fix(toolchain): pass -L as well as an rpath for a toolchain outside /usr

Inside the Flatpak the bundled toolchain compiled every bootstrap object,
then glslang's configure failed its test link with `unable to find
library -lc++`. clang searches its per-target runtime directory
(lib/<triple>) and the system library paths for libc++, not
<prefix>/lib, so with the classic layout a toolchain outside /usr needs
the search path spelled out, just as its loader needs the rpath.
LibcxxLinkFlags and build.sh now emit both, the same pair an explicit
CRAFTER_BUILD_LIBCXX_DIR already got.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

build.sh

@@ -14,12 +14,13 @@ if [ -n "${CRAFTER_BUILD_LLVM_DIR:-}" ]; then
     CC="$CRAFTER_BUILD_LLVM_DIR/bin/clang"
     AR="$CRAFTER_BUILD_LLVM_DIR/bin/llvm-ar"
     LLVM_CONFIG="$CRAFTER_BUILD_LLVM_DIR/bin/llvm-config"
-    # A prefix other than the system one keeps its libc++ off the loader's
-    # default path; the rpath is what lets the binary start.
+    # A prefix other than the system one keeps its libc++ off both the
+    # linker's search path (clang looks in lib/<triple> and the system dirs,
+    # not <prefix>/lib) and the loader's; -L and an rpath cover the two.
     if [ "$CRAFTER_BUILD_LLVM_DIR" != /usr ]; then
-        RPATH_FLAG="-Wl,-rpath,$CRAFTER_BUILD_LLVM_DIR/lib"
+        LIBCXX_LINK_FLAGS="-L$CRAFTER_BUILD_LLVM_DIR/lib -Wl,-rpath,$CRAFTER_BUILD_LLVM_DIR/lib"
     else
-        RPATH_FLAG=""
+        LIBCXX_LINK_FLAGS=""
     fi
     if [ -f "$CRAFTER_BUILD_LLVM_DIR/share/libc++/v1/std.cppm" ]; then
         STD_CPPM="$CRAFTER_BUILD_LLVM_DIR/share/libc++/v1/std.cppm"
@@ -31,7 +32,7 @@ else
     CC=clang
     AR=llvm-ar
     LLVM_CONFIG=llvm-config
-    RPATH_FLAG=""
+    LIBCXX_LINK_FLAGS=""
     STD_CPPM=/usr/share/libc++/v1/std.cppm
 fi
 
@@ -65,8 +66,8 @@ cmake -B build \
   -DCMAKE_C_COMPILER="$CC" \
   -DCMAKE_CXX_COMPILER="$CXX" \
   -DCMAKE_CXX_FLAGS="-stdlib=libc++" \
-  -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++ $RPATH_FLAG" \
-  -DCMAKE_SHARED_LINKER_FLAGS="-stdlib=libc++ $RPATH_FLAG" \
+  -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++ $LIBCXX_LINK_FLAGS" \
+  -DCMAKE_SHARED_LINKER_FLAGS="-stdlib=libc++ $LIBCXX_LINK_FLAGS" \
   -DCMAKE_BUILD_TYPE=Release \
   -DCMAKE_ARCHIVE_OUTPUT_DIRECTORY="$current_dir/build" \
   -DCMAKE_LIBRARY_OUTPUT_DIRECTORY="$current_dir/build" \
@@ -117,7 +118,7 @@ rm -f ./lib/libcrafter-build.a
 "$AR" rcs ./lib/libcrafter-build.a $OBJECTS
 
