feat(requires): support variable references in enum constraints

Add support for referencing variables in enum validation:

```yaml
vars:
  ALLOWED_ENVS: ["dev", "staging", "prod"]

tasks:
  deploy:
    requires:
      vars:
        - name: ENV
          enum:
            ref: .ALLOWED_ENVS
```

This allows enum values to be defined once and reused, or computed
dynamically using template expressions like `| fromYaml`.

Changes:
- Add Enum type with Ref/Value fields in taskfile/ast/requires.go
- Add resolveEnumRefs() to resolve refs at task compilation time
- Add getEnumValues() helper in requires.go
- Only resolve enum refs when shell variables are evaluated

executor_test.go

@@ -351,6 +351,32 @@ func TestRequires(t *testing.T) {
 		),
 		WithTask("var-defined-in-task"),
 	)
+	NewExecutorTest(t,
+		WithName("enum ref - passes validation"),
+		WithExecutorOptions(
+			task.WithDir("testdata/requires"),
+		),
+		WithTask("validation-var-ref"),
+		WithVar("ENV", "dev"),
+	)
+	NewExecutorTest(t,
+		WithName("enum ref - fails validation"),
+		WithExecutorOptions(
+			task.WithDir("testdata/requires"),
+		),
+		WithTask("validation-var-ref"),
+		WithVar("ENV", "invalid"),
+		WithRunError(),
+	)
+	NewExecutorTest(t,
+		WithName("enum ref - ref to non-list"),
+		WithExecutorOptions(
+			task.WithDir("testdata/requires"),
+		),
+		WithTask("validation-var-ref-invalid"),
+		WithVar("VALUE", "test"),
+		WithRunError(),
+	)
 }
 
 // TODO: mock fs

requires.go

@@ -81,7 +81,7 @@ func (e *Executor) promptDepsVars(calls []*Call) error {
 	e.promptedVars = ast.NewVars()
 
 	for _, v := range varsMap {
-		value, err := prompter.Prompt(v.Name, v.Enum)
+		value, err := prompter.Prompt(v.Name, getEnumValues(v.Enum))
 		if err != nil {
 			if errors.Is(err, input.ErrCancelled) {
 				return &errors.TaskCancelledByUserError{TaskName: "interactive prompt"}
@@ -120,7 +120,7 @@ func (e *Executor) promptTaskVars(t *ast.Task, call *Call) (bool, error) {
 	prompter := e.newPrompter()
 
 	for _, v := range missing {
-		value, err := prompter.Prompt(v.Name, v.Enum)
+		value, err := prompter.Prompt(v.Name, getEnumValues(v.Enum))
 		if err != nil {
 			if errors.Is(err, input.ErrCancelled) {
 				return false, &errors.TaskCancelledByUserError{TaskName: t.Name()}
@@ -168,7 +168,7 @@ func (e *Executor) areTaskRequiredVarsSet(t *ast.Task) error {
 	for i, v := range missing {
 		missingVars[i] = errors.MissingVar{
 			Name:          v.Name,
-			AllowedValues: v.Enum,
+			AllowedValues: getEnumValues(v.Enum),
 		}
 	}
 
@@ -187,11 +187,12 @@ func (e *Executor) areTaskRequiredVarsAllowedValuesSet(t *ast.Task) error {
 	for _, requiredVar := range t.Requires.Vars {
 		varValue, _ := t.Vars.Get(requiredVar.Name)
 
+		enumValues := getEnumValues(requiredVar.Enum)
 		value, isString := varValue.Value.(string)
-		if isString && requiredVar.Enum != nil && !slices.Contains(requiredVar.Enum, value) {
+		if isString && len(enumValues) > 0 && !slices.Contains(enumValues, value) {
 			notAllowedValuesVars = append(notAllowedValuesVars, errors.NotAllowedVar{
 				Value: value,
-				Enum:  requiredVar.Enum,
+				Enum:  enumValues,
 				Name:  requiredVar.Name,
 			})
 		}
@@ -206,3 +207,11 @@ func (e *Executor) areTaskRequiredVarsAllowedValuesSet(t *ast.Task) error {
 
 	return nil
 }
+
+// getEnumValues returns the enum values from an Enum struct, or nil if the enum is nil.
+func getEnumValues(e *ast.Enum) []string {
+	if e == nil {
+		return nil
+	}
+	return e.Value
+}

taskfile/ast/requires.go

@@ -22,9 +22,53 @@ func (r *Requires) DeepCopy() *Requires {
 	}
 }
 
+// Enum represents an enum constraint for a required variable.
+// It can either be a static list of values or a reference to another variable.
+type Enum struct {
+	Ref   string   // Reference to a variable containing the allowed values
+	Value []string // Static list of allowed values
+}
+
+func (e *Enum) DeepCopy() *Enum {
+	if e == nil {
+		return nil
+	}
+	return &Enum{
+		Ref:   e.Ref,
+		Value: deepcopy.Slice(e.Value),
+	}
+}
+
+// UnmarshalYAML implements yaml.Unmarshaler interface.
+func (e *Enum) UnmarshalYAML(node *yaml.Node) error {
+	switch node.Kind {
+	case yaml.SequenceNode:
+		// Static list of values: enum: ["a", "b"]
+		var values []string
+		if err := node.Decode(&values); err != nil {
+			return errors.NewTaskfileDecodeError(err, node)
+		}
+		e.Value = values
+		return nil
+
+	case yaml.MappingNode:
+		// Reference to another variable: enum: { ref: .VAR }
+		var refStruct struct {
+			Ref string
+		}
+		if err := node.Decode(&refStruct); err != nil {
+			return errors.NewTaskfileDecodeError(err, node)
+		}
+		e.Ref = refStruct.Ref
+		return nil
+	}
+
+	return errors.NewTaskfileDecodeError(nil, node).WithTypeMessage("enum")
+}
+
 type VarsWithValidation struct {
 	Name string
-	Enum []string
+	Enum *Enum
 }
 
