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#![feature(associated_type_defaults)]
#![feature(fmt_helpers_for_derive)]
#![feature(min_specialization)]
#![feature(never_type)]
#![feature(rustc_attrs)]
#![feature(unwrap_infallible)]
#![deny(rustc::untranslatable_diagnostic)]
#![deny(rustc::diagnostic_outside_of_impl)]
#![allow(internal_features)]
#[macro_use]
extern crate bitflags;
#[macro_use]
extern crate rustc_macros;
use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
use rustc_data_structures::unify::{EqUnifyValue, UnifyKey};
use smallvec::SmallVec;
use std::fmt;
use std::fmt::Debug;
use std::hash::Hash;
use std::mem::discriminant;
pub mod codec;
pub mod fold;
pub mod sty;
pub mod ty_info;
pub mod visit;
#[macro_use]
mod macros;
mod structural_impls;
pub use codec::*;
pub use structural_impls::{DebugWithInfcx, InferCtxtLike, OptWithInfcx};
pub use sty::*;
pub use ty_info::*;
/// Needed so we can use #[derive(HashStable_Generic)]
pub trait HashStableContext {}
pub trait Interner: Sized {
type AdtDef: Clone + Debug + Hash + Ord;
type GenericArgsRef: Clone
+ DebugWithInfcx<Self>
+ Hash
+ Ord
+ IntoIterator<Item = Self::GenericArg>;
type GenericArg: Clone + DebugWithInfcx<Self> + Hash + Ord;
type DefId: Clone + Debug + Hash + Ord;
type Binder<T>;
type Ty: Clone + DebugWithInfcx<Self> + Hash + Ord;
type Const: Clone + DebugWithInfcx<Self> + Hash + Ord;
type Region: Clone + DebugWithInfcx<Self> + Hash + Ord;
type Predicate;
type TypeAndMut: Clone + Debug + Hash + Ord;
type Mutability: Clone + Debug + Hash + Ord;
type Movability: Clone + Debug + Hash + Ord;
type PolyFnSig: Clone + DebugWithInfcx<Self> + Hash + Ord;
type ListBinderExistentialPredicate: Clone + DebugWithInfcx<Self> + Hash + Ord;
type BinderListTy: Clone + DebugWithInfcx<Self> + Hash + Ord;
type ListTy: Clone + Debug + Hash + Ord + IntoIterator<Item = Self::Ty>;
type AliasTy: Clone + DebugWithInfcx<Self> + Hash + Ord;
type ParamTy: Clone + Debug + Hash + Ord;
type BoundTy: Clone + Debug + Hash + Ord;
type PlaceholderType: Clone + Debug + Hash + Ord;
type InferTy: Clone + DebugWithInfcx<Self> + Hash + Ord;
type ErrorGuaranteed: Clone + Debug + Hash + Ord;
type PredicateKind: Clone + Debug + Hash + PartialEq + Eq;
type AllocId: Clone + Debug + Hash + Ord;
type InferConst: Clone + DebugWithInfcx<Self> + Hash + Ord;
type AliasConst: Clone + DebugWithInfcx<Self> + Hash + Ord;
type PlaceholderConst: Clone + Debug + Hash + Ord;
type ParamConst: Clone + Debug + Hash + Ord;
type BoundConst: Clone + Debug + Hash + Ord;
type ValueConst: Clone + Debug + Hash + Ord;
type ExprConst: Clone + DebugWithInfcx<Self> + Hash + Ord;
type EarlyBoundRegion: Clone + Debug + Hash + Ord;
type BoundRegion: Clone + Debug + Hash + Ord;
type FreeRegion: Clone + Debug + Hash + Ord;
type RegionVid: Clone + DebugWithInfcx<Self> + Hash + Ord;
type PlaceholderRegion: Clone + Debug + Hash + Ord;
fn ty_and_mut_to_parts(ty_and_mut: Self::TypeAndMut) -> (Self::Ty, Self::Mutability);
fn mutability_is_mut(mutbl: Self::Mutability) -> bool;
}
/// Imagine you have a function `F: FnOnce(&[T]) -> R`, plus an iterator `iter`
/// that produces `T` items. You could combine them with
/// `f(&iter.collect::<Vec<_>>())`, but this requires allocating memory for the
/// `Vec`.
