build.rs

extern crate gcc;

fn main() {
    gcc::compile_library("libtest.a", &["test.c"]);
}

Cargo.toml

[package]
name = "segv"
version = "0.1.0"
build = "build.rs"

[[bin]]
name = "segv"
path = "main.rs"

[build-dependencies]
gcc = "0.3"

main.rs

extern {
    fn test(str: *const u8, len: usize) -> *mut u8;
}

fn main() {
    let s = "hello";
    unsafe {
        test(s.as_ptr(), s.len());
    }
}

test.c

#include <sys/types.h>
#include <string.h>
#include <stdlib.h>

char *test(const char *str, size_t len)
{
    char *new_str;

    new_str = malloc(len + 1);
    if (!new_str)
        return NULL;

    memcpy(new_str, str, len);
    new_str[len] = '\0';
    return new_str;
}