[dev.typeparams] cmd/compile: refactor irgen's handling of ":="

The previous code was stylized after noder, which was written when it
was more idiomatic to simple create a gc.Node and then populate and
shuffle around its fields as appropriate.

Now with package ir, it's somewhat nicer to compute all the fields up
front and pass them to the constructor functions, rather than passing
nil and populating the fields afterwards.

Net addition of lines of code, but I think the new code is overall
still somewhat simpler, and will be easier to refactor out into code
for helpers.go.

Change-Id: I8c6f6b65e0a8317129655a0fc493d8af75527b97
Reviewed-on: https://go-review.googlesource.com/c/go/+/285732
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/noder/stmt.go

@@ -59,19 +59,17 @@ func (g *irgen) stmt0(stmt syntax.Stmt) ir.Node {
 			return ir.NewAssignOpStmt(g.pos(stmt), op, g.expr(stmt.Lhs), g.expr(stmt.Rhs))
 		}
 
+		names, lhs := g.assignList(stmt.Lhs, stmt.Op == syntax.Def)
 		rhs := g.exprList(stmt.Rhs)
-		if list, ok := stmt.Lhs.(*syntax.ListExpr); ok && len(list.ElemList) != 1 || len(rhs) != 1 {
-			n := ir.NewAssignListStmt(g.pos(stmt), ir.OAS2, nil, nil)
-			n.Def = stmt.Op == syntax.Def
-			n.Lhs = g.assignList(stmt.Lhs, n, n.Def)
-			n.Rhs = rhs
+
+		if len(lhs) == 1 && len(rhs) == 1 {
+			n := ir.NewAssignStmt(g.pos(stmt), lhs[0], rhs[0])
+			n.Def = initDefn(n, names)
 			return n
 		}
 
-		n := ir.NewAssignStmt(g.pos(stmt), nil, nil)
-		n.Def = stmt.Op == syntax.Def
-		n.X = g.assignList(stmt.Lhs, n, n.Def)[0]
-		n.Y = rhs[0]
+		n := ir.NewAssignListStmt(g.pos(stmt), ir.OAS2, lhs, rhs)
+		n.Def = initDefn(n, names)
 		return n
 
 	case *syntax.BranchStmt:
@@ -119,9 +117,9 @@ func (g *irgen) op(op syntax.Operator, ops []ir.Op) ir.Op {
 	return ops[op]
 }
 
-func (g *irgen) assignList(expr syntax.Expr, defn ir.InitNode, colas bool) []ir.Node {
-	if !colas {
-		return g.exprList(expr)
+func (g *irgen) assignList(expr syntax.Expr, def bool) ([]*ir.Name, []ir.Node) {
+	if !def {
+		return nil, g.exprList(expr)
 	}
 
 	var exprs []syntax.Expr
@@ -131,6 +129,7 @@ func (g *irgen) assignList(expr syntax.Expr, defn ir.InitNode, colas bool) []ir.
 		exprs = []syntax.Expr{expr}
 	}
 
+	var names []*ir.Name
 	res := make([]ir.Node, len(exprs))
 	for i, expr := range exprs {
 		expr := expr.(*syntax.Name)
@@ -145,11 +144,28 @@ func (g *irgen) assignList(expr syntax.Expr, defn ir.InitNode, colas bool) []ir.
 		}
 
 		name, _ := g.def(expr)
-		name.Defn = defn
-		defn.PtrInit().Append(ir.NewDecl(name.Pos(), ir.ODCL, name))
+		names = append(names, name)
 		res[i] = name
 	}
-	return res
+
+	return names, res
+}
+
+// initDefn marks the given names as declared by defn and populates
+// its Init field with ODCL nodes. It then reports whether any names
+// were so declared, which can be used to initialize defn.Def.
+func initDefn(defn ir.InitNode, names []*ir.Name) bool {
+	if len(names) == 0 {
+		return false
+	}
+
+	init := make([]ir.Node, len(names))
+	for i, name := range names {
+		name.Defn = defn
+		init[i] = ir.NewDecl(name.Pos(), ir.ODCL, name)
+	}
+	defn.SetInit(init)
+	return true
 }
 
 func (g *irgen) blockStmt(stmt *syntax.BlockStmt) []ir.Node {
@@ -171,18 +187,25 @@ func (g *irgen) ifStmt(stmt *syntax.IfStmt) ir.Node {
 	return g.init(init, n)
 }
 
+// unpackTwo returns the first two nodes in list. If list has fewer
+// than 2 nodes, then the missing nodes are replaced with nils.
+func unpackTwo(list []ir.Node) (fst, snd ir.Node) {
+	switch len(list) {
+	case 0:
+		return nil, nil
+	case 1:
+		return list[0], nil
+	default:
+		return list[0], list[1]
+	}
+}
+
 func (g *irgen) forStmt(stmt *syntax.ForStmt) ir.Node {
 	if r, ok := stmt.Init.(*syntax.RangeClause); ok {
-		n := ir.NewRangeStmt(g.pos(r), nil, nil, g.expr(r.X), nil)
-		if r.Lhs != nil {
-			n.Def = r.Def
-			lhs := g.assignList(r.Lhs, n, n.Def)
-			n.Key = lhs[0]
-			if len(lhs) > 1 {
-				n.Value = lhs[1]
-			}
-		}
-		n.Body = g.blockStmt(stmt.Body)
+		names, lhs := g.assignList(r.Lhs, r.Def)
+		key, value := unpackTwo(lhs)
+		n := ir.NewRangeStmt(g.pos(r), key, value, g.expr(r.X), g.blockStmt(stmt.Body))
+		n.Def = initDefn(n, names)
 		return n
 	}