Enum rustc_middle::mir::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>>),
    AscribeUserType(Box<(Place<'tcx>, UserTypeProjection)>, Variance),
    Coverage(Box<Coverage>),
    Intrinsic(Box<NonDivergingIntrinsic<'tcx>>),
    Nop,
}

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 paragragh 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>)>)

This represents all the reading that a pattern match may do (e.g., inspecting constants and discriminant values), and the kind of pattern it comes from. This is in order to adapt potential error messages to these specific patterns.

Note that this also is emitted for regular let bindings to ensure that locals that are never accessed still get some sanity checks for, e.g., let x: ! = ..;

When executed at runtime this is a nop.

Disallowed after drop elaboration.

SetDiscriminant

Fields

place: Box<Place<'tcx>>

variant_index: VariantIdx

Write the discriminant for a variant to the enum Place.

This is permitted for both generators 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.

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. 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 is UB. However, for an unallocated local an additional StorageDead all is simply 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.

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(Box<Coverage>)

Marks the start of a “coverage region”, injected with ‘-Cinstrument-coverage’. A Coverage statement carries metadata about the coverage region, used to inject a coverage map into the binary. If Coverage::kind is a Counter, the statement also generates executable code, to increment a counter variable at runtime, each time the code region is executed.

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.

Nop

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

Implementations

impl<'tcx> StatementKind<'tcx>

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 copy 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<I = TyCtxt<'tcx>>> Decodable<__D> for StatementKind<'tcx>

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

impl<'tcx, __E: TyEncoder<I = TyCtxt<'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,

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<StatementKind<'tcx>> for StatementKind<'tcx>

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

This method tests for self and other values to be equal, and is used by ==.

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.

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

fn try_fold_with<__F: FallibleTypeFolder<'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<'tcx>>(self, folder: &mut F) -> Self

A convenient alternative to try_fold_with for use with infallible folders. Do not override this method, to ensure coherence with try_fold_with.

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

fn visit_with<__V: TypeVisitor<'tcx>>(
    &self,
    __visitor: &mut __V
) -> ControlFlow<__V::BreakTy>

The entry point for visiting. To visit a value t with a visitor v call: t.visit_with(v).

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.

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

Returns true if this self has any regions that escape binder (and hence are not bound by it).

fn has_escaping_bound_vars(&self) -> bool

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

fn has_projections(&self) -> bool

fn has_opaque_types(&self) -> bool

fn references_error(&self) -> bool

fn error_reported(&self) -> Option<ErrorGuaranteed>

fn has_param_types_or_consts(&self) -> bool

fn has_infer_regions(&self) -> bool

fn has_infer_types(&self) -> bool

fn has_infer_types_or_consts(&self) -> bool

fn needs_infer(&self) -> bool

fn has_placeholders(&self) -> bool

fn needs_subst(&self) -> bool

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.

fn has_erased_regions(&self) -> bool

fn has_erasable_regions(&self) -> bool

True if there are any un-erased free regions.

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.

fn has_late_bound_regions(&self) -> bool

True if there are any late-bound regions

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.

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
• AscribeUserType: 15 bytes
• Coverage: 15 bytes
• Intrinsic: 15 bytes
• Nop: 0 bytes