How do I convert a Rust string to a C string?

Question

I'm using Rust's FFI to call LLVM. Here's a minimal working example, based on the example provided with the library.

extern crate llvm_sys as llvm;

fn main() {
    unsafe {
        let module = llvm::core::LLVMModuleCreateWithName(b"nop\0".as_ptr() as *const _);
        llvm::core::LLVMDumpModule(module);
        llvm::core::LLVMDisposeModule(module);
    }
}

This works, and valgrind reports no errors. However, I want to use Rust strings in my code, so I wrote this:

extern crate llvm_sys as llvm;
use std::ffi::CString;

/// Convert a Rust string to a C char pointer.
fn cstr(s: &str) -> *const i8 {
    let cstring = CString::new(s).unwrap();
    cstring.to_bytes_with_nul().as_ptr() as *const _
}

fn main() {
    unsafe {
        let module = llvm::core::LLVMModuleCreateWithName(cstr("nop"));
        llvm::core::LLVMDumpModule(module);
        llvm::core::LLVMDisposeModule(module);
    }
}

However, valgrind now reports:

==7147== ERROR SUMMARY: 6 errors from 4 contexts (suppressed: 0 from 0)

Is this because cstring is being dropped when cstr returns? Since cstr is safe, why does the compiler not complain about cstr? What's the correct way to pass arbitrary Rust strings to C libraries?