.{ x } or .{ x, y, z } etc... are adhoc-constructed anonymous struct-values which can be inspected at compile time by the standard library's string formatting code.
It all makes a lot more sense after understanding Zig's comptime features:
https://ziglang.org/documentation/master/#comptime
The end of the comptime section also explains how string formatting works that way.
Why is just "x" not allowed? What does ".{x}" actually mean?