Fix #13875 (Document and rewrite cstyleCast) [ci skip]

Author: danmar

man/checkers/cstyleCast.md

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+
+# cstyleCast
+
+**Category**: Type Safety<br/>
+**Severity**: Style<br/>
+**Language**: C++, not applicable for C code
+
+## Motivation
+
+Casts can be dangerous:
+ * there can be unintentional loss of precision
+ * there can be unintentional loss of sign
+ * there can be unintentional loss of constness
+ * there can be invalid type conversions
+
+## Philosophy
+
+This checker warns about old style C casts.
+
+This checker is not about readability. It is about safety.
+
+We only want to warn if there are possible safety issues and a C++ cast can provide better safety.
+
+## Other tools / complements
+
+Related checkers:
+ * Misra C 11.8 is violated if there is "loss of constness"
+ * Misra C++ 8.2.2 is more pedantic than this checker and warns about all C style casts.
+ * Warnings about "loss of constness" may be written by compilers (gcc/clang reports Wcast-qual).
+
+## Example
+
+### Compliant C style cast
+
+The goal is to not warn about "safe" and/or "intentional" casts. For example:
+```
+int *p = (int*)0;
+```
+Replacing the C style cast with a corresponding C++ cast would not make the code safer.
+
+### Non compliant C style cast
+
+A cast from a base class pointer to a derived class pointer is potentially unsafe:
+```
+Derived *p = (Derived*)base;
+```
+If it is obvious that `base` always point at a Derived object the cast is safe and ideally no warning should be reported.
+But otherwise a `dynamic_cast` is safer.
+
+## How to fix
+
+You can use `dynamic_cast` or `static_cast` to fix warnings.
+
+Before:
+```
+Derived *p = (Derived*)base;
+```
+
+After:
+```
+Derived *p = dynamic_cast<Derived*>(base);
+```
+
+## Enable
+
+--enable=style
+