From the discussion linked[1] in another reply at your level, I believe the proposed automatic type conversions are only for when there's no data loss, and there needs to be explicit conversions whenever loss of information is possible. That is, it's implicit for i32 -> u32, or i32 -> i64, but not for something like i32 -> i8 (such as the int to char conversion you mentioned), which would need to be explicitly converted.
i32 -> u32 is not lossless, since u32 cannot represent negative numbers. And indeed that's not listed in the implicit conversion table in the page you linked to.
1: https://internals.rust-lang.org/t/implicit-widening-polymorp...