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digitalmars.D.learn - Optimal implementation of identity function

For the default case (`return key`) in

     auto adjustKeyType(SomeKey)(const return scope SomeKey key) 
const  trusted
         pragma(inline, true);            // must be inlined
         static if (is(SomeKey : U[], U)) // is array
             /* because return value is used only temporarily it's 
ok to cast to
              * `immutable` to prevent GC-allocations in types 
such as
              * `sso_string.SSOString` */
             return cast(immutable(typeof(key[0]))[])key;
             return key;

I want this function to be optimized away completely when inlined 
in release mode.

Is this the best of achieving this with regards to

1. its return type: auto (or is better auto ref) and
2. parameter `key` being qualified as `return scope`

Dec 20 2018