compiler: fix error message on missing with target (#4458)

"with keyword target must start with input or data"
=>
"with keyword target must reference existing input or data"

Fixes #4431

Signed-off-by: Anders Eknert <anders@eknert.com>

ast/compile.go

@@ -4615,7 +4615,7 @@ func rewriteWithModifier(c *Compiler, f *equalityFactory, expr *Expr) ([]*Expr,
 
 func validateTarget(c *Compiler, term *Term) *Error {
 	if !isInputRef(term) && !isDataRef(term) {
-		return NewError(TypeErr, term.Location, "with keyword target must start with %v or %v", InputRootDocument, DefaultRootDocument)
+		return NewError(TypeErr, term.Location, "with keyword target must reference existing %v or %v", InputRootDocument, DefaultRootDocument)
 	}
 
 	if isDataRef(term) {

ast/compile_test.go

@@ -3710,7 +3710,7 @@ func TestCompilerRewriteWithValue(t *testing.T) {
 		{
 			note:    "invalid target",
 			input:   `p { true with foo.q as 1 }`,
-			wantErr: fmt.Errorf("rego_type_error: with keyword target must start with input or data"),
+			wantErr: fmt.Errorf("rego_type_error: with keyword target must reference existing input or data"),
 		},
 	}
 
@@ -5786,7 +5786,7 @@ func TestQueryCompiler(t *testing.T) {
 			q:        "x = 1 with foo.p as null",
 			pkg:      "",
 			imports:  nil,
-			expected: fmt.Errorf("1 error occurred: 1:12: rego_type_error: with keyword target must start with input or data"),
+			expected: fmt.Errorf("1 error occurred: 1:12: rego_type_error: with keyword target must reference existing input or data"),
 		},
 		{
 			note:     "rewrite with value",