interpret: Fix writing uninit to an allocation

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -892,8 +892,11 @@ impl<'tcx, 'a, Tag: Provenance, Extra> AllocRefMut<'a, 'tcx, Tag, Extra> {
     }
 
     /// Mark the entire referenced range as uninitalized
-    pub fn write_uninit(&mut self) {
-        self.alloc.mark_init(self.range, false);
+    pub fn write_uninit(&mut self) -> InterpResult<'tcx> {
+        Ok(self
+            .alloc
+            .write_uninit(&self.tcx, self.range)
+            .map_err(|e| e.to_interp_error(self.alloc_id))?)
     }
 }
 
@@ -1053,8 +1056,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // This also avoids writing to the target bytes so that the backing allocation is never
             // touched if the bytes stay uninitialized for the whole interpreter execution. On contemporary
             // operating system this can avoid physically allocating the page.
-            dest_alloc.mark_init(dest_range, false); // `Size` multiplication
-            dest_alloc.mark_relocation_range(relocations);
+            dest_alloc
+                .write_uninit(&tcx, dest_range)
+                .map_err(|e| e.to_interp_error(dest_alloc_id))?;
+            // We can forget about the relocations, this is all not initialized anyway.
             return Ok(());
         }
 

compiler/rustc_const_eval/src/interpret/place.rs

@@ -823,7 +823,7 @@ where
             // Zero-sized access
             return Ok(());
         };
-        alloc.write_uninit();
+        alloc.write_uninit()?;
         Ok(())
     }
 

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

@@ -269,7 +269,7 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
     /// `get_bytes_with_uninit_and_ptr` instead,
     ///
     /// This function also guarantees that the resulting pointer will remain stable
-    /// even when new allocations are pushed to the `HashMap`. `copy_repeatedly` relies
+    /// even when new allocations are pushed to the `HashMap`. `mem_copy_repeatedly` relies
     /// on that.
     ///
     /// It is the caller's responsibility to check bounds and alignment beforehand.
@@ -429,8 +429,7 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
         let val = match val {
             ScalarMaybeUninit::Scalar(scalar) => scalar,
             ScalarMaybeUninit::Uninit => {
-                self.mark_init(range, false);
-                return Ok(());
+                return self.write_uninit(cx, range);
             }
         };
 
@@ -455,6 +454,13 @@ impl<Tag: Provenance, Extra> Allocation<Tag, Extra> {
 
         Ok(())
     }
+
+    /// Write "uninit" to the given memory range.
+    pub fn write_uninit(&mut self, cx: &impl HasDataLayout, range: AllocRange) -> AllocResult {
+        self.mark_init(range, false);
+        self.clear_relocations(cx, range)?;
+        return Ok(());
+    }
 }
 
 /// Relocations.
@@ -599,6 +605,9 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
     /// Applies a relocation copy.
     /// The affected range, as defined in the parameters to `prepare_relocation_copy` is expected
     /// to be clear of relocations.
+    ///
+    /// This is dangerous to use as it can violate internal `Allocation` invariants!
+    /// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
     pub fn mark_relocation_range(&mut self, relocations: AllocationRelocations<Tag>) {
         self.relocations.0.insert_presorted(relocations.relative_relocations);
     }
@@ -1056,7 +1065,7 @@ impl<Tag: Copy, Extra> Allocation<Tag, Extra> {
         })
     }
 
-    pub fn mark_init(&mut self, range: AllocRange, is_init: bool) {
+    fn mark_init(&mut self, range: AllocRange, is_init: bool) {
         if range.size.bytes() == 0 {
             return;
         }
@@ -1118,6 +1127,9 @@ impl<Tag, Extra> Allocation<Tag, Extra> {
     }
 
     /// Applies multiple instances of the run-length encoding to the initialization mask.
+    ///
+    /// This is dangerous to use as it can violate internal `Allocation` invariants!
+    /// It only exists to support an efficient implementation of `mem_copy_repeatedly`.
     pub fn mark_compressed_init_range(
         &mut self,
         defined: &InitMaskCompressed,