You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: from_form_field.rs
    path of file: ./repos/Rocket/core/codegen/src/derive
    
    the code of the file until where you have to start completion: use devise::{*, ext::SpanDiagnosticExt};
use proc_macro2::TokenStream;

use crate::exports::*;
use crate::derive::form_field::{VariantExt, first_duplicate};

pub fn derive_from_form_field(input: proc_macro::TokenStream) -> TokenStream {
    DeriveGenerator::build_for(input, quote!(impl<'__v> #_form::FromFormField<'__v>))
        .support(Support::Enum)
        .validator(ValidatorBuild::new()
            // We only accept C-like enums with at least one variant.
            .fields_validate(|_, fields| {
                if !fields.is_empty() {
                    return Err(fields.span().error("variants cannot have fields"));
                }

                Ok(())
            })
            .enum_validate(|_, data| {
                if data.variants.is_empty() {
                    return Err(data.span().error("enum must have at least one variant"));
                }

                if let Some(d) = first_duplicate(data.variants(), |v| v.form_field_values())? {
                    let (variant_a_i, variant_a, value_a) = d.0;
                    let (variant_b_i, variant_b, value_b) = d.1;

                    if variant_a_i == variant_b_i {
                        return Err(variant_a.error("variant has conflicting values")
                            .span_note(value_a, "this value...")
                            .span_note(value_b, "...conflicts with this value"));
                    }

                    return Err(value_b.error("field value conflicts with previous value")
                        .span_help(variant_b, "...declared in this variant")
                        .span_note(variant_a, "previous field with conflicting name"));
                }

                Ok(())
            })
        )
        .outer_mapper(quote! {
            #[allow(unused_imports)]
            use #_http::uncased::AsUncased;
        })
        .inner_mapper(MapperBuild::new()
            .with_output(|_, output| quote! {
                fn from_value(
                    __f: #_form::ValueField<'__v>
                ) -> #_Result<Self, #_form::Errors<'__v>> {

                    #output
                }
            })
            .try_enum_map(|mapper, data| {
                let mut variant_value = vec![];
                for v in data.variants().map(|v| v.form_field_values()) {
                    variant_value.append(&mut v?);
                }

                let variant_condition = data.variants()
                    .map(|v| mapper.map_variant(v))
                    .collect::<Result<Vec<_>>>()?;

                let (_ok, _cow) = (std::iter::repeat(_Ok), std::iter::repeat(_Cow));
                Ok(quote! {
                    #(#variant_condition)*

                    const OPTS: &'static [#_Cow<'static, str>] =
                        &[#(#_cow::Borrowed(#variant_value)),*];

                    let _error = #_form::Error::from(OPTS)
                        .with_name(__f.name)
                        .with_value(__f.value);

                    #_Err(_error)?
                })
            })
            .try_variant_map(|_, variant| {
                let builder = variant.builder(|_| unreachable!("fieldless"));
                let value = variant.form_field_values()?;

                Ok(quote_spanned! { variant.span
    
pub fn derive_from_form_field(input: proc_macro::TokenStream) -> TokenStream {
    DeriveGenerator::build_for(input, quote!(impl<'__v> #_form::FromFormField<'__v>))
        .support(Support::Enum)
        .validator(ValidatorBuild::new()
            // We only accept C-like enums with at least one variant.
            .fields_validate(|_, fields| {
                if !fields.is_empty() {
                    return Err(fields.span().error("variants cannot have fields"));
                }

                Ok(())
            })
            .enum_validate(|_, data| {
                if data.variants.is_empty() {
                    return Err(data.span().error("enum must have at least one variant"));
                }

                if let Some(d) = first_duplicate(data.variants(), |v| v.form_field_values())? {
                    let (variant_a_i, variant_a, value_a) = d.0;
                    let (variant_b_i, variant_b, value_b) = d.1;

                    if variant_a_i == variant_b_i {
                        return Err(variant_a.error("variant has conflicting values")
                            .span_note(value_a, "this value...")
                            .span_note(value_b, "...conflicts with this value"));
                    }

                    return Err(value_b.error("field value conflicts with previous value")
                        .span_help(variant_b, "...declared in this variant")
                        .span_note(variant_a, "previous field with conflicting name"));
                }

                Ok(())
            })
        )
        .outer_mapper(quote! {
            #[allow(unused_imports)]
            use #_http::uncased::AsUncased;
        })
        .inner_mapper(MapperBuild::new()
            .with_output(|_, output| quote! {
                fn from_value(
                    __f: #_form::ValueField<'__v>
                ) -> #_Result<Self, #_form::Errors<'__v>> {

                    #output
                }
            })
            .try_enum_map(|mapper, data| {
                let mut variant_value = vec![];
                for v in data.variants().map(|v| v.form_field_values()) {
                    variant_value.append(&mut v?);
                }

                let variant_condition = data.variants()
                    .map(|v| mapper.map_variant(v))
                    .collect::<Result<Vec<_>>>()?;

                let (_ok, _cow) = (std::iter::repeat(_Ok), std::iter::repeat(_Cow));
                Ok(quote! {
                    #(#variant_condition)*

                    const OPTS: &'static [#_Cow<'static, str>] =
                        &[#(#_cow::Borrowed(#variant_value)),*];

                    let _error = #_form::Error::from(OPTS)
                        .with_name(__f.name)
                        .with_value(__f.value);

                    #_Err(_error)?
                })
            })
            .try_variant_map(|_, variant| {
                let builder = variant.builder(|_| unreachable!("fieldless"));
                let value = variant.form_field_values()?;

                Ok(quote_spanned! { variant.span() =>
                    if #(__f.value.as_uncased() == #value)||* {
                        return #_Ok(#builder);
                    }
                })
            })
        )
        .to_tokens()
}