///
/// This trait allows for faster implementations, intended for cases where the
/// number of items produced by the iterator is small. There is a blanket impl
/// for `T` items, but there is also a fallible impl for `Result<T, E>` items.
pub trait CollectAndApply<T, R>: Sized {
type Output;
/// Produce a result of type `Self::Output` from `iter`. The result will
/// typically be produced by applying `f` on the elements produced by
/// `iter`, though this may not happen in some impls, e.g. if an error
/// occurred during iteration.
fn collect_and_apply<I, F>(iter: I, f: F) -> Self::Output
where
I: Iterator<Item = Self>,
F: FnOnce(&[T]) -> R;
}
/// The blanket impl that always collects all elements and applies `f`.
impl<T, R> CollectAndApply<T, R> for T {
type Output = R;
/// Equivalent to `f(&iter.collect::<Vec<_>>())`.
fn collect_and_apply<I, F>(mut iter: I, f: F) -> R
where
I: Iterator<Item = T>,
F: FnOnce(&[T]) -> R,
{
// This code is hot enough that it's worth specializing for the most
// common length lists, to avoid the overhead of `SmallVec` creation.
// Lengths 0, 1, and 2 typically account for ~95% of cases. If
// `size_hint` is incorrect a panic will occur via an `unwrap` or an
// `assert`.
match iter.size_hint() {
(0, Some(0)) => {
assert!(iter.next().is_none());
f(&[])
}
(1, Some(1)) => {
let t0 = iter.next().unwrap();
assert!(iter.next().is_none());
f(&[t0])
}
(2, Some(2)) => {
let t0 = iter.next().unwrap();
let t1 = iter.next().unwrap();
assert!(iter.next().is_none());
f(&[t0, t1])
}
_ => f(&iter.collect::<SmallVec<[_; 8]>>()),
}
}
}
/// A fallible impl that will fail, without calling `f`, if there are any
/// errors during collection.
impl<T, R, E> CollectAndApply<T, R> for Result<T, E> {
type Output = Result<R, E>;
/// Equivalent to `Ok(f(&iter.collect::<Result<Vec<_>>>()?))`.
fn collect_and_apply<I, F>(mut iter: I, f: F) -> Result<R, E>
where
I: Iterator<Item = Result<T, E>>,
F: FnOnce(&[T]) -> R,
{
// This code is hot enough that it's worth specializing for the most
// common length lists, to avoid the overhead of `SmallVec` creation.
// Lengths 0, 1, and 2 typically account for ~95% of cases. If
// `size_hint` is incorrect a panic will occur via an `unwrap` or an
// `assert`, unless a failure happens first, in which case the result
// will be an error anyway.
Ok(match iter.size_hint() {
(0, Some(0)) => {
assert!(iter.next().is_none());
f(&[])
}
(1, Some(1)) => {
let t0 = iter.next().unwrap()?;
assert!(iter.next().is_none());
f(&[t0])
}
(2, Some(2)) => {
let t0 = iter.next().unwrap()?;
let t1 = iter.next().unwrap()?;
assert!(iter.next().is_none());
f(&[t0, t1])
}
_ => f(&iter.collect::<Result<SmallVec<[_; 8]>, _>>()?),
})
}
}
bitflags! {
/// Flags that we track on types. These flags are propagated upwards
/// through the type during type construction, so that we can quickly check
/// whether the type has various kinds of types in it without recursing
/// over the type itself.
pub struct TypeFlags: u32 {
// Does this have parameters? Used to determine whether substitution is
// required.
