Make engine.#Scratch a specialization of #FS, rather than a task

Signed-off-by: Joel Longtine <joel@dagger.io>

europa-universe/yarn/tests/simple/simple.cue

@@ -5,7 +5,7 @@ import (
 )
 
 b: #Build & {
-	source: engine.#Scratch.output
+	source: engine.#Scratch
 }
 
 out: b.output

plan/task/scratch.go

@@ -1,24 +0,0 @@
-package task
-
-import (
-	"context"
-
-	"go.dagger.io/dagger/compiler"
-	"go.dagger.io/dagger/plancontext"
-	"go.dagger.io/dagger/solver"
-)
-
-func init() {
-	Register("Scratch", func() Task { return &scratchTask{} })
-}
-
-type scratchTask struct {
-}
-
-func (t *scratchTask) Run(ctx context.Context, pctx *plancontext.Context, s solver.Solver, v *compiler.Value) (*compiler.Value, error) {
-	fs := pctx.FS.New(nil)
-
-	return compiler.NewValue().FillFields(map[string]interface{}{
-		"output": fs.MarshalCUE(),
-	})
-}

plancontext/fs.go

@@ -71,6 +71,11 @@ func (c *fsContext) FromValue(v *compiler.Value) (*FS, error) {
 	c.l.RLock()
 	defer c.l.RUnlock()
 
+	// This is #Scratch, so we'll return an empty FS
+	if v.LookupPath(fsIDPath).Kind() == cue.NullKind {
+		return &FS{}, nil
+	}
+
 	id, err := v.LookupPath(fsIDPath).String()
 	if err != nil {
 		return nil, fmt.Errorf("invalid FS %q: %w", v.Path(), err)

stdlib/europa/dagger/engine/fs.cue

@@ -49,11 +49,7 @@ package engine
 }
 
 // Produce an empty directory
-#Scratch: {
+#Scratch: {$dagger: fs: _id: null}
-	$dagger: task: _name: "Scratch"
-
-	output: #FS
-}
 
 // Copy files from one FS tree to another
 #Copy: {

stdlib/europa/dagger/engine/types.cue

@@ -7,7 +7,7 @@ package engine
 //  - A directory containing binary artifacts
 // Rule of thumb: if it fits in a tar archive, it fits in a #FS.
 #FS: {
-	$dagger: fs: _id: string
+	$dagger: fs: _id: string | null
 }
 
 // A reference to an external secret, for example:

stdlib/europa/dagger/utils.cue

@@ -16,12 +16,9 @@ import (
 	// Subdirectory tree
 	output: #FS & copy.output
 
-	// Base image
-	scratch: engine.#Scratch
-
 	// Copy action
 	copy: engine.#Copy & {
-		"input": scratch.output
+		"input": engine.#Scratch
 		source: {
 			root:   input
 			"path": path

tests/cue.mod/module.cue

@@ -0,0 +1 @@
+module: ""

tests/cue.mod/pkg/.gitignore

@@ -0,0 +1,3 @@
+# generated by dagger
+alpha.dagger.io
+dagger.lock

tests/plan/outputs/outputs.cue

@@ -4,10 +4,8 @@ import "alpha.dagger.io/europa/dagger/engine"
 
 engine.#Plan & {
 	actions: {
-		scratch: engine.#Scratch
-
 		data: engine.#WriteFile & {
-			input:       scratch.output
+			input:       engine.#Scratch
 			path:        "/test"
 			permissions: 0o600
 			contents:    "foobar"

tests/tasks/scratch/scratch.cue

@@ -10,13 +10,11 @@ engine.#Plan & {
 			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
 		}
 
-		scratch: engine.#Scratch
-
 		exec: engine.#Exec & {
 			input: image.output
 			mounts: fs: {
 				dest:     "/scratch"
-				contents: scratch.output
+				contents: engine.#Scratch
 			}
 			workdir: "/"
 			args: [