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//! Traits for transforming bits of IR. use crate::interner::TargetInterner; use crate::*; use std::fmt::Debug; mod binder_impls; mod boring_impls; pub mod shift; mod subst; pub use self::shift::Shift; pub use self::subst::Subst; /// A "folder" is a transformer that can be used to make a copy of /// some term -- that is, some bit of IR, such as a `Goal` -- with /// certain changes applied. The idea is that it contains methods that /// let you swap types/lifetimes for new types/lifetimes; meanwhile, /// each bit of IR implements the `Fold` trait which, given a /// `Folder`, will reconstruct itself, invoking the folder's methods /// to transform each of the types/lifetimes embedded within. /// /// # Interners /// /// The Folder trait has two type parameters, `I` and `TI`: /// /// * `I` is the "source interner" that we are folding *from* /// * `TI` is the "target interner" that we are folding *into* /// /// Often, both are the same. /// /// # Usage patterns /// /// ## Substituting for free variables /// /// Most of the time, though, we are not interested in adjust /// arbitrary types/lifetimes, but rather just free variables (even /// more often, just free existential variables) that appear within /// the term. /// /// For this reason, the `Folder` trait extends two other traits that /// contain methods that are invoked when just those particular /// /// In particular, folders can intercept references to free variables /// (either existentially or universally quantified) and replace them /// with other types/lifetimes as appropriate. /// /// To create a folder `F`, one never implements `Folder` directly, but instead /// implements one of each of these three sub-traits: /// /// - `FreeVarFolder` -- folds `BoundVar` instances that appear free /// in the term being folded (use `DefaultFreeVarFolder` to /// ignore/forbid these altogether) /// - `InferenceFolder` -- folds existential `InferenceVar` instances /// that appear in the term being folded (use /// `DefaultInferenceFolder` to ignore/forbid these altogether) /// - `PlaceholderFolder` -- folds universal `Placeholder` instances /// that appear in the term being folded (use /// `DefaultPlaceholderFolder` to ignore/forbid these altogether) /// /// To **apply** a folder, use the `Fold::fold_with` method, like so /// /// ```rust,ignore /// let x = x.fold_with(&mut folder, 0); /// ``` pub trait Folder<'i, I: Interner, TI: TargetInterner<I> = I> where I: 'i, TI: 'i, { /// Creates a `dyn` value from this folder. Unfortunately, this /// must be added manually to each impl of Folder; it permits the /// default implements below to create a `&mut dyn Folder` from /// `Self` without knowing what `Self` is (by invoking this /// method). Effectively, this limits impls of `Folder` to types /// for which we are able to create a dyn value (i.e., not `[T]` /// types). fn as_dyn(&mut self) -> &mut dyn Folder<'i, I, TI>; /// Top-level callback: invoked for each `Ty<I>` that is /// encountered when folding. By default, invokes /// `super_fold_with`, which will in turn invoke the more /// specialized folding methods below, like `fold_free_var_ty`. fn fold_ty(&mut self, ty: &Ty<I>, outer_binder: DebruijnIndex) -> Fallible<Ty<TI>> { ty.super_fold_with(self.as_dyn(), outer_binder) } /// Top-level callback: invoked for each `Lifetime<I>` that is /// encountered when folding. By default, invokes /// `super_fold_with`, which will in turn invoke the more /// specialized folding methods below, like `fold_free_var_lifetime`. fn fold_lifetime( &mut self, lifetime: &Lifetime<I>, outer_binder: DebruijnIndex, ) -> Fallible<Lifetime<TI>> { lifetime.super_fold_with(self.as_dyn(), outer_binder) } /// Top-level callback: invoked for each `Const<I>` that is /// encountered when folding. By default, invokes /// `super_fold_with`, which will in turn invoke the more /// specialized folding methods below, like `fold_free_var_const`. fn fold_const( &mut self, constant: &Const<I>, outer_binder: DebruijnIndex, ) -> Fallible<Const<TI>> { constant.super_fold_with(self.as_dyn(), outer_binder) } /// Invoked for every program clause. By default, recursively folds the goals contents. fn fold_program_clause( &mut self, clause: &ProgramClause<I>, outer_binder: DebruijnIndex, ) -> Fallible<ProgramClause<TI>> { clause.super_fold_with(self.as_dyn(), outer_binder) } /// Invoked