lang: Further limit the console-only type function

This commit introduces a capsule type, `TypeType`, which is used to
extricate type information from the console-only `type` function. In
combination with the `TypeType` mark, this allows us to restrict the use
of this function to top-level display of a value's type. Any other use
of `type()` will result in an error diagnostic.

internal/lang/funcs/conversion.go

@@ -1,12 +1,10 @@
 package funcs
 
 import (
-	"fmt"
-	"sort"
 	"strconv"
-	"strings"
 
 	"github.com/hashicorp/terraform/internal/lang/marks"
+	"github.com/hashicorp/terraform/internal/lang/types"
 	"github.com/zclconf/go-cty/cty"
 	"github.com/zclconf/go-cty/cty/convert"
 	"github.com/zclconf/go-cty/cty/function"
@@ -97,6 +95,9 @@ func MakeToFunc(wantTy cty.Type) function.Function {
 	})
 }
 
+// TypeFunc returns an encapsulated value containing its argument's type. This
+// value is marked to allow us to limit the use of this function at the moment
+// to only a few supported use cases.
 var TypeFunc = function.New(&function.Spec{
 	Params: []function.Parameter{
 		{
@@ -107,117 +108,13 @@ var TypeFunc = function.New(&function.Spec{
 			AllowNull:        true,
 		},
 	},
-	Type: function.StaticReturnType(cty.String),
+	Type: function.StaticReturnType(types.TypeType),
 	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
-		return cty.StringVal(TypeString(args[0].Type())).Mark(marks.Raw), nil
+		givenType := args[0].Type()
+		return cty.CapsuleVal(types.TypeType, &givenType).Mark(marks.TypeType), nil
 	},
 })
 
-// Modified copy of TypeString from go-cty:
-// https://github.com/zclconf/go-cty-debug/blob/master/ctydebug/type_string.go
-//
-// TypeString returns a string representation of a given type that is
-// reminiscent of Go syntax calling into the cty package but is mainly
-// intended for easy human inspection of values in tests, debug output, etc.
-//
-// The resulting string will include newlines and indentation in order to
-// increase the readability of complex structures. It always ends with a
-// newline, so you can print this result directly to your output.
-func TypeString(ty cty.Type) string {
-	var b strings.Builder
-	writeType(ty, &b, 0)
-	return b.String()
-}
-
-func writeType(ty cty.Type, b *strings.Builder, indent int) {
-	switch {
-	case ty == cty.NilType:
-		b.WriteString("nil")
-		return
-	case ty.IsObjectType():
-		atys := ty.AttributeTypes()
-		if len(atys) == 0 {
-			b.WriteString("object({})")
-			return
-		}
-		attrNames := make([]string, 0, len(atys))
-		for name := range atys {
-			attrNames = append(attrNames, name)
-		}
-		sort.Strings(attrNames)
-		b.WriteString("object({\n")
-		indent++
-		for _, name := range attrNames {
-			aty := atys[name]
-			b.WriteString(indentSpaces(indent))
-			fmt.Fprintf(b, "%s: ", name)
-			writeType(aty, b, indent)
-			b.WriteString(",\n")
-		}
-		indent--
-		b.WriteString(indentSpaces(indent))
-		b.WriteString("})")
-	case ty.IsTupleType():
-		etys := ty.TupleElementTypes()
-		if len(etys) == 0 {
-			b.WriteString("tuple([])")
-			return
-		}
-		b.WriteString("tuple([\n")
-		indent++
-		for _, ety := range etys {
-			b.WriteString(indentSpaces(indent))
-			writeType(ety, b, indent)
-			b.WriteString(",\n")
-		}
-		indent--
-		b.WriteString(indentSpaces(indent))
-		b.WriteString("])")
-	case ty.IsCollectionType():
-		ety := ty.ElementType()
-		switch {
-		case ty.IsListType():
-			b.WriteString("list(")
-		case ty.IsMapType():
-			b.WriteString("map(")
-		case ty.IsSetType():
-			b.WriteString("set(")
-		default:
-			// At the time of writing there are no other collection types,
-			// but we'll be robust here and just pass through the GoString
-			// of anything we don't recognize.
-			b.WriteString(ty.FriendlyName())
-			return
-		}
-		// Because object and tuple types render split over multiple
-		// lines, a collection type container around them can end up
-		// being hard to see when scanning, so we'll generate some extra
-		// indentation to make a collection of structural type more visually
-		// distinct from the structural type alone.
-		complexElem := ety.IsObjectType() || ety.IsTupleType()
-		if complexElem {
-			indent++
-			b.WriteString("\n")
-			b.WriteString(indentSpaces(indent))
-		}
-		writeType(ty.ElementType(), b, indent)
-		if complexElem {
-			indent--
-			b.WriteString(",\n")
-			b.WriteString(indentSpaces(indent))
-		}
-		b.WriteString(")")
-	default:
-		// For any other type we'll just use its GoString and assume it'll
-		// follow the usual GoString conventions.
-		b.WriteString(ty.FriendlyName())
-	}
-}
-
-func indentSpaces(level int) string {
-	return strings.Repeat("    ", level)
-}
-
 func Type(input []cty.Value) (cty.Value, error) {
 	return TypeFunc.Call(input)
 }

internal/lang/funcs/conversion_test.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"testing"
 
