[flang] Improve presentation of errors after last source line (llvm#161391)

We don't emit source file names or line numbers for error messages at
EOF. Detect these and handle them a little better, pointing at the
newline at the end of the last source line instead.

Author: klausler

flang/include/flang/Parser/message.h

@@ -65,6 +65,8 @@ class MessageFixedText {
     return severity_ == Severity::Error || severity_ == Severity::Todo;
   }
 
+  static const MessageFixedText endOfFileMessage; // "end of file"_err_en_US
+
 private:
   CharBlock text_;
   Severity severity_{Severity::None};

flang/lib/Parser/basic-parsers.h

@@ -828,7 +828,7 @@ struct NextCh {
     if (std::optional<const char *> result{state.GetNextChar()}) {
       return result;
     }
-    state.Say("end of file"_err_en_US);
+    state.Say(MessageFixedText::endOfFileMessage);
     return std::nullopt;
   }
 };

flang/lib/Parser/message.cpp

@@ -21,6 +21,10 @@
 
 namespace Fortran::parser {
 
+// The nextCh parser emits this, and Message::GetProvenanceRange() looks for it.
+const MessageFixedText MessageFixedText::endOfFileMessage{
+    "end of file"_err_en_US};
+
 llvm::raw_ostream &operator<<(llvm::raw_ostream &o, const MessageFixedText &t) {
   std::size_t n{t.text().size()};
   for (std::size_t j{0}; j < n; ++j) {
@@ -232,7 +236,20 @@ std::optional<ProvenanceRange> Message::GetProvenanceRange(
     const AllCookedSources &allCooked) const {
   return common::visit(
       common::visitors{
-          [&](CharBlock cb) { return allCooked.GetProvenanceRange(cb); },
+          [&](CharBlock cb) -> std::optional<ProvenanceRange> {
+            if (auto pr{allCooked.GetProvenanceRange(cb)}) {
+              return pr;
+            } else if (const auto *fixed{std::get_if<MessageFixedText>(&text_)};
+                fixed &&
+                fixed->text() == MessageFixedText::endOfFileMessage.text() &&
+                cb.begin() && cb.size() == 1) {
+              // Failure from "nextCh" due to reaching EOF.  Back up one byte
+              // to the terminal newline so that the output looks better.
+              return allCooked.GetProvenanceRange(CharBlock{cb.begin() - 1, 1});
+            } else {
+              return std::nullopt;
+            }
+          },
           [](const ProvenanceRange &pr) { return std::make_optional(pr); },
       },
       location_);

flang/test/Parser/recovery08.f90

@@ -0,0 +1,11 @@
+! RUN: not %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+! CHECK: error: end of file
+! CHECK: ^
+! CHECK: in the context: END PROGRAM statement
+! CHECK: in the context: main program
+
+  integer :: i
+
+  ! Add empty lines for emphasis
+
+  i = 5