Rust requires explicit conversions for numeric types for exactly the reasons you mention.
Most systems languages (that aren't C) force you to be explicit about demoting casts between formal variables; that's the easy part. But no language I know of (not even Rust) gets rid of implicit promotion during math operations, in favor of 1. forcing you to specify the size of the destination register you want the instruction to use (i.e. the working precision), and then 2. requiring an explicit demoting cast if the destination formal variable is smaller than the working precision you specified.
I mean, C's inline assembler, and LLVM IR, both require this; but they also require you to specify which work register you're talking about, rather than just the size, which breaks the conceit of a "portable systems language."
IMHO C is far too lax with things like this; an ideal systems language would force you to annotate maths operations with the "working precision" for each operation, making explicit any promotion done to the inputs before the math happens. And then, if your "working precision" exceeded the destination variable's size, you'd also be forced to do an explicit demoting cast, just as if you were assigning from a declared larger-sized variable.
Yes, it'd be mostly irrelevant for everyday int math stuff; but it'd make things so much clearer as soon as you started working with floats (where you'd have to explicitly opt into Intel's 80-bit extended "working precision.")