Type Alias ProjectionKind 

pub type ProjectionKind = ProjectionElem<(), ()>;

Alias for projections as they appear in UserTypeProjection, where we need neither the V parameter for Index nor the T for Field.

Aliased Type

pub enum ProjectionKind {
    Deref,
    Field(FieldIdx, ()),
    Index(()),
    ConstantIndex {
        offset: u64,
        min_length: u64,
        from_end: bool,
    },
    Subslice {
        from: u64,
        to: u64,
        from_end: bool,
    },
    Downcast(Option<Symbol>, VariantIdx),
    OpaqueCast(()),
    UnwrapUnsafeBinder(()),
    PhantomDeref,
}

Variants

Deref

Field(FieldIdx, ())

A field (e.g., f in _1.f) is one variant of ProjectionElem. Conceptually, rustc can identify that a field projection refers to either two different regions of memory or the same one between the base and the ‘projection element’. Read more about projections in the rustc-dev-guide

Index(())

Index into a slice/array.

Note that this does not also dereference, and so it does not exactly correspond to slice indexing in Rust. In other words, in the below Rust code:

let x = &[1, 2, 3, 4];
let i = 2;
x[i];

The x[i] is turned into a Deref followed by an Index, not just an Index. The same thing is true of the ConstantIndex and Subslice projections below.

ConstantIndex

These endpoint-relative indices are generated by slice/array patterns.

For array types, offset is always relative to the start of the array. For slice types, from_end determines whether offset is relative to the start or the end of the slice being inspected.

Slice-pattern indices are easiest to explain by the position of X in these examples:

[X, _, .., _, _] => { offset: 0, min_length: 4, from_end: false },
[_, X, .., _, _] => { offset: 1, min_length: 4, from_end: false },
[_, _, .., X, _] => { offset: 2, min_length: 4, from_end: true },
[_, _, .., _, X] => { offset: 1, min_length: 4, from_end: true },

Fields

offset: u64

• If from_end == false, this is a 0-based offset from the start of the array/slice.
• If from_end == true, this is a 1-based offset from the end of the slice.

min_length: u64

The thing being indexed must be at least this long – otherwise, the projection is UB.

For arrays this is always the exact length.

from_end: bool

If true, offset is a 1-based offset from the end of the slice. Always false when indexing an array.

Subslice

These indices are generated by slice patterns.

If from_end is true slice[from..slice.len() - to]. Otherwise array[from..to].

This projection cannot have ConstantIndex or additional Subslice projections after it before runtime MIR.

Fields

from: u64

to: u64

from_end: bool

Whether to counts from the start or end of the array/slice. For PlaceElems this is true if and only if the base is a slice. For ProjectionKind, this can also be true for arrays.

Downcast(Option<Symbol>, VariantIdx)

“Downcast” to a variant of an enum or a coroutine.

The included Symbol is the name of the variant, used for printing MIR.

This operation itself is never UB, all it does is change the type of the place.

OpaqueCast(())

Like an explicit cast from an opaque type to a concrete type, but without requiring an intermediate variable.

This is unused with -Znext-solver.

UnwrapUnsafeBinder(())

A transmute from an unsafe binder to the type that it wraps. This is a projection of a place, so it doesn’t necessarily constitute a move out of the binder.

PhantomDeref

A symbolic dereference of a Reborrow type that does not contain any &mut T fields.

If a type is Reborrow and contains a &mut T field then reborrowing it reborrows the T, producing a borrow on an indirect place, producing a value that can be returned from the function since it does not capture any local place. If no such field exists, then reborrowing the type must dereference the type itself to find an indirect place, but generally such types will not implement Deref. Therefore, in borrow checking we instead perform a “phantom dereference” (named so because the type will usually contain some PhantomData<&'a ()> or equivalent that captures the lifetime) to access an indeterminate indirect place.

FIXME(reborrow): currently this variant is not considered an indirect place for whatever reason. This variant makes no sense if that cannot be fixed.

FIXME(reborrow): if the Reborrow traits experiment is rejected, this variant can be removed: see the PR.

Layout

Note: Unable to compute type layout, possibly due to this type having generic parameters. Layout can only be computed for concrete, fully-instantiated types.