[clang-format] Add `AllowShortNamespacesOnASingleLine` option (#105597)

This fixes #101363 which is a resurrection of a previously opened but
never completed review: https://reviews.llvm.org/D11851

The feature is to allow code like the following not to be broken across
multiple lines:

```
namespace foo { class bar; }
namespace foo { namespace bar { class baz; } }
```

Code like this is commonly used for forward declarations, which are
ideally kept compact. This is also apparently the format that
include-what-you-use will insert for forward declarations.

Also, fix an off-by-one error in `CompactNamespaces` code. For nested
namespaces with 3 or more namespaces, it was incorrectly compacting
lines which were 1 or two spaces over the `ColumnLimit`, leading to
incorrect formatting results.

7 files changed, 221 insertions, 1 deletions

diff --git a/clang/docs/ClangFormatStyleOptions.rst b/clang/docs/ClangFormatStyleOptions.rst
index 4be4481..c175436 100644
--- a/clang/docs/ClangFormatStyleOptions.rst
+++ b/clang/docs/ClangFormatStyleOptions.rst
@@ -2088,6 +2088,11 @@ the configuration (without a prefix: ``Auto``).
   If ``true``, ``while (true) continue;`` can be put on a single
   line.
 
+.. _AllowShortNamespacesOnASingleLine:
+
+**AllowShortNamespacesOnASingleLine** (``Boolean``) :versionbadge:`clang-format 20` :ref:`¶ <AllowShortNamespacesOnASingleLine>`
+  If ``true``, ``namespace a { class b; }`` can be put on a single line.
+
 .. _AlwaysBreakAfterDefinitionReturnType:
 
 **AlwaysBreakAfterDefinitionReturnType** (``DefinitionReturnTypeBreakingStyle``) :versionbadge:`clang-format 3.7` :ref:`¶ <AlwaysBreakAfterDefinitionReturnType>`
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 210ccc1..b7da12bc 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -1123,6 +1123,7 @@ clang-format
   ``Never``, and ``true`` to ``Always``.
 - Adds ``RemoveEmptyLinesInUnwrappedLines`` option.
 - Adds ``KeepFormFeed`` option and set it to ``true`` for ``GNU`` style.
+- Adds ``AllowShortNamespacesOnASingleLine`` option.
 
 libclang
 --------
diff --git a/clang/include/clang/Format/Format.h b/clang/include/clang/Format/Format.h
index 6383934..eefaabf 100644
--- a/clang/include/clang/Format/Format.h
+++ b/clang/include/clang/Format/Format.h
@@ -988,6 +988,10 @@ struct FormatStyle {
   /// \version 3.7
   bool AllowShortLoopsOnASingleLine;
 
