lint hints should use attribute syntax, not command line syntax

[brson]

I see output like this: warning: unnecessaryunsafeblock [-W unused-unsafe (default)].

rustc is telling you the command line argument to control this warning, but command line arguments are not our preferred way of turning on warnings.

[catamorphism]

(bug triage) Easy to fix; doesn't look like a nominatable one to me.

[metajack]

triage bump