more precise message for the ptr access check on deref

Author: RalfJung

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

@@ -170,6 +170,8 @@ impl fmt::Display for InvalidProgramInfo<'_> {
 /// Details of why a pointer had to be in-bounds.
 #[derive(Debug, Copy, Clone, TyEncodable, TyDecodable, HashStable)]
 pub enum CheckInAllocMsg {
+    /// We are dereferencing a pointer (i.e., creating a place).
+    DerefTest,
     /// We are access memory.
     MemoryAccessTest,
     /// We are doing pointer arithmetic.
@@ -186,6 +188,7 @@ impl fmt::Display for CheckInAllocMsg {
             f,
             "{}",
             match *self {
+                CheckInAllocMsg::DerefTest => "dereferencing pointer failed: ",
                 CheckInAllocMsg::MemoryAccessTest => "memory access failed: ",
                 CheckInAllocMsg::PointerArithmeticTest => "pointer arithmetic failed: ",
                 CheckInAllocMsg::InboundsTest => "",

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

@@ -428,7 +428,11 @@ crate struct AllocMap<'tcx> {
 
 impl<'tcx> AllocMap<'tcx> {
     crate fn new() -> Self {
-        AllocMap { alloc_map: Default::default(), dedup: Default::default(), next_id: AllocId(NonZeroU64::new(1).unwrap()) }
+        AllocMap {
+            alloc_map: Default::default(),
+            dedup: Default::default(),
+            next_id: AllocId(NonZeroU64::new(1).unwrap()),
+        }
     }
     fn reserve(&mut self) -> AllocId {
         let next = self.next_id;

compiler/rustc_mir/src/interpret/place.rs

@@ -373,7 +373,7 @@ where
         let val = self.read_immediate(src)?;
         trace!("deref to {} on {:?}", val.layout.ty, *val);
         let mplace = self.ref_to_mplace(&val)?;
-        self.check_mplace_access(mplace)?;
+        self.check_mplace_access(mplace, CheckInAllocMsg::DerefTest)?;
         Ok(mplace)
     }
 
@@ -400,18 +400,17 @@ where
     }
 
     /// Check if this mplace is dereferencable and sufficiently aligned.
-    pub fn check_mplace_access(&self, mplace: MPlaceTy<'tcx, M::PointerTag>) -> InterpResult<'tcx> {
+    fn check_mplace_access(
+        &self,
+        mplace: MPlaceTy<'tcx, M::PointerTag>,
+        msg: CheckInAllocMsg,
+    ) -> InterpResult<'tcx> {
         let (size, align) = self
             .size_and_align_of_mplace(&mplace)?
             .unwrap_or((mplace.layout.size, mplace.layout.align.abi));
         assert!(mplace.mplace.align <= align, "dynamic alignment less strict than static one?");
         let align = M::enforce_alignment(&self.memory.extra).then_some(align);
-        self.memory.check_ptr_access_align(
-            mplace.ptr,
-            size,
-            align.unwrap_or(Align::ONE),
-            CheckInAllocMsg::MemoryAccessTest, // FIXME sth more specific?
-        )?;
+        self.memory.check_ptr_access_align(mplace.ptr, size, align.unwrap_or(Align::ONE), msg)?;
         Ok(())
     }
 

src/test/ui/consts/const-eval/ub-nonnull.32bit.stderr

@@ -13,7 +13,7 @@ error[E0080]: evaluation of constant value failed
   --> $DIR/ub-nonnull.rs:19:30
    |
 LL |     let out_of_bounds_ptr = &ptr[255];
-   |                              ^^^^^^^^ memory access failed: pointer must be in-bounds for 256 bytes at offset 0, but alloc11 has size 1
+   |                              ^^^^^^^^ dereferencing pointer failed: pointer must be in-bounds for 256 bytes at offset 0, but alloc11 has size 1
 
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-nonnull.rs:23:1

src/test/ui/consts/const-eval/ub-nonnull.64bit.stderr

@@ -13,7 +13,7 @@ error[E0080]: evaluation of constant value failed
   --> $DIR/ub-nonnull.rs:19:30
    |
 LL |     let out_of_bounds_ptr = &ptr[255];
-   |                              ^^^^^^^^ memory access failed: pointer must be in-bounds for 256 bytes at offset 0, but alloc11 has size 1
+   |                              ^^^^^^^^ dereferencing pointer failed: pointer must be in-bounds for 256 bytes at offset 0, but alloc11 has size 1
 
 error[E0080]: it is undefined behavior to use this value
   --> $DIR/ub-nonnull.rs:23:1

src/test/ui/consts/ptr_comparisons.stderr

@@ -16,7 +16,7 @@ error[E0080]: evaluation of constant value failed
   --> $DIR/ptr_comparisons.rs:64:33
    |
 LL |     unsafe { std::ptr::addr_of!((*(FOO as *const usize as *const [u8; 1000]))[999]) };
-   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ memory access failed: pointer must be in-bounds for 1000 bytes at offset 0, but alloc3 has size $WORD
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: pointer must be in-bounds for 1000 bytes at offset 0, but alloc3 has size $WORD
 
 error: any use of this value will cause an error
   --> $DIR/ptr_comparisons.rs:68:27