Move variable assignment handling to generation context

This allows the parsing code to be cleaner, as it
doesn't have to care about variable assignments.

Bug: 228518745
Test: go test
Change-Id: I33425c2fb51acab4901bfa82a53d337b75210f8e

mk2rbc/node.go

@@ -196,7 +196,6 @@ type assignmentNode struct {
 	flavor   assignmentFlavor
 	location ErrorLocation
 	isTraced bool
-	previous *assignmentNode
 }
 
 func (asgn *assignmentNode) emit(gctx *generationContext) {
@@ -209,7 +208,7 @@ func (asgn *assignmentNode) emit(gctx *generationContext) {
 		gctx.newLine()
 		gctx.tracedCount++
 		gctx.writef(`print("%s.%d: %s := ", `, gctx.starScript.mkFile, gctx.tracedCount, asgn.lhs.name())
-		asgn.lhs.emitGet(gctx, true)
+		asgn.lhs.emitGet(gctx)
 		gctx.writef(")")
 	}
 }
@@ -271,6 +270,7 @@ type switchCase struct {
 func (cb *switchCase) emit(gctx *generationContext) {
 	cb.gate.emit(gctx)
 	gctx.indentLevel++
+	gctx.pushVariableAssignments()
 	hasStatements := false
 	for _, node := range cb.nodes {
 		if _, ok := node.(*commentNode); !ok {
@@ -282,6 +282,7 @@ func (cb *switchCase) emit(gctx *generationContext) {
 		gctx.emitPass()
 	}
 	gctx.indentLevel--
+	gctx.popVariableAssignments()
 }
 
 // A single complete if ... elseif ... else ... endif sequences
@@ -302,6 +303,7 @@ type foreachNode struct {
 }
 
 func (f *foreachNode) emit(gctx *generationContext) {
+	gctx.pushVariableAssignments()
 	gctx.newLine()
 	gctx.writef("for %s in ", f.varName)
 	f.list.emit(gctx)
@@ -318,4 +320,5 @@ func (f *foreachNode) emit(gctx *generationContext) {
 		gctx.emitPass()
 	}
 	gctx.indentLevel--
+	gctx.popVariableAssignments()
 }