Compile message contains reference to an identifier not used by the erraneous code

[svenschmidt75]

rustc 1.37.0 (eae3437 2019-08-13)

Repo: https://github.com/svenschmidt75/Rust
Commit: svenschmidt75/Rust@23ec342

When building, I get error message

   Compiling ann v0.1.0 (/home/svenschmidt75/Develop/Rust/NeuralNetwork/lib/ann)
error[E0282]: type annotations needed
  --> lib/ann/src/ann/cost_function.rs:25:9
   |
25 |         diff2.iter().sum() as f64 / 2.0
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `S`
   |
   = note: type must be known at this point

error: aborting due to previous error

For more information about this error, try `rustc --explain E0282`.
error: Could not compile `ann`.
warning: build failed, waiting for other jobs to finish...
error[E0282]: type annotations needed
  --> lib/ann/src/ann/cost_function.rs:25:9
   |
25 |         diff2.iter().sum() as f64 / 2.0
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for `S`
   |
   = note: type must be known at this point

error: aborting due to previous error

For more information about this error, try `rustc --explain E0282`.
error: Could not compile `ann`.

To learn more, run the command again with --verbose.

Process finished with exit code 101

Where does S come from?

Thanks,

[jonas-schievink]

S is the generic parameter of Iterator::sum().

[estebank]

@svenschmidt75 would the following output be enough for you to understand what was happening?

error[E0282]: type annotations needed
  --> lib/ann/src/ann/cost_function.rs:25:9
   |
25 |         diff2.iter().sum() as f64 / 2.0
   |                      ^^^ cannot infer type for `S`
   |
   = note: type must be known at this point

IMO it is better to see where the problem is coming from. But there is still no obvious connection between 'S' and sum().
But the explanation from @jonas-schievink and the doc does explain it good.

fn sum<S>(self) -> S

[jonas-schievink]

It would probably help a lot if the message tells you what S is. Something like

error[E0282]: type annotations needed
  --> lib/ann/src/ann/cost_function.rs:25:9
   |
25 |         diff2.iter().sum() as f64 / 2.0
   |                      ^^^ cannot infer type of type parameter `S`
   |
   = note: type must be known at this point

[svenschmidt75]

@estebank It is definitely an improvement. Is there any way that the trait can be mentioned as well? I agree with @BO41...

[estebank]

@svenschmidt75 I've been looking at doing that but will require a much larger refactor to mention either Iterator::sum or Sum<Iterator::Item>. I'll leave this ticket open, but I feel that the small PR I put together is a good stop gap to marginally improve things.

[compiler-errors]

This has improved in the mean time slightly:

error[[E0282]](https://doc.rust-lang.org/stable/error-index.html#E0282): type annotations needed
 --> src/lib.rs:2:18
  |
2 |     diff2.iter().sum() as f64 / 2.0
  |                  ^^^ cannot infer type of the type parameter `S` declared on the associated function `sum`
  |
help: consider specifying the generic argument
  |
2 |     diff2.iter().sum::<S>() as f64 / 2.0
  |                     +++++

For more information about this error, try `rustc --explain E0282`.

I'm probably gonna close this because it suggests the turbofish and mentions that S is a parameter of sum