PI constant is ambiguous

Question

Consider following code:

fn main() {
    let i = f32::consts::PI;
}

With following error:

$ rustc --version
rustc 1.0.0 (a59de37e9 2015-05-13) (built 2015-05-14)
$ rustc -
<anon>:2:13: 2:28 error: ambiguous associated type; specify the type using the syntax `<f32 as Trait>::consts` [E0223]
<anon>:2     let i = f32::consts::PI;
                     ^~~~~~~~~~~~~~~
error: aborting due to previous error

Why does it complain about an "associated type"? The only type I see here is f32, which is not associated.
Why is this ambigious? I clearly specified the f32.
What is this <f32 as Trait>::consts syntax? I've never seen it before.
And, obviously, what can I do to fix this error and get my variable set to PI?

the error is surprising (and is a bug IMHO). To make the code work you can specify `std::f32::consts::PI` or add a use `std::f32` — Paolo Falabella, Jul 03 '15 at 14:05
It's working, thanks! This solves the fourth question and maybe even 1 and 2 them if it really is a bug (which it looks like) — Kapichu, Jul 03 '15 at 14:11
I reported it as an issue https://github.com/rust-lang/rust/issues/26760, so a Rust dev can look into it. — Paolo Falabella, Jul 03 '15 at 14:30
I'm voting to close this question as off-topic because Stack Overflow isn’t the place for compiler bugs like this has been identified as. — Chris Morgan, Jul 03 '15 at 14:47
It's confusing, but I don't think the difference between the **module** `std::f32` and the **type** `f32` will ever disappear. — bluss, Jul 03 '15 at 15:19
@bluss ...but can we agree that the error message could be better? — Paolo Falabella, Jul 03 '15 at 15:47

score 18 · Accepted Answer · answered Jul 03 '15 at 15:46

18

To solve the issue, add use std::f32 or use std::f32::consts::PI, so that the compiler knows we're talking about the module f32 here, not the type f32.

answered Jul 03 '15 at 15:46

Kapichu

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It's actually bad design if the module `f32` and type `f32` conflict **by default**, though this is even forbidden in Rust! – Kapichu Jul 03 '15 at 15:49

score 4 · Answer 2 · edited Jul 03 '15 at 22:14

4

What is this <f32 as Trait>::consts syntax? I've never seen it before.

This is currently called "universal function call syntax" http://doc.rust-lang.org/stable/book/ufcs.html, but we're talking about not calling it that anymore, since this isn't a function... it's more of an expanded, unambiguous form.

edited Jul 03 '15 at 22:14

Francis Gagné

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answered Jul 03 '15 at 16:43

Steve Klabnik

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PI constant is ambiguous

2 Answers2