nix: send the Git date to build.rs

Thanks to @thoughtpolice for providing the original version of the nix code; I
edited it a bit.

cli/build.rs

@@ -69,13 +69,20 @@ fn timestamp_to_date(ts_str: &str) -> Option<DateTime<Utc>> {
         .and_then(|ts| DateTime::<Utc>::from_timestamp(ts, 0))
 }
 
-/// Return the git hash and the committer timestamp
+/// Return the UTC committer date (maybe) and the git hash
 fn get_git_timestamp_and_hash() -> Option<(String, Option<DateTime<Utc>>)> {
-    if let Some(nix_hash) = std::env::var("NIX_JJ_GIT_HASH")
-        .ok()
-        .filter(|s| !s.is_empty())
-    {
-        return Some((nix_hash, None));
+    if let (Ok(nix_hash), maybe_nix_timestamp) = (
+        std::env::var("NIX_JJ_GIT_HASH"),
+        std::env::var("NIX_JJ_GIT_TIMESTAMP"),
+    ) {
+        if !nix_hash.is_empty() {
+            return Some((
+                nix_hash,
+                maybe_nix_timestamp
+                    .ok()
+                    .and_then(|ts_str| timestamp_to_date(&ts_str)),
+            ));
+        }
     }
 
     fn parse_timestamp_vbar_hash(bytes: &[u8]) -> (String, Option<DateTime<Utc>>) {

flake.nix

@@ -18,6 +18,11 @@
   } //
   (flake-utils.lib.eachDefaultSystem (system:
     let
+      # TODO(aseipp): Use dirtyRev and dirtyShortRev to record dirty checkout
+      # when we update `build.rs` to understand dirty checkouts.
+      gitRev = self.rev or "";
+      gitTimestamp = toString self.lastModified;
+
       pkgs = import nixpkgs {
         inherit system;
         overlays = [
@@ -100,7 +105,8 @@
           ZSTD_SYS_USE_PKG_CONFIG = "1";
           LIBSSH2_SYS_USE_PKG_CONFIG = "1";
           RUSTFLAGS = pkgs.lib.optionalString useMoldLinker "-C link-arg=-fuse-ld=mold";
-          NIX_JJ_GIT_HASH = self.rev or "";
+          NIX_JJ_GIT_HASH = gitRev;
+          NIX_JJ_GIT_TIMESTAMP = gitTimestamp;
           CARGO_INCREMENTAL = "0";
 
           preCheck = ''
@@ -188,6 +194,8 @@
           export RUST_BACKTRACE=1
           export ZSTD_SYS_USE_PKG_CONFIG=1
           export LIBSSH2_SYS_USE_PKG_CONFIG=1
+          export NIX_JJ_GIT_HASH="${gitRev}"
+          export NIX_JJ_GIT_TIMESTAMP="${gitTimestamp}";
         '' + pkgs.lib.optionalString useMoldLinker ''
           export RUSTFLAGS="-C link-arg=-fuse-ld=mold"
         '' + darwinNextestHack;