gccrs: Document proc macro token tree indices

Multiple references to procedural macro token trees were left as magic
number in the code. This commit introduces some constexpr for those along
some explanation for the selected value.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc (get_attributes): Add documentation for
	indices 3 and 4.
	(get_trait_name): Add documentation for index 1.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

1 files changed, 23 insertions, 4 deletions

diff --git a/gcc/rust/backend/rust-compile-base.cc b/gcc/rust/backend/rust-compile-base.cc
index fe6d5e3..b4a3685 100644
--- a/gcc/rust/backend/rust-compile-base.cc
+++ b/gcc/rust/backend/rust-compile-base.cc
@@ -165,12 +165,24 @@ get_attributes (const AST::Attribute &attr)
 
   // TODO: Should we rely on fixed index ? Should we search for the
   // attribute tokentree instead ?
-  if (tt.get_token_trees ().size () > 3)
+
+  // Derive proc macros have the following format:
+  // #[proc_macro_derive(TraitName, attributes(attr1, attr2, attr3))]
+  //                    -~~~~~~~~ - ~~~~~~~~~~---------------------
+  //                    ^0  ^1    ^2     ^3           ^4
+  // - "attributes" is stored at position 3 in the token tree
+  // - attribute are stored in the delimited token tree in position 4
+  constexpr size_t attr_kw_pos = 3;
+  constexpr size_t attribute_list_pos = 4;
+
+  if (tt.get_token_trees ().size () > attr_kw_pos)
     {
-      rust_assert (tt.get_token_trees ()[3]->as_string () == "attributes");
+      rust_assert (tt.get_token_trees ()[attr_kw_pos]->as_string ()
+		   == "attributes");
 
       auto attributes = static_cast<const AST::DelimTokenTree *> (
-	tt.get_token_trees ()[4].get ());
+	tt.get_token_trees ()[attribute_list_pos].get ());
+
       auto &token_trees = attributes->get_token_trees ();
 
       for (auto i = token_trees.cbegin () + 1; // Skip opening parenthesis
@@ -186,11 +198,18 @@ get_attributes (const AST::Attribute &attr)
 static std::string
 get_trait_name (const AST::Attribute &attr)
 {
+  // Derive proc macros have the following format:
+  // #[proc_macro_derive(TraitName, attributes(attr1, attr2, attr3))]
+  //                    -~~~~~~~~ - ~~~~~~~~~~---------------------
+  //                    ^0  ^1    ^2     ^3           ^4
+  // - The trait name is stored at position 1
+  constexpr size_t trait_name_pos = 1;
+
   rust_assert (attr.get_attr_input ().get_attr_input_type ()
 	       == Rust::AST::AttrInput::TOKEN_TREE);
   const auto &tt
     = static_cast<const AST::DelimTokenTree &> (attr.get_attr_input ());
-  return tt.get_token_trees ()[1]->as_string ();
+  return tt.get_token_trees ()[trait_name_pos]->as_string ();
 }
 
 void