# Enum rustc_middle::mir::ProjectionElem

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    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§

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### Deref

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### 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

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### 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.

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### 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. For arrays this is always the exact length.

§`from_end: bool`

Counting backwards from end? This is always false when indexing an array.

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### 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 `PlaceElem`s this is `true` if and only if the base is a slice. For `ProjectionKind`, this can also be `true` for arrays.

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### 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.

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### OpaqueCast(T)

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

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### 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`.

## Implementations§

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### impl<V, T> ProjectionElem<V, T>

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#### fn is_indirect(&self) -> bool

Returns `true` if the target of this projection may refer to a different region of memory than the base.

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#### pub fn is_stable_offset(&self) -> bool

Returns `true` if the target of this projection always refers to the same memory region whatever the state of the program.

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#### pub fn is_downcast_to(&self, v: VariantIdx) -> bool

Returns `true` if this is a `Downcast` projection with the given `VariantIdx`.

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#### pub fn is_field_to(&self, f: FieldIdx) -> bool

Returns `true` if this is a `Field` projection with the given index.

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#### pub fn can_use_in_debuginfo(&self) -> bool

Returns `true` if this is accepted inside `VarDebugInfoContents::Place`.

## Trait Implementations§

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### impl<V: Clone, T: Clone> Clone for ProjectionElem<V, T>

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#### fn clone(&self) -> ProjectionElem<V, T>

Returns a copy of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl<V: Debug, T: Debug> Debug for ProjectionElem<V, T>

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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### impl<V, T, __D: TyDecoder> Decodable<__D> for ProjectionElem<V, T>
where T: Decodable<__D>, V: Decodable<__D>,

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#### fn decode(__decoder: &mut __D) -> Self

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### impl<V, T, __E: TyEncoder> Encodable<__E> for ProjectionElem<V, T>
where T: Encodable<__E>, V: Encodable<__E>,

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#### fn encode(&self, __encoder: &mut __E)

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### impl<V: Hash, T: Hash> Hash for ProjectionElem<V, T>

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#### fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given `Hasher`. Read more
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#### fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given `Hasher`. Read more
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### impl<'__ctx, V, T> HashStable<StableHashingContext<'__ctx>> for ProjectionElem<V, T>
where T: HashStable<StableHashingContext<'__ctx>>, V: HashStable<StableHashingContext<'__ctx>>,

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#### fn hash_stable( &self, __hcx: &mut StableHashingContext<'__ctx>, __hasher: &mut StableHasher, )

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### impl<V: Ord, T: Ord> Ord for ProjectionElem<V, T>

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#### fn cmp(&self, other: &ProjectionElem<V, T>) -> Ordering

This method returns an `Ordering` between `self` and `other`. Read more
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#### fn max(self, other: Self) -> Self
where Self: Sized,

Compares and returns the maximum of two values. Read more
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#### fn min(self, other: Self) -> Self
where Self: Sized,

Compares and returns the minimum of two values. Read more
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#### fn clamp(self, min: Self, max: Self) -> Self
where Self: Sized + PartialOrd,

Restrict a value to a certain interval. Read more
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### impl<V: PartialEq, T: PartialEq> PartialEq for ProjectionElem<V, T>

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#### fn eq(&self, other: &ProjectionElem<V, T>) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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### impl<V: PartialOrd, T: PartialOrd> PartialOrd for ProjectionElem<V, T>

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#### fn partial_cmp(&self, other: &ProjectionElem<V, T>) -> Option<Ordering>

This method returns an ordering between `self` and `other` values if one exists. Read more
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#### fn lt(&self, other: &Rhs) -> bool

This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more
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#### fn le(&self, other: &Rhs) -> bool

This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
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#### fn gt(&self, other: &Rhs) -> bool

This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
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#### fn ge(&self, other: &Rhs) -> bool

This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
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### impl<'tcx, V, T> TypeFoldable<TyCtxt<'tcx>> for ProjectionElem<V, T>
where T: TypeFoldable<TyCtxt<'tcx>>, V: TypeFoldable<TyCtxt<'tcx>>,

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#### fn try_fold_with<__F: FallibleTypeFolder<TyCtxt<'tcx>>>( self, __folder: &mut __F, ) -> Result<Self, __F::Error>

The entry point for folding. To fold a value `t` with a folder `f` call: `t.try_fold_with(f)`. Read more
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#### fn fold_with<F>(self, folder: &mut F) -> Self
where F: TypeFolder<I>,

A convenient alternative to `try_fold_with` for use with infallible folders. Do not override this method, to ensure coherence with `try_fold_with`.
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### impl<'tcx, V, T> TypeVisitable<TyCtxt<'tcx>> for ProjectionElem<V, T>
where T: TypeVisitable<TyCtxt<'tcx>>, V: TypeVisitable<TyCtxt<'tcx>>,

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#### fn visit_with<__V: TypeVisitor<TyCtxt<'tcx>>>( &self, __visitor: &mut __V, ) -> __V::Result