for every goal. By default, recursively folds the goals contents. fn fold_goal(&mut self, goal: &Goal<I>, outer_binder: DebruijnIndex) -> Fallible<Goal<TI>> { goal.super_fold_with(self.as_dyn(), outer_binder) } /// If overridden to return true, then folding will panic if a /// free variable is encountered. This should be done if free /// type/lifetime variables are not expected. fn forbid_free_vars(&self) -> bool { false } /// Invoked for `TyData::BoundVar` instances that are not bound /// within the type being folded over: /// /// - `depth` is the depth of the `TyData::BoundVar`; this has /// been adjusted to account for binders in scope. /// - `binders` is the number of binders in scope. /// /// This should return a type suitable for a context with /// `binders` in scope. fn fold_free_var_ty( &mut self, bound_var: BoundVar, outer_binder: DebruijnIndex, ) -> Fallible<Ty<TI>> { if self.forbid_free_vars() { panic!( "unexpected free variable with depth `{:?}` with outer binder {:?}", bound_var, outer_binder ) } else { let bound_var = bound_var.shifted_in_from(outer_binder); Ok(TyData::<TI>::BoundVar(bound_var).intern(self.target_interner())) } } /// As `fold_free_var_ty`, but for lifetimes. fn fold_free_var_lifetime( &mut self, bound_var: BoundVar, outer_binder: DebruijnIndex, ) -> Fallible<Lifetime<TI>> { if self.forbid_free_vars() { panic!( "unexpected free variable with depth `{:?}` with outer binder {:?}", bound_var, outer_binder ) } else { let bound_var = bound_var.shifted_in_from(outer_binder); Ok(LifetimeData::<TI>::BoundVar(bound_var).intern(self.target_interner())) } } /// As `fold_free_var_ty`, but for constants. fn fold_free_var_const( &mut self, ty: &Ty<I>, bound_var: BoundVar, outer_binder: DebruijnIndex, ) -> Fallible<Const<TI>> { if self.forbid_free_vars() { panic!( "unexpected free variable with depth `{:?}` with outer binder {:?}", bound_var, outer_binder ) } else { let bound_var = bound_var.shifted_in_from(outer_binder); Ok(ConstData { ty: ty.fold_with(self.as_dyn(), outer_binder)?, value: ConstValue::<TI>::BoundVar(bound_var), } .intern(self.target_interner())) } } /// If overridden to return true, we will panic when a free /// placeholder type/lifetime/const is encountered. fn forbid_free_placeholders(&self) -> bool { false } /// Invoked for each occurrence of a placeholder type; these are /// used when we instantiate binders universally. Returns a type /// to use instead, which should be suitably shifted to account /// for `binders`. /// /// - `universe` is the universe of the `TypeName::ForAll` that was found /// - `binders` is the number of binders in scope #[allow(unused_variables)] fn fold_free_placeholder_ty( &mut self, universe: PlaceholderIndex, outer_binder: DebruijnIndex, ) -> Fallible<Ty<TI>> { if self.forbid_free_placeholders() { panic!("unexpected placeholder type `{:?}`", universe) } else { Ok(universe.to_ty::<TI>(self.target_interner())) } } /// As with `fold_free_placeholder_ty`, but for lifetimes. #[allow(unused_variables)] fn fold_free_placeholder_lifetime( &mut self, universe: PlaceholderIndex, outer_binder: DebruijnIndex, ) -> Fallible<Lifetime<TI>> { if self.forbid_free_placeholders() { panic!("unexpected placeholder lifetime `{:?}`", universe) } else { Ok(universe.to_lifetime(self.target_interner())) } } /// As with `fold_free_placeholder_ty`, but for constants. #[allow(unused_variables)] fn fold_free_placeholder_const( &mut self, ty: &Ty<I>, universe: PlaceholderIndex, outer_binder: DebruijnIndex, ) -> Fallible<Const<TI>> { if self.forbid_free_placeholders() { panic!("unexpected placeholder const `{:?}`", universe) } else { Ok(universe.to_const( self.target_interner(), ty.fold_with(self.as_dyn(), outer_binder)?, )) } } /// If overridden to return true, inference variables will trigger /// panics when folded. Used when inference variables are /// unexpected. fn forbid_inference_vars(&self) -> bool { false } /// Invoked for each occurrence of a inference type; these are /// used when we instantiate binders universally. Returns a type /// to use instead, which should be suitably shifted to account /// for `binders`. /// /// - `universe` is the universe of the `TypeName::ForAll` that was found /// - `binders` is the number of binders in scope #[allow(unused_variables)] fn fold_inference_ty( &mut self, var: InferenceVar, kind: TyKind, outer_binder: DebruijnIndex, ) -> Fallible<Ty<TI>> { if self.forbid_inference_vars() { panic!