offset, offset_from: allow zero-byte offset on arbitrary pointers

Author: RalfJung

tests/fail/dangling_pointers/dangling_zst_deref.rs

@@ -1,13 +1,13 @@
 error: Undefined Behavior: memory access failed: ALLOC has size 0, so pointer to 1 byte starting at offset 0 is out-of-bounds
-  --> $DIR/zst1.rs:LL:CC
+  --> $DIR/zst_local_oob.rs:LL:CC
    |
 LL |     let _val = unsafe { *x };
    |                         ^^ memory access failed: ALLOC has size 0, so pointer to 1 byte starting at offset 0 is out-of-bounds
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: BACKTRACE:
-   = note: inside `main` at $DIR/zst1.rs:LL:CC
+   = note: inside `main` at $DIR/zst_local_oob.rs:LL:CC
 
 note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
 

tests/fail/dangling_pointers/dangling_zst_deref.stderr

@@ -118,10 +118,9 @@ fn vtable() {
     let parts: (*const (), *const u8) = unsafe { mem::transmute(ptr) };
     let vtable = parts.1;
     let offset = vtable.align_offset(mem::align_of::<TWOPTR>());
-    let _vtable_aligned = vtable.wrapping_add(offset) as *const [TWOPTR; 0];
-    // FIXME: we can't actually do the access since vtable pointers act like zero-sized allocations.
-    // Enable the next line once https://github.com/rust-lang/rust/issues/117945 is implemented.
-    //let _place = unsafe { &*vtable_aligned };
+    let vtable_aligned = vtable.wrapping_add(offset) as *const [TWOPTR; 0];
+    // Zero-sized deref, so no in-bounds requirement.
+    let _place = unsafe { &*vtable_aligned };
 }
 
 fn main() {