rustc_type_ir/

ty_kind.rs

use std::fmt;
use std::marker::PhantomData;
use std::ops::Deref;

use derive_where::derive_where;
use rustc_abi::ExternAbi;
use rustc_ast_ir::Mutability;
#[cfg(feature = "nightly")]
use rustc_data_structures::stable_hash::{StableHash, StableHashCtxt, StableHasher};
#[cfg(feature = "nightly")]
use rustc_macros::{Decodable_NoContext, Encodable_NoContext, StableHash_NoContext};
use rustc_type_ir::data_structures::{NoError, UnifyKey, UnifyValue};
use rustc_type_ir_macros::{
    GenericTypeVisitable, Lift_Generic, TypeFoldable_Generic, TypeVisitable_Generic,
};

use self::TyKind::*;
pub use self::closure::*;
use crate::inherent::*;
use crate::ty::AliasTy;
#[cfg(feature = "nightly")]
use crate::visit::TypeVisitable;
use crate::{
    self as ty, BoundVarIndexKind, FloatTy, FreeAliasTy, InherentAliasTy, IntTy, Interner,
    OpaqueAliasTy, ProjectionAliasTy, UintTy, Unnormalized,
};

mod closure;

#[derive_where(Clone, Copy, Hash, PartialEq, Eq, Debug; I: Interner)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic, Lift_Generic)]
#[cfg_attr(
    feature = "nightly",
    derive(Encodable_NoContext, Decodable_NoContext, StableHash_NoContext)
)]
pub enum AliasTyKind<I: Interner> {
    /// A projection `<Type as Trait>::AssocType`.
    ///
    /// Can get normalized away if monomorphic enough.
    ///
    /// The `def_id` is the `DefId` of the `TraitItem` for the associated type.
    ///
    /// Note that the `def_id` is not the `DefId` of the `TraitRef` containing this
    /// associated type, which is in `interner.associated_item(def_id).container`,
    /// aka. `interner.parent(def_id)`.
    Projection { def_id: I::TraitAssocTyId },

    /// An associated type in an inherent `impl`
    ///
    /// The `def_id` is the `DefId` of the `ImplItem` for the associated type.
    Inherent { def_id: I::InherentAssocTyId },

    /// An opaque type (usually from `impl Trait` in type aliases or function return types)
    ///
    /// `def_id` is the `DefId` of the `OpaqueType` item.
    ///
    ///
    /// Can only be normalized away in `PostAnalysis` mode or its defining scope.
    ///
    /// During codegen, `interner.type_of(def_id)` can be used to get the type of the
    /// underlying type if the type is an opaque.
    Opaque { def_id: I::OpaqueTyId },

    /// A type alias that actually checks its trait bounds.
    ///
    /// Currently only used if the type alias references opaque types.
    /// Can always be normalized away.
    Free { def_id: I::FreeTyAliasId },
}

impl<I: Interner> AliasTyKind<I> {
    pub fn descr(self) -> &'static str {
        match self {
            AliasTyKind::Projection { .. } => "associated type",
            AliasTyKind::Inherent { .. } => "inherent associated type",
            AliasTyKind::Opaque { .. } => "opaque type",
            AliasTyKind::Free { .. } => "type alias",
        }
    }

    pub fn try_to_projection(self) -> Option<I::TraitAssocTyId> {
        match self {
            AliasTyKind::Projection { def_id } => Some(def_id),
            _ => None,
        }
    }

    pub fn try_to_inherent(self) -> Option<I::InherentAssocTyId> {
        match self {
            AliasTyKind::Inherent { def_id } => Some(def_id),
            _ => None,
        }
    }

    pub fn try_to_opaque(self) -> Option<I::OpaqueTyId> {
        match self {
            AliasTyKind::Opaque { def_id } => Some(def_id),
            _ => None,
        }
    }

    pub fn try_to_free(self) -> Option<I::FreeTyAliasId> {
        match self {
            AliasTyKind::Free { def_id } => Some(def_id),
            _ => None,
        }
    }
}

/// Whether an alias type is rigid or potentially normalizeable.
///
/// This is not used by the old solver and is always `IsRigid::No` there. In the new solver,
/// aliases which cannot be further normalized in their current scope normalize to themselves
/// with `IsRigid::Yes`. At this point we no longer have to try and renormalize this alias
/// later on.
///
/// FIXME(#155345): Alias handling is currently still in flux for the new trait
/// solver and this is currently somewhat messy. Please reach out on
/// #t-types/trait-system-refactor-initiative if you encounter this and it isn't
/// immediately clear what to do.
#[derive(Debug, Clone, Copy, Hash, PartialEq)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic)]
#[cfg_attr(
    feature = "nightly",
    derive(Decodable_NoContext, Encodable_NoContext, StableHash_NoContext)
)]
pub enum IsRigid {
    Yes,
    No,
}

impl IsRigid {
    pub fn yes_if_next_solver<I: Interner>(interner: I) -> IsRigid {
        if interner.next_trait_solver_globally() { IsRigid::Yes } else { IsRigid::No }
    }
}

/// Defines the kinds of types used by the type system.
///
/// Types written by the user start out as `hir::TyKind` and get
/// converted to this representation using `<dyn HirTyLowerer>::lower_ty`.
#[cfg_attr(feature = "nightly", rustc_diagnostic_item = "IrTyKind")]
#[derive_where(Clone, Copy, Hash, PartialEq; I: Interner)]
#[derive(GenericTypeVisitable)]
#[cfg_attr(
    feature = "nightly",
    derive(Encodable_NoContext, Decodable_NoContext, StableHash_NoContext)
)]
pub enum TyKind<I: Interner> {
    /// The primitive boolean type. Written as `bool`.
    Bool,

    /// The primitive character type; holds a Unicode scalar value
    /// (a non-surrogate code point). Written as `char`.
    Char,

