[clang] [C2y] Implement WG14 N3411 (PR #130180)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman created https://github.com/llvm/llvm-project/pull/130180

This paper (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3411.pdf) allows a source file to end without a newline. Clang has supported this as a conforming extension for a long time, so this suppresses the diagnotic in C2y mode but continues to diagnose as an extension in earlier language modes. It also continues to diagnose if the user passes -Wnewline-eof explicitly.

>From 517b2b64e8dc67cf33b5e1aaef8d17c567b6b5d7 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Thu, 6 Mar 2025 16:43:38 -0500
Subject: [PATCH] [C2y] Implement WG14 N3411

This paper allows a source file to end without a newline. Clang has
supported this as a conforming extension for a long time, so this
suppresses the diagnotic in C2y mode but continues to diagnose as an
extension in earlier language modes. It also continues to diagnose if
the user passes -Wnewline-eof explicitly.
---
 clang/docs/ReleaseNotes.rst |  3 +++
 clang/lib/Lex/Lexer.cpp     | 15 +++++++--------
 clang/test/C/C2y/n3411.c    | 14 ++++++++++++++
 clang/www/c_status.html     |  2 +-
 4 files changed, 25 insertions(+), 9 deletions(-)
 create mode 100644 clang/test/C/C2y/n3411.c

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 38b5bdf1dd46b..36aeb6c736627 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -108,6 +108,9 @@ C Language Changes
 
 C2y Feature Support
 ^^^^^^^^^^^^^^^^^^^
+- Implemented N3411 which allows a source file to not end with a newline
+  character. This is still reported as a conforming extension in earlier
+  language modes.
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
index 087c6f13aea66..c62a9f5041183 100644
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -3192,23 +3192,22 @@ bool Lexer::LexEndOfFile(Token &Result, const char *CurPtr) {
   if (CurPtr != BufferStart && (CurPtr[-1] != '\n' && CurPtr[-1] != '\r')) {
     DiagnosticsEngine &Diags = PP->getDiagnostics();
     SourceLocation EndLoc = getSourceLocation(BufferEnd);
-    unsigned DiagID;
+    unsigned DiagID = diag::warn_no_newline_eof;
 
     if (LangOpts.CPlusPlus11) {
       // C++11 [lex.phases] 2.2 p2
       // Prefer the C++98 pedantic compatibility warning over the generic,
       // non-extension, user-requested "missing newline at EOF" warning.
-      if (!Diags.isIgnored(diag::warn_cxx98_compat_no_newline_eof, EndLoc)) {
+      if (!Diags.isIgnored(diag::warn_cxx98_compat_no_newline_eof, EndLoc))
         DiagID = diag::warn_cxx98_compat_no_newline_eof;
-      } else {
-        DiagID = diag::warn_no_newline_eof;
-      }
     } else {
-      DiagID = diag::ext_no_newline_eof;
+      // This is conforming in C2y, but is an extension in earlier language
+      // modes.
+      if (!LangOpts.C2y)
+        DiagID = diag::ext_no_newline_eof;
     }
 
-    Diag(BufferEnd, DiagID)
-      << FixItHint::CreateInsertion(EndLoc, "\n");
+    Diag(BufferEnd, DiagID) << FixItHint::CreateInsertion(EndLoc, "\n");
   }
 
   BufferPtr = CurPtr;
diff --git a/clang/test/C/C2y/n3411.c b/clang/test/C/C2y/n3411.c
new file mode 100644
index 0000000000000..edfc1c16fe365
--- /dev/null
+++ b/clang/test/C/C2y/n3411.c
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -verify=c2y -std=c2y -Wall -pedantic %s
+// RUN: %clang_cc1 -verify -Wnewline-eof -std=c2y -Wall -pedantic %s
+// RUN: %clang_cc1 -verify -std=c23 -Wall -pedantic %s
+
+/* WG14 N3411: Yes
+ * Slay Some Earthly Demons XII
+ *
+ * Allow a non-empty source file to end without a final newline character. Note
+ * that this file intentionally does not end with a trailing newline.
+ */
+// c2y-no-diagnostics
+
+int x; // Ensure the file contains at least one declaration.
+// expected-warning {{no newline at end of file}}
\ No newline at end of file
diff --git a/clang/www/c_status.html b/clang/www/c_status.html
index 4e912987b69c6..e8b50a37093f9 100644
--- a/clang/www/c_status.html
+++ b/clang/www/c_status.html
@@ -294,7 +294,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Slay Some Earthly Demons XII</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3411.pdf">N3411</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="unreleased" align="center">Clang 21</td>
 	</tr>
     <tr>
       <td>Slay Some Earthly Demons XIII</td>