core: Add list() interpolation function

The list() interpolation function provides a way to add support for list
literals (of strings) to HIL without having to invent new syntax for it
and modify the HIL parser.

It presents as a function, thus:

    - list() -> []
    - list("a") -> ["a"]
    - list("a", "b") -> ["a", "b"]

Thanks to @wr0ngway for the idea of this approach, fixes #7460.

config/interpolate_funcs.go

@@ -69,6 +69,7 @@ func Funcs() map[string]ast.Function {
 		"join":         interpolationFuncJoin(),
 		"jsonencode":   interpolationFuncJSONEncode(),
 		"length":       interpolationFuncLength(),
+		"list":         interpolationFuncList(),
 		"lower":        interpolationFuncLower(),
 		"md5":          interpolationFuncMd5(),
 		"uuid":         interpolationFuncUUID(),
@@ -83,6 +84,26 @@ func Funcs() map[string]ast.Function {
 	}
 }
 
+// interpolationFuncList creates a list from the parameters passed
+// to it.
+func interpolationFuncList() ast.Function {
+	return ast.Function{
+		ArgTypes:     []ast.Type{},
+		ReturnType:   ast.TypeList,
+		Variadic:     true,
+		VariadicType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			var outputList []string
+
+			for _, val := range args {
+				outputList = append(outputList, val.(string))
+			}
+
+			return stringSliceToVariableValue(outputList), nil
+		},
+	}
+}
+
 // interpolationFuncCompact strips a list of multi-variable values
 // (e.g. as returned by "split") of any empty strings.
 func interpolationFuncCompact() ast.Function {

config/interpolate_funcs_test.go

@@ -12,6 +12,55 @@ import (
 	"github.com/hashicorp/hil/ast"
 )
 
+func TestInterpolateFuncList(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			// empty input returns empty list
+			{
+				`${list()}`,
+				[]interface{}{},
+				false,
+			},
+
+			// single input returns list of length 1
+			{
+				`${list("hello")}`,
+				[]interface{}{"hello"},
+				false,
+			},
+
+			// two inputs returns list of length 2
+			{
+				`${list("hello", "world")}`,
+				[]interface{}{"hello", "world"},
+				false,
+			},
+
+			// not a string input gives error
+			{
+				`${list("hello", "${var.list}")}`,
+				nil,
+				true,
+			},
+		},
+		Vars: map[string]ast.Variable{
+			"var.list": {
+				Type: ast.TypeList,
+				Value: []ast.Variable{
+					{
+						Type:  ast.TypeString,
+						Value: "Hello",
+					},
+					{
+						Type:  ast.TypeString,
+						Value: "World",
+					},
+				},
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncCompact(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{

website/source/docs/configuration/interpolation.html.md

@@ -168,6 +168,11 @@ The supported built-in functions are:
       * `${length(split(",", "a,b,c"))}` = 3
       * `${length("a,b,c")}` = 5
 
+  * `list(items...)` - Returns a list consisting of the arguments to the function.
+      This function provides a way of representing list literals in interpolation.
+      * `${list("a", "b", "c")}` returns a list of `"a", "b", "c"`.
+      * `${list()}` returns an empty list.
+ 
   * `lookup(map, key [, default])` - Performs a dynamic lookup into a mapping
       variable. The `map` parameter should be another variable, such
       as `var.amis`. If `key` does not exist in `map`, the interpolation will