/// Does this have `Param`?
const HAS_TY_PARAM = 1 << 0;
/// Does this have `ReEarlyBound`?
const HAS_RE_PARAM = 1 << 1;
/// Does this have `ConstKind::Param`?
const HAS_CT_PARAM = 1 << 2;
const HAS_PARAM = TypeFlags::HAS_TY_PARAM.bits
| TypeFlags::HAS_RE_PARAM.bits
| TypeFlags::HAS_CT_PARAM.bits;
/// Does this have `Infer`?
const HAS_TY_INFER = 1 << 3;
/// Does this have `ReVar`?
const HAS_RE_INFER = 1 << 4;
/// Does this have `ConstKind::Infer`?
const HAS_CT_INFER = 1 << 5;
/// Does this have inference variables? Used to determine whether
/// inference is required.
const HAS_INFER = TypeFlags::HAS_TY_INFER.bits
| TypeFlags::HAS_RE_INFER.bits
| TypeFlags::HAS_CT_INFER.bits;
/// Does this have `Placeholder`?
const HAS_TY_PLACEHOLDER = 1 << 6;
/// Does this have `RePlaceholder`?
const HAS_RE_PLACEHOLDER = 1 << 7;
/// Does this have `ConstKind::Placeholder`?
const HAS_CT_PLACEHOLDER = 1 << 8;
/// Does this have placeholders?
const HAS_PLACEHOLDER = TypeFlags::HAS_TY_PLACEHOLDER.bits
| TypeFlags::HAS_RE_PLACEHOLDER.bits
| TypeFlags::HAS_CT_PLACEHOLDER.bits;
/// `true` if there are "names" of regions and so forth
/// that are local to a particular fn/inferctxt
const HAS_FREE_LOCAL_REGIONS = 1 << 9;
/// `true` if there are "names" of types and regions and so forth
/// that are local to a particular fn
const HAS_FREE_LOCAL_NAMES = TypeFlags::HAS_TY_PARAM.bits
| TypeFlags::HAS_CT_PARAM.bits
| TypeFlags::HAS_TY_INFER.bits
| TypeFlags::HAS_CT_INFER.bits
| TypeFlags::HAS_TY_PLACEHOLDER.bits
| TypeFlags::HAS_CT_PLACEHOLDER.bits
// We consider 'freshened' types and constants
// to depend on a particular fn.
// The freshening process throws away information,
// which can make things unsuitable for use in a global
// cache. Note that there is no 'fresh lifetime' flag -
// freshening replaces all lifetimes with `ReErased`,
// which is different from how types/const are freshened.
| TypeFlags::HAS_TY_FRESH.bits
| TypeFlags::HAS_CT_FRESH.bits
| TypeFlags::HAS_FREE_LOCAL_REGIONS.bits
| TypeFlags::HAS_RE_ERASED.bits;
/// Does this have `Projection`?
const HAS_TY_PROJECTION = 1 << 10;
/// Does this have `Inherent`?
const HAS_TY_INHERENT = 1 << 11;
/// Does this have `Opaque`?
const HAS_TY_OPAQUE = 1 << 12;
/// Does this have `ConstKind::Unevaluated`?
const HAS_CT_PROJECTION = 1 << 13;
/// Could this type be normalized further?
const HAS_PROJECTION = TypeFlags::HAS_TY_PROJECTION.bits
| TypeFlags::HAS_TY_OPAQUE.bits
| TypeFlags::HAS_TY_INHERENT.bits
| TypeFlags::HAS_CT_PROJECTION.bits;
/// Is an error type/const reachable?
const HAS_ERROR = 1 << 14;
/// Does this have any region that "appears free" in the type?
/// Basically anything but `ReLateBound` and `ReErased`.
const HAS_FREE_REGIONS = 1 << 15;
/// Does this have any `ReLateBound` regions?
const HAS_RE_LATE_BOUND = 1 << 16;
/// Does this have any `Bound` types?
const HAS_TY_LATE_BOUND = 1 << 17;
/// Does this have any `ConstKind::Bound` consts?
const HAS_CT_LATE_BOUND = 1 << 18;
/// Does this have any bound variables?
