[jandrewrogers]

> starting playing around with std::launder and std::byte and strict aliasing rules and lifetime rules, and you'll yearn for the simplicity of Rust

Annotations like std::launder, lifetime manipulation, etc solve a class of problems that exist in every systems language. They inform the compiler of properties that cannot be known by analyzing the code. Rust isn't special in this regard, it has the same issues.

Without these features, we either relied on unofficial compiler-specific behavior or used unnecessarily conservative code that was safe but slower.

[tialaramex]

> Rust isn't special in this regard, it has the same issues.

This is both fundamentally true and misleading. Rust has to solve the same issues but isn't obliged to make all the same bad choices to do that and so the results are much better.

For example C++ dare not perform compile time transmutations so, it just forbids them and a whole bunch of extra stuff landed to work around that, but in Rust they're actually fine and so you can just:

    const FOO: bool = unsafe { core::mem::transmute::<i8, bool>(2) };

That blows up at compile time because we claimed the bit pattern for the integer 2 is a valid boolean and it isn't. If we choose instead 0 (or 1) this works and we get the expected false (or true) boolean instead of a compiler diagnostic.

C++ could allow this but it doesn't, rather than figure out all the tricky edge cases they just said no, use this other new thing we made.

[jandrewrogers]

> For example C++ dare not perform compile time transmutations

I am confused by this assertion. You can abuse the hell out of transformations in a constexpr context. The gap between what is possible at compile-time and run-time became vanishingly small a while ago.

I think your example is not illustrative in any case. Many C++ code bases work exactly like your example, enforced at compile-time. That this can be an issue is a hangover from retaining compatibility with C-style code which conflates comparison operators and cast operators. It is a choice.

C++ can enforce many type constraints beyond this at compile-time that Rust cannot, with zero effort or explicit type creation. No one should be passing ints around.

[gpderetta]

First of all mem::transmute is like bit_cast (which works perfectly fine in constexpr context), not reinterpret cast.

Second, this compiles just fine:

   constexpr int ivalue = 1;
   constexpr bool bvalue {ivalue};

This fails at compile time (invalid narrowing):

    constexpr int ivalue = 2;
    constexpr bool bvalue {ivalue};

Note we don't need bit_cast for this example as int to bool conversions are allowed in C++.

[tialaramex]

Surely "We have many different ways to do this, each with different rules" is exactly the point? C++ 20's std::bit_cast isn't necessarily constexpr by the way although it is for the trivial byte <-> boolean transmutation I mentioned here.

I see that C++ people were more comfortable with the "We have far too many ways to initialize things" examples of this problem but I think transmutation hits harder precisely because it sneaks up on you.

[gpderetta]

bit_cast and reinterpret_cast do different things: one works at the value level, the second preserves address identity (and it is problematic from an aliasing point of view).

Not sure what any of this has to do with initialization though.

FWIW, the direct translation of your rust code is:

    constexpr char y = 2;
    constexpr bool x = std::bit_cast<bool>(y);

It fails on clang for y=2 and works for y=1, exactly like rust;

GCC produces UB for y=2, I don't know if it is a GCC bug or the standard actually allows this form of UB to be ignored at contexpr time.

What is the rust equivalent of reinterpret_cast and does it work at constexpr time?

edit: I guess it would be an unsafe dereference of a casted pointer. Does it propagate constants?

[tialaramex]

Firstly, that's not a direct translation because you're making two variables and I made none at all. Rust's const is an actual constant, it's not an immutable variable. We have both, but they're different. The analogous Rust for your bit cast example would make two immutable variables that we promise have constant values, maybe:

    static y: u8 = 2;
    static x: bool = unsafe { core::mem::transmute(y) };

Of course this also won't compile, because the representation for 2 still isn't a boolean. If it did compile you'd also (by default) get angry warnings because it's bad style to give these lowercase names.

I also don't know if you found a GCC bug but it seems likely from your description. I can't see a way to have UB, a runtime phenomenon, at compile time in C++ as the committee imagines their language. Of course "UB? In my lexer?" is an example of how the ISO document doesn't understand intention, but I'd be surprised if the committee would resolve a DR with "That's fine, UB at compile time is intentional".

I understand that "these are different things" followed by bafflegab is how C++ gets here but the whole point of this sub-thread is that Rust didn't do that, so in Rust these aren't "different things". They're both transmutation, they don't emit CPU instructions because they happen in the type system and the type system evaporates at runtime.

So this is an impedance mismatch, you've got Roman numerals and you can't see why metric units are a good idea, and I've got the positional notation and so it's obvious to me. I am not going to be able to explain why this is a good idea in your notation, the brilliance vanishes during translation.

[gpderetta]

I'm using two variables because numeric literals have the wrong type and bit_cast rejects transmutations between differently sized types.

I could have written it as x = bit_cast<bool>(char{2}), but does it really make a difference?

I don't know enough rust to know what's the difference between its const and c++ constexpr. It might not be a meaningful difference in C++.

> So this is an impedance mismatch, you've got Roman numerals and you can't see why metric units are a good idea, and I've got the positional notation and so it's obvious to me. I am not going to be able to explain why this is a good idea in your notation, the brilliance vanishes during translation.

There are plenty of rust users on HN that are capable of kind, constructive, and technically interesting conversations. Unfortunately there are a small few that will destroy any goodwill the rest of the community works hard to generate.

[germandiago]

In my experience conversions is one of the things that maximum warning levels do excellent static analysis for nowadays. In the last 15 years I hardly had a couole problems (init vs paren initialization). All narrowing etc. is caught out of the box with warnings.

[poppadom1982]

I'm not sure what you're getting at but

const bool z = (const bool)((int8_t)2);

Is perfectly valid C++.

[debugnik]

That's a conversion, not the same. The naive equivalent to transmute would be

    int8_t x = 2;
    bool y = *reinterpret_cast<bool *>(&x);

But reinterpret_cast isn't valid in a constexpr scope.

[poppadom1982]

My point is, in your exact example both reinterpret_cast and C-style casts have the exact same behavior, making the example bad. If you want to showcase a deficiency of C++, it would make sense to pick something where the difference between cast types actually matters.

[TuxSH]

> But reinterpret_cast isn't valid in a constexpr scope.

std::bit_cast is

[debugnik]

Oh cool, and it behaves like memcpy, not like pointer aliasing! I'm stuck with C++14 at work so I missed that one.