Add a proc_macro_test crate

This exports all 3 kinds of proc macros and is useful for testing

crates/proc_macro_test/src/lib.rs

@@ -0,0 +1,18 @@
+//! Exports a few trivial procedural macros for testing.
+
+use proc_macro::TokenStream;
+
+#[proc_macro]
+pub fn function_like_macro(args: TokenStream) -> TokenStream {
+    args
+}
+
+#[proc_macro_attribute]
+pub fn attribute_macro(_args: TokenStream, item: TokenStream) -> TokenStream {
+    item
+}
+
+#[proc_macro_derive(DummyTrait)]
+pub fn derive_macro(_item: TokenStream) -> TokenStream {
+    TokenStream::new()
+}