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use rustc_hir::def_id::DefId;
use rustc_middle::ty::{self, Ty, TyVid};
use rustc_span::symbol::Symbol;
use rustc_span::Span;
use crate::infer::InferCtxtUndoLogs;
use rustc_data_structures::snapshot_vec as sv;
use rustc_data_structures::unify as ut;
use std::cmp;
use std::marker::PhantomData;
use std::ops::Range;
use rustc_data_structures::undo_log::{Rollback, UndoLogs};
/// Represents a single undo-able action that affects a type inference variable.
#[derive(Clone)]
pub(crate) enum UndoLog<'tcx> {
EqRelation(sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>),
SubRelation(sv::UndoLog<ut::Delegate<ty::TyVid>>),
Values(sv::UndoLog<Delegate>),
}
/// Convert from a specific kind of undo to the more general UndoLog
impl<'tcx> From<sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>> for UndoLog<'tcx> {
fn from(l: sv::UndoLog<ut::Delegate<TyVidEqKey<'tcx>>>) -> Self {
UndoLog::EqRelation(l)
}
}
/// Convert from a specific kind of undo to the more general UndoLog
impl<'tcx> From<sv::UndoLog<ut::Delegate<ty::TyVid>>> for UndoLog<'tcx> {
fn from(l: sv::UndoLog<ut::Delegate<ty::TyVid>>) -> Self {
UndoLog::SubRelation(l)
}
}
/// Convert from a specific kind of undo to the more general UndoLog
impl<'tcx> From<sv::UndoLog<Delegate>> for UndoLog<'tcx> {
fn from(l: sv::UndoLog<Delegate>) -> Self {
UndoLog::Values(l)
}
}
/// Convert from a specific kind of undo to the more general UndoLog
impl<'tcx> From<Instantiate> for UndoLog<'tcx> {
fn from(l: Instantiate) -> Self {
UndoLog::Values(sv::UndoLog::Other(l))
}
}
impl<'tcx> Rollback<UndoLog<'tcx>> for TypeVariableStorage<'tcx> {
fn reverse(&mut self, undo: UndoLog<'tcx>) {
match undo {
UndoLog::EqRelation(undo) => self.eq_relations.reverse(undo),
UndoLog::SubRelation(undo) => self.sub_relations.reverse(undo),
UndoLog::Values(undo) => self.values.reverse(undo),
}
}
}
#[derive(Clone)]
pub struct TypeVariableStorage<'tcx> {
values: sv::SnapshotVecStorage<Delegate>,
/// Two variables are unified in `eq_relations` when we have a
/// constraint `?X == ?Y`. This table also stores, for each key,
/// the known value.
eq_relations: ut::UnificationTableStorage<TyVidEqKey<'tcx>>,
/// Two variables are unified in `sub_relations` when we have a
/// constraint `?X <: ?Y` *or* a constraint `?Y <: ?X`. This second
/// table exists only to help with the occurs check. In particular,
/// we want to report constraints like these as an occurs check
/// violation:
/// ``` text
/// ?1 <: ?3
/// Box<?3> <: ?1
/// ```
/// Without this second table, what would happen in a case like
/// this is that we would instantiate `?1` with a generalized
/// type like `Box<?6>`. We would then relate `Box<?3> <: Box<?6>`
/// and infer that `?3 <: ?6`. Next, since `?1` was instantiated,
/// we would process `?1 <: ?3`, generalize `?1 = Box<?6>` to `Box<?9>`,
/// and instantiate `?3` with `Box<?9>`. Finally, we would relate
/// `?6 <: ?9`. But now that we instantiated `?3`, we can process
/// `?3 <: ?6`, which gives us `Box<?9> <: ?6`... and the cycle
/// continues. (This is `occurs-check-2.rs`.)
///
/// What prevents this cycle is that when we generalize
/// `Box<?3>` to `Box<?6>`, we also sub-unify `?3` and `?6`
/// (in the generalizer). When we then process `Box<?6> <: ?3`,
/// the occurs check then fails because `?6` and `?3` are sub-unified,
/// and hence generalization fails.
///
/// This is reasonable because, in Rust, subtypes have the same
/// "skeleton" and hence there is no possible type such that
/// (e.g.) `Box<?3> <: ?3` for any `?3`.
