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points by jordan_bonecut 4 days ago | 0 comments
The compiler statically tracks “where” everything lives. It also knows “when” the “where” is valid.

Each “where” is called a plane, which is a tree. The stack plane contains all your function calls memory, a structure allocated on the stack has a child plane for each field. The compiler tags which plane something is from at compile time and tracks when it dies.

Things do get complicated in scenarios like:

fn max(a: &usize, b: &usize) -> &usize {

  if *a > *b { a } else { b }
}

In such a scenario the compiler tags the result with both of the input planes. However, you cannot define add a postcondition ensures clause to make it more precise

[–] tancop 3 days ago
Sounds a lot like group borrowing from Valen, but that one still has annotations like `entity in world.entities[] mut`. Are you tracking the same things implicitly with post conditions?
[–] 0c3ca83 3 days ago
And what about allocations and threads?
[–] jordan_bonecut 3 days ago
Allocations live in their own plane, the root allocated typed being the root node of that plane. For threads, I’m still working on that, don’t have a super cool answer yet
[–] 0c3ca83 3 days ago
how does that prevent use after free? pretty sure your algorithm is either very poorly explained or doesnt' actually work.
[–] jordan_bonecut 3 days ago
Another thing - when a value is moved or dropped the plane’s type changes from T to shape(T). shape(T) contains unspecified data but has the size and alignment of a T. So you couldn’t safely prove that a moved out/dropped shape(T) is a T
[–] 0c3ca83 3 days ago
This only works if you make every allocation its own plane, and scope all borrows to the lifetime of the plane, with a move terminating all borrows, at which point this is equivalent to rust, just with confusing jargon.
[–] jordan_bonecut 3 days ago
Correct, every allocation is its own plane. The nice thing about this model is that you don't need to explicitly call out lifetimes. I also believe that lifetimes aren't really a good model of what actually happens in memory. This model is more powerful because it understands relationships between planes in memory, which Rust's model doesn't. This is why it requires mutable borrows to be unique which is oftentimes overly restrictive. For instance this is not valid Rust:

let x = vec![1, 2, 3]; let a = &mut x[0]; let b = &mut x[1]; swap(a, b);

Because Rust doesn't really know the difference between the whole Vec an element within the vec. Therefore it treats any mutation as one that could potentially invalidate anything with the same lifetime and doesn't compile. So I believe the plane model to be strictly better because it allows for this and still prevents the real scary thing:

let x = vec![1, 2]; let a = &mut x[0]; x.push(3); a = a + 1;

[–] 0c3ca83 3 days ago
How about `x[i] = 7`? What if x is a vector of values that contain allocations?
[–] jordan_bonecut 3 days ago
The compiler tracks all of the currently living planes. The compiler tracks from which plane each borrow is borrowed. Once the plane it is borrowed from dies that borrow is no longer valid
[–] 0c3ca83 3 days ago
If you can replace elements in a vector you can make elements unreachable.