Handle doc-tests

[CAD97]

Potential (hacky) way to do so:

xd009642/tarpaulin#13 (comment)

I was able to run kcov on doctests in the past. My quick and dirty solution was to use LD_PRELOAD and inject a execvp function.

Here is the code that I used to create a .so file:

#[macro_use]
extern crate redhook;
extern crate libc;

use libc::c_char;
use std::ffi::CString;
use std::slice;
use std::env;

hook! {
    unsafe fn execvp(filename: *const c_char, argv: *const *const c_char) => my_execvp {
        let file = slice::from_raw_parts(filename as *const u8, libc::strlen(filename));
        if let Ok(run) = env::var("TEST_WRAPPER") {
            if let Ok(cmd) = run.split_whitespace()
                                .map(|s| CString::new(s))
                                .collect::<Result<Vec<_>, _>>() {
                if !cmd.is_empty() &&
                    file.starts_with(b"/tmp/rustdoctest") &&
                    file.ends_with(b"rust_out")
                {
                    let mut args: Vec<_> = cmd.iter().map(|s| s.as_ptr()).collect();
                    let mut x = argv;
                    while !(*x).is_null() {
                        args.push(*x);
                        x = x.offset(1);
                    }
                    args.push(*x);
                    real!(execvp)(cmd[0].as_ptr(), args.as_ptr());
                    return;
                }
            }
        }

        real!(execvp)(filename, argv);
    }
}

To run doctests with kcov, execute LD_PRELOAD=path-to-lib.so TEST_WRAPPER=kcov cargo test --doc