Rollup merge of rust-lang#126308 - scottmcm:ban-some-coercions, r=sae…

…thlin

Ban `ArrayToPointer` and `MutToConstPointer` from runtime MIR

Zulip conversation: <https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/CastKind.3A.3APointerCoercion.20in.20Runtime.20MIR/near/443955195>

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.)

r? mir

compiler/rustc_codegen_cranelift/src/base.rs

@@ -677,21 +677,22 @@ fn codegen_stmt<'tcx>(
  CastKind::PointerCoercion(PointerCoercion::UnsafeFnPointer),
  ref operand,
  to_ty,
- )
- | Rvalue::Cast(
- CastKind::PointerCoercion(PointerCoercion::MutToConstPointer),
- ref operand,
- to_ty,
- )
- | Rvalue::Cast(
- CastKind::PointerCoercion(PointerCoercion::ArrayToPointer),
- ref operand,
- to_ty,
  ) => {
  let to_layout = fx.layout_of(fx.monomorphize(to_ty));
  let operand = codegen_operand(fx, operand);
  lval.write_cvalue(fx, operand.cast_pointer_to(to_layout));
  }
+ Rvalue::Cast(
+ CastKind::PointerCoercion(
+ PointerCoercion::MutToConstPointer | PointerCoercion::ArrayToPointer,
+ ),
+ ..,
+ ) => {
+ bug!(
+ "{:?} is for borrowck, and should never appear in codegen",
+ to_place_and_rval.1
+ );
+ }
  Rvalue::Cast(
  CastKind::IntToInt
  | CastKind::FloatToFloat

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -456,8 +456,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
  base::unsize_ptr(bx, lldata, operand.layout.ty, cast.ty, llextra);
  OperandValue::Pair(lldata, llextra)
  }
- mir::CastKind::PointerCoercion(PointerCoercion::MutToConstPointer)
- | mir::CastKind::PtrToPtr
+ mir::CastKind::PointerCoercion(
+ PointerCoercion::MutToConstPointer | PointerCoercion::ArrayToPointer,
+ ) => {
+ bug!("{kind:?} is for borrowck, and should never appear in codegen");
+ }
+ mir::CastKind::PtrToPtr
  if bx.cx().is_backend_scalar_pair(operand.layout) =>
  {
  if let OperandValue::Pair(data_ptr, meta) = operand.val {
@@ -477,9 +481,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
  base::cast_to_dyn_star(bx, lldata, operand.layout, cast.ty, llextra);
  OperandValue::Pair(lldata, llextra)
  }
- mir::CastKind::PointerCoercion(
- PointerCoercion::MutToConstPointer | PointerCoercion::ArrayToPointer,
- )
  | mir::CastKind::IntToInt
  | mir::CastKind::FloatToInt
  | mir::CastKind::FloatToFloat

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -70,9 +70,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
  CastKind::PointerCoercion(
  PointerCoercion::MutToConstPointer | PointerCoercion::ArrayToPointer,
  ) => {
- // These are NOPs, but can be wide pointers.
- let v = self.read_immediate(src)?;
- self.write_immediate(*v, dest)?;
+ bug!("{cast_kind:?} casts are for borrowck only, not runtime MIR");
  }
 
  CastKind::PointerCoercion(PointerCoercion::ReifyFnPointer) => {

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);

tests/mir-opt/instsimplify/casts.rs

@@ -21,7 +21,7 @@ pub fn roundtrip(x: *const u8) -> *const u8 {
  // CHECK-LABEL: fn roundtrip(
  // CHECK: _4 = _1;
  // CHECK: _3 = move _4 as *mut u8 (PtrToPtr);
- // CHECK: _2 = move _3 as *const u8 (PointerCoercion(MutToConstPointer));
+ // CHECK: _2 = move _3 as *const u8 (PtrToPtr);
  x as *mut u8 as *const u8
 }
 

tests/mir-opt/pre-codegen/slice_iter.enumerated_loop.PreCodegen.after.panic-abort.mir

@@ -79,7 +79,7 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
  _7 = _4 as *mut T (PtrToPtr);
  _8 = Offset(_7, _3);
  StorageDead(_7);
- _9 = move _8 as *const T (PointerCoercion(MutToConstPointer));
+ _9 = move _8 as *const T (PtrToPtr);
  StorageDead(_8);
  goto -> bb3;
  }

tests/mir-opt/pre-codegen/slice_iter.enumerated_loop.PreCodegen.after.panic-unwind.mir