+  /// If ``true``, ``namespace a { class b; }`` can be put on a single line.
+  /// \version 20
+  bool AllowShortNamespacesOnASingleLine;
+
   /// Different ways to break after the function definition return type.
   /// This option is **deprecated** and is retained for backwards compatibility.
   enum DefinitionReturnTypeBreakingStyle : int8_t {
@@ -5168,6 +5172,8 @@ struct FormatStyle {
                R.AllowShortIfStatementsOnASingleLine &&
            AllowShortLambdasOnASingleLine == R.AllowShortLambdasOnASingleLine &&
            AllowShortLoopsOnASingleLine == R.AllowShortLoopsOnASingleLine &&
+           AllowShortNamespacesOnASingleLine ==
+               R.AllowShortNamespacesOnASingleLine &&
            AlwaysBreakBeforeMultilineStrings ==
                R.AlwaysBreakBeforeMultilineStrings &&
            AttributeMacros == R.AttributeMacros &&
diff --git a/clang/lib/Format/Format.cpp b/clang/lib/Format/Format.cpp
index 95129a8..8f44e9f 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -975,6 +975,8 @@ template <> struct MappingTraits<FormatStyle> {
                    Style.AllowShortLambdasOnASingleLine);
     IO.mapOptional("AllowShortLoopsOnASingleLine",
                    Style.AllowShortLoopsOnASingleLine);
+    IO.mapOptional("AllowShortNamespacesOnASingleLine",
+                   Style.AllowShortNamespacesOnASingleLine);
     IO.mapOptional("AlwaysBreakAfterDefinitionReturnType",
                    Style.AlwaysBreakAfterDefinitionReturnType);
     IO.mapOptional("AlwaysBreakBeforeMultilineStrings",
@@ -1480,6 +1482,7 @@ FormatStyle getLLVMStyle(FormatStyle::LanguageKind Language) {
   LLVMStyle.AllowShortIfStatementsOnASingleLine = FormatStyle::SIS_Never;
   LLVMStyle.AllowShortLambdasOnASingleLine = FormatStyle::SLS_All;
   LLVMStyle.AllowShortLoopsOnASingleLine = false;
+  LLVMStyle.AllowShortNamespacesOnASingleLine = false;
   LLVMStyle.AlwaysBreakAfterDefinitionReturnType = FormatStyle::DRTBS_None;
   LLVMStyle.AlwaysBreakBeforeMultilineStrings = false;
   LLVMStyle.AttributeMacros.push_back("__capability");
diff --git a/clang/lib/Format/UnwrappedLineFormatter.cpp b/clang/lib/Format/UnwrappedLineFormatter.cpp
index 1804c14..803c600 100644
--- a/clang/lib/Format/UnwrappedLineFormatter.cpp
+++ b/clang/lib/Format/UnwrappedLineFormatter.cpp
@@ -361,9 +361,18 @@ private:
     const auto *FirstNonComment = TheLine->getFirstNonComment();
     if (!FirstNonComment)
       return 0;
+
     // FIXME: There are probably cases where we should use FirstNonComment
     // instead of TheLine->First.
 
+    if (Style.AllowShortNamespacesOnASingleLine &&
+        TheLine->First->is(tok::kw_namespace) &&
+        TheLine->Last->is(tok::l_brace)) {
+      const auto result = tryMergeNamespace(I, E, Limit);
+      if (result > 0)
+        return result;
+    }
+
     if (Style.CompactNamespaces) {
       if (const auto *NSToken = TheLine->First->getNamespaceToken()) {
         int J = 1;
@@ -373,7 +382,7 @@ private:
              ClosingLineIndex == I[J]->MatchingClosingBlockLineIndex &&
              I[J]->Last->TotalLength < Limit;
              ++J, --ClosingLineIndex) {
-          Limit -= I[J]->Last->TotalLength;
+          Limit -= I[J]->Last->TotalLength + 1;
 
           // Reduce indent level for bodies of namespaces which were compacted,
           // but only if their content was indented in the first place.
@@ -420,6 +429,7 @@ private:
         TheLine->First != LastNonComment) {
       return MergeShortFunctions ? tryMergeSimpleBlock(I, E, Limit) : 0;
     }
+
     // Try to merge a control statement block with left brace unwrapped.
     if (TheLine->Last->is(tok::l_brace) && FirstNonComment != TheLine->Last &&
         FirstNonComment->isOneOf(tok::kw_if, tok::kw_while, tok::kw_for,
@@ -616,6 +626,72 @@ private:
     return 1;
   }
 