/// Used to check if a global bound is safe to evaluate.
const HAS_LATE_BOUND = TypeFlags::HAS_RE_LATE_BOUND.bits
| TypeFlags::HAS_TY_LATE_BOUND.bits
| TypeFlags::HAS_CT_LATE_BOUND.bits;
/// Does this have any `ReErased` regions?
const HAS_RE_ERASED = 1 << 19;
/// Does this value have parameters/placeholders/inference variables which could be
/// replaced later, in a way that would change the results of `impl` specialization?
const STILL_FURTHER_SPECIALIZABLE = 1 << 20;
/// Does this value have `InferTy::FreshTy/FreshIntTy/FreshFloatTy`?
const HAS_TY_FRESH = 1 << 21;
/// Does this value have `InferConst::Fresh`?
const HAS_CT_FRESH = 1 << 22;
/// Does this have `Generator` or `GeneratorWitness`?
const HAS_TY_GENERATOR = 1 << 23;
}
}
rustc_index::newtype_index! {
/// A [De Bruijn index][dbi] is a standard means of representing
/// regions (and perhaps later types) in a higher-ranked setting. In
/// particular, imagine a type like this:
/// ```ignore (illustrative)
/// for<'a> fn(for<'b> fn(&'b isize, &'a isize), &'a char)
/// // ^ ^ | | |
/// // | | | | |
/// // | +------------+ 0 | |
/// // | | |
/// // +----------------------------------+ 1 |
/// // | |
/// // +----------------------------------------------+ 0
/// ```
/// In this type, there are two binders (the outer fn and the inner
/// fn). We need to be able to determine, for any given region, which
/// fn type it is bound by, the inner or the outer one. There are
/// various ways you can do this, but a De Bruijn index is one of the
/// more convenient and has some nice properties. The basic idea is to
/// count the number of binders, inside out. Some examples should help
/// clarify what I mean.
///
/// Let's start with the reference type `&'b isize` that is the first
/// argument to the inner function. This region `'b` is assigned a De
/// Bruijn index of 0, meaning "the innermost binder" (in this case, a
/// fn). The region `'a` that appears in the second argument type (`&'a
/// isize`) would then be assigned a De Bruijn index of 1, meaning "the
/// second-innermost binder". (These indices are written on the arrows
/// in the diagram).
///
/// What is interesting is that De Bruijn index attached to a particular
/// variable will vary depending on where it appears. For example,
/// the final type `&'a char` also refers to the region `'a` declared on
/// the outermost fn. But this time, this reference is not nested within
/// any other binders (i.e., it is not an argument to the inner fn, but
/// rather the outer one). Therefore, in this case, it is assigned a
/// De Bruijn index of 0, because the innermost binder in that location
/// is the outer fn.
///
/// [dbi]: https://en.wikipedia.org/wiki/De_Bruijn_index
#[derive(HashStable_Generic)]
#[debug_format = "DebruijnIndex({})"]
pub struct DebruijnIndex {
const INNERMOST = 0;
}
}
impl DebruijnIndex {
/// Returns the resulting index when this value is moved into
/// `amount` number of new binders. So, e.g., if you had
///
/// for<'a> fn(&'a x)
///
/// and you wanted to change it to
///
/// for<'a> fn(for<'b> fn(&'a x))
///
/// you would need to shift the index for `'a` into a new binder.
#[inline]
#[must_use]
pub fn shifted_in(self, amount: u32) -> DebruijnIndex {
DebruijnIndex::from_u32(self.as_u32() + amount)
}
/// Update this index in place by shifting it "in" through
/// `amount` number of binders.
#[inline]
pub fn shift_in(&mut self, amount: u32) {
*self = self.shifted_in(amount);
}
/// Returns the resulting index when this value is moved out from
/// `amount` number of new binders.
#[inline]
#[must_use]
pub fn shifted_out(self, amount: u32) -> DebruijnIndex {
DebruijnIndex::from_u32(self.as_u32() - amount)
}
/// Update in place by shifting out from `amount` binders.
