| //! Freshening is the process of replacing unknown variables with fresh types. The idea is that |
| //! the type, after freshening, contains no inference variables but instead contains either a |
| //! value for each variable or fresh "arbitrary" types wherever a variable would have been. |
| //! |
| //! Freshening is used primarily to get a good type for inserting into a cache. The result |
| //! summarizes what the type inferencer knows "so far". The primary place it is used right now is |
| //! in the trait matching algorithm, which needs to be able to cache whether an `impl` self type |
| //! matches some other type X -- *without* affecting `X`. That means if that if the type `X` is in |
| //! fact an unbound type variable, we want the match to be regarded as ambiguous, because depending |
| //! on what type that type variable is ultimately assigned, the match may or may not succeed. |
| //! |
| //! To handle closures, freshened types also have to contain the signature and kind of any |
| //! closure in the local inference context, as otherwise the cache key might be invalidated. |
| //! The way this is done is somewhat hacky - the closure signature is appended to the args, |
| //! as well as the closure kind "encoded" as a type. Also, special handling is needed when |
| //! the closure signature contains a reference to the original closure. |
| //! |
| //! Note that you should be careful not to allow the output of freshening to leak to the user in |
| //! error messages or in any other form. Freshening is only really useful as an internal detail. |
| //! |
| //! Because of the manipulation required to handle closures, doing arbitrary operations on |
| //! freshened types is not recommended. However, in addition to doing equality/hash |
| //! comparisons (for caching), it is possible to do a `ty::_match` operation between |
| //! 2 freshened types - this works even with the closure encoding. |
| //! |
| //! __An important detail concerning regions.__ The freshener also replaces *all* free regions with |
| //! 'erased. The reason behind this is that, in general, we do not take region relationships into |
| //! account when making type-overloaded decisions. This is important because of the design of the |
| //! region inferencer, which is not based on unification but rather on accumulating and then |
| //! solving a set of constraints. In contrast, the type inferencer assigns a value to each type |
| //! variable only once, and it does so as soon as it can, so it is reasonable to ask what the type |
| //! inferencer knows "so far". |
| use super::InferCtxt; |
| use rustc_data_structures::fx::FxHashMap; |
| use rustc_middle::infer::unify_key::ToType; |
| use rustc_middle::ty::fold::TypeFolder; |
| use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable, TypeSuperFoldable, TypeVisitableExt}; |
| use std::collections::hash_map::Entry; |
| |
| pub struct TypeFreshener<'a, 'tcx> { |
| infcx: &'a InferCtxt<'tcx>, |
| ty_freshen_count: u32, |
| const_freshen_count: u32, |
| ty_freshen_map: FxHashMap<ty::InferTy, Ty<'tcx>>, |
| const_freshen_map: FxHashMap<ty::InferConst<'tcx>, ty::Const<'tcx>>, |
| } |
| |
| impl<'a, 'tcx> TypeFreshener<'a, 'tcx> { |
| pub fn new(infcx: &'a InferCtxt<'tcx>) -> TypeFreshener<'a, 'tcx> { |
| TypeFreshener { |
| infcx, |
| ty_freshen_count: 0, |
| const_freshen_count: 0, |
| ty_freshen_map: Default::default(), |
| const_freshen_map: Default::default(), |
| } |
| } |
| |
| fn freshen_ty<F>(&mut self, opt_ty: Option<Ty<'tcx>>, key: ty::InferTy, mk_fresh: F) -> Ty<'tcx> |
| where |
| F: FnOnce(u32) -> Ty<'tcx>, |
| { |
| if let Some(ty) = opt_ty { |
| return ty.fold_with(self); |
| } |
| |
| match self.ty_freshen_map.entry(key) { |
| Entry::Occupied(entry) => *entry.get(), |
| Entry::Vacant(entry) => { |
| let index = self.ty_freshen_count; |
| self.ty_freshen_count += 1; |
| let t = mk_fresh(index); |