+  unsigned tryMergeNamespace(SmallVectorImpl<AnnotatedLine *>::const_iterator I,
+                             SmallVectorImpl<AnnotatedLine *>::const_iterator E,
+                             unsigned Limit) {
+    if (Limit == 0)
+      return 0;
+
+    assert(I[1]);
+    const auto &L1 = *I[1];
+    if (L1.InPPDirective != (*I)->InPPDirective ||
+        (L1.InPPDirective && L1.First->HasUnescapedNewline)) {
+      return 0;
+    }
+
+    if (std::distance(I, E) <= 2)
+      return 0;
+
+    assert(I[2]);
+    const auto &L2 = *I[2];
+    if (L2.Type == LT_Invalid)
+      return 0;
+
+    Limit = limitConsideringMacros(I + 1, E, Limit);
+
+    if (!nextTwoLinesFitInto(I, Limit))
+      return 0;
+
+    // Check if it's a namespace inside a namespace, and call recursively if so.
+    // '3' is the sizes of the whitespace and closing brace for " _inner_ }".
+    if (L1.First->is(tok::kw_namespace)) {
+      if (L1.Last->is(tok::comment) || !Style.CompactNamespaces)
+        return 0;
+
+      assert(Limit >= L1.Last->TotalLength + 3);
+      const auto InnerLimit = Limit - L1.Last->TotalLength - 3;
+      const auto MergedLines = tryMergeNamespace(I + 1, E, InnerLimit);
+      if (MergedLines == 0)
+        return 0;
+      const auto N = MergedLines + 2;
+      // Check if there is even a line after the inner result.
+      if (std::distance(I, E) <= N)
+        return 0;
+      // Check that the line after the inner result starts with a closing brace
+      // which we are permitted to merge into one line.
+      if (I[N]->First->is(tok::r_brace) && !I[N]->First->MustBreakBefore &&
+          I[MergedLines + 1]->Last->isNot(tok::comment) &&
+          nextNLinesFitInto(I, I + N + 1, Limit)) {
+        return N;
+      }
+      return 0;
+    }
+
+    // There's no inner namespace, so we are considering to merge at most one
+    // line.
+
+    // The line which is in the namespace should end with semicolon.
+    if (L1.Last->isNot(tok::semi))
+      return 0;
+
+    // Last, check that the third line starts with a closing brace.
+    if (L2.First->isNot(tok::r_brace) || L2.First->MustBreakBefore)
+      return 0;
+
+    // If so, merge all three lines.
+    return 2;
+  }
+
   unsigned tryMergeSimpleControlStatement(
       SmallVectorImpl<AnnotatedLine *>::const_iterator I,
       SmallVectorImpl<AnnotatedLine *>::const_iterator E, unsigned Limit) {
@@ -916,6 +992,21 @@ private:
     return 1 + I[1]->Last->TotalLength + 1 + I[2]->Last->TotalLength <= Limit;
   }
 
+  bool nextNLinesFitInto(SmallVectorImpl<AnnotatedLine *>::const_iterator I,
+                         SmallVectorImpl<AnnotatedLine *>::const_iterator E,
+                         unsigned Limit) {
+    unsigned JoinedLength = 0;
+    for (const auto *J = I + 1; J != E; ++J) {
+      if ((*J)->First->MustBreakBefore)
+        return false;
+
+      JoinedLength += 1 + (*J)->Last->TotalLength;
+      if (JoinedLength > Limit)
+        return false;
+    }
+    return true;
+  }
+
   bool containsMustBreak(const AnnotatedLine *Line) {
     assert(Line->First);
     // Ignore the first token, because in this situation, it applies more to the
diff --git a/clang/unittests/Format/ConfigParseTest.cpp b/clang/unittests/Format/ConfigParseTest.cpp
index 7fc7492..b249bf0 100644
--- a/clang/unittests/Format/ConfigParseTest.cpp
+++ b/clang/unittests/Format/ConfigParseTest.cpp
@@ -159,6 +159,7 @@ TEST(ConfigParseTest, ParsesConfigurationBools) {
   CHECK_PARSE_BOOL(AllowShortCompoundRequirementOnASingleLine);
   CHECK_PARSE_BOOL(AllowShortEnumsOnASingleLine);
   CHECK_PARSE_BOOL(AllowShortLoopsOnASingleLine);
+  CHECK_PARSE_BOOL(AllowShortNamespacesOnASingleLine);
   CHECK_PARSE_BOOL(BinPackArguments);
   CHECK_PARSE_BOOL(BreakAdjacentStringLiterals);
   CHECK_PARSE_BOOL(BreakAfterJavaFieldAnnotations);
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 47465a1..22b6f7e 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -4476,6 +4476,7 @@ TEST_F(FormatTest, FormatsCompactNamespaces) {
                "int k; } // namespace out",
                Style);
 
+  Style.ColumnLimit = 41;
   verifyFormat("namespace A { namespace B { namespace C {\n"
                "}}} // namespace A::B::C",
                "namespace A { namespace B {\n"
@@ -4504,6 +4505,12 @@ TEST_F(FormatTest, FormatsCompactNamespaces) {
                "} // namespace bbbbbb\n"
                "} // namespace aaaaaa",
                Style);
+
+  verifyFormat("namespace a { namespace b {\n"
+               "namespace c {\n"
+               "}}} // namespace a::b::c",
+               Style);
+
   Style.ColumnLimit = 80;
 