@@ -79,7 +79,7 @@ fn enumerated_loop(_1: &[T], _2: impl Fn(usize, &T)) -> () {
  _7 = _4 as *mut T (PtrToPtr);
  _8 = Offset(_7, _3);
  StorageDead(_7);
- _9 = move _8 as *const T (PointerCoercion(MutToConstPointer));
+ _9 = move _8 as *const T (PtrToPtr);
  StorageDead(_8);
  goto -> bb3;
  }

tests/mir-opt/pre-codegen/slice_iter.forward_loop.PreCodegen.after.panic-abort.mir

@@ -71,7 +71,7 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
  _7 = _4 as *mut T (PtrToPtr);
  _8 = Offset(_7, _3);
  StorageDead(_7);
- _9 = move _8 as *const T (PointerCoercion(MutToConstPointer));
+ _9 = move _8 as *const T (PtrToPtr);
  StorageDead(_8);
  goto -> bb3;
  }

tests/mir-opt/pre-codegen/slice_iter.forward_loop.PreCodegen.after.panic-unwind.mir

@@ -71,7 +71,7 @@ fn forward_loop(_1: &[T], _2: impl Fn(&T)) -> () {
  _7 = _4 as *mut T (PtrToPtr);
  _8 = Offset(_7, _3);
  StorageDead(_7);
- _9 = move _8 as *const T (PointerCoercion(MutToConstPointer));
+ _9 = move _8 as *const T (PtrToPtr);
  StorageDead(_8);
  goto -> bb3;
  }

tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.panic-abort.mir

@@ -79,7 +79,7 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
  _7 = _4 as *mut T (PtrToPtr);
  _8 = Offset(_7, _3);
  StorageDead(_7);
- _9 = move _8 as *const T (PointerCoercion(MutToConstPointer));
+ _9 = move _8 as *const T (PtrToPtr);
  StorageDead(_8);
  goto -> bb3;
  }

tests/mir-opt/pre-codegen/slice_iter.reverse_loop.PreCodegen.after.panic-unwind.mir

@@ -79,7 +79,7 @@ fn reverse_loop(_1: &[T], _2: impl Fn(&T)) -> () {
  _7 = _4 as *mut T (PtrToPtr);
  _8 = Offset(_7, _3);
  StorageDead(_7);
- _9 = move _8 as *const T (PointerCoercion(MutToConstPointer));
+ _9 = move _8 as *const T (PtrToPtr);
  StorageDead(_8);
  goto -> bb3;
  }

tests/mir-opt/retag.array_casts.SimplifyCfg-pre-optimizations.after.panic-abort.mir

@@ -64,7 +64,7 @@ fn array_casts() -> () {
  StorageLive(_4);
  _4 = &mut _1;
  _3 = &raw mut (*_4);
- _2 = move _3 as *mut usize (PointerCoercion(ArrayToPointer));
+ _2 = move _3 as *mut usize (PtrToPtr);
  StorageDead(_3);
  StorageDead(_4);
  StorageLive(_5);
@@ -87,7 +87,7 @@ fn array_casts() -> () {
  StorageLive(_11);
  _11 = &_8;
  _10 = &raw const (*_11);
- _9 = move _10 as *const usize (PointerCoercion(ArrayToPointer));
+ _9 = move _10 as *const usize (PtrToPtr);
  StorageDead(_10);
  StorageDead(_11);
  StorageLive(_12);

tests/mir-opt/retag.array_casts.SimplifyCfg-pre-optimizations.after.panic-unwind.mir

@@ -64,7 +64,7 @@ fn array_casts() -> () {
  StorageLive(_4);
  _4 = &mut _1;
  _3 = &raw mut (*_4);
- _2 = move _3 as *mut usize (PointerCoercion(ArrayToPointer));
+ _2 = move _3 as *mut usize (PtrToPtr);
  StorageDead(_3);
  StorageDead(_4);
  StorageLive(_5);
@@ -87,7 +87,7 @@ fn array_casts() -> () {
  StorageLive(_11);
  _11 = &_8;
  _10 = &raw const (*_11);
- _9 = move _10 as *const usize (PointerCoercion(ArrayToPointer));
+ _9 = move _10 as *const usize (PtrToPtr);
  StorageDead(_10);
  StorageDead(_11);
  StorageLive(_12);