Configure nix development shell environment

This gets used by `nix develop`, or automatically by `direnv` if you have it
installed.

The binary versions are pinned to the versions recommended by `contributing.md`.

```
>> cargo --version
cargo 1.60.0 (d1fd9fe 2022-03-01)

>> rustc --version
rustc 1.60.0 (7737e0b5c 2022-04-04)

>> cargo fmt --version
rustfmt 1.5.1-nightly (3984bc5 2023-01-17)

>> cargo clippy --version
clippy 0.1.60 (7737e0b 2022-04-04)
```

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -3,3 +3,4 @@
 result
 # generated by the insta crate
 *.pending-snap
+.direnv

Cargo.toml

@@ -3,7 +3,7 @@ name = "jujutsu"
 version = "0.6.1"
 authors = ["Martin von Zweigbergk <martinvonz@google.com>"]
 edition = "2021"
-rust-version = "1.60"  # Remember to update CI
+rust-version = "1.60"  # Remember to update CI, contributing.md, and flake.nix
 license = "Apache-2.0"
 description = "Jujutsu (an experimental VCS)"
 homepage = "https://github.com/martinvonz/jj"

flake.lock

@@ -1,5 +1,20 @@
 {
   "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1659877975,
+        "narHash": "sha256-zllb8aq3YO3h8B/U0/J1WBgAL8EX5yWf5pMj3G0NAmc=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "c0e246b9b83f637f4681389ecabcb2681b4f3af0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1651634615,
@@ -16,7 +31,29 @@
     },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "rust-overlay": "rust-overlay"
+      }
+    },
+    "rust-overlay": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1674008920,
+        "narHash": "sha256-ugwPxKjvmJ5GpzN/MHg8tuhe8nYi3SbJm5nWNy7CB0Q=",
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "rev": "eecc44934a0f6c02c02856b38bd3b6af3bec0870",
+        "type": "github"
+      },
+      "original": {
+        "owner": "oxalica",
+        "repo": "rust-overlay",
+        "type": "github"
       }
     }
   },

flake.nix

@@ -1,7 +1,13 @@
 {
   description = "jujutsu";
 
-  outputs = { self, nixpkgs, ... }:
+  inputs = {
+    # For installing non-standard rustc versions
+    rust-overlay.url = "github:oxalica/rust-overlay";
+    rust-overlay.inputs.nixpkgs.follows = "nixpkgs";
+  };
+
+  outputs = { self, nixpkgs, rust-overlay, ... }:
     let
       lib = nixpkgs.lib;
       systems = [
@@ -83,7 +89,10 @@
       let
         pkgs = import nixpkgs {
           inherit system;
-          overlays = [ self.overlays.default ];
+          overlays = [
+            self.overlays.default
+            rust-overlay.overlays.default
+          ];
         };
       in
       {
@@ -103,5 +112,30 @@
           '';
         });
         formatter.${system} = pkgs.nixpkgs-fmt;
+        devShells.${system}.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            # Using the minimal profile with explicit "clippy" extension to avoid
+            # two versions of rustfmt
+            (rust-bin.stable."1.60.0".minimal.override {
+              extensions = [
+                "rust-src" # for rust-analyzer
+                "clippy"
+              ];
+            })
+
+            # The CI checks against the latest nightly rustfmt, so we should too.
+            (rust-bin.selectLatestNightlyWith (toolchain: toolchain.rustfmt))
+
+            # Required build dependencies
+            openssl
+            pkg-config # to find openssl
+
+            # Additional tools recommended by contributing.md
+            cargo-deny
+            cargo-insta
+            cargo-nextest
+            cargo-watch
+          ];
+        };
       }));
 }