eclass/cargo.eclass: add CARGO_OPTIONAL condition

Signed-off-by: Georgy Yakovlev <[email protected]>

eclass/cargo.eclass

@@ -15,20 +15,41 @@ _CARGO_ECLASS=1
 # we need this for 'cargo vendor' subcommand and net.offline config knob
 RUST_DEPEND=">=virtual/rust-1.37.0"
 
-case ${EAPI} in
- 7) BDEPEND="${RUST_DEPEND}";;
- *) die "EAPI=${EAPI:-0} is not supported" ;;
+case "${EAPI:-0}" in
+ 0|1|2|3|4|5|6)
+ die "Unsupported EAPI=${EAPI:-0} (too old) for ${ECLASS}"
+ ;;
+ 7)
+ ;;
+ *)
+ die "Unsupported EAPI=${EAPI} (unknown) for ${ECLASS}"
+ ;;
 esac
 
 inherit multiprocessing toolchain-funcs
 
-EXPORT_FUNCTIONS src_unpack src_configure src_compile src_install src_test
+if [[ ! ${CARGO_OPTIONAL} ]]; then
+ BDEPEND="${RUST_DEPEND}"
+ EXPORT_FUNCTIONS src_unpack src_configure src_compile src_install src_test
+fi
 
 IUSE="${IUSE} debug"
 
 ECARGO_HOME="${WORKDIR}/cargo_home"
 ECARGO_VENDOR="${ECARGO_HOME}/gentoo"
 
+# @ECLASS-VARIABLE: CARGO_OPTIONAL
+# @DEFAULT_UNSET
+# @PRE_INHERIT
+# @DESCRIPTION:
+# If set to a non-null value, before inherit cargo part of the ebuild will
+# be considered optional. No dependencies will be added and no phase
+# functions will be exported.
+#
+# If you enable CARGO_OPTIONAL, you have to set BDEPEND on virtual/rust
+# for your package and call at least cargo_gen_config manually before using
+# other src_ functions of this eclass.
+
 # @VARIABLE: myfeatures
 # @DEFAULT_UNSET
 # @DESCRIPTION: