>
55    ///
56    /// Formally, given `let (shape, shape_args) = get_mutability_shape(args);`, we have the following:
57    /// - `shape.substitute(shape_args) == args`;
58    /// - `shape_args` contains no infected types;
59    /// - `shape` is as shallow as possible (i.e. takes just enough to get all the infected types
60    ///   and not more).
61    ///
62    /// Note: the input arguments are assumed to have been already partially monomorphized, in the
63    /// sense that we won't recurse inside ADT args because we assume any ADT applied to infected
64    /// args to have been replaced with a fresh infected ADT.
65    fn compute_shape(
66        pm: &'pm PartialMonomorphizer<'ctx>,
67        target_params: &GenericParams,
68        args: &GenericArgs,
69    ) -> (MutabilityShape, GenericArgs) {
70        // We start with the implicit parameters from the original item. We'll need to substitute
71        // them once we've figured out the mapping of explicit parameters, but we'll also be adding
72        // new trait clauses potentially so we can't leave the vector empty (the ids would be
73        // wrong).
74        let mut shape_contents = args.clone();
75        let mut builder = Self {
76            pm,
77            params: GenericParams {
78                regions: IndexVec::new(),
79                types: IndexVec::new(),
80                const_generics: IndexVec::new(),
81                ..target_params.clone()
82            },
83            extracted: GenericArgs {
84                regions: IndexVec::new(),
85                types: IndexVec::new(),
86                const_generics: IndexVec::new(),
87                trait_refs: mem::take(&mut shape_contents.trait_refs),
88            },
89            binder_depth: DeBruijnId::zero(),
90        };
91
92        // Traverse the generics and replace any non-infected type, region or const generic with a
93        // fresh variable.
94        let _ = VisitWithBinderDepth::new(&mut builder).visit(&mut shape_contents);
95
96        let shape_params = {
97            let mut shape_params = builder.params;
98            // Now the explicit params in `shape_params` are correct, and the implicit params are a mix
99            // of the old params and new trait clauses. The old params may refer to the old explicit
100            // params which is wrong and must be fixed up.
101            shape_params.trait_clauses = shape_params.trait_clauses.map_indexed(|i, x| {
102                if i.index() < target_params.trait_clauses.len() {
103                    x.substitute_explicits(&shape_contents)
104                } else {
105                    x
106                }
107            });
108            shape_params.trait_type_constraints =
109                shape_params.trait_type_constraints.map_indexed(|i, x| {
110                    if i.index() < target_params.trait_type_constraints.len() {
111                        x.substitute_explicits(&shape_contents)
112                    } else {
113                        x
114                    }
115                });
116            shape_params.regions_outlive = shape_params
117                .regions_outlive
118                .into_iter()
119                .enumerate()
120                .map(|(i, x)| {
121                    if i < target_params.regions_outlive.len() {
122                        x.substitute_explicits(&shape_contents)
123                    } else {
124                        x
125                    }
126                })
127                .collect();
128            shape_params.types_outlive = shape_params
129                .types_outlive
130                .into_iter()
131                .enumerate()
132                .map(|(i, x)| {
133                    if i < target_params.types_outlive.len() {
134                        x.substitute_explicits(&shape_contents)
135                    } else {
136                        x
137                    }
138                })
139                .collect();
140            shape_params
141        };
142
143        // The first half of the trait params correspond to the original item clauses so we can
144        // pass them unmodified.
145        shape_contents.trait_refs = shape_params.identity_args().trait_refs;
146        shape_contents
147            .trait_refs
148            .truncate(target_params.trait_clauses.len());
149
150        let shape_args = builder.extracted;
151        let shape = Binder::new(BinderKind::Other, shape_params, shape_contents);
152        (shape, shape_args)
153    }
154
155    /// Replace this value with a fresh variable, and record that we did so.
156    fn replace_with_fresh_var<Id, Param, Arg>(
157        &mut self,
158        val: &mut Arg,
159        mk_param: impl FnOnce(Id) -> Param,
160        mk_value: impl FnOnce(DeBruijnVar<Id>) -> Arg,
161    ) where
162        Id: Idx + Display,
163        Arg: TyVisitable + Clone,
164        GenericParams: HasIdxVecOf<Id, Output = Param>,
165        GenericArgs: HasIdxVecOf<Id, Output = Arg>,
166    {
167        let Some(shifted_val) = val.clone().move_from_under_binders(self.binder_depth) else {
168            // Give up on this value.
169            return;
170        };
171        // Record the mapping in the output `GenericArgs`.
172        self.extracted.get_idx_vec_mut().push(shifted_val);
173        // Put a fresh param in place of `val`.
174        let id = self.params.get_idx_vec_mut().push_with(mk_param);
175        *val = mk_value(DeBruijnVar::bound(self.binder_depth, id));
176    }
177}
178
179impl<'pm, 'ctx> VisitorWithBinderDepth for MutabilityShapeBuilder<'pm, 'ctx> {
180    fn binder_depth_mut(&mut self) -> &mut DeBruijnId {
181        &mut self.binder_depth
182    }
183}
184
185impl<'pm, 'ctx> VisitAstMut for MutabilityShapeBuilder<'pm, 'ctx> {
186    fn visit<T: AstVisitable>(&mut self, x: &mut T) -> ControlFlow<Self::Break> {
187        VisitWithBinderDepth::new(self).visit(x)
188    }
189
190    fn enter_ty(&mut self, ty: &mut Ty) {
191        if !self.pm.is_infected(ty) {
192            self.replace_with_fresh_var(
193                ty,
194                |id| TypeParam::new(id, format!("T{id}"), Variance::Unknown),
195                |v| v.into(),
196            );
197        }
198    }
199    fn exit_ty_kind(&mut self, kind: &mut TyKind) {
200        if let TyKind::Adt(TypeDeclRef {
201            id: TypeId::Adt(id),
202            generics,
203        }) = kind
204        {
205            // Since the type was not replaced with a type var, it's an infected type. We've
206            // traversed it so we have its final explicit arguments. Now we need to satisfy its
207            // predicates. For that we add all its predicates to the new item, and pass those new
208            // trait clauses to it.
209            let Some(target_params) = self.pm.generic_params.get(&(*id).into()) else {
210                return;
211            };
212            let Some(shifted_generics) =
213                generics.clone().move_from_under_binders(self.binder_depth)
214            else {
215                // Give up on this value.
216                return;
217            };
218
219            // Add the target predicates (properly substituted) to the new item params.
220            let num_clauses_before_merge = self.params.trait_clauses.len();
221            self.params.merge_predicates_from(
222                target_params
223                    .clone()
224                    .substitute_explicits(&shifted_generics),
225            );
226
227            // Record the trait arguments in the output `GenericArgs`.
228            self.extracted
229                .trait_refs
230                .extend(shifted_generics.trait_refs);
231
232            // Replace each trait ref with a clause var.
233            for (target_clause_id, tref) in generics.trait_refs.iter_mut_enumerated() {
234                let clause_id = target_clause_id + num_clauses_before_merge;
235                *tref =
236                    self.params.trait_clauses[clause_id].identity_tref_at_depth(self.binder_depth);
237            }
238        }
239    }
240    fn enter_region(&mut self, r: &mut Region) {
241        self.replace_with_fresh_var(
242            r,
243            |id| RegionParam::new(id, None, Variance::Unknown),
244            |v| v.into(),
245        );
246    }
247    // TODO: we're missing type info for this
248    // fn enter_const_generic(&mut self, cg: &mut ConstGeneric) {
249    //     self.replace_with_fresh_var(cg, |id| {
250    //         ConstGenericParam::new(id, format!("N{id}"), cg.ty().clone())
251    //     });
252    // }
253    fn visit_trait_ref(&mut self, _tref: &mut TraitRef) -> ControlFlow<Self::Break> {
254        // We don't touch trait refs or we'd risk adding duplicated extra params. Instead, we fix
255        // them up in `exit_ty_kind` and `compute_shape`.
256        ControlFlow::Continue(())
257    }
258
259    fn visit_constant_expr(
260        &mut self,
261        _: &mut ConstantExpr,
262    ) -> ::std::ops::ControlFlow<Self::Break> {
263        ControlFlow::Continue(())
264    }
265}
266
267#[derive(Visitor)]
268struct PartialMonomorphizer<'a> {
269    ctx: &'a mut TransformCtx,
270    /// Tracks the closest span to emit useful errors.
271    span: Span,
272    /// Whether we partially-monomorphize type declarations.
