Errormessage regarding curly braces in for statement

[Bendrien]

I got the following code:

struct StructWithCurlyBraces {
    _foo: usize
}

impl Iterator for StructWithCurlyBraces {
    type Item = ();
    
    fn next(&mut self) -> Option<Self::Item> {
        None
    }
}

pub fn main() {
    // with surrounding parentheses it works
    for _ in (StructWithCurlyBraces { _foo: 42 }) { }
    
    // without it doesn't
    for _ in StructWithCurlyBraces { _foo: 42 } { }
}

I don't know if the case without surrounding parentheses is intended to work but apart from that the error message is somewhat missleading.

error: expected type, found `42`
  --> <anon>:18:44
   |
18 |     for _ in StructWithCurlyBraces { _foo: 42 } { }
   |                                            ^^

error[E0423]: expected value, found struct `StructWithCurlyBraces`
  --> <anon>:18:14
   |
18 |     for _ in StructWithCurlyBraces { _foo: 42 } { }
   |              ^^^^^^^^^^^^^^^^^^^^^ did you mean `StructWithCurlyBraces { /* fields */ }`?

error: aborting due to 2 previous errors

It seems that the tokenizer doesn't get this right?

Edit: Include error message

[fintelia]

The same thing happens with an if statement:

struct X {}
if let X {} = X {} {}

error[E0423]: expected value, found struct `X`
  |
4 |     if let X {} = X {} {}
  |                   ^ did you mean `X { /* fields */ }`?

[estebank]

The parser cannot differentiate between a struct literal and a "path" (a variable) followed by a block in these places. There's no way to do so at parse time, as you don't yet know wether the found path is a struct so it could be parsed as such, so the parser just flat out doesn't allow that construct.

What could be improved is

1. not suggesting the struct literal (remove the "did you mean..." if tcx.sess.get_parent_node(id) is an if/while/for statement)

2. suggesting this instead:

error[E0423]: expected value, found struct `X`
  |
4 |     if let X {} = X {} {}
  |                   ^
help: if you wanted to use a literal struct constructor here, try surrounding it with parenthesis
4 |     if let X {} = (X {}) {}
  |                   ^^^^^^

The second would be harder as at this point the compiler already decided that we meant X as the struct itself, not as the struct constructor, so we do not know where the span for its block ends, if it would parse correctly as such and even if does end. Instead of the suggestion a help could be displayed instead of the current suggestion stating something along the lines of "struct literals are forbidden here due to ambiguous parsing, surround with parenthesis to avoid this".

[estebank]

Fixed by #51360, #59981 and #60118