Nomenclature question: what is the `EXPR` in `for PATTERN in EXPR { }` called?

Location

https://doc.rust-lang.org/reference/expressions/loop-expr.html#iterator-loops

Context

I was reviewing a test cleanup PR https://github.com/rust-lang/rust/pull/141974, and I was trying to say something about the EXPR in

for PATTERN in EXPR {
    //         ^--- what is this expr called?
    // ...
}

but I couldn't find a good term for this[^1][^2]. The current grammar calls this EXPR

Expression_{except struct expression}

whereas for e.g. match EXPR { .. } or if let PATTERN = EXPR { .. }, there's a more "meaningful name" for that EXPR, which is the scrutinee expr.

[^1]: In the community discord, terms like "interatee" or "iterable" or "into-iterable" were suggested, but yeah.
[^2]: Nadrieril came up with "loopee", which is an excellent term 😆

Start with the iterator-loops section of the Rust Reference at the linked location, then compare its EXPR wording with the scrutinee terminology used for match and if let. The work is done when the project has agreed on a term for the expression and the relevant reference wording reflects that decision.