Invalid format string detection is over-eager

[coriolinus]

Example

// for purposes of this example, this struct is defined in an external crate and cannot be modified.
#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

// therefore, this wrapper exists to add `FromStr` and `Display` implementations.
#[derive(Debug, parse_display::FromStr, parse_display::Display)]
#[display("<{0.x}, {0.y}>")]
struct PointWrapper(Point);

Expected Behavior

As all fields of the wrapped type are specified, no error occurs.

Observed Behavior

error: field `0` is not appear in format.
 --> src/main.rs:8:11
  |
8 | #[display("<{0.x}, {0.y}>")]
  |           ^^^^^^^^^^^^^^^^

[frozenlib]

The ability to initialize a field with a constructor of its type does not exist, which is why this error is generated.

If a variable such as "{a.b}" is used in a format string, as in your example, I can implement the initialization ability using the constructor of the field type.

However, when using a variable such as "{a.b.c}" as shown in the following example, I cannot implement it because the macro cannot obtain the constructor name of the "a.b" part.

struct Point {
    x: i32,
    y: i32,
}
struct KeyValue {
    key: i32,
    value: Point,
}

// The macro can't obtain the constructor name `Point`.
#[derive(parse_display::FromStr)]
#[display("key = {0.key}, value = <{0.value.x}, {0.value.y}>")]
struct Wrapper(KeyValue);

For this reason, I have not implemented it until now, as it would be a strange "only available when the field nest is shallow" specification.

However, I think this specification is still useful, and if I can't think of a better one, I will try to implement it.

If Pre-RFC: Struct constructor name inference is available, then I can support variable such as {a.b.c}.