Clang does not warn for unused variables in included files, but only main file

[kayhayen]

Hello,

I have tried to get a warning about an unused variable in included code. I drilled it down to this, which was created with -E from an example:

# 1 "test.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 360 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "test.c" 2

static int x_top = 1;

# 1 "included.c" 1
static int x_middle = 1;
# 5 "test.c" 2

static int x_bottom = 1;

Compiling with clang-21 and older gives:

gcc -o test.o -c -I/usr/include/python3.11/ -I./nuitka/build/include/nuitka -I./nuitka/build/include/ -IAssertsTest.build -Wall -Wunused-variable -Werror -Wextra preprocessed.c

test.c:2:12: error: unused variable 'x_top' [-Werror,-Wunused-variable]
static int x_top = 1;
           ^
test.c:6:12: error: unused variable 'x_bottom' [-Werror,-Wunused-variable]
static int x_bottom = 1;

Which was very unexpected for me. Another C compiler gave warnings for all 3 variables, including x_middle.

Using a header file "included.h" doesn't change this. I just had a theory that maybe .h files are treated differently, it is applied for .c files as well, which is a bit of a problem due to how I use includes in my project Nuitka.

I didn't succeed in finding if this is expected behavior or not. I didn't succeed in any way with options like -isystem --Wsystem-header, not with warning mapping files, etc. it appears to me that warnings are only given for the main file. Is that itentional?

I have seen it with

clang --version
Ubuntu clang version 21.0.0 (++20250529012635+c474f8f2404d-1~exp1~20250529132841.938)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-21/bin

clang --version
Debian clang version 14.0.6
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

clang-19 --version
Debian clang version 19.1.4 (1~deb12u1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin

[shafik]

x_middle is being kicked out of consideration here:

llvm-project/clang/lib/Sema/SemaDecl.cpp

Lines 1879 to 1883 in eed98e1

	// Constants and utility variables are defined in headers with internal
	// linkage; don't warn. (Unlike functions, there isn't a convenient marker
	// like "inline".)
	if (!isMainFileLoc(*this, VD->getLocation()))
	return false;

Added here: 5ef2175

a long time ago and looks intentional.

CC @AaronBallman @erichkeane

[AaronBallman]

CC @efriedma-quic

We now have inline variables in C++17, so perhaps there is now a convenient marker for us to use?

[efriedma-quic]

We don't have inline variables in C.

We could maybe consider being more aggressive with variables that aren't "const": sticking a definition of non-const variable in a header is an uncommon pattern.

[kayhayen]

It's not necessarily a header file, I am using #include on C files with most of the implementation behind them, with static functions and variables. While I get that .h files cannot be expected to do this, for the .c files I really want to know about unused stuff.