[clang-format] allow short function body on a single line #151428

brandb97 · 2025-07-31T00:16:37Z

Fix issue #145161

llvmbot · 2025-07-31T00:17:08Z

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-format

Author: Lidong Yan (brandb97)

Changes

Fix issue #145161

Full diff: https://github.com/llvm/llvm-project/pull/151428.diff

4 Files Affected:

(modified) clang/include/clang/Format/Format.h (+6)
(modified) clang/lib/Format/Format.cpp (+3)
(modified) clang/lib/Format/UnwrappedLineFormatter.cpp (+20)
(modified) clang/lib/Format/UnwrappedLineParser.cpp (+4)

diff --git a/clang/include/clang/Format/Format.h b/clang/include/clang/Format/Format.h
index 31582a40de866..e13df9df83e18 100644
--- a/clang/include/clang/Format/Format.h
+++ b/clang/include/clang/Format/Format.h
@@ -878,6 +878,12 @@ struct FormatStyle {
   /// \version 3.5
   ShortFunctionStyle AllowShortFunctionsOnASingleLine;
 
+  /// Dependent on the value, function body like ``{ return 0; }`` can be
+  /// put on a single line. Only when AllowShortFunctionsOnASingleLine = None
+  /// and AllowShortBlocksOnASingleLine != Never, the value of this option
+  /// is true.
+  bool AllowShortFunctionBodiesOnASingleLine;
+
   /// Different styles for handling short if statements.
   enum ShortIfStyle : int8_t {
     /// Never put short ifs on the same line.
diff --git a/clang/lib/Format/Format.cpp b/clang/lib/Format/Format.cpp
index 063780721423f..61c0d399ca796 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -3828,6 +3828,9 @@ reformat(const FormatStyle &Style, StringRef Code,
   default:
     break;
   }
+  Expanded.AllowShortFunctionBodiesOnASingleLine =
+      Expanded.AllowShortFunctionsOnASingleLine == FormatStyle::SFS_None &&
+      Expanded.AllowShortBlocksOnASingleLine != FormatStyle::SBS_Never;
 
   if (Expanded.DisableFormat)
     return {tooling::Replacements(), 0};
diff --git a/clang/lib/Format/UnwrappedLineFormatter.cpp b/clang/lib/Format/UnwrappedLineFormatter.cpp
index 0adf7ee9ed543..44c767a0db9b8 100644
--- a/clang/lib/Format/UnwrappedLineFormatter.cpp
+++ b/clang/lib/Format/UnwrappedLineFormatter.cpp
@@ -257,6 +257,12 @@ class LineJoiner {
       return tryMergeSimpleBlock(I, E, Limit);
     }
 
+    if (TheLine->Last->is(TT_FunctionLBrace) &&
+        TheLine->First == TheLine->Last &&
+        Style.AllowShortFunctionBodiesOnASingleLine) {
+      return tryMergeSimpleBlock(I, E, Limit);
+    }
+
     const auto *PreviousLine = I != AnnotatedLines.begin() ? I[-1] : nullptr;
     // Handle empty record blocks where the brace has already been wrapped.
     if (PreviousLine && TheLine->Last->is(tok::l_brace) &&
@@ -532,6 +538,20 @@ class LineJoiner {
       }
       return MergedLines;
     }
+
+    // Previously, UnwrappedLineParser would move the left brace to a new line
+    // when AllowShortFunctionBodiesOnASingleLine is enabled. However, if the
+    // function body cannot fit on a single line, and Style.BraceWrapping.AfterFunction
+    // is false, we should merge the function name and the left brace back onto
+    // the same line
+    if (NextLine.First->is(TT_FunctionLBrace) &&
+        Style.AllowShortFunctionBodiesOnASingleLine &&
+        !Style.BraceWrapping.AfterFunction) {
+      unsigned MergedLines = 0;
+      MergedLines = tryMergeSimpleBlock(I + 1, E, Limit);
+      return MergedLines > 0 ? 0 : 1;
+    }
+
     auto IsElseLine = [&TheLine]() -> bool {
       const FormatToken *First = TheLine->First;
       if (First->is(tok::kw_else))
diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
index 91b8fdc8a3c38..7dd59c1cbd678 100644
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -1921,6 +1921,10 @@ void UnwrappedLineParser::parseStructuralElement(
           }
         } else if (Style.BraceWrapping.AfterFunction) {
           addUnwrappedLine();
+        } else if (Style.AllowShortFunctionBodiesOnASingleLine) {
+          // Wrap the left brace here; we'll try to merge it back
+          // later if needed.
+          addUnwrappedLine();
         }
         if (!Previous || Previous->isNot(TT_TypeDeclarationParen))
           FormatTok->setFinalizedType(TT_FunctionLBrace);

github-actions · 2025-07-31T00:18:57Z

✅ With the latest revision this PR passed the C/C++ code formatter.

