Auto merge of #121199 - RalfJung:ref-to-vtable-fn, r=<try>

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

Such references sound like an exceedingly bad idea... just use raw pointers if you must do things like this.

But we should crater this, to be on the safe side.

Author: bors

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

@@ -22,9 +22,11 @@ const UNALIGNED_BOX: Box<u16> = unsafe { mem::transmute(&[0u8; 4]) };
 
 const NULL: &u16 = unsafe { mem::transmute(0usize) };
 //~^ ERROR it is undefined behavior to use this value
+//~| null reference
 
 const NULL_BOX: Box<u16> = unsafe { mem::transmute(0usize) };
 //~^ ERROR it is undefined behavior to use this value
+//~| null box
 
 
 // It is very important that we reject this: We do promote `&(4 * REF_AS_USIZE)`,
@@ -41,23 +43,28 @@ const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute::<&[usize],
 
 const USIZE_AS_REF: &'static u8 = unsafe { mem::transmute(1337usize) };
 //~^ ERROR it is undefined behavior to use this value
+//~| dangling reference
 
 const USIZE_AS_BOX: Box<u8> = unsafe { mem::transmute(1337usize) };
 //~^ ERROR it is undefined behavior to use this value
+//~| dangling box
 
 const UNINIT_PTR: *const i32 = unsafe { MaybeUninit { uninit: () }.init };
 //~^ ERROR evaluation of constant value failed
 //~| uninitialized
 
 const NULL_FN_PTR: fn() = unsafe { mem::transmute(0usize) };
 //~^ ERROR it is undefined behavior to use this value
+//~| encountered null pointer, but expected a function pointer
 const UNINIT_FN_PTR: fn() = unsafe { MaybeUninit { uninit: () }.init };
 //~^ ERROR evaluation of constant value failed
 //~| uninitialized
 const DANGLING_FN_PTR: fn() = unsafe { mem::transmute(13usize) };
 //~^ ERROR it is undefined behavior to use this value
+//~| expected a function pointer
 const DATA_FN_PTR: fn() = unsafe { mem::transmute(&13) };
 //~^ ERROR it is undefined behavior to use this value
+//~| expected a function pointer
 
 
 const UNALIGNED_READ: () = unsafe {
@@ -66,5 +73,16 @@ 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

@@ -32,7 +32,7 @@ LL | const NULL: &u16 = unsafe { mem::transmute(0usize) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref-ptr.rs:26:1
+  --> $DIR/ub-ref-ptr.rs:27:1
    |
 LL | const NULL_BOX: Box<u16> = unsafe { mem::transmute(0usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a null box
@@ -43,7 +43,7 @@ LL | const NULL_BOX: Box<u16> = unsafe { mem::transmute(0usize) };
            }
 
 error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-ref-ptr.rs:33:1
+  --> $DIR/ub-ref-ptr.rs:35:1
    |
 LL | const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^ unable to turn pointer into integer
@@ -52,7 +52,7 @@ LL | const REF_AS_USIZE: usize = unsafe { mem::transmute(&0) };
    = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
 
 error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-ref-ptr.rs:36:39
+  --> $DIR/ub-ref-ptr.rs:38:39
    |
 LL | const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
    |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unable to turn pointer into integer
@@ -61,13 +61,13 @@ LL | const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
    = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
 
 note: erroneous constant encountered
-  --> $DIR/ub-ref-ptr.rs:36:38
+  --> $DIR/ub-ref-ptr.rs:38:38
    |
 LL | const REF_AS_USIZE_SLICE: &[usize] = &[unsafe { mem::transmute(&0) }];
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-ref-ptr.rs:39:86
+  --> $DIR/ub-ref-ptr.rs:41:86
    |
 LL | const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute::<&[usize], _>(&[mem::transmute(&0)]) };
    |                                                                                      ^^^^^^^^^^^^^^^^^^^^ unable to turn pointer into integer
@@ -76,13 +76,13 @@ LL | const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute::<&[us
    = help: the absolute address of a pointer is not known at compile-time, so such operations are not supported
 
 note: erroneous constant encountered
-  --> $DIR/ub-ref-ptr.rs:39:85
+  --> $DIR/ub-ref-ptr.rs:41:85
    |
 LL | const REF_AS_USIZE_BOX_SLICE: Box<[usize]> = unsafe { mem::transmute::<&[usize], _>(&[mem::transmute(&0)]) };
    |                                                                                     ^^^^^^^^^^^^^^^^^^^^^
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref-ptr.rs:42:1
+  --> $DIR/ub-ref-ptr.rs:44:1
    |
 LL | const USIZE_AS_REF: &'static u8 = unsafe { mem::transmute(1337usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a dangling reference (0x539[noalloc] has no provenance)
@@ -93,7 +93,7 @@ LL | const USIZE_AS_REF: &'static u8 = unsafe { mem::transmute(1337usize) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref-ptr.rs:45:1
+  --> $DIR/ub-ref-ptr.rs:48:1
    |
 LL | const USIZE_AS_BOX: Box<u8> = unsafe { mem::transmute(1337usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a dangling box (0x539[noalloc] has no provenance)
@@ -104,13 +104,13 @@ LL | const USIZE_AS_BOX: Box<u8> = unsafe { mem::transmute(1337usize) };
            }
 
 error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-ref-ptr.rs:48:41
+  --> $DIR/ub-ref-ptr.rs:52:41
    |
 LL | const UNINIT_PTR: *const i32 = unsafe { MaybeUninit { uninit: () }.init };
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref-ptr.rs:52:1
+  --> $DIR/ub-ref-ptr.rs:56:1
    |
 LL | const NULL_FN_PTR: fn() = unsafe { mem::transmute(0usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered null pointer, but expected a function pointer
@@ -121,13 +121,13 @@ LL | const NULL_FN_PTR: fn() = unsafe { mem::transmute(0usize) };
            }
 
 error[E0080]: evaluation of constant value failed
-  --> $DIR/ub-ref-ptr.rs:54:38
+  --> $DIR/ub-ref-ptr.rs:59:38
    |
 LL | const UNINIT_FN_PTR: fn() = unsafe { MaybeUninit { uninit: () }.init };
    |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ using uninitialized data, but this operation requires initialized memory
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref-ptr.rs:57:1
+  --> $DIR/ub-ref-ptr.rs:62:1
    |
 LL | const DANGLING_FN_PTR: fn() = unsafe { mem::transmute(13usize) };
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered 0xd[noalloc], but expected a function pointer
@@ -138,7 +138,7 @@ LL | const DANGLING_FN_PTR: fn() = unsafe { mem::transmute(13usize) };
            }
 
 error[E0080]: it is undefined behavior to use this value
-  --> $DIR/ub-ref-ptr.rs:59:1
+  --> $DIR/ub-ref-ptr.rs:65:1
    |
 LL | const DATA_FN_PTR: fn() = unsafe { mem::transmute(&13) };
    | ^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered ALLOC2<imm>, but expected a function pointer
@@ -158,11 +158,33 @@ note: inside `std::ptr::read::<u32>`
 note: inside `std::ptr::const_ptr::<impl *const u32>::read`
   --> $SRC_DIR/core/src/ptr/const_ptr.rs:LL:COL
 note: inside `UNALIGNED_READ`
-  --> $DIR/ub-ref-ptr.rs:66:5
+  --> $DIR/ub-ref-ptr.rs:73:5
    |
 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:76:1
+   |
+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:81:1
+   |
+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`.