# Struct rustc_middle::mir::syntax::Place

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    pub struct Place<'tcx> {
        pub local: Local,
        pub projection: &'tcx List<PlaceElem<'tcx>>,
    }

Expand description

Places roughly correspond to a “location in memory.” Places in MIR are the same mathematical object as places in Rust. This of course means that what exactly they are is undecided and part of the Rust memory model. However, they will likely contain at least the following pieces of information in some form:

1. The address in memory that the place refers to.
2. The provenance with which the place is being accessed.
3. The type of the place and an optional variant index. See `PlaceTy`.
4. Optionally, some metadata. This exists if and only if the type of the place is not `Sized`.

We’ll give a description below of how all pieces of the place except for the provenance are calculated. We cannot give a description of the provenance, because that is part of the undecided aliasing model - we only include it here at all to acknowledge its existence.

Each local naturally corresponds to the place `Place { local, projection: [] }`. This place has the address of the local’s allocation and the type of the local.

Needs clarification: Unsized locals seem to present a bit of an issue. Their allocation can’t actually be created on `StorageLive`, because it’s unclear how big to make the allocation. Furthermore, MIR produces assignments to unsized locals, although that is not permitted under `#![feature(unsized_locals)]` in Rust. Besides just putting “unsized locals are special and different” in a bunch of places, I (JakobDegen) don’t know how to incorporate this behavior into the current MIR semantics in a clean way - possibly this needs some design work first.

For places that are not locals, ie they have a non-empty list of projections, we define the values as a function of the parent place, that is the place with its last `ProjectionElem` stripped. The way this is computed of course depends on the kind of that last projection element:

- `Downcast`: This projection sets the place’s variant index to the given one, and makes no other changes. A `Downcast` projection on a place with its variant index already set is not well-formed.

- `Field`: `Field` projections take their parent place and create a place referring to one of the fields of the type. The resulting address is the parent address, plus the offset of the field. The type becomes the type of the field. If the parent was unsized and so had metadata associated with it, then the metadata is retained if the field is unsized and thrown out if it is sized.

  These projections are only legal for tuples, ADTs, closures, and coroutines. If the ADT or coroutine has more than one variant, the parent place’s variant index must be set, indicating which variant is being used. If it has just one variant, the variant index may or may not be included - the single possible variant is inferred if it is not included.

- `OpaqueCast`: This projection changes the place’s type to the given one, and makes no other changes. A `OpaqueCast` projection on any type other than an opaque type from the current crate is not well-formed.

- `ConstantIndex`: Computes an offset in units of `T` into the place as described in the documentation for the `ProjectionElem`. The resulting address is the parent’s address plus that offset, and the type is `T`. This is only legal if the parent place has type `[T; N]` or `[T]` (not `&[T]`). Since such a `T` is always sized, any resulting metadata is thrown out.

- `Subslice`: This projection calculates an offset and a new address in a similar manner as `ConstantIndex`. It is also only legal on `[T; N]` and `[T]`. However, this yields a `Place` of type `[T]`, and additionally sets the metadata to be the length of the subslice.

- `Index`: Like `ConstantIndex`, only legal on `[T; N]` or `[T]`. However, `Index` additionally takes a local from which the value of the index is computed at runtime. Computing the value of the index involves interpreting the `Local` as a `Place { local, projection: [] }`, and then computing its value as if done via `Operand::Copy`. The array/slice is then indexed with the resulting value. The local must have type `usize`.

- `Deref`: Derefs are the last type of projection, and the most complicated. They are only legal on parent places that are references, pointers, or `Box`. A `Deref` projection begins by loading a value from the parent place, as if by `Operand::Copy`. It then dereferences the resulting pointer, creating a place of the pointee’s type. The resulting address is the address that was stored in the pointer. If the pointee type is unsized, the pointer additionally stored the value of the metadata.

The “validity invariant” of places is the same as that of raw pointers, meaning that e.g. `*ptr` on a dangling or unaligned pointer is never UB. (Later doing a load/store on that place or turning it into a reference can be UB though!) The only ways for a place computation can cause UB are:

- On a `Deref` projection, we do an actual load of the inner place, with all the usual consequences (the inner place must be based on an aligned pointer, it must point to allocated memory, the aliasig model must allow reads, this must not be a data race).
- For the projections that perform pointer arithmetic, the offset must in-bounds of an allocation (i.e., the preconditions of `ptr::offset` must be met).

