[clang] [analyzer] Refactor CallDescription match mode (NFC) (PR #83432)

[Balázs Kéri via cfe-commits]

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@@ -27,20 +27,48 @@ class IdentifierInfo;
 
 namespace clang {
 namespace ento {
-
-enum CallDescriptionFlags : unsigned {
-  CDF_None = 0,
-
-  /// Describes a C standard function that is sometimes implemented as a macro
-  /// that expands to a compiler builtin with some __builtin prefix.
-  /// The builtin may as well have a few extra arguments on top of the requested
-  /// number of arguments.
-  CDF_MaybeBuiltin = 1 << 0,
-};
-
-/// This class represents a description of a function call using the number of
-/// arguments and the name of the function.
+/// A `CallDescription` is a pattern that can be used to _match_ calls
+/// based on the qualified name and the argument/parameter counts.
 class CallDescription {
+public:
+  enum class Mode {
+    /// Match calls to functions from the C standard library. On some platforms
+    /// some functions may be implemented as macros that expand to calls to
+    /// built-in variants of the given functions, so in this mode we use some
+    /// heuristics to recognize these implementation-defined variants:
+    ///  - We also accept calls where the name is derived from the specified
+    ///    name by adding "__builtin" or similar prefixes/suffixes.
+    ///  - We also accept calls where the number of arguments or parameters is
+    ///    greater than the specified value.
+    /// For the exact heuristics, see CheckerContext::isCLibraryFunction().
+    /// Note that functions whose declaration context is not a TU (e.g.
+    /// methods, functions in namespaces) are not accepted as C library
+    /// functions.
+    /// FIXME: If I understand it correctly, this discards calls where C++ code
+    /// refers a C library function through the namespace `std::` via headers
+    /// like <cstdlib>.
+    CLibrary,
----------------
balazske wrote:

It is good if it can be used for any C library (or like) function. I saw cases at MallocChecker where it is not used, then it should be used in these cases too. If for some reason we want to find C library functions that are not "builtin" another mode (the "Unspecified") can be used. Probably it is OK if later this mode can find functions in the `std::` namespace.

https://github.com/llvm/llvm-project/pull/83432