Filter out proc_macro_derive functions (fixes #1615)

clippy_lints/src/needless_pass_by_value.rs

@@ -55,8 +55,19 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NeedlessPassByValue {
             return;
         }
 
-        if !matches!(kind, FnKind::ItemFn(..)) {
+        match kind {
+            FnKind::ItemFn(.., attrs) => {
+                for a in attrs {
+                    if_let_chain!{[
+                        a.meta_item_list().is_some(),
+                        let Some(name) = a.name(),
+                        &*name.as_str() == "proc_macro_derive",
+                    ], {
                         return;
+                    }}
+                }
+            },
+            _ => return,
         }
 
         // Allows these to be passed by value.

tests/ui/needless_pass_by_value_proc_macro.rs

@@ -0,0 +1,11 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+#![crate_type = "proc-macro"]
+#![deny(needless_pass_by_value)]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_derive(Foo)]
+pub fn foo(_input: TokenStream) -> TokenStream { unimplemented!() }