Add flake.templates, justfile, and .envrc for direnv

Compared this template against the older nix-templates/templates/rust
("rrrr") ahead of eventually replacing it. Ported the pieces that were
genuinely missing and non-controversial:

- flake.templates.default, so `nix flake init -t <url>` works once
  this repo is published — the actual mechanism a "template" needs.
- justfile wrapping the cargo/nix commands for discoverability.
- .envrc (use flake) for direnv auto-activation.
- `just` added to the dev shell so the justfile is usable without a
  separate install.

Left out on purpose: workspace crate split, tracing/serde/tokio-heavy
example code, docker.nix, nightly coverage toolchain, and copier.yaml —
those make it an opinionated async-service starter rather than a blank
Rust template, and copier.yaml in the old repo was dead configuration
(no file actually referenced its Jinja variables).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018ALEXkc7pTro7tF1WcUuKb

.envrc

@@ -0,0 +1,11 @@
+# Automatically enter the Nix dev shell on `cd`. Requires direnv: https://direnv.net/
+if ! has nix_direnv_version || ! nix_direnv_version 3.0.4; then
+  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/3.0.4/direnvrc" "sha256-DzlYZ33mWF/Gs8DDeyjr8mnVmQGx7ASYqA5WlxwvBG4="
+fi
+
+use flake
+
+watch_file flake.nix
+watch_file flake.lock
+watch_file nix/*.nix
+watch_file Cargo.toml

CLAUDE.md

@@ -50,6 +50,10 @@ Both paths are meant to converge on the same result — the Nix devShell and the
 are built from one shared toolchain/build-args definition (see Architecture below), so what
 compiles under `nix build` should also compile under plain `cargo build` inside `nix develop`.
 
+`justfile` wraps the commands above (`just build`, `just test`, `just ci`, `just nix-check`, ...;
+run `just` with no args to list them) — available inside `nix develop`. `.envrc` (`use flake`) lets
+direnv users skip typing `nix develop` entirely.
+
 ## IDE / LSP support
 
 `rust-analyzer` is included in the fenix toolchain (`nix/rust.nix`), and `nix develop`'s
@@ -74,7 +78,8 @@ install, so the LSP sees the same compiler version as the build.
   treefmt-nix, wired into both `nix fmt` and the `formatting` check in `flake.nix`.
 - **`flake.nix`** ties it together with `flake-parts`: it imports the treefmt module, evaluates
   `nix/rust.nix` per-system, and exposes `packages.default`, `checks`, `apps.default`, and
-  `devShells.default` from its outputs.
+  `devShells.default` from its outputs. It also declares `flake.templates.default`, so once this
+  repo is published, `nix flake init -t <url>` scaffolds a new project straight from it.
 - **`src/lib.rs`** holds the actual logic (currently just `greet`); `src/main.rs` is a thin `clap`
   wrapper around it. Keep new functionality in the lib crate, not `main.rs`, so it stays reachable
   from `tests/` and `benches/` (both depend on the `rust_template` lib crate by name, not on the

README.md

@@ -6,7 +6,7 @@ way with plain `cargo` or with `nix`.
 
 ## Using this template
 
-1. Copy/clone this repo.
+1. Copy/clone this repo (or, once published, `nix flake init -t <this-repo-url>`).
 2. Rename the placeholder crate name:
    ```bash
    ./scripts/rename-project.sh my_project
@@ -34,4 +34,9 @@ nix flake check              # clippy, fmt, doc, audit, tests — all in one go
 nix fmt                       # format Rust, Nix, and TOML files
 ```
 
+If you have [direnv](https://direnv.net/) installed, `direnv allow` activates the dev shell
+automatically on `cd` (see `.envrc`).
+
+`just` (available inside `nix develop`) wraps the common commands above — run `just` to list them.
+
 See `CLAUDE.md` for more detail on the project layout and how the pieces fit together.

flake.nix

@@ -76,5 +76,10 @@
 
           devShells.default = import ./nix/devshell.nix { inherit pkgs lib rust; };
         };
+
+      flake.templates.default = {
+        path = ./.;
+        description = "Rust CLI + lib template with crane, fenix, and treefmt-nix";
+      };
     };
 }

justfile

@@ -0,0 +1,71 @@
+# justfile — task runner. See: https://just.systems/
+# Available via `nix develop` (or install `just` yourself).
+
+default:
+    @just --list
+
+# --- cargo ---
+
+build:
+    cargo build
+
+build-release:
+    cargo build --release
+
+run *args:
+    cargo run -- {{ args }}
+
+test:
+    cargo test
+
+bench:
+    cargo bench
+
+clippy:
+    cargo clippy --all-targets -- --deny warnings
+
+fmt:
+    nix fmt
+
+fmt-check:
+    nix fmt -- --fail-on-change
+
+audit:
+    cargo audit
+
+doc:
+    cargo doc --no-deps
+
+doc-open:
+    cargo doc --no-deps --open
+
+# Everything nix flake check runs, but via cargo for a faster local loop
+ci: fmt-check clippy test audit doc
+
+clean:
+    cargo clean
+    rm -rf result result-*
+
+# Background code checker (bacon)
+watch:
+    bacon
+
+# --- nix ---
+
+nix-build:
+    nix build
+
+nix-check:
+    nix flake check
+
+nix-develop:
+    nix develop
+
+nix-update:
+    nix flake update
+
+# --- template ---
+
+# Rename the "rust_template" placeholder to <name> (snake_case)
+rename name:
+    ./scripts/rename-project.sh {{ name }}

nix/devshell.nix

@@ -18,6 +18,7 @@ rust.craneLib.devShell {
       cargo-expand
       taplo
       bacon
+      just
     ]
     ++ lib.optionals pkgs.stdenv.isLinux [ mold ]
     ++ lib.optionals pkgs.stdenv.isDarwin [