Is Roslyn available only for .NET?

Yes it's the name of the .NET compiler API.

It was code-named to disambiguate it from the old compiler. But Roslyn is almost 15 years old now, so I can't call it new, but it's newer than the really legacy stuff.

It essentially lets you operate on the abstract snytax tree itself, so there is background compilation that powers inspection and transformation.

Instant renaming is an obvious benefit, but you can do more powerful transformations, such as removing redundant code or transforming one syntax style into another, e.g. tranforming from a Fluent API into a procedural one or vice-versa.