## MirPhase

# Enum MirPhase

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    pub enum MirPhase {
        Built,
        Analysis(AnalysisPhase),
        Runtime(RuntimePhase),
    }

Expand description

Represents the “flavors” of MIR.

The MIR pipeline is structured into a few major dialects, with one or more phases within each dialect. A MIR flavor is identified by a dialect-phase pair. A single `MirPhase` value specifies such a pair. All flavors of MIR use the same data structure to represent the program.

Different MIR dialects have different semantics. (The differences between dialects are small, but they do exist.) The progression from one MIR dialect to the next is technically a lowering from one IR to another. In other words, a single well-formed `Body` might have different semantic meaning and different behavior at runtime in the different dialects. The specific differences between dialects are described on the variants below.

Phases exist only to place restrictions on what language constructs are permitted in well-formed MIR, and subsequent phases mostly increase those restrictions. I.e. to convert MIR from one phase to the next might require removing/replacing certain MIR constructs.

When adding dialects or phases, remember to update `MirPhase::index`.

## Variants§

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### Built

The “built MIR” dialect, as generated by MIR building.

The only things that operate on this dialect are unsafeck, the various MIR lints, and const qualifs.

This dialect has just the one (implicit) phase, which places few restrictions on what MIR constructs are allowed.

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### Analysis(AnalysisPhase)

The “analysis MIR” dialect, used for borrowck and friends.

The only semantic difference between built MIR and analysis MIR relates to constant promotion. In built MIR, sequences of statements that would generally be subject to constant promotion are semantically constants, while in analysis MIR all constants are explicit.

The result of const promotion is available from the `mir_promoted` and `promoted_mir` queries.

The phases of this dialect are described in `AnalysisPhase`.

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### Runtime(RuntimePhase)

The “runtime MIR” dialect, used for CTFE, optimizations, and codegen.

The semantic differences between analysis MIR and runtime MIR are as follows.

- Drops: In analysis MIR, `Drop` terminators represent conditional drops; roughly speaking, if dataflow analysis determines that the place being dropped is uninitialized, the drop will not be executed. The exact semantics of this aren’t written down anywhere, which means they are essentially “what drop elaboration does.” In runtime MIR, the drops are unconditional; when a `Drop` terminator is reached, if the type has drop glue that drop glue is always executed. This may be UB if the underlying place is not initialized.
- Packed drops: Places might in general be misaligned - in most cases this is UB, the exception is fields of packed structs. In analysis MIR, `Drop(P)` for a `P` that might be misaligned for this reason implicitly moves `P` to a temporary before dropping. Runtime MIR has no such rules, and dropping a misaligned place is simply UB.
- Async drops: after drop elaboration some drops may become async (`drop`, `async_fut` fields). StateTransform pass will expand those async drops or reset to sync.
- Unwinding: in analysis MIR, unwinding from a function which may not unwind aborts. In runtime MIR, this is UB.
- Retags: If `-Zmir-emit-retag` is enabled, analysis MIR has “implicit” retags in the same way that Rust itself has them. Where exactly these are is generally subject to change, and so we don’t document this here. Runtime MIR has most retags explicit (though implicit retags can still occur at `Rvalue::{Ref,AddrOf}`).
- Coroutine bodies: In analysis MIR, locals may actually be behind a pointer that user code has access to. This occurs in coroutine bodies. Such locals do not behave like other locals, because they e.g. may be aliased in surprising ways. Runtime MIR has no such special locals. All coroutine bodies are lowered and so all places that look like locals really are locals.

Also note that the lint pass which reports eg `200_u8 + 200_u8` as an error is run as a part of analysis to runtime MIR lowering. To ensure lints are reported reliably, this means that transformations that can suppress such errors should not run on analysis MIR.

The phases of this dialect are described in `RuntimePhase`.

## Implementations§

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### impl MirPhase

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

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#### pub fn index(&self) -> (usize, usize)

Gets the (dialect, phase) index of the current `MirPhase`. Both numbers are 1-indexed.

## Trait Implementations§

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### impl Clone for MirPhase

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#### fn clone(&self) -> MirPhase

Returns a duplicate of the value. Read more
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#### fn clone_from(&mut self, source: &Self)

Performs copy-assignment from `source`. Read more
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### impl Debug for MirPhase

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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### impl<'tcx, __D: TyDecoder<'tcx>> Decodable<__D> for MirPhase

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

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### impl<'tcx, __E: TyEncoder<'tcx>> Encodable<__E> for MirPhase

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#### fn encode(&self, __encoder: &mut __E)

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### impl<'__ctx> HashStable<StableHashingContext<'__ctx>> for MirPhase

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#### fn hash_stable( &self, __hcx: &mut StableHashingContext<'__ctx>, __hasher: &mut StableHasher, )

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### impl Ord for MirPhase

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#### fn cmp(&self, other: &MirPhase) -> Ordering

This method returns an `Ordering` between `self` and `other`. Read more
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#### fn max(self, other: Self) -> Self
where Self: Sized,

Compares and returns the maximum of two values. Read more
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#### fn min(self, other: Self) -> Self
where Self: Sized,

Compares and returns the minimum of two values. Read more
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#### fn clamp(self, min: Self, max: Self) -> Self
where Self: Sized,

Restrict a value to a certain interval. Read more
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### impl PartialEq for MirPhase

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#### fn eq(&self, other: &MirPhase) -> bool

Tests for `self` and `other` values to be equal, and is used by `==`.
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#### fn ne(&self, other: &Rhs) -> bool

Tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.
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### impl PartialOrd for MirPhase

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#### fn partial_cmp(&self, other: &MirPhase) -> Option<Ordering>

This method returns an ordering between `self` and `other` values if one exists. Read more
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#### fn lt(&self, other: &Rhs) -> bool

Tests less than (for `self` and `other`) and is used by the `<` operator. Read more
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#### fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
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#### fn gt(&self, other: &Rhs) -> bool

Tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
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#### fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more
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### impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for MirPhase

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#### fn try_fold_with<F: FallibleTypeFolder<TyCtxt<'tcx>>>( self, _: &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)`. Read more
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#### fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, _: &mut F) -> Self

The entry point for folding. To fold a value `t` with a folder `f` call: `t.fold_with(f)`. Read more
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### impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for MirPhase

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#### fn visit_with<F: TypeVisitor<TyCtxt<'tcx>>>(&self, _: &mut F) -> F::Result

The entry point for visiting. To visit a value `t` with a visitor `v` call: `t.visit_with(v)`. Read more
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### impl Copy for MirPhase

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### impl Eq for MirPhase

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### impl StructuralPartialEq for MirPhase

## Auto Trait Implementations§

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### impl DynSend for MirPhase

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### impl DynSync for MirPhase

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### impl Freeze for MirPhase

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### impl RefUnwindSafe for MirPhase

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### impl Send for MirPhase

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### impl Sync for MirPhase

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### impl Unpin for MirPhase

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### impl UnwindSafe for MirPhase

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## 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: 2 bytes

Size for each variant:

- `Built`: 0 bytes
- `Analysis`: 1 byte
- `Runtime`: 1 byte