On Borrow Checking

[Walter Bright]

On 5/2/2025 12:44 AM, Manu wrote:
> On a slight tangent; why do I need to attribute the function at all for it to 
> check `scope` args don't escape?

Because otherwise the compiler has to examine the function implementation to 
verify this. Doing such is called attribute inference, and D does quite a bit of 
that. Problems are:

1. Function declarations don't have a body, so there's no way to inspect the body.

2. Inspecting all the bodies means compilation gets slowed down quite a bit.

3. Chicken-and-egg problems when inferring attributes when there are loops in 
the function call flow graph.

> I do want to add `scope` to pointers and ref arguments, but I don't see any 
> reason that I should have to attribute `live`... why isn't `scope` enough to 
> enable escape analysis?

The DFA (Data Flow Analysis) required to determine whether a pointer is the 
"owner" or not at each point in the function is considerably more complex than 
just checking for scope-ness. DFA equations are generated and then solved 
iteratively.

The code required to do it is:

https://github.com/dlang/dmd/blob/master/compiler/src/dmd/ob.d

which is very similar to the DFA performed by the optimizer:

https://github.com/dlang/dmd/blob/master/compiler/src/dmd/backend/gflow.d