    /// A primitive signed integer type. For example, `i32`.
    Int(IntTy),

    /// A primitive unsigned integer type. For example, `u32`.
    Uint(UintTy),

    /// A primitive floating-point type. For example, `f64`.
    Float(FloatTy),

    /// Algebraic data types (ADT). For example: structures, enumerations and unions.
    ///
    /// For example, the type `List<i32>` would be represented using the `AdtDef`
    /// for `struct List<T>` and the args `[i32]`.
    ///
    /// Note that generic parameters in fields only get lazily instantiated
    /// by using something like `adt_def.all_fields().map(|field| field.ty(interner, args))`.
    Adt(I::AdtDef, I::GenericArgs),

    /// An unsized FFI type that is opaque to Rust. Written as `extern type T`.
    Foreign(I::ForeignId),

    /// The pointee of a string slice. Written as `str`.
    Str,

    /// An array with the given length. Written as `[T; N]`.
    Array(I::Ty, I::Const),

    /// A pattern newtype.
    ///
    /// Takes any type and restricts its valid values to its pattern.
    /// This will also change the layout to take advantage of this restriction.
    /// Only `Copy` and `Clone` will automatically get implemented for pattern types.
    /// Auto-traits treat this as if it were an aggregate with a single nested type.
    /// Only supports integer range patterns for now.
    Pat(I::Ty, I::Pat),

    /// The pointee of an array slice. Written as `[T]`.
    Slice(I::Ty),

    /// A raw pointer. Written as `*mut T` or `*const T`
    RawPtr(I::Ty, Mutability),

    /// A reference; a pointer with an associated lifetime. Written as
    /// `&'a mut T` or `&'a T`.
    Ref(I::Region, I::Ty, Mutability),

    /// The anonymous type of a function declaration/definition.
    ///
    /// Each function has a unique type.
    ///
    /// For the function `fn foo() -> i32 { 3 }` this type would be
    /// shown to the user as `fn() -> i32 {foo}`.
    ///
    /// For example the type of `bar` here:
    /// ```rust
    /// fn foo() -> i32 { 1 }
    /// let bar = foo; // bar: fn() -> i32 {foo}
    /// ```
    FnDef(I::FunctionId, ty::Binder<I, I::GenericArgs>),

    /// A pointer to a function.
    ///
    /// Written as `fn() -> i32`.
    ///
    /// Note that both functions and closures start out as either
    /// [FnDef] or [Closure] which can be then be coerced to this variant.
    ///
    /// For example the type of `bar` here:
    ///
    /// ```rust
    /// fn foo() -> i32 { 1 }
    /// let bar: fn() -> i32 = foo;
    /// ```
    ///
    /// These two fields are equivalent to a `ty::Binder<I, FnSig<I>>`. But by
    /// splitting that into two pieces, we get a more compact data layout that
    /// reduces the size of `TyKind` by 8 bytes. It is a very hot type, so it's
    /// worth the mild inconvenience.
    FnPtr(ty::Binder<I, FnSigTys<I>>, FnHeader<I>),

    /// An unsafe binder type.
    ///
    /// A higher-ranked type used to represent a type which has had some of its
    /// lifetimes erased. This can be used to represent types in positions where
    /// a lifetime is literally inexpressible, such as self-referential types.
    UnsafeBinder(UnsafeBinderInner<I>),

    /// A trait object. Written as `dyn for<'b> Trait<'b, Assoc = u32> + Send + 'a`.
    Dynamic(I::BoundExistentialPredicates, I::Region),

    /// The anonymous type of a closure. Used to represent the type of `|a| a`.
    ///
    /// Closure args contain both the - potentially instantiated - generic parameters
    /// of its parent and some synthetic parameters. See the documentation for
    /// `ClosureArgs` for more details.
    Closure(I::ClosureId, I::GenericArgs),

    /// The anonymous type of a closure. Used to represent the type of `async |a| a`.
    ///
    /// Coroutine-closure args contain both the - potentially instantiated - generic
    /// parameters of its parent and some synthetic parameters. See the documentation
    /// for `CoroutineClosureArgs` for more details.
    CoroutineClosure(I::CoroutineClosureId, I::GenericArgs),

    /// The anonymous type of a coroutine. Used to represent the type of
    /// `|a| yield a`.
    ///
    /// For more info about coroutine args, visit the documentation for
    /// `CoroutineArgs`.
    Coroutine(I::CoroutineId, I::GenericArgs),

    /// A type representing the types stored inside a coroutine.
    ///
    /// This should only appear as part of the `CoroutineArgs`.
    ///
    /// Unlike upvars, the witness can reference lifetimes from
    /// inside of the coroutine itself. To deal with them in
    /// the type of the coroutine, we convert them to higher ranked
    /// lifetimes bound by the witness itself.
    ///
    /// This contains the `DefId` and the `GenericArgsRef` of the coroutine.
    /// The actual witness types are computed on MIR by the `mir_coroutine_witnesses` query.
    ///
    /// Looking at the following example, the witness for this coroutine
    /// may end up as something like `for<'a> [Vec<i32>, &'a Vec<i32>]`:
    ///
    /// ```
    /// #![feature(coroutines)]
    /// #[coroutine] static |a| {
    ///     let x = &vec![3];
    ///     yield a;
    ///     yield x[0];
    /// }
    /// # ;
    /// ```
    CoroutineWitness(I::CoroutineId, I::GenericArgs),

    /// The never type `!`.
    Never,

    /// A tuple type. For example, `(i32, bool)`.
    Tuple(I::Tys),

    /// A projection, opaque type, free type alias, or inherent associated type.
    ///
    /// All of these types are represented as pairs of def-id and args, and can
    /// be normalized, so they are grouped conceptually.
    Alias(IsRigid, AliasTy<I>),

    /// A type parameter; for example, `T` in `fn f<T>(x: T) {}`.
    Param(I::ParamTy),

