fix: build when target_arch==build_arch

In #82 we fixed the build when the target arch is the same as the build arch. A
side effect of the fix is that the rust build-scripts (which are intended to run
on the build host) are now built with the target compiler and cflags.

This is fine in most cases but it breaks when the host system is older than the
system we are building. For example, we might build for a newer glibc that will
be available on the target but when we run the build script on the host, this
version does not exist which leads to the error described in #83.

Another version of the same problem is that we may try to use some build flags
that are available on the compiler for the target (`aarch64-poky-linux-gcc`) but
not on the host compiler (`gcc`) because the host compiler is older than the one
we are using for the target.

This patch fixes the problem by using two unstable features of cargo:

 - [`UNSTABLE_TARGET_APPLIES_TO_HOST`](https://doc.rust-lang.org/cargo/reference/unstable.html#target-applies-to-host)
 allows us to pass the `CARGO_TARGET_APPLIES_TO_HOST=false` setting so that the
 target build settings are not used when building for the host.
 - [`UNSTABLE_HOST_CONFIG`](https://doc.rust-lang.org/cargo/reference/unstable.html#host-config)
 allows us to set build flags for the host in a `[host]` section of the
 `Cargo.toml` file.

The `__CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS` environment variable is
required to use these unstable features on a stable rust. This patch works with
stable rust and does not require unstable.

The inspiration for this solution [comes from buildroot](https://github.com/buildroot/buildroot/blob/25d865996d1d9753fe7d4dfe39cf18c7e9f91224/package/pkg-cargo.mk#L26-L47).

fixes #83

Author: sarfata

classes/cargo.bbclass

@@ -64,6 +64,10 @@ create_cargo_config() {
         echo > ${CARGO_HOME}/config
         echo "[target.${RUST_TARGET}]" >> ${CARGO_HOME}/config
         echo "linker = '${WRAPPER_DIR}/linker-wrapper.sh'" >> ${CARGO_HOME}/config
+
+        # (Rust unstable) - See do_compile below.
+        echo "[host]" >> ${CARGO_HOME}/config
+        echo "linker = '${WRAPPER_DIR}/linker-native-wrapper.sh'" >> ${CARGO_HOME}/config
     fi
 
     echo >> ${CARGO_HOME}/config
@@ -116,12 +120,23 @@ cargo_do_compile() {
     export TARGET_CXX="${WRAPPER_DIR}/cxx-wrapper.sh"
     export CC="${WRAPPER_DIR}/cc-native-wrapper.sh"
     export CXX="${WRAPPER_DIR}/cxx-native-wrapper.sh"
-    export TARGET_LD="${WRAPPER_DIR}/ld-wrapper.sh"
-    export LD="${WRAPPER_DIR}/ld-native-wrapper.sh"
     export PKG_CONFIG_ALLOW_CROSS="1"
     export LDFLAGS=""
     export RUSTFLAGS="${RUSTFLAGS}"
     export SSH_AUTH_SOCK="${SSH_AUTH_SOCK}"
+    # When RUST_BUILD == RUST_TARGET, we need to use an unstable Rust feature
+    # to specify different build options for the target and the Host.
+    # The Host configuration is set in do_configure() above.
+    if [ "${RUST_BUILD}" = "${RUST_TARGET}" ]; then
+        export __CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS="nightly"
+        export CARGO_UNSTABLE_TARGET_APPLIES_TO_HOST="true"
+        export CARGO_UNSTABLE_HOST_CONFIG="true"
+        export CARGO_TARGET_APPLIES_TO_HOST="false"
+
+        # Make sure the cc crate does not use CFLAGS when building for the host.
+        export HOST_CFLAGS=""
+        export HOST_CXXFLAGS=""
+    fi
     bbnote "which rustc:" `which rustc`
     bbnote "rustc --version" `rustc --version`
     bbnote "which cargo:" `which cargo`