Rust compiler errors

First moves

# The full explanation, with a worked example. Works offline.
rustc --explain E0499

# One line per diagnostic, for a wall of output.
cargo build --message-format=short

# Count the error codes in a large failure.
cargo build --message-format=json 2>/dev/null | grep -o '"code":{"code":"E[0-9]*"' | sort | uniq -c | sort -rn

Fix the first error, then rebuild. One move or type error produces a cascade downstream, and most of the cascade disappears with the first fix.

Prove the fix