///
/// In practice, we sometimes sub-unify variables in other spots, such
/// as when processing subtype predicates. This is not necessary but is
/// done to aid diagnostics, as it allows us to be more effective when
/// we guide the user towards where they should insert type hints.
sub_relations: ut::UnificationTableStorage<ty::TyVid>,
}
pub struct TypeVariableTable<'a, 'tcx> {
storage: &'a mut TypeVariableStorage<'tcx>,
undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
}
#[derive(Copy, Clone, Debug)]
pub struct TypeVariableOrigin {
pub kind: TypeVariableOriginKind,
pub span: Span,
}
/// Reasons to create a type inference variable
#[derive(Copy, Clone, Debug)]
pub enum TypeVariableOriginKind {
MiscVariable,
NormalizeProjectionType,
TypeInference,
OpaqueTypeInference(DefId),
TypeParameterDefinition(Symbol, DefId),
/// One of the upvars or closure kind parameters in a `ClosureArgs`
/// (before it has been determined).
// FIXME(eddyb) distinguish upvar inference variables from the rest.
ClosureSynthetic,
AutoDeref,
AdjustmentType,
/// In type check, when we are type checking a function that
/// returns `-> dyn Foo`, we substitute a type variable for the
/// return type for diagnostic purposes.
DynReturnFn,
LatticeVariable,
}
#[derive(Clone)]
pub(crate) struct TypeVariableData {
origin: TypeVariableOrigin,
}
#[derive(Copy, Clone, Debug)]
pub enum TypeVariableValue<'tcx> {
Known { value: Ty<'tcx> },
Unknown { universe: ty::UniverseIndex },
}
impl<'tcx> TypeVariableValue<'tcx> {
/// If this value is known, returns the type it is known to be.
/// Otherwise, `None`.
pub fn known(&self) -> Option<Ty<'tcx>> {
match *self {
TypeVariableValue::Unknown { .. } => None,
TypeVariableValue::Known { value } => Some(value),
}
}
pub fn is_unknown(&self) -> bool {
match *self {
TypeVariableValue::Unknown { .. } => true,
TypeVariableValue::Known { .. } => false,
}
}
}
#[derive(Clone)]
pub(crate) struct Instantiate;
pub(crate) struct Delegate;
impl<'tcx> TypeVariableStorage<'tcx> {
pub fn new() -> TypeVariableStorage<'tcx> {
TypeVariableStorage {
values: sv::SnapshotVecStorage::new(),
eq_relations: ut::UnificationTableStorage::new(),
sub_relations: ut::UnificationTableStorage::new(),
}
}
#[inline]
pub(crate) fn with_log<'a>(
&'a mut self,
undo_log: &'a mut InferCtxtUndoLogs<'tcx>,
) -> TypeVariableTable<'a, 'tcx> {
TypeVariableTable { storage: self, undo_log }
}
#[inline]
pub(crate) fn eq_relations_ref(&self) -> &ut::UnificationTableStorage<TyVidEqKey<'tcx>> {
&self.eq_relations
}
}
impl<'tcx> TypeVariableTable<'_, 'tcx> {
/// Returns the origin that was given when `vid` was created.
///
/// Note that this function does not return care whether
/// `vid` has been unified with something else or not.
pub fn var_origin(&self, vid: ty::TyVid) -> &TypeVariableOrigin {
&self.storage.values.get(vid.as_usize()).origin
}
/// Records that `a == b`, depending on `dir`.
///
/// Precondition: neither `a` nor `b` are known.
pub fn equate(&mut self, a: ty::TyVid, b: ty::TyVid) {
debug_assert!(self.probe(a).is_unknown());
debug_assert!(self.probe(b).is_unknown());
self.eq_relations().union(a, b);
self.sub_relations().union(a, b);
}
/// Records that `a <: b`, depending on `dir`.
///
/// Precondition: neither `a` nor `b` are known.
pub fn sub(&mut self, a: ty::TyVid, b: ty::TyVid) {
debug_assert!(self.probe(a).is_unknown());
debug_assert!(self.probe(b).is_unknown());
self.sub_relations().union(a, b);
}
/// Instantiates `vid` with the type `ty`.
