Lexical lifetimes in Julia (simplified prototype)

lifetimes.jl

abstract type GlobalScope end
abstract type _LifeTime{S<:GlobalScope} end
const LifeTime{S<:GlobalScope} = _LifeTime{>:S}

struct LivesExactly{L<:LifeTime,T}
    val::T
    LivesExactly(lt::Type{L},val::T) where {T,L<:LifeTime} = new{L,T}(val)
end

const LivesAtLeast{L,T} = LivesExactly{<:L,T}


# Example in use:

# Examples of lifetimes generated by scope macro.
abstract type Scope1 <: GlobalScope end
abstract type Scope2 <: Scope1 end
abstract type Scope3 <: GlobalScope end

# Simplified version of what would be generated by the @! function   macro
function foo(context_lifetime::Type{L} ,x::LivesAtLeast{L,Int}) where {L<:LifeTime} 
    x.val 
end

g = LivesExactly(LifeTime{GlobalScope}, 10)
a = LivesExactly(LifeTime{Scope1}, 10)
b = LivesExactly(LifeTime{Scope2}, 10)
c = LivesExactly(LifeTime{Scope3}, 10)

# Example of context-aware function calls with @!
foo(LifeTime{Scope1},a)           # Works (exactly correct lifetime)
foo(LifeTime{Scope1},g)           # Works (g outlives the context of the call)

foo(LifeTime{Scope1},b)          # MethodError (possible use after free on b)
foo(LifeTime{Scope1},c)          # MethodError  (unrelated lifetimes)