Quantity(100) is counterproductive here, as that doesn't narrow the type, it does the opposite, it casts whatever value is given to the type, so even Quantity(100.5) will still work, while just plain 100.5 would have given an error with '-Wconversion'.
I do run clippy on my Rust projects, but that's a matter of style and readability, not correctness (for the most part!).
Conversions may be fine and even useful in many cases, in this case it isn’t. Converting to std::variant or std::optional are some of those cases that are really nice.
I appreciate that there are guardrails in a tool like rust, I also appreciate that sharp tools like c exist, they both have advantages.
There are also more type-safe conversion methods that perform a more focused conversion. Eg a widening conversion from i8 -> i16 can be done with .into(), a narrowing conversion from i16 -> i8 can be done with .try_into() (which returns a Result and forces you to handle the overflow case), a signed to unsigned reinterpretation like i64 -> u64 can be done with .cast_unsigned(), and so on. Unlike `as` these have the advantage that they stop compiling if the original value changes type; eg if you refactor something and the i8 in the first example becomes an i32, the i32 -> i16 conversion is no longer a widening conversion so the `.into()` will fail to compile.