feat: Assert no attributes used on self and ctx

sylvia-derive/src/parser/mod.rs

@@ -67,6 +67,29 @@ pub fn process_fields<'s, Generic>(
 where
     Generic: GetPath + PartialEq,
 {
+    // Assert no accidental usage of Sylvia attributes on `self` and `ctx` parameters.
+    sig.inputs.iter().take(2).for_each(|arg| match arg {
+        FnArg::Receiver(item) => {
+            item.attrs.iter().for_each(|attr| {
+                emit_error!(attr.span(),
+                    "Invalid usage of Sylvia attribute.";
+                    note = "First and second arguments of the method should be `self` and `ctx` respectively.";
+                    note = "Unexpected attribute on `self` parameter."
+                );
+            });
+        }
+
+        FnArg::Typed(item) => {
+            item.attrs.iter().for_each(|attr| {
+                emit_error!(attr.span(),
+                    "Invalid usage of Sylvia attribute.";
+                    note = "First and second arguments of the method should be `self` and `ctx` respectively.";
+                    note = "Unexpected attribute on `ctx` parameter."
+                );
+            });
+        }
+    });
+
     sig.inputs
         .iter()
         .skip(2)

sylvia/tests/ui/attributes/payload/invalid_usage.stderr

@@ -1,3 +1,13 @@
+error: Invalid usage of Sylvia attribute.
+
+         = note: First and second arguments of the method should be `self` and `ctx` respectively.
+         = note: Unexpected attribute on `ctx` parameter.
+
+  --> tests/ui/attributes/payload/invalid_usage.rs:23:25
+   |
+23 |         fn reply(&self, #[sv::payload(raw)] _ctx: ReplyCtx) -> StdResult<Response> {
+   |                         ^
+
 error: Missing payload parameter.
 
          = note: Expected at least one payload parameter at the end of parameter list.