introduce #[fbinit::nested_test] that nests well with other #[test] a…

…ttribute wrapper

Summary:
`#[fbinit::test]` does not nest well with other test attributes e.g. `rstest`. This change introduces a new attribute `#[fbinit::nested_test]` to solve this problem. Example usage

```
#[rstest]
#[fbinit::nested_test]
fn some_test(fb: FacebookInit, some_fixture: u32) {
    // ...
}
```

Reviewed By: edward-shen

Differential Revision: D54487672

fbshipit-source-id: dbee66d38ac8719ded84944fa4f6218f1d39fbb3

shed/fbinit/fbinit_macros/expand.rs

@@ -22,6 +22,7 @@ use syn::Token;
 pub enum Mode {
     Main,
     Test,
+    NestedTest,
 }
 
 mod kw {
@@ -108,10 +109,10 @@ pub fn expand(
         }
     }
 
-    if function.sig.inputs.len() > 1 {
+    if mode != Mode::NestedTest && function.sig.inputs.len() > 1 {
         return Err(Error::new_spanned(
             function.sig,
-            "expected one argument of type fbinit::FacebookInit",
+            "expected one argument of type fbinit::FacebookInit unless #[fbinit::nested_test] is used",
         ));
     }
 
@@ -128,16 +129,24 @@ pub fn expand(
                 panic!("fbinit must be performed in the crate root on the main function");
             }
         }),
-        Mode::Test => None,
+        _ => None,
     };
 
     let assignment = function.sig.inputs.first().map(|arg| quote!(let #arg =));
-    function.sig.inputs = Punctuated::new();
+    match mode {
+        Mode::NestedTest => {
+            // remove the first input (fb: FacebookInit) from function signature
+            function.sig.inputs = function.sig.inputs.into_iter().skip(1).collect();
+        }
+        _ => {
+            function.sig.inputs = Punctuated::new();
+        }
+    }
 
     let block = function.block;
 
     let body = match (function.sig.asyncness.is_some(), mode) {
-        (true, Mode::Test) => quote! {
+        (true, Mode::Test | Mode::NestedTest) => quote! {
             fbinit_tokio::tokio_test(async #block )
         },
         (true, Mode::Main) => quote! {

shed/fbinit/fbinit_macros/lib.rs

@@ -93,3 +93,29 @@ pub fn test(args: TokenStream, input: TokenStream) -> TokenStream {
     .unwrap_or_else(|err| err.to_compile_error())
     .into()
 }
+
+// Similar to #[fbinit::test], but allows for nesting with other test attributes (e.g. #[rstest]) that wraps #[test].
+//
+//     #[rstest]
+//     #[fbinit::nested_test]
+//     fn name_of_test(fb: FacebookInit, some_fixture: u32) {
+//         ...
+//     }
+//
+// to:
+//
+//     #[rstest]
+//     fn name_of_test(some_fixture: u32) {
+//         let fb: FacebookInit = fbinit::perform_init();
+//         ...
+//     }
+#[proc_macro_attribute]
+pub fn nested_test(args: TokenStream, input: TokenStream) -> TokenStream {
+    expand(
+        Mode::NestedTest,
+        parse_macro_input!(args with Punctuated::parse_terminated),
+        parse_macro_input!(input),
+    )
+    .unwrap_or_else(|err| err.to_compile_error())
+    .into()
+}

shed/fbinit/src/lib.rs

@@ -20,6 +20,7 @@
 mod oss;
 
 pub use fbinit_macros::main;
+pub use fbinit_macros::nested_test;
 pub use fbinit_macros::test;
 #[cfg(not(fbcode_build))]
 pub use oss::*;