1 files changed, 96 insertions, 0 deletions
diff --git a/syn/examples/heapsize/heapsize_derive/src/lib.rs b/syn/examples/heapsize/heapsize_derive/src/lib.rs
new file mode 100644
index 0000000..9176b29
--- /dev/null
+++ b/syn/examples/heapsize/heapsize_derive/src/lib.rs
@@ -0,0 +1,96 @@
+extern crate proc_macro;
+
+use proc_macro2::TokenStream;
+use quote::{quote, quote_spanned};
+use syn::spanned::Spanned;
+use syn::{parse_macro_input, parse_quote, Data, DeriveInput, Fields, GenericParam, Generics, Index};
+
+#[proc_macro_derive(HeapSize)]
+pub fn derive_heap_size(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    // Parse the input tokens into a syntax tree.
+    let input = parse_macro_input!(input as DeriveInput);
+
+    // Used in the quasi-quotation below as `#name`.
+    let name = input.ident;
+
+    // Add a bound `T: HeapSize` to every type parameter T.
+    let generics = add_trait_bounds(input.generics);
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+
+    // Generate an expression to sum up the heap size of each field.
+    let sum = heap_size_sum(&input.data);
+
+    let expanded = quote! {
+        // The generated impl.
+        impl #impl_generics heapsize::HeapSize for #name #ty_generics #where_clause {
+            fn heap_size_of_children(&self) -> usize {
+                #sum
+            }
+        }
+    };
+
+    // Hand the output tokens back to the compiler.
+    proc_macro::TokenStream::from(expanded)
+}
+
+// Add a bound `T: HeapSize` to every type parameter T.
+fn add_trait_bounds(mut generics: Generics) -> Generics {
+    for param in &mut generics.params {
+        if let GenericParam::Type(ref mut type_param) = *param {
+            type_param.bounds.push(parse_quote!(heapsize::HeapSize));
+        }
+    }
+    generics
+}
+
+// Generate an expression to sum up the heap size of each field.
+fn heap_size_sum(data: &Data) -> TokenStream {
+    match *data {
+        Data::Struct(ref data) => {
+            match data.fields {
+                Fields::Named(ref fields) => {
+                    // Expands to an expression like
+                    //
+                    //     0 + self.x.heap_size() + self.y.heap_size() + self.z.heap_size()
+                    //
+                    // but using fully qualified function call syntax.
+                    //
+                    // We take some care to use the span of each `syn::Field` as
+                    // the span of the corresponding `heap_size_of_children`
+                    // call. This way if one of the field types does not
+                    // implement `HeapSize` then the compiler's error message
+                    // underlines which field it is. An example is shown in the
+                    // readme of the parent directory.
+                    let recurse = fields.named.iter().map(|f| {
+                        let name = &f.ident;
+                        quote_spanned! {f.span()=>
+                            heapsize::HeapSize::heap_size_of_children(&self.#name)
+                        }
+                    });
+                    quote! {
+                        0 #(+ #recurse)*
+                    }
+                }
+                Fields::Unnamed(ref fields) => {
+                    // Expands to an expression like
+                    //
+                    //     0 + self.0.heap_size() + self.1.heap_size() + self.2.heap_size()
+                    let recurse = fields.unnamed.iter().enumerate().map(|(i, f)| {
+                        let index = Index::from(i);
+                        quote_spanned! {f.span()=>
+                            heapsize::HeapSize::heap_size_of_children(&self.#index)
+                        }
+                    });
+                    quote! {
+                        0 #(+ #recurse)*
+                    }
+                }
+                Fields::Unit => {
+                    // Unit structs cannot own more than 0 bytes of heap memory.
+                    quote!(0)
+                }
+            }
+        }
+        Data::Enum(_) | Data::Union(_) => unimplemented!(),
+    }
+}