Add test-suite for failure modes

Also, discover that apparently `momo` isn't effective where it should be
*or* the implementation is more complex.

Author: Byron

Cargo.lock

@@ -20,3 +20,6 @@ proc_macro = true
 syn = { version = "2.0", features = ["full", "fold"] }
 quote = "1.0"
 proc-macro2 = "1.0"
+
+[dev-dependencies]
+trybuild = "1.0"

gix-macros/Cargo.toml

@@ -11,6 +11,14 @@ pub(crate) fn inner(code: proc_macro2::TokenStream) -> proc_macro2::TokenStream
 
     if let Item::Fn(item_fn) = fn_item {
         let ty_conversions = parse_generics(&item_fn.sig);
+        // TODO: uncomment and see it fail in places where it should actually succeed.
+        // if ty_conversions.is_empty() {
+        //     return Error::new(
+        //         item_fn.span(),
+        //         "Couldn't apply a single conversion - momo is ineffective here",
+        //     )
+        //     .to_compile_error();
+        // }
         let (argtypes, argexprs, has_self) = convert(&item_fn.sig.inputs, &ty_conversions);
 
         let uses_self = has_self

gix-macros/src/momo.rs

@@ -302,3 +302,16 @@ fn struct_fn() {
     )
     .unwrap_err();
 }
+
+#[test]
+fn ux() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/momo/ux/error_on_struct.rs");
+}
+
+#[test]
+#[ignore = "needs work"]
+fn ux_todo() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/momo/ux/error_if_ineffective.rs");
+}

gix-macros/tests/momo/mod.rs

@@ -0,0 +1,3 @@
+/// If nothing is done, momo should fail.
+#[gix_macros::momo]
+fn main() {}

gix-macros/tests/momo/ux/error_if_ineffective.rs

@@ -0,0 +1,5 @@
+fn main() {}
+
+/// Only functions work with momo
+#[gix_macros::momo]
+struct S;

gix-macros/tests/momo/ux/error_on_struct.rs

@@ -0,0 +1,5 @@
+error: expect a function
+ --> tests/momo/ux/error_on_struct.rs:3:1
+  |
+3 | /// Only functions work with momo
+  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^