#[inline]
pub fn shift_out(&mut self, amount: u32) {
*self = self.shifted_out(amount);
}
/// Adjusts any De Bruijn indices so as to make `to_binder` the
/// innermost binder. That is, if we have something bound at `to_binder`,
/// it will now be bound at INNERMOST. This is an appropriate thing to do
/// when moving a region out from inside binders:
///
/// ```ignore (illustrative)
/// for<'a> fn(for<'b> for<'c> fn(&'a u32), _)
/// // Binder: D3 D2 D1 ^^
/// ```
///
/// Here, the region `'a` would have the De Bruijn index D3,
/// because it is the bound 3 binders out. However, if we wanted
/// to refer to that region `'a` in the second argument (the `_`),
/// those two binders would not be in scope. In that case, we
/// might invoke `shift_out_to_binder(D3)`. This would adjust the
/// De Bruijn index of `'a` to D1 (the innermost binder).
///
/// If we invoke `shift_out_to_binder` and the region is in fact
/// bound by one of the binders we are shifting out of, that is an
/// error (and should fail an assertion failure).
#[inline]
pub fn shifted_out_to_binder(self, to_binder: DebruijnIndex) -> Self {
self.shifted_out(to_binder.as_u32() - INNERMOST.as_u32())
}
}
pub fn debug_bound_var<T: std::fmt::Write>(
fmt: &mut T,
debruijn: DebruijnIndex,
var: impl std::fmt::Debug,
) -> Result<(), std::fmt::Error> {
if debruijn == INNERMOST {
write!(fmt, "^{var:?}")
} else {
write!(fmt, "^{}_{:?}", debruijn.index(), var)
}
}
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[derive(Encodable, Decodable, HashStable_Generic)]
pub enum IntTy {
Isize,
I8,
I16,
I32,
I64,
I128,
}
impl IntTy {
pub fn name_str(&self) -> &'static str {
match *self {
IntTy::Isize => "isize",
IntTy::I8 => "i8",
IntTy::I16 => "i16",
IntTy::I32 => "i32",
IntTy::I64 => "i64",
IntTy::I128 => "i128",
}
}
pub fn bit_width(&self) -> Option<u64> {
Some(match *self {
IntTy::Isize => return None,
IntTy::I8 => 8,
IntTy::I16 => 16,
IntTy::I32 => 32,
IntTy::I64 => 64,
IntTy::I128 => 128,
})
}
pub fn normalize(&self, target_width: u32) -> Self {
match self {
IntTy::Isize => match target_width {
16 => IntTy::I16,
32 => IntTy::I32,
64 => IntTy::I64,
_ => unreachable!(),
},
_ => *self,
}
}
pub fn to_unsigned(self) -> UintTy {
match self {
IntTy::Isize => UintTy::Usize,
IntTy::I8 => UintTy::U8,
IntTy::I16 => UintTy::U16,
IntTy::I32 => UintTy::U32,
IntTy::I64 => UintTy::U64,
IntTy::I128 => UintTy::U128,
}
}
}
#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Copy)]
#[derive(Encodable, Decodable, HashStable_Generic)]
pub enum UintTy {
Usize,
U8,
U16,
U32,
U64,
U128,
}
impl UintTy {
pub fn name_str(&self) -> &'static str {
match *self {
UintTy::Usize => "usize",
UintTy::U8 => "u8",
UintTy::U16 => "u16",
UintTy::U32 => "u32",
UintTy::U64 => "u64",
UintTy::U128 => "u128",
}
}
pub fn bit_width(&self) -> Option<u64> {
Some(match *self {
UintTy::Usize => return None,
UintTy::U8 => 8,
UintTy::U16 => 16,
UintTy::U32 => 32,
UintTy::U64 => 64,
UintTy::U128 => 128,
})
}
pub fn normalize(&self, target_width: u32) -> Self {
match self {
UintTy::Usize => match target_width {
16 => UintTy::U16,
32 => UintTy::U32,
64 => UintTy::U64,
_ => unreachable!(),
},
_ => *self,
}
}
pub fn to_signed(self) -> IntTy {
match self {
UintTy::Usize => IntTy::Isize,
UintTy::U8 => IntTy::I8,
UintTy::U16 => IntTy::I16,
UintTy::U32 => IntTy::I32,
UintTy::U64 => IntTy::I64,
UintTy::U128 => IntTy::I128,
}
}
}
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[derive(Encodable, Decodable, HashStable_Generic)]
pub enum FloatTy {
F32,
F64,
}
impl FloatTy {
pub fn name_str(self) -> &'static str {
match self {
FloatTy::F32 => "f32",
FloatTy::F64 => "f64",
}
}
pub fn bit_width(self) -> u64 {
match self {
FloatTy::F32 => 32,
FloatTy::F64 => 64,
}
}
}
#[derive(Clone, Copy, PartialEq, Eq)]
pub enum IntVarValue {
IntType(IntTy),
UintType(UintTy),
}
#[derive(Clone, Copy, PartialEq, Eq)]
pub struct FloatVarValue(pub FloatTy);
rustc_index::newtype_index! {
/// A **ty**pe **v**ariable **ID**.
