cbindgen: Add version defines

This will allow users to check the version of rustls-ffi without
resorting to checking if functions are defined.

In addition to RUSTLS_VERSION_{MAJOR,MINOR,PATCH}, also define
RUSTLS_VERSION_NUMBER, which includes each version part in it, bit shifted to the
left. This is inspired by openssl[0], c-ares[1]. There are other options for
this, for example zstd multiplies each part by a power of 100[2].

We might want to also have the entire version string here, which could
help tools that need to parse the header file itself.

[0] https://github.com/openssl/openssl/blob/cdd01b5e0734b0324251b32a8edd97f42ba90429/include/openssl/opensslv.h.in#L92-L102
[1] https://github.com/c-ares/c-ares/blob/42ddbc14ec008e738fa44aa2c16e74cad93742c2/include/ares_version.h#L43-L45
[2] https://github.com/facebook/zstd/blob/3c3b8274c517727952927c705940eb90c10c736f/lib/zstd.h#L115

Fixes #557

Author: yedayak

librustls/Cargo.toml

@@ -1,5 +1,6 @@
 [package]
 name = "rustls-ffi"
+# Keep in sync with defines in cbindgen.toml
 version = "0.15.0"
 license = "Apache-2.0 OR ISC OR MIT"
 readme = "../README-crates.io.md"

librustls/cbindgen.toml

@@ -3,6 +3,21 @@ language = "C"
 
 usize_is_size_t = true
 
+# Keep in sync with "package.version" in Cargo.toml
+after_includes = """
+
+#define RUSTLS_VERSION_MAJOR 0
+#define RUSTLS_VERSION_MINOR 15
+#define RUSTLS_VERSION_PATCH 0
+
+/**
+ * This gives each version part 8 bits, and leaves the 8 least significant bits
+ * empty for future additions, for example pre-release versions.
+ */
+#define RUSTLS_VERSION_NUMBER ((RUSTLS_VERSION_MAJOR << 24)   \\
+                               |(RUSTLS_VERSION_MINOR << 16)  \\
+                               |(RUSTLS_VERSION_MINOR << 8))"""
+
 [enum]
 prefix_with_name = true
 rename_variants = "ScreamingSnakeCase"

librustls/src/rustls.h

@@ -7,6 +7,18 @@
 #include <stdint.h>
 #include <stdlib.h>
 
+#define RUSTLS_VERSION_MAJOR 0
+#define RUSTLS_VERSION_MINOR 15
+#define RUSTLS_VERSION_PATCH 0
+
+/**
+ * This gives each version part 8 bits, and leaves the 8 least significant bits
+ * empty for future additions, for example pre-release versions.
+ */
+#define RUSTLS_VERSION_NUMBER ((RUSTLS_VERSION_MAJOR << 24)   \
+                               |(RUSTLS_VERSION_MINOR << 16)  \
+                               |(RUSTLS_VERSION_MINOR << 8))
+
 /**
  * Describes which sort of handshake happened.
  */