'attempt to use non-constant in a constant' should point to the constant

[jyn514]

(playground)

extern crate reqwest; // 0.10.9

fn main() {
    let s = String::from("http://example.com");
    const response = reqwest::get(&s);
}

error[E0435]: attempt to use a non-constant value in a constant
 --> src/main.rs:5:36
  |
5 |     const response = reqwest::get(&s);
  |                                    ^ non-constant value

I had const response = self.request(...) when I should have had let response = self.request(...) ... too much Javascript.
I don't think I equated "a constant" with const foo = ... in my head, because I'm used to doing const foo = bar(...) in Javascript all the time

It would be nice for the error to point to response as the constant:

5 |     const response = reqwest::get(&s);
  |           ^^^^^^^^ constant        ^ non-constant value

[sasurau4]

I'll try it
@rustbot claim

[estebank]

My position,from my comment in the PR:

The code changes themselves look reasonable. The ui change I'm ambivalent about. I see in the original report why this might be a problem for people coming from JS, but I don't know that this change will be a solution. I can see other possibilities, like detecting that the const is in an item body (in other words, inside an fn) and then potentially suggest let instead of const. For other cases (like the one mentioned in the help for E0435, we could emit a label pointing at the definition of the non-const binding and suggesting turning that into a const (if the value can be const). If none of these cases fit, I think that the current output is reasonable enough, but could be expanded by saying "this binding can't be const, consider using a literal of the appropriate type" or something along those lines.