Upgrade toolchain to 2/6

- contract changes, c.f. rust-lang/rust@bcb8565

Author: Carolyn Zech

kani-compiler/src/codegen_cprover_gotoc/codegen/rvalue.rs

@@ -851,7 +851,7 @@ impl GotocCtx<'_> {
                             .bytes(),
                         Type::size_t(),
                     ),
-                    NullOp::UbChecks => Expr::c_false(),
+                    NullOp::ContractChecks | NullOp::UbChecks => Expr::c_false(),
                 }
             }
             Rvalue::ShallowInitBox(ref operand, content_ty) => {

kani-compiler/src/kani_middle/transform/internal_mir.rs

@@ -215,6 +215,7 @@ impl RustcInternalMir for NullOp {
                 ),
             ),
             NullOp::UbChecks => rustc_middle::mir::NullOp::UbChecks,
+            NullOp::ContractChecks => rustc_middle::mir::NullOp::ContractChecks,
         }
     }
 }

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2025-02-05"
+channel = "nightly-2025-02-06"
 components = ["llvm-tools", "rustc-dev", "rust-src", "rustfmt"]