273    specialize_adts: bool,
274    /// Types that contain mutable references.
275    infected_types: HashSet<TypeDeclId>,
276    /// Map of generic params for each item. We can't use `ctx.translated` because while iterating
277    /// over items the current item isn't available anymore, which would break recursive types.
278    /// This also makes it possible to record the generics of our to-be-added items without adding
279    /// them.
280    generic_params: HashMap<ItemId, GenericParams>,
281    /// Map of partial monomorphizations. The source item applied with the generic params gives the
282    /// target item. The resulting partially-monomorphized item will have the binder params as
283    /// generic params.
284    partial_mono_shapes: SeqHashMap<(ItemId, MutabilityShape), ItemId>,
285    /// Reverse of `partial_mono_shapes`.
286    reverse_shape_map: HashMap<ItemId, (ItemId, MutabilityShape)>,
287    /// Items that need to be processed.
288    to_process: VecDeque<ItemId>,
289}
290
291impl<'a> PartialMonomorphizer<'a> {
292    pub fn new(ctx: &'a mut TransformCtx, specialize_adts: bool) -> Self {
293        // Compute the types that contain `&mut` (even indirectly). We actually can ignore
294        // `&'static mut`, so we simply rely on our "lifetime mutability" computation.
295        let infected_types: HashSet<_> = ctx
296            .translated
297            .type_decls
298            .iter()
299            .filter(|tdecl| {
300                tdecl
301                    .generics
302                    .regions
303                    .iter()
304                    .any(|r| r.mutability.is_mutable())
305            })
306            .map(|tdecl| tdecl.def_id)
307            .collect();
308
309        // Record the generic params of all items.
310        let generic_params: HashMap<ItemId, GenericParams> = ctx
311            .translated
312            .all_items()
313            .map(|item| (item.id(), item.generic_params().clone()))
314            .collect();
315
316        // Enqueue all items to be processed.
317        let to_process = ctx.translated.all_ids().collect();
318        PartialMonomorphizer {
319            ctx,
320            span: Span::dummy(),
321            specialize_adts,
322            infected_types,
323            generic_params,
324            to_process,
325            partial_mono_shapes: SeqHashMap::default(),
326            reverse_shape_map: Default::default(),
327        }
328    }
329
330    /// Whether this type is or contains a `&mut`. This assumes that we've already visited this
331    /// type and partially monomorphized any ADT references.
332    fn is_infected(&self, ty: &Ty) -> bool {
333        match ty.kind() {
334            TyKind::Ref(_, _, RefKind::Mut) => true,
335            TyKind::Ref(_, ty, _)
336            | TyKind::RawPtr(ty, _)
337            | TyKind::Array(ty, _)
338            | TyKind::Pattern(ty, _)
339            | TyKind::Slice(ty) => self.is_infected(ty),
340            TyKind::Adt(tref) => match tref.id {
341                TypeId::Adt(id) => {
342                    let ty_infected = self.infected_types.contains(&id);
343                    let args_infected = if self.specialize_adts {
344                        // Since we make sure to only call the method on a processed type, any type
345                        // with infected arguments would have been replaced with a fresh instantiated
346                        // (and infected type). Hence we don't need to check the arguments here, only
347                        // the type id.
348                        false
349                    } else {
350                        tref.generics.types.iter().any(|ty| self.is_infected(ty))
351                    };
352                    ty_infected || args_infected
353                }
354                TypeId::Tuple | TypeId::Builtin(_) => {
355                    // Builtin types have no declaration to specialize, so infected arguments stay
356                    // visible inside them.
357                    tref.generics.types.iter().any(|ty| self.is_infected(ty))
358                }
359            },
360            // A function pointer/item by itself doesn't carry any mutable reference, even if it
361            // uses some in its signature. Compare with closures: a closure without captures
362            // doesn't trigger partial mono regardless of its signature.
363            TyKind::FnDef(..) | TyKind::FnPtr(..) => false,
364            TyKind::DynTrait(_) => {
365                register_error!(
366                    self.ctx,
367                    self.span,
368                    "`dyn Trait` is unsupported with `--monomorphize-mut`"
369                );
370                false
371            }
372            TyKind::TypeVar(..)
373            | TyKind::Literal(..)