    /// Bound type variable, used to represent the `'a` in `for<'a> fn(&'a ())`.
    ///
    /// For canonical queries, we replace inference variables with bound variables,
    /// so e.g. when checking whether `&'_ (): Trait<_>` holds, we canonicalize that to
    /// `for<'a, T> &'a (): Trait<T>` and then convert the introduced bound variables
    /// back to inference variables in a new inference context when inside of the query.
    ///
    /// It is conventional to render anonymous bound types like `^N` or `^D_N`,
    /// where `N` is the bound variable's anonymous index into the binder, and
    /// `D` is the debruijn index, or totally omitted if the debruijn index is zero.
    ///
    /// See the `rustc-dev-guide` for more details about
    /// [higher-ranked trait bounds][1] and [canonical queries][2].
    ///
    /// [1]: https://rustc-dev-guide.rust-lang.org/traits/hrtb.html
    /// [2]: https://rustc-dev-guide.rust-lang.org/traits/canonical-queries.html
    Bound(BoundVarIndexKind, ty::BoundTy<I>),

    /// A placeholder type, used during higher ranked subtyping to instantiate
    /// bound variables.
    ///
    /// It is conventional to render anonymous placeholder types like `!N` or `!U_N`,
    /// where `N` is the placeholder variable's anonymous index (which corresponds
    /// to the bound variable's index from the binder from which it was instantiated),
    /// and `U` is the universe index in which it is instantiated, or totally omitted
    /// if the universe index is zero.
    Placeholder(ty::PlaceholderType<I>),

    /// A type variable used during type checking.
    ///
    /// Similar to placeholders, inference variables also live in a universe to
    /// correctly deal with higher ranked types. Though unlike placeholders,
    /// that universe is stored in the `InferCtxt` instead of directly
    /// inside of the type.
    Infer(InferTy),

    /// A placeholder for a type which could not be computed.
    ///
    /// This is propagated to avoid useless error messages.
    Error(I::ErrorGuaranteed),
}

impl<I: Interner> Eq for TyKind<I> {}

impl<I: Interner> TyKind<I> {
    pub fn fn_sig(self, interner: I) -> ty::Binder<I, ty::FnSig<I>> {
        self.unnormalized_fn_sig(interner).skip_normalization()
    }

    pub fn unnormalized_fn_sig(self, interner: I) -> Unnormalized<I, ty::Binder<I, ty::FnSig<I>>> {
        match self {
            ty::FnPtr(sig_tys, hdr) => Unnormalized::new_wip(sig_tys.with(hdr)),
            ty::FnDef(def_id, args) => {
                interner.fn_sig(def_id).instantiate(interner, args.no_bound_vars().unwrap())
            }
            ty::Error(_) => {
                // ignore errors (#54954)
                Unnormalized::dummy(ty::Binder::dummy(ty::FnSig::dummy()))
            }
            ty::Closure(..) => panic!(
                "to get the signature of a closure, use `args.as_closure().sig()` not `fn_sig()`",
            ),
            _ => panic!("Ty::fn_sig() called on non-fn type: {:?}", self),
        }
    }

    /// Returns `true` when the outermost type cannot be further normalized,
    /// resolved, or instantiated.
    ///
    /// This includes all primitive types, but also
    /// things like ADTs and trait objects, since even if their arguments or
    /// nested types may be further simplified, the outermost [`ty::TyKind`] or
    /// type constructor remains the same.
    pub fn is_known_rigid(self) -> bool {
        match self {
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Adt(_, _)
            | ty::Foreign(_)
            | ty::Str
            | ty::Array(_, _)
            | ty::Pat(_, _)
            | ty::Slice(_)
            | ty::RawPtr(_, _)
            | ty::Ref(_, _, _)
            | ty::FnDef(_, _)
            | ty::FnPtr(..)
            | ty::UnsafeBinder(_)
            | ty::Dynamic(_, _)
            | ty::Closure(_, _)
            | ty::CoroutineClosure(_, _)
            | ty::Coroutine(_, _)
            | ty::CoroutineWitness(..)
            | ty::Never
            | ty::Tuple(_) => true,

            ty::Error(_)
            | ty::Infer(_)
            | ty::Alias(ty::IsRigid::No | ty::IsRigid::Yes, _)
            | ty::Param(_)
            | ty::Bound(_, _)
            | ty::Placeholder(_) => false,
        }
    }
}

// This is manually implemented because a derive would require `I: Debug`
impl<I: Interner> fmt::Debug for TyKind<I> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Bool => write!(f, "bool"),
            Char => write!(f, "char"),
            Int(i) => write!(f, "{i:?}"),
            Uint(u) => write!(f, "{u:?}"),
            Float(float) => write!(f, "{float:?}"),
            Adt(d, s) => {
                write!(f, "{d:?}")?;
                let mut s = s.iter();
                let first = s.next();
                match first {
                    Some(first) => write!(f, "<{:?}", first)?,
                    None => return Ok(()),
                };

                for arg in s {
                    write!(f, ", {:?}", arg)?;
                }

                write!(f, ">")
            }
            Foreign(d) => f.debug_tuple("Foreign").field(d).finish(),
            Str => write!(f, "str"),
            Array(t, c) => write!(f, "[{t:?}; {c:?}]"),
            Pat(t, p) => write!(f, "pattern_type!({t:?} is {p:?})"),
            Slice(t) => write!(f, "[{:?}]", &t),
            RawPtr(ty, mutbl) => write!(f, "*{} {:?}", mutbl.ptr_str(), ty),
            Ref(r, t, m) => write!(f, "&{:?} {}{:?}", r, m.prefix_str(), t),
            FnDef(d, s) => f.debug_tuple("FnDef").field(d).field(&s).finish(),
            FnPtr(sig_tys, hdr) => write!(f, "{:?}", sig_tys.with(*hdr)),
            // FIXME(unsafe_binder): print this like `unsafe<'a> T<'a>`.
            UnsafeBinder(binder) => write!(f, "{:?}", binder),
            Dynamic(p, r) => write!(f, "dyn {p:?} + {r:?}"),
            Closure(d, s) => f.debug_tuple("Closure").field(d).field(&s).finish(),
            CoroutineClosure(d, s) => f.debug_tuple("CoroutineClosure").field(d).field(&s).finish(),
            Coroutine(d, s) => f.debug_tuple("Coroutine").field(d).field(&s).finish(),
            CoroutineWitness(d, s) => f.debug_tuple("CoroutineWitness").field(d).field(&s).finish(),
            Never => write!(f, "!"),
            Tuple(t) => {
                write!(f, "(")?;
                let mut count = 0;
                for ty in t.iter() {
                    if count > 0 {
                        write!(f, ", ")?;
                    }
                    write!(f, "{ty:?}")?;
                    count += 1;
                }
                // unary tuples need a trailing comma
                if count == 1 {
                    write!(f, ",")?;
                }
                write!(f, ")")
            }
            Alias(is_rigid, a) => f.debug_tuple("Alias").field(&is_rigid).field(&a).finish(),
            Param(p) => write!(f, "{p:?}"),
            Bound(d, b) => crate::debug_bound_var(f, *d, b),
            Placeholder(p) => write!(f, "{p:?}"),
            Infer(t) => write!(f, "{:?}", t),
            TyKind::Error(_) => write!(f, "{{type error}}"),
        }
    }
}