#[debug_format = "?{}t"]
pub struct TyVid {}
}
rustc_index::newtype_index! {
/// An **int**egral (`u32`, `i32`, `usize`, etc.) type **v**ariable **ID**.
#[debug_format = "?{}i"]
pub struct IntVid {}
}
rustc_index::newtype_index! {
/// A **float**ing-point (`f32` or `f64`) type **v**ariable **ID**.
#[debug_format = "?{}f"]
pub struct FloatVid {}
}
/// A placeholder for a type that hasn't been inferred yet.
///
/// E.g., if we have an empty array (`[]`), then we create a fresh
/// type variable for the element type since we won't know until it's
/// used what the element type is supposed to be.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Encodable, Decodable)]
pub enum InferTy {
/// A type variable.
TyVar(TyVid),
/// An integral type variable (`{integer}`).
///
/// These are created when the compiler sees an integer literal like
/// `1` that could be several different types (`u8`, `i32`, `u32`, etc.).
/// We don't know until it's used what type it's supposed to be, so
/// we create a fresh type variable.
IntVar(IntVid),
/// A floating-point type variable (`{float}`).
///
/// These are created when the compiler sees an float literal like
/// `1.0` that could be either an `f32` or an `f64`.
/// We don't know until it's used what type it's supposed to be, so
/// we create a fresh type variable.
FloatVar(FloatVid),
/// A [`FreshTy`][Self::FreshTy] is one that is generated as a replacement
/// for an unbound type variable. This is convenient for caching etc. See
/// `rustc_infer::infer::freshen` for more details.
///
/// Compare with [`TyVar`][Self::TyVar].
FreshTy(u32),
/// Like [`FreshTy`][Self::FreshTy], but as a replacement for [`IntVar`][Self::IntVar].
FreshIntTy(u32),
/// Like [`FreshTy`][Self::FreshTy], but as a replacement for [`FloatVar`][Self::FloatVar].
FreshFloatTy(u32),
}
/// Raw `TyVid` are used as the unification key for `sub_relations`;
/// they carry no values.
impl UnifyKey for TyVid {
type Value = ();
#[inline]
fn index(&self) -> u32 {
self.as_u32()
}
#[inline]
fn from_index(i: u32) -> TyVid {
TyVid::from_u32(i)
}
fn tag() -> &'static str {
"TyVid"
}
}
impl EqUnifyValue for IntVarValue {}
impl UnifyKey for IntVid {
type Value = Option<IntVarValue>;
#[inline] // make this function eligible for inlining - it is quite hot.
