build: Use `git describe` to generate a version

Only if `RADICLE_VERSION` is not set, we ask Git to desribe the commit
we are building. This is better than having all these cases just be
"pre-release", which is not very informative.

build.rs

@@ -21,10 +21,44 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
             .unwrap_or("unknown".into())
     });
 
-    let version = if let Ok(version) = env::var("RADICLE_VERSION") {
-        version
+    let version = env::var("RADICLE_VERSION").unwrap_or_else(|_| {
+        // If `RADICLE_VERSION` is not set, we still try our best to
+        // describe this version by asking git. The result will in many
+        // cases be a reference to the last released version, and how
+        // many commits we are ahead, plus a short version of the
+        // object ID of `HEAD`, e.g. `releases/x.y.z-80-gefe10f95be-dirty`
+        // which would mean that we built 80 commits ahead of release
+        // x.y.z, with efe10f95be being a unique prefix of the OID of
+        // `HEAD`, and the working directory was dirty.
+        // If this is a build pointing to a commit that has release tag, this
+        // will just return the tag name itelf, e.g. `releases/x.y.z`.
+        // If all fails, we just use `hash`, which, in the worst case is
+        // still "unknown" (see above) but in most cases will just be
+        // the short OID of `HEAD`.
+        Command::new("git")
+            .arg("describe")
+            .arg("--always")
+            .arg("--broken")
+            .arg("--dirty")
+            .output()
+            .ok()
+            .and_then(|output| {
+                if output.status.success() {
+                    String::from_utf8(output.stdout).ok()
                 } else {
-        "pre-release".to_owned()
+                    None
+                }
+            })
+            .unwrap_or(hash.clone())
+    });
+
+    // Since in the previous step we are likely to almost always end up with
+    // a prefix of `releases/`, as this is the scheme we use in this
+    // repository, we remove this common prefix, to get nice version numbers.
+    let version = if let Some(stripped) = version.strip_prefix("releases/") {
+        stripped.to_owned()
+    } else {
+        version
     };
 
     // Set a build-time `SOURCE_DATE_EPOCH` env var which includes the commit time.