# Struct rustc_mir_transform::ref_prop::ReferencePropagation

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    pub struct ReferencePropagation;

Expand description

Propagate references using SSA analysis.

MIR building may produce a lot of borrow-dereference patterns.

This pass aims to transform the following pattern: _1 = &raw? mut? PLACE; _3 = *_1; _4 = &raw? mut? *_1;

Into _1 = &raw? mut? PLACE; _3 = PLACE; _4 = &raw? mut? PLACE;

where `PLACE` is a direct or an indirect place expression.

There are 3 properties that need to be upheld for this transformation to be legal:

- place stability: `PLACE` must refer to the same memory wherever it appears;
- pointer liveness: we must not introduce dereferences of dangling pointers;
- `&mut` borrow uniqueness.

## Stability

If `PLACE` is an indirect projection, if its of the form `(*LOCAL).PROJECTIONS` where:

- `LOCAL` is SSA;
- all projections in `PROJECTIONS` have a stable offset (no dereference and no indexing).

If `PLACE` is a direct projection of a local, we consider it as constant if:

- the local is always live, or it has a single `StorageLive`;
- all projections have a stable offset.

## Liveness

When performing a substitution, we must take care not to introduce uses of dangling locals. To ensure this, we walk the body with the `MaybeStorageDead` dataflow analysis:

- if we want to replace `*x` by reborrow `*y` and `y` may be dead, we allow replacement and mark storage statements on `y` for removal;
- if we want to replace `*x` by non-reborrow `y` and `y` must be live, we allow replacement;
- if we want to replace `*x` by non-reborrow `y` and `y` may be dead, we do not replace.

## Uniqueness

For `&mut` borrows, we also need to preserve the uniqueness property: we must avoid creating a state where we interleave uses of `*_1` and `_2`. To do it, we only perform full substitution of mutable borrows: we replace either all or none of the occurrences of `*_1`.

Some care has to be taken when `_1` is copied in other locals. _1 = &raw? mut? _2; _3 = *_1; _4 = _1 _5 = *_4 In such cases, fully substituting `_1` means fully substituting all of the copies.

For immutable borrows, we do not need to preserve such uniqueness property, so we perform all the possible substitutions without removing the `_1 = &_2` statement.

## Trait Implementations§

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### impl<'tcx> MirPass<'tcx> for ReferencePropagation

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#### fn is_enabled(&self, sess: &Session) -> bool

Returns `true` if this pass is enabled with the current combination of compiler flags.
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#### fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>)

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#### fn name(&self) -> &'static str

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#### fn profiler_name(&self) -> &'static str

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#### fn is_mir_dump_enabled(&self) -> bool

## Auto Trait Implementations§

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### impl RefUnwindSafe for ReferencePropagation

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### impl Send for ReferencePropagation

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### impl Sync for ReferencePropagation

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### impl Unpin for ReferencePropagation

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### impl UnwindSafe for ReferencePropagation

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

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

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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::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.

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