Ban ArrayToPointer and MutToConstPointer from runtime MIR

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.)

Author: scottmcm

compiler/rustc_middle/src/mir/syntax.rs

@@ -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,

compiler/rustc_mir_transform/src/cleanup_post_borrowck.rs

@@ -18,7 +18,8 @@
 
 use crate::MirPass;
 use rustc_middle::mir::coverage::CoverageKind;
-use rustc_middle::mir::{Body, BorrowKind, Rvalue, StatementKind, TerminatorKind};
+use rustc_middle::mir::{Body, BorrowKind, CastKind, Rvalue, StatementKind, TerminatorKind};
+use rustc_middle::ty::adjustment::PointerCoercion;
 use rustc_middle::ty::TyCtxt;
 
 pub struct CleanupPostBorrowck;
@@ -36,6 +37,22 @@ impl<'tcx> MirPass<'tcx> for CleanupPostBorrowck {
                         CoverageKind::BlockMarker { .. } | CoverageKind::SpanMarker { .. },
                     )
                     | StatementKind::FakeRead(..) => statement.make_nop(),
+                    StatementKind::Assign(box (
+                        _,
+                        Rvalue::Cast(
+                            ref mut cast_kind @ CastKind::PointerCoercion(
+                                PointerCoercion::ArrayToPointer
+                                | PointerCoercion::MutToConstPointer,
+                            ),
+                            ..,
+                        ),
+                    )) => {
+                        // BorrowCk needed to track whether these cases were coercions or casts,
+                        // to know whether to check lifetimes in their pointees,
+                        // but from now on that distinction doesn't matter,
+                        // so just make them ordinary pointer casts instead.
+                        *cast_kind = CastKind::PtrToPtr;
+                    }
                     _ => (),
                 }
             }

compiler/rustc_mir_transform/src/gvn.rs

@@ -571,11 +571,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     let ret = self.ecx.ptr_to_ptr(&src, to).ok()?;
                     ret.into()
                 }
-                CastKind::PointerCoercion(
-                    ty::adjustment::PointerCoercion::MutToConstPointer
-                    | ty::adjustment::PointerCoercion::ArrayToPointer
-                    | ty::adjustment::PointerCoercion::UnsafeFnPointer,
-                ) => {
+                CastKind::PointerCoercion(ty::adjustment::PointerCoercion::UnsafeFnPointer) => {
                     let src = self.evaluated[value].as_ref()?;
                     let src = self.ecx.read_immediate(src).ok()?;
                     let to = self.ecx.layout_of(to).ok()?;
@@ -1164,10 +1160,10 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
             }
         }
 
-        if let PtrToPtr | PointerCoercion(MutToConstPointer) = kind
+        if let PtrToPtr = kind
             && let Value::Cast { kind: inner_kind, value: inner_value, from: inner_from, to: _ } =
                 *self.get(value)
-            && let PtrToPtr | PointerCoercion(MutToConstPointer) = inner_kind
+            && let PtrToPtr = inner_kind
         {
             from = inner_from;
             value = inner_value;

compiler/rustc_mir_transform/src/lib.rs

@@ -51,11 +51,11 @@ mod abort_unwinding_calls;
 mod add_call_guards;
 mod add_moves_for_packed_drops;
 mod add_retag;
+mod add_subtyping_projections;
+mod check_alignment;
 mod check_const_item_mutation;
 mod check_packed_ref;
-mod remove_place_mention;
 // This pass is public to allow external drivers to perform MIR cleanup
-mod add_subtyping_projections;
 pub mod cleanup_post_borrowck;
 mod copy_prop;
 mod coroutine;
@@ -94,6 +94,7 @@ mod prettify;
 mod promote_consts;
 mod ref_prop;
 mod remove_noop_landing_pads;
+mod remove_place_mention;
 mod remove_storage_markers;
 mod remove_uninit_drops;
 mod remove_unneeded_drops;
@@ -103,7 +104,6 @@ mod reveal_all;
 mod shim;
 mod ssa;
 // This pass is public to allow external drivers to perform MIR cleanup
-mod check_alignment;
 pub mod simplify;
 mod simplify_branches;
 mod simplify_comparison_integral;

