Enum rustc_middle::mir::Rvalue

source ·
pub enum Rvalue<'tcx> {
Show 15 variants Use(Operand<'tcx>), Repeat(Operand<'tcx>, Const<'tcx>), Ref(Region<'tcx>, BorrowKindPlace<'tcx>), ThreadLocalRef(DefId), AddressOf(MutabilityPlace<'tcx>), Len(Place<'tcx>), Cast(CastKindOperand<'tcx>, Ty<'tcx>), BinaryOp(BinOpBox<(Operand<'tcx>, Operand<'tcx>)>), CheckedBinaryOp(BinOpBox<(Operand<'tcx>, Operand<'tcx>)>), NullaryOp(NullOpTy<'tcx>), UnaryOp(UnOpOperand<'tcx>), Discriminant(Place<'tcx>), Aggregate(Box<AggregateKind<'tcx>>, Vec<Operand<'tcx>>), ShallowInitBox(Operand<'tcx>, Ty<'tcx>), CopyForDeref(Place<'tcx>),
}
Expand description

The various kinds of rvalues that can appear in MIR.

Not all of these are allowed at every MirPhase - when this is the case, it’s stated below.

Computing any rvalue begins by evaluating the places and operands in some order (Needs clarification: Which order?). These are then used to produce a “value” - the same kind of value that an Operand produces.

Variants§

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Use(Operand<'tcx>)

Yields the operand unchanged

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Repeat(Operand<'tcx>, Const<'tcx>)

Creates an array where each element is the value of the operand.

This is the cause of a bug in the case where the repetition count is zero because the value is not dropped, see #74836.

Corresponds to source code like [x; 32].

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Ref(Region<'tcx>, BorrowKindPlace<'tcx>)

Creates a reference of the indicated kind to the place.

There is not much to document here, because besides the obvious parts the semantics of this are essentially entirely a part of the aliasing model. There are many UCG issues discussing exactly what the behavior of this operation should be.

Shallow borrows are disallowed after drop lowering.

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ThreadLocalRef(DefId)

Creates a pointer/reference to the given thread local.

The yielded type is a *mut T if the static is mutable, otherwise if the static is extern a *const T, and if neither of those apply a &T.

Note: This is a runtime operation that actually executes code and is in this sense more like a function call. Also, eliminating dead stores of this rvalue causes fn main() {} to SIGILL for some reason that I (JakobDegen) never got a chance to look into.

Needs clarification: Are there weird additional semantics here related to the runtime nature of this operation?

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AddressOf(MutabilityPlace<'tcx>)

Creates a pointer with the indicated mutability to the place.

This is generated by pointer casts like &v as *const _ or raw address of expressions like &raw v or addr_of!(v).

Like with references, the semantics of this operation are heavily dependent on the aliasing model.

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Len(Place<'tcx>)

Yields the length of the place, as a usize.

If the type of the place is an array, this is the array length. For slices ([T], not &[T]) this accesses the place’s metadata to determine the length. This rvalue is ill-formed for places of other types.

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Cast(CastKindOperand<'tcx>, Ty<'tcx>)

Performs essentially all of the casts that can be performed via as.

This allows for casts from/to a variety of types.

FIXME: Document exactly which CastKinds allow which types of casts. Figure out why ArrayToPointer and MutToConstPointer are special.

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BinaryOp(BinOpBox<(Operand<'tcx>, Operand<'tcx>)>)

  • Offset has the same semantics as offset, except that the second parameter may be a usize as well.
  • The comparison operations accept bools, chars, signed or unsigned integers, floats, raw pointers, or function pointers and return a bool. The types of the operands must be matching, up to the usual caveat of the lifetimes in function pointers.
  • Left and right shift operations accept signed or unsigned integers not necessarily of the same type and return a value of the same type as their LHS. Like in Rust, the RHS is truncated as needed.
  • The Bit* operations accept signed integers, unsigned integers, or bools with matching types and return a value of that type.
  • The remaining operations accept signed integers, unsigned integers, or floats with matching types and return a value of that type.
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CheckedBinaryOp(BinOpBox<(Operand<'tcx>, Operand<'tcx>)>)

Same as BinaryOp, but yields (T, bool) with a bool indicating an error condition.

When overflow checking is disabled and we are generating run-time code, the error condition is false. Otherwise, and always during CTFE, the error condition is determined as described below.

