Treat local mutable vars as capabilities #24815
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Move operations a bit higher up the callchain and allow for more modifications what is allowed.
When we have
var x: A^{c} = ...
where `x` is a local variable then when dereferencing `x` we also need to charge `c`.
This was forgotten before. For fields it's not a problem since `c` would already have been
charged for the prefix `p` in `p.x`.
An alternative would be to consider mutable variables to have boxed types. Then the same
capture set would be charged on unboxing.
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bracevac
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Dec 22, 2025
| var r: Int = 0 | ||
| (() => r, (x: Int) => r = x) | ||
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| class Struct: |
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The asymmetry between pairs and such a custom class is jarring.
I'm wondering if we could have a less boilerplate-y way of achieving the Struct case with standard pairs? Could we have a scoped delimiter that
says "We allow construction of data with shared components here"?
def foo() =
val r = Ref()
shared:
(() => r.get, (x: Int) => r.set(x)) // ok, nowMaking sure that the cap of the pair object subsumes the component caps?
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The problem is that the pair object is pure, so no cap to hang this on.
Co-authored-by: Oliver Bračevac <bracevac@users.noreply.github.com>
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Under separation checking: A mutable var owned by a term that is not annotated with @untrackedCaptures gives rise to a Mutable capability. Since a mutable variable is not trackable, we do this by adding a varMirror symbol to the variable which represents the capability.