[clang-tools-extra] [clang-tidy] new check readability-mark-static (PR #90830)

[Piotr Zegar via cfe-commits]

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+.. title:: clang-tidy - readability-mark-static
+
+readability-mark-static
+=======================
+
+Detects variable and function can be marked as static.
+
+Static functions and variables are scoped to a single file. Marking functions
+and variables as static helps to better remove dead code. In addition, it gives
+the compiler more information and can help compiler make more aggressive
+optimizations.
+
----------------
PiotrZSL wrote:

Yes it does, because you cannot get access to such variable/function via extern in separate TU.

[(2.3)](https://eel.is/c++draft/basic.link#2.3)
When a name has [internal linkage](https://eel.is/c++draft/basic.link#def:linkage,internal), the entity it denotes can be referred to by names from other scopes in the same translation unit[.](https://eel.is/c++draft/basic.link#2.3.sentence-1)

The name of an entity that belongs to a [namespace scope](https://eel.is/c++draft/basic.scope.namespace) has internal linkage if it is the name of
[(3.1)](https://eel.is/c++draft/basic.link#3.1)
a variable, variable template, function, or function template that is explicitly declared static; or


https://github.com/llvm/llvm-project/pull/90830