Expect an array when expected and acutal types are both arrays during cast

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

compiler/rustc_hir_typeck/src/cast.rs

@@ -1062,8 +1062,22 @@ impl<'a, 'tcx> CastCheck<'tcx> {
                     )
                     });
 
+                let erased_ety = fcx.tcx.erase_regions(*ety);
+                let erased_m_cast = fcx.tcx.erase_regions(m_cast.ty);
+
+                let expected = if matches!(m_cast.ty.kind(), ty::Array(_, _))
+                    && !erased_ety.eq(&erased_m_cast)
+                {
+                    // if the expected type is [ty1; N] and the actual type is [ty2; M], and ty is not [ty2; M]
+                    // then we need to use [ty1; N] as the expected type
+                    m_expr.ty
+                } else {
+                    // otherwise, use the expected type as the expected type
+                    *ety
+                };
+
                 // this will report a type mismatch if needed
-                fcx.demand_eqtype(self.span, *ety, m_cast.ty);
+                fcx.demand_eqtype(self.span, expected, m_cast.ty);
                 return Ok(CastKind::ArrayPtrCast);
             }
         }