Make libnixrust a dynamic library

This is a hack to fix the build on macOS, which was failing because
libnixrust.a contains compiler builtins that clash with
libclang_rt.osx.a. There's probably a better solution...

https://hydra.nixos.org/build/107473280

nix-rust/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2018"
 
 [lib]
 name = "nixrust"
-crate-type = ["staticlib"]
+crate-type = ["cdylib"]
 
 [dependencies]
 tar = "0.4"

nix-rust/local.mk

@@ -6,15 +6,29 @@ else
   RUST_DIR = debug
 endif
 
-libnixrust_PATH := $(d)/target/$(RUST_DIR)/libnixrust.a
-libnixrust_INSTALL_PATH := $(libnixrust_PATH)
+libnixrust_PATH := $(d)/target/$(RUST_DIR)/libnixrust.$(SO_EXT)
+libnixrust_INSTALL_PATH := $(libdir)/libnixrust.$(SO_EXT)
 libnixrust_LDFLAGS_USE := -L$(d)/target/$(RUST_DIR) -lnixrust -ldl
-libnixrust_LDFLAGS_USE_INSTALLED := $(libnixrust_LDFLAGS_USE)
+libnixrust_LDFLAGS_USE_INSTALLED := -L$(libdir) -lnixrust -ldl
 
-$(d)/target/$(RUST_DIR)/libnixrust.a: $(wildcard $(d)/src/*.rs) $(d)/Cargo.toml
+ifeq ($(OS), Darwin)
+libnixrust_BUILD_FLAGS = NIX_LDFLAGS="-undefined dynamic_lookup"
+else
+libnixrust_LDFLAGS_USE += -Wl,-rpath,$(abspath $(d)/target/$(RUST_DIR))
+libnixrust_LDFLAGS_USE_INSTALLED += -Wl,-rpath,$(libdir)
+endif
+
+$(libnixrust_PATH): $(wildcard $(d)/src/*.rs) $(d)/Cargo.toml
 	$(trace-gen) cd nix-rust && CARGO_HOME=$$(if [[ -d vendor ]]; then echo vendor; fi) \
-	cargo build $(RUST_MODE) $$(if [[ -d vendor ]]; then echo --offline; fi) \
-	&& touch target/$(RUST_DIR)/libnixrust.a
+	$(libnixrust_BUILD_FLAGS) \
+	  cargo build $(RUST_MODE) $$(if [[ -d vendor ]]; then echo --offline; fi) \
+	&& touch target/$(RUST_DIR)/libnixrust.$(SO_EXT)
+
+$(libnixrust_INSTALL_PATH): $(libnixrust_PATH)
+	$(target-gen) cp $^ $@
+ifeq ($(OS), Darwin)
+	install_name_tool -id $@ $@
+endif
 
 dist-files += $(d)/vendor