374            | TyKind::Never
375            | TyKind::TraitType(..)
376            | TyKind::PtrMetadata(..)
377            | TyKind::Error(_) => false,
378        }
379    }
380
381    /// Given that `generics` apply to item `id`, if any of the generics is infected we generate a
382    /// reference to a new item obtained by partially instantiating item `id`. (That new item isn't
383    /// added immediately but is added to the `to_process` queue to be created later).
384    fn process_generics(&mut self, id: ItemId, generics: &GenericArgs) -> Option<DeclRef<ItemId>> {
385        if !generics.types.iter().any(|ty| self.is_infected(ty)) {
386            return None;
387        }
388
389        // If the type is already an instantiation, transform this reference into a reference to
390        // the original type so we don't instantiate the instantiation.
391        let mut new_generics;
392        let (id, generics) = if let Some(&(base_id, ref shape)) = self.reverse_shape_map.get(&id) {
393            new_generics = shape.clone().apply(generics);
394            let _ = self.visit(&mut new_generics); // New instantiation may require cleanup.
395            (base_id, &new_generics)
396        } else {
397            (id, generics)
398        };
399
400        // Split the args between the infected part and the non-infected part.
401        let item_params = self.generic_params.get(&id)?;
402        let (shape, shape_args) =
403            MutabilityShapeBuilder::compute_shape(self, item_params, generics);
404
405        // Create a new type id.
406        let new_params = shape.params.clone();
407        let key: (ItemId, MutabilityShape) = (id, shape);
408        let new_id = *self
409            .partial_mono_shapes
410            .entry(key.clone())
411            .or_insert_with(|| {
412                let new_id = match id {
413                    ItemId::Type(_) => {
414                        let new_id = self.ctx.translated.type_decls.reserve_slot();
415                        self.infected_types.insert(new_id);
416                        new_id.into()
417                    }
418                    ItemId::Fun(_) => self.ctx.translated.fun_decls.reserve_slot().into(),
419                    ItemId::Global(_) => self.ctx.translated.global_decls.reserve_slot().into(),
420                    ItemId::TraitDecl(_) => self.ctx.translated.trait_decls.reserve_slot().into(),
421                    ItemId::TraitImpl(_) => self.ctx.translated.trait_impls.reserve_slot().into(),
422                };
423                self.generic_params.insert(new_id, new_params);
424                self.reverse_shape_map.insert(new_id, key);
425                self.to_process.push_back(new_id);
426                new_id
427            });
428
429        let fmt_ctx = self.ctx.into_fmt();
430        trace!(
431            "processing {}{}\n output: {}{}",
432            id.with_ctx(&fmt_ctx),
433            generics.with_ctx(&fmt_ctx),
434            new_id.with_ctx(&fmt_ctx),
435            shape_args.with_ctx(&fmt_ctx),
436        );
437        Some(DeclRef {
438            id: new_id,
439            generics: Box::new(shape_args),
440            trait_ref: None,
441        })
442    }
443
444    /// Traverse the item, replacing any type instantiations we don't want with references to
445    /// soon-to-be-created partially-monomorphized types. This does not access the items in
446    /// `self.translated`, which may be missing since we took `item` out for processing.
447    pub fn process_item(&mut self, item: &mut ItemRefMut<'_>) {
448        let _ = item.drive_mut(self);
449    }
450
451    /// Creates the item corresponding to this id by instantiating the item it is based on.
452    ///
453    /// This accesses the items in `self.translated`, which must therefore all be there.
454    /// That's why items are created outside of `process_item`.
