> I'm not a compiler theory expert but my hunch is that ExpiredLink's premise is wrong. I'm guessing that any non-trivial program requires a priori knowledge from the programmer's mind to properly inform the compiler which variables do what and when.
I also think the thesis is wrong, but i'd put it differently. I suspect that in many cases, a sufficiently smart compiler could analyse a program and assign plausible classifications to every variable. However, i suspect that there will be some cases which are not - there is no set of classifications which can be assigned which will conform to the rules. Inevitably, these would be exactly the cases you actually hit when writing real programs. In other words, i think this ends up looking a bit like the halting problem.
As a thought experiment, imagine Rust, but without any of the memory management classifications on variables: no tildes, ampersands, or single quotes. You could compile and execute this with fairly conventional garbage-collected memory management semantics. But you could also try to assign memory management classifications to every variable, and the compile it as Rust. The number of possible assignments over the whole program is clearly finite - there are a finite number of variables, a finite number of pointer types, a finite number of lifetimes, and so a finite number of arrangements of all those. Large, but finite. So you could, in principle, enumerate them by brute force.
If you took an existing, correct, Rust program, erased all the classifications, and fed it into this brute force re-classifying compiler, it would eventually find a legal classification, and compile it. It might be the original classification, or it might not. This means that there isn't a requirement for "a priori knowledge from the programmer's mind". Any program which could be given a plausible classification can be given one automatically.
However, i think there is no such certainty that it would find a legal classification for a arbitrary program. Indeed, i suspect it wouldn't be too hard to find a counterexample: you just need a Rust program which doesn't compile, and can't be fixed purely by changing memory management classifications. Anyone want to give that a go?
I also think the thesis is wrong, but i'd put it differently. I suspect that in many cases, a sufficiently smart compiler could analyse a program and assign plausible classifications to every variable. However, i suspect that there will be some cases which are not - there is no set of classifications which can be assigned which will conform to the rules. Inevitably, these would be exactly the cases you actually hit when writing real programs. In other words, i think this ends up looking a bit like the halting problem.
As a thought experiment, imagine Rust, but without any of the memory management classifications on variables: no tildes, ampersands, or single quotes. You could compile and execute this with fairly conventional garbage-collected memory management semantics. But you could also try to assign memory management classifications to every variable, and the compile it as Rust. The number of possible assignments over the whole program is clearly finite - there are a finite number of variables, a finite number of pointer types, a finite number of lifetimes, and so a finite number of arrangements of all those. Large, but finite. So you could, in principle, enumerate them by brute force.
If you took an existing, correct, Rust program, erased all the classifications, and fed it into this brute force re-classifying compiler, it would eventually find a legal classification, and compile it. It might be the original classification, or it might not. This means that there isn't a requirement for "a priori knowledge from the programmer's mind". Any program which could be given a plausible classification can be given one automatically.
However, i think there is no such certainty that it would find a legal classification for a arbitrary program. Indeed, i suspect it wouldn't be too hard to find a counterexample: you just need a Rust program which doesn't compile, and can't be fixed purely by changing memory management classifications. Anyone want to give that a go?