zclsyntax: template single-interpolation case as separate AST node

Rather than setting an "Unwrap" field on the existing TemplateExpr, we'll
instead use a separate, simpler AST node.

There is very little in common between these two cases, so overloading the
TemplateExpr node doesn't buy much. A separate node is needed here,
rather than just returning the wrapped node directly, to give us somewhere
to capture the full source range extent of the wrapping template, whereas
the inner expression only captures the range of itself. This is important
both for good diagnostics and for transforming zclsyntax AST into
zclwrite AST.

zcl/zclsyntax/expression_template.go

@@ -11,7 +11,6 @@ import (
 
 type TemplateExpr struct {
 	Parts []Expression
-	Unwrap bool
 
 	SrcRange zcl.Range
 }
@@ -23,14 +22,6 @@ func (e *TemplateExpr) walkChildNodes(w internalWalkFunc) {
 }
 
 func (e *TemplateExpr) Value(ctx *zcl.EvalContext) (cty.Value, zcl.Diagnostics) {
-	if e.Unwrap {
-		if len(e.Parts) != 1 {
-			// should never happen - parser bug, if so
-			panic("Unwrap set with len(e.Parts) != 1")
-		}
-		return e.Parts[0].Value(ctx)
-	}
-
 	buf := &bytes.Buffer{}
 	var diags zcl.Diagnostics
 	isKnown := true
@@ -93,3 +84,29 @@ func (e *TemplateExpr) Range() zcl.Range {
 func (e *TemplateExpr) StartRange() zcl.Range {
 	return e.Parts[0].StartRange()
 }
+
+// TemplateWrapExpr is used instead of a TemplateExpr when a template
+// consists _only_ of a single interpolation sequence. In that case, the
+// template's result is the single interpolation's result, verbatim with
+// no type conversions.
+type TemplateWrapExpr struct {
+	Wrapped Expression
+
+	SrcRange zcl.Range
+}
+
+func (e *TemplateWrapExpr) walkChildNodes(w internalWalkFunc) {
+	e.Wrapped = w(e.Wrapped).(Expression)
+}
+
+func (e *TemplateWrapExpr) Value(ctx *zcl.EvalContext) (cty.Value, zcl.Diagnostics) {
+	return e.Wrapped.Value(ctx)
+}
+
+func (e *TemplateWrapExpr) Range() zcl.Range {
+	return e.SrcRange
+}
+
+func (e *TemplateWrapExpr) StartRange() zcl.Range {
+	return e.SrcRange
+}

zcl/zclsyntax/expression_vars.go

@@ -55,6 +55,10 @@ func (e *TemplateExpr) Variables() []zcl.Traversal {
 	return Variables(e)
 }
 
+func (e *TemplateWrapExpr) Variables() []zcl.Traversal {
+	return Variables(e)
+}
+
 func (e *TupleConsExpr) Variables() []zcl.Traversal {
 	return Variables(e)
 }

zcl/zclsyntax/parser.go

@@ -761,14 +761,22 @@ func (p *parser) parseExpressionTerm() (Expression, zcl.Diagnostics) {
 	case TokenOQuote, TokenOHeredoc:
 		open := p.Read() // eat opening marker
 		closer := p.oppositeBracket(open.Type)
-		exprs, unwrap, _, diags := p.parseTemplateInner(closer)
+		exprs, passthru, _, diags := p.parseTemplateInner(closer)
 
 		closeRange := p.PrevRange()
 
+		if passthru {
+			if len(exprs) != 1 {
+				panic("passthru set with len(exprs) != 1")
+			}
+			return &TemplateWrapExpr{
+				Wrapped:  exprs[0],
+				SrcRange: zcl.RangeBetween(open.Range, closeRange),
+			}, diags
+		}
+
 		return &TemplateExpr{
 			Parts:    exprs,
-			Unwrap: unwrap,
-
 			SrcRange: zcl.RangeBetween(open.Range, closeRange),
 		}, diags
 

zcl/zclsyntax/parser_template.go

@@ -14,12 +14,20 @@ func (p *parser) ParseTemplate() (Expression, zcl.Diagnostics) {
 }
 
 func (p *parser) parseTemplate(end TokenType) (Expression, zcl.Diagnostics) {
-	exprs, unwrap, rng, diags := p.parseTemplateInner(end)
+	exprs, passthru, rng, diags := p.parseTemplateInner(end)
+
+	if passthru {
+		if len(exprs) != 1 {
+			panic("passthru set with len(exprs) != 1")
+		}
+		return &TemplateWrapExpr{
+			Wrapped:  exprs[0],
+			SrcRange: rng,
+		}, diags
+	}
 
 	return &TemplateExpr{
 		Parts:    exprs,
-		Unwrap: unwrap,
-
 		SrcRange: rng,
 	}, diags
 }
@@ -33,14 +41,14 @@ func (p *parser) parseTemplateInner(end TokenType) ([]Expression, bool, zcl.Rang
 	exprs, exprsDiags := tp.parseRoot()
 	diags = append(diags, exprsDiags...)
 
-	unwrap := false
+	passthru := false
 	if len(parts.Tokens) == 2 { // one real token and one synthetic "end" token
 		if _, isInterp := parts.Tokens[0].(*templateInterpToken); isInterp {
-			unwrap = true
+			passthru = true
 		}
 	}
 
-	return exprs, unwrap, parts.SrcRange, diags
+	return exprs, passthru, parts.SrcRange, diags
 }
 
 type templateParser struct {
@@ -87,8 +95,7 @@ func (p *templateParser) parseExpr() (Expression, zcl.Diagnostics) {
 		return p.parseIf()
 
 	case *templateForToken:
-		// TODO: implement
-		panic("template for token not yet implemented")
+		return p.parseFor()
 
 	case *templateEndToken:
 		p.Read() // eat erroneous token

zcl/zclsyntax/parser_test.go

@@ -433,7 +433,6 @@ block "valid" {}
 									},
 								},
 							},
-							Unwrap: false,
 
 							SrcRange: zcl.Range{
 								Start: zcl.Pos{Line: 1, Column: 5, Byte: 4},