Optionally beefed-up shadowing-prevention

[Nick Sabalausky]

bearophile just put up an interesting enhancement request, and I wanted to 
discuss it, but figured this would be a better place to discuss:

bearophile's original message reproduced here:
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@outer() attribute:  http://d.puremagic.com/issues/show_bug.cgi?id=5007
----------------------------
Generally it's not a good practice to use global values (or values from 
outer scopes, D has nested functions too, so names may come from the outer 
function too), but passed arguments increase the amount of used stack and 
they may slow down the code a little where high-performance is very 
important.

So in some situations the programmer may need to use global/outer names. But 
allowing functions to freely access global scope as in C language may lead 
to bugs, because there is no control over the flow of information between 
the subsystems of the program, and also because accidental masking of an 
outer name
is allowed:

int x = 100;
int foo(int y) {
    int x = 5;
    return x + y; // silently uses local x
}
void main() {
    assert(foo(10) == 15);
}

For this (and for other purposes) D has introduced the 'pure' attribute for 
functions that disallows the access to mutable outer state. But 'pure' is a 
blunt tool, and in some situations it can't be used. To avoid bugs in such
situations, caused by unwanted usage of outer state, an attribute may be 
defined, it may be named "@outer".

The purpose of the (optional) @outer attribute is similar to the 'global' 
attribute in the SPARK language:

# global in out CallCount;

A D function that is annotated with @outer must specify all global variables 
it uses, and if each of them is just read (in), written to (out), or both 
(inout).

An example of its possible syntax:

int x = 100;
int y = 200;

@outer(in x, inout y)
int foo(int z) {
    y = x + z;
    return y;
}

Here the compiler enforces that foo() uses only the x and y outer defined
variables, that x is just read and y is both read and written inside foo().
This tidies up the flow of information.

The @outer attribute is optional, and you may avoid its usage in small
script-like D programs. But in situations where the D code must be very
reliable, a simple automatic code review tool may require the usage of 
@outer
by all functions/methods.

The @outer(...) need to be shown both in the documentation produced by -D 
and
-X (Json too) dmd compilation switches.
=========================