[clangd] Change line break behaviour for hoverinfo

`parseDocumentation` retains hard line breaks and removes soft line
breaks inside documentation comments.
Wether a line break is hard or soft is determined by the following rules
(some of which have been discussed in
https://github.com/clangd/clangd/issues/95):

Line breaks that are preceded by a punctuation are retained
Line breaks that are followed by "interesting characters" (e.g. Markdown
syntax, doxygen commands) are retained
All other line breaks are removed

Related issue: https://github.com/clangd/clangd/issues/95

Differential Revision: https://reviews.llvm.org/D76094

clang-tools-extra/clangd/Hover.cpp

@@ -520,6 +520,49 @@ llvm::Optional<HoverInfo> getHoverContents(const Expr *E, ParsedAST &AST) {
   }
   return llvm::None;
 }
+
+bool isParagraphLineBreak(llvm::StringRef Str, size_t LineBreakIndex) {
+  return Str.substr(LineBreakIndex + 1)
+      .drop_while([](auto C) { return C == ' ' || C == '\t'; })
+      .startswith("\n");
+};
+
+bool isPunctuationLineBreak(llvm::StringRef Str, size_t LineBreakIndex) {
+  constexpr llvm::StringLiteral Punctuation = R"txt(.:,;!?)txt";
+
+  return LineBreakIndex > 0 && Punctuation.contains(Str[LineBreakIndex - 1]);
+};
+
+bool isFollowedByHardLineBreakIndicator(llvm::StringRef Str,
+                                        size_t LineBreakIndex) {
+  // '-'/'*' md list, '@'/'\' documentation command, '>' md blockquote,
+  // '#' headings, '`' code blocks
+  constexpr llvm::StringLiteral LinbreakIdenticators = R"txt(-*@\>#`)txt";
+
+  auto NextNonSpaceCharIndex = Str.find_first_not_of(' ', LineBreakIndex + 1);
+
+  if (NextNonSpaceCharIndex == llvm::StringRef::npos) {
+    return false;
+  }
+
+  auto FollowedBySingleCharIndicator =
+      LinbreakIdenticators.find(Str[NextNonSpaceCharIndex]) !=
+      llvm::StringRef::npos;
+
+  auto FollowedByNumberedListIndicator =
+      llvm::isDigit(Str[NextNonSpaceCharIndex]) &&
+      NextNonSpaceCharIndex + 1 < Str.size() &&
+      (Str[NextNonSpaceCharIndex + 1] == '.' ||
+       Str[NextNonSpaceCharIndex + 1] == ')');
+
+  return FollowedBySingleCharIndicator || FollowedByNumberedListIndicator;
+};
+
+bool isHardLineBreak(llvm::StringRef Str, size_t LineBreakIndex) {
+  return isPunctuationLineBreak(Str, LineBreakIndex) ||
+         isFollowedByHardLineBreakIndicator(Str, LineBreakIndex);
+}
+
 } // namespace
 
 llvm::Optional<HoverInfo> getHover(ParsedAST &AST, Position Pos,
@@ -652,7 +695,7 @@ markup::Document HoverInfo::present() const {
   }
 
   if (!Documentation.empty())
-    Output.addParagraph().appendText(Documentation);
+    parseDocumentation(Documentation, Output);
 
   if (!Definition.empty()) {
     Output.addRuler();
@@ -675,6 +718,45 @@ markup::Document HoverInfo::present() const {
   return Output;
 }
 