-	"github.com/google/go-cmp/cmp"
 	"github.com/hashicorp/terraform/internal/lang/marks"
 	"github.com/zclconf/go-cty/cty"
 )
@@ -191,92 +190,3 @@ func TestTo(t *testing.T) {
 		})
 	}
 }
-
-func TestType(t *testing.T) {
-	tests := []struct {
-		Input cty.Value
-		Want  string
-	}{
-		// Primititves
-		{
-			cty.StringVal("a"),
-			"string",
-		},
-		{
-			cty.NumberIntVal(42),
-			"number",
-		},
-		{
-			cty.BoolVal(true),
-			"bool",
-		},
-		// Collections
-		{
-			cty.EmptyObjectVal,
-			`object({})`,
-		},
-		{
-			cty.EmptyTupleVal,
-			`tuple([])`,
-		},
-		{
-			cty.ListValEmpty(cty.String),
-			`list(string)`,
-		},
-		{
-			cty.MapValEmpty(cty.String),
-			`map(string)`,
-		},
-		{
-			cty.SetValEmpty(cty.String),
-			`set(string)`,
-		},
-		{
-			cty.ListVal([]cty.Value{cty.StringVal("a")}),
-			`list(string)`,
-		},
-		{
-			cty.ListVal([]cty.Value{cty.ListVal([]cty.Value{cty.NumberIntVal(42)})}),
-			`list(list(number))`,
-		},
-		{
-			cty.ListVal([]cty.Value{cty.MapValEmpty(cty.String)}),
-			`list(map(string))`,
-		},
-		{
-			cty.ListVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
-				"foo": cty.StringVal("bar"),
-			})}),
-			"list(\n    object({\n        foo: string,\n    }),\n)",
-		},
-		// Unknowns and Nulls
-		{
-			cty.UnknownVal(cty.String),
-			"string",
-		},
-		{
-			cty.NullVal(cty.Object(map[string]cty.Type{
-				"foo": cty.String,
-			})),
-			"object({\n    foo: string,\n})",
-		},
-		{ // irrelevant marks do nothing
-			cty.ListVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
-				"foo": cty.StringVal("bar").Mark("ignore me"),
-			})}),
-			"list(\n    object({\n        foo: string,\n    }),\n)",
-		},
-	}
-	for _, test := range tests {
-		got, err := Type([]cty.Value{test.Input})
-		if err != nil {
-			t.Fatalf("unexpected error: %s", err)
-		}
-		// The value is marked to help with formatting
-		got, _ = got.Unmark()
-
-		if got.AsString() != test.Want {
-			t.Errorf("wrong result:\n%s", cmp.Diff(got.AsString(), test.Want))
-		}
-	}
-}

internal/lang/marks/marks.go

@@ -38,6 +38,7 @@ func Contains(val cty.Value, mark valueMark) bool {
 // Terraform.
 var Sensitive = valueMark("sensitive")
 
-// Raw is used to indicate to the repl that the value should be written without
-// any formatting.
-var Raw = valueMark("raw")
+// TypeType is used to indicate that the value contains a representation of
+// another value's type. This is part of the implementation of the console-only
+// `type` function.
+var TypeType = valueMark("typeType")

internal/lang/types/type_type.go

@@ -0,0 +1,12 @@
+package types
+
+import (
+	"reflect"
+
+	"github.com/zclconf/go-cty/cty"
+)
+
+// TypeType is a capsule type used to represent a cty.Type as a cty.Value. This
+// is used by the `type()` console function to smuggle cty.Type values to the
+// REPL session, where it can be displayed to the user directly.
+var TypeType = cty.Capsule("type", reflect.TypeOf(cty.Type{}))

internal/lang/types/types.go

@@ -0,0 +1,2 @@
+// Package types contains non-standard cty types used only within Terraform.
+package types

internal/repl/session.go

@@ -1,6 +1,8 @@
 package repl
 
 import (
+	"fmt"
+	"sort"
 	"strings"
 
