Enum MirPhase

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

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

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.

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.

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.
• 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

impl MirPhase

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

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

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

pub fn parse(dialect: String, phase: Option<String>) -> Self

Parses a MirPhase from a pair of strings. Panics if this isn’t possible for any reason.

Trait Implementations

impl Clone for MirPhase

fn clone(&self) -> MirPhase

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for MirPhase

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

Formats the value using the given formatter.

impl<'tcx, __D: TyDecoder<'tcx>> Decodable<__D> for MirPhase

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

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

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

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

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

impl Ord for MirPhase

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

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl PartialEq for MirPhase

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

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

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

impl PartialOrd for MirPhase

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

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for MirPhase

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).

fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, _: &mut F) -> Self

A convenient alternative to try_fold_with for use with infallible folders. Do not override this method, to ensure coherence with try_fold_with.

impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for MirPhase

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).

impl Copy for MirPhase

impl Eq for MirPhase

impl StructuralPartialEq for MirPhase

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