The "structure := default(TStructure);" has a number of problems, among them, the programmer has to remember (and type) that structure is of type TStructure
We are talking about local vars. And only local vars. And about code that initializes them, code that is at the very top of the routine.
Code that is immediately below the declaration of the variables.
If the amount of local vars is so big, that you can't see their declaration, then maybe - just maybe - there is a different issue.....
Otherwise, the type-name you need, is right there.
and, are the "default" values always binary zeroes ?
Is all zero always the correct behaviour?
type
TFoo = (foo1 = 11, foo2 = 21);
TBar = 100..110;
var
aFoo: TFoo;
aBar: TBar;
Initializing aFoo/aBar with zero creates an invalid value. And invalid values can lead to unpredictable behaviour. Upgrade your compiler, or change some settings and the behaviour of your app may change unexpectedly.
And if - hypothetically - an option as you advertise should ever come to be, then it should never set invalid values. So it could never be "zero all locals", but if it were to be it could only be "set all locals to default".
and, lastly, it still requires ensuring that every structure has been "defaulted".
As I said "a greater need than exists". This leaves the possibility of an existing need (Clearly you have this need, and others do not). But there are needs for many a thing. Not all of them are significant enough to warrant a new feature. [1]
Also, the compiler does already provide the tools to "ensure all have been defaulted" (or rather initialized).
If you miss one, you get a compiler hint.
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[1] This is a general statement. This does not place you request into either group of features. However, my personal opinion (which is of no matter towards any decision that may be made on the topic) is that if there were some feature (related to local initialization) that would bring forward such a warrant, it should not be a compiler directive, or any form were *all* locals would be affected by a single command.