Add a small convenience macro for including generated scaffolding. (#397

)

docs/manual/src/tutorial/Rust_scaffolding.md

@@ -30,10 +30,21 @@ fn main() {
 
 **Note:** This is the equivalent of calling (and does it under the hood) `uniffi-bindgen scaffolding src/math.udl --out-dir <OUT_DIR>`.
 
-Lastly, we include the generated scaffolding code in our `lib.rs`:
+Lastly, we include the generated scaffolding code in our `lib.rs`. If you've used the default build
+settings then this can be done using a handy macro:
+
+```rust
+uniffi_macros::include_scaffolding!("math");
+```
+
+If you have generated the scaffolding in a custom location, use the standard `!include` macro
+to include the generated file by name, like this:
+
+
 ```rust
 include!(concat!(env!("OUT_DIR"), "/math.uniffi.rs"));
 ```
+
 **Note:** The file name is always `<namespace>.uniffi.rs`.
 
 Great! `add` is ready to see the outside world!

examples/arithmetic/src/lib.rs

@@ -31,4 +31,4 @@ fn equal(a: u64, b: u64) -> bool {
 
 type Result<T, E = ArithmeticError> = std::result::Result<T, E>;
 
-include!(concat!(env!("OUT_DIR"), "/arithmetic.uniffi.rs"));
+uniffi_macros::include_scaffolding!("arithmetic");

uniffi_macros/src/lib.rs

@@ -84,3 +84,31 @@ impl syn::parse::Parse for FilePaths {
         })
     }
 }
+
+/// A helper macro to include generated component scaffolding.
+///
+/// This is a simple convenience macro to include the UniFFI component
+/// scaffolding as built by `uniffi_build::generate_scaffolding`.
+/// Use it like so:
+///
+/// ```rs
+/// uniffi_macros::include_scaffolding!("my_component_name");
+/// ```
+///
+/// This will expand to the appropriate `include!` invocation to include
+/// the generated `my_component_name.uniffi.rs` (which it assumes has
+/// been successfully built by your crate's `build.rs` script).
+//
+#[proc_macro]
+pub fn include_scaffolding(component_name: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let name = syn::parse_macro_input!(component_name as syn::LitStr);
+    if std::env::var("OUT_DIR").is_err() {
+        quote! {
+            compile_error!("This macro assumes the crate has a build.rs script, but $OUT_DIR is not present");
+        }
+    } else {
+        quote! {
+            include!(concat!(env!("OUT_DIR"), "/", #name, ".uniffi.rs"));
+        }
+    }.into()
+}