Ninja: Do not add empty custom command for file(GENERATE) outputs

Internally we mark `file(GENERATE)` outputs as `GENERATED` in order
to tell custom command dependency tracing logic not to expect the
files to exist on disk yet.  This is because we do not generate the
files until after that tracing is done.

The Ninja generator also interprets the `GENERATED` property to mean
that it is expected that some build rule will generate the file if
another build rule depends on it.  If the generator does not know of a
custom command that generates the file then it adds an empty one so that
the `ninja` build tool does not complain about a dependency on a file
that does not exist and has no rule to generate it.  However, this step
is not necessary for `file(GENERATE)` outputs because there is no build
rule to generate them and they will exist before `ninja` runs.

Add an additional `__CMAKE_GENERATED_BY_CMAKE` property internally to
tell the Ninja generator that a `GENERATED` file will exist before the
build starts and is not expected to have a build rule producing it.

Fixes: #17942

2 files changed, 9 insertions, 1 deletions

diff --git a/Source/cmGeneratorExpressionEvaluationFile.cxx b/Source/cmGeneratorExpressionEvaluationFile.cxx
index c544141932..99b926126f 100644
--- a/Source/cmGeneratorExpressionEvaluationFile.cxx
+++ b/Source/cmGeneratorExpressionEvaluationFile.cxx
@@ -105,8 +105,14 @@ void cmGeneratorExpressionEvaluationFile::CreateOutputFile(
       lg, config, false, nullptr, nullptr, nullptr, le);
     cmSourceFile* sf = lg->GetMakefile()->GetOrCreateSource(
       name, false, cmSourceFileLocationKind::Known);
+    // Tell TraceDependencies that the file is not expected to exist
+    // on disk yet.  We generate it after that runs.
     sf->SetProperty("GENERATED", "1");
 
+    // Tell the build system generators that there is no build rule
+    // to generate the file.
+    sf->SetProperty("__CMAKE_GENERATED_BY_CMAKE", "1");
+
     gg->SetFilenameTargetDepends(
       sf, this->OutputFileExpr->GetSourceSensitiveTargets());
   }
diff --git a/Source/cmNinjaTargetGenerator.cxx b/Source/cmNinjaTargetGenerator.cxx
index f4faf47a20..a6a3efbfbc 100644
--- a/Source/cmNinjaTargetGenerator.cxx
+++ b/Source/cmNinjaTargetGenerator.cxx
@@ -896,7 +896,9 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatement(
   // (either attached to this source file or another one), assume that one of
   // the target dependencies, OBJECT_DEPENDS or header file custom commands
   // will rebuild the file.
-  if (source->GetPropertyAsBool("GENERATED") && !source->GetCustomCommand() &&
+  if (source->GetPropertyAsBool("GENERATED") &&
+      !source->GetPropertyAsBool("__CMAKE_GENERATED_BY_CMAKE") &&
+      !source->GetCustomCommand() &&
       !this->GetGlobalGenerator()->HasCustomCommandOutput(sourceFileName)) {
     this->GetGlobalGenerator()->AddAssumedSourceDependencies(sourceFileName,
                                                              orderOnlyDeps);