lang: Improve string interpolation

The original string interpolation was based on hil which didn't allow
proper escaping, since they used a different escape pattern. Secondly,
the golang Unquote function didn't deal with the variable substitution,
which meant it had to be performed in a second step.

Most importantly, because we did this partial job in Unquote (the fact
that is strips the leading and trailing quotes tricked me into thinking
I was done with interpolation!) it was impossible to remedy the
remaining parts in a second pass with hil. Both operations needs to be
done in a single step. This is logical when you aren't tunnel visioned.

This patch replaces both of these so that string interpolation works
properly. This removes the ability to allow inline function calls in a
string, however this was an incidental feature, and it's not clear that
having it is a good idea. It also requires you wrap the var name with
curly braces. (They are not optional.)

This comes with a load of tests, but I think I got some of it wrong,
since I'm quite new at ragel. If you find something, please say so =D In
any case, this is much better than the original hil implementation, and
easy for a new contributor to patch to make the necessary fixes.

lang/interpolate_test.go

@@ -30,6 +30,7 @@ import (
 
 	"github.com/davecgh/go-spew/spew"
 	"github.com/kylelemons/godebug/pretty"
+	"github.com/sanity-io/litter"
 )
 
 func TestInterpolate0(t *testing.T) {
@@ -127,6 +128,66 @@ func TestInterpolate0(t *testing.T) {
 			ast:  ast,
 		})
 	}
+	{
+		ast := &StmtProg{
+			Prog: []interfaces.Stmt{
+				&StmtRes{
+					Kind: "test",
+					Name: &ExprStr{
+						V: "t1",
+					},
+					Contents: []StmtResContents{
+						&StmtResField{
+							Field: "stringptr",
+							Value: &ExprStr{
+								V: "${hello}",
+							},
+						},
+					},
+				},
+			},
+		}
+		testCases = append(testCases, test{
+			name: "variable escaping 1",
+			code: `
+			test "t1" {
+				stringptr => "\${hello}",
+			}
+			`,
+			fail: false,
+			ast:  ast,
+		})
+	}
+	{
+		ast := &StmtProg{
+			Prog: []interfaces.Stmt{
+				&StmtRes{
+					Kind: "test",
+					Name: &ExprStr{
+						V: "t1",
+					},
+					Contents: []StmtResContents{
+						&StmtResField{
+							Field: "stringptr",
+							Value: &ExprStr{
+								V: `\` + `$` + `{hello}`,
+							},
+						},
+					},
+				},
+			},
+		}
+		testCases = append(testCases, test{
+			name: "variable escaping 2",
+			code: `
+			test "t1" {
+				stringptr => "` + `\\` + `\$` + "{hello}" + `",
+			}
+			`,
+			fail: false,
+			ast:  ast,
+		})
+	}
 
 	names := []string{}
 	for index, tc := range testCases { // run all the tests
@@ -189,14 +250,28 @@ func TestInterpolate0(t *testing.T) {
 				return
 			}
 			// double check because DeepEqual is different since the logf exists
+			lo := &litter.Options{
+				//Compact: false,
+				StripPackageNames: true,
+				HidePrivateFields: true,
+				HideZeroValues:    true,
+				//FieldExclusions: regexp.MustCompile(`^(data)$`),
+				//FieldFilter       func(reflect.StructField, reflect.Value) bool
+				//HomePackage       string
+				//Separator         string
+			}
+			if lo.Sdump(iast) == lo.Sdump(exp) { // simple diff
+				return
+			}
+
 			diff := pretty.Compare(iast, exp)
 			if diff == "" { // bonus
 				return
 			}
 			t.Errorf("test #%d: AST did not match expected", index)
 			// TODO: consider making our own recursive print function
-			t.Logf("test #%d:   actual: \n%s", index, spew.Sdump(iast))
-			t.Logf("test #%d: expected: \n%s", index, spew.Sdump(exp))
+			t.Logf("test #%d:   actual: \n%s", index, lo.Sdump(iast))
+			t.Logf("test #%d: expected: \n%s", index, lo.Sdump(exp))
 			t.Logf("test #%d: diff:\n%s", index, diff)
 		})
 	}
@@ -341,7 +416,7 @@ func TestInterpolateBasicStmt(t *testing.T) {
 		}
 		resName := &ExprCall{
 			Name: operatorFuncName,
-			// incorrect sig for this function, but correct interpolation
+			// incorrect sig for this function, and now invalid interpolation
 			Args: []interfaces.Expr{
 				&ExprStr{
 					V: "+",
@@ -370,11 +445,12 @@ func TestInterpolateBasicStmt(t *testing.T) {
 				},
 			},
 		}
+		_ = exp // historical
 		testCases = append(testCases, test{
 			name: "expanded invalid resource name",
 			ast:  ast,
-			fail: false,
-			exp:  exp,
+			fail: true,
+			//exp:  exp,
 		})
 	}
 
@@ -524,7 +600,7 @@ func TestInterpolateBasicExpr(t *testing.T) {
 	//}
 	{
 		ast := &ExprStr{
-			V: "sweetie${3.14159}", // invalid but only at type check
+			V: "sweetie${3.14159}", // invalid
 		}
 		exp := &ExprCall{
 			Name: operatorFuncName,
@@ -540,11 +616,11 @@ func TestInterpolateBasicExpr(t *testing.T) {
 				},
 			},
 		}
+		_ = exp // historical
 		testCases = append(testCases, test{
 			name: "float expansion",
 			ast:  ast,
-			fail: false,
-			exp:  exp,
+			fail: true,
 		})
 	}
 	{
@@ -566,11 +642,11 @@ func TestInterpolateBasicExpr(t *testing.T) {
 				},
 			},
 		}
+		_ = exp // historical
 		testCases = append(testCases, test{
 			name: "function expansion",
 			ast:  ast,
-			fail: false,
-			exp:  exp,
+			fail: true,
 		})
 	}
 	{
@@ -599,11 +675,11 @@ func TestInterpolateBasicExpr(t *testing.T) {
 				},
 			},
 		}
+		_ = exp // historical
 		testCases = append(testCases, test{
 			name: "function expansion arg",
 			ast:  ast,
-			fail: false,
-			exp:  exp,
+			fail: true,
 		})
 	}
 	// FIXME: i am broken, i don't deal well with negatives for some reason