   // Extra semicolon after 'inner' closing brace prevents merging
@@ -28314,6 +28321,112 @@ TEST_F(FormatTest, KeepFormFeed) {
                Style);
 }
 
+TEST_F(FormatTest, ShortNamespacesOption) {
+  auto Style = getLLVMStyle();
+  Style.AllowShortNamespacesOnASingleLine = true;
+  Style.CompactNamespaces = true;
+  Style.FixNamespaceComments = false;
+
+  // Basic functionality.
+  verifyFormat("namespace foo { class bar; }", Style);
+  verifyFormat("namespace foo::bar { class baz; }", Style);
+  verifyFormat("namespace { class bar; }", Style);
+  verifyFormat("namespace foo {\n"
+               "class bar;\n"
+               "class baz;\n"
+               "}",
+               Style);
+
+  // Trailing comments prevent merging.
+  verifyFormat("namespace foo { namespace baz {\n"
+               "class qux;\n"
+               "} // comment\n"
+               "}",
+               Style);
+
+  // Make sure code doesn't walk too far on unbalanced code.
+  verifyFormat("namespace foo {", Style);
+  verifyFormat("namespace foo {\n"
+               "class baz;",
+               Style);
+  verifyFormat("namespace foo {\n"
+               "namespace bar { class baz; }",
+               Style);
+
+  // Nested namespaces.
+  verifyFormat("namespace foo { namespace bar { class baz; } }", Style);
+
+  // Without CompactNamespaces, we won't merge consecutive namespace
+  // declarations.
+  Style.CompactNamespaces = false;
+  verifyFormat("namespace foo {\n"
+               "namespace bar { class baz; }\n"
+               "}",
+               Style);
+
+  verifyFormat("namespace foo {\n"
+               "namespace bar { class baz; }\n"
+               "namespace qux { class quux; }\n"
+               "}",
+               Style);
+
+  Style.CompactNamespaces = true;
+
+  // Varying inner content.
+  verifyFormat("namespace foo {\n"
+               "int f() { return 5; }\n"
+               "}",
+               Style);
+  verifyFormat("namespace foo { template <T> struct bar; }", Style);
+  verifyFormat("namespace foo { constexpr int num = 42; }", Style);
+
+  // Validate nested namespace wrapping scenarios around the ColumnLimit.
+  Style.ColumnLimit = 64;
+
+  // Validate just under the ColumnLimit.
+  verifyFormat(
+      "namespace foo { namespace bar { namespace baz { class qux; } } }",
+      Style);
+
+  // Validate just over the ColumnLimit.
+  verifyFormat("namespace foo { namespace baar { namespace baaz {\n"
+               "class quux;\n"
+               "}}}",
+               Style);
+
+  verifyFormat(
+      "namespace foo { namespace bar { namespace baz { namespace qux {\n"
+      "class quux;\n"
+      "}}}}",
+      Style);
+
+  // Validate that the ColumnLimit logic accounts for trailing content as well.
+  verifyFormat("namespace foo { namespace bar { class qux; } } // extra",
+               Style);
+
+  verifyFormat("namespace foo { namespace bar { namespace baz {\n"
+               "class qux;\n"
+               "}}} // extra",
+               Style);
+
+  // FIXME: Ideally AllowShortNamespacesOnASingleLine would disable the trailing
+  // namespace comment from 'FixNamespaceComments', as it's not really necessary
+  // in this scenario, but the two options work at very different layers of the
+  // formatter, so I'm not sure how to make them interact.
+  //
+  // As it stands, the trailing comment will be added and likely make the line
+  // too long to fit within the ColumnLimit, reducing the how likely the line
+  // will still fit on a single line. The recommendation for now is to use the
+  // concatenated namespace syntax instead. e.g. 'namespace foo::bar'
+  Style.FixNamespaceComments = true;
+  verifyFormat(
+      "namespace foo { namespace bar { namespace baz {\n"
+      "class qux;\n"
+      "}}} // namespace foo::bar::baz",
+      "namespace foo { namespace bar { namespace baz { class qux; } } }",
+      Style);
+}
+
 } // namespace
 } // namespace test
 } // namespace format