Enum ProjectionElem

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

Variants

Deref

Field(FieldIdx, T)

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(V)

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 indices are generated by slice patterns. Easiest to explain by example:

[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

index or -index (in Python terms), depending on from_end

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

Counting backwards from end? This is 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].

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(T)

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

Subtype(T)

A Subtype(T) projection is applied to any StatementKind::Assign where type of lvalue doesn’t match the type of rvalue, the primary goal is making subtyping explicit during optimizations and codegen.

This projection doesn’t impact the runtime behavior of the program except for potentially changing some type metadata of the interpreter or codegen backend.

This goal is achieved with mir_transform pass Subtyper, which runs right after borrowchecker, as we only care about subtyping that can affect trait selection and TypeId.