Feature request: controlling tracking from inside the program

[CAD97]

Specifically,

#[cfg(miri)]
extern "Rust" {
    /// Start tracking the pointee alloc id as if -Zmiri-track-alloc-id was used
    fn miri_start_tracking_alloc(ptr: *mut ());
    /// Stop tracking the pointee alloc id
    fn miri_stop_tracking_alloc(ptr: *mut ());
    /// Print the alloc id as if tracking was triggered
    fn miri_track_alloc(ptr: *mut ());
    
    /// Start tracking the pointer tag as if -Zmiri-track-pointer-tag was used
    fn miri_start_tracking_tag(ptr: *mut ());
    /// Stop tracking the pointer tag
    fn miri_stop_tracking_tag(ptr: *mut ());
    /// Print the pointer tag as if tracking was triggered
    fn miri_track_tag(ptr: *mut ());
}

Names are bikeshedable of course.

The primary advantage of this is being able to track behavior (so long as source edits are workable) without "guessing" the correct alloc/tag id.

[RalfJung]

I agree something like this makes sense. However I think the details are not entirely obvious.

• Part of the point of tracking an alloc ID / ptr tag is to figure out when it got created. I that sense, these runtime APIs can never be the same kind of tracking as what the flags provide. miri_start_tracking_alloc would literally just be miri_tell_me_when_this_gets_deallocated.
• "Print the alloc id as if tracking was triggered" sounds like you want a miri_get_alloc_id(ptr: *mut ()) -> u64? This is certainly not "tracking" anything in any sense of the word "tracking" that I am aware of. ;)
• For pointer tags, the suggested types would be it impossible or at least very hard to track a reference. Casting it to a raw ptr will change its tag! So I think we need to make these functions take a ptr: T with a comment saying ptr must be a reference or raw pointer. But even then you won't be able to do this on a mutable reference since you would lose the reference (or reborrow it, which changes the tag). So maybe ref_to_ptr: &T, and the relevant tag is that of the pointer that the reference points to?