HazardyKnusperkeks

You are missing unit tests. clang/unittests/Format/FromatTest.cpp.

clang/include/clang/Format/Format.h

clang/lib/Format/UnwrappedLineFormatter.cpp

clang/lib/Format/UnwrappedLineParser.cpp

brandb97 · 2025-08-10T14:28:34Z

ping

clang/lib/Format/UnwrappedLineFormatter.cpp

clang/include/clang/Format/Format.h

owenca · 2025-08-19T08:22:47Z

Can we fix the bug instead of adding an option?

brandb97 · 2025-08-19T11:11:52Z

Sorry, I promised to add enum PutShortFunctionBodiesOnASingleLine option, but I haven’t touched this code in quite a while (due to summer vacation). It seems the requirements have changed since then, so maybe it was a good thing I didn’t write it after all? ;-)

brandb97 · 2025-08-19T11:15:35Z

I can just fix the bug by moving PutShortFunctionBodiesOnASingleLine to private area in FormatStyle and set PutShortFunctionBodiesOnASingleLine to Always if and only if AllowShortFunctionsOnASingleLine is Never and AllowShortBlocksOnASingleLine is Always. Is that what I should do?

brandb97 · 2025-08-19T11:33:57Z

@owenca I actually think adding a new option would be better. The reason is that when users want to specify putting function bodies on a single line, without a new option, they would have to set
AllowShortFunctionsOnASingleLine: Never
AllowShortBlocksOnASingleLine: Always
But such a setting would also cause shorter if and while blocks to be placed on a single line, which users might not want. With a new option, this problem wouldn’t occur.

owenca · 2025-08-20T02:35:10Z

Sorry, I promised to add enum PutShortFunctionBodiesOnASingleLine option, but I haven’t touched this code in quite a while (due to summer vacation). It seems the requirements have changed since then, so maybe it was a good thing I didn’t write it after all? ;-)

I don't think the requirements have changed from my perspective. See #145161 (comment).

@owenca I actually think adding a new option would be better. The reason is that when users want to specify putting function bodies on a single line, without a new option, they would have to set AllowShortFunctionsOnASingleLine: Never AllowShortBlocksOnASingleLine: Always But such a setting would also cause shorter if and while blocks to be placed on a single line, which users might not want. With a new option, this problem wouldn’t occur.

We can't keep adding all kinds of options people can come up with. See https://clang.llvm.org/docs/ClangFormatStyleOptions.html#adding-additional-style-options.

brandb97 · 2025-08-20T06:56:10Z

I see, I’ll rewrite this part of the code.

brandb97 · 2025-08-22T15:38:01Z

@owenca Could you help review my code? Thank you very much.

HazardyKnusperkeks · 2025-08-25T09:08:51Z

clang/lib/Format/UnwrappedLineFormatter.cpp

+      const bool NextShortBlock = NextLine.First == NextLine.Last &&
+                                  I + 3 != E && I[3]->First->is(tok::r_brace);


If I + 2 == E this is wrong.

Ah, I am being stupid, thanks.

I didn't quite understand why the LLDB formatter failed. Could you help me analyze the possible reasons for the error? Thank you very much!

I don't understand the question, where do you get LLDB from?

When set AllowShortBlocksOnASingleLine as Empty or Always and we can't put the whole function on a single line, clang-format doesn't tries to put the function body on a seperate line from function signature. Add tryMergeLines to put function bodies on a single line if possible. Signed-off-by: Lidong Yan <yldhome2d2@gmail.com>

llvmbot added the clang-format label Jul 31, 2025

HazardyKnusperkeks reviewed Jul 31, 2025

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brandb97 force-pushed the fix-clang-format branch from 7a074db to 25d45a4 Compare August 5, 2025 07:54

llvmbot added the clang Clang issues not falling into any other category label Aug 5, 2025

brandb97 requested a review from HazardyKnusperkeks August 5, 2025 09:34

HazardyKnusperkeks reviewed Aug 11, 2025

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brandb97 force-pushed the fix-clang-format branch from 25d45a4 to 04cd330 Compare August 22, 2025 15:36

brandb97 requested a review from HazardyKnusperkeks August 22, 2025 15:38

brandb97 force-pushed the fix-clang-format branch 2 times, most recently from e5a880b to 7cdbee9 Compare August 25, 2025 05:20

HazardyKnusperkeks reviewed Aug 25, 2025

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brandb97 force-pushed the fix-clang-format branch from 7cdbee9 to 0c5bc1a Compare August 25, 2025 10:56

brandb97 force-pushed the fix-clang-format branch from 0c5bc1a to 8df95ae Compare August 25, 2025 11:42

Merge branch 'main' into fix-clang-format

8de5291

		const bool NextShortBlock = NextLine.First == NextLine.Last &&
		I + 3 != E && I[3]->First->is(tok::r_brace);

[clang-format] allow short function body on a single line #151428

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[clang-format] allow short function body on a single line #151428

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