classes: create proper compiler and linker wrappers

Rust builds (custom build.rs and gcc-rs users) expect to have the
compiler and the linker available to them as a single command with no
white space trailing. Yocto unfortunately does not conform to that. The
build compiler and linker almost always have a trailing space due to
how the variables are composed and the target compiler and linker are
almost always more than a single command. Then if you throw ccache into
the mix you'll get another command. As a result to handle all these
cases properly there need to be wrappers created. This change creates
wrappers for both build and target and both the linker and compiler.
This likely fixes #76.

classes/rust-triples.bbclass

@@ -56,3 +56,51 @@ def rust_base_triple(d, thing):
 RUST_BUILD_SYS = "${@rust_base_triple(d, 'BUILD')}"
 RUST_HOST_SYS = "${@rust_base_triple(d, 'HOST')}"
 RUST_TARGET_SYS = "${@rust_base_triple(d, 'TARGET')}"
+
+# wrappers to get around the fact that Rust needs a single
+# binary but Yocto's compiler and linker commands have
+# arguments. Technically the archiver is always one command but
+# this is necessary for builds that determine the prefix and then
+# use those commands based on the prefix.
+WRAPPER_DIR = "${WORKDIR}/wrapper"
+RUST_BUILD_CC = "${WRAPPER_DIR}/build-rust-cc"
+RUST_BUILD_CCLD = "${WRAPPER_DIR}/build-rust-ccld"
+RUST_BUILD_AR = "${WRAPPER_DIR}/build-rust-ar"
+RUST_TARGET_CC = "${WRAPPER_DIR}/target-rust-cc"
+RUST_TARGET_CCLD = "${WRAPPER_DIR}/target-rust-ccld"
+RUST_TARGET_AR = "${WRAPPER_DIR}/target-rust-ar"
+
+create_wrapper () {
+	file="$1"
+	shift
+
+	cat <<- EOF > "${file}"
+	#!/bin/sh
+	$@ "\$@"
+	EOF
+	chmod +x "${file}"
+}
+
+# compiler is used by gcc-rs
+# linker is used by rustc/cargo
+# archiver is used by the build of libstd-rs
+do_rust_create_wrappers () {
+	mkdir -p "${WRAPPER_DIR}"
+
+	# Yocto Build / Rust Host compiler
+	create_wrapper "${RUST_BUILD_CC}" "${BUILD_CC}"
+	# Yocto Build / Rust Host linker
+	create_wrapper "${RUST_BUILD_CCLD}" "${BUILD_CCLD}" "${BUILD_LDFLAGS}"
+	# Yocto Build / Rust Host archiver
+	create_wrapper "${RUST_BUILD_AR}" "${BUILD_AR}"
+
+	# Yocto Target / Rust Target compiler
+	create_wrapper "${RUST_TARGET_CC}" "${CC}"
+	# Yocto Target / Rust Target linker
+	create_wrapper "${RUST_TARGET_CCLD}" "${CCLD}" "${LDFLAGS}"
+	# Yocto Target / Rust Target archiver
+	create_wrapper "${RUST_TARGET_AR}" "${AR}"
+}
+
+addtask rust_create_wrappers before do_configure after do_patch
+do_rust_create_wrappers[dirs] += "${WRAPPER_DIR}"