Implement 'extern' declaration modifier.

The 'extern' declaration modifier provides a more convenient
way of interfacing Dafny code with C# source files and
.Net DLLs.

We support an 'extern' keyword on a module, class, function method,
or method (cannot extern ghost). We check the CompileNames of
all modules to make sure there are no duplicate names.

Every Dafny-generated C# class is marked partial, so it can
potentially be extended.

The extern keyword could be accompanied by an optional string naming
the corresponding C# method/class to connect to. If not given
the name of the method/class is used.

An 'extern' keyword implies an {:axiom} attribute for functions
and methods, so their ensures clauses are assumed to be true
without proof.

In addition to the .dfy files, the user may supply
C# files (.cs) and dynamic linked libraries (.dll) on
command line. These will be passed onto the C# compiler,
the .cs files as sources, and the .dll files as references.

As part of this change the grammar was refactored some.
New productions are

- TopDecls - a list of top-level declarations.
- TopDecl - a single top-level declaration
- DeclModifiers - a list of declaration modifiers which are
  either 'abstract', 'ghost', 'static', 'protected', or 'extern'.
  They can be in any order and we diagnose duplicates.
  In addition, since they are not all allowed in all contexts,
  we check and give diagnostics if an DeclModifier appears
  where it is not allowed.

1 files changed, 26 insertions, 0 deletions

diff --git a/Test/dafny0/ExternNegative2.dfy b/Test/dafny0/ExternNegative2.dfy
new file mode 100644
index 00000000..3d09913b
--- /dev/null
+++ b/Test/dafny0/ExternNegative2.dfy
@@ -0,0 +1,26 @@
+// RUN: %dafny /compile:0 /print:"%t.print" /dprint:"%t.dprint" "%s" > "%t"

+// RUN: %diff "%s.expect" "%t"

+extern "Modx" module Mod1 

+{

+  extern "classx" class Class1

+  {

+    extern "Fun1x" static function method Fun1() : int

+      ensures Fun1() > 0

+    extern "Method1x" static method Method1() returns (x: int)

+      ensures x > 0

+    static function method Fun2() : int

+	ensures Fun2() > 0

+    {

+	Fun1()

+    }

+    static method Method2() returns (x: int)

+      ensures x > 0

+    {

+	x := Method1();

+    }

+  }

+}

+// Will give error about duplicate CompileName for module.

+extern "Modx" module Mod2

+{

+}