[PATCH] Documentation: warn against using int, hex, string options as expressions in Kconfig

I've been working on type checking Kconfig, and found some instances of
dead code due to conditions that always evaluate to false.

For example:

config FOO
  int

config BAR
  bool
  default 'y' if FOO

This 'if FOO' will always be false, regardless of FOO's value (even when
FOO=1). The same goes for strings and hex.

See also this related patch:
https://lore.kernel.org/all/[email protected]/

Let's warn users against this usage.

Signed-off-by: Julian Braha <[email protected]>
---
 Documentation/kbuild/kconfig-language.rst | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
index ab2822d7ca1c..aee84b294a56 100644
--- a/Documentation/kbuild/kconfig-language.rst
+++ b/Documentation/kbuild/kconfig-language.rst
@@ -320,6 +320,17 @@ An expression can have a value of 'n', 'm' or 'y' (or 0, 1, 2
 respectively for calculations). A menu entry becomes visible when its
 expression evaluates to 'm' or 'y'.
 
+Note that options of int, hex, or string type should not be used directly
+as expressions; these will always evaluate to 'n'. Instead, to check if
+one of these options is in use, consider using one of these heuristics::
+
+  <symbol> != 0
+  <symbol> != 0x0
+  <symbol> != ""
+
+because int, hex, and string options will take these values of '0', '0x0',
+and the empty string, respectively, when the dependencies are unmet.
+
 There are two types of symbols: constant and non-constant symbols.
 Non-constant symbols are the most common ones and are defined with the
 'config' statement. Non-constant symbols consist entirely of alphanumeric
-- 
2.55.0