enable -Werror=unused-result

Inspired by
010ff57ebb

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

@@ -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
 

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

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

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

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

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',

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

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