impl<I: Interner> AliasTy<I> {
    pub fn new_from_args(interner: I, kind: AliasTyKind<I>, args: I::GenericArgs) -> AliasTy<I> {
        if cfg!(debug_assertions) {
            let def_id = match kind {
                AliasTyKind::Projection { def_id } => def_id.into(),
                AliasTyKind::Inherent { def_id } => def_id.into(),
                AliasTyKind::Opaque { def_id } => def_id.into(),
                AliasTyKind::Free { def_id } => def_id.into(),
            };
            interner.debug_assert_args_compatible(def_id, args);
        }
        AliasTy { kind, args, _use_alias_new_instead: () }
    }

    pub fn new(
        interner: I,
        kind: AliasTyKind<I>,
        args: impl IntoIterator<Item: Into<I::GenericArg>>,
    ) -> AliasTy<I> {
        let args = interner.mk_args_from_iter(args.into_iter().map(Into::into));
        Self::new_from_args(interner, kind, args)
    }

    /// Whether this alias type is an opaque.
    pub fn is_opaque(self) -> bool {
        matches!(self.kind, AliasTyKind::Opaque { .. })
    }

    pub fn to_ty(self, interner: I, is_rigid: ty::IsRigid) -> I::Ty {
        Ty::new_alias(interner, is_rigid, self)
    }

    pub fn try_to_projection(self) -> Option<ProjectionAliasTy<I>> {
        self.kind.try_to_projection().map(|kind| ty::Alias {
            kind,
            args: self.args,
            _use_alias_new_instead: (),
        })
    }

    pub fn try_to_inherent(self) -> Option<InherentAliasTy<I>> {
        self.kind.try_to_inherent().map(|kind| ty::Alias {
            kind,
            args: self.args,
            _use_alias_new_instead: (),
        })
    }

    pub fn try_to_opaque(self) -> Option<OpaqueAliasTy<I>> {
        self.kind.try_to_opaque().map(|kind| ty::Alias {
            kind,
            args: self.args,
            _use_alias_new_instead: (),
        })
    }

    pub fn try_to_free(self) -> Option<FreeAliasTy<I>> {
        self.kind.try_to_free().map(|kind| ty::Alias {
            kind,
            args: self.args,
            _use_alias_new_instead: (),
        })
    }
}

impl<I: Interner> ProjectionAliasTy<I> {
    pub fn new_projection_from_args(
        interner: I,
        kind: I::TraitAssocTyId,
        args: I::GenericArgs,
    ) -> Self {
        interner.debug_assert_args_compatible(kind.into(), args);
        Self { kind, args, _use_alias_new_instead: () }
    }

    pub fn projection_to_alias_ty(self) -> AliasTy<I> {
        AliasTy {
            kind: AliasTyKind::Projection { def_id: self.kind },
            args: self.args,
            _use_alias_new_instead: (),
        }
    }

    #[track_caller]
    pub fn projection_self_ty(self) -> I::Ty {
        self.args.type_at(0)
    }
}

/// The following methods work only with (trait) associated type projections.
// FIXME: Migrate these to the impl on ProjectionAliasTy (by making callers use `try_to_projection`
// or similar when they guard for projections)
impl<I: Interner> AliasTy<I> {
    #[track_caller]
    pub fn self_ty(self) -> I::Ty {
        self.args.type_at(0)
    }

    pub fn with_replaced_self_ty(self, interner: I, self_ty: I::Ty) -> Self {
        AliasTy::new(
            interner,
            self.kind,
            [self_ty.into()].into_iter().chain(self.args.iter().skip(1)),
        )
    }

    pub fn trait_def_id(self, interner: I) -> I::TraitId {
        let AliasTyKind::Projection { def_id } = self.kind else { panic!("expected a projection") };

        interner.projection_parent(def_id.into())
    }

    /// Extracts the underlying trait reference and own args from this projection.
    ///
    /// For example, if this is a projection of `<T as StreamingIterator>::Item<'a>`,
    /// then this function would return a `T: StreamingIterator` trait reference and
    /// `['a]` as the own args.
    pub fn trait_ref_and_own_args(self, interner: I) -> (ty::TraitRef<I>, I::GenericArgsSlice) {
        let AliasTyKind::Projection { def_id } = self.kind else { panic!("expected a projection") };

        interner.trait_ref_and_own_args_for_alias(def_id.into(), self.args)
    }

    /// Extracts the underlying trait reference from this projection.
    ///
    /// For example, if this is a projection of `<T as Iterator>::Item`,
    /// then this function would return a `T: Iterator` trait reference.
    ///
    /// WARNING: This will drop the args for generic associated types
    /// consider calling [Self::trait_ref_and_own_args] to get those
    /// as well.
    pub fn trait_ref(self, interner: I) -> ty::TraitRef<I> {
        self.trait_ref_and_own_args(interner).0
    }
}

impl<I: Interner> InherentAliasTy<I> {
    pub fn new_inherent_from_args(
        interner: I,
        kind: I::InherentAssocTyId,
        args: I::GenericArgs,
    ) -> Self {
        interner.debug_assert_args_compatible(kind.into(), args);
        Self { kind, args, _use_alias_new_instead: () }
    }

    pub fn inherent_to_alias_ty(self) -> AliasTy<I> {
        AliasTy {
            kind: AliasTyKind::Inherent { def_id: self.kind },
            args: self.args,
            _use_alias_new_instead: (),
        }
    }
}