 	"github.com/zclconf/go-cty/cty"
@@ -9,6 +11,7 @@ import (
 	"github.com/hashicorp/hcl/v2/hclsyntax"
 	"github.com/hashicorp/terraform/internal/lang"
 	"github.com/hashicorp/terraform/internal/lang/marks"
+	"github.com/hashicorp/terraform/internal/lang/types"
 	"github.com/hashicorp/terraform/internal/tfdiags"
 )
 
@@ -55,20 +58,19 @@ func (s *Session) handleEval(line string) (string, tfdiags.Diagnostics) {
 		return "", diags
 	}
 
-	// The raw mark is used only by the console-only `type` function, in order
-	// to allow display of a string value representation of the type without the
-	// usual HCL formatting. If we receive a string value with this mark, we do
-	// not want to format it any further.
-	//
-	// Due to mark propagation in cty, calling `type` as part of a larger
-	// expression can lead to other values being marked, which can in turn lead
-	// to unpredictable results. If any non-string value has the raw mark, we
-	// return a diagnostic explaining that this use of `type` is not permitted.
-	if marks.Contains(val, marks.Raw) {
-		if val.Type().Equals(cty.String) {
-			raw, _ := val.Unmark()
-			return raw.AsString(), diags
-		} else {
+	// The TypeType mark is used only by the console-only `type` function, in
+	// order to smuggle the type of a given value back here. We can then
+	// display a representation of the type directly.
+	if marks.Contains(val, marks.TypeType) {
+		val, _ = val.UnmarkDeep()
+
+		valType := val.Type()
+		switch {
+		case valType.Equals(types.TypeType):
+			// An encapsulated type value, which should be displayed directly.
+			valType := val.EncapsulatedValue().(*cty.Type)
+			return typeString(*valType), diags
+		default:
 			diags = diags.Append(tfdiags.Sourceless(
 				tfdiags.Error,
 				"Invalid use of type function",
@@ -96,3 +98,108 @@ Control-D.
 
 	return strings.TrimSpace(text), nil
 }
+
+// Modified copy of TypeString from go-cty:
+// https://github.com/zclconf/go-cty-debug/blob/master/ctydebug/type_string.go
+//
+// TypeString returns a string representation of a given type that is
+// reminiscent of Go syntax calling into the cty package but is mainly
+// intended for easy human inspection of values in tests, debug output, etc.
+//
+// The resulting string will include newlines and indentation in order to
+// increase the readability of complex structures. It always ends with a
+// newline, so you can print this result directly to your output.
+func typeString(ty cty.Type) string {
+	var b strings.Builder
+	writeType(ty, &b, 0)
+	return b.String()
+}
+
+func writeType(ty cty.Type, b *strings.Builder, indent int) {
+	switch {
+	case ty == cty.NilType:
+		b.WriteString("nil")
+		return
+	case ty.IsObjectType():
+		atys := ty.AttributeTypes()
+		if len(atys) == 0 {
+			b.WriteString("object({})")
+			return
+		}
+		attrNames := make([]string, 0, len(atys))
+		for name := range atys {
+			attrNames = append(attrNames, name)
+		}
+		sort.Strings(attrNames)
+		b.WriteString("object({\n")
+		indent++
+		for _, name := range attrNames {
+			aty := atys[name]
+			b.WriteString(indentSpaces(indent))
+			fmt.Fprintf(b, "%s: ", name)
+			writeType(aty, b, indent)
+			b.WriteString(",\n")
+		}
+		indent--
+		b.WriteString(indentSpaces(indent))
+		b.WriteString("})")
+	case ty.IsTupleType():
+		etys := ty.TupleElementTypes()
+		if len(etys) == 0 {
+			b.WriteString("tuple([])")
+			return
+		}
+		b.WriteString("tuple([\n")
+		indent++
+		for _, ety := range etys {
+			b.WriteString(indentSpaces(indent))
+			writeType(ety, b, indent)
+			b.WriteString(",\n")
+		}
+		indent--
+		b.WriteString(indentSpaces(indent))
+		b.WriteString("])")
+	case ty.IsCollectionType():
+		ety := ty.ElementType()
+		switch {
+		case ty.IsListType():
+			b.WriteString("list(")
+		case ty.IsMapType():
+			b.WriteString("map(")
+		case ty.IsSetType():
+			b.WriteString("set(")
+		default:
+			// At the time of writing there are no other collection types,
+			// but we'll be robust here and just pass through the GoString
+			// of anything we don't recognize.
+			b.WriteString(ty.FriendlyName())
+			return
+		}
+		// Because object and tuple types render split over multiple
+		// lines, a collection type container around them can end up
+		// being hard to see when scanning, so we'll generate some extra
+		// indentation to make a collection of structural type more visually
+		// distinct from the structural type alone.
+		complexElem := ety.IsObjectType() || ety.IsTupleType()
+		if complexElem {
+			indent++
+			b.WriteString("\n")
+			b.WriteString(indentSpaces(indent))
+		}
+		writeType(ty.ElementType(), b, indent)
+		if complexElem {
+			indent--
+			b.WriteString(",\n")
+			b.WriteString(indentSpaces(indent))
+		}
+		b.WriteString(")")
+	default:
+		// For any other type we'll just use its GoString and assume it'll
+		// follow the usual GoString conventions.
+		b.WriteString(ty.FriendlyName())
+	}
+}
+
+func indentSpaces(level int) string {
+	return strings.Repeat("    ", level)
+}

