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//! **Canonicalization** is the key to constructing a query in the
//! middle of type inference. Ordinarily, it is not possible to store
//! types from type inference in query keys, because they contain
//! references to inference variables whose lifetimes are too short
//! and so forth. Canonicalizing a value T1 using `canonicalize_query`
//! produces two things:
//!
//! - a value T2 where each unbound inference variable has been
//! replaced with a **canonical variable**;
//! - a map M (of type `CanonicalVarValues`) from those canonical
//! variables back to the original.
//!
//! We can then do queries using T2. These will give back constraints
//! on the canonical variables which can be translated, using the map
//! M, into constraints in our source context. This process of
//! translating the results back is done by the
//! `instantiate_query_result` method.
//!
//! For a more detailed look at what is happening here, check
//! out the [chapter in the rustc dev guide][c].
//!
//! [c]: https://rust-lang.github.io/chalk/book/canonical_queries/canonicalization.html
use crate::infer::MemberConstraint;
use crate::mir::ConstraintCategory;
use crate::ty::GenericArg;
use crate::ty::{self, BoundVar, List, Region, Ty, TyCtxt};
use rustc_macros::HashStable;
use smallvec::SmallVec;
use std::fmt::Display;
use std::ops::Index;
/// A "canonicalized" type `V` is one where all free inference
/// variables have been rewritten to "canonical vars". These are
/// numbered starting from 0 in order of first appearance.
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable)]
#[derive(HashStable, TypeFoldable, TypeVisitable)]
pub struct Canonical<'tcx, V> {
pub value: V,
pub max_universe: ty::UniverseIndex,
pub variables: CanonicalVarInfos<'tcx>,
}
impl<'tcx, V: Display> std::fmt::Display for Canonical<'tcx, V> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(
f,
"Canonical {{ value: {}, max_universe: {:?}, variables: {:?} }}",
self.value, self.max_universe, self.variables
)
}
}
pub type CanonicalVarInfos<'tcx> = &'tcx List<CanonicalVarInfo<'tcx>>;
impl<'tcx> ty::TypeFoldable<TyCtxt<'tcx>> for CanonicalVarInfos<'tcx> {
fn try_fold_with<F: ty::FallibleTypeFolder<TyCtxt<'tcx>>>(
self,
folder: &mut F,
) -> Result<Self, F::Error> {
ty::util::fold_list(self, folder, |tcx, v| tcx.mk_canonical_var_infos(v))
}
}
/// A set of values corresponding to the canonical variables from some
/// `Canonical`. You can give these values to
/// `canonical_value.substitute` to substitute them into the canonical
/// value at the right places.
///
/// When you canonicalize a value `V`, you get back one of these
/// vectors with the original values that were replaced by canonical
/// variables. You will need to supply it later to instantiate the
/// canonicalized query response.
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable)]
#[derive(HashStable, TypeFoldable, TypeVisitable)]
pub struct CanonicalVarValues<'tcx> {
pub var_values: ty::GenericArgsRef<'tcx>,
}
impl CanonicalVarValues<'_> {
pub fn is_identity(&self) -> bool {
self.var_values.iter().enumerate().all(|(bv, arg)| match arg.unpack() {
ty::GenericArgKind::Lifetime(r) => {
matches!(*r, ty::ReLateBound(ty::INNERMOST, br) if br.var.as_usize() == bv)
}
ty::GenericArgKind::Type(ty) => {
matches!(*ty.kind(), ty::Bound(ty::INNERMOST, bt) if bt.var.as_usize() == bv)
}
ty::GenericArgKind::Const(ct) => {
matches!(ct.kind(), ty::ConstKind::Bound(ty::INNERMOST, bc) if bc.as_usize() == bv)
}
})
}
pub fn is_identity_modulo_regions(&self) -> bool {
let mut var = ty::BoundVar::from_u32(0);
for arg in self.var_values {
match arg.unpack() {
ty::GenericArgKind::Lifetime(r) => {
if let ty::ReLateBound(ty::INNERMOST, br) = *r
&& var == br.var
{
var = var + 1;
} else {
// It's ok if this region var isn't unique
}
},
ty::GenericArgKind::Type(ty) => {
if let ty::Bound(ty::INNERMOST, bt) = *ty.kind()
&& var == bt.var
{
var = var + 1;
} else {
return false;
}
}
ty::GenericArgKind::Const(ct) => {
if let ty::ConstKind::Bound(ty::INNERMOST, bc) = ct.kind()
&& var == bc
{
var = var + 1;
} else {
return false;
}
}
}
}
true
}
}
/// When we canonicalize a value to form a query, we wind up replacing
/// various parts of it with canonical variables. This struct stores
/// those replaced bits to remember for when we process the query
/// result.
