Function expr 

pub fn expr(node: &Expr) -> String

Render one expression, for a caller that has only one.

What a generator building C++ by hand needs where the value is already a tree: the read harness’s union arms compare against the same label the generated codec does, and the label’s written form — which suffix a wide one takes — is CodecTarget::wide_int’s answer. Asking for the tree’s text is how the two stay one rule rather than two that agree today.