bjoernager/rust
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Ban ArrayToPointer and MutToConstPointer from runtime MIR

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Apparently MIR borrowck cares about at least one of these for checking variance.

In runtime MIR, though, there's no need for them as `PtrToPtr` does the same thing.

(Banning them simplifies passes like GVN that no longer need to handle multiple cast possibilities.)
This commit is contained in:
Scott McMurray 2024-06-12 01:22:31 -07:00
parent 894f7a4ba6
commit 4630d1b23b
21 changed files with 60 additions and 32 deletions
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13
compiler/rustc_middle/src/mir/syntax.rs
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@ -127,6 +127,9 @@ pub enum AnalysisPhase {
/// * [`StatementKind::AscribeUserType`]
/// * [`StatementKind::Coverage`] with [`CoverageKind::BlockMarker`] or [`CoverageKind::SpanMarker`]
/// * [`Rvalue::Ref`] with `BorrowKind::Fake`
/// * [`CastKind::PointerCoercion`] with any of the following:
/// * [`PointerCoercion::ArrayToPointer`]
/// * [`PointerCoercion::MutToConstPointer`]
///
/// Furthermore, `Deref` projections must be the first projection within any place (if they
/// appear at all)
@ -1284,8 +1287,7 @@ pub enum Rvalue<'tcx> {
///
/// This allows for casts from/to a variety of types.
///
/// **FIXME**: Document exactly which `CastKind`s allow which types of casts. Figure out why
/// `ArrayToPointer` and `MutToConstPointer` are special.
/// **FIXME**: Document exactly which `CastKind`s allow which types of casts.
Cast(CastKind, Operand<'tcx>, Ty<'tcx>),
/// * `Offset` has the same semantics as [`offset`](pointer::offset), except that the second
@ -1365,6 +1367,13 @@ pub enum CastKind {
PointerWithExposedProvenance,
/// Pointer related casts that are done by coercions. Note that reference-to-raw-ptr casts are
/// translated into `&raw mut/const *r`, i.e., they are not actually casts.
///
/// The following are allowed in [`AnalysisPhase::Initial`] as they're needed for borrowck,
/// but after that are forbidden (including in all phases of runtime MIR):
/// * [`PointerCoercion::ArrayToPointer`]
/// * [`PointerCoercion::MutToConstPointer`]
///
/// Both are runtime nops, so should be [`CastKind::PtrToPtr`] instead in runtime MIR.
PointerCoercion(PointerCoercion),
/// Cast into a dyn* object.
DynStar,