stdlib: add use_flake function (#847)

I noticed some projects in the wild calling `use_flake` without sourcing
the `use_flake` function. It's time to add this to the stdlib.

man/direnv-stdlib.1

@@ -397,6 +397,19 @@ If you have a \fB\fCdefault.nix\fR or \fB\fCshell.nix\fR these will be used by d
 .PP
 See http://nixos.org/nix/manual/#sec-nix-shell
 
+.SS \fB\fCuse flake [<installable>]\fR
+.PP
+Load the build environment of a derivation similar to \fB\fCnix develop\fR\&.
+
+.PP
+By default it will load the current folder flake.nix devShell attribute. Or
+pass an "installable" like "nixpkgs#hello" to load all the build dependencies
+of the hello package from the latest nixpkgs.
+
+.PP
+Note that the flakes feature is hidden behind an experimental flag, which you
+will have to enable on your own. Flakes is not considered stable yet.
+
 .SS \fB\fCuse guix [...]\fR
 .PP
 Load environment variables from \fB\fCguix environment\fR\&.

man/direnv-stdlib.1.md

@@ -288,6 +288,17 @@ If you have a `default.nix` or `shell.nix` these will be used by default, but yo
 
 See http://nixos.org/nix/manual/#sec-nix-shell
 
+### `use flake [<installable>]`
+
+Load the build environment of a derivation similar to `nix develop`.
+
+By default it will load the current folder flake.nix devShell attribute. Or
+pass an "installable" like "nixpkgs#hello" to load all the build dependencies
+of the hello package from the latest nixpkgs.
+
+Note that the flakes feature is hidden behind an experimental flag, which you
+will have to enable on your own. Flakes is not considered stable yet.
+
 ### `use guix [...]`
 
 Load environment variables from `guix environment`.

stdlib.go

@@ -1144,6 +1144,22 @@ const StdLib = "#!/usr/bin/env bash\n" +
 	"  fi\n" +
 	"}\n" +
 	"\n" +
+	"# Usage: use_flake [<installable>]\n" +
+	"#\n" +
+	"# Load the build environment of a derivation similar to `nix develop`.\n" +
+	"#\n" +
+	"# By default it will load the current folder flake.nix devShell attribute. Or\n" +
+	"# pass an \"installable\" like \"nixpkgs#hello\" to load all the build\n" +
+	"# dependencies of the hello package from the latest nixpkgs.\n" +
+	"#\n" +
+	"# Note that the flakes feature is hidden behind an experimental flag, which\n" +
+	"# you will have to enable on your own. Flakes is not considered stable yet.\n" +
+	"use_flake() {\n" +
+	"  watch_file flake.nix\n" +
+	"  watch_file flake.lock\n" +
+	"  eval \"$(nix print-dev-env --profile \"$(direnv_layout_dir)/flake-profile\" \"$@\")\"\n" +
+	"}\n" +
+	"\n" +
 	"# Usage: use_guix [...]\n" +
 	"#\n" +
 	"# Load environment variables from `guix environment`.\n" +

stdlib.sh

@@ -1141,6 +1141,22 @@ use_nix() {
   fi
 }
 
+# Usage: use_flake [<installable>]
+#
+# Load the build environment of a derivation similar to `nix develop`.
+#
+# By default it will load the current folder flake.nix devShell attribute. Or
+# pass an "installable" like "nixpkgs#hello" to load all the build
+# dependencies of the hello package from the latest nixpkgs.
+#
+# Note that the flakes feature is hidden behind an experimental flag, which
+# you will have to enable on your own. Flakes is not considered stable yet.
+use_flake() {
+  watch_file flake.nix
+  watch_file flake.lock
+  eval "$(nix print-dev-env --profile "$(direnv_layout_dir)/flake-profile" "$@")"
+}
+
 # Usage: use_guix [...]
 #
 # Load environment variables from `guix environment`.