Merge #1044

1044: Restrict follow-up tokens on `expr` and `stmt` r=CohenArthur a=CohenArthur

This adds a base for respecting the [Macro Follow-Set Ambiguity specification](https://doc.rust-lang.org/reference/macro-ambiguity.html).

If the design is validated, adding more restrictions on other fragment specifiers should not be difficult

Addresses #947 

Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>

1 files changed, 141 insertions, 0 deletions

diff --git a/gcc/rust/parse/rust-parse.cc b/gcc/rust/parse/rust-parse.cc
index f995e4b..16ed4a0 100644
--- a/gcc/rust/parse/rust-parse.cc
+++ b/gcc/rust/parse/rust-parse.cc
@@ -92,4 +92,145 @@ extract_module_path (const AST::AttrVec &inner_attrs,
 
   return path;
 }
+
+static bool
+peculiar_fragment_match_compatible (AST::MacroMatchFragment &last_match,
+				    AST::MacroMatch &match)
+{
+  static std::unordered_map<AST::MacroFragSpec::Kind, std::vector<TokenId>>
+    follow_set = {
+      {AST::MacroFragSpec::EXPR, {MATCH_ARROW, COMMA, SEMICOLON}},
+      {AST::MacroFragSpec::STMT, {MATCH_ARROW, COMMA, SEMICOLON}},
+    };
+
+  Location error_locus = match.get_match_locus ();
+
+  // There are two behaviors to handle here: If the follow-up match is a token,
+  // we want to check if it is allowed.
+  // If it is a fragment, repetition or matcher then we know that it will be
+  // an error.
+  // For repetitions and matchers we want to extract a proper location to report
+  // the error.
+  switch (match.get_macro_match_type ())
+    {
+      case AST::MacroMatch::Tok: {
+	auto tok = static_cast<AST::Token *> (&match);
+	auto &allowed_toks
+	  = follow_set[last_match.get_frag_spec ().get_kind ()];
+	auto is_valid = std::find (allowed_toks.begin (), allowed_toks.end (),
+				   tok->get_id ())
+			!= allowed_toks.end ();
+	if (!is_valid)
+	  // FIXME: Add hint about allowed fragments
+	  rust_error_at (tok->get_match_locus (),
+			 "token %<%s%> is not allowed after %<%s%> fragment",
+			 tok->get_str ().c_str (),
+			 last_match.get_frag_spec ().as_string ().c_str ());
+	return is_valid;
+      }
+      break;
+      case AST::MacroMatch::Repetition: {
+	auto repetition = static_cast<AST::MacroMatchRepetition *> (&match);
+	auto &matches = repetition->get_matches ();
+	if (!matches.empty ())
+	  error_locus = matches.front ()->get_match_locus ();
+	break;
+      }
+      case AST::MacroMatch::Matcher: {
+	auto matcher = static_cast<AST::MacroMatcher *> (&match);
+	auto &matches = matcher->get_matches ();
+	if (!matches.empty ())
+	  error_locus = matches.front ()->get_match_locus ();
+	break;
+      }
+    default:
+      break;
+    }
+
+  rust_error_at (error_locus, "fragment not allowed after %<%s%> fragment",
+		 last_match.get_frag_spec ().as_string ().c_str ());
+
+  return false;
+}
+
+/**
+ * Avoid UB by calling .front() and .back() on empty containers...
+ */
+
+template <typename T>
+static T *
+get_back_ptr (std::vector<std::unique_ptr<T>> &values)
+{
+  if (values.empty ())
+    return nullptr;
+
+  return values.back ().get ();
+}
+
+template <typename T>
+static T *
+get_front_ptr (std::vector<std::unique_ptr<T>> &values)
+{
+  if (values.empty ())
+    return nullptr;
+
+  return values.front ().get ();
+}
+
+bool
+is_match_compatible (AST::MacroMatch &last_match, AST::MacroMatch &match)
+{
+  AST::MacroMatch *new_last = nullptr;
+
+  // We want to "extract" the concerning matches. In cases such as matchers and
+  // repetitions, we actually store multiple matchers, but are only concerned
+  // about the follow-set ambiguities of certain elements.
+  // There are some cases where we can short-circuit the algorithm: There will
+  // never be restrictions on token literals, or on certain fragments which do
+  // not have a set of follow-restrictions.
+
+  switch (last_match.get_macro_match_type ())
+    {
+      // This is our main stop condition: When we are finally looking at the
+      // last match (or its actual last component), and it is a fragment, it
+      // may contain some follow up restrictions.
+      case AST::MacroMatch::Fragment: {
+	auto fragment = static_cast<AST::MacroMatchFragment *> (&last_match);
+	if (fragment->get_frag_spec ().has_follow_set_restrictions ())
+	  return peculiar_fragment_match_compatible (*fragment, match);
+	else
+	  return true;
+      }
+      case AST::MacroMatch::Repetition: {
+	// A repetition on the left hand side means we want to make sure the
+	// last match of the repetition is compatible with the new match
+	auto repetition
+	  = static_cast<AST::MacroMatchRepetition *> (&last_match);
+	new_last = get_back_ptr (repetition->get_matches ());
+	// If there are no matches in the matcher, then it can be followed by
+	// anything
+	if (!new_last)
+	  return true;
+	break;
+      }
+      case AST::MacroMatch::Matcher: {
+	// Likewise for another matcher
+	auto matcher = static_cast<AST::MacroMatcher *> (&last_match);
+	new_last = get_back_ptr (matcher->get_matches ());
+	// If there are no matches in the matcher, then it can be followed by
+	// anything
+	if (!new_last)
+	  return true;
+	break;
+      }
+    case AST::MacroMatch::Tok:
+      return true;
+    }
+
+  rust_assert (new_last);
+
+  // We check recursively until we find a terminating condition
+  // FIXME: Does expansion depth/limit matter here?
+  return is_match_compatible (*new_last, match);
+}
 } // namespace Rust