#[derive(Clone, Debug)]
pub struct OriginalQueryValues<'tcx> {
/// Map from the universes that appear in the query to the universes in the
/// caller context. For all queries except `evaluate_goal` (used by Chalk),
/// we only ever put ROOT values into the query, so this map is very
/// simple.
pub universe_map: SmallVec<[ty::UniverseIndex; 4]>,
/// This is equivalent to `CanonicalVarValues`, but using a
/// `SmallVec` yields a significant performance win.
pub var_values: SmallVec<[GenericArg<'tcx>; 8]>,
}
impl<'tcx> Default for OriginalQueryValues<'tcx> {
fn default() -> Self {
let mut universe_map = SmallVec::default();
universe_map.push(ty::UniverseIndex::ROOT);
Self { universe_map, var_values: SmallVec::default() }
}
}
/// Information about a canonical variable that is included with the
/// canonical value. This is sufficient information for code to create
/// a copy of the canonical value in some other inference context,
/// with fresh inference variables replacing the canonical values.
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
#[derive(TypeFoldable, TypeVisitable)]
pub struct CanonicalVarInfo<'tcx> {
pub kind: CanonicalVarKind<'tcx>,
}
impl<'tcx> CanonicalVarInfo<'tcx> {
pub fn universe(&self) -> ty::UniverseIndex {
self.kind.universe()
}
#[must_use]
pub fn with_updated_universe(self, ui: ty::UniverseIndex) -> CanonicalVarInfo<'tcx> {
CanonicalVarInfo { kind: self.kind.with_updated_universe(ui) }
}
pub fn is_existential(&self) -> bool {
match self.kind {
CanonicalVarKind::Ty(_) => true,
CanonicalVarKind::PlaceholderTy(_) => false,
CanonicalVarKind::Region(_) => true,
CanonicalVarKind::PlaceholderRegion(..) => false,
CanonicalVarKind::Const(..) => true,
CanonicalVarKind::PlaceholderConst(_, _) => false,
CanonicalVarKind::Effect => true,
}
}
pub fn is_region(&self) -> bool {
match self.kind {
CanonicalVarKind::Region(_) | CanonicalVarKind::PlaceholderRegion(_) => true,
CanonicalVarKind::Ty(_)
| CanonicalVarKind::PlaceholderTy(_)
| CanonicalVarKind::Const(_, _)
| CanonicalVarKind::PlaceholderConst(_, _)
| CanonicalVarKind::Effect => false,
}
}
pub fn expect_placeholder_index(self) -> usize {
match self.kind {
CanonicalVarKind::Ty(_)
| CanonicalVarKind::Region(_)
| CanonicalVarKind::Const(_, _)
| CanonicalVarKind::Effect => bug!("expected placeholder: {self:?}"),
CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.bound.var.as_usize(),
CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.bound.var.as_usize(),
CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.bound.as_usize(),
}
}
}
/// Describes the "kind" of the canonical variable. This is a "kind"
/// in the type-theory sense of the term -- i.e., a "meta" type system
/// that analyzes type-like values.
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
#[derive(TypeFoldable, TypeVisitable)]
pub enum CanonicalVarKind<'tcx> {
/// Some kind of type inference variable.
Ty(CanonicalTyVarKind),
/// A "placeholder" that represents "any type".
PlaceholderTy(ty::PlaceholderType),
/// Region variable `'?R`.
Region(ty::UniverseIndex),
/// A "placeholder" that represents "any region". Created when you
/// are solving a goal like `for<'a> T: Foo<'a>` to represent the
/// bound region `'a`.
PlaceholderRegion(ty::PlaceholderRegion),
/// Some kind of const inference variable.
Const(ty::UniverseIndex, Ty<'tcx>),
/// Effect variable `'?E`.
Effect,
/// A "placeholder" that represents "any const".
PlaceholderConst(ty::PlaceholderConst<'tcx>, Ty<'tcx>),
}
impl<'tcx> CanonicalVarKind<'tcx> {
pub fn universe(self) -> ty::UniverseIndex {
match self {
CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui)) => ui,
CanonicalVarKind::Ty(CanonicalTyVarKind::Float | CanonicalTyVarKind::Int) => {
ty::UniverseIndex::ROOT
}
CanonicalVarKind::Effect => ty::UniverseIndex::ROOT,
CanonicalVarKind::PlaceholderTy(placeholder) => placeholder.universe,
CanonicalVarKind::Region(ui) => ui,
CanonicalVarKind::PlaceholderRegion(placeholder) => placeholder.universe,
CanonicalVarKind::Const(ui, _) => ui,
CanonicalVarKind::PlaceholderConst(placeholder, _) => placeholder.universe,
}
}
/// Replaces the universe of this canonical variable with `ui`.