("unexpected inference type `{:?}`", var) } else { Ok(var.to_ty(self.target_interner(), kind)) } } /// As with `fold_inference_ty`, but for lifetimes. #[allow(unused_variables)] fn fold_inference_lifetime( &mut self, var: InferenceVar, outer_binder: DebruijnIndex, ) -> Fallible<Lifetime<TI>> { if self.forbid_inference_vars() { panic!("unexpected inference lifetime `'{:?}`", var) } else { Ok(var.to_lifetime(self.target_interner())) } } /// As with `fold_inference_ty`, but for constants. #[allow(unused_variables)] fn fold_inference_const( &mut self, ty: &Ty<I>, var: InferenceVar, outer_binder: DebruijnIndex, ) -> Fallible<Const<TI>> { if self.forbid_inference_vars() { panic!("unexpected inference const `{:?}`", var) } else { Ok(var.to_const( self.target_interner(), ty.fold_with(self.as_dyn(), outer_binder)?, )) } } /// Gets the interner that is being folded from. fn interner(&self) -> &'i I; /// Gets the interner that is being folded to. fn target_interner(&self) -> &'i TI; } /// Applies the given `Folder` to a value, producing a folded result /// of type `Self::Result`. The result is in the interner /// `TI`. The result type is typically the same as the source type /// (modulo interner), but in some cases we convert from borrowed /// to owned as well (e.g., the folder for `&T` will fold to a fresh /// `T`; well, actually `T::Result`). /// /// # Interners /// /// The `Fold` trait has two type parameters, `I` and `TI`: /// /// * `I` is the "source interner" that we are folding *from* /// * `TI` is the "target interner" that we are folding *into* /// /// Often, both are the same. pub trait Fold<I: Interner, TI: TargetInterner<I> = I>: Debug { /// The type of value that will be produced once folding is done. /// Typically this is `Self`, unless `Self` contains borrowed /// values, in which case owned values are produced (for example, /// one can fold over a `&T` value where `T: Fold`, in which case /// you get back a `T`, not a `&T`). type Result; /// Apply the given folder `folder` to `self`; `binders` is the /// number of binders that are in scope when beginning the /// folder. Typically `binders` starts as 0, but is adjusted when /// we encounter `Binders<T>` in the IR or other similar /// constructs. fn fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Self::Result> where I: 'i, TI: 'i; } /// For types where "fold" invokes a callback on the `Folder`, the /// `SuperFold` trait captures the recursive behavior that folds all /// the contents of the type. pub trait SuperFold<I: Interner, TI: TargetInterner<I> = I>: Fold<I, TI> { /// Recursively folds the value. fn super_fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Self::Result> where I: 'i, TI: 'i; } /// "Folding" a type invokes the `fold_ty` method on the folder; this /// usually (in turn) invokes `super_fold_ty` to fold the individual /// parts. impl<I: Interner, TI: TargetInterner<I>> Fold<I, TI> for Ty<I> { type Result = Ty<TI>; fn fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Self::Result> where I: 'i, TI: 'i, { folder.fold_ty(self, outer_binder) } } /// "Super fold" for a type invokes te more detailed callbacks on the type impl<I, TI> SuperFold<I, TI> for Ty<I> where I: Interner, TI: TargetInterner<I>, { fn super_fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Ty<TI>> where I: 'i, TI: 'i, { let interner = folder.interner(); match self.data(interner) { TyData::BoundVar(bound_var) => { if let Some(bound_var1) = bound_var.shifted_out_to(outer_binder) { // This variable was bound outside of the binders // that we have traversed during folding; // therefore, it is free. Let the folder have a // crack at it. folder.fold_free_var_ty(bound_var1, outer_binder) } else { // This variable was bound within the binders that // we folded over, so just return a bound // variable. Ok(TyData::<TI>::BoundVar(*bound_var).intern(folder.target_interner())) } } TyData::Dyn(clauses) => Ok(TyData::Dyn(clauses.fold_with(folder, outer_binder)?) .intern(folder.target_interner())), TyData::InferenceVar(var, kind) => folder.fold_inference_ty(*var, *kind, outer_binder), TyData::Apply(apply) => Ok(TyData::Apply(apply.fold_with(folder, outer_binder)?) .intern(folder.target_interner())), TyData::Placeholder(ui) => Ok(folder.fold_free_placeholder_ty(*ui, outer_binder)?), TyData::Alias(proj) => Ok(TyData::Alias(proj.fold_with(folder, outer_binder)?) .intern(folder.target_interner())), TyData::Function(fun) => Ok(TyData::Function(fun.fold_with(folder, outer_binder)?) .intern(folder.target_interner())), } } } /// "Folding" a