///
/// Precondition: `vid` must not have been previously instantiated.
pub fn instantiate(&mut self, vid: ty::TyVid, ty: Ty<'tcx>) {
let vid = self.root_var(vid);
debug_assert!(self.probe(vid).is_unknown());
debug_assert!(
self.eq_relations().probe_value(vid).is_unknown(),
"instantiating type variable `{:?}` twice: new-value = {:?}, old-value={:?}",
vid,
ty,
self.eq_relations().probe_value(vid)
);
self.eq_relations().union_value(vid, TypeVariableValue::Known { value: ty });
// Hack: we only need this so that `types_escaping_snapshot`
// can see what has been unified; see the Delegate impl for
// more details.
self.undo_log.push(Instantiate);
}
/// Creates a new type variable.
///
/// - `diverging`: indicates if this is a "diverging" type
/// variable, e.g., one created as the type of a `return`
/// expression. The code in this module doesn't care if a
/// variable is diverging, but the main Rust type-checker will
/// sometimes "unify" such variables with the `!` or `()` types.
/// - `origin`: indicates *why* the type variable was created.
/// The code in this module doesn't care, but it can be useful
/// for improving error messages.
pub fn new_var(
&mut self,
universe: ty::UniverseIndex,
origin: TypeVariableOrigin,
) -> ty::TyVid {
let eq_key = self.eq_relations().new_key(TypeVariableValue::Unknown { universe });
let sub_key = self.sub_relations().new_key(());
assert_eq!(eq_key.vid, sub_key);
let index = self.values().push(TypeVariableData { origin });
assert_eq!(eq_key.vid.as_u32(), index as u32);
debug!("new_var(index={:?}, universe={:?}, origin={:?})", eq_key.vid, universe, origin);
eq_key.vid
}
/// Returns the number of type variables created thus far.
pub fn num_vars(&self) -> usize {
self.storage.values.len()
}
/// Returns the "root" variable of `vid` in the `eq_relations`
/// equivalence table. All type variables that have been equated
/// will yield the same root variable (per the union-find
/// algorithm), so `root_var(a) == root_var(b)` implies that `a ==
/// b` (transitively).
pub fn root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
self.eq_relations().find(vid).vid
}
/// Returns the "root" variable of `vid` in the `sub_relations`
/// equivalence table. All type variables that have been are
/// related via equality or subtyping will yield the same root
/// variable (per the union-find algorithm), so `sub_root_var(a)
/// == sub_root_var(b)` implies that:
/// ```text
/// exists X. (a <: X || X <: a) && (b <: X || X <: b)
/// ```
pub fn sub_root_var(&mut self, vid: ty::TyVid) -> ty::TyVid {
self.sub_relations().find(vid)
}
/// Returns `true` if `a` and `b` have same "sub-root" (i.e., exists some
/// type X such that `forall i in {a, b}. (i <: X || X <: i)`.
pub fn sub_unified(&mut self, a: ty::TyVid, b: ty::TyVid) -> bool {
self.sub_root_var(a) == self.sub_root_var(b)
}
/// Retrieves the type to which `vid` has been instantiated, if
/// any.
pub fn probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
self.inlined_probe(vid)
}
/// An always-inlined variant of `probe`, for very hot call sites.
#[inline(always)]
pub fn inlined_probe(&mut self, vid: ty::TyVid) -> TypeVariableValue<'tcx> {
self.eq_relations().inlined_probe_value(vid)
}
/// If `t` is a type-inference variable, and it has been
/// instantiated, then return the with which it was
/// instantiated. Otherwise, returns `t`.
pub fn replace_if_possible(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
match *t.kind() {
ty::Infer(ty::TyVar(v)) => match self.probe(v) {
TypeVariableValue::Unknown { .. } => t,
TypeVariableValue::Known { value } => value,
},
_ => t,
}
}
#[inline]
fn values(
&mut self,
) -> sv::SnapshotVec<Delegate, &mut Vec<TypeVariableData>, &mut InferCtxtUndoLogs<'tcx>> {
self.storage.values.with_log(self.undo_log)
}
#[inline]
fn eq_relations(&mut self) -> super::UnificationTable<'_, 'tcx, TyVidEqKey<'tcx>> {
self.storage.eq_relations.with_log(self.undo_log)
}
#[inline]
fn sub_relations(&mut self) -> super::UnificationTable<'_, 'tcx, ty::TyVid> {
self.storage.sub_relations.with_log(self.undo_log)
}
/// Returns a range of the type variables created during the snapshot.
