Rollup merge of #87020 - RalfJung:const_raw_ptr_to_usize_cast, r=oli-obk

remove const_raw_ptr_to_usize_cast feature

This feature currently has the strange status of "const-only `unsafe`", which was an experiment that we no longer think is a good idea. We need to find better ways to enable things like "messing with the low bits of a pointer" during CTFE.

r? `@oli-obk`

Author: JohnTitor

compiler/rustc_feature/src/active.rs

@@ -416,9 +416,6 @@ declare_features! (
     /// Allows accessing fields of unions inside `const` functions.
     (active, const_fn_union, "1.27.0", Some(51909), None),
 
-    /// Allows casting raw pointers to `usize` during const eval.
-    (active, const_raw_ptr_to_usize_cast, "1.27.0", Some(51910), None),
-
     /// Allows dereferencing raw pointers during const eval.
     (active, const_raw_ptr_deref, "1.27.0", Some(51911), None),
 

compiler/rustc_feature/src/removed.rs

@@ -144,6 +144,10 @@ declare_features! (
     (removed, external_doc, "1.54.0", Some(44732), None,
      Some("use #[doc = include_str!(\"filename\")] instead, which handles macro invocations")),
 
+     /// Allows casting raw pointers to `usize` during const eval.
+    (removed, const_raw_ptr_to_usize_cast, "1.55.0", Some(51910), None,
+     Some("at compile-time, pointers do not have an integer value, so these casts cannot be properly supported")),
+
     // -------------------------------------------------------------------------
     // feature-group-end: removed features
     // -------------------------------------------------------------------------

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -397,6 +397,9 @@ impl NonConstOp for PanicNonStr {
     }
 }
 
+/// Comparing raw pointers for equality.
+/// Not currently intended to ever be allowed, even behind a feature gate: operation depends on
+/// allocation base addresses that are not known at compile-time.
 #[derive(Debug)]
 pub struct RawPtrComparison;
 impl NonConstOp for RawPtrComparison {
@@ -430,20 +433,22 @@ impl NonConstOp for RawPtrDeref {
     }
 }
 
+/// Casting raw pointer or function pointer to an integer.
+/// Not currently intended to ever be allowed, even behind a feature gate: operation depends on
+/// allocation base addresses that are not known at compile-time.
 #[derive(Debug)]
 pub struct RawPtrToIntCast;
 impl NonConstOp for RawPtrToIntCast {
-    fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
-        Status::Unstable(sym::const_raw_ptr_to_usize_cast)
-    }
-
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-        feature_err(
-            &ccx.tcx.sess.parse_sess,
-            sym::const_raw_ptr_to_usize_cast,
-            span,
-            &format!("casting pointers to integers in {}s is unstable", ccx.const_kind(),),
-        )
+        let mut err = ccx
+            .tcx
+            .sess
+            .struct_span_err(span, "pointers cannot be cast to integers during const eval.");
+        err.note("at compile-time, pointers do not have an integer value");
+        err.note(
+            "avoiding this restriction via `transmute`, `union`, or raw pointers leads to compile-time undefined behavior",
+        );
+        err
     }
 }
 

compiler/rustc_mir/src/transform/check_unsafety.rs

@@ -7,7 +7,6 @@ use rustc_hir::intravisit;
 use rustc_hir::Node;
 use rustc_middle::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
-use rustc_middle::ty::cast::CastTy;
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_session::lint::builtin::{UNSAFE_OP_IN_UNSAFE_FN, UNUSED_UNSAFE};
@@ -18,7 +17,6 @@ use std::ops::Bound;
 pub struct UnsafetyChecker<'a, 'tcx> {
     body: &'a Body<'tcx>,
     body_did: LocalDefId,
-    const_context: bool,
     violations: Vec<UnsafetyViolation>,
     source_info: SourceInfo,
     tcx: TyCtxt<'tcx>,
@@ -30,7 +28,6 @@ pub struct UnsafetyChecker<'a, 'tcx> {
 
 impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
     fn new(
-        const_context: bool,
         body: &'a Body<'tcx>,
         body_did: LocalDefId,
         tcx: TyCtxt<'tcx>,
@@ -39,7 +36,6 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
         Self {
             body,
             body_did,
-            const_context,
             violations: vec![],
             source_info: SourceInfo::outermost(body.span),
             tcx,
@@ -136,25 +132,6 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
                     self.register_violations(&violations, &unsafe_blocks);
                 }
             },
-            // casting pointers to ints is unsafe in const fn because the const evaluator cannot
-            // possibly know what the result of various operations like `address / 2` would be
-            // pointers during const evaluation have no integral address, only an abstract one
-            Rvalue::Cast(CastKind::Misc, ref operand, cast_ty)
-                if self.const_context && self.tcx.features().const_raw_ptr_to_usize_cast =>
-            {
-                let operand_ty = operand.ty(self.body, self.tcx);
-                let cast_in = CastTy::from_ty(operand_ty).expect("bad input type for cast");
-                let cast_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
-                match (cast_in, cast_out) {
-                    (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Int(_)) => {
-                        self.require_unsafe(
-                            UnsafetyViolationKind::General,
-                            UnsafetyViolationDetails::CastOfPointerToInt,
-                        );
-                    }
-                    _ => {}
-                }
-            }
             _ => {}
         }
         self.super_rvalue(rvalue, location);
@@ -469,13 +446,7 @@ fn unsafety_check_result<'tcx>(
 
     let param_env = tcx.param_env(def.did);
 
-    let id = tcx.hir().local_def_id_to_hir_id(def.did);
-    let const_context = match tcx.hir().body_owner_kind(id) {
-        hir::BodyOwnerKind::Closure => false,
-        hir::BodyOwnerKind::Fn => tcx.is_const_fn_raw(def.did.to_def_id()),
-        hir::BodyOwnerKind::Const | hir::BodyOwnerKind::Static(_) => true,
-    };
-    let mut checker = UnsafetyChecker::new(const_context, body, def.did, tcx, param_env);
+    let mut checker = UnsafetyChecker::new(body, def.did, tcx, param_env);
     checker.visit_body(&body);
 
     check_unused_unsafe(tcx, def.did, &checker.used_unsafe, &mut checker.inherited_blocks);

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -25,7 +25,6 @@ struct UnsafetyVisitor<'a, 'tcx> {
     /// The `#[target_feature]` attributes of the body. Used for checking
     /// calls to functions with `#[target_feature]` (RFC 2396).
     body_target_features: &'tcx Vec<Symbol>,
-    is_const: bool,
     in_possible_lhs_union_assign: bool,
     in_union_destructure: bool,
 }
@@ -315,16 +314,6 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 (Bound::Unbounded, Bound::Unbounded) => {}
                 _ => self.requires_unsafe(expr.span, InitializingTypeWith),
             },
-            ExprKind::Cast { source } => {
-                let source = &self.thir[source];
-                if self.tcx.features().const_raw_ptr_to_usize_cast
-                    && self.is_const
-                    && (source.ty.is_unsafe_ptr() || source.ty.is_fn_ptr())
-                    && expr.ty.is_integral()
-                {
-                    self.requires_unsafe(expr.span, CastOfPointerToInt);
-                }
-            }
             ExprKind::Closure {
                 closure_id,
                 substs: _,
@@ -413,7 +402,6 @@ enum UnsafeOpKind {
     CallToUnsafeFunction,
     UseOfInlineAssembly,
     InitializingTypeWith,
-    CastOfPointerToInt,
     UseOfMutableStatic,
     UseOfExternStatic,
     DerefOfRawPointer,
@@ -446,9 +434,6 @@ impl UnsafeOpKind {
                 "initializing a layout restricted type's field with a value outside the valid \
                  range is undefined behavior",
             ),
-            CastOfPointerToInt => {
-                ("cast of pointer to int", "casting pointers to integers in constants")
-            }
             UseOfMutableStatic => (
                 "use of mutable static",
                 "mutable statics can be mutated by multiple threads: aliasing violations or data \
@@ -526,19 +511,13 @@ pub fn check_unsafety<'tcx>(tcx: TyCtxt<'tcx>, def: ty::WithOptConstParam<LocalD
     let body_target_features = &tcx.codegen_fn_attrs(def.did).target_features;
     let safety_context =
         if body_unsafety.is_unsafe() { SafetyContext::UnsafeFn } else { SafetyContext::Safe };
-    let is_const = match tcx.hir().body_owner_kind(hir_id) {
-        hir::BodyOwnerKind::Closure => false,
-        hir::BodyOwnerKind::Fn => tcx.is_const_fn_raw(def.did.to_def_id()),
-        hir::BodyOwnerKind::Const | hir::BodyOwnerKind::Static(_) => true,
-    };
     let mut visitor = UnsafetyVisitor {
         tcx,
         thir,
         safety_context,
         hir_context: hir_id,
         body_unsafety,
         body_target_features,
-        is_const,
         in_possible_lhs_union_assign: false,
         in_union_destructure: false,
     };

