Specify property name in exception message on nested type build failure about freebuilder HOT 2 OPEN

Nice idea, though we risk breaking enclosing error-handling code by throwing away the exception type.

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That's true, it's a breaking change. But I would consider an unbuildable nested property as an illegal state of top-level builder (caused by an illegal state of the nested builder), just like an unset property. And in some (typical?) cases it conceptually is unset, like nested2 above. From this perspective just passing nested builder errors is inconsistent to handling of concretely unset properties, making this issue look like a bug rather than an enhancement.

In cases where retaining compatibility with enclosing error-handling code is required, I think all build() methods might be overridden to catch the exception and rethrow the cause (recursively if nested builders are out of own control). To identify such exceptions, the message could (and probably should anyway) be formatted something like Unbuildable: <property-name>; <cause-message>.

from freebuilder.