For addition, subtraction, and multiplication on integers the error condition is set when the infinite precision result would be unequal to the actual result.

For shift operations on integers the error condition is set when the value of right-hand side is greater than or equal to the number of bits in the type of the left-hand side, or when the value of right-hand side is negative.

Other combinations of types and operators are unsupported.

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NullaryOp(NullOpTy<'tcx>)

Computes a value as described by the operation.

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UnaryOp(UnOpOperand<'tcx>)

Exactly like BinaryOp, but less operands.

Also does two’s-complement arithmetic. Negation requires a signed integer or a float; bitwise not requires a signed integer, unsigned integer, or bool. Both operation kinds return a value with the same type as their operand.

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Discriminant(Place<'tcx>)

Computes the discriminant of the place, returning it as an integer of type discriminant_ty. Returns zero for types without discriminant.

The validity requirements for the underlying value are undecided for this rvalue, see #91095. Note too that the value of the discriminant is not the same thing as the variant index; use discriminant_for_variant to convert.

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Aggregate(Box<AggregateKind<'tcx>>, Vec<Operand<'tcx>>)

Creates an aggregate value, like a tuple or struct.

This is needed because dataflow analysis needs to distinguish dest = Foo { x: ..., y: ... } from dest.x = ...; dest.y = ...; in the case that Foo has a destructor.

Disallowed after deaggregation for all aggregate kinds except Array and Generator. After generator lowering, Generator aggregate kinds are disallowed too.

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ShallowInitBox(Operand<'tcx>, Ty<'tcx>)

Transmutes a *mut u8 into shallow-initialized Box<T>.

This is different from a normal transmute because dataflow analysis will treat the box as initialized but its content as uninitialized. Like other pointer casts, this in general affects alias analysis.

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CopyForDeref(Place<'tcx>)

A CopyForDeref is equivalent to a read from a place at the codegen level, but is treated specially by drop elaboration. When such a read happens, it is guaranteed (via nature of the mir_opt Derefer in rustc_mir_transform/src/deref_separator) that the only use of the returned value is a deref operation, immediately followed by one or more projections. Drop elaboration treats this rvalue as if the read never happened and just projects further. This allows simplifying various MIR optimizations and codegen backends that previously had to handle deref operations anywhere in a place.

Implementations§

Returns true if this rvalue is deeply initialized (most rvalues) or whether its only shallowly initialized (Rvalue::Box).

Rvalues

Returns true if rvalue can be safely removed when the result is unused.

Trait Implementations§

Returns a copy of the value. Read more
Performs copy-assignment from source. Read more
Formats the value using the given formatter. Read more
Feeds this value into the given Hasher. Read more
Feeds a slice of this type into the given Hasher. Read more
This method tests for self and other values to be equal, and is used by ==. Read more
This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
The entry point for folding. To fold a value t with a folder f call: t.try_fold_with(f). Read more
A convenient alternative to try_fold_with for use with infallible folders. Do not override this method, to ensure coherence with try_fold_with. Read more
The entry point for visiting. To visit a value t with a visitor v call: t.visit_with(v). Read more
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. Read more
Returns true if this self has any regions that escape binder (and hence are not bound by it). Read more
“Free” regions in this context means that it has any region that is not (a) erased or (b) late-bound. Read more
True if there are any un-erased free regions.
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. Read more
True if there are any late-bound regions
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. Read more

Auto Trait Implementations§

Blanket Implementations§

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more
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). Read more
This method tries to recover the query key from the given DepNode, something which is needed when forcing DepNodes 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. Read more

Returns the argument unchanged.

Calls U::from(self).

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

The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.

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: 40 bytes

Size for each variant:

  • Use: 31 bytes
  • Repeat: 39 bytes
  • Ref: 31 bytes
  • ThreadLocalRef: 11 bytes
  • AddressOf: 23 bytes
  • Len: 23 bytes
  • Cast: 39 bytes
  • BinaryOp: 15 bytes
  • CheckedBinaryOp: 15 bytes
  • NullaryOp: 15 bytes
  • UnaryOp: 31 bytes
  • Discriminant: 23 bytes
  • Aggregate: 39 bytes
  • ShallowInitBox: 39 bytes
  • CopyForDeref: 23 bytes