Form derivation macros insists on string literals.

[horned-sphere]

Describe the bug
When using the Form derivation macro and specifying field names/enumeration tags etc, for example:

pub enum Envelope {
    #[form(tag = "auth")]
    Auth {
        #[form(body)]
        body: Option<Value>,
    },
    ...
}

the macro will only work if them name is a literal. If a constant is specified:

const AUTH: &str = "auth";
pub enum Envelope {
    #[form(tag = AUTH)]
    Auth {
        #[form(body)]
        body: Option<Value>,
    },
    ...
}

a compiler error is produced.

Expected behavior
The macro should accept a path to a constant string in addition to a literal.

[DobromirM]

Unfortunately it looks like this is not supported by the Rust compiler:
rust-lang/rust#52393 (comment)

There is a workaround using proc macros:
rust-lang/rust#78835 (comment)

but apparently it is deliberately implemented to work only for the top level and not with nested attributes:
rust-lang/rust#78835 (comment)

[DobromirM]

As a potential workaround we can treat any uppercase string as a constant, but I am not sure how intuitive it will be.

[DobromirM]

I've tested more things and it is possible to add support for custom attributes by creating our own attribute parser instead of using the default syn::Meta parser. However that leads to a more complex structure and duplicates some of the code from the syn library. Furthermore, because constants are evaluated much later than macros there is no way to get their actual value and validation for duplicate tags will not be possible.

Example:

const TEST_TAG: &str = "test";

#[derive(Form)]
enum Foo{
    #[form(tag = "test")]
    A,
    /// There is no way to check what the actual value of TEST_TAG is.
      #[form(tag = TEST_TAG)]
    B,
}