internal/repl/session_test.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/zclconf/go-cty/cty"
 
 	"github.com/hashicorp/terraform/internal/addrs"
@@ -193,11 +194,59 @@ func TestSession_stateless(t *testing.T) {
 		})
 	})
 
+	t.Run("type function", func(t *testing.T) {
+		testSession(t, testSessionTest{
+			Inputs: []testSessionInput{
+				{
+					Input:  `type("foo")`,
+					Output: "string",
+				},
+			},
+		})
+	})
+
+	t.Run("type type is type", func(t *testing.T) {
+		testSession(t, testSessionTest{
+			Inputs: []testSessionInput{
+				{
+					Input:  `type(type("foo"))`,
+					Output: "type",
+				},
+			},
+		})
+	})
+
+	t.Run("interpolating type with strings is not possible", func(t *testing.T) {
+		testSession(t, testSessionTest{
+			Inputs: []testSessionInput{
+				{
+					Input:         `"quin${type([])}"`,
+					Error:         true,
+					ErrorContains: "Invalid template interpolation value",
+				},
+			},
+		})
+	})
+
 	t.Run("type function cannot be used in expressions", func(t *testing.T) {
 		testSession(t, testSessionTest{
 			Inputs: []testSessionInput{
 				{
 					Input:         `[for i in [1, "two", true]: type(i)]`,
+					Output:        "",
+					Error:         true,
+					ErrorContains: "Invalid use of type function",
+				},
+			},
+		})
+	})
+
+	t.Run("type equality checks are not permitted", func(t *testing.T) {
+		testSession(t, testSessionTest{
+			Inputs: []testSessionInput{
+				{
+					Input:         `type("foo") == type("bar")`,
+					Output:        "",
 					Error:         true,
 					ErrorContains: "Invalid use of type function",
 				},
@@ -311,3 +360,86 @@ type testSessionInput struct {
 	Exit           bool // Exit is true if exiting is expected
 	ErrorContains  string
 }
+
+func TestTypeString(t *testing.T) {
+	tests := []struct {
+		Input cty.Value
+		Want  string
+	}{
+		// Primititves
+		{
+			cty.StringVal("a"),
+			"string",
+		},
+		{
+			cty.NumberIntVal(42),
+			"number",
+		},
+		{
+			cty.BoolVal(true),
+			"bool",
+		},
+		// Collections
+		{
+			cty.EmptyObjectVal,
+			`object({})`,
+		},
+		{
+			cty.EmptyTupleVal,
+			`tuple([])`,
+		},
+		{
+			cty.ListValEmpty(cty.String),
+			`list(string)`,
+		},
+		{
+			cty.MapValEmpty(cty.String),
+			`map(string)`,
+		},
+		{
+			cty.SetValEmpty(cty.String),
+			`set(string)`,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.StringVal("a")}),
+			`list(string)`,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.ListVal([]cty.Value{cty.NumberIntVal(42)})}),
+			`list(list(number))`,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.MapValEmpty(cty.String)}),
+			`list(map(string))`,
+		},
+		{
+			cty.ListVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
+				"foo": cty.StringVal("bar"),
+			})}),
+			"list(\n    object({\n        foo: string,\n    }),\n)",
+		},
+		// Unknowns and Nulls
+		{
+			cty.UnknownVal(cty.String),
+			"string",
+		},
+		{
+			cty.NullVal(cty.Object(map[string]cty.Type{
+				"foo": cty.String,
+			})),
+			"object({\n    foo: string,\n})",
+		},
+		{ // irrelevant marks do nothing
+			cty.ListVal([]cty.Value{cty.ObjectVal(map[string]cty.Value{
+				"foo": cty.StringVal("bar").Mark("ignore me"),
+			})}),
+			"list(\n    object({\n        foo: string,\n    }),\n)",
+		},
+	}
+	for _, test := range tests {
+		got := typeString(test.Input.Type())
+		if got != test.Want {
+			t.Errorf("wrong result:\n%s", cmp.Diff(got, test.Want))
+		}
+	}
+}