[llvm] [Support][YamlTraits] Add quoting for keys in textual YAML representation (PR #88763)

[Matt Arsenault via llvm-commits]

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@@ -801,6 +769,45 @@ void Output::output(StringRef s) {
   Out << s;
 }
 
+void Output::output(StringRef S, QuotingType MustQuote) {
+  if (MustQuote == QuotingType::None) {
+    // Only quote if we must.
+    output(S);
+    return;
+  }
+
+  const char *const Quote = MustQuote == QuotingType::Single ? "'" : "\"";
+  output(Quote); // Starting quote.
+
+  // When using double-quoted strings (and only in that case), non-printable
+  // characters may be present, and will be escaped using a variety of
+  // unicode-scalar and special short-form escapes. This is handled in
+  // yaml::escape.
+  if (MustQuote == QuotingType::Double) {
+    output(yaml::escape(S, /* EscapePrintable= */ false));
+    output(Quote);
+    return;
+  }
+
+  unsigned i = 0;
+  unsigned j = 0;
+  unsigned End = S.size();
+  const char *Base = S.data();
+
+  // When using single-quoted strings, any single quote ' must be doubled to be
+  // escaped.
+  while (j < End) {
+    if (S[j] == '\'') {                   // Escape quotes.
+      output(StringRef(&Base[i], j - i)); // "flush".
+      output(StringLiteral("''"));        // Print it as ''
+      i = j + 1;
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arsenm wrote:

Can you use a StringRef with take_back incrementing? (I guess this is just moving code and shouldn't change here) 

https://github.com/llvm/llvm-project/pull/88763