`T: 'a` means that you may keep values of type T around for lifetime 'a, but not that they will. So `T: 'static` means that you may keep values of that type around forever, as they do not refer to anything that doesn't live forever.
So neither `&'a i32` nor `MutexGuard<'a, i32>` are 'static (unless 'a is), because you shouldn't keep references like that around longer than the stuff it points to. But i32 by itself satisfies i32: 'static, because it's perfectly fine to keep an i32 around forever. (But that doesn't mean that every i32 will stick around forever.)
So neither `&'a i32` nor `MutexGuard<'a, i32>` are 'static (unless 'a is), because you shouldn't keep references like that around longer than the stuff it points to. But i32 by itself satisfies i32: 'static, because it's perfectly fine to keep an i32 around forever. (But that doesn't mean that every i32 will stick around forever.)