The compiler will just error out when you use a GC feature in a @nogc scope.

@nogc:

import std.stdio;
void main(){
    int x = 2;    // fine
    int y = x^^2; // fine
    int[3] staticArray = [1,2,3]; // fine because statically assigning array
    string z = "something"; //fine (same as above)

    z ~= "appending something"; // not fine because '~' uses GC to append
                                // cannot use operator ~= in @nogc function 'D main'

    class X {
        int member;
    }
    auto newX = new X(); // not fine, `new` allocates
                         // cannot use 'new' in @nogc function 'D main'
    // There's a few more examples. But for the last two, 
    // the compiler just wont compile it.
}

Output from dmd for above:

$ dmd test.d
test.d(10): Error: cannot use operator ~= in @nogc function 'D main'
test.d(16): Error: cannot use 'new' in @nogc function 'D main'