[PATCH 2/8] auto: add "auto" keyword as alias for __auto_type

[Alexey Dobriyan <adobriyan@gmail.com>]

It has similar semantics to "auto" keyword from a language
which can not be named on this mailing list, in particular:

	{
		int a;
		const auto b = a; // const int b = a;
		b = 1;	// compile error
	}
	{
		char a;
		auto b = a; // char b = a;
		// no integer promotions
		static_assert(sizeof(b) == 1);
	}
	{
		int a;
		const auto p = &a; // int *const p = &a;
		*p = 1;	// works because const is applied only to top-level
	}

It can be used to save on macroexpansion inside macro forests which
use typeof() somewhere deep enough. It is cool regardless.

gcc 5.1 supports __auto_type.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---
 Documentation/process/coding-style.rst | 31 +++++++++++++++++++++-----
 include/linux/compiler_types.h         |  2 ++
 2 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 6db37a46d305..5bfa605c8bac 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -1018,7 +1018,26 @@ result.  Typical examples would be functions that return pointers; they use
 NULL or the ERR_PTR mechanism to report failure.
 
 
-17) Using bool
+17) Using auto
+--------------
+
+Use ``auto`` macro-keyword (alias for ``__auto_type`` extension) in macros
+with "evaluate argument once" idiom:
+
+.. code-block:: c
+
+        #define min2(a, b)              \
+        ({                              \
+                auto a_ = (a);          \
+                auto b_ = (b);          \
+                a_ < b_ ? a_ : b_;      \
+        })
+
+Read https://gcc.gnu.org/onlinedocs/gcc/Typeof.html before using ``auto`` or
+changing anything with ``auto`` in it.
+
+
+18) Using bool
 --------------
 
 The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
@@ -1048,7 +1067,7 @@ readable alternative if the call-sites have naked true/false constants.
 Otherwise limited use of bool in structures and arguments can improve
 readability.
 
-18) Don't re-invent the kernel macros
+19) Don't re-invent the kernel macros
 -------------------------------------
 
 The header file include/linux/kernel.h contains a number of macros that
@@ -1071,7 +1090,7 @@ need them.  Feel free to peruse that header file to see what else is already
 defined that you shouldn't reproduce in your code.
 
 
-19) Editor modelines and other cruft
+20) Editor modelines and other cruft
 ------------------------------------
 
 Some editors can interpret configuration information embedded in source files,
@@ -1105,7 +1124,7 @@ own custom mode, or may have some other magic method for making indentation
 work correctly.
 
 
-20) Inline assembly
+21) Inline assembly
 -------------------
 
 In architecture-specific code, you may need to use inline assembly to interface
@@ -1137,7 +1156,7 @@ the next instruction in the assembly output:
 	     : /* outputs */ : /* inputs */ : /* clobbers */);
 
 
-21) Conditional Compilation
+22) Conditional Compilation
 ---------------------------
 
 Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
@@ -1186,7 +1205,7 @@ expression used.  For instance:
 	#endif /* CONFIG_SOMETHING */
 
 
-22) Do not crash the kernel
+23) Do not crash the kernel
 ---------------------------
 
 In general, the decision to crash the kernel belongs to the user, rather
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 547ea1ff806e..bf0ac2a04154 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -4,6 +4,8 @@
 
 #ifndef __ASSEMBLY__
 
+#define auto __auto_type
+
 /*
  * Skipped when running bindgen due to a libclang issue;
  * see https://github.com/rust-lang/rust-bindgen/issues/2244.
-- 
2.40.1