zclsyntax: Generate "Variables" implementations for all Expressions

The implementation of Variables will be identical for every Expression
implementation since we just wrap our AST-walk-based "Variables" function
to do the work.

Rather than manually copy-pasting the declaration for each expression
type, instead we'll generate this programmatically using "go generate".
This will need to be re-run each time a new expression node type is
added, in order to make it actually implement the Expression interface.

zcl/zclsyntax/expression.go

@@ -43,10 +43,6 @@ func (e *LiteralValueExpr) StartRange() zcl.Range {
 	return e.SrcRange
 }
 
-func (e *LiteralValueExpr) Variables() []zcl.Traversal {
-	return Variables(e)
-}
-
 // ScopeTraversalExpr is an Expression that retrieves a value from the scope
 // using a traversal.
 type ScopeTraversalExpr struct {
@@ -69,7 +65,3 @@ func (e *ScopeTraversalExpr) Range() zcl.Range {
 func (e *ScopeTraversalExpr) StartRange() zcl.Range {
 	return e.SrcRange
 }
-
-func (e *ScopeTraversalExpr) Variables() []zcl.Traversal {
-	return Variables(e)
-}

zcl/zclsyntax/expression_vars.go

@@ -0,0 +1,16 @@
+package zclsyntax
+
+// Generated by expression_vars_get.go. DO NOT EDIT.
+// Run 'go generate' on this package to update the set of functions here.
+
+import (
+	"github.com/apparentlymart/go-zcl/zcl"
+)
+
+func (e *LiteralValueExpr) Variables() []zcl.Traversal {
+	return Variables(e)
+}
+
+func (e *ScopeTraversalExpr) Variables() []zcl.Traversal {
+	return Variables(e)
+}

zcl/zclsyntax/expression_vars_gen.go

@@ -0,0 +1,99 @@
+// This is a 'go generate'-oriented program for producing the "Variables"
+// method on every Expression implementation found within this package.
+// All expressions share the same implementation for this method, which
+// just wraps the package-level function "Variables" and uses an AST walk
+// to do its work.
+
+// +build ignore
+
+package main
+
+import (
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"os"
+	"sort"
+)
+
+func main() {
+	fs := token.NewFileSet()
+	pkgs, err := parser.ParseDir(fs, ".", nil, 0)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error while parsing: %s\n", err)
+		os.Exit(1)
+	}
+	pkg := pkgs["zclsyntax"]
+
+	// Walk all the files and collect the receivers of any "Value" methods
+	// that look like they are trying to implement Expression.
+	var recvs []string
+	for _, f := range pkg.Files {
+		for _, decl := range f.Decls {
+			fd, ok := decl.(*ast.FuncDecl)
+			if !ok {
+				continue
+			}
+			if fd.Name.Name != "Value" {
+				continue
+			}
+			results := fd.Type.Results.List
+			if len(results) != 2 {
+				continue
+			}
+			valResult := fd.Type.Results.List[0].Type.(*ast.SelectorExpr).X.(*ast.Ident)
+			diagsResult := fd.Type.Results.List[1].Type.(*ast.SelectorExpr).X.(*ast.Ident)
+
+			if valResult.Name != "cty" && diagsResult.Name != "zcl" {
+				continue
+			}
+
+			// If we have a method called Value and its returns something in
+			// cty followed by something in zcl then that's specific enough
+			// for now, even though this is not 100% exact as a correct
+			// implementation of Value.
+
+			recvTy := fd.Recv.List[0].Type
+
+			switch rtt := recvTy.(type) {
+			case *ast.StarExpr:
+				name := rtt.X.(*ast.Ident).Name
+				recvs = append(recvs, fmt.Sprintf("*%s", name))
+			default:
+				fmt.Fprintf(os.Stderr, "don't know what to do with a %T receiver\n", recvTy)
+			}
+
+		}
+	}
+
+	sort.Strings(recvs)
+
+	of, err := os.OpenFile("expression_vars.go", os.O_WRONLY|os.O_CREATE|os.O_TRUNC, os.ModePerm)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to open output file: %s\n", err)
+		os.Exit(1)
+	}
+
+	fmt.Fprint(of, outputPreamble)
+	for _, recv := range recvs {
+		fmt.Fprintf(of, outputMethodFmt, recv)
+	}
+	fmt.Fprint(of, "\n")
+
+}
+
+const outputPreamble = `package zclsyntax
+
+// Generated by expression_vars_get.go. DO NOT EDIT.
+// Run 'go generate' on this package to update the set of functions here.
+
+import (
+	"github.com/apparentlymart/go-zcl/zcl"
+)`
+
+const outputMethodFmt = `
+
+func (e %s) Variables() []zcl.Traversal {
+	return Variables(e)
+}`

zcl/zclsyntax/generate.go

@@ -0,0 +1,3 @@
+package zclsyntax
+
+//go:generate go run expression_vars_gen.go