fn index(&self) -> u32 {
self.as_u32()
}
#[inline]
fn from_index(i: u32) -> IntVid {
IntVid::from_u32(i)
}
fn tag() -> &'static str {
"IntVid"
}
}
impl EqUnifyValue for FloatVarValue {}
impl UnifyKey for FloatVid {
type Value = Option<FloatVarValue>;
#[inline]
fn index(&self) -> u32 {
self.as_u32()
}
#[inline]
fn from_index(i: u32) -> FloatVid {
FloatVid::from_u32(i)
}
fn tag() -> &'static str {
"FloatVid"
}
}
#[derive(Copy, Clone, PartialEq, Eq, Decodable, Encodable, Hash, HashStable_Generic)]
#[rustc_pass_by_value]
pub enum Variance {
Covariant, // T<A> <: T<B> iff A <: B -- e.g., function return type
Invariant, // T<A> <: T<B> iff B == A -- e.g., type of mutable cell
Contravariant, // T<A> <: T<B> iff B <: A -- e.g., function param type
Bivariant, // T<A> <: T<B> -- e.g., unused type parameter
}
impl Variance {
/// `a.xform(b)` combines the variance of a context with the
/// variance of a type with the following meaning. If we are in a
/// context with variance `a`, and we encounter a type argument in
/// a position with variance `b`, then `a.xform(b)` is the new
/// variance with which the argument appears.
///
/// Example 1:
/// ```ignore (illustrative)
/// *mut Vec<i32>
/// ```
/// Here, the "ambient" variance starts as covariant. `*mut T` is
/// invariant with respect to `T`, so the variance in which the
/// `Vec<i32>` appears is `Covariant.xform(Invariant)`, which
/// yields `Invariant`. Now, the type `Vec<T>` is covariant with
/// respect to its type argument `T`, and hence the variance of
/// the `i32` here is `Invariant.xform(Covariant)`, which results
/// (again) in `Invariant`.
///
/// Example 2:
/// ```ignore (illustrative)
/// fn(*const Vec<i32>, *mut Vec<i32)
/// ```
/// The ambient variance is covariant. A `fn` type is
/// contravariant with respect to its parameters, so the variance
/// within which both pointer types appear is
/// `Covariant.xform(Contravariant)`, or `Contravariant`. `*const
/// T` is covariant with respect to `T`, so the variance within
/// which the first `Vec<i32>` appears is
/// `Contravariant.xform(Covariant)` or `Contravariant`. The same
/// is true for its `i32` argument. In the `*mut T` case, the
/// variance of `Vec<i32>` is `Contravariant.xform(Invariant)`,
/// and hence the outermost type is `Invariant` with respect to
/// `Vec<i32>` (and its `i32` argument).
///
/// Source: Figure 1 of "Taming the Wildcards:
/// Combining Definition- and Use-Site Variance" published in PLDI'11.
pub fn xform(self, v: Variance) -> Variance {
match (self, v) {
// Figure 1, column 1.
(Variance::Covariant, Variance::Covariant) => Variance::Covariant,
(Variance::Covariant, Variance::Contravariant) => Variance::Contravariant,
(Variance::Covariant, Variance::Invariant) => Variance::Invariant,
(Variance::Covariant, Variance::Bivariant) => Variance::Bivariant,
// Figure 1, column 2.
(Variance::Contravariant, Variance::Covariant) => Variance::Contravariant,
(Variance::Contravariant, Variance::Contravariant) => Variance::Covariant,
(Variance::Contravariant, Variance::Invariant) => Variance::Invariant,
(Variance::Contravariant, Variance::Bivariant) => Variance::Bivariant,
// Figure 1, column 3.
(Variance::Invariant, _) => Variance::Invariant,
// Figure 1, column 4.
