TWiR quote of the week

I wonder if quote of the week can be used to bust some myth.

But what's the point of a const function then?

It makes code like this compile:

const FOO: i32 = my_const_fn(13);

That code is rejected unless my_const_fn is a const fn .

That is the only effect const has on fn . And AFAIK we never say anything else anywhere in the docs, so I do not comprehend where all these extra assumptions come from that you have been making. I'd really like to know as I'd like to avoid people making such false assumptions. :slight_smile:

I always thought that const means all of these happens in compile time and none of it goes to runtime except those called in non-const places.

Code in const "places" (usually called const contexts) must run at compiletime. So this statement is true but I don't think it means what you think it means. In particular, format is impossible to use in const contexts:

const FOO: String = format!("abc"); // does not compile

Since format! can only be used in non-const-contexts, that means it never happens at compiletime. Or rather, it is never guaranteed to happen at compiletime -- optimizations can do whatever they want, but there's no guarantee they ever happen.

-- @RalfJung github comments

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I like the idea, but it's there's not really a pointed quote in that comment I think.^^ Maybe except for this part.

But what's the point of a const function then?

It makes code like this compile:

const FOO: i32 = my_const_fn(13);

That code is rejected unless my_const_fn is a const fn .

That is the only effect const has on fn . There is no effect on optimizations or runtime code.

(Slightly edited as well.) But still that's rather long. And we're starting to artifically construct a quote rather than take a real one.^^

I don't know if it qualifies for quote of the week but I'm glad it was posted here so I could read it


@zrk are you perhaps shocked by it like me and another member did, thinking that const can allow generating better code without side effects?

In response to In this mesh class, what's wrong with my use of lifetimes?

Many hours of discussion with the borrow checker leads me to concur with @trentj here.


Disappointed with Path


Advice by @alice, relevant to every Rust nauplius who attemps self-referential structs. (emphasis added)
From How to resolve "error[E0499]: cannot borrow ... as mutable more than once at a time" in this case


Slightly OT here, but should that compile error say something about self-referential structs?

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Figuring out the exact conditions is probably difficult, but I think it would be a good idea.

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Sage advice for anyone programming in Rust: first get it working correctly, then measure, then tune performance only if necessary.


from IRLO


You want to be looking into languages that tell you not "programming is so easy with me" but "programming is actually fiendishly hard but I'm going to try my best to help you get through it in one piece."

Bodil Stokke on twitter


Rust has a curse (it has many, but this one is critical): inefficient code is generally visible. Experienced developers hate to notice that their code is inefficient. They will recoil at seeing Arc<RefCell<T>> , but won't bat an eye at using Python.

by ekuber


The whole post is worth reading, I'd say :slightly_smiling_face:

  • for those wanting to discuss about it / challenge that assertion, the quote of the tweek thread is not for that. Instead, take advantage of there being a dedicated thread for it.

Unfortunately it's from the survey, so I don't know who to attribute it to, but this made me chuckle:

[...] clippy is for people who find a certain emptiness inside when they finally get code through the compiler. :wink:


Just because Rust allows you to write super cool non-allocating zero-copy algorithms safely, doesn’t mean every algorithm you write should be super cool, zero-copy and non-allocating.

trentj on URLO


And it's true that a lot of stuff requires a "sufficiently smart compiler" but really it's 2020, if your compiler isn't serving you breakfast in bed you need to be upping your expectations.

-- Jubilee on the Rust Zulip


Historically, C has been the best fit for these applications just because it so lean: by providing essentially nothing other than the portable assembler that is the language itself, it avoids the implicit assumptions (and girth) of a complicated runtime. But the nothing that C provides reflects history more than minimalism; it is not an elegant nothing, but rather an ill-considered nothing

-- Bryan Cantrill on what no_std is competing with


Alternative title: mediocre C++ is arguably worse than mediocre rust.

I’d agree with that? I think the power and danger of C++ is that it assumes the programmer knows best. Rust assumes the programmer is wrong if it looks dangerous. So.. yes?

By sesuximo in


Think of it like this: When Rust passes control to C it's like you jumping into a pit full of vipers in the hope that none of them bytes you. I might put up a sign "unsafe, vipers in pit".

ZiCog in Does Rust offer C interop?