impl<I: Interner> OpaqueAliasTy<I> {
    pub fn new_opaque_from_args(interner: I, kind: I::OpaqueTyId, args: I::GenericArgs) -> Self {
        interner.debug_assert_args_compatible(kind.into(), args);
        Self { kind, args, _use_alias_new_instead: () }
    }

    pub fn opaque_to_alias_ty(self) -> AliasTy<I> {
        AliasTy {
            kind: AliasTyKind::Opaque { def_id: self.kind },
            args: self.args,
            _use_alias_new_instead: (),
        }
    }
}

impl<I: Interner> FreeAliasTy<I> {
    pub fn new_free_from_args(interner: I, kind: I::FreeTyAliasId, args: I::GenericArgs) -> Self {
        interner.debug_assert_args_compatible(kind.into(), args);
        Self { kind, args, _use_alias_new_instead: () }
    }

    pub fn free_to_alias_ty(self) -> AliasTy<I> {
        AliasTy {
            kind: AliasTyKind::Free { def_id: self.kind },
            args: self.args,
            _use_alias_new_instead: (),
        }
    }
}

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum IntVarValue {
    Unknown,
    IntType(IntTy),
    UintType(UintTy),
}

impl IntVarValue {
    pub fn is_known(self) -> bool {
        match self {
            IntVarValue::IntType(_) | IntVarValue::UintType(_) => true,
            IntVarValue::Unknown => false,
        }
    }

    pub fn is_unknown(self) -> bool {
        !self.is_known()
    }
}

#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum FloatVarValue {
    Unknown,
    Known(FloatTy),
}

impl FloatVarValue {
    pub fn is_known(self) -> bool {
        match self {
            FloatVarValue::Known(_) => true,
            FloatVarValue::Unknown => false,
        }
    }

    pub fn is_unknown(self) -> bool {
        !self.is_known()
    }
}

rustc_index::newtype_index! {
    /// A **ty**pe **v**ariable **ID**.
    #[encodable]
    #[orderable]
    #[debug_format = "?{}t"]
    #[gate_rustc_only]
    pub struct TyVid {}
}

rustc_index::newtype_index! {
    /// An **int**egral (`u32`, `i32`, `usize`, etc.) type **v**ariable **ID**.
    #[encodable]
    #[orderable]
    #[debug_format = "?{}i"]
    #[gate_rustc_only]
    pub struct IntVid {}
}

rustc_index::newtype_index! {
    /// A **float**ing-point (`f32` or `f64`) type **v**ariable **ID**.
    #[encodable]
    #[orderable]
    #[debug_format = "?{}f"]
    #[gate_rustc_only]
    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)]
#[cfg_attr(feature = "nightly", derive(Encodable_NoContext, Decodable_NoContext))]
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 `TypeFreshener` 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),
}

impl UnifyValue for IntVarValue {
    type Error = NoError;

    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, Self::Error> {
        match (*value1, *value2) {
            (IntVarValue::Unknown, IntVarValue::Unknown) => Ok(IntVarValue::Unknown),
            (
                IntVarValue::Unknown,
                known @ (IntVarValue::UintType(_) | IntVarValue::IntType(_)),
            )
            | (
                known @ (IntVarValue::UintType(_) | IntVarValue::IntType(_)),
                IntVarValue::Unknown,
            ) => Ok(known),
            _ => panic!("differing ints should have been resolved first"),
        }
    }
}

impl UnifyKey for IntVid {
    type Value = 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 UnifyValue for FloatVarValue {
    type Error = NoError;

    fn unify_values(value1: &Self, value2: &Self) -> Result<Self, Self::Error> {
        match (*value1, *value2) {
            (FloatVarValue::Unknown, FloatVarValue::Unknown) => Ok(FloatVarValue::Unknown),
            (FloatVarValue::Unknown, FloatVarValue::Known(known))
            | (FloatVarValue::Known(known), FloatVarValue::Unknown) => {
                Ok(FloatVarValue::Known(known))
            }
            (FloatVarValue::Known(_), FloatVarValue::Known(_)) => {
                panic!("differing floats should have been resolved first")
            }
        }
    }
}

impl UnifyKey for FloatVid {
    type Value = 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"
    }
}

#[cfg(feature = "nightly")]
impl StableHash for InferTy {
    fn stable_hash<Hcx: StableHashCtxt>(&self, hcx: &mut Hcx, hasher: &mut StableHasher) {
        use InferTy::*;
        std::mem::discriminant(self).stable_hash(hcx, hasher);
        match self {
            TyVar(_) | IntVar(_) | FloatVar(_) => {
                panic!("type variables should not be hashed: {self:?}")
            }
            FreshTy(v) | FreshIntTy(v) | FreshFloatTy(v) => v.stable_hash(hcx, hasher),
        }
    }
}

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})"),
        }
    }
}

impl fmt::Debug for InferTy {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        use InferTy::*;
        match *self {
            TyVar(ref v) => v.fmt(f),
            IntVar(ref v) => v.fmt(f),
            FloatVar(ref v) => v.fmt(f),
            FreshTy(v) => write!(f, "FreshTy({v:?})"),
            FreshIntTy(v) => write!(f, "FreshIntTy({v:?})"),
            FreshFloatTy(v) => write!(f, "FreshFloatTy({v:?})"),
        }
    }
}

#[derive_where(Clone, Copy, PartialEq, Hash, Debug; I: Interner)]
#[cfg_attr(
    feature = "nightly",
    derive(Encodable_NoContext, Decodable_NoContext, StableHash_NoContext)
)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic)]
pub struct TypeAndMut<I: Interner> {
    pub ty: I::Ty,
    pub mutbl: Mutability,
}

impl<I: Interner> Eq for TypeAndMut<I> {}

/// Error type for splatted argument index errors.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SplattedArgIndexError {
    /// The splatted argument index is invalid.
    /// A `u8::MAX` argument index used to indicate that no argument is splatted.
    /// Higher values are also not supported, for performance reasons.
    InvalidIndex { splatted_arg_index: u8 },

