fix(parser): disallow from attributes with args

fixes #16

macros/src/parser/field.rs

@@ -108,6 +108,14 @@ impl FromAttributeCheck {
                     return Err(Self::err_too_many_from_attributes(field_span));
                 }
 
+                // check if the attr has some args
+                match attr.meta {
+                    syn::Meta::List(_) | syn::Meta::NameValue(_) => {
+                        return Err(Self::err_from_attribute_has_args(&attr.span()))
+                    }
+                    syn::Meta::Path(_) => (), // good. there are no args in `#[from]`
+                }
+
                 already_found_from_attribute = true;
             }
         }
@@ -127,6 +135,13 @@ impl FromAttributeCheck {
         )
     }
 
+    pub fn err_from_attribute_has_args(attribute_span: &Span) -> syn::Error {
+        syn::Error::new(
+            *attribute_span,
+            "The `#[from]` attribute does not take any arguments, but some were found.",
+        )
+    }
+
     /// Returns the created WrappedField, consuming `self`.
     pub fn finish(self) -> WrappedField {
         self.wrapped_field

src/_doctests.rs

@@ -0,0 +1,13 @@
+/**
+```compile_fail
+use super::Error;
+use std::error::Error;
+
+#[derive(Debug, Error)]
+enum FromAttrWithArgs {
+    #[error(transparent)]
+    ErrorVariant(#[from("this shouldn't work")] std::io::Error),
+}
+```
+*/
+pub fn from_attr_args_shouldnt_have_args() {}

src/lib.rs

@@ -45,4 +45,7 @@ There are a few ground rules, though:
 The only note I have for now is to generate the README using `cargo-rdme`. The crate includes a configuration file for this tool, so it should just work.
 */
 
+#[cfg(doctest)]
+pub mod _doctests;
+
 pub use macros::Error;