fix(rt): handle SingletonArray+Field(0)+ConstantIndex(0) in traversal

kmir/src/kmir/kdist/mir-semantics/rt/data.md

@@ -452,6 +452,17 @@ This is done without consideration of the validity of the Downcast[^downcast].
 [^downcast]: See discussion in https://github.com/rust-lang/rust/issues/93688#issuecomment-1032929496.
 
 ```k
+  // fast elimination: SingletonArray + Field(0) + ConstantIndex(0) on a single-element Aggregate
+  rule <k> #traverseProjection(
+             DEST,
+             Aggregate(IDX, ARGS),
+             projectionElemSingletonArray projectionElemField(fieldIdx(0), TY) projectionElemConstantIndex(0, 0, false) PROJS,
+             CTXTS
+           )
+        => #traverseProjection(DEST, Aggregate(IDX, ARGS), projectionElemField(fieldIdx(0), TY) PROJS, CTXTS) ... </k>
+    requires size(ARGS) ==Int 1
+    [preserves-definedness]
+
   rule <k> #traverseProjection(
              DEST,
              Aggregate(IDX, ARGS),

kmir/src/tests/integration/data/prove-rs/write_volatile_pubkey_array.rs

@@ -0,0 +1,37 @@
+#![allow(invalid_reference_casting)]
+
+fn main() {
+    let _keep: fn(&AccountInfo<'_>, Pubkey) = repro;
+}
+
+#[inline(never)]
+#[no_mangle]
+pub fn repro(account: &AccountInfo<'_>, replacement: Pubkey) {
+    account.assign(replacement);
+    assert_eq!(*account.owner, replacement);
+}
+
+struct AccountInfo<'a> {
+    owner: &'a Pubkey,
+}
+
+impl<'a> AccountInfo<'a> {
+    #[inline(never)]
+    fn assign(&self, new_owner: Pubkey) {
+        unsafe {
+            std::ptr::write_volatile(
+                self.owner as *const Pubkey as *mut [u8; 32],
+                new_owner.to_bytes(),
+            );
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+struct Pubkey([u8; 32]);
+
+impl Pubkey {
+    fn to_bytes(self) -> [u8; 32] {
+        self.0
+    }
+}

kmir/src/tests/integration/test_integration.py

@@ -40,6 +40,7 @@
     'test_offset_from-fail': ['testing'],
     'iter-eq-copied-take-dereftruncate': ['repro'],
     'spl-multisig-iter-eq-copied-next': ['repro'],
+    'write_volatile_pubkey_array': ['repro'],
 }
 PROVE_SHOW_SPECS = [
     'local-raw-fail',