[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.

1 files changed, 3 insertions, 0 deletions

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");