 func (v *VarsWithValidation) DeepCopy() *VarsWithValidation {
@@ -33,7 +77,7 @@ func (v *VarsWithValidation) DeepCopy() *VarsWithValidation {
 	}
 	return &VarsWithValidation{
 		Name: v.Name,
-		Enum: v.Enum,
+		Enum: v.Enum.DeepCopy(),
 	}
 }
 
@@ -53,7 +97,7 @@ func (v *VarsWithValidation) UnmarshalYAML(node *yaml.Node) error {
 	case yaml.MappingNode:
 		var vv struct {
 			Name string
-			Enum []string
+			Enum *Enum
 		}
 		if err := node.Decode(&vv); err != nil {
 			return errors.NewTaskfileDecodeError(err, node)

testdata/requires/Taskfile.yml

@@ -1,5 +1,9 @@
 version: '3'
 
+vars:
+  ALLOWED_ENVS: ["dev", "staging", "prod"]
+  NOT_A_LIST: "this is a string"
+
 tasks:
   default:
     - task: missing-var
@@ -41,3 +45,19 @@ tasks:
       {{range .MY_VAR | splitList " " }}
         echo {{.}}
       {{end}}
+
+  validation-var-ref:
+    requires:
+      vars:
+        - name: ENV
+          enum:
+            ref: .ALLOWED_ENVS
+    cmd: echo "{{.ENV}}"
+
+  validation-var-ref-invalid:
+    requires:
+      vars:
+        - name: VALUE
+          enum:
+            ref: .NOT_A_LIST
+    cmd: echo "{{.VALUE}}"

testdata/requires/testdata/TestRequires-enum_ref_-_fails_validation-err-run.golden

@@ -0,0 +1,2 @@
+task: Task "validation-var-ref" cancelled because it is missing required variables:
+  - ENV has an invalid value : 'invalid' (allowed values : [dev staging prod])

testdata/requires/testdata/TestRequires-enum_ref_-_passes_validation.golden

@@ -0,0 +1,2 @@
+task: [validation-var-ref] echo "dev"
+dev

testdata/requires/testdata/TestRequires-enum_ref_-_ref_to_non-list-err-run.golden

@@ -0,0 +1 @@
+enum reference ".NOT_A_LIST" must resolve to a list

variables.go

@@ -98,6 +98,17 @@ func (e *Executor) compiledTask(call *Call, evaluateShVars bool) (*ast.Task, err
 	}
 
 	cache := &templater.Cache{Vars: vars}
+
+	// Resolve enum refs in requires only when evaluateShVars is true,
+	// since enum refs may depend on shell variables
+	requires := origTask.Requires
+	if evaluateShVars {
+		requires = origTask.Requires.DeepCopy()
+		if err := resolveEnumRefs(requires, cache); err != nil {
+			return nil, err
+		}
+	}
+
 	new := ast.Task{
 		Task:                 origTask.Task,
 		Label:                templater.Replace(origTask.Label, cache),
@@ -125,7 +136,7 @@ func (e *Executor) compiledTask(call *Call, evaluateShVars bool) (*ast.Task, err
 		Platforms:            origTask.Platforms,
 		If:                   templater.Replace(origTask.If, cache),
 		Location:             origTask.Location,
-		Requires:             origTask.Requires,
+		Requires:             requires,
 		Watch:                origTask.Watch,
 		Failfast:             origTask.Failfast,
 		Namespace:            origTask.Namespace,
@@ -431,6 +442,32 @@ func resolveMatrixRefs(matrix *ast.Matrix, cache *templater.Cache) error {
 	return nil
 }
 
+func resolveEnumRefs(requires *ast.Requires, cache *templater.Cache) error {
+	if requires == nil || len(requires.Vars) == 0 {
+		return nil
+	}
+	for _, v := range requires.Vars {
+		if v.Enum == nil || v.Enum.Ref == "" {
+			continue
+		}
+		resolved := templater.ResolveRef(v.Enum.Ref, cache)
+		arr, ok := resolved.([]any)
+		if !ok {
+			return fmt.Errorf("enum reference %q must resolve to a list", v.Enum.Ref)
+		}
+		strValues := make([]string, 0, len(arr))
+		for _, item := range arr {
+			s, ok := item.(string)
+			if !ok {
+				return fmt.Errorf("enum reference %q must contain only strings", v.Enum.Ref)
+			}
+			strValues = append(strValues, s)
+		}
+		v.Enum.Value = strValues
+	}
+	return nil
+}
+
 // product generates the cartesian product of the input map of slices.
 func product(matrix *ast.Matrix) []map[string]any {
 	if matrix.Len() == 0 {