StatementKind in rustc_middle::mir::syntax - Rust
Description: The various kinds of statements that can appear in MIR.

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## StatementKind

# Enum StatementKind

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    pub enum StatementKind<'tcx> {
    Show 13 variants    Assign(Box<(Place<'tcx>, Rvalue<'tcx>)>),
        FakeRead(Box<(FakeReadCause, Place<'tcx>)>),
        SetDiscriminant {
            place: Box<Place<'tcx>>,
            variant_index: VariantIdx,
        },
        StorageLive(Local),
        StorageDead(Local),
        Retag(RetagKind, Box<Place<'tcx>>),
        PlaceMention(Box<Place<'tcx>>),
        AscribeUserType(Box<(Place<'tcx>, UserTypeProjection)>, Variance),
        Coverage(CoverageKind),
        Intrinsic(Box<NonDivergingIntrinsic<'tcx>>),
        ConstEvalCounter,
        Nop,
        BackwardIncompatibleDropHint {
            place: Box<Place<'tcx>>,
            reason: BackwardIncompatibleDropReason,
        },
    }

Expand description

The various kinds of statements that can appear in MIR.

Not all of these are allowed at every `MirPhase`. Check the documentation there to see which ones you do not have to worry about. The MIR validator will generally enforce such restrictions, causing an ICE if they are violated.

## Variants§

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### Assign(Box<(Place<'tcx>, Rvalue<'tcx>)>)

Assign statements roughly correspond to an assignment in Rust proper (`x = ...`) except without the possibility of dropping the previous value (that must be done separately, if at all). The exact way this works is undecided. It probably does something like evaluating the LHS to a place and the RHS to a value, and then storing the value to the place. Various parts of this may do type specific things that are more complicated than simply copying bytes.

Needs clarification: The implication of the above idea would be that assignment implies that the resulting value is initialized. I believe we could commit to this separately from committing to whatever part of the memory model we would need to decide on to make the above paragraph precise. Do we want to?

Assignments in which the types of the place and rvalue differ are not well-formed.

Needs clarification: Do we ever want to worry about non-free (in the body) lifetimes for the typing requirement in post drop-elaboration MIR? I think probably not - I’m not sure we could meaningfully require this anyway. How about free lifetimes? Is ignoring this interesting for optimizations? Do we want to allow such optimizations?

Needs clarification: We currently require that the LHS place not overlap with any place read as part of computation of the RHS for some rvalues. This requirement is under discussion in #68364. Specifically, overlap is permitted only for assignments of a type with `BackendRepr::Scalar | BackendRepr::ScalarPair` where all the scalar fields are `Scalar::Initialized`. As a part of this discussion, it is also unclear in what order the components are evaluated.

See `Rvalue` documentation for details on each of those.

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### FakeRead(Box<(FakeReadCause, Place<'tcx>)>)

When executed at runtime, this is a nop.

During static analysis, a fake read:

- requires that the value being read is initialized (or, in the case of closures, that it was fully initialized at some point in the past)
- constitutes a use of a value for the purposes of NLL (i.e. if the value being fake-read is a reference, the lifetime of that reference will be extended to cover the `FakeRead`)
- but, unlike an actual read, does not invalidate any exclusive borrows.

See `FakeReadCause` for more details on the situations in which a `FakeRead` is emitted.

Disallowed after drop elaboration.

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### SetDiscriminant

Write the discriminant for a variant to the enum Place.

This is permitted for both coroutines and ADTs. This does not necessarily write to the entire place; instead, it writes to the minimum set of bytes as required by the layout for the type.

#### Fields

§`place: Box<Place<'tcx>>`
§`variant_index: VariantIdx`
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### StorageLive(Local)

`StorageLive` and `StorageDead` statements mark the live range of a local.

At any point during the execution of a function, each local is either allocated or unallocated. Except as noted below, all locals except function parameters are initially unallocated. `StorageLive` statements cause memory to be allocated for the local while `StorageDead` statements cause the memory to be freed. In other words, `StorageLive`/`StorageDead` act like the heap operations `allocate`/`deallocate`, but for stack-allocated local variables. Using a local in any way (not only reading/writing from it) while it is unallocated is UB.

Some locals have no `StorageLive` or `StorageDead` statements within the entire MIR body. These locals are implicitly allocated for the full duration of the function. There is a convenience method at `rustc_mir_dataflow::storage::always_storage_live_locals` for computing these locals.

If the local is already allocated, calling `StorageLive` again will implicitly free the local and then allocate fresh uninitialized memory. If a local is already deallocated, calling `StorageDead` again is a NOP.

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### StorageDead(Local)

See `StorageLive` above.

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### Retag(RetagKind, Box<Place<'tcx>>)

Retag references in the given place, ensuring they got fresh tags.

This is part of the Stacked Borrows model. These statements are currently only interpreted by miri and only generated when `-Z mir-emit-retag` is passed. See https://internals.rust-lang.org/t/stacked-borrows-an-aliasing-model-for-rust/8153/ for more details.

For code that is not specific to stacked borrows, you should consider retags to read and modify the place in an opaque way.

Only `RetagKind::Default` and `RetagKind::FnEntry` are permitted.

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### PlaceMention(Box<Place<'tcx>>)

This statement exists to preserve a trace of a scrutinee matched against a wildcard binding. This is especially useful for `let _ = PLACE;` bindings that desugar to a single `PlaceMention(PLACE)`.