    /// The splatted argument index is outside the bounds of the function arguments.
    OutOfBounds { splatted_arg_index: u8, args_len: u16 },
}

/// Contains the packed non-type fields of a function signature.
#[derive_where(Copy, Clone, PartialEq, Eq, Hash; I: Interner)]
#[derive(TypeVisitable_Generic, TypeFoldable_Generic, Lift_Generic)]
#[cfg_attr(
    feature = "nightly",
    derive(Encodable_NoContext, Decodable_NoContext, StableHash_NoContext)
)]
pub struct FnSigKind<I: Interner> {
    /// Holds the c_variadic and safety bitflags, and 6 bits for the `ExternAbi` variant and unwind
    /// flag.
    #[lift(identity)]
    #[type_visitable(ignore)]
    #[type_foldable(identity)]
    flags: u8,

    /// Which function argument is splatted into multiple arguments in callers, if any?
    /// Splatting functions with `>= u8::MAX` arguments is not supported, for performance reasons.
    /// (And spending an extra byte on an edge case is not worth the perf.)
    #[lift(identity)]
    #[type_visitable(ignore)]
    #[type_foldable(identity)]
    splatted: u8,

    #[type_visitable(ignore)]
    #[type_foldable(identity)]
    _marker: PhantomData<fn() -> I>,
}

impl<I: Interner> fmt::Debug for FnSigKind<I> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut f = f.debug_tuple("FnSigKind");

        if self.is_safe() {
            f.field(&"Safe");
        } else {
            f.field(&"Unsafe");
        }

        f.field(&self.abi());

        if self.c_variadic() {
            f.field(&"CVariadic");
        }

        if let Some(index) = self.splatted() {
            f.field(&format!("Splatted({})", index));
        }

        f.finish()
    }
}

impl<I: Interner> Default for FnSigKind<I> {
    /// Create a new FnSigKind with the "Rust" ABI, "Unsafe" safety, and no C-style variadic or splatted arguments.
    /// To modify these flags, use the `set_*` methods, for readability.
    fn default() -> Self {
        Self { flags: 0, splatted: 0, _marker: PhantomData }
            .set_abi(ExternAbi::Rust)
            .set_safety(I::Safety::unsafe_mode())
            .set_c_variadic(false)
            .set_no_splatted_args()
    }
}

impl<I: Interner> FnSigKind<I> {
    /// Mask for the `ExternAbi` variant, including the unwind flag.
    const EXTERN_ABI_MASK: u8 = 0b111111;

    /// Bitflag for `Safety::Safe`. The default is `Unsafe`.
    const SAFE_FLAG: u8 = 1 << 6;

    /// Bitflag for a trailing C-style variadic argument.
    const C_VARIADIC_FLAG: u8 = 1 << 7;

    /// The marker index for "no splatted arguments". Higher values are also not supported, for
    /// performance reasons.
    ///
    /// Must have the same value as `FnDeclFlags::NO_SPLATTED_ARG_INDEX` and
    /// `rustc_ast::FnDecl::NO_SPLATTED_ARG_INDEX`.
    ///
    /// This is an implementation detail, which should only be used in low-level encoding.
    pub const NO_SPLATTED_ARG_INDEX: u8 = u8::MAX;

    /// Create a new FnSigKind with the given ABI, safety, C-style variadic, and splatted argument
    /// index.
    pub fn new(
        abi: ExternAbi,
        safety: I::Safety,
        c_variadic: bool,
        splatted: Option<u8>,
        args_len: usize,
    ) -> Result<Self, SplattedArgIndexError> {
        Self::default()
            .set_abi(abi)
            .set_safety(safety)
            .set_c_variadic(c_variadic)
            .set_splatted(splatted, args_len)
    }

    /// Create a new safe FnSigKind with the `extern "Rust"` ABI, that isn't C-style variadic or splatted.
    pub fn dummy() -> Self {
        Self::default().set_safety(I::Safety::safe())
    }

    /// Set the ABI, including the unwind flag.
    #[must_use = "this method does not modify the receiver"]
    pub fn set_abi(mut self, abi: ExternAbi) -> Self {
        let abi_index = abi.as_packed();
        assert!(abi_index <= Self::EXTERN_ABI_MASK);

        self.flags &= !Self::EXTERN_ABI_MASK;
        self.flags |= abi_index;

        self
    }

    /// Set the safety flag, `true` is `Safe`.
    #[must_use = "this method does not modify the receiver"]
    pub fn set_safety(mut self, safety: I::Safety) -> Self {
        if safety.is_safe() {
            self.flags |= Self::SAFE_FLAG;
        } else {
            self.flags &= !Self::SAFE_FLAG;
        }

        self
    }

    /// Set the C-style variadic argument flag.
    #[must_use = "this method does not modify the receiver"]
    pub fn set_c_variadic(mut self, c_variadic: bool) -> Self {
        if c_variadic {
            self.flags |= Self::C_VARIADIC_FLAG;
        } else {
            self.flags &= !Self::C_VARIADIC_FLAG;
        }

        self
    }

    /// Set the splatted argument index.
    /// The number of function arguments is used for error checking.
    #[must_use = "this method does not modify the receiver"]
    pub fn set_splatted(
        mut self,
        splatted: Option<u8>,
        args_len: usize,
    ) -> Result<Self, SplattedArgIndexError> {
        if let Some(splatted_arg_index) = splatted {
            if splatted_arg_index == Self::NO_SPLATTED_ARG_INDEX {
                // This index value is used as a marker for "no splatting", so it is unsupported.
                // Higher values are also not supported, for performance reasons.
                return Err(SplattedArgIndexError::InvalidIndex { splatted_arg_index });
            } else if usize::from(splatted_arg_index) >= args_len {
                return Err(SplattedArgIndexError::OutOfBounds {
                    splatted_arg_index,
                    args_len: args_len as u16,
                });
            }

            self.splatted = splatted_arg_index;
        } else {
            self.splatted = Self::NO_SPLATTED_ARG_INDEX;
        }

