component_model_meta 0.16.0

A flexible implementation of the Builder pattern supporting nested builders and collection-specific subcomponent_models. Implementation of its derive macro. Should not be used independently, instead use module::component_model which relies on the module.
Documentation 
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use super :: *;
// Use re-exports from macro_tools
use macro_tools :: { attr, diag, item_struct, Result, proc_macro2 ::TokenStream };

///
/// Generates an implementation of the `From< T >` trait for a custom struct, enabling
/// type-based conversion from `T` to the struct. This function parses the given
/// `TokenStream` representing a struct, and produces code that allows for its
/// fields to be initialized from an instance of type `T`, assuming `T` can be
/// converted into each of the struct's field types.
///
/// # Example of generated code for a tuple struct
///
/// ```ignore
/// impl< T > From< T > for TargetTuple
/// where
///   T: Clone,
///   T: Into< i32 >,
///   T: Into< String >,
/// {
///   #[ inline( always ) ]
///   fn from( src: T ) -> Self
///   {
///     let field_0 = Into :: < i32 > ::into( src.clone() );
///     let field_1 = Into :: < String > ::into( src.clone() );
///     Self( field_0, field_1 ) // Uses tuple construction
/// }
/// }
/// ```
///
#[ inline ]
pub fn from_components(input: proc_macro ::TokenStream) -> Result< proc_macro2 ::TokenStream > 
{
  let original_input = input.clone();
  let parsed = syn ::parse :: < syn ::ItemStruct >(input)?;
  let has_debug = attr ::has_debug(parsed.attrs.iter())?;

  // Struct name
  let item_name = &parsed.ident;

  // Generate snippets based on whether fields are named or unnamed
  let (field_assigns, final_construction) : (Vec< TokenStream >, TokenStream) =  match &parsed.fields 
  {
  syn ::Fields ::Named(fields_named) =>
  {
   let assigns = field_assign_named(fields_named.named.iter());
   let names: Vec< _ > = fields_named.named.iter().map(|f| f.ident.as_ref().unwrap()).collect();
   let construction = quote! { Self { #( #names, )* } };
   (assigns, construction)
 }
  syn ::Fields ::Unnamed(fields_unnamed) =>
  {
   let (assigns, temp_names) = field_assign_unnamed(fields_unnamed.unnamed.iter().enumerate());
   let construction = quote! { Self ( #( #temp_names, )* ) };
   (assigns, construction)
 }
  syn ::Fields ::Unit =>
  {
   // No fields to assign, construct directly
   (vec![], quote! { Self })
 }
 };

  // Extract field types for trait bounds
  let field_types = item_struct ::field_types(&parsed);
  let trait_bounds = trait_bounds(field_types);

  // Generate the From< T > trait implementation
  let result = qt! {
  impl< T > From< T > for #item_name
  where
   T: Clone,
   #( #trait_bounds )*
  {
   #[ inline( always ) ]
   fn from( src: T ) -> Self
   {
  #( #field_assigns )*
  #final_construction // Use the determined construction syntax
 }
 }
 };

  if has_debug 
  {
  let about = format!("derive: FromComponents\nstructure: {0}", &parsed.ident);
  diag ::report_print(about, &original_input, &result);
 }

  Ok(result)
}

/// Generates trait bounds for the `From< T >` implementation. (Same as before)
#[ inline ]
fn trait_bounds< 'a >(field_types: impl macro_tools ::IterTrait< 'a, &'a syn ::Type >) -> Vec< proc_macro2 ::TokenStream > 
{
  field_types
  .map(|field_type| {
   qt! {
  T: Into< #field_type >,
 }
 })
  .collect()
}

/// Generates assignment snippets for named fields.
#[ inline ]
fn field_assign_named< 'a >(fields: impl Iterator< Item = &'a syn ::Field >) -> Vec< proc_macro2 ::TokenStream > 
{
  fields
  .map(|field| {
   let field_ident = field.ident.as_ref().unwrap(); // Safe because we are in Named fields
   let field_type = &field.ty;
   qt! {
  let #field_ident = Into :: < #field_type > ::into( src.clone() );
 }
 })
  .collect()
}

/// Generates assignment snippets for unnamed fields and returns temporary variable names.
#[ inline ]
fn field_assign_unnamed< 'a >(
  fields: impl Iterator< Item = (usize, &'a syn ::Field) >,
) -> (Vec< proc_macro2 ::TokenStream >, Vec< proc_macro2 ::Ident >) {
  fields
  .map(|(index, field)| {
   let temp_var_name = format_ident!("field_{}", index); // Create temp name like field_0
   let field_type = &field.ty;
   let assign_snippet = qt! {
  let #temp_var_name = Into :: < #field_type > ::into( src.clone() );
 };
   (assign_snippet, temp_var_name)
 })
  .unzip() // Unzip into two vectors: assignments and temp names
}