Cast and Coercion enums duplicate a lot of structure

[oli-obk]

The Adjust enum in src/librustc/ty/adjustment.rs and the CastKind enum in src/librustc/mir/mod.rs have all fields but one of the CastKind enum in common. We should pull them out into a single enum that both use. Additionally this will allow merging some variants of ExprKind in src/librustc_mir/hair/mod.rs by having a single Coerce variant that has a field of the newly created enum.

[saleemjaffer]

@oli-obk Can i pick this up?

[oli-obk]

It's all yours!

[saleemjaffer]

pub enum Adjust<'tcx> {
    /// Go from ! to any type.
    NeverToAny,

    /// Go from a fn-item type to a fn-pointer type.
    ReifyFnPointer,

    /// Go from a safe fn pointer to an unsafe fn pointer.
    UnsafeFnPointer,

    /// Go from a non-capturing closure to an fn pointer or an unsafe fn pointer.
    /// It cannot convert a closure that requires unsafe.
    ClosureFnPointer(hir::Unsafety),

    /// Go from a mut raw pointer to a const raw pointer.
    MutToConstPointer,

    /// Dereference once, producing a place.
    Deref(Option<OverloadedDeref<'tcx>>),

    /// Take the address and produce either a `&` or `*` pointer.
    Borrow(AutoBorrow<'tcx>),

    /// Unsize a pointer/reference value, e.g., `&[T; n]` to
    /// `&[T]`. Note that the source could be a thin or fat pointer.
    /// This will do things like convert thin pointers to fat
    /// pointers, or convert structs containing thin pointers to
    /// structs containing fat pointers, or convert between fat
    /// pointers. We don't store the details of how the transform is
    /// done (in fact, we don't know that, because it might depend on
    /// the precise type parameters). We just store the target
    /// type. Codegen backends and miri figure out what has to be done
    /// based on the precise source/target type at hand.
    Unsize,
}

pub enum CastKind {
    Misc,

    /// Converts unique, zero-sized type for a fn to fn()
    ReifyFnPointer,

    /// Converts non capturing closure to fn() or unsafe fn().
    /// It cannot convert a closure that requires unsafe.
    ClosureFnPointer(hir::Unsafety),

    /// Converts safe fn() to unsafe fn()
    UnsafeFnPointer,

    /// Coerces *mut T to *const T, preserving T.
    MutToConstPointer,

    /// "Unsize" -- convert a thin-or-fat pointer to a fat pointer.
    /// codegen must figure out the details once full monomorphization
    /// is known. For example, this could be used to cast from a
    /// `&[i32;N]` to a `&[i32]`, or a `Box<T>` to a `Box<dyn Trait>`
    /// (presuming `T: Trait`).
    Unsize,
}

Adjust has 2 extra fieldsDeref and Borrowand CastKind has a Misc extra.

[oli-obk]

Yes, that's expected. The shared variants can be moved to a new enum so e.g. CastKind could be

enum CastKind {
    Misc,
    Pointer(PointerCast),
}