ffi: Fold signal_ffi.h generation into the Rust build

We're going to need nightly Rust for other things anyway, so we may as
well drop the extra Makefile and just require Cargo.

Also, delete the pkg-config file now that we no longer need to access
libsignal_ffi from arbitrary locations.

rust/bridge/ffi/Cargo.toml

@@ -14,3 +14,6 @@ rand = "0.7.3"
 libc = "0.2"
 num-traits = "0.2"
 num-derive = "0.3"
+
+[build-dependencies]
+cbindgen = "0.14"

rust/bridge/ffi/Makefile

@@ -1,35 +0,0 @@
-CONFIG ?= debug
-
-TARGET_DIR := target/$(CARGO_BUILD_TARGET)/$(CONFIG)
-
-ifeq ($(CONFIG),debug)
-  CARGO_CONFIG_FLAG :=
-else ifeq ($(CONFIG),release)
-  CARGO_CONFIG_FLAG := --release
-else
-  $(error CONFIG can be "debug" or "release" (defaults to "debug"))
-endif
-
-
-default: rust cbindgen pkg-config
-
-clean:
-	cargo clean
-
-rust: src/*.rs Cargo.toml
-	cargo build $(CARGO_CONFIG_FLAG)
-
-cbindgen: $(TARGET_DIR)/signal_ffi.h
-
-target/%/signal_ffi.h: signal_ffi.h
-	cp $^ $@
-
-signal_ffi.h: src/*.rs Cargo.toml cbindgen.toml
-	unset CARGO_BUILD_TARGET && rustup run nightly cbindgen -o $@
-
-pkg-config: $(TARGET_DIR)/signal_ffi.pc
-
-target/%/signal_ffi.pc: signal_ffi.pc
-	cp $^ $@
-
-.PHONY: default clean rust cbindgen pkg-config

rust/bridge/ffi/build.rs

@@ -0,0 +1,26 @@
+//
+// Copyright (C) 2020 Signal Messenger, LLC.
+// All rights reserved.
+//
+// SPDX-License-Identifier: GPL-3.0-only
+//
+
+extern crate cbindgen;
+
+use std::env;
+use std::path::Path;
+
+fn main() {
+    let crate_dir = env::var("CARGO_MANIFEST_DIR").unwrap();
+
+    // Walk up from OUT_DIR to find where to put signal_ffi.h.
+    // Cargo doesn't officially support this, but we'll know if it breaks.
+    let out_dir = env::var("OUT_DIR").unwrap();
+    let out_path = Path::new(&out_dir);
+    assert!(out_path.ancestors().nth(2).unwrap().ends_with("build"));
+    let header_path = out_path.ancestors().nth(3).unwrap().join("signal_ffi.h");
+
+    cbindgen::generate(crate_dir)
+        .unwrap()
+        .write_to_file(header_path);
+}

rust/bridge/ffi/signal_ffi.pc

@@ -1,3 +0,0 @@
-Name: signal_ffi
-Cflags: -I${pcfiledir}
-Libs: -L${pcfiledir} -lsignal_ffi