nix_completer: Add action for completing attributes on flakes

Fixes: carapace-sh#2374

This will only complete attributes for flakes that are in
the user's registry or are a local path. This means
that the flake eval cache should be very useful for these
completions, and that carapace won't go out and fetch
random flakes when the user is trying to tab complete.

completers/nix_completer/cmd/action/flake.go

@@ -0,0 +1,105 @@
+package action
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"slices"
+	"strings"
+
+	"github.com/carapace-sh/carapace"
+	"github.com/carapace-sh/carapace-bin/pkg/actions/tools/nix"
+)
+
+// ActionFlakeRefs completes a full flake reference
+// It will only complete attributes for local flakes or flakes
+// in the flake registry.
+// It takes in the subcommand being completed
+//
+// nixpkgs#hello
+// .#foo
+func ActionFlakeRefs(fullCmd []string) carapace.Action {
+	return carapace.ActionMultiPartsN("#", 2, func(c carapace.Context) carapace.Action {
+		switch len(c.Parts) {
+		case 0:
+			return nix.ActionFlakes().Suffix("#")
+		default:
+			return ActionFlakeAttributes(fullCmd, c.Parts[0], c.Value)
+		}
+	})
+}
+
+// ActionFlakeAttributes completes attributes on a flake
+// Completions are only supplied for local flakes or flakes
+// in the registry.
+//
+// hello
+// packages.x86_64-linux.hello
+func ActionFlakeAttributes(fullCmd []string, flake, flakePath string) carapace.Action {
+	return carapace.ActionCallback(func(c carapace.Context) carapace.Action {
+		// Get completions from the flake registry
+		c.Setenv("NIX_GET_COMPLETIONS", fmt.Sprint(len(fullCmd)))
+		completionArgs := append(fullCmd[1:], flake)
+
+		var stdout, stderr bytes.Buffer
+		cmd := c.Command("nix", completionArgs...)
+		cmd.Stdout = &stdout
+		cmd.Stderr = &stderr
+
+		if err := cmd.Run(); err != nil {
+			return carapace.ActionValues()
+		}
+
+		// Check if the given flake is in the registry or is a local path
+		// For some reason, nix adds \b to completion items
+		rawLines := strings.ReplaceAll(stdout.String(), "\r\b", "")
+		lines := strings.Split(rawLines, "\n")
+
+		flakeInRegistry := slices.ContainsFunc(lines[1:], func(s string) bool {
+			return strings.Trim(s, " \t") == flake
+		})
+		flakeIsLocal := directoryExists(flake)
+
+		if !flakeInRegistry && !flakeIsLocal {
+			return carapace.ActionValues()
+		}
+
+		// Get completions using the nix cli
+		stdout.Reset()
+		stderr.Reset()
+
+		completionArgs = append(fullCmd[1:], flake+"#"+flakePath)
+		cmd = c.Command("nix", completionArgs...)
+		cmd.Stdout = &stdout
+		cmd.Stderr = &stderr
+
+		if err := cmd.Run(); err != nil {
+			return carapace.ActionValues()
+		}
+
+		// Remove \r for consistency with line endings
+		// For some reason, nix adds \b to completion items
+		rawLines = strings.ReplaceAll(stdout.String(), "\r\b", "")
+		lines = strings.Split(rawLines, "\n")
+
+		// The first line is always "attrs"
+		completionResults := lines[1:]
+
+		// Remove the flake# prefix
+		for i := range completionResults {
+			completionResults[i] = strings.Trim(completionResults[i], " \t")
+			completionResults[i] = strings.TrimPrefix(completionResults[i], flake+"#")
+		}
+
+		return carapace.ActionValues(completionResults...)
+	})
+}
+
+func directoryExists(path string) bool {
+	info, err := os.Stat(path)
+	if err != nil {
+		return false
+	}
+
+	return info.IsDir()
+}

completers/nix_completer/cmd/build.go

@@ -2,6 +2,7 @@ package cmd
 
 import (
 	"github.com/carapace-sh/carapace"
+	"github.com/carapace-sh/carapace-bin/completers/nix_completer/cmd/action"
 	"github.com/carapace-sh/carapace-bin/pkg/actions/tools/nix"
 	"github.com/spf13/cobra"
 )
@@ -39,5 +40,5 @@ func init() {
 		"profile":             carapace.ActionFiles(),
 		"reference-lock-file": carapace.ActionFiles("lock"),
 	})
-	carapace.Gen(buildCmd).PositionalCompletion(nix.ActionFlakes())
+	carapace.Gen(buildCmd).PositionalCompletion(action.ActionFlakeRefs([]string{"nix", "build"}))
 }

completers/nix_completer/cmd/develop.go

@@ -2,6 +2,7 @@ package cmd
 
 import (
 	"github.com/carapace-sh/carapace"
+	"github.com/carapace-sh/carapace-bin/completers/nix_completer/cmd/action"
 	"github.com/carapace-sh/carapace-bin/pkg/actions/os"
 	"github.com/carapace-sh/carapace-bin/pkg/actions/tools/nix"
 	"github.com/spf13/cobra"
@@ -50,5 +51,5 @@ func init() {
 		"reference-lock-file": carapace.ActionFiles("lock"),
 		"unset":               os.ActionEnvironmentVariables(),
 	})
-	carapace.Gen(developCmd).PositionalCompletion(nix.ActionDevShells())
+	carapace.Gen(developCmd).PositionalCompletion(action.ActionFlakeRefs([]string{"nix", "develop"}))
 }

pkg/actions/tools/nix/flake.go

@@ -30,7 +30,7 @@ func ActionFlakes() carapace.Action {
 			carapace.ActionDirectories(),
 			carapace.ActionStyledValuesDescribed(vals...),
 		).ToA()
-	}).Cache(time.Minute).Suffix("#")
+	}).Cache(time.Minute)
 }
 
 func styleForRegistry(s string) string {