lifetime invokes the `fold_lifetime` method on the folder; this /// usually (in turn) invokes `super_fold_lifetime` to fold the individual /// parts. impl<I: Interner, TI: TargetInterner<I>> Fold<I, TI> for Lifetime<I> { type Result = Lifetime<TI>; fn fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Self::Result> where I: 'i, TI: 'i, { folder.fold_lifetime(self, outer_binder) } } impl<I, TI> SuperFold<I, TI> for Lifetime<I> where I: Interner, TI: TargetInterner<I>, { fn super_fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Lifetime<TI>> where I: 'i, TI: 'i, { let interner = folder.interner(); match self.data(interner) { LifetimeData::BoundVar(bound_var) => { if let Some(bound_var1) = bound_var.shifted_out_to(outer_binder) { // This variable was bound outside of the binders // that we have traversed during folding; // therefore, it is free. Let the folder have a // crack at it. folder.fold_free_var_lifetime(bound_var1, outer_binder) } else { // This variable was bound within the binders that // we folded over, so just return a bound // variable. Ok(LifetimeData::<TI>::BoundVar(*bound_var).intern(folder.target_interner())) } } LifetimeData::InferenceVar(var) => folder.fold_inference_lifetime(*var, outer_binder), LifetimeData::Placeholder(universe) => { folder.fold_free_placeholder_lifetime(*universe, outer_binder) } LifetimeData::Static => Ok(LifetimeData::<TI>::Static.intern(folder.target_interner())), LifetimeData::Phantom(..) => unreachable!(), } } } /// "Folding" a const invokes the `fold_const` method on the folder; this /// usually (in turn) invokes `super_fold_const` to fold the individual /// parts. impl<I: Interner, TI: TargetInterner<I>> Fold<I, TI> for Const<I> { type Result = Const<TI>; fn fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Self::Result> where I: 'i, TI: 'i, { folder.fold_const(self, outer_binder) } } impl<I, TI> SuperFold<I, TI> for Const<I> where I: Interner, TI: TargetInterner<I>, { fn super_fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Const<TI>> where I: 'i, TI: 'i, { let interner = folder.interner(); let target_interner = folder.target_interner(); let ConstData { ref ty, ref value } = self.data(interner); let mut fold_ty = || ty.fold_with(folder, outer_binder); match value { ConstValue::BoundVar(bound_var) => { if let Some(bound_var1) = bound_var.shifted_out_to(outer_binder) { folder.fold_free_var_const(ty, bound_var1, outer_binder) } else { Ok(bound_var.to_const(target_interner, fold_ty()?)) } } ConstValue::InferenceVar(var) => folder.fold_inference_const(ty, *var, outer_binder), ConstValue::Placeholder(universe) => { folder.fold_free_placeholder_const(ty, *universe, outer_binder) } ConstValue::Concrete(ev) => Ok(ConstData { ty: fold_ty()?, value: ConstValue::Concrete(ConcreteConst { interned: folder.target_interner().transfer_const(&ev.interned), }), } .intern(folder.target_interner())), } } } /// Folding a goal invokes the `fold_goal` callback (which will, by /// default, invoke super-fold). impl<I: Interner, TI: TargetInterner<I>> Fold<I, TI> for Goal<I> { type Result = Goal<TI>; fn fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Self::Result> where I: 'i, TI: 'i, { folder.fold_goal(self, outer_binder) } } /// Superfold folds recursively. impl<I: Interner, TI: TargetInterner<I>> SuperFold<I, TI> for Goal<I> { fn super_fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Self::Result> where I: 'i, TI: 'i, { let interner = folder.interner(); let target_interner = folder.target_interner(); Ok(Goal::new( target_interner, self.data(interner).fold_with(folder, outer_binder)?, )) } } /// Folding a program clause invokes the `fold_program_clause` /// callback on the folder (which will, by default, invoke the /// `super_fold_with` method on the program clause). impl<I: Interner, TI: TargetInterner<I>> Fold<I, TI> for ProgramClause<I> { type Result = ProgramClause<TI>; fn fold_with<'i>( &self, folder: &mut dyn Folder<'i, I, TI>, outer_binder: DebruijnIndex, ) -> Fallible<Self::Result> where I: 'i, TI: 'i, { folder.fold_program_clause(self, outer_binder) } }