I'm not parsing expert, but being hard to parse makes it harder to write tools that work with the language.
Personally I find rust to be not easy at all for humans to read, and so it's interesting that it's also hard for parsers to parse. Not sure what was optimized for in the design.
I've coded a moderate amount in Rust since it was born, I think the prevailing rustfmt style is too bent towards machine-perfect neatness (one example: function definitions are split so that every parameter has a new line).
This machine perfect neatness comes from lots of small places in the syntax where Rust decided let's make the syntax permissive so that it's easy to autogenerate code with some macro or to make good diffs.
Then a systematic implementation of trailing commas and allowing leading/trailing separators in some places. The result is quite sterile and to me is no longer organically nicely readable.
