borrow-checker allows partial reinit of struct that has been moved away, but no use of it.

[pnkfelix]

Here is some sample code:

#[derive(Show)]
struct Pair { lft: u8, rgt: u8, }

fn main() {
    let mut p = Pair { lft: 1, rgt: 2 };
    let mut f = move |&mut:| { p.lft = 3; p.rgt };
    p.rgt = 4;
    println!("f: {:?}", f());
    p.lft = 5;
    // println!("p: {:?}", p);
}

This compiles without an error; running it prints f: 2u8.

If you uncomment the last line, you get an error saying "error: use of moved value: p". Likewise if you just attempt to print individual fields.

While I do see the logic being used here ("we are not overwriting the value within the closure itself; we are writing into the uninitialized memory that resulted from moving p into the closure"), it seems potentially confusing.

Is there any reason that we allowing these writes, which cannot be subsequently read? It seems like a situation analogous to moving into a moved-away array (see rust-lang/rfcs#533 )

[pnkfelix]

(however, unlike rust-lang/rfcs#533, it would not be all that terrible for non-zeroing dynamic-drop if we did not get around to making this illegal; we're already going to have to support representing structure fragments to implement non-zeroing dynamic drop, and initializations like the one in this ticket are just another case of that. Plus we might in the future add the ability to track such partial initializations and allow reads from initialized fields in partially-initialized structs.)

[mbrubeck]

If this doesn't become an error, it should at least be a warning. Code like this is almost certainly a logic error of some kind, but currently compiles without error or warning:

struct Foo { x: i32 }
let mut f = Foo { x: 0 };
drop(f);
f.x = 1;

[pnkfelix]

triage: P-backcompat-lang, 1.0 beta

(but do see comments above noting that its not the end of the world if we did not fix this.)

[pnkfelix]

triage: P-backcompat-lang (1.0 beta)

just trying to accommodate highfive...

[nikomatsakis]

Interesting, I see the bug being that you get an error at all -- that is, I think that once you have fully "reinitialized" the structure, you should be able to read it again. (Rather than the ability to partially reinit the struct as being an error.)

[nikomatsakis]

I'm going to tag this as I-Needs-Decision.

[mbrubeck]

Note that this is currently an error if the moved value has a destructor. For example, if you add impl Drop for Foo {fn drop(&mut self) {}} to the previous example:

struct Foo { x: i32 }
impl Drop for Foo { fn drop(&mut self) {} }
let mut f = Foo { x: 0 };
drop(f);
f.x = 1;

then it is rejected with "error: partial reinitialization of uninitialized structure f."

[pnkfelix]

at this point I think I would be in favor saying that we can live with the current semantics as is today, and fix things in the future backwards-compatibly to either 1. track the initialized parts and allow reading from them, as I mentioned at the end of my comment, or 2. the slightly more limited (but perhaps more in line with our current compiler infrastructure) approach of tracking when the entire (non-Drop) structure is reinitialized and then allow reads from it, as outlined in niko's comment.

So, bascially, I'm willing to reclassify this as a P-low, not 1.0 bug.