///
/// In case this is a float or int variable, this causes an ICE if
/// the updated universe is not the root.
pub fn with_updated_universe(self, ui: ty::UniverseIndex) -> CanonicalVarKind<'tcx> {
match self {
CanonicalVarKind::Ty(CanonicalTyVarKind::General(_)) => {
CanonicalVarKind::Ty(CanonicalTyVarKind::General(ui))
}
CanonicalVarKind::Ty(CanonicalTyVarKind::Int | CanonicalTyVarKind::Float)
| CanonicalVarKind::Effect => {
assert_eq!(ui, ty::UniverseIndex::ROOT);
self
}
CanonicalVarKind::PlaceholderTy(placeholder) => {
CanonicalVarKind::PlaceholderTy(ty::Placeholder { universe: ui, ..placeholder })
}
CanonicalVarKind::Region(_) => CanonicalVarKind::Region(ui),
CanonicalVarKind::PlaceholderRegion(placeholder) => {
CanonicalVarKind::PlaceholderRegion(ty::Placeholder { universe: ui, ..placeholder })
}
CanonicalVarKind::Const(_, ty) => CanonicalVarKind::Const(ui, ty),
CanonicalVarKind::PlaceholderConst(placeholder, ty) => {
CanonicalVarKind::PlaceholderConst(
ty::Placeholder { universe: ui, ..placeholder },
ty,
)
}
}
}
}
/// Rust actually has more than one category of type variables;
/// notably, the type variables we create for literals (e.g., 22 or
/// 22.) can only be instantiated with integral/float types (e.g.,
/// usize or f32). In order to faithfully reproduce a type, we need to
/// know what set of types a given type variable can be unified with.
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, TyDecodable, TyEncodable, HashStable)]
pub enum CanonicalTyVarKind {
/// General type variable `?T` that can be unified with arbitrary types.
General(ty::UniverseIndex),
/// Integral type variable `?I` (that can only be unified with integral types).
Int,
/// Floating-point type variable `?F` (that can only be unified with float types).
Float,
}
/// After we execute a query with a canonicalized key, we get back a
/// `Canonical<QueryResponse<..>>`. You can use
/// `instantiate_query_result` to access the data in this result.
#[derive(Clone, Debug, HashStable, TypeFoldable, TypeVisitable)]
pub struct QueryResponse<'tcx, R> {
pub var_values: CanonicalVarValues<'tcx>,
pub region_constraints: QueryRegionConstraints<'tcx>,
pub certainty: Certainty,
/// List of opaque types which we tried to compare to another type.
/// Inside the query we don't know yet whether the opaque type actually
/// should get its hidden type inferred. So we bubble the opaque type
/// and the type it was compared against upwards and let the query caller
/// handle it.
pub opaque_types: Vec<(ty::OpaqueTypeKey<'tcx>, Ty<'tcx>)>,
pub value: R,
}
#[derive(Clone, Debug, Default, PartialEq, Eq, Hash)]
#[derive(HashStable, TypeFoldable, TypeVisitable)]
pub struct QueryRegionConstraints<'tcx> {
pub outlives: Vec<QueryOutlivesConstraint<'tcx>>,
pub member_constraints: Vec<MemberConstraint<'tcx>>,
}
impl QueryRegionConstraints<'_> {
/// Represents an empty (trivially true) set of region
/// constraints.
pub fn is_empty(&self) -> bool {
self.outlives.is_empty() && self.member_constraints.is_empty()
}
}
pub type CanonicalQueryResponse<'tcx, T> = &'tcx Canonical<'tcx, QueryResponse<'tcx, T>>;
/// Indicates whether or not we were able to prove the query to be
/// true.
#[derive(Copy, Clone, Debug, HashStable)]
pub enum Certainty {
/// The query is known to be true, presuming that you apply the
/// given `var_values` and the region-constraints are satisfied.
Proven,
/// The query is not known to be true, but also not known to be
/// false. The `var_values` represent *either* values that must
/// hold in order for the query to be true, or helpful tips that
/// *might* make it true. Currently rustc's trait solver cannot
/// distinguish the two (e.g., due to our preference for where
/// clauses over impls).
///
/// After some unification and things have been done, it makes
/// sense to try and prove again -- of course, at that point, the
/// canonical form will be different, making this a distinct
/// query.
