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

[ghost]

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