This is minor and probably completely inconsequential to performance,
but I find vec::map to be more clear than vec::each and a push.

rustc: Fix calls to the logging function when the must_cast flag is true; stop using shape code for logging

This reverts commit ac822a52.

convert doc-attributes to doc-comments using ./src/etc/sugarise-doc-comments.py (and manually tweaking) - for issue #2498

These affect logging output to stdout globally, and turning the console off
has no effect when overridden by RUST_LOG.