        Ok(self)
    }

    /// Set the splatted argument index to "no splatted arguments".
    #[must_use = "this method does not modify the receiver"]
    pub fn set_no_splatted_args(mut self) -> Self {
        self.splatted = Self::NO_SPLATTED_ARG_INDEX;
        self
    }

    /// Get the ABI, including the unwind flag.
    pub fn abi(self) -> ExternAbi {
        let abi_index = self.flags & Self::EXTERN_ABI_MASK;
        ExternAbi::from_packed(abi_index)
    }

    /// Get the safety flag.
    pub fn is_safe(self) -> bool {
        self.flags & Self::SAFE_FLAG != 0
    }

    /// Returns the safety mode.
    pub fn safety(self) -> I::Safety {
        if self.is_safe() { I::Safety::safe() } else { I::Safety::unsafe_mode() }
    }

    /// Do the function arguments end with a C-style variadic argument?
    pub fn c_variadic(self) -> bool {
        self.flags & Self::C_VARIADIC_FLAG != 0
    }

    /// Get the index of the splatted argument, if any.
    pub fn splatted(self) -> Option<u8> {
        if self.splatted == Self::NO_SPLATTED_ARG_INDEX { None } else { Some(self.splatted) }
    }
}

#[derive_where(Clone, Copy, PartialEq, Hash; I: Interner)]
#[cfg_attr(
    feature = "nightly",
    derive(Encodable_NoContext, Decodable_NoContext, StableHash_NoContext)
)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic, Lift_Generic)]
pub struct FnSig<I: Interner> {
    pub inputs_and_output: I::Tys,
    #[type_visitable(ignore)]
    #[type_foldable(identity)]
    pub fn_sig_kind: FnSigKind<I>,
}

impl<I: Interner> Eq for FnSig<I> {}

impl<I: Interner> FnSig<I> {
    pub fn inputs(self) -> I::FnInputTys {
        self.inputs_and_output.inputs()
    }

    pub fn output(self) -> I::Ty {
        self.inputs_and_output.output()
    }

    pub fn is_fn_trait_compatible(self) -> bool {
        !self.c_variadic() && self.safety().is_safe() && self.abi() == ExternAbi::Rust
    }

    /// Set the safety flag.
    #[must_use = "this method does not modify the receiver"]
    pub fn set_safety(self, safety: I::Safety) -> Self {
        Self { fn_sig_kind: self.fn_sig_kind.set_safety(safety), ..self }
    }

    /// Set the splatted argument index.
    /// The number of function arguments is used for error checking.
    #[must_use = "this method does not modify the receiver"]
    pub fn set_splatted(
        self,
        splatted: Option<u8>,
        args_len: usize,
    ) -> Result<Self, SplattedArgIndexError> {
        Ok(Self { fn_sig_kind: self.fn_sig_kind.set_splatted(splatted, args_len)?, ..self })
    }

    pub fn safety(self) -> I::Safety {
        self.fn_sig_kind.safety()
    }

    pub fn abi(self) -> ExternAbi {
        self.fn_sig_kind.abi()
    }

    pub fn c_variadic(self) -> bool {
        self.fn_sig_kind.c_variadic()
    }

    pub fn splatted(self) -> Option<u8> {
        self.fn_sig_kind.splatted()
    }

    /// Create a new safe FnSig with no arguments or return type, using the `extern "Rust"` ABI,
    /// that isn't C-style variadic or splatted.
    pub fn dummy() -> Self {
        Self { inputs_and_output: Default::default(), fn_sig_kind: FnSigKind::dummy() }
    }
}

impl<I: Interner> ty::Binder<I, FnSig<I>> {
    #[inline]
    pub fn inputs(self) -> ty::Binder<I, I::FnInputTys> {
        self.map_bound(|fn_sig| fn_sig.inputs())
    }

    #[inline]
    #[track_caller]
    pub fn input(self, index: usize) -> ty::Binder<I, I::Ty> {
        self.map_bound(|fn_sig| fn_sig.inputs().get(index).unwrap())
    }

    pub fn inputs_and_output(self) -> ty::Binder<I, I::Tys> {
        self.map_bound(|fn_sig| fn_sig.inputs_and_output)
    }

    #[inline]
    pub fn output(self) -> ty::Binder<I, I::Ty> {
        self.map_bound(|fn_sig| fn_sig.output())
    }

    pub fn fn_sig_kind(self) -> FnSigKind<I> {
        self.skip_binder().fn_sig_kind
    }

    pub fn c_variadic(self) -> bool {
        self.skip_binder().c_variadic()
    }

    pub fn splatted(self) -> Option<u8> {
        self.skip_binder().splatted()
    }

    pub fn safety(self) -> I::Safety {
        self.skip_binder().safety()
    }

    pub fn abi(self) -> ExternAbi {
        self.skip_binder().abi()
    }

    pub fn is_fn_trait_compatible(&self) -> bool {
        self.skip_binder().is_fn_trait_compatible()
    }

    // Used to split a single value into the two fields in `TyKind::FnPtr`.
    pub fn split(self) -> (ty::Binder<I, FnSigTys<I>>, FnHeader<I>) {
        let hdr = FnHeader { fn_sig_kind: self.fn_sig_kind() };
        (self.map_bound(|sig| FnSigTys { inputs_and_output: sig.inputs_and_output }), hdr)
    }
}

impl<I: Interner> fmt::Debug for FnSig<I> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let sig = self;
        let FnSig { inputs_and_output: _, fn_sig_kind } = sig;

        write!(f, "{}", fn_sig_kind.safety().prefix_str())?;
        if fn_sig_kind.abi() != ExternAbi::Rust {
            write!(f, "extern \"{:?}\" ", fn_sig_kind.abi())?;
        }

        write!(f, "fn(")?;
        let inputs = sig.inputs();
        for (i, ty) in inputs.iter().enumerate() {
            if i > 0 {
                write!(f, ", ")?;
            }
            if Some(i) == fn_sig_kind.splatted().map(usize::from) {
                write!(f, "#[splat] ")?;
            }
            write!(f, "{ty:?}")?;
        }
        if fn_sig_kind.c_variadic() {
            if inputs.is_empty() {
                write!(f, "...")?;
            } else {
                write!(f, ", ...")?;
            }
        }
        write!(f, ")")?;

        let output = sig.output();
        match output.kind() {
            Tuple(list) if list.is_empty() => Ok(()),
            _ => write!(f, " -> {:?}", sig.output()),
        }
    }
}