+void parseDocumentation(llvm::StringRef Input, markup::Document &Output) {
+
+  constexpr auto WhiteSpaceChars = "\t\n\v\f\r ";
+
+  auto TrimmedInput = Input.trim();
+
+  std::string CurrentLine;
+
+  for (size_t CharIndex = 0; CharIndex < TrimmedInput.size();) {
+    if (TrimmedInput[CharIndex] == '\n') {
+      // Trim whitespace infront of linebreak
+      const auto LastNonSpaceCharIndex =
+          CurrentLine.find_last_not_of(WhiteSpaceChars) + 1;
+      CurrentLine.erase(LastNonSpaceCharIndex);
+
+      if (isParagraphLineBreak(TrimmedInput, CharIndex) ||
+          isHardLineBreak(TrimmedInput, CharIndex)) {
+        // FIXME: maybe distinguish between line breaks and paragraphs
+        Output.addParagraph().appendText(CurrentLine);
+        CurrentLine = "";
+      } else {
+        // Ommit linebreak
+        CurrentLine += ' ';
+      }
+
+      CharIndex++;
+      // After a linebreak always remove spaces to avoid 4 space markdown code
+      // blocks, also skip all additional linebreaks since they have no effect
+      CharIndex = TrimmedInput.find_first_not_of(WhiteSpaceChars, CharIndex);
+    } else {
+      CurrentLine += TrimmedInput[CharIndex];
+      CharIndex++;
+    }
+  }
+  if (!CurrentLine.empty()) {
+    Output.addParagraph().appendText(CurrentLine);
+  }
+}
+
 llvm::raw_ostream &operator<<(llvm::raw_ostream &OS,
                               const HoverInfo::Param &P) {
   std::vector<llvm::StringRef> Output;

clang-tools-extra/clangd/Hover.h

@@ -74,6 +74,10 @@ struct HoverInfo {
   /// Produce a user-readable information.
   markup::Document present() const;
 };
+
+// Try to infer structure of a documentation comment (e.g. line breaks).
+void parseDocumentation(llvm::StringRef Input, markup::Document &Output);
+
 llvm::raw_ostream &operator<<(llvm::raw_ostream &, const HoverInfo::Param &);
 inline bool operator==(const HoverInfo::Param &LHS,
                        const HoverInfo::Param &RHS) {

clang-tools-extra/clangd/unittests/HoverTests.cpp

@@ -1883,6 +1883,71 @@ def)",
   }
 }
 
+TEST(Hover, DocCommentLineBreakConversion) {
+  struct Case {
+    llvm::StringRef Documentation;
+    llvm::StringRef ExpectedRenderMarkdown;
+    llvm::StringRef ExpectedRenderPlainText;
+  } Cases[] = {{
+                   " \n foo\nbar",
+                   "foo bar",
+                   "foo bar",
+               },
+               {
+                   "foo\nbar \n  ",
+                   "foo bar",
+                   "foo bar",
+               },
+               {
+                   "foo  \nbar",
+                   "foo bar",
+                   "foo bar",
+               },
+               {
+                   "foo    \nbar",
+                   "foo bar",
+                   "foo bar",
+               },
+               {
+                   "foo\n\n\nbar",
+                   "foo  \nbar",
+                   "foo\nbar",
+               },
+               {
+                   "foo\n\n\n\tbar",
+                   "foo  \nbar",
+                   "foo\nbar",
+               },
+               {
+                   "foo\n\n\n bar",
+                   "foo  \nbar",
+                   "foo\nbar",
+               },
+               {
+                   "foo.\nbar",
+                   "foo.  \nbar",
+                   "foo.\nbar",
+               },
+               {
+                   "foo\n*bar",
+                   "foo  \n\\*bar",
+                   "foo\n*bar",
+               },
+               {
+                   "foo\nbar",
+                   "foo bar",
+                   "foo bar",
+               }};
+
+  for (const auto &C : Cases) {
+    markup::Document Output;
+    parseDocumentation(C.Documentation, Output);
+
+    EXPECT_EQ(Output.asMarkdown(), C.ExpectedRenderMarkdown);
+    EXPECT_EQ(Output.asPlainText(), C.ExpectedRenderPlainText);
+  }
+}
+
 // This is a separate test as headings don't create any differences in plaintext
 // mode.
 TEST(Hover, PresentHeadings) {