Upgrade Rust toolchain to 2025-06-27 (#4182)

Relevant upstream PRs:
- rust-lang/rust#141061 (Change
  __rust_no_alloc_shim_is_unstable to be a function)
- rust-lang/rust#142650 (Refactor Translator)

Resolves: #4176

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

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(),
         ])
     };
 }

kani-compiler/src/session.rs

@@ -4,9 +4,10 @@
 //! Module used to configure a compiler session.
 
 use crate::args::Arguments;
+use rustc_driver::default_translator;
 use rustc_errors::{
-    ColorConfig, DiagInner, emitter::Emitter, emitter::HumanReadableErrorType,
-    fallback_fluent_bundle, json::JsonEmitter, registry::Registry as ErrorRegistry,
+    ColorConfig, DiagInner, emitter::Emitter, emitter::HumanReadableErrorType, json::JsonEmitter,
+    registry::Registry as ErrorRegistry,
 };
 use rustc_session::EarlyDiagCtxt;
 use rustc_session::config::ErrorOutputType;
@@ -52,13 +53,11 @@ static JSON_PANIC_HOOK: LazyLock<Box<dyn Fn(&panic::PanicHookInfo<'_>) + Sync +
         panic::set_hook(Box::new(|info| {
             // Print stack trace.
             let msg = format!("Kani unexpectedly panicked at {info}.",);
-            let fallback_bundle =
-                fallback_fluent_bundle(rustc_driver::DEFAULT_LOCALE_RESOURCES.to_vec(), false);
             let mut emitter = JsonEmitter::new(
                 Box::new(io::BufWriter::new(io::stderr())),
                 #[allow(clippy::arc_with_non_send_sync)]
                 Some(Arc::new(SourceMap::new(FilePathMapping::empty()))),
-                fallback_bundle,
+                default_translator(),
                 false,
                 HumanReadableErrorType::Default,
                 ColorConfig::Never,

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-2025-06-27"
 components = ["llvm-tools", "rustc-dev", "rust-src", "rustfmt"]