var r: T = undefined, then setting fields anyways in std.json

[dire edge]

https://github.com/ziglang/zig/blob/master/lib/std/json/static.zig#L336

I'm trying to understand the ownership of json parsing, and in reading the code im puzzled by how like 336 declares r as undefined, then line 370 sets a field on r (where its type is a struct) even though r seemingly has never been initialized or had memory allocated for it.

[merry gale]

r is a variable on the stack, undefined means dont initialize iit but it always exists