## Fields§

§`local: Local`§`projection: &'tcx List<PlaceElem<'tcx>>`

projection out of a place (access a field, deref a pointer, etc)

## Implementations§

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### impl<'tcx> Place<'tcx>

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#### pub fn return_place() -> Place<'tcx>

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

Returns `true` if this `Place` contains a `Deref` projection.

If `Place::is_indirect` returns false, the caller knows that the `Place` refers to the same region of memory as its base.

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

Returns `true` if this `Place`’s first projection is `Deref`.

This is useful because for MIR phases `AnalysisPhase::PostCleanup` and later, `Deref` projections can only occur as the first projection. In that case this method is equivalent to `is_indirect`, but faster.

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#### pub fn local_or_deref_local(&self) -> Option<Local>

Finds the innermost `Local` from this `Place`, if it is either a local itself or a single deref of a local.

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#### pub fn as_local(&self) -> Option<Local>

If this place represents a local variable like `_X` with no projections, return `Some(_X)`.

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#### pub fn as_ref(&self) -> PlaceRef<'tcx>

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#### pub fn iter_projections( self ) -> impl Iterator<Item = (PlaceRef<'tcx>, PlaceElem<'tcx>)> + DoubleEndedIterator

Iterate over the projections in evaluation order, i.e., the first element is the base with its projection and then subsequently more projections are added. As a concrete example, given the place a.b.c, this would yield:

- (a, .b)
- (a.b, .c)

Given a place without projections, the iterator is empty.

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#### pub fn project_deeper( self, more_projections: &[PlaceElem<'tcx>], tcx: TyCtxt<'tcx> ) -> Self

Generates a new place by appending `more_projections` to the existing ones and interning the result.

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### impl<'tcx> Place<'tcx>

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#### pub fn ty_from<D>( local: Local, projection: &[PlaceElem<'tcx>], local_decls: &D, tcx: TyCtxt<'tcx> ) -> PlaceTy<'tcx>
where D: HasLocalDecls<'tcx> + ?Sized,

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#### pub fn ty<D>(&self, local_decls: &D, tcx: TyCtxt<'tcx>) -> PlaceTy<'tcx>
where D: HasLocalDecls<'tcx> + ?Sized,

## Trait Implementations§

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### impl<'tcx> Clone for Place<'tcx>

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#### fn clone(&self) -> Place<'tcx>

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 Debug for Place<'_>

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

Formats the value using the given formatter. Read more
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### impl<'tcx, D: TyDecoder<I = TyCtxt<'tcx>>> Decodable<D> for Place<'tcx>

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#### fn decode(decoder: &mut D) -> Self

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### impl<'tcx, __E: TyEncoder<I = TyCtxt<'tcx>>> Encodable<__E> for Place<'tcx>

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

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### impl From<Local> for Place<'_>

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#### fn from(local: Local) -> Self

Converts to this type from the input type.
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### impl<'tcx> Hash for Place<'tcx>

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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<'tcx, '__ctx> HashStable<StableHashingContext<'__ctx>> for Place<'tcx>

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

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### impl<'tcx> PartialEq for Place<'tcx>

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#### fn eq(&self, other: &Place<'tcx>) -> 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<'tcx> TypeFoldable<TyCtxt<'tcx>> for Place<'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> TypeVisitable<TyCtxt<'tcx>> for Place<'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<'tcx> Copy for Place<'tcx>

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### impl<'tcx> Eq for Place<'tcx>

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### impl<'tcx> StructuralPartialEq for Place<'tcx>

## Auto Trait Implementations§

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### impl<'tcx> DynSend for Place<'tcx>

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### impl<'tcx> DynSync for Place<'tcx>

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### impl<'tcx> Freeze for Place<'tcx>

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### impl<'tcx> !RefUnwindSafe for Place<'tcx>

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### impl<'tcx> Send for Place<'tcx>

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### impl<'tcx> Sync for Place<'tcx>

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### impl<'tcx> Unpin for Place<'tcx>

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### impl<'tcx> !UnwindSafe for Place<'tcx>

## 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, 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<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<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<'tcx, T> ToPredicate<'tcx, T> for T

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#### fn to_predicate(self, _tcx: TyCtxt<'tcx>) -> T

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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<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: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain `repr(...)` attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 16 bytes