Better diagnostics for missing iterator in for-loop

[jyn514]

Spot the bug:

fn f() -> usize {
    for i in {
        println!("{}", i);
    }
    1
}

Unfortunately, instead of pointing to the missing iterator, rust points to the 1:

error: expected `{`, found `1`
 --> src/lib.rs:5:5
  |
5 |     1
  |     ^
  |     |
  |     expected `{`
  |     help: try placing this code inside a block: `{ 1 }`

I'd love it to instead say 'missing iterator for loop':

error: expected iterator, found block
 --> src/lib.rs
  |
2 |    for i in {
  |             ^ this starts a block expression ...
5 |     1
  |     ^ ... so there is no body for the loop
  =     help: try adding an iterator to the for-loop: `for i in iter {`

Other variants of this with less than ideal errors:

fn f() {
    for i in {
        println!("{}", i);
    }
}

error: expected `{`, found `}`
 --> src/lib.rs:5:1
  |
5 | }
  | ^ expected `{`

fn f() -> usize {
    for i in {
        println!("{}", i);
    }
    return 1;
}

error: expected `{`, found keyword `return`
 --> src/lib.rs:5:5
  |
5 |     return 1;
  |     ^^^^^^---
  |     |
  |     expected `{`
  |     help: try placing this code inside a block: `{ return 1; }`

[camelid]

I'd love it to instead say 'missing iterator for loop':

error: expected iterator, found block
 --> src/lib.rs
  |
2 |    for i in {
  |             ^ this starts a block expression ...
5 |     1
  |     ^ ... so there is no body for the loop
  =     help: try adding an iterator to the for-loop: `for i in iter {`

Well, it thinks the block is the iterator.

[jyn514]

Yes, I understand, but a) () is not an iterator, and b) since there's no body of the loop I was hoping the parser could recover somehow and understand that the block expression was intended to be the body.

[camelid]

Yes, I just wanted to make sure we were on the same page :)

[estebank]

I could have sworn there was an old [A-diagnostics] ticket for this already but I can't find it 😅

[obeis]

@rustbot claim