Ambiguous,
}
impl Certainty {
pub fn is_proven(&self) -> bool {
match self {
Certainty::Proven => true,
Certainty::Ambiguous => false,
}
}
}
impl<'tcx, R> QueryResponse<'tcx, R> {
pub fn is_proven(&self) -> bool {
self.certainty.is_proven()
}
}
impl<'tcx, R> Canonical<'tcx, QueryResponse<'tcx, R>> {
pub fn is_proven(&self) -> bool {
self.value.is_proven()
}
pub fn is_ambiguous(&self) -> bool {
!self.is_proven()
}
}
impl<'tcx, V> Canonical<'tcx, V> {
/// Allows you to map the `value` of a canonical while keeping the
/// same set of bound variables.
///
/// **WARNING:** This function is very easy to mis-use, hence the
/// name! In particular, the new value `W` must use all **the
/// same type/region variables** in **precisely the same order**
/// as the original! (The ordering is defined by the
/// `TypeFoldable` implementation of the type in question.)
///
/// An example of a **correct** use of this:
///
/// ```rust,ignore (not real code)
/// let a: Canonical<'_, T> = ...;
/// let b: Canonical<'_, (T,)> = a.unchecked_map(|v| (v, ));
/// ```
///
/// An example of an **incorrect** use of this:
///
/// ```rust,ignore (not real code)
/// let a: Canonical<'tcx, T> = ...;
/// let ty: Ty<'tcx> = ...;
/// let b: Canonical<'tcx, (T, Ty<'tcx>)> = a.unchecked_map(|v| (v, ty));
/// ```
pub fn unchecked_map<W>(self, map_op: impl FnOnce(V) -> W) -> Canonical<'tcx, W> {
let Canonical { max_universe, variables, value } = self;
Canonical { max_universe, variables, value: map_op(value) }
}
/// Allows you to map the `value` of a canonical while keeping the same set of
/// bound variables.
///
/// **WARNING:** This function is very easy to mis-use, hence the name! See
/// the comment of [Canonical::unchecked_map] for more details.
pub fn unchecked_rebind<W>(self, value: W) -> Canonical<'tcx, W> {
let Canonical { max_universe, variables, value: _ } = self;
Canonical { max_universe, variables, value }
}
}
pub type QueryOutlivesConstraint<'tcx> =
(ty::OutlivesPredicate<GenericArg<'tcx>, Region<'tcx>>, ConstraintCategory<'tcx>);
TrivialTypeTraversalImpls! {
crate::infer::canonical::Certainty,
crate::infer::canonical::CanonicalTyVarKind,
}
impl<'tcx> CanonicalVarValues<'tcx> {
// Given a list of canonical variables, construct a set of values which are
// the identity response.
pub fn make_identity(
tcx: TyCtxt<'tcx>,
infos: CanonicalVarInfos<'tcx>,
) -> CanonicalVarValues<'tcx> {
CanonicalVarValues {
var_values: tcx.mk_args_from_iter(infos.iter().enumerate().map(
|(i, info)| -> ty::GenericArg<'tcx> {
match info.kind {
CanonicalVarKind::Ty(_) | CanonicalVarKind::PlaceholderTy(_) => {
Ty::new_bound(tcx, ty::INNERMOST, ty::BoundVar::from_usize(i).into())
.into()
}
CanonicalVarKind::Region(_) | CanonicalVarKind::PlaceholderRegion(_) => {
let br = ty::BoundRegion {
var: ty::BoundVar::from_usize(i),
kind: ty::BrAnon,
};
ty::Region::new_late_bound(tcx, ty::INNERMOST, br).into()
}
CanonicalVarKind::Effect => ty::Const::new_bound(
tcx,
ty::INNERMOST,
ty::BoundVar::from_usize(i),
tcx.types.bool,
)
.into(),
CanonicalVarKind::Const(_, ty)
| CanonicalVarKind::PlaceholderConst(_, ty) => ty::Const::new_bound(
tcx,
ty::INNERMOST,
ty::BoundVar::from_usize(i),
ty,
)
.into(),
}
},
)),
}
}
/// Creates dummy var values which should not be used in a
/// canonical response.
pub fn dummy() -> CanonicalVarValues<'tcx> {
CanonicalVarValues { var_values: ty::List::empty() }
}
#[inline]
pub fn len(&self) -> usize {
self.var_values.len()
}
}
impl<'a, 'tcx> IntoIterator for &'a CanonicalVarValues<'tcx> {
type Item = GenericArg<'tcx>;
type IntoIter = ::std::iter::Copied<::std::slice::Iter<'a, GenericArg<'tcx>>>;
fn into_iter(self) -> Self::IntoIter {
self.var_values.iter()
}
}
impl<'tcx> Index<BoundVar> for CanonicalVarValues<'tcx> {
type Output = GenericArg<'tcx>;
fn index(&self, value: BoundVar) -> &GenericArg<'tcx> {
&self.var_values[value.as_usize()]
}
}