Struct rustc_middle::mir::syntax::Place

source ·
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:

  1. The address in memory that the place refers to.
  2. The provenance with which the place is being accessed.
  3. The type of the place and an optional variant index. See PlaceTy.
  4. 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. A Downcast projection on a place with its variant index already set is not well-formed.

  • 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 generators. If the ADT or generator 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. A OpaqueCast projection on any type other than an opaque type from the current crate is not well-formed.

  • ConstantIndex: Computes an offset in units of T into the place as described in the documentation for the ProjectionElem. The resulting address is the parent’s address plus that offset, and the type is T. This is only legal if the parent place has type [T; N] or [T] (not &[T]). Since such a T is always sized, any resulting metadata is thrown out.

  • Subslice: This projection calculates an offset and a new address in a similar manner as ConstantIndex. It is also only legal on [T; N] and [T]. However, this yields a Place of type [T], and additionally sets the metadata to be the length of the subslice.

  • Index: Like ConstantIndex, 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 the Local as a Place { local, projection: [] }, and then computing its value as if done via Operand::Copy. The array/slice is then indexed with the resulting value. The local must have type usize.

  • Deref: Derefs are the last type of projection, and the most complicated. They are only legal on parent places that are references, pointers, or Box. A Deref projection begins by loading a value from the parent place, as if by Operand::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.

Computing a place may cause UB. One possibility is that the pointer used for a Deref may not be suitably aligned. Another possibility is that the place is not in bounds, meaning it does not point to an actual allocation.

However, if this is actually UB and when the UB kicks in is undecided. This is being discussed in UCG#319. The options include that every place must obey those rules, that only some places must obey them, or that places impose no rules of their own.

Rust currently requires that every place obey those two rules. This is checked by MIRI and taken advantage of by codegen (via gep inbounds). That is possibly subject to change.


§local: Local§projection: &'tcx List<PlaceElem<'tcx>>

projection out of a place (access a field, deref a pointer, etc)


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.

If MirPhase >= Derefered and if projection contains Deref, It’s guaranteed to be in the first place

Finds the innermost Local from this Place, if it is either a local itself or a single deref of a local.

If this place represents a local variable like _X with no projections, return Some(_X).

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.

Generates a new place by appending more_projections to the existing ones and interning the result.

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
Converts to this type from the input type.
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 type has any regions that escape binder (and hence are not bound by it). Read more
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
“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.


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.

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