From 101915c9b73cfe434457f380de4fbaea03430851 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B6rg=20Thalheim?= <joerg@thalheim.io>
Date: Tue, 2 Jul 2024 08:35:56 +0200
Subject: [PATCH] enable -Werror=unused-result

Inspired by
https://git.lix.systems/lix-project/lix/commit/010ff57ebb40f1a9aaff99867d2886f0e59f774a

From the original PR:

> We do not have any of these warnings appearing at the moment, but
> it seems like a good idea to enable [[nodiscard]] checking anyway.
> Once we start introducing more functions with must-use conditions we will
> need such checking, and the rust stdlib has proven them very useful.
---
 Makefile                               | 2 +-
 src/libfetchers/meson.build            | 1 +
 src/libstore/meson.build               | 1 +
 src/libutil-c/meson.build              | 1 +
 src/libutil/meson.build                | 1 +
 src/perl/meson.build                   | 5 ++++-
 tests/unit/libutil-support/meson.build | 1 +
 tests/unit/libutil/meson.build         | 1 +
 8 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 132fe29cc..bb64a104e 100644
--- a/Makefile
+++ b/Makefile
@@ -92,7 +92,7 @@ ifdef HOST_WINDOWS
   GLOBAL_LDFLAGS += -Wl,--export-all-symbols
 endif
 
-GLOBAL_CXXFLAGS += -g -Wall -Wdeprecated-copy -Wignored-qualifiers -Wimplicit-fallthrough -include $(buildprefix)config.h -std=c++2a -I src
+GLOBAL_CXXFLAGS += -g -Wall -Wdeprecated-copy -Wignored-qualifiers -Wimplicit-fallthrough -Werror=unused-result -include $(buildprefix)config.h -std=c++2a -I src
 
 # Include the main lib, causing rules to be defined
 
diff --git a/src/libfetchers/meson.build b/src/libfetchers/meson.build
index cab8d63eb..d7975ac65 100644
--- a/src/libfetchers/meson.build
+++ b/src/libfetchers/meson.build
@@ -58,6 +58,7 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  '-Werror=unused-result',
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
   # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
diff --git a/src/libstore/meson.build b/src/libstore/meson.build
index a605b43e1..d9237c55a 100644
--- a/src/libstore/meson.build
+++ b/src/libstore/meson.build
@@ -164,6 +164,7 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  '-Werror=unused-result',
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
   # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
diff --git a/src/libutil-c/meson.build b/src/libutil-c/meson.build
index a2b020818..5de288e18 100644
--- a/src/libutil-c/meson.build
+++ b/src/libutil-c/meson.build
@@ -34,6 +34,7 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  '-Werror=unused-result',
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
   # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
diff --git a/src/libutil/meson.build b/src/libutil/meson.build
index 9d0b28539..099c0c65f 100644
--- a/src/libutil/meson.build
+++ b/src/libutil/meson.build
@@ -143,6 +143,7 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  '-Werror=unused-result',
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
   # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
diff --git a/src/perl/meson.build b/src/perl/meson.build
index 06abb4f2e..5fe7e1e28 100644
--- a/src/perl/meson.build
+++ b/src/perl/meson.build
@@ -23,11 +23,14 @@ nix_perl_conf.set('PACKAGE_VERSION', meson.project_version())
 # set error arguments
 #-------------------------------------------------
 error_args = [
+  '-Werror=unused-result',
+  '-Wdeprecated-copy',
+  '-Wdeprecated-declarations',
+  '-Wignored-qualifiers',
   '-Wno-pedantic',
   '-Wno-non-virtual-dtor',
   '-Wno-unused-parameter',
   '-Wno-variadic-macros',
-  '-Wdeprecated-declarations',
   '-Wno-missing-field-initializers',
   '-Wno-unknown-warning-option',
   '-Wno-unused-variable',
diff --git a/tests/unit/libutil-support/meson.build b/tests/unit/libutil-support/meson.build
index 5912c00e0..c0345a6ee 100644
--- a/tests/unit/libutil-support/meson.build
+++ b/tests/unit/libutil-support/meson.build
@@ -28,6 +28,7 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  '-Werror=unused-result',
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
   # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked
diff --git a/tests/unit/libutil/meson.build b/tests/unit/libutil/meson.build
index 56147686b..cc13c4364 100644
--- a/tests/unit/libutil/meson.build
+++ b/tests/unit/libutil/meson.build
@@ -34,6 +34,7 @@ add_project_arguments(
   '-Wimplicit-fallthrough',
   '-Werror=switch',
   '-Werror=switch-enum',
+  '-Werror=unused-result',
   '-Wdeprecated-copy',
   '-Wignored-qualifiers',
   # Enable assertions in libstdc++ by default. Harmless on libc++. Benchmarked