Safety, undefined behavior, @safe, @trusted

[Don]

Walter Bright wrote:
> Following the safe D discussions, I've had a bit of a change of mind. 
> Time for a new strawman.
> 
> Based on Andrei's and Cardelli's ideas, I propose that Safe D be defined 
> as the subset of D that guarantees no undefined behavior. Implementation 
> defined behavior (such as varying pointer sizes) is still allowed.
> 
> Memory safety is a subset of this. Undefined behavior nicely covers 
> things like casting away const and shared.
> 
> Safety has a lot in common with function purity, which is set by an 
> attribute and verified by the compiler. Purity is a subset of safety.
> 
> Safety seems more and more to be a characteristic of a function, rather 
> than a module or command line switch. To that end, I propose two new 
> attributes:
> 
> @safe
> @trusted
> 
> A function marked as @safe cannot use any construct that could result in 
> undefined behavior. An @safe function can only call other @safe 
> functions or @trusted functions.
> 
> A function marked as @trusted is assumed to be safe by the compiler, but 
> is not checked. It can call any function.

> 
> Functions not marked as @safe or @trusted can call any function.
> 
> To mark an entire module as safe, add the line:
> 
>    @safe:
> 
> after the module statement. Ditto for marking the whole module as 
> @trusted. An entire application can be checked for safety by making 
> main() safe:
> 
>     @safe int main() { ... }
> 
> This proposal eliminates the need for command line switches, and 
> versioning based on safety.

I think it's important to also have @unsafe. These are functions which 
are unsafe, and not trusted. Like free(), for example. They're usually 
easy to identify, and should be small in number.
They should only be callable from @trusted functions.
That's different from unmarked functions, which generally just haven't 
been checked for safety.
I want to able to find the cases where I'm calling those guys, without 
having to mark every function in the program with an @safe attribute.