如何在Rust中释放通过FFI分配的*字符

我通过Rust的FFI调用LLVM API.LLVMPrintModuleToString uses strdup to create a string. 然而，当我将指针包装在CString中时，如果 rust 掉了它，我会得到一个错误.

#![feature(cstr_memory)]
use std::ffi::CString;

extern crate llvm_sys as llvm;

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

    println!("llvm_ir_cstring: {:?}", llvm_ir_cstring);
}

Valgrind错误:

==10413== Invalid read of size 4
==10413==    at 0x4870586: pthread_mutex_lock (in /usr/lib/libpthread-2.21.so)
==10413==    by 0x17F89B: je_arena_dalloc_small (in /home/wilfred/projects/bfc/target/debug/bfc)
==10413==    by 0x178C30: je_sdallocx (in /home/wilfred/projects/bfc/target/debug/bfc)
==10413==    by 0x10FA57: heap::imp::deallocate::h1fb92d59333c497bkja (heap.rs:268)
==10413==    by 0x10F999: heap::deallocate::h5680e3fedc9e96320da (heap.rs:89)
==10413==    by 0x10F929: heap::exchange_free::h452463f962f7ec23kfa (heap.rs:131)
==10413==    by 0x10F8C5: Box$LT$$u5b$u8$u5d$$GT$::drop.1769::haf7017472635c7cf (in /home/wilfred/projects/bfc/target/debug/bfc)
==10413==    by 0x10F836: std..ffi..c_str..CString::drop.1766::h04d2b3db8d468f0c (in /home/wilfred/projects/bfc/target/debug/bfc)
==10413==    by 0x10F5FF: main::h04b7feb343e229ccgaa (in /home/wilfred/projects/bfc/target/debug/bfc)
==10413==    by 0x16DBCA: rt::unwind::try::try_fn::h2403085009213705177 (in /home/wilfred/projects/bfc/target/debug/bfc)
==10413==    by 0x16FF5A: rust_try_inner (in /home/wilfred/projects/bfc/target/debug/bfc)
==10413==    by 0x16FF33: rust_try (in /home/wilfred/projects/bfc/target/debug/bfc)
==10413==  Address 0x1d684 is not stack'd, malloc'd or (recently) free'd

这是为什么？从C API处理*char的正确方法是什么？