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> The underspecified part should not be a deal breaker, even though it's certainly a difficulty. But it will also be an opportunity to spot suboptimal implement

> The underspecified part should not be a deal breaker, even though it's certainly a difficulty. But it will also be an opportunity to spot suboptimal implementation choices in rustc. Exposing these will improve the language as a whole. but... we don't want same sourcecode compile with one compiler implementation, but not the other... right? When I say "under-specified", I meant something like this: https://github.com/rust-lang/rust/pull/105300/ https://github.com/rust-lang/rust/pull/105300/ quote> I claim it's more correct as well because it fixes #104639. There is no spec. Just a bunch of implementation-defined behavior. Without a spec, this would be a forever catching up game. It is not easy to come up with a spec -- the implement is filled with heuristics because the intrinsic NP-completeness.

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tialaramex 4y ago
> we don't want same sourcecode compile with one compiler implementation, but not the other... right? It's not ideal, but I can live with it so long as when I write a program which compiles it has the desired behaviour. That's the problem in C++ and to a lesser extent C which ultimately falls out of this same constraint. In C++ it's easy to write programs which compiler A and compiler B will both compile but they have different results and neither of them is wrong. I have no use for this whatsoever. If some Rust compilers say my program is unacceptable, and others build it correctly, obviously that's not brilliant news but I can choose whether to fix the program to be acceptable to more compilers, or not worry - the results at least are correct.
samus 4y ago
Specifying the heuristics would actually impede future implementations. Even if they would be willing to engage in spending more computational effort on proving that a controversial block of code is type-safe. And it's always possible that a smart PhD student comes up with a better heuristics for certain kinds of programs. Implementations accepting different sets of inputs is certainly annoying, but acceptable as long as they are erring on the side of caution. We definitely don't want code to compile which is actually not type-safe. At a quick glance, the PR can be understood as automating a workaround that programmers had to do by themselves before. > This PR (similarly, and building on top of #104765) allows RPITs and async fns to untangle their lifetime bounds to figure out the set of lifetimes that actually get caputed instead of falling back to a larger one or even 'static. For most cases it was was already possible for users to manually reorder their lifetimes to make their code compile, this PR just makes that automatic, as the order of generic params and bounds should not matter.

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tialaramex 4y ago

> we don't want same sourcecode compile with one compiler implementation, but not the other... right? It's not ideal, but I can live with it so long as when I write a program which compiles it has the desired behaviour. That's the problem in C++ and to a lesser extent C which ultimately falls out of this same constraint. In C++ it's easy to write programs which compiler A and compiler B will both compile but they have different results and neither of them is wrong. I have no use for this whatsoever. If some Rust compilers say my program is unacceptable, and others build it correctly, obviously that's not brilliant news but I can choose whether to fix the program to be acceptable to more compilers, or not worry - the results at least are correct.

samus 4y ago

Specifying the heuristics would actually impede future implementations. Even if they would be willing to engage in spending more computational effort on proving that a controversial block of code is type-safe. And it's always possible that a smart PhD student comes up with a better heuristics for certain kinds of programs. Implementations accepting different sets of inputs is certainly annoying, but acceptable as long as they are erring on the side of caution. We definitely don't want code to compile which is actually not type-safe. At a quick glance, the PR can be understood as automating a workaround that programmers had to do by themselves before. > This PR (similarly, and building on top of #104765) allows RPITs and async fns to untangle their lifetime bounds to figure out the set of lifetimes that actually get caputed instead of falling back to a larger one or even 'static. For most cases it was was already possible for users to manually reorder their lifetimes to make their code compile, this PR just makes that automatic, as the order of generic params and bounds should not matter.