The entry point for visiting. To visit a value `t` with a visitor `v` call: `t.visit_with(v)`. Read more
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### impl<V: Copy, T: Copy> Copy for ProjectionElem<V, T>

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### impl<V: Eq, T: Eq> Eq for ProjectionElem<V, T>

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### impl<V, T> StructuralPartialEq for ProjectionElem<V, T>

## Auto Trait Implementations§

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### impl<V, T> DynSend for ProjectionElem<V, T>
where T: DynSend, V: DynSend,

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### impl<V, T> DynSync for ProjectionElem<V, T>
where T: DynSync, V: DynSync,

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### impl<V, T> Freeze for ProjectionElem<V, T>
where T: Freeze, V: Freeze,

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### impl<V, T> RefUnwindSafe for ProjectionElem<V, T>
where T: RefUnwindSafe, V: RefUnwindSafe,

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### impl<V, T> Send for ProjectionElem<V, T>
where T: Send, V: Send,

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### impl<V, T> Sync for ProjectionElem<V, T>
where T: Sync, V: Sync,

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### impl<V, T> Unpin for ProjectionElem<V, T>
where T: Unpin, V: Unpin,

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### impl<V, T> UnwindSafe for ProjectionElem<V, T>
where T: UnwindSafe, V: UnwindSafe,

## Blanket Implementations§

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### impl<T> Aligned for T

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#### const ALIGN: Alignment = _

Alignment of `Self`.
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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> AnyEq for T
where T: Any + PartialEq,

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#### fn equals(&self, other: &(dyn Any + 'static)) -> bool

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#### fn as_any(&self) -> &(dyn Any + 'static)

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### impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for T
where T: Copy,

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#### fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut T

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#### fn allocate_from_iter<'a>( arena: &'a Arena<'tcx>, iter: impl IntoIterator<Item = T>, ) -> &'a mut [T]

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### impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for T
where T: Copy,

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#### fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut T

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#### fn allocate_from_iter<'a>( arena: &'a Arena<'tcx>, iter: impl IntoIterator<Item = T>, ) -> &'a mut [T]

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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Copy,

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#### unsafe fn clone_to_uninit(&self, dst: *mut T)

🔬This is a nightly-only experimental API. (`clone_to_uninit`)
Performs copy-assignment from `self` to `dst`. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### default unsafe fn clone_to_uninit(&self, dst: *mut T)

🔬This is a nightly-only experimental API. (`clone_to_uninit`)
Performs copy-assignment from `self` to `dst`. Read more
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### impl<T, R> CollectAndApply<T, R> for T

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#### fn collect_and_apply<I, F>(iter: I, f: F) -> R
where I: Iterator<Item = T>, F: FnOnce(&[T]) -> R,

Equivalent to `f(&iter.collect::<Vec<_>>())`.

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#### type Output = R

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### impl<Q, K> Comparable<K> for Q
where Q: Ord + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn compare(&self, key: &K) -> Ordering

Compare self to `key` and return their ordering.
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### impl<Tcx, T> DepNodeParams<Tcx> for T
where Tcx: DepContext, T: for<'a> HashStable<StableHashingContext<'a>> + Debug,

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#### default fn fingerprint_style() -> FingerprintStyle

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#### default fn to_fingerprint(&self, tcx: Tcx) -> Fingerprint

This method turns the parameters of a DepNodeConstructor into an opaque Fingerprint to be used in DepNode. Not all DepNodeParams support being turned into a Fingerprint (they don’t need to if the corresponding DepNode is anonymous).
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#### default fn to_debug_str(&self, _: Tcx) -> String

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#### default fn recover(_: Tcx, _: &DepNode) -> Option<T>

This method tries to recover the query key from the given `DepNode`, something which is needed when forcing `DepNode`s during red-green evaluation. The query system will only call this method if `fingerprint_style()` is not `FingerprintStyle::Opaque`. It is always valid to return `None` here, in which case incremental compilation will treat the query as having changed instead of forcing it.
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Checks if this value is equivalent to the given key. Read more
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### impl<Q, K> Equivalent<K> for Q
where Q: Eq + ?Sized, K: Borrow<Q> + ?Sized,

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#### fn equivalent(&self, key: &K) -> bool

Compare self to `key` and return `true` if they are equal.
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### impl<T> Filterable for T

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#### fn filterable( self, filter_name: &'static str, ) -> RequestFilterDataProvider<T, fn(_: DataRequest<'_>) -> bool>

Creates a filterable data provider with the given name for debugging. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T> Instrument for T

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#### fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided `Span`, returning an `Instrumented` wrapper. Read more
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#### fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current `Span`, returning an `Instrumented` wrapper. Read more
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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> IntoEither for T

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#### fn into_either(self, into_left: bool) -> Either<Self, Self>

Converts `self` into a `Left` variant of `Either<Self, Self>` if `into_left` is `true`. Converts `self` into a `Right` variant of `Either<Self, Self>` otherwise. Read more
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#### fn into_either_with<F>(self, into_left: F) -> Either<Self, Self>
where F: FnOnce(&Self) -> bool,

