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Ruby: AST ref docs - note about desugaring and synthesized AstNodes

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Alex Ford
2022-11-04 15:36:18 +00:00
parent 530b29ccdf
commit a77fc96067
1 changed files with 23 additions and 0 deletions
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docs/codeql/codeql-language-guides/abstract-syntax-tree-classes-for-working-with-ruby-programs.rst
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@@ -416,6 +416,27 @@ All classes in this subsection are subclasses of VariableAccess_.
| ``self`` | SelfVariableReadAccess_ | VariableReadAccess_, SelfVariableAccess_ | |
+----------------------------+------------------------------+-----------------------------------------------+------------------+
Desugaring
~~~~~~~~~~
Certain Ruby language features are implemented using syntactic sugar. For example, supposing that ``x`` is an object with an attribute ``foo``, the assignment::
x.foo = y
is desugared to code similar to::
x.foo=(__synth_0 = y);
__synth_0;
In other words, there is effectively a call to the SetterMethodCall_ ``foo=`` on ``x`` with argument ``__synth_0 = y``, followed by a read of the ``__synth_0`` variable.
In CodeQL, this is implemented by syntheisizing AstNode_ instances corresponding to this desugared version of the code.
Note that both the original AssignExpr_ and the desugared SetterMethodCall_ versions are both available to CodeQL queries, and it is usually not necessary to be aware of any desugaring that may take place. However, if a codebase explicitly uses ``x.foo=(y)`` SetterMethodCall_ syntax, then this will not be found by a query for AssignExpr_ instances.
Other synthesized AstNode_ instances exist, see the isSynthesized_ and getDesugared_ predicates for details.
.. _BlockArgument: https://codeql.github.com/codeql-standard-libraries/ruby/codeql/ruby/ast/Call.qll/type.Call$BlockArgument.html
.. _Call: https://codeql.github.com/codeql-standard-libraries/ruby/codeql/ruby/ast/Call.qll/type.Call$Call.html
.. _ElementReference: https://codeql.github.com/codeql-standard-libraries/ruby/codeql/ruby/ast/Call.qll/type.Call$ElementReference.html
@@ -618,3 +639,5 @@ All classes in this subsection are subclasses of VariableAccess_.
.. _VariableAccess: https://codeql.github.com/codeql-standard-libraries/ruby/codeql/ruby/ast/Variable.qll/type.Variable$VariableAccess.html
.. _VariableReadAccess: https://codeql.github.com/codeql-standard-libraries/ruby/codeql/ruby/ast/Variable.qll/type.Variable$VariableReadAccess.html
.. _VariableWriteAccess: https://codeql.github.com/codeql-standard-libraries/ruby/codeql/ruby/ast/Variable.qll/type.Variable$VariableWriteAccess.html
.. _isSynthesized: https://codeql.github.com/codeql-standard-libraries/ruby/codeql/ruby/AST.qll/predicate.AST$AstNode$isSynthesized.0.html
.. _getDesugared: https://codeql.github.com/codeql-standard-libraries/ruby/codeql/ruby/AST.qll/predicate.AST$AstNode$getDesugared.0.html