CodeLens is great for finding references within the IDE. It also provides information about the history, which Sourcetrail doesn't do yet.
I would say we go a step further, by also providing a visual representation of the references to symbol, which quickly conveys how a symbol is embedded into the rest of the code base, without even reading a line of code. That makes exploring a lot faster.
CodeLens => https://msdn.microsoft.com/en-us/library/dn269218.aspx