Upgrade Rust toolchain to 2025-06-20

Relevant upstream PR: rust-lang/rust#141061
(Change __rust_no_alloc_shim_is_unstable to be a function)

Resolves: model-checking#4176

Author: tautschnig

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

@@ -35,6 +35,7 @@ lazy_static! {
             "__rust_alloc_zeroed".into(),
             "__rust_dealloc".into(),
             "__rust_realloc".into(),
+            "__rust_no_alloc_shim_is_unstable_v2".into(),
         ])
     };
 }

library/kani/kani_lib.c

@@ -117,3 +117,9 @@ uint8_t *__rust_realloc(uint8_t *ptr, size_t old_size, size_t align, size_t new_
 
     return result;
 }
+
+// Function required by the linker, see https://github.com/rust-lang/rust/pull/141061
+struct Unit __rust_no_alloc_shim_is_unstable_v2(void)
+{
+    return VoidUnit;
+}

rust-toolchain.toml

@@ -2,5 +2,5 @@
 # SPDX-License-Identifier: Apache-2.0 OR MIT
 
 [toolchain]
-channel = "nightly-2025-06-18"
+channel = "nightly-2025-06-20"
 components = ["llvm-tools", "rustc-dev", "rust-src", "rustfmt"]