[clang] [clang][analyzer][doc] Migrate ClangSA www FAQ section (PR #112831)

[Donát Nagy via cfe-commits]

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+FAQ and How to Deal with Common False Positives
+===============================================
+
+.. contents::
+   :local:
+
+Custom Assertions
+-----------------
+
+Q: How do I tell the analyzer that I do not want the bug being reported here since my custom error handler will safely end the execution before the bug is reached?
+
+You can tell the analyzer that this path is unreachable by teaching it about your `custom assertion handlers <annotations.html#custom_assertions>`_. For example, you can modify the code segment as following:
+
+.. code-block:: c
+
+   void customAssert() __attribute__((analyzer_noreturn));
+   int foo(int *b) {
+     if (!b)
+       customAssert();
+     return *b;
+   }
+
+Null Pointer Dereference
+------------------------
+
+Q: The analyzer reports a null dereference, but I know that the pointer is never null. How can I tell the analyzer that a pointer can never be null?
+
+The reason the analyzer often thinks that a pointer can be null is because the preceding code checked compared it against null. If you are absolutely sure that it cannot be null, remove the preceding check and, preferably, add an assertion as well. For example:
+
+.. code-block:: c
+
+   void usePointer(int *b);
+   int foo(int *b) {
+     usePointer(b);
+     return *b;
+   }
+
+Dead Store
+----------
+
+Q: How do I tell the static analyzer that I don't care about a specific dead store?
+
+When the analyzer sees that a value stored into a variable is never used, it's going to produce a message similar to this one:
+
+.. code-block:: none
+
+   Value stored to 'x' is never read
+
+You can use the ``(void)x;`` idiom to acknowledge that there is a dead store in your code but you do not want it to be reported in the future.
+
+Unused Instance Variable
+------------------------
+
+Q: How do I tell the static analyzer that I don't care about a specific unused instance variable in Objective-C?
+
+When the analyzer sees that a value stored into a variable is never used, it is going to produce a message similar to this one:
+
+.. code-block:: none
+
+   Instance variable 'commonName' in class 'HappyBird' is never used by the methods in its @implementation
+
+You can add ``__attribute__((unused))`` to the instance variable declaration to suppress the warning.
+
+Unlocalized String
+------------------
+
+Q: How do I tell the static analyzer that I don't care about a specific unlocalized string?
----------------
NagyDonat wrote:

This is oddly specific advice, I don't see why is it in the "frequently" asked questions. (Although I'm not native to ObjCLand, maybe this is a common issue there...)

https://github.com/llvm/llvm-project/pull/112831