coccinelle: misc: add uninitialized_var.cocci script

uninitialized_var() macro was removed from the sources [1] and
other warning-silencing tricks were deprecated [2]. The purpose of this
cocci script is to prevent new occurrences of uninitialized_var()
open-coded variants.

[1] commit 63a0895d ("compiler: Remove uninitialized_var() macro")
[2] commit 4b19bec9 ("docs: deprecated.rst: Add uninitialized_var()")

Cc: Kees Cook <keescook@chromium.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

scripts/coccinelle/misc/uninitialized_var.cocci

+// SPDX-License-Identifier: GPL-2.0-only
+///
+/// Please, don't reintroduce uninitialized_var().
+/// From Documentation/process/deprecated.rst:
+///  For any compiler warnings about uninitialized variables, just add
+///  an initializer. Using warning-silencing tricks is dangerous as it
+///  papers over real bugs (or can in the future), and suppresses unrelated
+///  compiler warnings (e.g. "unused variable"). If the compiler thinks it
+///  is uninitialized, either simply initialize the variable or make compiler
+///  changes. Keep in mind that in most cases, if an initialization is
+///  obviously redundant, the compiler's dead-store elimination pass will make
+///  sure there are no needless variable writes.
+///
+// Confidence: High
+// Copyright: (C) 2020 Denis Efremov ISPRAS
+// Options: --no-includes --include-headers
+//
+
+virtual context
+virtual report
+virtual org
+
+@r@
+identifier var;
+type T;
+position p;
+@@
+
+(
+* T var =@p var;
+|
+* T var =@p *(&(var));
+|
+* var =@p var
+|
+* var =@p *(&(var))
+)
+
+@script:python depends on report@
+p << r.p;
+@@
+
+coccilib.report.print_report(p[0],
+  "WARNING this kind of initialization is deprecated (https://www.kernel.org/doc/html/latest/process/deprecated.html#uninitialized-var)")
+
+@script:python depends on org@
+p << r.p;
+@@
+
+coccilib.org.print_todo(p[0],
+  "WARNING this kind of initialization is deprecated (https://www.kernel.org/doc/html/latest/process/deprecated.html#uninitialized-var)")