build: Error when attempting to set USECLANG/USEGCC (#58795)

Way back in the good old days, these used to switch between GCC and
Clang. I guess these days we always auto-switch based on the CC value.
If you try to directly set USECLANG, things get into a bad state. Give a
better error message for that case.

Author: Keno

Make.inc

@@ -521,6 +521,10 @@ endif
 
 # Compiler specific stuff
 
+ifneq ($(USECLANG)$(USEGCC),)
+$(error "These internal variables are not overridable. Set CC instead.")
+endif
+
 ifeq (default,$(origin CC))
 CC := $(CROSS_COMPILE)$(CC) # attempt to add cross-compiler prefix, if the user
                             # is not overriding the default, to form target-triple-cc (which
@@ -1661,8 +1665,8 @@ endif
 # Note: we're passing *FLAGS here computed based on your system compiler to
 # clang. If that causes you problems, you might want to build and/or run
 # specific clang-sa-* files with clang explicitly selected:
-#   make CC=~+/../usr/tools/clang CXX=~+/../usr/tools/clang USECLANG=1 analyzegc
-#   make USECLANG=1 clang-sa-*
+#   make CC=~+/../usr/tools/clang CXX=~+/../usr/tools/clang analyzegc
+#   make clang-sa-*
 CLANGSA_FLAGS :=
 CLANGSA_CXXFLAGS :=
 ifeq ($(OS), Darwin) # on new XCode, the files are hidden

contrib/asan/Make.user.asan

@@ -3,7 +3,6 @@ BINDIR=$(TOOLCHAIN)/usr/bin
 TOOLDIR=$(TOOLCHAIN)/usr/tools
 
 # use our new toolchain
-USECLANG=1
 override CC=$(TOOLDIR)/clang
 override CXX=$(TOOLDIR)/clang++
 override PATCHELF=$(TOOLDIR)/patchelf

contrib/pgo-lto/Makefile

@@ -32,7 +32,7 @@ STAGE2_FLAGS:=LDFLAGS="-fuse-ld=lld -flto=thin -Wl,--undefined-version -fprofile
 			CFLAGS="-fprofile-use=$(PROFILE_FILE)" $\
 			CXXFLAGS="-fprofile-use=$(PROFILE_FILE)"
 
-COMMON_FLAGS:=USECLANG=1 USE_BINARYBUILDER_LLVM=0
+COMMON_FLAGS:=USE_BINARYBUILDER_LLVM=0
 
 all: stage2 # Default target as first in file
 

contrib/tsan/Make.user.tsan

@@ -3,7 +3,6 @@ BINDIR=$(TOOLCHAIN)/usr/bin
 TOOLDIR=$(TOOLCHAIN)/usr/tools
 
 # use our new toolchain
-USECLANG=1
 override CC=$(TOOLDIR)/clang
 override CXX=$(TOOLDIR)/clang++
 

doc/src/devdocs/sanitizers.md

@@ -21,14 +21,14 @@ If you require customization or further detail, see the documentation below.
 
 ## General considerations
 
-Using Clang's sanitizers obviously requires you to use Clang (`USECLANG=1`), but there's another
+Using Clang's sanitizers obviously requires you to use Clang, but there's another
 catch: most sanitizers require a run-time library, provided by the host compiler, while the instrumented
 code generated by Julia's JIT relies on functionality from that library. This implies that the
 LLVM version of your host compiler must match that of the LLVM library used within Julia.
 
 An easy solution is to have a dedicated build folder for providing a matching toolchain, by building
 with `BUILD_LLVM_CLANG=1`. You can then refer to this toolchain from another build
-folder by specifying `USECLANG=1` while overriding the `CC` and `CXX` variables.
+folder by overriding the `CC` and `CXX` variables.
 
 The sanitizers error out when they detect a shared library being opened using `RTLD_DEEPBIND`
 (ref: [google/sanitizers#611](https://github.com/google/sanitizers/issues/611)).
@@ -44,7 +44,7 @@ look like this, plus one or more of the `SANITIZE_*` flags listed below:
 
     make -C deps USE_BINARYBUILDER_LLVM=0 LLVM_VER=svn stage-llvm
 
-    make -C src SANITIZE=1 USECLANG=1 \
+    make -C src SANITIZE=1 \
         CC=~+/deps/scratch/llvm-svn/build_Release/bin/clang \
         CXX=~+/deps/scratch/llvm-svn/build_Release/bin/clang++ \
         CPPFLAGS="-isysroot $(xcode-select -p)/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk" \
@@ -99,7 +99,6 @@ Checkout a Git worktree (or create out-of-tree build directory) at
 TOOLCHAIN=$(TOOLCHAIN_WORKTREE)/usr/tools
 
 # use our new toolchain
-USECLANG=1
 override CC=$(TOOLCHAIN)/clang
 override CXX=$(TOOLCHAIN)/clang++
 export ASAN_SYMBOLIZER_PATH=$(TOOLCHAIN)/llvm-symbolizer