Better error messages: Tell programmer about forgotten let

[Kaligule]

As a python programmer, forgetting a let in an assignment to a variable that didn't exist before is an easy mistake to make.

example code

fn main() {
    // forgot the "let" here
    horses = ["Secretariat", "Seabiscuit", "Binky"];
    println!("I have {} horses.", horses.len());
}

(link to playground)

Rust sees two uses of an undefined variable here (E0425 twice):

error[[E0425]](https://doc.rust-lang.org/stable/error-index.html#E0425): cannot find value `horses` in this scope
 --> src/main.rs:3:5
  |
3 |     horses = ["Secretariat", "Seabiscuit", "Binky"];
  |     ^^^^^^ not found in this scope

error[[E0425]](https://doc.rust-lang.org/stable/error-index.html#E0425): cannot find value `horses` in this scope
 --> src/main.rs:4:35
  |
4 |     println!("I have {} horses.", horses.len());
  |                                   ^^^^^^ not found in this scope

For more information about this error, try `rustc --explain E0425`.

What I would hope for

While the second one is fine, in the first case I would have really appreciated a sentence like this:

Did you mean to use `let` to introduce a new variable? Try `let horses = ...`

This would helped me as a newcomer to the language, because it does an educated guess what the problem might be and tells me how to fix it.

I think the hint should be added if the value is assigned to (first error), but not if it is used as an argument (second error).

I hope this is the right place and format to file this suggestion, I would appreciate hints if this is not the case.

[Rageking8]

I hope this is the right place and format to file this suggestion, I would appreciate hints if this is not the case.

This is the right place, however you should use the "diagnostic" template when creating issues like this. Thanks for the issue though.

@rustbot label +T-compiler +A-diagnostics

[chenyukang]

@rustbot claim