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Comment by otabdeveloper4 | Hacker News Reader

> undefined behavior

What does that even mean in the absence of a formal standard and competing compiler implementations? (Hint: nothing.)

If you refuse to define anything then you automatically make the "undefinedness" problem go away. (But not the pain it causes.)

> Rust's type system and safety guarantees put a floor on how crappy the code can be

No. The floor on crappiness is defined by the problem domain. Powerful features allow for powerful takes on crappiness.

Unless you want Rust to stay a teaching language forever, you must necessarily introduce abusable features. (See Python for a real-time slow-motion elaboration of this train wreck if you don't believe me.)

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> What does that even mean in the absence of a formal standard and competing compiler implementations? (Hint: nothing.)

Undefined behaviour is stuff that you compiler's optimizers have been promised can never occur, so they are allowed to transform your code based on that assumption.

Here's a post I made on Reddit in 2019 with a list of resources on what undefined behaviour is:

https://www.reddit.com/r/rust/comments/dpxswt/rust_2020_lets...

...and here's the link to an example of what invoking undefined behaviour can do in C++:

https://gcc.godbolt.org/z/0ubnbS

...and here's an explanation of why it does what it does:

https://kristerw.blogspot.com/2017/09/why-undefined-behavior...

(TL;DR: It injects a call to EraseAll because calling Do while it's still null is undefined behaviour, and Do is a static, so the optimizer determines that the only possible answer within the rules it was given is that code outside that compilation unit will have called NeverCalled to set Do = EraseAll before invoking main().)

> What does that even mean in the absence of a formal standard and competing compiler implementations?

It means that when the language designers are asked "what is the behavior of this code (that compiles and doesn't use 'unsafe')?" they never throw up their hands and say "it's undefined behaviour, you must not write that code" (as the C++ definition often says).

They may say "oops, we're not sure what it should do, we need to clarify the language definition and write some tests to ensure the compiler does that". (This happens in C++ too.)

> Powerful features allow for powerful takes on crappiness.

I don't know what this means.

> Unless you want Rust to stay a teaching language forever

A lot of companies big and small are using Rust in production so this is not a compelling premise.

"Undefined behavior" in a C++ context is a legalese feature of the ISO standard. The standard defines, very precisely, the behavior of a compliant C++ compiler. In places where behavior cannot be specified (for logical or practical reasons), the behavior is marked as "undefined".

In absence of a standard effectively every language construct is 'undefined behavior'.

Again, look at Python for a vivid example of this. (We were discussing just this is a neighboring thread: https://news.ycombinator.com/item?id=26826158)

I have zero reason to believe that Rust won't meet Python's fate.

The Rust people don't have a standard and don't understand why they need one; in fact, their lack of standards is somehow touted as a benefit. Apparently, people think that if there is no standard for "defined" and "undefined" behavior that everything is "defined" by default.

(They're absolutely wrong, of course; it's actually the opposite.)

What you seem to be describing is "unspecified behaviour" or "implementation defined behaviour". If your program contains undefined behaviour the compiler makes no guarantees about how your program will behave.

For more see: https://blog.regehr.org/archives/213 and https://raphlinus.github.io/programming/rust/2018/08/17/unde... .

If there is one implementation as the definition then by default all behaviour is defined by the implementation, and hence parent is correct.

No. If we accept your argument then this would be correct:

> In absence of a standard effectively every language construct is 'implementation defined behaviour'.

But the parent said:

> In absence of a standard effectively every language construct is 'undefined behavior'.

Undefined behaviour != implementation defined behaviour. Both of these things exist in C and are separate. You can rely on implementation defined behaviour giving some consistent result on a given, compiler and hardware. You cannot rely on the behaviour of your program if you hit undefined behaviour.

I didn't state 'implementation defined behavior' which encompasses more than the Rust implementation. The Rust implementation defines the behavior, for example signed int overflow.

The "Rust Book" says signed integer overflow to be two's complement in release builds and panic in debug builds: https://doc.rust-lang.org/book/ch03-02-data-types.html

That's clear enough for programmers to rely on. It is nonsense to argue that this is equivalent to C's "the compiler may do anything it wants" just because the "Rust Book" is not called the "Rust Standard" or not "formal" enough.

this magical thinking about standards documents is one of the primary incentives to not have one

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