(Variance::Bivariant, _) => Variance::Bivariant,
}
}
}
impl<CTX> HashStable<CTX> for InferTy {
fn hash_stable(&self, ctx: &mut CTX, hasher: &mut StableHasher) {
use InferTy::*;
discriminant(self).hash_stable(ctx, hasher);
match self {
TyVar(_) | IntVar(_) | FloatVar(_) => {
panic!("type variables should not be hashed: {self:?}")
}
FreshTy(v) | FreshIntTy(v) | FreshFloatTy(v) => v.hash_stable(ctx, hasher),
}
}
}
impl fmt::Debug for IntVarValue {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match *self {
IntVarValue::IntType(ref v) => v.fmt(f),
IntVarValue::UintType(ref v) => v.fmt(f),
}
}
}
impl fmt::Debug for FloatVarValue {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
self.0.fmt(f)
}
}
impl fmt::Debug for Variance {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str(match *self {
Variance::Covariant => "+",
Variance::Contravariant => "-",
Variance::Invariant => "o",
Variance::Bivariant => "*",
})
}
}
impl fmt::Display for InferTy {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
use InferTy::*;
match *self {
TyVar(_) => write!(f, "_"),
IntVar(_) => write!(f, "{}", "{integer}"),
FloatVar(_) => write!(f, "{}", "{float}"),
FreshTy(v) => write!(f, "FreshTy({v})"),
FreshIntTy(v) => write!(f, "FreshIntTy({v})"),
FreshFloatTy(v) => write!(f, "FreshFloatTy({v})"),
}
}
}
rustc_index::newtype_index! {
/// "Universes" are used during type- and trait-checking in the
/// presence of `for<..>` binders to control what sets of names are
/// visible. Universes are arranged into a tree: the root universe
/// contains names that are always visible. Each child then adds a new
/// set of names that are visible, in addition to those of its parent.
/// We say that the child universe "extends" the parent universe with
/// new names.
///
/// To make this more concrete, consider this program:
///
/// ```ignore (illustrative)
/// struct Foo { }
/// fn bar<T>(x: T) {
/// let y: for<'a> fn(&'a u8, Foo) = ...;
/// }
/// ```
///
/// The struct name `Foo` is in the root universe U0. But the type
/// parameter `T`, introduced on `bar`, is in an extended universe U1
/// -- i.e., within `bar`, we can name both `T` and `Foo`, but outside
/// of `bar`, we cannot name `T`. Then, within the type of `y`, the
/// region `'a` is in a universe U2 that extends U1, because we can
/// name it inside the fn type but not outside.
///
/// Universes are used to do type- and trait-checking around these
/// "forall" binders (also called **universal quantification**). The
/// idea is that when, in the body of `bar`, we refer to `T` as a
/// type, we aren't referring to any type in particular, but rather a
/// kind of "fresh" type that is distinct from all other types we have
/// actually declared. This is called a **placeholder** type, and we
/// use universes to talk about this. In other words, a type name in
/// universe 0 always corresponds to some "ground" type that the user
/// declared, but a type name in a non-zero universe is a placeholder
/// type -- an idealized representative of "types in general" that we
/// use for checking generic functions.
#[derive(HashStable_Generic)]
#[debug_format = "U{}"]
pub struct UniverseIndex {}
}
impl UniverseIndex {
pub const ROOT: UniverseIndex = UniverseIndex::from_u32(0);
/// Returns the "next" universe index in order -- this new index
/// is considered to extend all previous universes. This
/// corresponds to entering a `forall` quantifier. So, for
/// example, suppose we have this type in universe `U`:
///
/// ```ignore (illustrative)
/// for<'a> fn(&'a u32)
/// ```
///
/// Once we "enter" into this `for<'a>` quantifier, we are in a
/// new universe that extends `U` -- in this new universe, we can
/// name the region `'a`, but that region was not nameable from
/// `U` because it was not in scope there.
pub fn next_universe(self) -> UniverseIndex {
UniverseIndex::from_u32(self.private.checked_add(1).unwrap())
}
/// Returns `true` if `self` can name a name from `other` -- in other words,
/// if the set of names in `self` is a superset of those in
/// `other` (`self >= other`).
pub fn can_name(self, other: UniverseIndex) -> bool {
self.private >= other.private
}
/// Returns `true` if `self` cannot name some names from `other` -- in other
/// words, if the set of names in `self` is a strict subset of
/// those in `other` (`self < other`).
pub fn cannot_name(self, other: UniverseIndex) -> bool {
self.private < other.private
}
}