According to Makefile $(RUST_LIB) target depends on $(LIB_FILE):

$(RUST_LIB): Cargo.toml $(RUST_SOURCES) $(LIB_FILE)

but is that really the case? As far as I can tell $(RUST_LIB) does not
use $(LIB_FILE) in any way and there's no such dependency in
meson.build.