GenerateDepsFile Task should be internally-incremental

[baronfel]

Is your feature request related to a problem? Please describe.

The GenerateDepsFile always writes a new deps.json file, even if the content is the same. This leads to rebuilds all down the target chain. We shouldn't write a deps.json unless the content would be materially different.

sdk/src/Tasks/Microsoft.NET.Build.Tasks/GenerateDepsFile.cs

Lines 253 to 258 in 5457ad3

	var writer = new DependencyContextWriter();
	using (var fileStream = File.Create(depsFilePath))
	{
	writer.Write(dependencyContext, fileStream);
	}
	_filesWritten.Add(new TaskItem(depsFilePath));

[rainersigwald]

Same in GenerateRuntimeConfigurationFiles:

sdk/src/Tasks/Microsoft.NET.Build.Tasks/GenerateRuntimeConfigurationFiles.cs

Lines 393 to 396 in 5457ad3

	using (JsonTextWriter writer = new(new StreamWriter(File.Create(fileName))))
	{
	serializer.Serialize(writer, value);
	}

Note though that if the task doesn't unconditionally update the timestamp of its output, the target can become always out of date, since it has the .json file as an output.

[baronfel]

Neither of these structures implement structural equality, so we'll need to define what that means as part of this work.

[dsplaisted]

Can we generate the output in memory and compare that to the current file contents to determine if we need to write out the file or not? It seems like that way we don't have to worry about structural equality, it's just whether the output is the same.

[baronfel]

Good point!