Confusing error when populating a struct with a field called 'start' without an explicit value and a variable called start is not defined

[jjl]

Given the following code:

struct Wat {
    start: (),
}
impl Wat {
    fn new() -> Wat {
      Wat { start }
    }
}

The current output is:

expected value, found built-in attribute `start`
a field by this name exists in `Self` [E0423]

I frankly have no idea what it's on about and I was surprised that it doesn't just tell me it can't find the variable of that name.

[mbartlett21]

This is because there is a builtin attribute #[start] that can be applied to a function to make it the entry point, which is what it resolves start to, since you have no variable shadowing it.

[jjl]

Ahh, gotcha. I don't think it's helpful to note that it found an attribute of that name, or at least it confused me.

[kpreid]

If the field name does not happen to be an attribute, the note is less confusing but still unhelpful:

struct Foo {
    bar: i32,
}

impl Foo {
    fn new() -> Self {
        Self {
            bar,
        }
    }
}

error[E0425]: cannot find value `bar` in this scope
 --> src/lib.rs:8:13
  |
8 |             bar,
  |             ^^^ a field by this name exists in `Self`

For more information about this error, try `rustc --explain E0425`.
error: could not compile `playground` due to previous error

It seems to me that there is little point in saying “a field by this name exists”, since we're in a struct initializer — if there wasn't such a field, we'd be getting a different error. Or perhaps it would be helpful to say “a field by this name exists in Self, but there is no variable by this name to initialize it with; try writing bar: something_else_here”.

[AnthonyMikh]

Hit it today with a code that can be minified to this:

fn f() {
    path
}

Giving

error[E0423]: expected value, found built-in attribute `path`
 --> src/lib.rs:2:5
  |
2 |     path
  |     ^^^^ not a value

In my case it was even more confusing because I was rearranging my code and I actually had a variable named path, just in a nested scope.

Interestingly enough, Rust prior to version 1.38.0 gives a more sensible error. For example, Rust 1.37.0 gives the following:

error[E0425]: cannot find value `path` in this scope
 --> <source>:2:5
  |
2 |     path
  |     ^^^^ not found in this scope