malloc and calloc should be @trusted because by specification they cannot corrupt memory regardless of what arguments they are passed. core.stdc.stdlib.d has this comment explaining why they aren't @trusted:

// We don't mark these @trusted. Given that they return a void*, one has
// to do a pointer cast to do anything sensible with the result. Thus,
// functions using these already have to be @trusted, allowing them to
// call @system stuff anyway.

That comment is longer than just writing @trusted twice. Moreover since that comment was written Phobos has adopted the style of wrapping individual statements in @trusted lambdas instead of marking entire functions as @trusted.