pub fn vars_since_snapshot(
&mut self,
value_count: usize,
) -> (Range<TyVid>, Vec<TypeVariableOrigin>) {
let range = TyVid::from_usize(value_count)..TyVid::from_usize(self.num_vars());
(
range.start..range.end,
(range.start.as_usize()..range.end.as_usize())
.map(|index| self.storage.values.get(index).origin)
.collect(),
)
}
/// Returns indices of all variables that are not yet
/// instantiated.
pub fn unsolved_variables(&mut self) -> Vec<ty::TyVid> {
(0..self.storage.values.len())
.filter_map(|i| {
let vid = ty::TyVid::from_usize(i);
match self.probe(vid) {
TypeVariableValue::Unknown { .. } => Some(vid),
TypeVariableValue::Known { .. } => None,
}
})
.collect()
}
}
impl sv::SnapshotVecDelegate for Delegate {
type Value = TypeVariableData;
type Undo = Instantiate;
fn reverse(_values: &mut Vec<TypeVariableData>, _action: Instantiate) {
// We don't actually have to *do* anything to reverse an
// instantiation; the value for a variable is stored in the
// `eq_relations` and hence its rollback code will handle
// it. In fact, we could *almost* just remove the
// `SnapshotVec` entirely, except that we would have to
// reproduce *some* of its logic, since we want to know which
// type variables have been instantiated since the snapshot
// was started, so we can implement `types_escaping_snapshot`.
//
// (If we extended the `UnificationTable` to let us see which
// values have been unified and so forth, that might also
// suffice.)
}
}
///////////////////////////////////////////////////////////////////////////
/// These structs (a newtyped TyVid) are used as the unification key
/// for the `eq_relations`; they carry a `TypeVariableValue` along
/// with them.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(crate) struct TyVidEqKey<'tcx> {
vid: ty::TyVid,
// in the table, we map each ty-vid to one of these:
phantom: PhantomData<TypeVariableValue<'tcx>>,
}
impl<'tcx> From<ty::TyVid> for TyVidEqKey<'tcx> {
#[inline] // make this function eligible for inlining - it is quite hot.
fn from(vid: ty::TyVid) -> Self {
TyVidEqKey { vid, phantom: PhantomData }
}
}
impl<'tcx> ut::UnifyKey for TyVidEqKey<'tcx> {
type Value = TypeVariableValue<'tcx>;
#[inline(always)]
fn index(&self) -> u32 {
self.vid.as_u32()
}
#[inline]
fn from_index(i: u32) -> Self {
TyVidEqKey::from(ty::TyVid::from_u32(i))
}
fn tag() -> &'static str {
"TyVidEqKey"
}
}
impl<'tcx> ut::UnifyValue for TypeVariableValue<'tcx> {
type Error = ut::NoError;
fn unify_values(value1: &Self, value2: &Self) -> Result<Self, ut::NoError> {
match (value1, value2) {
// We never equate two type variables, both of which
// have known types. Instead, we recursively equate
// those types.
(&TypeVariableValue::Known { .. }, &TypeVariableValue::Known { .. }) => {
bug!("equating two type variables, both of which have known types")
}
// If one side is known, prefer that one.
(&TypeVariableValue::Known { .. }, &TypeVariableValue::Unknown { .. }) => Ok(*value1),
(&TypeVariableValue::Unknown { .. }, &TypeVariableValue::Known { .. }) => Ok(*value2),
// If both sides are *unknown*, it hardly matters, does it?
(
&TypeVariableValue::Unknown { universe: universe1 },
&TypeVariableValue::Unknown { universe: universe2 },
) => {
// If we unify two unbound variables, ?T and ?U, then whatever
// value they wind up taking (which must be the same value) must
// be nameable by both universes. Therefore, the resulting
// universe is the minimum of the two universes, because that is
// the one which contains the fewest names in scope.
let universe = cmp::min(universe1, universe2);
Ok(TypeVariableValue::Unknown { universe })
}
}
}
}