diagnostics: better warning in case of let declaration and assignment operator: let y += 4

[matthiaskrgr]

fn main() {
    let y = 3;
    let y = y + 4;

this code compiles fine, but if I try to use something like let y += 4; rustc has no idea what is going on:

fn main() {
    let y = 3;
    let y += 4;
}

    Compiling mul v0.1.0 (file:///tmp/mul)
error: expected one of `:`, `;`, `=`, or `@`, found `+=`
 --> src/main.rs:3:11
  |
3 |     let y += 4;
  |           ^^ expected one of `:`, `;`, `=`, or `@` here
error: aborting due to previous error

I wonder if we could figure out that the user wants let y = y + 4 in this case and suggest it?

[memoryruins]

Instead of shadowing, I would first think they meant to type y += 4; without the let and to make the first y mutable.

[memoryruins]

But if that’s assuming too much, then I think your suggestion is better than nothing.

[JohnTitor]

The current output:

error: can't reassign to an uninitialized variable
 --> src/main.rs:3:11
  |
3 |     let y += 4;
  |           ^^ help: initialize the variable
  |
  = help: if you meant to overwrite, remove the `let` binding

The diagnostics have improved by #83062 and others. Closing as fixed.