Enum StatementKind

pub enum StatementKind<'tcx> {
    Assign(Box<(Place<'tcx>, Rvalue<'tcx>)>),
    FakeRead(Box<(FakeReadCause, Place<'tcx>)>),
    SetDiscriminant {
        place: Box<Place<'tcx>>,
        variant_index: VariantIdx,
    },
    Deinit(Box<Place<'tcx>>),
    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,
    },
}

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

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 (generally those not producing primitives). This requirement is under discussion in #68364. 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.

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.

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

Deinit(Box<Place<'tcx>>)

Deinitializes the place.

This writes uninit bytes to the entire place.

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.

StorageDead(Local)

See StorageLive above.

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.

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.

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.

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.

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.

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.

Nop

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

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

impl<'tcx> StatementKind<'tcx>

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").

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

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

Trait Implementations

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

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

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<'tcx> Debug for StatementKind<'tcx>

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

Formats the value using the given formatter.

impl<'tcx, __D: TyDecoder<'tcx>> Decodable<__D> for StatementKind<'tcx>

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

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

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

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

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

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<'tcx, '__ctx> HashStable<StableHashingContext<'__ctx>> for StatementKind<'tcx>

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

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

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

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for StatementKind<'tcx>

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).

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).

impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for StatementKind<'tcx>

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).

impl<'tcx> StructuralPartialEq for StatementKind<'tcx>

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
• Deinit: 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