[flang][preprocessor] Support \ as line continuation (#89970)

When prescanning a Fortran source file with preprocessing enabled in
free source form, interpret a line-ending backslash as a source line
continuation marker as a C preprocessor would. This usage isn't
completely portable, but it is supported by GNU Fortran and appears in
the source for FPM package manager.

Author: klausler

flang/docs/Preprocessing.md

@@ -93,6 +93,9 @@ local:
 * If a `#define` or `#undef` directive appears among continuation
   lines, it may or may not affect text in the continued statement that
   appeared before the directive.
+* A backslash at the end of a free form source line is a continuation
+  marker, with no space skipping or special handling of a leading `&`
+  on the next line.
 
 ## Behavior that few compilers properly support (or none), but should:
 

flang/include/flang/Parser/preprocessor.h

@@ -81,6 +81,7 @@ class Preprocessor {
   void Undefine(std::string macro);
   bool IsNameDefined(const CharBlock &);
   bool IsFunctionLikeDefinition(const CharBlock &);
+  bool AnyDefinitions() const { return !definitions_.empty(); }
 
   // When called with partialFunctionLikeMacro not null, MacroReplacement()
   // and ReplaceMacros() handle an unclosed function-like macro reference

flang/lib/Parser/prescan.cpp

@@ -29,15 +29,18 @@ Prescanner::Prescanner(Messages &messages, CookedSource &cooked,
     Preprocessor &preprocessor, common::LanguageFeatureControl lfc)
     : messages_{messages}, cooked_{cooked}, preprocessor_{preprocessor},
       allSources_{preprocessor_.allSources()}, features_{lfc},
+      backslashFreeFormContinuation_{preprocessor.AnyDefinitions()},
       encoding_{allSources_.encoding()} {}
 
 Prescanner::Prescanner(const Prescanner &that)
     : messages_{that.messages_}, cooked_{that.cooked_},
       preprocessor_{that.preprocessor_}, allSources_{that.allSources_},
-      features_{that.features_}, inFixedForm_{that.inFixedForm_},
+      features_{that.features_},
+      backslashFreeFormContinuation_{that.backslashFreeFormContinuation_},
+      inFixedForm_{that.inFixedForm_},
       fixedFormColumnLimit_{that.fixedFormColumnLimit_},
-      encoding_{that.encoding_}, prescannerNesting_{that.prescannerNesting_ +
-                                     1},
+      encoding_{that.encoding_},
+      prescannerNesting_{that.prescannerNesting_ + 1},
       skipLeadingAmpersand_{that.skipLeadingAmpersand_},
       compilerDirectiveBloomFilter_{that.compilerDirectiveBloomFilter_},
       compilerDirectiveSentinels_{that.compilerDirectiveSentinels_} {}
@@ -1226,9 +1229,14 @@ bool Prescanner::Continuation(bool mightNeedFixedFormSpace) {
     } else {
       return FreeFormContinuation();
     }
-  } else {
-    return false;
+  } else if (*at_ == '\\' && at_ + 2 == nextLine_ &&
+      backslashFreeFormContinuation_ && !inFixedForm_ && nextLine_ < limit_) {
+    // cpp-like handling of \ at end of a free form source line
+    BeginSourceLine(nextLine_);
+    NextLine();
+    return true;
   }
+  return false;
 }
 
 std::optional<Prescanner::LineClassification>

flang/lib/Parser/prescan.h

@@ -197,6 +197,7 @@ class Prescanner {
   Preprocessor &preprocessor_;
   AllSources &allSources_;
   common::LanguageFeatureControl features_;
+  bool backslashFreeFormContinuation_{false};
   bool inFixedForm_{false};
   int fixedFormColumnLimit_{72};
   Encoding encoding_{Encoding::UTF_8};

flang/test/Preprocessing/backslash-contin1.F90

@@ -0,0 +1,8 @@
+! RUN: %flang -E %s | FileCheck %s
+print *, \
+  "hello, \
+world"
+end
+!CHECK:      print *, "hello, world"
+!CHECK:      end
+