Many UI changes

devenv.nix

@@ -1,16 +1,22 @@
-{ pkgs, ... }:
+{ config, lib, pkgs, ... }:
 
+let
+  sourceRoot = builtins.toString ./.;
+  sourceKey = builtins.substring 0 12 (builtins.hashString "sha256" sourceRoot);
+  transientDotfile = "/tmp/furumi-tui-devenv-${sourceKey}";
+  rustToolchain = builtins.fromTOML (builtins.readFile ./rust-toolchain.toml);
+  rustVersion = rustToolchain.toolchain.channel;
+in
 {
+  # Pure flakes only expose their read-only store source. Keep devenv's own
+  # task/profile state writable, then restore the real checkout path in-shell.
+  devenv.root = sourceRoot;
+  devenv.dotfile = transientDotfile;
+  devenv.state = "${transientDotfile}/state";
+
   languages.rust = {
     enable = true;
-    channel = "stable";
-    version = "1.97.0";
-    components = [
-      "cargo"
-      "clippy"
-      "rust-src"
-      "rustfmt"
-    ];
+    toolchainFile = ./rust-toolchain.toml;
   };
 
   packages = [
@@ -20,4 +26,15 @@
   ];
 
   env.RUST_BACKTRACE = "1";
+
+  enterShell = lib.mkAfter ''
+    export DEVENV_ROOT="$PWD"
+    # Keep Nix builds separate from artifacts produced by rustup or another
+    # devenv generation. Cargo metadata is not compatible across compilers.
+    export CARGO_TARGET_DIR="$PWD/target/devenv-rust-${rustVersion}"
+    export PATH="${config.languages.rust.toolchainPackage}/bin:$PATH"
+    export RUSTC="${config.languages.rust.toolchainPackage}/bin/rustc"
+    export RUSTDOC="${config.languages.rust.toolchainPackage}/bin/rustdoc"
+    hash -r
+  '';
 }