less unsupported errors in Miri, and clarifying comments

Author: RalfJung

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

@@ -16,7 +16,8 @@ pub struct ProvenanceMap<Prov = AllocId> {
     /// bytes. Two entires in this map are always at least a pointer size apart.
     ptrs: SortedMap<Size, Prov>,
     /// Provenance in this map only applies to the given single byte.
-    /// This map is disjoint from the previous.
+    /// This map is disjoint from the previous. It will always be empty when
+    /// `Prov::OFFSET_IS_ADDR` is false.
     bytes: SortedMap<Size, Prov>,
 }
 
@@ -39,7 +40,7 @@ impl ProvenanceMap {
     /// Only exposed with `AllocId` provenance, since it panics if there is bytewise provenance.
     #[inline]
     pub fn ptrs(&self) -> &SortedMap<Size, AllocId> {
-        debug_assert!(self.bytes.is_empty());
+        debug_assert!(self.bytes.is_empty()); // `AllocId::OFFSET_IS_ADDR` is false so this cannot fail
         &self.ptrs
     }
 }

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

@@ -401,16 +401,15 @@ impl fmt::Display for UndefinedBehaviorInfo {
 pub enum UnsupportedOpInfo {
     /// Free-form case. Only for errors that are never caught!
     Unsupported(String),
-    /// Overwriting parts of a pointer; the resulting state cannot be represented in our
-    /// `Allocation` data structure. See <https://github.com/rust-lang/miri/issues/2181>.
-    PartialPointerOverwrite(Pointer<AllocId>),
-    /// Attempting to `copy` parts of a pointer to somewhere else; the resulting state cannot be
-    /// represented in our `Allocation` data structure. See
-    /// <https://github.com/rust-lang/miri/issues/2181>.
-    PartialPointerCopy(Pointer<AllocId>),
     //
     // The variants below are only reachable from CTFE/const prop, miri will never emit them.
     //
+    /// Overwriting parts of a pointer; without knowing absolute addresses, the resulting state
+    /// cannot be represented by the CTFE interpreter.
+    PartialPointerOverwrite(Pointer<AllocId>),
+    /// Attempting to `copy` parts of a pointer to somewhere else; without knowing absolute
+    /// addresses, the resulting state cannot be represented by the CTFE interpreter.
+    PartialPointerCopy(Pointer<AllocId>),
     /// Encountered a pointer where we needed raw bytes.
     ReadPointerAsBytes,
     /// Accessing thread local statics

src/tools/miri/src/diagnostics.rs

@@ -229,13 +229,13 @@ pub fn report_error<'tcx, 'mir>(
                 Unsupported(
                     UnsupportedOpInfo::ThreadLocalStatic(_) |
                     UnsupportedOpInfo::ReadExternStatic(_) |
-                    UnsupportedOpInfo::PartialPointerOverwrite(_) | // we make memory uninit instead
+                    UnsupportedOpInfo::PartialPointerOverwrite(_) |
+                    UnsupportedOpInfo::PartialPointerCopy(_) |
                     UnsupportedOpInfo::ReadPointerAsBytes
                 ) =>
                     panic!("Error should never be raised by Miri: {kind:?}", kind = e.kind()),
                 Unsupported(
-                    UnsupportedOpInfo::Unsupported(_) |
-                    UnsupportedOpInfo::PartialPointerCopy(_)
+                    UnsupportedOpInfo::Unsupported(_)
                 ) =>
                     vec![(None, format!("this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support"))],
                 UndefinedBehavior(UndefinedBehaviorInfo::AlignmentCheckFailed { .. })