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| -rw-r--r-- | doc/manual_experimental.rst | 25 |
1 files changed, 25 insertions, 0 deletions
diff --git a/doc/manual_experimental.rst b/doc/manual_experimental.rst index 228c20339..5c4059e2d 100644 --- a/doc/manual_experimental.rst +++ b/doc/manual_experimental.rst @@ -1606,3 +1606,28 @@ validation errors: If the taint mode is turned off, ``TaintedString`` is simply an alias for ``string``. + + +Aliasing restrictions in parameter passing +========================================== + +**Note**: The aliasing restrictions are currently not enforced by the +implementation and need to be fleshed out futher. + +"Aliasing" here means that the underlying storage locations overlap in memory +at runtime. An "output parameter" is a parameter of type ``var T``, an input +parameter is any parameter that is not of type ``var``. + +1. Two output parameters should never be aliased. +2. An input and an output parameter should not be aliased. +3. An output parameter should never be aliased with a global or thread local + variable referenced by the called proc. +4. An input parameter should not be aliased with a global or thread local + variable updated by the called proc. + +One problem with rules 3 and 4 is that they affect specific global or thread +local variables, but Nim's effect tracking only tracks "uses no global variable" +via ``.noSideEffect``. The rules 3 and 4 can also be approximated by a different rule: + +5. A global or thread local variable (or a location derived from such a location) + can only passed to a parameter of a ``.noSideEffect`` proc. |