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|
//! 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 substs,
//! 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, TypeVisitable};
use std::collections::hash_map::Entry;
pub struct TypeFreshener<'a, 'tcx> {
infcx: &'a InferCtxt<'a, '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>>,
keep_static: bool,
}
impl<'a, 'tcx> TypeFreshener<'a, 'tcx> {
pub fn new(infcx: &'a InferCtxt<'a, 'tcx>, keep_static: bool) -> TypeFreshener<'a, 'tcx> {
TypeFreshener {
infcx,
ty_freshen_count: 0,
const_freshen_count: 0,
ty_freshen_map: Default::default(),
const_freshen_map: Default::default(),
keep_static,
}
}
fn freshen_ty<F>(
&mut self,
opt_ty: Option<Ty<'tcx>>,
key: ty::InferTy,
freshener: F,
) -> Ty<'tcx>
where
F: FnOnce(u32) -> ty::InferTy,
{
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 = self.infcx.tcx.mk_ty_infer(freshener(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 = self.infcx.tcx.mk_const_infer(freshener(index), ty);
entry.insert(ct);
ct
}
}
}
}
impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
fn tcx<'b>(&'b 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::ReErased => {
// replace all free regions with 'erased
self.tcx().lifetimes.re_erased
}
ty::ReStatic => {
if self.keep_static {
r
} else {
self.tcx().lifetimes.re_erased
}
}
}
}
fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
if !t.needs_infer() && !t.has_erasable_regions() {
return t;
}
let tcx = self.infcx.tcx;
match *t.kind() {
ty::Infer(ty::TyVar(v)) => {
let opt_ty = self.infcx.inner.borrow_mut().type_variables().probe(v).known();
self.freshen_ty(opt_ty, ty::TyVar(v), ty::FreshTy)
}
ty::Infer(ty::IntVar(v)) => self.freshen_ty(
self.infcx
.inner
.borrow_mut()
.int_unification_table()
.probe_value(v)
.map(|v| v.to_type(tcx)),
ty::IntVar(v),
ty::FreshIntTy,
),
ty::Infer(ty::FloatVar(v)) => self.freshen_ty(
self.infcx
.inner
.borrow_mut()
.float_unification_table()
.probe_value(v)
.map(|v| v.to_type(tcx)),
ty::FloatVar(v),
ty::FreshFloatTy,
),
ty::Infer(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
);
}
t
}
ty::Generator(..)
| ty::Bool
| ty::Char
| ty::Int(..)
| ty::Uint(..)
| ty::Float(..)
| ty::Adt(..)
| ty::Str
| ty::Error(_)
| ty::Array(..)
| ty::Slice(..)
| ty::RawPtr(..)
| ty::Ref(..)
| ty::FnDef(..)
| ty::FnPtr(_)
| ty::Dynamic(..)
| ty::Never
| ty::Tuple(..)
| ty::Projection(..)
| ty::Foreign(..)
| ty::Param(..)
| ty::Closure(..)
| ty::GeneratorWitness(..)
| ty::Opaque(..) => t.super_fold_with(self),
ty::Placeholder(..) | ty::Bound(..) => bug!("unexpected type {:?}", t),
}
}
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::Error(_) => ct.super_fold_with(self),
}
}
}
|
