How to printf-debug alloc/core

[the8472]

credit for the hack goes to @nnethercote

[nnethercote]

Credit also to @bjorn3 who gave me the idea and an earlier version of the code.

[nnethercote]

This is very useful info and I have been using this hack repeatedly since I learned about it.

I have a slightly modified version I've been using:

extern "C" { fn dprintf(fd: i32, s: *const u8, ...); }
macro_rules! d { ($s:expr) => { unsafe { dprintf(2, $s.as_ptr() as *const u8); } } }

fn function_to_debug() {
    // Note: trailing '\0' is critical!
    d!("hello world\n\0");
    d!(format!("debug: {}\n\0", "hello world"));
}

It's more concise, easier to copy and paste, and easier to comment out.

[nnethercote]

Another comment: if you use this in code that format! relies on you'll get infinite loops, which can be confusing. E.g. I had this problem when I had a d! call in the RawVec growth code path.