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:
- The address in memory that the place refers to.
- The provenance with which the place is being accessed.
- The type of the place and an optional variant index. See
PlaceTy
. - 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. ADowncast
projection must always be followed immediately by aField
projection. -
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. AOpaqueCast
projection on any type other than an opaque type from the current crate is not well-formed. -
ConstantIndex
: Computes an offset in units ofT
into the place as described in the documentation for theProjectionElem
. The resulting address is the parent’s address plus that offset, and the type isT
. This is only legal if the parent place has type[T; N]
or[T]
(not&[T]
). Since such aT
is always sized, any resulting metadata is thrown out. -
Subslice
: This projection calculates an offset and a new address in a similar manner asConstantIndex
. It is also only legal on[T; N]
and[T]
. However, this yields aPlace
of type[T]
, and additionally sets the metadata to be the length of the subslice. -
Index
: LikeConstantIndex
, 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 theLocal
as aPlace { local, projection: [] }
, and then computing its value as if done viaOperand::Copy
. The array/slice is then indexed with the resulting value. The local must have typeusize
. -
Deref
: Derefs are the last type of projection, and the most complicated. They are only legal on parent places that are references, pointers, orBox
. ADeref
projection begins by loading a value from the parent place, as if byOperand::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§
Source§impl<'tcx> Place<'tcx>
impl<'tcx> Place<'tcx>
pub fn return_place() -> Place<'tcx>
Sourcepub fn is_indirect(&self) -> bool
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.
Sourcepub fn is_indirect_first_projection(&self) -> bool
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.
Sourcepub fn local_or_deref_local(&self) -> Option<Local>
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.
Sourcepub fn as_local(&self) -> Option<Local>
pub fn as_local(&self) -> Option<Local>
If this place represents a local variable like _X
with no
projections, return Some(_X)
.
pub fn as_ref(&self) -> PlaceRef<'tcx>
Sourcepub fn iter_projections(
self,
) -> impl Iterator<Item = (PlaceRef<'tcx>, PlaceElem<'tcx>)> + DoubleEndedIterator
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.
Sourcepub fn project_deeper(
self,
more_projections: &[PlaceElem<'tcx>],
tcx: TyCtxt<'tcx>,
) -> Self
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.
Trait Implementations§
Source§impl<'tcx, '__ctx> HashStable<StableHashingContext<'__ctx>> for Place<'tcx>
impl<'tcx, '__ctx> HashStable<StableHashingContext<'__ctx>> for Place<'tcx>
fn hash_stable( &self, __hcx: &mut StableHashingContext<'__ctx>, __hasher: &mut StableHasher, )
Source§impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for Place<'tcx>
impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for Place<'tcx>
Source§fn try_fold_with<__F: FallibleTypeFolder<TyCtxt<'tcx>>>(
self,
__folder: &mut __F,
) -> Result<Self, __F::Error>
fn try_fold_with<__F: FallibleTypeFolder<TyCtxt<'tcx>>>( self, __folder: &mut __F, ) -> Result<Self, __F::Error>
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try_fold_with
for use with infallible
folders. Do not override this method, to ensure coherence with
try_fold_with
.Source§impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for Place<'tcx>
impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for Place<'tcx>
Source§fn visit_with<__V: TypeVisitor<TyCtxt<'tcx>>>(
&self,
__visitor: &mut __V,
) -> __V::Result
fn visit_with<__V: TypeVisitor<TyCtxt<'tcx>>>( &self, __visitor: &mut __V, ) -> __V::Result
impl<'tcx> Copy for Place<'tcx>
impl<'tcx> Eq for Place<'tcx>
impl<'tcx> StructuralPartialEq for Place<'tcx>
Auto Trait Implementations§
impl<'tcx> DynSend for Place<'tcx>
impl<'tcx> DynSync for Place<'tcx>
impl<'tcx> Freeze for Place<'tcx>
impl<'tcx> !RefUnwindSafe for Place<'tcx>
impl<'tcx> Send for Place<'tcx>
impl<'tcx> Sync for Place<'tcx>
impl<'tcx> Unpin for Place<'tcx>
impl<'tcx> !UnwindSafe for Place<'tcx>
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T: Copy,
impl<'tcx, T> ArenaAllocatable<'tcx, IsCopy> for Twhere
T: Copy,
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fn allocate_from_iter( arena: &'tcx Arena<'tcx>, iter: impl IntoIterator<Item = T>, ) -> &'tcx mut [T]
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fn allocate_from_iter( arena: &'tcx Arena<'tcx>, iter: impl IntoIterator<Item = T>, ) -> &'tcx mut [T]
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T: ?Sized,
impl<T> BorrowMut<T> for Twhere
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Size: 16 bytes