I'm aware of a similar question, but that refers to ? used in function definitions with a set pattern (pinned).
I looked up the implementation of lib.makeOverridable and came across this line:
${if result ? overrideAttrs then "overrideAttrs" else null} = fdrv:
(Also learned that this kind of dynamic assignment is possible, so that's a bonus.)
According to the table at 15.3. Operators (pinned) in the Nix manual:
Table 15.1. Operators
| Name | Syntax | Description |
+---------------+--------------+---------------------------------+
| | | Test whether set e contains the |
| Has Attribute | e ? attrpath | attribute denoted by attrpath; |
| | | return true or false. |
+---------------+--------------+---------------------------------+
Examples:
$ nix repl
Welcome to Nix version 2.3.10. Type :? for help.
nix-repl> set = { a = 27; b = "lofa"; }
nix-repl> set ? a
true
nix-repl> set ? lofa
false
nix-repl> supset = { c = set; d = 7; }
nix-repl> supset ? c.a
true