compiler/rustc_mir_transform/src/validate.rs

@@ -1188,6 +1188,9 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                             "CastKind::{kind:?} output must be a raw const pointer, not {:?}",
                             ty::RawPtr(_, Mutability::Not)
                         );
+                        if self.mir_phase >= MirPhase::Analysis(AnalysisPhase::PostCleanup) {
+                            self.fail(location, format!("After borrowck, MIR disallows {kind:?}"));
+                        }
                     }
                     CastKind::PointerCoercion(PointerCoercion::ArrayToPointer) => {
                         // FIXME: Check pointee types
@@ -1201,6 +1204,9 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                             "CastKind::{kind:?} output must be a raw pointer, not {:?}",
                             ty::RawPtr(..)
                         );
+                        if self.mir_phase >= MirPhase::Analysis(AnalysisPhase::PostCleanup) {
+                            self.fail(location, format!("After borrowck, MIR disallows {kind:?}"));
+                        }
                     }
                     CastKind::PointerCoercion(PointerCoercion::Unsize) => {
                         // This is used for all `CoerceUnsized` types,
@@ -1212,7 +1218,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         if !input_valid || !target_valid {
                             self.fail(
                                 location,
-                                format!("Wrong cast kind {kind:?} for the type {op_ty}",),
+                                format!("Wrong cast kind {kind:?} for the type {op_ty}"),
                             );
                         }
                     }

tests/mir-opt/dataflow-const-prop/default_boxed_slice.main.GVN.32bit.panic-abort.diff

@@ -80,7 +80,7 @@
 
       bb4: {
           StorageDead(_8);
--         _11 = _6 as *const [bool; 0] (PointerCoercion(MutToConstPointer));
+-         _11 = _6 as *const [bool; 0] (PtrToPtr);
 -         _5 = NonNull::<[bool; 0]> { pointer: _11 };
 +         _11 = const {0x1 as *const [bool; 0]};
 +         _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};

tests/mir-opt/dataflow-const-prop/default_boxed_slice.main.GVN.32bit.panic-unwind.diff

@@ -84,7 +84,7 @@
 
       bb5: {
           StorageDead(_8);
--         _11 = _6 as *const [bool; 0] (PointerCoercion(MutToConstPointer));
+-         _11 = _6 as *const [bool; 0] (PtrToPtr);
 -         _5 = NonNull::<[bool; 0]> { pointer: _11 };
 +         _11 = const {0x1 as *const [bool; 0]};
 +         _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};

tests/mir-opt/dataflow-const-prop/default_boxed_slice.main.GVN.64bit.panic-abort.diff

@@ -80,7 +80,7 @@
 
       bb4: {
           StorageDead(_8);
--         _11 = _6 as *const [bool; 0] (PointerCoercion(MutToConstPointer));
+-         _11 = _6 as *const [bool; 0] (PtrToPtr);
 -         _5 = NonNull::<[bool; 0]> { pointer: _11 };
 +         _11 = const {0x1 as *const [bool; 0]};
 +         _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};

tests/mir-opt/dataflow-const-prop/default_boxed_slice.main.GVN.64bit.panic-unwind.diff

@@ -84,7 +84,7 @@
 
       bb5: {
           StorageDead(_8);
--         _11 = _6 as *const [bool; 0] (PointerCoercion(MutToConstPointer));
+-         _11 = _6 as *const [bool; 0] (PtrToPtr);
 -         _5 = NonNull::<[bool; 0]> { pointer: _11 };
 +         _11 = const {0x1 as *const [bool; 0]};
 +         _5 = const NonNull::<[bool; 0]> {{ pointer: {0x1 as *const [bool; 0]} }};

tests/mir-opt/instsimplify/casts.roundtrip.InstSimplify.diff

@@ -14,7 +14,7 @@
           StorageLive(_4);
           _4 = _1;
           _3 = move _4 as *mut u8 (PtrToPtr);
-          _2 = move _3 as *const u8 (PointerCoercion(MutToConstPointer));
+          _2 = move _3 as *const u8 (PtrToPtr);
           StorageDead(_4);
           StorageDead(_3);
 -         _0 = move _2 as *const u8 (PtrToPtr);