core: allow partially-unknown lists from splat syntax

This was actually redundant anyway since HIL itself applied a similar
rule where any partially-unknown list would be automatically flattened
to a single unknown value.

However, now we're changing HIL to explicitly permit partially-unknown
lists so that we can allow the index operator [...] to succeed when
applied to one of the elements that _is_ known.

This, in conjunction with hashicorp/hil#51 and hashicorp/hil#52,
fixes #3449.

terraform/interpolate.go

@@ -594,10 +594,6 @@ func (i *Interpolater) computeResourceMultiVariable(
 		}
 
 		if singleAttr, ok := r.Primary.Attributes[v.Field]; ok {
-			if singleAttr == config.UnknownVariableValue {
-				return &unknownVariable, nil
-			}
-
 			values = append(values, singleAttr)
 			continue
 		}
@@ -613,10 +609,6 @@ func (i *Interpolater) computeResourceMultiVariable(
 			return nil, err
 		}
 
-		if multiAttr == unknownVariable {
-			return &unknownVariable, nil
-		}
-
 		values = append(values, multiAttr)
 	}
 

terraform/interpolate_test.go

@@ -359,8 +359,81 @@ func TestInterpolater_resourceVariableMulti(t *testing.T) {
 	}
 
 	testInterpolate(t, i, scope, "aws_instance.web.*.foo", ast.Variable{
-		Value: config.UnknownVariableValue,
+		Type: ast.TypeList,
+		Value: []ast.Variable{
+			{
 				Type:  ast.TypeUnknown,
+				Value: config.UnknownVariableValue,
+			},
+		},
+	})
+}
+
+func TestInterpolater_resourceVariableMultiPartialUnknown(t *testing.T) {
+	lock := new(sync.RWMutex)
+	state := &State{
+		Modules: []*ModuleState{
+			&ModuleState{
+				Path: rootModulePath,
+				Resources: map[string]*ResourceState{
+					"aws_instance.web.0": &ResourceState{
+						Type: "aws_instance",
+						Primary: &InstanceState{
+							ID: "bar",
+							Attributes: map[string]string{
+								"foo": "1",
+							},
+						},
+					},
+					"aws_instance.web.1": &ResourceState{
+						Type: "aws_instance",
+						Primary: &InstanceState{
+							ID: "bar",
+							Attributes: map[string]string{
+								"foo": config.UnknownVariableValue,
+							},
+						},
+					},
+					"aws_instance.web.2": &ResourceState{
+						Type: "aws_instance",
+						Primary: &InstanceState{
+							ID: "bar",
+							Attributes: map[string]string{
+								"foo": "2",
+							},
+						},
+					},
+				},
+			},
+		},
+	}
+
+	i := &Interpolater{
+		Module:    testModule(t, "interpolate-resource-variable-multi"),
+		State:     state,
+		StateLock: lock,
+	}
+
+	scope := &InterpolationScope{
+		Path: rootModulePath,
+	}
+
+	testInterpolate(t, i, scope, "aws_instance.web.*.foo", ast.Variable{
+		Type: ast.TypeList,
+		Value: []ast.Variable{
+			{
+				Type:  ast.TypeString,
+				Value: "1",
+			},
+			{
+				Type:  ast.TypeUnknown,
+				Value: config.UnknownVariableValue,
+			},
+			{
+				Type:  ast.TypeString,
+				Value: "2",
+			},
+		},
 	})
 }
 
@@ -408,8 +481,13 @@ func TestInterpolater_resourceVariableMultiList(t *testing.T) {
 	}
 
 	testInterpolate(t, i, scope, "aws_instance.web.*.ip", ast.Variable{
-		Value: config.UnknownVariableValue,
+		Type: ast.TypeList,
+		Value: []ast.Variable{
+			{
 				Type:  ast.TypeUnknown,
+				Value: config.UnknownVariableValue,
+			},
+		},
 	})
 }
 

terraform/test-fixtures/interpolate-resource-variable-multi/main.tf

@@ -0,0 +1,3 @@
+resource "aws_instance" "web" {
+    count = 3
+}