Converts `self` into a `Left` variant of `Either<Self, Self>` if `into_left(&self)` returns `true`. Converts `self` into a `Right` variant of `Either<Self, Self>` otherwise. Read more
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### impl<P> IntoQueryParam<P> for P

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#### fn into_query_param(self) -> P

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### impl<'tcx, T> IsSuggestable<'tcx> for T
where T: TypeVisitable<TyCtxt<'tcx>> + TypeFoldable<TyCtxt<'tcx>>,

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#### fn is_suggestable(self, tcx: TyCtxt<'tcx>, infer_suggestable: bool) -> bool

Whether this makes sense to suggest in a diagnostic. Read more
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#### fn make_suggestable( self, tcx: TyCtxt<'tcx>, infer_suggestable: bool, placeholder: Option<Ty<'tcx>>, ) -> Option<T>

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### impl<T> MaybeResult<T> for T

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#### type Error = !

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#### fn from(_: Result<T, <T as MaybeResult<T>>::Error>) -> T

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#### fn to_result(self) -> Result<T, <T as MaybeResult<T>>::Error>

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### impl<T> Pointable for T

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#### const ALIGN: usize = _

The alignment of pointer.
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#### type Init = T

The type for initializers.
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#### unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
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#### unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
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#### unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
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#### unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
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### impl<T> Same for T

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#### type Output = T

Should always be `Self`
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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
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### impl<I, T> TypeVisitableExt<I> for T
where I: Interner, T: TypeVisitable<I>,

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#### fn has_type_flags(&self, flags: TypeFlags) -> bool

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#### fn has_vars_bound_at_or_above(&self, binder: DebruijnIndex) -> bool

Returns `true` if `self` has any late-bound regions that are either bound by `binder` or bound by some binder outside of `binder`. If `binder` is `ty::INNERMOST`, this indicates whether there are any late-bound regions that appear free.
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#### fn error_reported(&self) -> Result<(), <I as Interner>::ErrorGuaranteed>

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#### fn has_vars_bound_above(&self, binder: DebruijnIndex) -> bool

Returns `true` if this type has any regions that escape `binder` (and hence are not bound by it).
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#### fn has_escaping_bound_vars(&self) -> bool

Return `true` if this type has regions that are not a part of the type. For example, `for<'a> fn(&'a i32)` return `false`, while `fn(&'a i32)` would return `true`. The latter can occur when traversing through the former. Read more
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#### fn has_aliases(&self) -> bool

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#### fn has_inherent_projections(&self) -> bool

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#### fn has_opaque_types(&self) -> bool

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#### fn has_coroutines(&self) -> bool

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#### fn references_error(&self) -> bool

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#### fn has_non_region_param(&self) -> bool

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#### fn has_infer_regions(&self) -> bool

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#### fn has_infer_types(&self) -> bool

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#### fn has_non_region_infer(&self) -> bool

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#### fn has_infer(&self) -> bool

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#### fn has_placeholders(&self) -> bool

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#### fn has_non_region_placeholders(&self) -> bool

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#### fn has_param(&self) -> bool

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#### fn has_free_regions(&self) -> bool

“Free” regions in this context means that it has any region that is not (a) erased or (b) late-bound.
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#### fn has_erased_regions(&self) -> bool

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#### fn has_erasable_regions(&self) -> bool

True if there are any un-erased free regions.
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#### fn is_global(&self) -> bool

Indicates whether this value references only ‘global’ generic parameters that are the same regardless of what fn we are in. This is used for caching.
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#### fn has_bound_regions(&self) -> bool

True if there are any late-bound regions
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#### fn has_non_region_bound_vars(&self) -> bool

True if there are any late-bound non-region variables
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#### fn has_bound_vars(&self) -> bool

True if there are any bound variables
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#### fn still_further_specializable(&self) -> bool

Indicates whether this value still has parameters/placeholders/inference variables which could be replaced later, in a way that would change the results of `impl` specialization.
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### impl<I, T, U> Upcast<I, U> for T
where U: UpcastFrom<I, T>,

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#### fn upcast(self, interner: I) -> U

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### impl<I, T> UpcastFrom<I, T> for T

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#### fn upcast_from(from: T, _tcx: I) -> T

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### impl<V, T> VZip<V> for T
where V: MultiLane<T>,

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#### fn vzip(self) -> V

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### impl<Tcx, T> Value<Tcx> for T
where Tcx: DepContext,

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#### default fn from_cycle_error( tcx: Tcx, cycle_error: &CycleError, _guar: ErrorGuaranteed, ) -> T

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### impl<T> WithSubscriber for T

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#### fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>
where S: Into<Dispatch>,

Attaches the provided `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more
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#### fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default `Subscriber` to this type, returning a `WithDispatch` wrapper. Read more
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### impl<'a, T> Captures<'a> for T
where T: ?Sized,

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### impl<T> ErasedDestructor for T
where T: 'static,

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### impl<T> MaybeSendSync for T
where T: Send + Sync,

## 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.