Avoid stringifying and reparsing a TokenStream

This PR changes removes uses of the pattern `syn::parse_str(tokens.to_string())`,
which loses `Span` information. In addition to making error messages
less precise, it can break code that depends on the hygiene information
carried by `Span`. Instead, `syn::parse_quote` is used to parse a
`TokenStream` into the desired AST struct.

I've added a test which demonstrates one of the problems with
the previous code - using `$crate` would previously cause
a panic, but now compiles successfully.

src/utils.rs

@@ -3,7 +3,7 @@
 use proc_macro2::{Span, TokenStream};
 use quote::{quote, ToTokens};
 use syn::{
-    parse_str, punctuated::Punctuated, spanned::Spanned, Attribute, Data, DeriveInput,
+    parse_quote, punctuated::Punctuated, spanned::Spanned, Attribute, Data, DeriveInput,
     Error, Field, Fields, FieldsNamed, FieldsUnnamed, GenericParam, Generics, Ident,
     ImplGenerics, Index, Meta, NestedMeta, Result, Token, Type, TypeGenerics,
     TypeParamBound, Variant, WhereClause,
@@ -121,10 +121,9 @@ pub fn add_extra_type_param_bound_op_output<'a>(
     let mut generics = generics.clone();
     for type_param in &mut generics.type_params_mut() {
         let type_ident = &type_param.ident;
-        let bound: TypeParamBound = parse_str(
-            &quote!(::core::ops::#trait_ident<Output=#type_ident>).to_string(),
-        )
-        .unwrap();
+        let bound: TypeParamBound = parse_quote! {
+            ::core::ops::#trait_ident<Output=#type_ident>
+        };
         type_param.bounds.push(bound)
     }
 
@@ -143,7 +142,7 @@ pub fn add_extra_ty_param_bound<'a>(
     bound: &'a TokenStream,
 ) -> Generics {
     let mut generics = generics.clone();
-    let bound: TypeParamBound = parse_str(&bound.to_string()).unwrap();
+    let bound: TypeParamBound = parse_quote! { #bound };
     for type_param in &mut generics.type_params_mut() {
         type_param.bounds.push(bound.clone())
     }
@@ -176,7 +175,7 @@ pub fn add_extra_generic_param(
     generics: &Generics,
     generic_param: TokenStream,
 ) -> Generics {
-    let generic_param: GenericParam = parse_str(&generic_param.to_string()).unwrap();
+    let generic_param: GenericParam = parse_quote! { #generic_param };
     let mut generics = generics.clone();
     generics.params.push(generic_param);
 
@@ -187,8 +186,7 @@ pub fn add_extra_where_clauses(
     generics: &Generics,
     type_where_clauses: TokenStream,
 ) -> Generics {
-    let mut type_where_clauses: WhereClause =
-        parse_str(&type_where_clauses.to_string()).unwrap();
+    let mut type_where_clauses: WhereClause = parse_quote! { #type_where_clauses };
     let mut new_generics = generics.clone();
     if let Some(old_where) = new_generics.where_clause {
         type_where_clauses.predicates.extend(old_where.predicates)

tests/lib.rs

@@ -153,6 +153,20 @@ struct DoubleUIntStruct {
 #[derive(From, Into, Constructor)]
 struct Unit;
 
+// Tests that we can forward to a path
+// containing `$crate`
+macro_rules! use_dollar_crate {
+    () => {
+        struct Foo;
+        #[derive(From)]
+        enum Bar {
+            First(#[from(forward)] $crate::Foo)
+        }
+    }
+}
+
+use_dollar_crate!();
+
 #[test]
 fn main() {
     let mut myint: MyInt = 5.into();