rust: Make dependencies visible

The Cargo.lock of rust_riotmodules_standalone should explicitly list all
versions of software that would ever be pulled in by enabling RIOT
modules backed by a Rust crate. This did not happen before, because
while the optional dependencies could all be selected on the command
line, they were not visible as possible features to `cargo update`, and
were thus not encoded in the Cargo.lock file.

sys/rust_riotmodules/Cargo.toml

@@ -12,3 +12,8 @@ publish = false
 
 riot-module-lsm303agr = { path = "../../drivers/lsm303agr", optional = true }
 riot-module-shell-democommands = { path = "../../sys/shell/democommands", optional = true }
+
+[features]
+# This feature is just there so that rust_riotmodules_standalone can use its
+# _all; see documentatio there.
+_all = [ "riot-module-lsm303agr", "riot-module-shell-democommands" ]

sys/rust_riotmodules_standalone/Cargo.toml

@@ -19,3 +19,11 @@ codegen-units = 1
 riot-wrappers = { version = "0.8", features = [ "set_panic_handler" ] }
 
 rust_riotmodules = { path = "../rust_riotmodules" }
+
+[features]
+# By having this explicit feature (comapred to what is enabled by the build
+# system as `--features rust_riotmodules/foo`), all possible dependencies are
+# visible to `cargo update` and in `Cargo.lock. This should not be enabled for
+# compilation, as the modules behind it likely have their own RIOT modules they
+# need enabled.
+_all = [ "rust_riotmodules/_all" ]