When executed at runtime, this computes the given place, but then discards it without doing a load. `let _ = *ptr;` is fine even if the pointer is dangling.

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### AscribeUserType(Box<(Place<'tcx>, UserTypeProjection)>, Variance)

Encodes a user’s type ascription. These need to be preserved intact so that NLL can respect them. For example:

ⓘ

    let a: T = y;

The effect of this annotation is to relate the type `T_y` of the place `y` to the user-given type `T`. The effect depends on the specified variance:

- `Covariant` – requires that `T_y <: T`
- `Contravariant` – requires that `T_y :> T`
- `Invariant` – requires that `T_y == T`
- `Bivariant` – no effect

When executed at runtime this is a nop.

Disallowed after drop elaboration.

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### Coverage(CoverageKind)

Carries control-flow-sensitive information injected by `-Cinstrument-coverage`, such as where to generate physical coverage-counter-increments during codegen.

Coverage statements are used in conjunction with the coverage mappings and other information stored in the function’s `mir::Body::function_coverage_info`. (For inlined MIR, take care to look up the original function’s coverage info.)

Interpreters and codegen backends that don’t support coverage instrumentation can usually treat this as a no-op.

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### Intrinsic(Box<NonDivergingIntrinsic<'tcx>>)

Denotes a call to an intrinsic that does not require an unwind path and always returns. This avoids adding a new block and a terminator for simple intrinsics.

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### ConstEvalCounter

Instructs the const eval interpreter to increment a counter; this counter is used to track how many steps the interpreter has taken. It is used to prevent the user from writing const code that runs for too long or infinitely. Other than in the const eval interpreter, this is a no-op.

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### Nop

No-op. Useful for deleting instructions without affecting statement indices.

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### BackwardIncompatibleDropHint

Marker statement indicating where `place` would be dropped. This is semantically equivalent to `Nop`, so codegen and MIRI should interpret this statement as such. The only use case of this statement is for linting in MIR to detect temporary lifetime changes.

#### Fields

§`place: Box<Place<'tcx>>`

Place to drop

§`reason: BackwardIncompatibleDropReason`

Reason for backward incompatibility

## Implementations§

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### impl<'tcx> StatementKind<'tcx>

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#### pub const fn name(&self) -> &'static str

Returns a simple string representation of a `StatementKind` variant, independent of any values it might hold (e.g. `StatementKind::Assign` always returns `"Assign"`).

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#### pub fn as_assign_mut(&mut self) -> Option<&mut (Place<'tcx>, Rvalue<'tcx>)>

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#### pub fn as_assign(&self) -> Option<&(Place<'tcx>, Rvalue<'tcx>)>

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#### pub fn as_debuginfo(&self) -> Option<StmtDebugInfo<'tcx>>

## Trait Implementations§

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

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

Returns a duplicate 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 StatementKind<'_>

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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<'tcx>> Decodable<__D> for StatementKind<'tcx>

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

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### impl<'tcx, __E: TyEncoder<'tcx>> Encodable<__E> for StatementKind<'tcx>

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

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### impl<'tcx> Hash for StatementKind<'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 StatementKind<'tcx>

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

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

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#### fn eq(&self, other: &StatementKind<'tcx>) -> bool

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

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 StatementKind<'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: TypeFolder<TyCtxt<'tcx>>>(self, __folder: &mut __F) -> Self

The entry point for folding. To fold a value `t` with a folder `f` call: `t.fold_with(f)`. Read more
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### impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for StatementKind<'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> StructuralPartialEq for StatementKind<'tcx>

## Auto Trait Implementations§

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

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

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

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

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

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

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

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### impl<'tcx> UnsafeUnpin for StatementKind<'tcx>

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### impl<'tcx> !UnwindSafe for StatementKind<'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<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: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit`)
Performs copy-assignment from `self` to `dest`. 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> DepNodeKey<'tcx> for T
where T: for<'a> HashStable<StableHashingContext<'a>> + Debug,

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#### default fn key_fingerprint_style() -> KeyFingerprintStyle

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#### default fn to_fingerprint(&self, tcx: TyCtxt<'tcx>) -> Fingerprint

This method turns a query key into an opaque `Fingerprint` to be used in `DepNode`.
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#### default fn try_recover_key(_: TyCtxt<'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<T> DynClone for T
where T: Clone,

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#### fn __clone_box(&self, _: Private) -> *mut ()

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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<K> IntoQueryKey<K> for K

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#### fn into_query_key(self) -> K

Argument conversion from `Self` to `K`. This should always be a very cheap conversion, e.g. `LocalDefId::to_def_id`.
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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 non_region_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_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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#### fn has_non_region_error(&self) -> bool

True if a type or const error is reachable
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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<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<T> ErasedDestructor for T
where T: 'static,

## 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

Size for each variant:

- `Assign`: 15 bytes
- `FakeRead`: 15 bytes
- `SetDiscriminant`: 15 bytes
- `StorageLive`: 7 bytes
- `StorageDead`: 7 bytes
- `Retag`: 15 bytes
- `PlaceMention`: 15 bytes
- `AscribeUserType`: 15 bytes
- `Coverage`: 11 bytes
- `Intrinsic`: 15 bytes
- `ConstEvalCounter`: 0 bytes
- `Nop`: 0 bytes
- `BackwardIncompatibleDropHint`: 15 bytes