Imagine something like NumPy. Go code to play with numbers; Rust code to actually do matrix multiplications et al. Since the "hot loop" of such code would be in the Rust part, not in the Go part, you don't see much overhead.
Or, consider Firefox. Firefox's rendering engine will soon be Servo (Rust). I presume that its networking and its Javascript engine will eventually go the same way. Eventually, the only stuff left written in C in the Firefox codebase will be "glue code": business-policy logic that doesn't do heavy lifting, and doesn't need speed. In other words, the exact type of code where Go's "you'll be safer without this" assertion applies. Firefox as Go + Rust: why not?
Or, consider Firefox. Firefox's rendering engine will soon be Servo (Rust). I presume that its networking and its Javascript engine will eventually go the same way. Eventually, the only stuff left written in C in the Firefox codebase will be "glue code": business-policy logic that doesn't do heavy lifting, and doesn't need speed. In other words, the exact type of code where Go's "you'll be safer without this" assertion applies. Firefox as Go + Rust: why not?