| entry.insert(t); |
| t |
| } |
| } |
| } |
| |
| fn freshen_const<F>( |
| &mut self, |
| opt_ct: Option<ty::Const<'tcx>>, |
| key: ty::InferConst<'tcx>, |
| freshener: F, |
| ty: Ty<'tcx>, |
| ) -> ty::Const<'tcx> |
| where |
| F: FnOnce(u32) -> ty::InferConst<'tcx>, |
| { |
| if let Some(ct) = opt_ct { |
| return ct.fold_with(self); |
| } |
| |
| match self.const_freshen_map.entry(key) { |
| Entry::Occupied(entry) => *entry.get(), |
| Entry::Vacant(entry) => { |
| let index = self.const_freshen_count; |
| self.const_freshen_count += 1; |
| let ct = ty::Const::new_infer(self.infcx.tcx, freshener(index), ty); |
| entry.insert(ct); |
| ct |
| } |
| } |
| } |
| } |
| |
| impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for TypeFreshener<'a, 'tcx> { |
| fn interner(&self) -> TyCtxt<'tcx> { |
| self.infcx.tcx |
| } |
| |
| fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> { |
| match *r { |
| ty::ReLateBound(..) => { |
| // leave bound regions alone |
| r |
| } |
| |
| ty::ReEarlyBound(..) |
| | ty::ReFree(_) |
| | ty::ReVar(_) |
| | ty::RePlaceholder(..) |
| | ty::ReStatic |
| | ty::ReError(_) |
| | ty::ReErased => self.interner().lifetimes.re_erased, |
| } |
| } |
| |
| #[inline] |
| fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> { |
| if !t.has_infer() && !t.has_erasable_regions() { |
| t |
| } else { |
| match *t.kind() { |
| ty::Infer(v) => self.fold_infer_ty(v).unwrap_or(t), |
| |
| // This code is hot enough that a non-debug assertion here makes a noticeable |
| // difference on benchmarks like `wg-grammar`. |
| #[cfg(debug_assertions)] |
| ty::Placeholder(..) | ty::Bound(..) => bug!("unexpected type {:?}", t), |
| |
| _ => t.super_fold_with(self), |
| } |
| } |
| } |
| |
| fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> { |
| match ct.kind() { |
| ty::ConstKind::Infer(ty::InferConst::Var(v)) => { |
| let opt_ct = self |
| .infcx |
| .inner |
| .borrow_mut() |
| .const_unification_table() |
| .probe_value(v) |
| .val |
| .known(); |
| self.freshen_const(opt_ct, ty::InferConst::Var(v), ty::InferConst::Fresh, ct.ty()) |
| } |
| ty::ConstKind::Infer(ty::InferConst::Fresh(i)) => { |
| if i >= self.const_freshen_count { |
| bug!( |
| "Encountered a freshend const with id {} \ |
| but our counter is only at {}", |
| i, |
| self.const_freshen_count, |
| ); |
| } |
| ct |
| } |
| |
| ty::ConstKind::Bound(..) | ty::ConstKind::Placeholder(_) => { |
| bug!("unexpected const {:?}", ct) |
| } |
| |
| ty::ConstKind::Param(_) |
| | ty::ConstKind::Value(_) |
| | ty::ConstKind::Unevaluated(..) |
| | ty::ConstKind::Expr(..) |
| | ty::ConstKind::Error(_) => ct.super_fold_with(self), |
| } |
| } |
| } |
| |
| impl<'a, 'tcx> TypeFreshener<'a, 'tcx> { |
| // This is separate from `fold_ty` to keep that method small and inlinable. |
| #[inline(never)] |
| fn fold_infer_ty(&mut self, v: ty::InferTy) -> Option<Ty<'tcx>> { |
| match v { |
| ty::TyVar(v) => { |
| let opt_ty = self.infcx.inner.borrow_mut().type_variables().probe(v).known(); |
| Some(self.freshen_ty(opt_ty, ty::TyVar(v), |n| Ty::new_fresh(self.infcx.tcx, n))) |
| } |
| |
| ty::IntVar(v) => Some( |
| self.freshen_ty( |
| self.infcx |
| .inner |
| .borrow_mut() |
| .int_unification_table() |
| .probe_value(v) |
| .map(|v| v.to_type(self.infcx.tcx)), |
| ty::IntVar(v), |
| |n| Ty::new_fresh_int(self.infcx.tcx, n), |
| ), |
| ), |
| |
| ty::FloatVar(v) => Some( |
| self.freshen_ty( |
| self.infcx |
| .inner |
| .borrow_mut() |
| .float_unification_table() |
| .probe_value(v) |
| .map(|v| v.to_type(self.infcx.tcx)), |
| ty::FloatVar(v), |
| |n| Ty::new_fresh_float(self.infcx.tcx, n), |
| ), |
| ), |
| |
| ty::FreshTy(ct) | ty::FreshIntTy(ct) | ty::FreshFloatTy(ct) => { |
| if ct >= self.ty_freshen_count { |
| bug!( |
| "Encountered a freshend type with id {} \ |
| but our counter is only at {}", |
| ct, |
| self.ty_freshen_count |
| ); |
| } |
| None |
| } |
| } |
| } |
| } |
