1 files changed, 81 insertions, 0 deletions
diff --git a/src/generate/into_heads.rs b/src/generate/into_heads.rs
new file mode 100644
index 0000000..5784e46
--- /dev/null
+++ b/src/generate/into_heads.rs
@@ -0,0 +1,81 @@
+use proc_macro2::TokenStream;
+use quote::quote;
+
+use crate::info_structures::{Options, StructInfo};
+
+/// Returns the Heads struct and a function to convert the original struct into a Heads instance.
+pub fn make_into_heads(info: &StructInfo, options: Options) -> (TokenStream, TokenStream) {
+    let visibility = if options.do_pub_extras {
+        info.vis.clone()
+    } else {
+        syn::parse_quote! { pub(super) }
+    };
+    let mut code = Vec::new();
+    let mut field_initializers = Vec::new();
+    let mut head_fields = Vec::new();
+    // Drop everything in the reverse order of what it was declared in. Fields that come later
+    // are only dependent on fields that came before them.
+    for field in info.fields.iter().rev() {
+        let field_name = &field.name;
+        if !field.self_referencing {
+            code.push(quote! { let #field_name = self.#field_name; });
+            if field.is_borrowed() {
+                field_initializers
+                    .push(quote! { #field_name: ::ouroboros::macro_help::unbox(#field_name) });
+            } else {
+                field_initializers.push(quote! { #field_name });
+            }
+            let field_type = &field.typ;
+            head_fields.push(quote! { #visibility #field_name: #field_type });
+        } else {
+            // Heads are fields that do not borrow anything.
+            code.push(quote! { ::core::mem::drop(self.#field_name); });
+        }
+    }
+    for (ty, ident) in info.generic_consumers() {
+        head_fields.push(quote! { #ident: ::core::marker::PhantomData<#ty> });
+        field_initializers.push(quote! { #ident: ::core::marker::PhantomData });
+    }
+    let documentation = format!(
+        concat!(
+            "A struct which contains only the ",
+            "[head fields](https://docs.rs/ouroboros/latest/ouroboros/attr.self_referencing.html#definitions) of [`{0}`]({0})."
+        ),
+        info.ident.to_string()
+    );
+    let generic_params = info.generic_params();
+    let generic_where = &info.generics.where_clause;
+    let heads_struct_def = quote! {
+        #[doc=#documentation]
+        #visibility struct Heads <#generic_params> #generic_where {
+            #(#head_fields),*
+        }
+    };
+    let documentation = concat!(
+        "This function drops all internally referencing fields and returns only the ",
+        "[head fields](https://docs.rs/ouroboros/latest/ouroboros/attr.self_referencing.html#definitions) of this struct."
+    ).to_owned();
+
+    let documentation = if !options.do_no_doc {
+        quote! {
+            #[doc=#documentation]
+        }
+    } else {
+        quote! { #[doc(hidden)] }
+    };
+
+    let generic_args = info.generic_arguments();
+    let into_heads_fn = quote! {
+        #documentation
+        #[allow(clippy::drop_ref)]
+        #[allow(clippy::drop_copy)]
+        #[allow(clippy::drop_non_drop)]
+        #visibility fn into_heads(self) -> Heads<#(#generic_args),*> {
+            #(#code)*
+            Heads {
+                #(#field_initializers),*
+            }
+        }
+    };
+    (heads_struct_def, into_heads_fn)
+}