rust bindings: keep argv[0] alive across benchmark reporting

[Alearner12]

Rust bindings: keep argv[0] alive across benchmark reporting

Summary

The Rust binding constructs temporary C strings for benchmark::Initialize()
and passes their raw pointers into the C++ API. The C++ benchmark library keeps
argv[0] in BenchmarkReporter::Context::executable_name and reads it later
when console, JSON, or CSV reporters emit benchmark context.

This change makes the Rust C++ bridge keep a persistent copy of argv[0]
before calling benchmark::Initialize(), matching the lifetime expectation of
the C++ reporter context.

Bug

google_benchmark_rs::initialize() currently builds temporary CString
storage:

let mut c_args: Vec<CString> = args.iter()
    .map(|arg| CString::new(arg.as_str()).unwrap())
    .collect();

let mut c_ptrs: Vec<*mut c_char> = c_args.iter_mut()
    .map(|c| c.as_ptr() as *mut c_char)
    .collect();

Those buffers are dropped when initialize() returns. However,
benchmark::Initialize() stores argv[0] as a raw pointer:

BenchmarkReporter::Context::executable_name =
    ((argc != nullptr) && *argc > 0) ? argv[0] : "unknown";

The retained pointer is dereferenced later while printing the benchmark
context. Under ASan/UBSan, a small reproducer following the Rust binding's
lifetime pattern reports:

ERROR: AddressSanitizer: heap-use-after-free
READ of size 2
    #0 __interceptor_strlen
    #1 std::operator<<(std::ostream&, char const*)
    #2 benchmark::BenchmarkReporter::PrintBasicContext
    #3 benchmark::ConsoleReporter::ReportContext
    #4 RunBenchmarks
    #5 benchmark::RunSpecifiedBenchmarks

In an unsanitized run, allocator reuse can cause the reporter to print unrelated
later heap contents as the executable name in the benchmark context.

Fix

Convert the incoming size_t back to char** once in the bridge, copy
argv[0] into static storage, and replace argv[0] with that stable buffer
before delegating to benchmark::Initialize().

All other command-line arguments continue to be parsed and copied by the normal
C++ flag parsing path. The only raw argument pointer retained after
initialization is Context::executable_name.

Verification

Ran the Rust binding tests:

cargo test

Result:

test test_bindings ... ok
test result: ok. 1 passed; 0 failed

Security Report

Related Google VRP / IssueTracker report: 529810915