Expand macros before generating the header

BREAKING CHANGE: This requires nightly for cbindgen

Makefile

@@ -36,6 +36,7 @@ test-rust-with-logs:
 .PHONY: test-rust-with-logs
 
 cbindgen:
+	RUSTUP_TOOLCHAIN=nightly \
 	cbindgen glean-core/ffi --lockfile Cargo.lock -o glean-core/ffi/examples/glean.h
 .PHONY: cbindgen
 

glean-core/ffi/cbindgen.toml

@@ -1,22 +1,27 @@
 header = """/* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */"""
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+typedef struct ExternError {
+    int32_t code;
+    char *message; // note: nullable
+} ExternError;
+typedef const char *FfiStr;
+typedef uint64_t TimerId;
+
+"""
 autogen_warning = """/* DO NOT MODIFY THIS MANUALLY! This file was generated using cbindgen.
  * To generate this file:
  *   1. Get the latest cbindgen using `cargo install --force cbindgen`
  *      a. Alternatively, you can clone `https://github.com/eqrion/cbindgen` and use a tagged release
- *   2. Run `cbindgen glean-core/ffi --lockfile Cargo.lock -o glean-core/ffi/examples/glean.h`
+ *   2. Run `RUSTUP_TOOLCHAIN=nightly cbindgen glean-core/ffi --lockfile Cargo.lock -o glean-core/ffi/examples/glean.h`
  */"""
-trailer = """
-void glean_destroy_glean(uint64_t handle, ExternError *error);
-void glean_destroy_boolean_metric(uint64_t handle, ExternError *error);
-void glean_destroy_string_metric(uint64_t handle, ExternError *error);
-void glean_destroy_counter_metric(uint64_t handle, ExternError *error);
-void glean_str_free(char *ptr);
-"""
 
 language = "C"
 
+[parse.expand]
+crates = ["glean_ffi"]
+
 [parse]
 parse_deps = true
 include = ["glean-core", "ffi-support"]