 "$CXX" --target="$TARGET" -std=c++26 -stdlib=libc++ -O3 -march="$MARCH" -mtune="$MTUNE" -fuse-ld=lld \
-    -Wl,--export-dynamic $RPATH_FLAG \
+    -Wl,--export-dynamic $LIBCXX_LINK_FLAGS \
     -L./build \
     $OBJECTS \
     ./build/main.o \

implementations/Crafter.Build-Toolchain.cpp

@@ -51,14 +51,17 @@ std::string Toolchain::LibcxxCompileFlags() const {
 }
 
 std::string Toolchain::LibcxxLinkFlags() const {
-    std::string flags;
+    // clang searches its per-target runtime directory and the system library
+    // paths for -lc++, not <prefix>/lib, so a toolchain outside /usr with the
+    // classic layout needs the search path as well as the run-time path.
+    fs::path lib;
     if (!libcxxDir.empty()) {
-        fs::path lib = libcxxDir / "lib";
-        flags += std::format(" -L{} -Wl,-rpath,{}", lib.string(), lib.string());
+        lib = libcxxDir / "lib";
     } else if (!prefix.empty() && prefix != "/usr") {
-        flags += std::format(" -Wl,-rpath,{}", (prefix / "lib").string());
+        lib = prefix / "lib";
     }
-    return flags;
+    if (lib.empty()) return {};
+    return std::format(" -L{} -Wl,-rpath,{}", lib.string(), lib.string());
 }
 
 std::string Toolchain::Key() const {

interfaces/Crafter.Build-Toolchain.cppm

@@ -44,10 +44,12 @@ export namespace Crafter {
 
         // Flags for compiling against libcxxDir; empty without one.
         std::string LibcxxCompileFlags() const;
-        // Flags for linking against the libc++ the compiler will use: an
-        // rpath into the prefix when that is not the system one (a bundled
-        // toolchain's libc++.so.1 is not on the loader's default path), plus
-        // -L and an rpath for an explicit libcxxDir.
+        // Flags for linking against the libc++ the compiler will use: a -L
+        // and an rpath for <prefix>/lib when the prefix is not the system one
+        // (clang searches only its per-target runtime directory and the
+        // system paths, and a bundled toolchain's libc++.so.1 is on neither
+        // the linker's nor the loader's default path), or for an explicit
+        // libcxxDir.
         std::string LibcxxLinkFlags() const;
         // Names this toolchain for cache directory names: the clang version
         // followed by a hash of what decides which headers and libraries a

tests/ToolchainResolve/main.cpp

@@ -73,7 +73,7 @@ int main() {
         Check(tc.stdCppm == full / "share/libc++/v1/std.cppm", "the prefix's std module source is used");
         Check(tc.wasiSysroot == full / "share/wasi-sysroot", "the prefix's wasi sysroot is used");
         Check(tc.LibcxxCompileFlags().empty(), "the compiler finds its own libc++ headers without flags");
-        Check(Contains(tc.LibcxxLinkFlags(), std::format("-Wl,-rpath,{}", (full / "lib").string())), "binaries get an rpath into the prefix's lib dir");
+        Check(Contains(tc.LibcxxLinkFlags(), std::format("-L{}", (full / "lib").string())) && Contains(tc.LibcxxLinkFlags(), std::format("-Wl,-rpath,{}", (full / "lib").string())), "binaries get -L and an rpath for the prefix's lib dir");
     }
     {
         ToolchainInputs in;
@@ -95,7 +95,7 @@ int main() {
         Check(tc.llvmIncludeDir.empty(), "no clang-c headers in the prefix: nothing added");
         Check(tc.stdCppm == "/usr/share/libc++/v1/std.cppm", "no std module in the prefix: the system copy");
         Check(tc.wasiSysroot == "/usr/share/wasi-sysroot", "no wasi sysroot in the prefix: the system one");
-        Check(Contains(tc.LibcxxLinkFlags(), std::format("-Wl,-rpath,{}", (bare / "lib").string())), "the rpath follows the explicit prefix");
+        Check(Contains(tc.LibcxxLinkFlags(), std::format("-L{}", (bare / "lib").string())) && Contains(tc.LibcxxLinkFlags(), std::format("-Wl,-rpath,{}", (bare / "lib").string())), "the link flags follow the explicit prefix");
     }
     {
         ToolchainInputs in;