Extend partial to properties to make Source Generators more useful

[b-straub]

Version Used:

VS 16.7 P2

Championed issue: #3301
Draft spec: #3417

With the introduction of SG a nice field to work on is the generation of properties with special skills, similar to the SWIFT PropertyWrappers. Currently a straightforward implementation is handicapped by the missing possiblity to use partial properties. To workaround an abstract helper class with abstract properties, or attributed private fields (see AutoNotifyGenerator) must be used. Both solutions are not very comfortable and concise.

With partial properties the sample would look much cleaner:

[AutoNotify]
public partial class ExampleViewModel
{
   public partial string Text { get; set; }
   public partial int Count { get; set; }
}

A working example SG using an abstract template class can be found here: ObservableTests. With partial properties the code would be way more concise.

[jaredpar]

This is a language request hence moving to csharplang.

[333fred]

Closing as a duplicate of #3412.