[clang] [Clang] Warn on backslash-newline-EOF (PR #97585)

[Mital Ashok via cfe-commits]

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@@ -3183,8 +3193,35 @@ bool Lexer::LexEndOfFile(Token &Result, const char *CurPtr) {
       DiagID = diag::ext_no_newline_eof;
     }
 
-    Diag(BufferEnd, DiagID)
-      << FixItHint::CreateInsertion(EndLoc, "\n");
+    if (LastNewline.empty()) {
+      Diag(BufferEnd, DiagID) << FixItHint::CreateInsertion(EndLoc, "\n");
+    } else {
+      // While the file physically ends in a newline, the previous
+      // line might have ended in a splice, so it would be deleted
+      StringRef WithoutLastNewline =
+          StringRef(BufferStart, LastNewline.data() - BufferStart);
+      while (!WithoutLastNewline.empty()) {
+        if (isHorizontalWhitespace(WithoutLastNewline.back())) {
+          WithoutLastNewline = WithoutLastNewline.drop_back();
+        } else {
+          break;
+        }
+      }
+
+      if (WithoutLastNewline.ends_with('\\') ||
+          (LangOpts.Trigraphs && WithoutLastNewline.ends_with("??"
+                                                              "/"))) {
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MitalAshok wrote:

That gives a "warning: trigraph ignored [-Wtrigraphs]" warning when compiling this

clang-format refuses to put concatenated string literals on the same line.

I've changed this and added a comment, but let me know if this could be done in a cleaner way

https://github.com/llvm/llvm-project/pull/97585