// FIXME: this is a distinct type because we need to define `Encode`/`Decode`
// impls in this crate for `Binder<I, I::Ty>`.
#[derive_where(Clone, Copy, PartialEq, Hash; I: Interner)]
#[cfg_attr(feature = "nightly", derive(StableHash_NoContext))]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic, Lift_Generic)]
pub struct UnsafeBinderInner<I: Interner>(ty::Binder<I, I::Ty>);

impl<I: Interner> Eq for UnsafeBinderInner<I> {}

impl<I: Interner> From<ty::Binder<I, I::Ty>> for UnsafeBinderInner<I> {
    fn from(value: ty::Binder<I, I::Ty>) -> Self {
        UnsafeBinderInner(value)
    }
}

impl<I: Interner> From<UnsafeBinderInner<I>> for ty::Binder<I, I::Ty> {
    fn from(value: UnsafeBinderInner<I>) -> Self {
        value.0
    }
}

impl<I: Interner> fmt::Debug for UnsafeBinderInner<I> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.0.fmt(f)
    }
}

impl<I: Interner> Deref for UnsafeBinderInner<I> {
    type Target = ty::Binder<I, I::Ty>;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

#[cfg(feature = "nightly")]
impl<I: Interner, E: rustc_serialize::Encoder> rustc_serialize::Encodable<E>
    for UnsafeBinderInner<I>
where
    I::Ty: rustc_serialize::Encodable<E>,
    I::BoundVarKinds: rustc_serialize::Encodable<E>,
{
    fn encode(&self, e: &mut E) {
        self.bound_vars().encode(e);
        self.as_ref().skip_binder().encode(e);
    }
}

#[cfg(feature = "nightly")]
impl<I: Interner, D: rustc_serialize::Decoder> rustc_serialize::Decodable<D>
    for UnsafeBinderInner<I>
where
    I::Ty: TypeVisitable<I> + rustc_serialize::Decodable<D>,
    I::BoundVarKinds: rustc_serialize::Decodable<D>,
{
    fn decode(decoder: &mut D) -> Self {
        let bound_vars = rustc_serialize::Decodable::decode(decoder);
        UnsafeBinderInner(ty::Binder::bind_with_vars(
            rustc_serialize::Decodable::decode(decoder),
            bound_vars,
        ))
    }
}

// This is just a `FnSig` without the `FnHeader` fields.
#[derive_where(Clone, Copy, Debug, PartialEq, Hash; I: Interner)]
#[cfg_attr(
    feature = "nightly",
    derive(Encodable_NoContext, Decodable_NoContext, StableHash_NoContext)
)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic, Lift_Generic)]
pub struct FnSigTys<I: Interner> {
    pub inputs_and_output: I::Tys,
}

impl<I: Interner> Eq for FnSigTys<I> {}

impl<I: Interner> FnSigTys<I> {
    pub fn inputs(self) -> I::FnInputTys {
        self.inputs_and_output.inputs()
    }

    pub fn output(self) -> I::Ty {
        self.inputs_and_output.output()
    }
}

impl<I: Interner> ty::Binder<I, FnSigTys<I>> {
    // Used to combine the two fields in `TyKind::FnPtr` into a single value.
    pub fn with(self, hdr: FnHeader<I>) -> ty::Binder<I, FnSig<I>> {
        self.map_bound(|sig_tys| FnSig {
            inputs_and_output: sig_tys.inputs_and_output,
            fn_sig_kind: hdr.fn_sig_kind,
        })
    }

    #[inline]
    pub fn inputs(self) -> ty::Binder<I, I::FnInputTys> {
        self.map_bound(|sig_tys| sig_tys.inputs())
    }

    #[inline]
    #[track_caller]
    pub fn input(self, index: usize) -> ty::Binder<I, I::Ty> {
        self.map_bound(|sig_tys| sig_tys.inputs().get(index).unwrap())
    }

    pub fn inputs_and_output(self) -> ty::Binder<I, I::Tys> {
        self.map_bound(|sig_tys| sig_tys.inputs_and_output)
    }

    #[inline]
    pub fn output(self) -> ty::Binder<I, I::Ty> {
        self.map_bound(|sig_tys| sig_tys.output())
    }
}

#[derive_where(Clone, Copy, Debug, PartialEq, Hash; I: Interner)]
#[cfg_attr(
    feature = "nightly",
    derive(Encodable_NoContext, Decodable_NoContext, StableHash_NoContext)
)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic, Lift_Generic)]
pub struct FnHeader<I: Interner> {
    #[type_visitable(ignore)]
    #[type_foldable(identity)]
    pub fn_sig_kind: FnSigKind<I>,
}

impl<I: Interner> FnHeader<I> {
    pub fn c_variadic(self) -> bool {
        self.fn_sig_kind.c_variadic()
    }

    pub fn safety(self) -> I::Safety {
        self.fn_sig_kind.safety()
    }

    pub fn abi(self) -> ExternAbi {
        self.fn_sig_kind.abi()
    }

    /// Create a new safe FnHeader with the `extern "Rust"` ABI, that isn't C-style variadic or splatted.
    pub fn dummy() -> Self {
        Self { fn_sig_kind: FnSigKind::dummy() }
    }
}

impl<I: Interner> Eq for FnHeader<I> {}

#[derive_where(Clone, Copy, Debug, PartialEq, Hash; I: Interner)]
#[cfg_attr(
    feature = "nightly",
    derive(Encodable_NoContext, Decodable_NoContext, StableHash_NoContext)
)]
#[derive(TypeVisitable_Generic, GenericTypeVisitable, TypeFoldable_Generic, Lift_Generic)]
pub struct CoroutineWitnessTypes<I: Interner> {
    pub types: I::Tys,
    pub assumptions: I::RegionAssumptions,
}

impl<I: Interner> Eq for CoroutineWitnessTypes<I> {}