JetBrains have always developed their own language analyzers from what I've seen. I would bet they have a clear API between a language analyzer and their suite of refactoring and code navigation tools, and it would probably end up being similar to work to support that API through an open source variant VS building their own.
Not to mention, JetBrains typically are at the forefront of IDE support for a language, they start working on it before there are any open source tools around to base work off of. Even with mature languages like C#, the JetBrains C# language backend predates Rosslyn by a good decade I think.
Not to mention, JetBrains typically are at the forefront of IDE support for a language, they start working on it before there are any open source tools around to base work off of. Even with mature languages like C#, the JetBrains C# language backend predates Rosslyn by a good decade I think.