Microsoft DevBlog: How to Write a Roslyn Analyzer (devblogs.microsoft.com)

Analyzers (but not Roslyn-based) can also be created for F# code using https://github.com/ionide/FSharp.Analyzers.SDK.

Here's a cool example that type checks SQL statements embedded in your code as you go: https://github.com/Zaid-Ajaj/Npgsql.FSharp.Analyzer

My team wrote an analyzer to prevent use of DateTime.Now and enforce DateTime.UtcNow instead. Helps us prevent weird time bugs. Neat stuff. The code for it took me a while to get used to, lots of strange looking syntax and types.