attr: Add ValueType method to Type interface

Reference: #496

This will allow the internal reflection package to return better error diagnostics.

.changelog/pending.txt

@@ -0,0 +1,7 @@
+```release-note:breaking-change
+attr: The `Type` interface now requires the `ValueType` method, which is used for enhancing error diagnostics from the framework
+```
+
+```release-note:enhancement
+internal/reflect: Added `attr.Value` type suggestions to error diagnostics
+```

attr/type.go

@@ -25,6 +25,12 @@ type Type interface {
 	// for the provider to consume the data with.
 	ValueFromTerraform(context.Context, tftypes.Value) (Value, error)
 
+	// ValueType should return the attr.Value type returned by
+	// ValueFromTerraform. The returned attr.Value can be any null, unknown,
+	// or known value for the type, as this is intended for type detection
+	// and improving error diagnostics.
+	ValueType(context.Context) Value
+
 	// Equal must return true if the Type is considered semantically equal
 	// to the Type passed as an argument.
 	Equal(Type) bool

internal/testing/types/bool.go

@@ -59,6 +59,11 @@ func (t BoolType) ValueFromTerraform(ctx context.Context, in tftypes.Value) (att
 	return Bool{Bool: types.Bool{Value: b}, CreatedBy: t}, nil
 }
 
+// ValueType returns the Value type.
+func (t BoolType) ValueType(_ context.Context) attr.Value {
+	return Bool{}
+}
+
 type Bool struct {
 	types.Bool
 

internal/testing/types/invalid.go

@@ -38,6 +38,11 @@ func (t InvalidType) ValueFromTerraform(ctx context.Context, in tftypes.Value) (
 	return nil, fmt.Errorf("intentional ValueFromTerraform error")
 }
 
+// ValueType returns the Value type.
+func (t InvalidType) ValueType(_ context.Context) attr.Value {
+	return Invalid{}
+}
+
 // Invalid is an attr.Value that returns errors for methods than can return errors.
 type Invalid struct{}
 

internal/testing/types/number.go

@@ -63,6 +63,11 @@ func (t NumberType) ValueFromTerraform(ctx context.Context, in tftypes.Value) (a
 	}, nil
 }
 
+// ValueType returns the Value type.
+func (t NumberType) ValueType(_ context.Context) attr.Value {
+	return Number{}
+}
+
 type Number struct {
 	types.Number
 

internal/testing/types/string.go

@@ -62,6 +62,11 @@ func (t StringType) ValueFromTerraform(ctx context.Context, in tftypes.Value) (a
 	}, nil
 }
 
+// ValueType returns the Value type.
+func (t StringType) ValueType(_ context.Context) attr.Value {
+	return String{}
+}
+
 type String struct {
 	InternalString types.String
 

types/list.go

@@ -163,6 +163,13 @@ func (l ListType) Validate(ctx context.Context, in tftypes.Value, path path.Path
 	return diags
 }
 
+// ValueType returns the Value type.
+func (t ListType) ValueType(_ context.Context) attr.Value {
+	return List{
+		ElemType: t.ElemType,
+	}
+}
+
 // List represents a list of attr.Values, all of the same type, indicated
 // by ElemType.
 type List struct {

types/map.go

@@ -167,6 +167,13 @@ func (m MapType) Validate(ctx context.Context, in tftypes.Value, path path.Path)
 	return diags
 }
 
+// ValueType returns the Value type.
+func (t MapType) ValueType(_ context.Context) attr.Value {
+	return Map{
+		ElemType: t.ElemType,
+	}
+}
+
 // Map represents a map of attr.Values, all of the same type, indicated by
 // ElemType. Keys for the map will always be strings.
 type Map struct {

types/object.go

@@ -145,6 +145,13 @@ func (o ObjectType) String() string {
 	return res.String()
 }
 
+// ValueType returns the Value type.
+func (t ObjectType) ValueType(_ context.Context) attr.Value {
+	return Object{
+		AttrTypes: t.AttrTypes,
+	}
+}
+
 // Object represents an object
 type Object struct {
 	// Unknown will be set to true if the entire object is an unknown value.

types/primitive.go

@@ -92,6 +92,25 @@ func (p primitive) ValueFromTerraform(ctx context.Context, in tftypes.Value) (at
 	}
 }
 
+// ValueType returns the Value type.
+func (p primitive) ValueType(_ context.Context) attr.Value {
+	// These Value do not need to be valid.
+	switch p {
+	case BoolType:
+		return Bool{}
+	case Float64Type:
+		return Float64{}
+	case Int64Type:
+		return Int64{}
+	case NumberType:
+		return Number{}
+	case StringType:
+		return String{}
+	default:
+		panic(fmt.Sprintf("unknown primitive %d", p))
+	}
+}
+
 // Equal returns true if `o` is also a primitive, and is the same type of
 // primitive as `p`.
 func (p primitive) Equal(o attr.Type) bool {

types/primitive_test.go

@@ -90,6 +90,11 @@ func (t testAttributeType) String() string {
 	panic("not implemented")
 }
 
+// ValueType returns the Value type.
+func (t testAttributeType) ValueType(_ context.Context) attr.Value {
+	panic("not implemented")
+}
+
 func TestPrimitiveEqual(t *testing.T) {
 	t.Parallel()
 

types/set.go

@@ -195,6 +195,13 @@ func (st SetType) Validate(ctx context.Context, in tftypes.Value, path path.Path
 	return diags
 }
 
+// ValueType returns the Value type.
+func (t SetType) ValueType(_ context.Context) attr.Value {
+	return Set{
+		ElemType: t.ElemType,
+	}
+}
+
 // Set represents a set of attr.Value, all of the same type,
 // indicated by ElemType.
 type Set struct {