Reflow MirPhase
comments.
Currently many of them exceed 100 chars, which makes them painful to read on a terminal that is 100 chars wide.
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@ -53,7 +53,8 @@ pub enum MirPhase {
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/// sequences of statements that would generally be subject to constant promotion are
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/// semantically constants, while in analysis MIR all constants are explicit.
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///
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/// The result of const promotion is available from the `mir_promoted` and `promoted_mir` queries.
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/// The result of const promotion is available from the `mir_promoted` and `promoted_mir`
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/// queries.
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///
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/// This is the version of MIR used by borrowck and friends.
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Analysis(AnalysisPhase),
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@ -61,26 +62,27 @@ pub enum MirPhase {
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///
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/// The semantic changes that occur in the lowering from analysis to runtime MIR are as follows:
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///
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/// - Drops: In analysis MIR, `Drop` terminators represent *conditional* drops; roughly speaking,
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/// if dataflow analysis determines that the place being dropped is uninitialized, the drop will
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/// not be executed. The exact semantics of this aren't written down anywhere, which means they
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/// are essentially "what drop elaboration does." In runtime MIR, the drops are unconditional;
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/// when a `Drop` terminator is reached, if the type has drop glue that drop glue is always
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/// executed. This may be UB if the underlying place is not initialized.
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/// - Packed drops: Places might in general be misaligned - in most cases this is UB, the exception
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/// is fields of packed structs. In analysis MIR, `Drop(P)` for a `P` that might be misaligned
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/// for this reason implicitly moves `P` to a temporary before dropping. Runtime MIR has no such
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/// rules, and dropping a misaligned place is simply UB.
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/// - Unwinding: in analysis MIR, unwinding from a function which may not unwind aborts. In runtime
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/// MIR, this is UB.
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/// - Retags: If `-Zmir-emit-retag` is enabled, analysis MIR has "implicit" retags in the same way
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/// that Rust itself has them. Where exactly these are is generally subject to change, and so we
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/// don't document this here. Runtime MIR has most retags explicit (though implicit retags
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/// can still occur at `Rvalue::{Ref,AddrOf}`).
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/// - Coroutine bodies: In analysis MIR, locals may actually be behind a pointer that user code has
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/// access to. This occurs in coroutine bodies. Such locals do not behave like other locals,
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/// because they eg may be aliased in surprising ways. Runtime MIR has no such special locals -
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/// all coroutine bodies are lowered and so all places that look like locals really are locals.
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/// - Drops: In analysis MIR, `Drop` terminators represent *conditional* drops; roughly
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/// speaking, if dataflow analysis determines that the place being dropped is uninitialized,
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/// the drop will not be executed. The exact semantics of this aren't written down anywhere,
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/// which means they are essentially "what drop elaboration does." In runtime MIR, the drops
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/// are unconditional; when a `Drop` terminator is reached, if the type has drop glue that
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/// drop glue is always executed. This may be UB if the underlying place is not initialized.
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/// - Packed drops: Places might in general be misaligned - in most cases this is UB, the
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/// exception is fields of packed structs. In analysis MIR, `Drop(P)` for a `P` that might be
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/// misaligned for this reason implicitly moves `P` to a temporary before dropping. Runtime
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/// MIR has no such rules, and dropping a misaligned place is simply UB.
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/// - Unwinding: in analysis MIR, unwinding from a function which may not unwind aborts. In
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/// runtime MIR, this is UB.
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/// - Retags: If `-Zmir-emit-retag` is enabled, analysis MIR has "implicit" retags in the same
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/// way that Rust itself has them. Where exactly these are is generally subject to change,
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/// and so we don't document this here. Runtime MIR has most retags explicit (though implicit
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/// retags can still occur at `Rvalue::{Ref,AddrOf}`).
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/// - Coroutine bodies: In analysis MIR, locals may actually be behind a pointer that user code
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/// has access to. This occurs in coroutine bodies. Such locals do not behave like other
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/// locals, because they eg may be aliased in surprising ways. Runtime MIR has no such
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/// special locals. All coroutine bodies are lowered and so all places that look like locals
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/// really are locals.
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///
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/// Also note that the lint pass which reports eg `200_u8 + 200_u8` as an error is run as a part
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/// of analysis to runtime MIR lowering. To ensure lints are reported reliably, this means that
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@ -111,7 +113,8 @@ pub enum AnalysisPhase {
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/// * [`TerminatorKind::FalseEdge`]
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/// * [`StatementKind::FakeRead`]
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/// * [`StatementKind::AscribeUserType`]
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/// * [`StatementKind::Coverage`] with [`CoverageKind::BlockMarker`] or [`CoverageKind::SpanMarker`]
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/// * [`StatementKind::Coverage`] with [`CoverageKind::BlockMarker`] or
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/// [`CoverageKind::SpanMarker`]
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/// * [`Rvalue::Ref`] with `BorrowKind::Fake`
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/// * [`CastKind::PointerCoercion`] with any of the following:
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/// * [`PointerCoercion::ArrayToPointer`]
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