src/test/ui/cast/cast-ptr-to-int-const.mir.stderr

@@ -1,15 +1,11 @@
-#![feature(const_raw_ptr_to_usize_cast, const_raw_ptr_deref)]
+#![feature(const_raw_ptr_deref)]
 
 fn main() {}
 
-// unconst and fine
-const Y: usize = unsafe { 42usize as *const i32 as usize + 1 };
-// unconst and bad, will thus error in miri
-const Y2: usize = unsafe { &1 as *const i32 as usize + 1 }; //~ ERROR any use of this
-//~| WARN this was previously accepted by the compiler but is being phased out
-// unconst and fine
+// fine
 const Z: i32 = unsafe { *(&1 as *const i32) };
-// unconst and bad, will thus error in miri
+
+// bad, will thus error in miri
 const Z2: i32 = unsafe { *(42 as *const i32) }; //~ ERROR any use of this value will cause
 //~| WARN this was previously accepted by the compiler but is being phased out
 const Z3: i32 = unsafe { *(44 as *const i32) }; //~ ERROR any use of this value will cause

src/test/ui/cast/cast-ptr-to-int-const.rs

@@ -1,28 +1,17 @@
 error: any use of this value will cause an error
-  --> $DIR/const_raw_ptr_ops2.rs:8:28
-   |
-LL | const Y2: usize = unsafe { &1 as *const i32 as usize + 1 };
-   | ---------------------------^^^^^^^^^^^^^^^^^^^^^^^^^-------
-   |                            |
-   |                            cannot cast pointer to integer because it was not created by cast from integer
-   |
-   = note: `#[deny(const_err)]` on by default
-   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
-   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
-
-error: any use of this value will cause an error
-  --> $DIR/const_raw_ptr_ops2.rs:13:26
+  --> $DIR/const_raw_ptr_ops2.rs:9:26
    |
 LL | const Z2: i32 = unsafe { *(42 as *const i32) };
    | -------------------------^^^^^^^^^^^^^^^^^^^---
    |                          |
    |                          unable to turn bytes into a pointer
    |
+   = note: `#[deny(const_err)]` on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
 error: any use of this value will cause an error
-  --> $DIR/const_raw_ptr_ops2.rs:15:26
+  --> $DIR/const_raw_ptr_ops2.rs:11:26
    |
 LL | const Z3: i32 = unsafe { *(44 as *const i32) };
    | -------------------------^^^^^^^^^^^^^^^^^^^---
@@ -32,5 +21,5 @@ LL | const Z3: i32 = unsafe { *(44 as *const i32) };
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
-error: aborting due to 3 previous errors
+error: aborting due to 2 previous errors