455    pub fn create_pending_instantiation(&mut self, new_id: ItemId) -> ItemByVal {
456        let (orig_id, shape) = &self.reverse_shape_map[&new_id];
457        let mut decl = self
458            .ctx
459            .translated
460            .get_item(*orig_id)
461            .unwrap()
462            .to_owned()
463            .substitute_with_self(&shape.skip_binder, &TraitRefKind::SelfId);
464
465        let mut decl_mut = decl.as_mut();
466        decl_mut.set_id(new_id);
467        *decl_mut.generic_params() = shape.params.clone();
468
469        let name_ref = &mut decl_mut.item_meta().name;
470        *name_ref = mem::take::<crate::ast::Name>(name_ref).instantiate(shape.clone());
471        self.ctx
472            .translated
473            .item_names
474            .insert(new_id, decl.as_ref().item_meta().name.clone());
475        if let (ItemId::TraitDecl(orig_trait_id), ItemId::TraitDecl(new_trait_id)) =
476            (*orig_id, new_id)
477        {
478            let names = self.ctx.translated.assoc_item_names[orig_trait_id].clone();
479            self.ctx
480                .translated
481                .assoc_item_names
482                .insert(new_trait_id, names);
483        }
484
485        decl
486    }
487}
488
489impl VisitorWithSpan for PartialMonomorphizer<'_> {
490    fn current_span(&mut self) -> &mut Span {
491        &mut self.span
492    }
493}
494impl VisitAstMut for PartialMonomorphizer<'_> {
495    fn visit<T: AstVisitable>(&mut self, x: &mut T) -> ControlFlow<Self::Break> {
496        // Track a useful enclosing span, for error messages.
497        VisitWithSpan::new(self).visit(x)
498    }
499
500    fn exit_type_decl_ref(&mut self, x: &mut TypeDeclRef) {
501        if self.specialize_adts
502            && let TypeId::Adt(id) = x.id
503            && let Some(new_decl_ref) = self.process_generics(id.into(), &x.generics)
504        {
505            *x = new_decl_ref.try_into().unwrap()
506        }
507    }
508    fn exit_fn_ptr(&mut self, x: &mut FnPtr) {
509        // TODO: methods. any `Trait::method<&mut A>` requires monomorphizing all the instances of
510        // that method just in case :>>>
511        if let FnPtrKind::Fun(FunId::Regular(id)) = *x.kind
512            && let Some(new_decl_ref) = self.process_generics(id.into(), &x.generics)
513        {
514            *x = new_decl_ref.try_into().unwrap()
515        }
516    }
517    fn exit_fun_decl_ref(&mut self, x: &mut FunDeclRef) {
518        if let Some(new_decl_ref) = self.process_generics(x.id.into(), &x.generics) {
519            *x = new_decl_ref.try_into().unwrap()
520        }
521    }
522    fn exit_global_decl_ref(&mut self, x: &mut GlobalDeclRef) {
523        if let Some(new_decl_ref) = self.process_generics(x.id.into(), &x.generics) {
524            *x = new_decl_ref.try_into().unwrap()
525        }
526    }
527    fn exit_trait_decl_ref(&mut self, x: &mut TraitDeclRef) {
528        if let Some(new_decl_ref) = self.process_generics(x.id.into(), &x.generics) {
529            *x = new_decl_ref.try_into().unwrap()
530        }
531    }
532    fn exit_trait_impl_ref(&mut self, x: &mut TraitImplRef) {
533        if let Some(new_decl_ref) = self.process_generics(x.id.into(), &x.generics) {
534            *x = new_decl_ref.try_into().unwrap()
535        }
536    }
537}
538
539pub struct Transform;
540impl TransformPass for Transform {
541    fn transform_ctx(&self, ctx: &mut TransformCtx) {
542        let Some(include_types) = ctx.options.monomorphize_mut else {
543            return;
544        };
545        // TODO: test name matcher, also with methods
546        let mut visitor =
547            PartialMonomorphizer::new(ctx, matches!(include_types, MonomorphizeMut::All));
548        while let Some(id) = visitor.to_process.pop_front() {
549            // Get the item corresponding to this id, either by creating it or by getting an
550            // existing one.
551            let mut decl = if visitor.reverse_shape_map.contains_key(&id) {
552                // Create the required item by instantiating the item it's based on.
553                visitor.create_pending_instantiation(id)
554            } else {
555                // Take the item out so we can modify it. Warning: don't look up other items in the
556                // meantime as this would break in recursive cases.
557                match visitor.ctx.translated.remove_item_temporarily(id) {
558                    Some(decl) => decl,
559                    None => continue,
560                }
561            };
562            // Visit the item, replacing type instantiations with references to soon-to-be-created
563            // partially-monomorphized types.
564            visitor.process_item(&mut decl.as_mut());
565            // Put the item back.
566            visitor.ctx.translated.put_item_back(id, decl);
567        }
568    }
569}
charon_lib/transform/normalize/partial_monomorphization.rs

charon_lib/transform/normalize/
partial_monomorphization.rs
