Make uniffi_build optional in uniffi_macros

As far as I can see, the dependency on uniffi_build is not needed for
the macros used by regular clients of UniFFI — it is only used to
create a convenience macro for the UI tests.

I named the new feature “trybuild” after the description of the macro.
Please let me know if you have preferences for a better name.

The overall goal here is to minimize the number of crates I need to
vendor to use UniFFI — I’m hoping to get away with checking in the
generated target language sources, so I’m currently focusing on the
dependencies in the rest of the code base.

fixtures/uitests/Cargo.toml

@@ -11,7 +11,7 @@ name = "uniffi_uitests"
 
 [dependencies]
 uniffi = { workspace = true }
-uniffi_macros = {path = "../../uniffi_macros"}
+uniffi_macros = { path = "../../uniffi_macros", features = ["trybuild"] }
 thiserror = "1.0"
 
 [dev-dependencies]

uniffi_macros/Cargo.toml

@@ -24,11 +24,13 @@ quote = "1.0"
 serde = "1.0.136"
 syn = { version = "2.0", features = ["full", "visit-mut"] }
 toml = "0.5.9"
-uniffi_build = { path = "../uniffi_build", version = "=0.26.0" }
+uniffi_build = { path = "../uniffi_build", version = "=0.26.0", optional = true }
 uniffi_meta = { path = "../uniffi_meta", version = "=0.26.0" }
 
 [features]
 default = []
+# Enable the generate_and_include_scaffolding! macro
+trybuild = [ "dep:uniffi_build" ]
 # Enable extra features that require a nightly compiler:
 # * Add the full module path of exported items to FFI metadata instead of just the crate name.
 #   This may be used by language backends to generate nested module structures in the future.

uniffi_macros/src/lib.rs

@@ -343,6 +343,7 @@ pub fn use_udl_object(tokens: TokenStream) -> TokenStream {
 /// uniffi_macros::generate_and_include_scaffolding!("path/to/my/interface.udl");
 /// ```
 #[proc_macro]
+#[cfg(features = "trybuild")]
 pub fn generate_and_include_scaffolding(udl_file: TokenStream) -> TokenStream {
     let udl_file = syn::parse_macro_input!(udl_file as LitStr);
     let udl_file_string = udl_file.value();