[flang] Warn about more continuation lines than the standard permits

[klausler]

Summary

f18 doesn't have any limit on continuation lines in fixed or free form
source (other than available memory), but the standard does. Emit
a portability warning when it is exceeded.

[clementval]

LGTM

flang/docs/Extensions.md

* Legacy names `AND`, `OR`, and `XOR` are accepted as aliases for
   respectively.
 * A digit count of d=0 is accepted in Ew.0, Dw.0, and Gw.0 output
   editing if no nonzero scale factor (kP) is in effect.
 * The name `IMAG` is accepted as an alias for the generic intrinsic
   function `AIMAG`.
 * The legacy extension intrinsic functions `IZEXT` and `JZEXT`
   are supported; `ZEXT` has different behavior with various older
   compilers, so it is not supported.
+* f18 doesn't impose a limit on the number of continuation lines
+  allowed for a single statement.
 
 ### Extensions supported when enabled by options
 
 * C-style backslash escape sequences in quoted CHARACTER literals
   (but not Hollerith) [-fbackslash]
 * Logical abbreviations `.T.`, `.F.`, `.N.`, `.A.`, `.O.`, and `.X.`
   [-flogical-abbreviations]
 * `.XOR.` as a synonym for `.NEQV.` [-fxor-operator]

flang/lib/Parser/prescan.h

void BeginSourceLineAndAdvance() {
     NextLine();
   }
 
   void BeginStatementAndAdvance() {
     BeginSourceLineAndAdvance();
     slashInCurrentStatement_ = false;
     preventHollerith_ = false;
     delimiterNesting_ = 0;
+    continuationLines_ = 0;
   }
 
   Provenance GetProvenance(const char *sourceChar) const {
     return startProvenance_ + (sourceChar - start_);
   }
 
   ProvenanceRange GetProvenanceRange(
       const char *first, const char *afterLast) const {
private:
   Preprocessor &preprocessor_;
   AllSources &allSources_;
   common::LanguageFeatureControl features_;
   bool inFixedForm_{false};
   int fixedFormColumnLimit_{72};
   Encoding encoding_{Encoding::UTF_8};
   int delimiterNesting_{0};
   int prescannerNesting_{0};
+  int continuationLines_{0};
 
   Provenance startProvenance_;
   const char *start_{nullptr}; // beginning of current source file content
   const char *limit_{nullptr}; // first address after end of current source
   const char *nextLine_{nullptr}; // next line to process; <= limit_
   const char *directiveSentinel_{nullptr}; // current compiler directive
 
   // This data members are state for processing the source line containing

flang/lib/Parser/prescan.cpp

if (inFixedForm_ != beganInFixedForm) {
     dir += '\n';
     TokenSequence tokens{dir, allSources_.AddCompilerInsertion(dir).start()};
     tokens.Emit(cooked_);
   }
 }
 
 void Prescanner::Statement() {
   TokenSequence tokens;
-  LineClassification line{ClassifyLine(nextLine_)};
+  const char *statementStart{nextLine_};
+  LineClassification line{ClassifyLine(statementStart)};
   switch (line.kind) {
   case LineClassification::Kind::Comment:
     nextLine_ += line.payloadOffset; // advance to '!' or newline
     NextLine();
     return;
   case LineClassification::Kind::IncludeLine:
     FortranInclude(nextLine_ + line.payloadOffset);
     NextLine();
case LineClassification::Kind::Source:
     } else {
       SkipSpaces();
     }
     break;
   }
 
   while (NextToken(tokens)) {
   }
+  if (continuationLines_ > 255) {
+    Say(GetProvenance(statementStart),
+        "%d continuation lines is more than the Fortran standard allows"_port_en_US,
+        continuationLines_);
+  }
 
   Provenance newlineProvenance{GetCurrentProvenance()};
   if (std::optional<TokenSequence> preprocessed{
           preprocessor_.MacroReplacement(tokens, *this)}) {
     // Reprocess the preprocessed line.  Append a newline temporarily.
     preprocessed->PutNextTokenChar('\n', newlineProvenance);
     preprocessed->CloseToken();
     const char *ppd{preprocessed->ToCharBlock().begin()};
if (outCol == 1) { // empty label field
     // a leading 'C' or 'D' won't be left-justified and then accidentally
     // misinterpreted as a comment card.
     EmitChar(token, ' ');
     ++outCol;
   }
   token.CloseToken();
   SkipToNextSignificantCharacter();
   if (IsDecimalDigit(*at_)) {
-    Say(GetProvenance(at_),
+    Say(GetCurrentProvenance(),
         "Label digit is not in fixed-form label field"_port_en_US);
   }
 }
 
 // 6.3.3.5: A program unit END statement, or any other statement whose
 // initial line resembles an END statement, shall not be continued in
 // fixed form source.
 void Prescanner::EnforceStupidEndStatementRules(const TokenSequence &tokens) {
void Prescanner::SkipToNextSignificantCharacter() {
   } else {
     bool mightNeedSpace{false};
     if (MustSkipToEndOfLine()) {
       SkipToEndOfLine();
     } else {
       mightNeedSpace = *at_ == '\n';
     }
     for (; Continuation(mightNeedSpace); mightNeedSpace = false) {
+      ++continuationLines_;
       if (MustSkipToEndOfLine()) {
         SkipToEndOfLine();
       }
     }
     if (*at_ == '\t') {
       tabInCurrentLine_ = true;
     }
   }
bool Prescanner::NextToken(TokenSequence &tokens) {
   CHECK(at_ >= start_ && at_ < limit_);
   if (InFixedFormSource()) {
     SkipSpaces();
   } else {
     if (*at_ == '/' && IsCComment(at_)) {
       // Recognize and skip over classic C style /*comments*/ when
       // outside a character literal.
       if (features_.ShouldWarn(LanguageFeature::ClassicCComments)) {
-        Say(GetProvenance(at_),
+        Say(GetCurrentProvenance(),
             "nonstandard usage: C-style comment"_port_en_US);
       }
       SkipCComments();
     }
     if (*at_ == ' ' || *at_ == '\t') {
       // Compress free-form white space into a single space character.
       const auto theSpace{at_};
       char previous{at_ <= start_ ? ' ' : at_[-1]};

flang/test/Parser/excessive-continuations.f90

+! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
+! CHECK: portability: 256 continuation lines is more than the Fortran standard allows
+! CHECK: LOGICAL, PARAMETER :: c255 = .true._4
+program test
+  logical, parameter :: c255 = 255 == len("&
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+end