This was left as a TODO in rust-lang#92268, and brings the trait more in
line with what was defined in MCP411.

`Assume::visibility` has been renamed to `Assume::safety`, as
library safety is what's actually being assumed; visibility is
just the mechanism by which it is currently checked (this may
change).

ref: rust-lang/compiler-team#411
ref: rust-lang#99571

resolves query instability issues, and probably better for performance

ref: rust-lang#100726 (comment)