[clang-tools-extra] [clangd] Improve Markup Rendering (PR #140498)

[Maksim Ivanov via cfe-commits]

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@@ -151,16 +151,91 @@ bool needsLeadingEscape(char C, llvm::StringRef Before, llvm::StringRef After,
   }
 }
 
+/// \brief Tests whether \p C should be backslash-escaped in markdown.
+///
+/// The MarkupContent LSP specification defines that `markdown` content needs to
+/// follow GFM (GitHub Flavored Markdown) rules. And we can assume that markdown
+/// is rendered on the client side. This means we do not need to escape any
+/// markdown constructs.
+/// The only exception is when the client does not support HTML rendering in
+/// markdown. In that case, we need to escape HTML tags and HTML entities.
+///
+/// **FIXME:** handle the case when the client does support HTML rendering in
+/// markdown. For this, the LSP server needs to check the
+/// [supportsHtml
+/// capability](https://github.com/microsoft/language-server-protocol/issues/1344)
+/// of the client.
+///
+/// \param C The character to check.
+/// \param After The string that follows \p C .
+//  This is used to determine if \p C is part of a tag or an entity reference.
+/// \returns true if \p C should be escaped, false otherwise.
+bool needsLeadingEscapeMarkdown(char C, llvm::StringRef After) {
+  switch (C) {
+  case '<': // HTML tag (or autolink, which we choose not to escape)
+    return looksLikeTag(After);
+  case '&': { // HTML entity reference
+    auto End = After.find(';');
+    if (End == llvm::StringRef::npos)
+      return false;
+    llvm::StringRef Content = After.substr(0, End);
+    if (Content.consume_front("#")) {
+      if (Content.consume_front("x") || Content.consume_front("X"))
+        return llvm::all_of(Content, llvm::isHexDigit);
+      return llvm::all_of(Content, llvm::isDigit);
+    }
+    return llvm::all_of(Content, llvm::isAlpha);
+  }
+  default:
+    return false;
+  }
+}
+
+bool needsLeadingEscape(char C, llvm::StringRef Before, llvm::StringRef After,
+                        bool StartsLine, bool EscapeMarkdown) {
+  if (EscapeMarkdown)
+    return needsLeadingEscapePlaintext(C, Before, After, StartsLine);
+  return needsLeadingEscapeMarkdown(C, After);
+}
+
 /// Escape a markdown text block. Ensures the punctuation will not introduce
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emaxx-google wrote:

nit: Update the comment (e.g., the second sentence is only applicable when `EscapeMarkdown` is `true` if I understand correctly).

https://github.com/llvm/llvm-project/pull/140498