Rollup merge of #128277 - RalfJung:offset_from_wildcard, r=oli-obk

miri: fix offset_from behavior on wildcard pointers

offset_from wouldn't behave correctly when the "end" pointer was a wildcard pointer (result of an int2ptr cast) just at the end of the allocation. Fix that by expressing the "same allocation" check in terms of two `check_ptr_access_signed` instead of something specific to offset_from, which is both more canonical and works better with wildcard pointers.

The second commit just improves diagnostics: I wanted the "pointer is dangling (has no provenance)" message to say how many bytes of memory it expected to see (since if it were 0 bytes, this would actually be legal, so it's good to tell the user that it's not 0 bytes). And then I was annoying that the error looks so different for when you deref a dangling pointer vs an out-of-bounds pointer so I made them more similar.

Fixes https://github.com/rust-lang/miri/issues/3767

2 files changed, 14 insertions, 6 deletions

diff --git a/compiler/rustc_middle/src/mir/interpret/error.rs b/compiler/rustc_middle/src/mir/interpret/error.rs
index 376ad19a749..d2d91333ffe 100644
--- a/compiler/rustc_middle/src/mir/interpret/error.rs
+++ b/compiler/rustc_middle/src/mir/interpret/error.rs
@@ -329,16 +329,21 @@ pub enum UndefinedBehaviorInfo<'tcx> {
     /// Using a pointer after it got freed.
     PointerUseAfterFree(AllocId, CheckInAllocMsg),
     /// Used a pointer outside the bounds it is valid for.
-    /// (If `ptr_size > 0`, determines the size of the memory range that was expected to be in-bounds.)
     PointerOutOfBounds {
         alloc_id: AllocId,
         alloc_size: Size,
         ptr_offset: i64,
-        ptr_size: Size,
+        /// The size of the memory range that was expected to be in-bounds.
+        inbounds_size: Size,
         msg: CheckInAllocMsg,
     },
     /// Using an integer as a pointer in the wrong way.
-    DanglingIntPointer(u64, CheckInAllocMsg),
+    DanglingIntPointer {
+        addr: u64,
+        /// The size of the memory range that was expected to be in-bounds (or 0 if we don't know).
+        inbounds_size: Size,
+        msg: CheckInAllocMsg,
+    },
     /// Used a pointer with bad alignment.
     AlignmentCheckFailed(Misalignment, CheckAlignMsg),
     /// Writing to read-only memory.
diff --git a/compiler/rustc_middle/src/mir/interpret/pointer.rs b/compiler/rustc_middle/src/mir/interpret/pointer.rs
index faacc245787..42f30c14cea 100644
--- a/compiler/rustc_middle/src/mir/interpret/pointer.rs
+++ b/compiler/rustc_middle/src/mir/interpret/pointer.rs
@@ -181,9 +181,12 @@ impl Provenance for CtfeProvenance {
     fn fmt(ptr: &Pointer<Self>, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         // Print AllocId.
         fmt::Debug::fmt(&ptr.provenance.alloc_id(), f)?; // propagates `alternate` flag
-        // Print offset only if it is non-zero.
-        if ptr.offset.bytes() > 0 {
-            write!(f, "+{:#x}", ptr.offset.bytes())?;
+        // Print offset only if it is non-zero. Print it signed.
+        let signed_offset = ptr.offset.bytes() as i64;
+        if signed_offset > 0 {
+            write!(f, "+{:#x}", signed_offset)?;
+        } else if signed_offset < 0 {
+            write!(f, "-{:#x}", signed_offset.unsigned_abs())?;
         }
         // Print immutable status.
         if ptr.provenance.immutable() {