const-eval: error when encountering references to functions / vtables

Author: RalfJung

compiler/rustc_const_eval/messages.ftl

@@ -464,8 +464,10 @@ const_eval_validation_out_of_range = {$front_matter}: encountered {$value}, but
 const_eval_validation_partial_pointer = {$front_matter}: encountered a partial pointer or a mix of pointers
 const_eval_validation_pointer_as_int = {$front_matter}: encountered a pointer, but {$expected}
 const_eval_validation_ptr_out_of_range = {$front_matter}: encountered a pointer, but expected something that cannot possibly fail to be {$in_range}
+const_eval_validation_ref_to_function = {$front_matter}: encountered a reference pointing to a function
 const_eval_validation_ref_to_static = {$front_matter}: encountered a reference pointing to a static variable in a constant
 const_eval_validation_ref_to_uninhabited = {$front_matter}: encountered a reference pointing to uninhabited type {$ty}
+const_eval_validation_ref_to_vtable = {$front_matter}: encountered a reference pointing to a vtable
 const_eval_validation_unaligned_box = {$front_matter}: encountered an unaligned box (required {$required_bytes} byte alignment but found {$found_bytes})
 const_eval_validation_unaligned_ref = {$front_matter}: encountered an unaligned reference (required {$required_bytes} byte alignment but found {$found_bytes})
 const_eval_validation_uninhabited_enum_variant = {$front_matter}: encountered an uninhabited enum variant

compiler/rustc_const_eval/src/errors.rs

@@ -614,6 +614,8 @@ impl<'tcx> ReportErrorExt for ValidationErrorInfo<'tcx> {
             PartialPointer => const_eval_validation_partial_pointer,
             ConstRefToMutable => const_eval_validation_const_ref_to_mutable,
             ConstRefToExtern => const_eval_validation_const_ref_to_extern,
+            RefToFunction => const_eval_validation_ref_to_function,
+            RefToVtable => const_eval_validation_ref_to_vtable,
             MutableRefInConstOrStatic => const_eval_validation_mutable_ref_in_const_or_static,
             MutableRefToImmutable => const_eval_validation_mutable_ref_to_immutable,
             NullFnPtr => const_eval_validation_null_fn_ptr,
@@ -771,6 +773,8 @@ impl<'tcx> ReportErrorExt for ValidationErrorInfo<'tcx> {
             | MutableRefInConstOrStatic
             | ConstRefToMutable
             | ConstRefToExtern
+            | RefToFunction
+            | RefToVtable
             | MutableRefToImmutable
             | NullFnPtr
             | NeverVal

compiler/rustc_const_eval/src/interpret/validity.rs

@@ -502,9 +502,11 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                         if is_mut { Mutability::Mut } else { Mutability::Not }
                     }
                     GlobalAlloc::Memory(alloc) => alloc.inner().mutability,
-                    GlobalAlloc::Function(..) | GlobalAlloc::VTable(..) => {
-                        // These are immutable, we better don't allow mutable pointers here.
-                        Mutability::Not
+                    GlobalAlloc::Function(..) => {
+                        throw_validation_failure!(self.path, RefToFunction);
+                    }
+                    GlobalAlloc::VTable(..) => {
+                        throw_validation_failure!(self.path, RefToVtable);
                     }
                 };
                 // Mutability check.

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -425,6 +425,8 @@ pub enum ValidationErrorKind<'tcx> {
     ConstRefToExtern,
     MutableRefToImmutable,
     UnsafeCellInImmutable,
+    RefToFunction,
+    RefToVtable,
     NullFnPtr,
     NeverVal,
     NullablePtrOutOfRange { range: WrappingRange, max_value: u128 },

tests/ui/consts/const-eval/ub-ref-ptr.rs

@@ -66,5 +66,16 @@ const UNALIGNED_READ: () = unsafe {
     ptr.read(); //~ inside `UNALIGNED_READ`
 };
 
+const POINTS_TO_FUNCTION: &() = unsafe {
+//~^ ERROR it is undefined behavior to use this value
+//~| function
+    mem::transmute(main as fn())
+};
+const POINTS_TO_VTABLE: (&(), &()) = unsafe {
+//~^ ERROR it is undefined behavior to use this value
+//~| vtable
+    mem::transmute(&() as &dyn Send)
+};
+
 
 fn main() {}

tests/ui/consts/const-eval/ub-ref-ptr.stderr

@@ -163,6 +163,28 @@ note: inside `UNALIGNED_READ`
 LL |     ptr.read();
    |     ^^^^^^^^^^
 
-error: aborting due to 15 previous errors
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/ub-ref-ptr.rs:69:1
+   |
+LL | const POINTS_TO_FUNCTION: &() = unsafe {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a function
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: $SIZE, align: $ALIGN) {
+               HEX_DUMP
+           }
+
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/ub-ref-ptr.rs:74:1
+   |
+LL | const POINTS_TO_VTABLE: (&(), &()) = unsafe {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value at .1: encountered a reference pointing to a vtable
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+   = note: the raw bytes of the constant (size: $SIZE, align: $ALIGN) {
+               HEX_DUMP
+           }
+
+error: aborting due to 17 previous errors
 
 For more information about this error, try `rustc --explain E0080`.