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Document proc macro token tree indices
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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 <[email protected]>
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P-E-P authored and philberty committed Oct 31, 2023
1 parent 4ac19a0 commit 5d622b6
Showing 1 changed file with 23 additions and 4 deletions.
27 changes: 23 additions & 4 deletions gcc/rust/backend/rust-compile-base.cc
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand All @@ -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
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