Just an aside, is there any way to know how many of those 16,000 compiler errors are independent. I mean, could it be that just by changing say 500 lines of code all those errors disappear?

Perhaps 16,000 could just measure cascade breakage, for example one lifetime mismatch can cause errors in every function that tries to use that reference.

Rust reference lifetime bookkeeping is a difficult task for LLMs. The LLM has to maintain, across multiple functions and structs, which references outlive which. Furthermore compiler messages are highly contextual and lifetime patterns are sparse in the training set.