Add the keyword trusted for specifying "friend modules"

active/0000-trusted-mod.md

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+- Start Date: 2014-04-16
+- RFC PR #: (leave this empty)
+- Rust Issue #: (leave this empty)
+
+# Summary
+
+Add the keyword ```trusted``` that can be used to specify that all private items and members of a certain module should be accessible from a different module.
+
+# Motivation
+
+This allows us to:  
+1) Keep distinct logical units of computation in their own modules.  
+2) Keep struct data fields private while allowing access to them from certain logically coupled modules.
+
+# Detailed design
+
+First of all, why the keyword ```trusted```? Why not use the word _friend_ as C++ does? It's because the word _friend_ implies a bidirectional relation: _"X is a friend of Y"_ also implies that _"Y is a friend of X"_.
+
+A _trusted module declaration_ such as ```trusted mod foo::bar;``` can be written only before any _use declarations_. Such a declaration would specify that all private items and members in the current module are accessible also from the module ```foo::bar```. If the module ```foo::bar``` doesn't exists, then this declaration would cause a compile-time error such as _"module foo::bar doesn't exist"_.  
+
+Multiple trusted modules can be specified at once using the following syntax:  
+```trusted mod { foo::bar, baz, bar };```
+
+# Alternatives
+
+If there is a desire to keep the number of keywords at a minimum, the following syntax could be used:  
+```pub for mod foo::bar;```  
+or, alternatively:  
+```pub in mod foo::bar;```  
+
+Another way to reduce the number of keywords would be to use keyword ```trusted```, but replace the keyword ```unsafe``` with ```trusted```. This follows from the logic that ```unsafe``` blocks and functions are _trusted_ not to cause any memory-safety errors because the author has promised that they won't.
+
+# Unresolved questions
+
+Is a more granular control over visibility desired? Specifying ```trusted``` on module boundaries follows the logic of other visibility rules, but should there be control over the visibility of specific items or members?